Stabilized multi-node quantum communication networks and related methods

Abstract

The present disclosure relates to techniques for stabilizing, synchronizing, or monitoring quantum communication networks, including multi-node networks. The quantum communication network includes a node array with a first node and a second node interconnected by an optical communication channel. A transmitter unit at the first node generates a quantum signal and a reference signal distinguishable across at least two degrees of freedom, multiplexes the signals, and transmits a multiplexed signal through the channel. A receiver unit at the second node receives and demultiplexes the multiplexed signal back into the quantum and reference signals. A stabilization feedback loop detects the reference signal, determines error signals indicative of network parameter errors associated with the channel, and controls at least one optical actuator to apply corrective operations to stabilize the network parameters. In some embodiments, a monitoring unit derives monitoring signals from the detected reference signal to provide status indicators of network performance.

Description

STABILIZED MULTI-NODE QUANTUM COMMUNICATION NETWORKS AND RELATEDMETHODSRELATED PATENT APPLICATION

[0001] The present application claims priority to U.S. Provisional Patent Application No. 63 / 728,856 filed on December 6, 2024, the disclosure of which is incorporated herein by reference in its entirety.TECHNICAL FIELD

[0002] The technical field relates to quantum photonics, and specifically to stabilization, synchronization, and monitoring techniques applicable to multi-node quantum communication networks.BACKGROUND

[0003] Quantum networks, which distribute quantum signals across spatially-separated nodes, underpin various emerging applications, including secure communications, distributed sensing, and distributed computing. These networks depend on precise and stable quantum signal transmission, requiring synchronization of multiple parameters across spatially distributed locations. The inherent fragility of quantum signals and their susceptibility to degradation present significant challenges for the development and practical deployment of quantum networks. Current implementations are often restricted to two-node, short-distance arrangements, limiting applications to basic two-user communication protocols. In contrast, multi-node quantum networks hold the potential for more sophisticated and impactful applications, such as secure multiuser communications, multi-party computing, blind quantum computing, multi-node clock transfer, and distributed sensing.

[0004] Scaling quantum networks to support multiple users introduces additional challenges, which are amplified by physical processes that degrade the distribution of quantum signals across nodes. These processes include fluctuations in temperature, stress, and length along optical channels; mismatches in polarization, timing, and frequency references between nodes; optical losses during quantum signal processing; and the presence of background noise photons. As networks expand, these disturbances intensify, leading to tighter timing constraints and an increase in the number of required rectification modules, thereby imposing more demanding performance standards. Despite ongoing efforts, significant challenges remain in developing technologies capable of stabilizing, synchronizing, and mitigating errors within multi-node quantum networks.SUMMARY

[0005] The present disclosure relates to techniques for stabilizing, synchronizing, or monitoring quantum communication networks.

[0006] 1. A quantum communication network, including: a node array having a plurality of nodes including a first node and a second node; an optical communication infrastructure interconnecting the plurality of nodes and including an optical communication channel linking the first and second nodes; a transmitter unit located at the first node and including: an optical source module configured to generate a quantum signal and a reference signal that are distinguishable across at least two degrees of freedom; a multiplexing module configured to combine the quantum and reference signals across the at least two degrees of freedom, thereby generating a multiplexed signal; and an output port configured to transmit the multiplexed signal for propagation through the optical communication channel; a receiver unit located at the second node and including: an input port configured to receive the multiplexed signal; and a demultiplexing module configured to separate the multiplexed signal into the quantum signal and the reference signal; and a stabilization feedback loop for network error compensation, including: a detection stage located at the second node and configured to detect the reference signal following demultiplexing; a processing stage configured to determine error signals from the detected reference signal, the error signals being related to the at least two degrees of freedom and indicative of errors in network parameters associated with the optical communication channel; and an actuation stage including at least one optical actuator configured to apply corrective operations within the quantum communication network, based on the error signals to stabilize the network parameters.

[0007] In some embodiments, the optical communication channel includes an optical fiber link, an optical waveguide, or a free-space link.

[0008] In some embodiments, the reference signal is a classical signal. In other embodiments, the reference signal is a quantum signal. In some embodiments, the quantum signal and the reference signal originate from a same light source within the optical source module. In other embodiments, the quantum signal and the reference signal originate from different light sources within the optical source module.

[0009] In some embodiments, the optical source module is configured to emit the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

[0010] In some embodiments, the at least two degrees of freedom include (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

[0011] In some embodiments, the receiver unit further includes (i) a quantum processor configured to perform operations on the quantum signal, and / or (ii) a quantum memory configured to store the quantum signal.

[0012] In some embodiments, the network parameters include optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

[0013] In some embodiments, the actuation stage is configured to apply the corrective operations in real-time or near real-time. In some embodiments, the processing stage is configured to compare the error signals against predetermined threshold levels, and the actuation stage is configured to apply the corrective operations upon determination that the error signals exceed the predetermined threshold levels. In some embodiments, the actuation stage is configured to apply the corrective operations upstream of the demultiplexing module . In some embodiments, the actuation stage is configured to apply the corrective operations within the receiver unit. In some embodiments, the at least one actuator includes an optical delay line for controlling a physical channel length, an electronically controlled polarization module for managing channel polarization, an optical interferometer for addressing phase errors, or any combination thereof.

[0014] In some embodiments, the second node is one of a plurality of second nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes; the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes; and the stabilization feedback loop is one of a plurality of stabilization feedback loops, each stabilization feedback loop being associated with a respective receiver unit. In some embodiments, the multiplexed signals sent by the transmitter unit are identical across all the receiver units. In other embodiments, the multiplexed signals sent by the transmitter unit are not identical across all the receiver units, differing in their quantum signals, their reference signals, or both.

[0015] In some embodiments, the first node is one of a plurality of first nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node; the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes; the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node; and the stabilization feedback loop is one of a plurality of stabilization feedback loops, each stabilization feedback loop being associated with a respective receiver unit. In some embodiments, the multiplexed signals are identical across all the transmitter units. In other embodiments, the multiplexed signals are not identical across all the transmitter units, differing in their quantum signals, their reference signals, or both.

[0016] In accordance with another aspect, there is provided a method for stabilization of a quantum communication network including a node array having a plurality of nodes including a first node and a second node, and an optical communication infrastructure interconnecting the plurality of nodes and including an optical communication channel linking the first and second nodes, the method including: generating a quantum signal and a reference signal at the first node, wherein the quantum signal and the reference signal are distinguishable across at least two degrees of freedom; multiplexing the quantum and reference signals across the at least two degrees of freedom, thereby generating a multiplexed signal; transmitting the multiplexed signal from the first node to the second node through the optical communication channel; receiving the multiplexed signal at the second node; demultiplexing the multiplexed signal into the quantum signal and the reference signal; detecting the reference signal following demultiplexing; determining error signals from the detected reference signal, wherein the error signals are associated with the at least two degrees of freedom and are indicative of errors in network parameters associated with the optical communication channel; and controlling at least one optical actuator to apply corrective operations within the quantum communication network, based on the error signals to stabilize the network parameters.

[0017] In some embodiments, the reference signal is a classical signal. In other embodiments, the reference signal is a quantum signal.

[0018] In some embodiments, generating the quantum signal and the reference signal includes emitting the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

[0019] In some embodiments, the at least two degrees of freedom include (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

[0020] In some embodiments, the network parameters include optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

[0021] In some embodiments, controlling the at least one optical actuator includes applying the corrective operations in real-time or near real-time.

[0022] In some embodiments, the method further includes comparing the error signals against predetermined threshold levels, and controlling the at least one optical actuator includes applying the corrective operations upon determination that the error signals exceed the predetermined threshold levels.

[0023] In some embodiments, controlling the at least one optical actuator includes applying the corrective operations upstream of the demultiplexing module. In some embodiments, controlling the at least one optical actuator includes applying the corrective operations within the receiver unit.

[0024] In some embodiments, the at least one actuator includes an optical delay line for controlling a physical channel length, an electronically controlled polarization module for managing channel polarization, an optical interferometer for addressing phase errors, or any combination thereof.

[0025] In some embodiments, the second node is one of a plurality of second nodes, the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes, and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes, and the error signals are indicative of errors in network parameters associated with the plurality of optical communication channels.

[0026] In some embodiments, the first node is one of a plurality of first nodes, the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node, the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes, the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node, and the error signals are indicative of errors in network parameters associated with the plurality of optical communication channels.

[0027] In accordance with another aspect, there is provided a network stabilization system for a quantum communication network including a node array having a plurality of nodes including a first node and a second node, and an optical communication infrastructure interconnecting the plurality of nodes and including an optical communication channel linking the first and second nodes, the network stabilization system including: a transmitter unit at the first node, including: an optical source module configured to generate a quantum signal and a reference signal that are distinguishable across at least two degrees of freedom; a multiplexing module configured to combine the quantum and reference signals across the at least two degrees of freedom, thereby generating a multiplexed signal; and an output port configured to transmit the multiplexed signal from the first node to the second node through the optical communication channel; and a receiver unit at the second node, including: an input port configured to receive the multiplexed signal; and a demultiplexing module configured to separate the multiplexed signal into the quantum signal and the reference signal; and a stabilization feedback loop for network error compensation, including:a detection stage configured to detect the reference signal after demultiplexing; a processing stage configured to determine error signals from the detected reference signal, the error signals being associated with the at least two degrees of freedom and indicative of errors in network parameters associated with the optical communication channel; and an actuation stage including at least one optical actuator configured to apply corrective operations within the quantum communication network, based on the error signals to stabilize the network parameters.

[0028] In some embodiments, the reference signal is a classical signal. In other embodiments, the reference signal is a quantum signal.

[0029] In some embodiments, the optical source module is configured to emit the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

[0030] In some embodiments, the at least two degrees of freedom include (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

[0031] In some embodiments, the receiver unit further includes (i) a quantum processor configured to perform operations on the quantum signal, and / or (ii) a quantum memory configured to store the quantum signal.

[0032] In some embodiments, the network parameters include optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

[0033] In some embodiments, the actuation stage is configured to apply the corrective operations in real-time or near real-time.

[0034] In some embodiments, the processing stage is configured to compare the error signals against predetermined threshold levels, and the actuation stage is configured to apply the corrective operations upon determination that the error signals exceed the predetermined threshold levels.

[0035] In some embodiments, the actuation stage is configured to apply the corrective operations upstream of the demultiplexing module. In some embodiments, the actuation stage is configured to apply the corrective operations within the receiver unit.

[0036] In some embodiments, the at least one actuator includes an optical delay line for controlling a physical channel length, an electronically controlled polarization module for managing channel polarization, an optical interferometer for addressing phase errors, or any combination thereof.

[0037] In some embodiments, the second node is one of a plurality of second nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linkingthe first node to a respective one of the second nodes; the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes; and the stabilization feedback loop is one of a plurality of stabilization feedback loops, each stabilization feedback loop being associated with a respective receiver unit. In some embodiments, the multiplexed signals sent by the transmitter unit are identical across all the receiver units. In other embodiments, the multiplexed signals sent by the transmitter unit are not identical across all the receiver units, differing in their quantum signals, their reference signals, or both.

[0038] In some embodiments, the first node is one of a plurality of first nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node; the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes; the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node; and the stabilization feedback loop is one of a plurality of stabilization feedback loops, each stabilization feedback loop being associated with a respective receiver unit.

[0039] In some embodiments, the multiplexed signals are identical across all the transmitter units. In other embodiments, the multiplexed signals are not identical across all the transmitter units, differing in their quantum signals, their reference signals, or both.

[0040] In accordance with another aspect, there is provided a quantum communication network, including: a node array with a plurality of nodes, the plurality of nodes including a first node and a second node; an optical communication infrastructure interconnecting the plurality of nodes, the optical communication infrastructure including an optical communication channel linking the first and second nodes; a transmitter unit at the first node, including: an optical source module configured to generate a quantum signal and a reference signal, the quantum and reference signals being distinguishable across at least two degrees of freedom; a multiplexing module configured to combine the quantum and reference signals across the at least two degrees of freedom to produce a multiplexed signal; and an output port configured to transmit the multiplexed signal through the optical communication channel; a receiver unit at the second node, including: an input port configured to receive the multiplexed signal; and a demultiplexing module configured to separate the multiplexed signal into the quantum signal and the reference signal; and a network monitoring unit, including: a detection stage located at the second node and configured to detect the reference signal after demultiplexing; anda processing stage configured to derive monitoring signals from the detected reference signal, the monitoring signals being related to the at least two degrees of freedom and providing status indicators of network parameters associated with the optical communication channel.

[0041] In some embodiments, the optical communication channel includes an optical fiber link, an optical waveguide, or a free-space link.

[0042] In some embodiments, the reference signal is a classical signal. In other embodiments, the reference signal is a quantum signal.

[0043] In some embodiments, the optical source module is configured to emit the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

[0044] In some embodiments, the at least two degrees of freedom include (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

[0045] In some embodiments, the network parameters include optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

[0046] In some embodiments, the second node is one of a plurality of second nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes; and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes.

[0047] In some embodiments, the first node is one of a plurality of first nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node; the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes; and the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node.

[0048] In accordance with another aspect, there is provided a method for monitoring a quantum communication network including a node array having a plurality of nodes including a first node and a second node, and an optical communication infrastructure interconnecting the plurality of nodes and including an optical communication channel linking the first and second nodes, the method including: generating a quantum signal and a reference signal at the first node, the quantum and reference signals being distinguishable across at least two degrees of freedom; multiplexing the quantum and reference signals across the at least two degrees of freedom to produce a multiplexed signal;transmitting the multiplexed signal from the first node to the second node through the optical communication channel; receiving the multiplexed signal at the second node; demultiplexing the multiplexed signal into the quantum signal and the reference signal; detecting the reference signal following demultiplexing; and deriving monitoring signals from the detected reference signal, the monitoring signals being related to the at least two degrees of freedom and providing status indicators of network parameters associated with the optical communication channel.

[0049] In some embodiments, the reference signal is a classical signal. In other embodiments, the reference signal is a quantum signal.

[0050] In some embodiments, generating the quantum signal and the reference signal includes emitting the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

[0051] In some embodiments, the at least two degrees of freedom include (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

[0052] In some embodiments, the network parameters include optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

[0053] In some embodiments, the second node is one of a plurality of second nodes, the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes, and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes, and the monitoring signals provide status indicators of network parameters associated with the plurality of optical communication channels.

[0054] In some embodiments, the first node is one of a plurality of first nodes, the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node, the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes, the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node, and the monitoring signals provide status indicators of network parameters associated with the plurality of optical communication channels.

[0055] In accordance with another aspect, there is provided a network monitoring system for a quantum communication network including a node array having a plurality of nodes including a first node and a second node, and an optical communication infrastructure interconnecting the plurality of nodes and including an optical communication channel linking the first and second nodes, the network monitoring system including:a transmiter unit at the first node, including: an optical source module configured to generate a quantum signal and a reference signal that are distinguishable across at least two degrees of freedom; a multiplexing module configured to combine the quantum and reference signals across the at least two degrees of freedom, thereby generating a multiplexed signal; and an output port configured to transmit the multiplexed signal from the first node to the second node through the optical communication channel; a receiver unit at the second node, including: an input port configured to receive the multiplexed signal; and a demultiplexing module configured to separate the multiplexed signal into the quantum signal and the reference signal; and a network monitoring unit, including: a detection stage located at the second node and configured to detect the reference signal after demultiplexing; and a processing stage configured to derive monitoring signals from the detected reference signal, the monitoring signals being related to the at least two degrees of freedom and providing status indicators of network parameters associated with the optical communication channel.

[0056] In some embodiments, the reference signal is a classical signal. In other embodiments, the reference signal is a quantum signal.

[0057] In some embodiments, the optical source module is configured to emit the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

[0058] In some embodiments, the at least two degrees of freedom include (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

[0059] In some embodiments, the network parameters include optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

[0060] In some embodiments, the second node is one of a plurality of second nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes; and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes. In some embodiments, the multiplexed signals sent by the transmiter unit are identical across all the receiver units. In other embodiments, the multiplexed signals sent by the transmiter unit are not identical across all the receiver units, differing in their quantum signals, their reference signals, or both.

[0061] In some embodiments, the first node is one of a plurality of first nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node; the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes; and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes.

[0062] In some embodiments, the multiplexed signals are identical across all the transmitter units. In other embodiments the multiplexed signals are not identical across all the transmitter units, differing in their quantum signals, their reference signals, or both.

[0063] In certain implementations, the transmitted signal of interest, while described above as a quantum signal, may instead be a classical signal within an optical communication network. In such cases, the stabilization, synchronization, and monitoring techniques described herein remain applicable, enabling error correction, maintenance and / or monitoring of network parameters across multiple degrees of freedom, and overall enhancement of network performance.

[0064] Other features and advantages of the present description will become more apparent upon reading the following non-restrictive description of specific embodiments, which are provided by way of example only, with reference to the appended drawings. Although certain features described in the above summary and the following detailed description may be associated with particular embodiments or aspects, these features may be combined with one another unless otherwise indicated.BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figs. 1 to 9 depict various aspects, features, and implementations of, or related to, the techniques disclosed herein.

[0066] Fig. 1 is a schematic representation of a quantum communication network, in accordance with an embodiment.

[0067] Fig. 2 is a schematic representation of a quantum communication network, in accordance with another embodiment.

[0068] Fig. 3 is a more detailed view of the quantum communication network of Fig. 1, illustrating signal propagation between a transmitter unit on a first node and a receiver unit on a second node.

[0069] Fig. 4 is a schematic representation of a quantum communication network, in accordance with another embodiment, in which the quantum and reference signals originate from the same light source.

[0070] Figs. 5A to 5C illustrate quantum and reference signal distinguishability and multiplexing across three degrees of freedom: frequency (Fig. 5A), time (Fig. 5B), and polarization (Fig. 5C). The upper panels show signal generation, and the lower panels show signal multiplexing.

[0071] Fig. 6 is a schematic representation of a quantum communication network, in accordance with another embodiment.

[0072] Fig. 7 is a schematic representation of a quantum communication network, in accordance with another embodiment.

[0073] Fig. 8 is a flow diagram illustrating a method for stabilization of a quantum communication network, in accordance with an embodiment.

[0074] Fig. 9 is a schematic representation of a quantum communication network, in accordance with another embodiment.

[0075] Fig. 10 is a flow diagram illustrating a method for monitoring a quantum communication network, in accordance with an embodiment.DETAILED DESCRIPTION

[0076] The present disclosure relates to techniques for stabilizing, synchronizing, or monitoring quantum communication networks, including multi-node network implementations. These techniques seek to address or mitigate limitations found in current systems, such as high crosstalk between quantum and stabilization / synchronization signals, the need for multiple parallel systems to handle different aspects of stabilization and synchronization, and the difficulty in probing certain parameters, such as optical channel length, due to the separation of quantum and stabilization / synchronization signal paths.

[0077] As used herein, the term “quantum communication network” is employed broadly to encompass any network architecture in which quantum signals are exchanged, distributed, or processed between nodes, including both networks whose primary function involves quantum communication and those whose primary function may not, such as quantum computing or quantum sensing networks.

[0078] The disclosed techniques employ generating and multiplexing of quantum and reference signals across two or more degrees of freedom, such as time, frequency, and polarization, while maintaining distinguishability. The quantum signals are used for quantum information exchange, while the reference signals are used for network stabilization, synchronization or monitoring. Transmitter nodes send these multiplexed signals for co-propagation along shared optical channels, providing distribution throughout the network. At receiver nodes, the signals are demultiplexed, allowing the detection and processing of the reference signals togenerate error signals that are used in feedback loops to stabilize and synchronize various network parameters associated with the optical channels. These parameters can include optical channel length, propagation time, polarization, clock frequency and phase, optical frequency and phase, and quantum state analyzer alignment. By utilizing multiplexing across multiple degrees of freedom, the disclosed techniques facilitate effective management, stabilization, and synchronization of numerous network parameters using a single reference signal, thereby increasing efficiency and reducing the physical resources required.

[0079] In certain embodiments, this integrated approach of using a common reference signal to measure and manage various network parameters improves the scalability and stability of multi-node quantum networks through several mechanisms. These mechanisms may include integrating quantum components with existing classical telecommunications infrastructure (including single-mode optical fiber networks), aligning quantum and reference signals with standard telecommunications frequency bands, reducing optical losses across network subsystems, and incorporating network node hardware with software for node-level reconfigurability and control.

[0080] The disclosed techniques have potential applications in various fields, particularly in configurations involving more than two network nodes. Examples of such applications include quantum networking, distributed quantum computing, distributed quantum sensing, quantum key distribution, entanglement distribution, quantum homomorphic encryption, multi-party quantum computing, quantum-enhanced geophysical measurements (e.g., geodesy, magnetometry), quantum memory integration, quantum teleportation, quantum-enhanced telescopes and gravitational wave detection, dark-matter search, time and clock synchronization, clock distribution and recovery, cybersecurity, quantum-secure links, remote sensing (e.g., quantum rangefinding and LiDAR), global positioning systems, network characterization, low-latency telecommunications, network-based sensing, quantum-enhanced biological imaging, entanglement-assisted spectroscopy, cryptographic protocols, quantum position verification, quantum secure time transfer, and telecommunications stabilization. In some cases, measured deviations of network parameters can serve as useful signals rather than issues to correct. For example, certain deviations in network parameters may be analyzed for applications such as anomaly detection, remote sensing, or seismology.

[0081] Various aspects, features, and implementations of the disclosed techniques are described below, with reference to the accompanying figures.

[0082] Fig. 1 illustrates an embodiment of a quantum communication network 100. This network 100 includes a node array 102 composed of a plurality of nodes 1041-1044, and an optical communication infrastructure 106 that interconnects these nodes 1041-1044. The nodes 1041-1044 enable the exchange of quantum information (e.g., entangled qubits) across pathways established by the optical communication infrastructure 106. They may perform quantum information processing tasks, including generating, detecting, processing, storing, anddistributing quantum information carriers. The nodes 1041-1044 may be categorized based on their functions within the network 100, including source nodes, repeater nodes, detector nodes, processor nodes, and storage nodes. Fig. 1, along with subsequent figures, serves as a schematic representation intended to illustrate various components and features of the quantum communication network 100. It is understood that additional components and features useful for practical operation may not be explicitly illustrated.

[0083] In the depicted network 100, the node array 102 includes four nodes 1041-1044 for illustrative purposes, although alternative embodiments may include two, three, or more than four nodes. In certain network architectures, the number of nodes may extend up to tens or even hundreds. However, the present techniques are not constrained to any particular number of nodes. The term “multi-node” refers herein to quantum networks including at least three nodes, distinguishing them from “two-node” networks (also known as “point-to-point” networks) that consist of two nodes. The distance between nodes can vary depending on the application, typically ranging from about tens of centimeters (e.g., for nodes linking quantum processors) to hundreds of kilometers for remotely located nodes, although distances outside this range are also feasible. Additionally, the distance between any two nodes may not be uniform throughout the network 100.

[0084] Various network topologies can be used for coupling the nodes 1041-1044 to allow the exchange of quantum information over the optical communication infrastructure 106. The illustrated embodiment employs a star topology, where a central node 104i is directly coupled to three peripheral nodes 1042-1044 through respective optical communication channels IO81.2, IO81.3, IO81.4 of the optical communication infrastructure 106. In this configuration, the peripheral nodes 1042-1044 are not interconnected. Other embodiments may utilize different network topologies, such as linear, tree, ring, mesh, bus, daisy-chain, hybrid, and fully-connected architectures. Another example of network topology is shown in Fig. 2, which includes five nodes 1041-1045, with transmitter node 104i connected to receiver nodes 1043, 1044 through optical communication channels IO81.3, IO81.4, and transmitter node 1042 connected to receiver nodes 1044, 104 through optical communication channels IO82-4, IO82-5. The disclosed techniques may be adapted to various network topologies with suitable modifications, including adjustments to reference signal encoding and interlocking, detection methodologies, and stabilization algorithms.

[0085] Returning to Fig. 1, the optical communication infrastructure 106 connecting the nodes 1041-1044 can be implemented using various technologies, including fiber-based technologies that use optical fibers to guide the transmission of quantum signals, and free-space technologies that rely on free-space optics for transmission through the air or space (e.g., satellite links). Employing a standard fiber-based telecommunications network with single-mode optical fiber links between nodes can offer advantages such as cost reduction, enhanced interoperability, and better integration of quantum communication networks with existing classical infrastructure. In certain embodiments, the optical communication infrastructure 106 may be a hybrid system combining both fiber-based and free-space links. Other technologies, such as photonic integrated circuits orwaveguides, may also be employed or combined, enabling the optical communication infrastructure 106 to adapt to specific operational conditions and application requirements.

[0086] Referring still to Fig. 1, as well as Fig. 3, the first and second nodes 104i, 1042 are used for implementing certain aspects of the disclosed techniques. These two nodes 1041-1042 are interconnected by optical communication channel IO81.2. The first node 104i functions as a transmitter node, while the second node 1042 serves as a receiver node. The first node 1041 incorporates a transmitter unit 110 that generates and transmits a multiplexed signal to the second node 1042 over the optical channel 1081-2. This multiplexed signal combines a quantum signal (the signal of interest) and a reference signal (for network stabilization). The second node 1042 features a receiver unit 112 that receives and processes the multiplexed signal, thereby separating the quantum and reference signals. The reference signal is detected and processed by a stabilization feedback loop 114 configured to determine error signals indicative of network parameter errors associated with the optical channel 1081 -2 and to apply corrective operations based on these indicators to stabilize the network 100. Although Fig . 1 depicts the first node 1041 with a transmitter unit 110 and the second node 1042 with a receiver unit 112, it should be noted that each node 1041-1044 within the network 100 can function solely as a transmitter, solely as a receiver, or as both, depending on the specific operational conditions and application requirements.

[0087] Fig. 3 provides a more detailed view of the first and second nodes 104i , 1042 from Fig. 1, which are configured to exchange quantum information over optical channel 1081.2 while implementing the stabilization techniques disclosed herein. The transmitter unit 110 within the first node 104i includes an optical source module 116, a multiplexing module 118, and an output port 120, while the receiver unit 112 within the second node 1042 includes an input port 122 and a demultiplexing module 124. The stabilization feedback loop 114 includes a detection stage 126, aprocessing stage 128, and an actuation stage 130. The structure, configuration, and operation of these components, along with other potential components, are detailed below. In some embodiments, the transmitter unit 110, the receiver unit 112, and the stabilization feedback loop 114 may collectively form a network stabilization system 200 designed for integration with and stabilization of a quantum communication network connecting an array of nodes through an optical communication infrastructure.

[0088] The optical source module 116 is configured to generate both a quantum signal 132 and a reference signal 134. It can be embodied by any device or combination of devices capable of producing these signals 132, 134 with characteristics suitable for enabling quantum information exchange and network stabilization. Depending on the application, the reference signal 134 may be either classical or quantum. In this context, a “classical signal” refers to an optical signal characterized by well-defined amplitude and phase properties described by classical electromagnetism, while a “quantum signal” refers to an optical signaldescribed by quantum states exhibiting properties that classical electromagnetism cannot fully capture, such as discrete energy levels, superposition, squeezing, and entanglement.

[0089] Depending on application requirements, the optical source module 116 may include one or multiple light sources, typically laser sources, for generating the quantum and reference signals 132, 134. Suitable laser sources for quantum communication include solid-state lasers (e.g., bulk crystal lasers and fiber lasers), semiconductor lasers (e.g., laser diodes and other on-chip laser sources), gas lasers (e.g., carbon dioxide lasers, chemical oxygen-iodine lasers, and dye lasers) and optical parametric oscillators. Non-laser light sources, such as light-emitting diodes (LEDs), may also be used in certain instances. These light sources may be operated in a continuous-wave, pulsed, or turbulent regime, with selection criteria based on factors such as operational wavelength, beam quality, degree of coherence, spatial and spectral profiles, and pulse characteristics (e.g., peak power, repetition rate, profile, and duration).

[0090] In certain configurations, the quantum and reference signals 132, 134 are generated independently by dedicated light sources 136i, 1362. This is illustrated in Fig. 3, where the optical source module 116 includes two distinct light sources 136i, 1362 (e.g., laser sources): one for generating the quantum signal 132 and another for the reference signal 134. The first light source 136i may produce the quantum signal 132 in forms such as single photons, entangled two- or multi-photon states, continuous-variable quantum states, and photonic graph states. The second light source 1362 may include a continuous-wave, mode-locked, or pulsed laser to generate the reference signal as a classical signal. In some cases, these dedicated light sources 1361 , 1362 may not operate entirely independently but may exhibit some degree of interdependency or controlled coupling, such as through optical injection locking, beat-note stabilization, optical reference comparison, or stabilized optical filtering.

[0091] Alternatively, the optical source module 116 may employ a single light source 136 to generate a primary signal 138, which is then split into two paths by a suitable optical splitter 140, as illustrated in Fig. 4. For example, in certain configurations, the quantum signal 132 is generated by pumping a nonlinear optical medium, such as a crystal, cavity, or fiber enabling spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM), or by using quantum dot sources or attenuated laser sources. In such a case, the pump source can also generate the reference signal 134. In some implementations, the single light source 136 first generates the quantum signal 132, with any residual light subsequently used to prepare the reference signal 134. It is understood that the approach of consolidating signal generation within a single device may help reduce the size, weight, power consumption, and overall cost of the optical source module 116.

[0092] Returning to Fig. 3, in some embodiments, the optical source module 116 is configured to emit the quantum and reference signals 132, 134 within a waveband ranging from about 700 nm to about 1700 nm, encompassing the near-infrared (IR) portion of the electromagnetic spectrum. However, the disclosed techniques may also operate outside this range, including at shorter wavelengths (e.g., visible range) or longerwavelengths (e.g., mid-IR, far-IR, and terahertz ranges). In some configurations, separating the wavelengths of the reference and quantum signals 132, 134, for example, within distinct standard telecommunications bands, may help reduce noise and crosstalk while maintaining low propagation losses. For example, the quantum and reference signals 132, 134 may be generated within the C-band (1530 nm to 1565 nm) and the O-band (1260 nm to 1360 nm), respectively, although other band allocations may be applied based on specific network requirements and applications.

[0093] The properties of the reference signal 134 may be tailored to meet the network’s stabilization needs. In certain embodiments, the reference signal 134 may be attenuated to low power levels to reduce crosstalk with the quantum signal 132, thereby limiting noise photons generated through nonlinear processes and scattering during co -propagation. For example, the reference signal 134 may be generated at standard telecommunications power levels (e.g., in the milliwatt range), with careful power control to reduce crosstalk during demultiplexing. In certain cases, the reference signal 134 may reach few-photon levels.

[0094] In some embodiments, the reference signal 134 may be generated as a single-frequency signal, enabling detection of the quantum state analyzer interferometer phase in various setups, such as Mach-Zehnder or Michelson configurations using single-mode or polarization-maintaining fibers. Alternatively, the reference signal 134 may be generated as a two-frequency signal, facilitating phase detection across the full range in Michelson-type, single-mode fiber interferometer configurations. Furthermore, in configurations where the reference signal 134 is a quantum signal, it may leverage quantum interference effects, such as Hong-Ou- Mandel interference. This capability can enhance temporal precision while reducing crosstalk and noise photons.

[0095] Referring still to Fig. 3, the optical source module 116 is configured to generate the quantum and reference signals 132, 134 to ensure distinguishability over at least two degrees of freedom. Specifically, the optical source module 116 is designed to prepare, encode, or condition the quantum and reference signals 132, 134 so that they differ in at least two degrees of freedom. This can be achieved using any device or combination of devices capable of manipulating the quantum and reference signals 132, 134 in this manner. Non-limiting examples of degrees of freedom include time, frequency, polarization, phase, spatial position, and angular orbital momentum. Various combinations are possible, such as any combination of two among time, frequency, and polarization, or all three.

[0096] The upper panels of Figs. 5A, 5B, and 5C illustrate cases where the quantum and reference signals 132, 134 occupy different spectral bands (Fig. 5A; distinguishability in frequency), are generated in distinct time windows (Fig. 5B; distinguishability in time), and exhibit different polarization states (Fig. 5C; distinguishability in polarization). Different techniques can be applied to prepare the quantum and reference signals 132, 134 with distinguishable characteristics across multiple degrees of freedom. For example, thequantum signal preparation may use a quantum signal conditioner 142 including a quantum frequency comb 176 placed between two encoding units 178i, 1782 (Figs. 3 and 4). In this configuration, the first encoding unit 178i may apply encoding to the classical signal used to generate the quantum signal 132, such as by employing an interferometer to create pulses with a stable and well-defined temporal separation. The second encoding unit 1782 may then act on the generated quantum signal 132, further refining its properties for transmission. For reference signal preparation, encoding may occur within a reference signal conditioner 144 including an encoder 180 having a single optical path (Fig. 3) or multiple paths (Fig. 4), depending on the specific needs of the system.

[0097] In some embodiments, the optical source module 116 includes dedicated signal conditioning units for each signal, such as the quantum signal conditioner 142 and the reference signal conditioner 144 illustrated in Figs. 3 and 4. Alternatively, for certain degrees of freedom, the conditioning of both signals 132, 134 may be performed using shared hardware components (e.g., a single interferometer) within the optical source module 116. It is understood that the principles and techniques for preparing and conditioning quantum and classical optical signals across different degrees of freedom are generally well established and need not be elaborated further, except as necessary for understanding the disclosed techniques.

[0098] Following conditioning of the quantum and reference signals 132, 134 to be distinguishable over the relevant degrees of freedom, both signals 132, 134 are directed to the multiplexing module 118. This module 118 is configured to combine the signals 132, 134 across these degrees of freedom to generate a multiplexed signal 146. In some embodiments, the multiplexing module 118 includes dedicated units for each degree of freedom. For example, in the setup shown in Fig. 3, the quantum and reference signals 132, 134 are prepared with distinct spectral bands, temporal windows, and polarization states. Consequently, the multiplexing module 118 includes a spectral multiplexer 148, a temporal multiplexer 150, and a polarization multiplexer 152 to multiplex the two signals 132, 134 in frequency, time, and polarization, respectively. The multiplexing processes for these three degrees of freedom are schematically represented in the lower panels of Figs. 5A, 5B, and 5C. Additionally, the order of the multiplexing components 148, 150, 152 within the multiplexing module 118 is not fixed and may vary according to the system’s configuration and processing requirements. In alternative configurations, multiplexing for certain degrees of freedom may be achieved using shared hardware components within the multiplexing module 118. It is understood that the principles and techniques for multiplexing quantum and classical optical signals across different degrees of freedom are generally well established and need not be elaborated further, except as necessary for understanding the disclosed techniques.

[0099] Temporal multiplexing can be implemented through several methods. One method may involve (i) using a common clock (e.g., an atomic clock) to synchronize the generation of quantum signals and the carving of reference signal pulses (e.g., via intensity modulation, direct modulation, or switching), (ii) employing anoptical delay line to introduce a temporal offset between the quantum and reference signals by a fraction of the clock period, and (iii) recombining the two signals onto a single optical path. In some cases, instead of pulse carving, both quantum and reference signals may be generated using a mode-locked laser. In another approach, synchronization and multiplexing can be achieved by using a pulsed reference signal that is deactivated during quantum signal transmission and activated when the quantum signal is absent, thereby reducing noise and crosstalk on the quantum channel.

[0100] Polarization multiplexing may employ a polarizing beamsplitter with two input ports, one for each signal type, and a single output port. This setup allows the quantum and reference signals to exit the beamsplitter along the same path with orthogonal polarization states. In the case of frequency multiplexing, certain implementations utilize wavelength-division multiplexing, a well-established technique in classical telecommunications .

[0101] Optical losses during the multiplexing of quantum and reference signals may arise from insertion losses, bend losses, and component-specific losses (e.g., from beamsplitters, circulators, and mirrors). Strategies to mitigate these losses include: using fiber-based platforms throughout the network; applying precise fiber splicing techniques; employing low-loss optical components and fiber-to-chip connections; and enhancing quantum signal strength by adjusting pump power or exploiting emitter nonlinearity.

[0102] As depicted in Fig. 3, the output port 120 directs the multiplexed signal 146 onto the optical communication channel 1081 _2for transmission to the receiver unit 112 at the second node 1042. For extended fiber links, strategically placed repeater modules along the channel 1081.2 can help counteract propagation losses. These modules may employ quantum repeater states with Bell state measurements or leverage matterbased quantum technologies capable of reviving quantum signals. Such modules can extend the effective transmission distance along the optical channel 1081.2.

[0103] In certain fiber-based configurations, noise photons generated through nonlinear interactions and crosstalk from the reference signal can degrade the signal -to-noise ratio of the quantum signal. This issue can be addressed through careful selection of spectral, temporal, and polarization multiplexing techniques. Effective strategies can include: choosing reference wavelengths that minimize nonlinear noise processes (e.g., Raman and Brillouin scattering); using pulsed reference signals with clock rates exceeding the timescales of relevant noise processes; and attenuating the reference signal (e.g., to few-photon levels).

[0104] Long-distance propagation in optical fibers may induce wavelength-dependent delays, leading to optical dispersion, which may interfere with temporal multiplexing and introduce noise detrimental to the quantum signal. Mitigation strategies for this issue can include: using either closely matched or well-separated wavelengths for the quantum and reference signals; employing dispersion-compensating fibers; and optimizing signal multiplexing based on fiber length considerations. Additionally, while standard single-mode fiberinfrastructure may be prone to polarization drifts, this issue can be effectively managed through robust polarization detection techniques, high-speed compensation rates, careful matching of quantum and reference signal wavelengths, or appropriate physical isolation of the fiber links.

[0105] Following propagation through the optical channel 1081_2, the multiplexed signal 146 reaches the input port 122 of the receiver unit 112 at the second node 1042 and proceeds to the demultiplexing module 124. This module 124 is configured to separate the multiplexed signal 146 according to each degree of freedom, thereby recovering the quantum signal 132 and the reference signal 134. The demultiplexing module is also configured to direct the two recovered signals 132, 134 onto distinct optical paths 154, 156 for further processing. Various demultiplexing techniques can be employed based on the involved degrees of freedom, operational conditions, and application requirements.

[0106] In some embodiments, the demultiplexing module 124 includes dedicated units for each degree of freedom. For example, as illustrated in Fig. 3, the demultiplexing module 124 incorporates a spectral demultiplexer 158, a temporal demultiplexer 160, and a polarization demultiplexer 162 to separate the multiplexed signal 146 in terms of frequency, time, and polarization, respectively. As described earlier, these three particular degrees of freedom represent only a subset of possible options, depending on the specific application. Additionally, the order of the demultiplexing components 158, 160, 162 within the demultiplexing module 124 is not fixed and may vary according to the system’s configuration and processing requirements. In alternative configurations, demultiplexing for certain degrees of freedom may be achieved using shared hardware components within the demultiplexing module 124. It is understood that the principles and techniques for demultiplexing quantum optical signals across multiple degrees of freedom are generally well established and need not be elaborated further, except as necessary for understanding the disclosed techniques.

[0107] Temporal demultiplexing can be performed using various techniques. One method involves applying digital post-processing with time-tagging and correlation electronics, while another employs a fast optical switch or modulator synchronized to the temporal multiplexing scheme to either route the signals to separate optical paths for further processing or selectively attenuate the reference signal. Polarization demultiplexing may use a polarizing beamsplitter with one input port for receiving the multiplexed signal and two output ports corresponding to two orthogonal polarization states: one for the quantum signal and the other for the reference signal. For frequency demultiplexing, certain implementations may employ wavelength-division demultiplexing techniques.

[0108] Following demultiplexing, the quantum and reference signals 132, 134 undergo separate processing. The receiver unit 112 may include a quantum processor 164 configured to perform operations on the quantum signal 132, such as projection measurements, entangling and non-entangling gate operations, quantum state estimation, error correction, or entanglement swapping. Alternatively, or additionally, the receiver unit 112may incorporate a quantum memory 166 to store the quantum signal 132 for subsequent processing. The receiver unit 112 may also transmit the quantum signal 132 to another device or node within the network 100. Independently, the reference signal 134 is routed to the stabilization feedback loop 114 for network error compensation, as detailed below.

[0109] The stabilization feedback loop 114 is designed to monitor and correct various network parameters associated with the operation of the quantum communication network 100 between the first and second nodes 104i, 1042. Non-limiting examples of network parameters that may be stabilized using the disclosed techniques include optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, and alignment between quantum state analyzers.

[0110] The network stabilization process may begin with preliminary operations, such as determining which network parameters should be monitored based on factors such as network connectivity, topology, and communication protocols. This phase may also involve establishing target values for each monitored parameter and defining acceptable deviations or threshold levels, beyond which the stabilization feedback loop 114 initiates corrective actions. In some cases, these target values may adjust dynamically over time as part of a communication protocol. For instance, in a quantum key distribution (QKD) protocol, the quantum state analyzers may change during key generation, depending on specific protocol requirements.

[0111] The stabilization feedback loop 114 includes the detection stage 126 located along the reference signal path 156 to receive and detect the reference signal 134 after demultiplexing. The detection stage 126 may include any suitable optical detector or combination of detectors capable of performing optical power measurements within the wavelength range of the reference signal 134. Examples of optical detectors include PIN photodiodes, amplified photodetectors, avalanche photodiodes, Schottky photodiodes, and superconducting nanowire single-photon detectors (SNSPDs). The selection of detectors depends on specific measurement requirements, such as power levels, wavelength range, measurement speed, and sensitivity.

[0112] The detection stage 126 performs measurements based on the specific degrees of freedom across which the reference signal 134 was multiplexed with the quantum signal 132. For example, if the degrees of freedom include frequency, time, and polarization, the detection stage 126 may detect properties related to the optical spectrum, arrival time, and polarization state of the reference signal 134. The specific measurements and detection configurations vary based on the network parameters being monitored. Certain setups may involve polarization-sensitive detection, while others may employ interferometric techniques for time -sensitive or frequency-sensitive measurements. An advantage of this approach is that a single reference signal can be used to monitor and stabilize multiple network parameters simultaneously due to the involvement of multiple degrees of freedom. For example, both the channel length and polarization stability of the optical channel 1081.2 between the first and second nodes 1041, 1042 can be monitored and stabilized based on measurements of the reference signal’s arrival time and polarization state.

[0113] The processing stage 128, which may include suitable processors and memory, receives the measurement data associated with the detected reference signal 134 from the detection stage 126 (e.g., in the form of digitized electrical signals) and processes it to generate error signals 168. These error signals 168 are related to the degrees of freedom involved and are indicative of errors in the monitored network parameters. The generated error signals 168 then serve as control signals, whether digital, electrical, or optical, to correct or update these network parameters. In addition to their role in stabilization, the error signals 168 can also be used for monitoring purposes, providing users with valuable insights into the state or health of the network 100. This monitoring aspect can be particularly useful for applications focused on network assessment and sensing. In certain cases, as mentioned earlier, the error signals may themselves represent useful signals rather than an issue to be corrected, such as for environmental sensing applications.

[0114] The actuation stage 130 includes at least one optical actuator 1701-1703, designed to apply corrective operations within the quantum communication network 100, based on the error signals 168, thereby stabilizing the monitored network parameters. Examples of such actuators include optical delay lines and piezoelectric fiber stretchers for controlling a physical channel length; electronically controlled polarization modules for managing channel polarization; and optical interferometers for addressing phase errors. As illustrated in Fig. 3, the optical actuators 1701-1703 may include a length compensation unit 170i, a polarization compensation unit 1702, and an interferometer unit 1703. While these are possible examples, other types of actuators can also be envisioned, such as electro-optic modulators for controlling optical phase and / or polarization between nodes or acousto-optic modulators for managing optical frequencies. It is understood that the principles and techniques for using optical actuators to apply corrections or compensation to optical channels (e.g., with respect to length, polarization, or phase) are well established and need not be elaborated further, except as necessary for understanding the disclosed techniques.

[0115] In some embodiments, the actuation stage 130 is configured to apply the corrective operations actively, in real-time or near real-time, to enable continuous network monitoring and stabilization. Alternatively, monitoring and stabilization may be performed intermittently, either at set intervals or upon request from an operator. While monitoring may occur continuously or intermittently, corrective stabilization might only be performed when the network parameters deviate from specified target values beyond predetermined thresholds, or at time intervals longer than those used for monitoring. For example, the processing stage 128 may be configured to compare the error signals 168 against predetermined threshold levels, and the actuation stage 130 may be configured to apply the corrective operations upon determination that the error signals exceed the predetermined threshold levels. In some cases, the monitored values can be stored for later use in digital postprocessing, eliminating the need for real-time corrections. The monitoring and stabilization routines can varyacross different network parameters. In some implementations, metrics derived from the reference signal can be calculated once or multiple times, resulting in a set of measurements that can be analyzed further (e.g., statistical analysis, averaging) to achieve higher estimation accuracy.

[0116] As shown in Fig. 3, in some embodiments, the optical actuators 1701-1703 operate on the optical communication channel 1081.2 or node equipment within or at the entrance of the receiver unit 112, upstream of the demultiplexing module 124. However, this is not strictly required, and other arrangements are possible. For example, in certain cases, the actuators 1701-1703 may act on the optical channel IO81.2 or node equipment within or near the transmitter unit 110, or at other relevant locations along this channel 1081.2, or elsewhere within the network 100. In other cases, different actuators 1701-1703 may be configured to operate at different locations. This flexibility in actuator placement allows for enhanced error correction and stabilization based on the specific network topology and the nature of the errors being addressed. The performance of stabilization and other technical requirements may depend on actuator placement. For example, if an actuator is positioned along a link between nodes, the system should ensure that the feedback signal is transmitted from the detection unit to the actuator within an acceptable latency to maintain effective correction. In some embodiments, feedback signals can be transmitted to the actuators as either analog or digital electronic signals.

[0117] Network parameter updates, including the target values of the monitored degrees of freedom, may be communicated using standard classical communication protocols (e.g., TCP / IP, UDP, HTTP) over the internet, leveraging existing infrastructure and protocols for ease of integration into networks with classical communication channels. Alternatively, network parameter updates can be transmitted through the same optical channels IO81.2 that carry the quantum and reference signals 132, 134, but at different frequencies, times, and / or polarizations. This approach can potentially reduce latency in parameter updates and simplify network architecture by eliminating the need for separate classical communication channels. However, careful design may be necessary to ensure that such update signals do not interfere with the quantum and reference signals 132, 134, thereby maintaining the signal integrity.

[0118] In some embodiments, the stabilization feedback loop 114 may also incorporate adaptive algorithms to optimize performance over time. For example, by analyzing historical data on network fluctuations and the effectiveness of past corrections, the system can refine the stabilization process, enabling more effective responses to recurring network instabilities. Besides stabilizing individual network parameters, a single stabilization feedback loop can also stabilize multiple network parameters or manage the interplay between them. For example, it can recognize that changes in one parameter (e.g., optical channel length) can affect others (e.g., propagation time, clock synchronization) and apply coordinated corrections across multiple parameters concurrently to enhance overall network stability. Additionally, one or more reference signals can be detected and combined within the digital stabilization algorithm, further refining the feedback process. This approach can lead to improved stability, reduced resource demands, and streamlined network operation.

[0119] While the preceding discussion focused on two-node implementations, the disclosed stabilization techniques can be extended to multi-node configurations, as outlined below.

[0120] In multi-node configurations, a transmitter unit at one node may be connected to multiple receiver units at different nodes. Fig. 6 illustrates a three-node embodiment where the first node 1041 includes a transmitter unit 110, while the second and third nodes 1042, 104; each feature a receiver unit 112 and a stabilization feedback loop 114. Separate optical channels IO81.2, IO81.3 link the first nodes 104i to the second and third nodes 1042, 1043, respectively. The structure, configuration, and operation of these components generally correspond to those previously described. The receiver units 112 and stabilization feedback loops 114 at the second and third nodes 1042, 104; may be identical or differ, based on the specific application. Although Fig. 6 shows only two receiving nodes 1042, 104; for simplicity, practical implementations may incorporate more nodes.

[0121] In certain configurations, the multiplexed signals 146 sent from the transmitter unit 110 at the first node 104i to the receiver units 112 at the second and third nodes 1042, 104; may be identical. For example, a beamsplitter 172 can be integrated within the transmitter unit 110, with its input port connected to the multiplexing module 118 and its two output ports directing identical reference signals 146 to the receiver units 112 at the second and third nodes 1042, 104s, respectively. Alternatively, the multiplexed signals 146 sent to the second and third nodes 1042, 104; may differ. In some cases, optical conditioning devices positioned along one or both output paths of the beamsplitter 172 can adjust either or both signals 146 before they enter the respective optical channels 1081.2, IO81.3. For example, the optical channel 1081.3 between the first and third nodes 104i, 1043 may require a different but synchronized clock frequency compared to the optical channel 1081.2 between the first and second nodes 104i, 1042, and it may also experience greater optical losses than this channel IO81.2. To address this, optical conditioning devices can be used to adjust the clock frequency and increase the intensity of the multiplexed signal 146 directed to the third node 1043. In other configurations, the multiplexed signals 146 sent to the second and third nodes 1042, 1043 may differ in their quantum signals, reference signals, or both. In yet other cases, the multiplexed signals 146 might be identical but activated at different times.

[0122] The number of light sources and signal conditioning components in the transmitter unit may vary based on whether the reference signals sent across distinct optical channels to receiver units at different nodes are the same, related (e.g., synchronized or originating from the same light source), or independent.

[0123] Utilizing the same or related reference signals across separate optical channels enables the stabilization and synchronization of optical channels linking a single transmitter unit to multiple receiver units across different nodes, even with variations in receiver and link characteristics. This approach can optimize network resources and simplify stabilization and synchronization compared to using independent reference signals fromseparate transmitter units. In such embodiments, the error signals that are determined from the various detected reference signals are indicative of errors in parameters associated with the plurality of optical communication channels connecting the multiple nodes. Corrective operations may be applied across the network or on a perchannel basis, depending on the application.

[0124] Other configurations may use multiple reference signals across the network. For example, each reference signal could correspond to a subset of linked nodes, with comparison modules synchronizing the multiple reference signals. In other scenarios, different reference signals may correspond to distinct quantum signals. Depending on the application, the network may distribute a single quantum signal or multiple quantum signals issued from either the same source (e.g., using a quantum frequency comb) or separate quantum sources.

[0125] In other embodiments, multiple transmitter units located at different nodes may connect to multiple receiver units located at the same node via separate optical channels. Such a setup is depicted in Fig. 7, where the first and third nodes 104i, 104; each include a transmitter unit 110, while the second node 1042 has two receiver units 112. One receiver unit 112 is connected to the transmitter unit 110 at the first node 104i via optical channel IO81.2, and the other is linked to the transmitter unit 110 at the third node 104; via optical channels IO83-2. Depending on the application, the two transmitter units 110 and their respective multiplexed signals 146 may be identical or differ. For example, while the multiplexed signals 146 might be synchronized or exhibit a controlled delay, they could vary in polarization and / or frequency according to communication protocols or specific network parameters. To accommodate these variations, the stabilization feedback loop 114 associated with each optical channel I O81.2, IO83-2 can be adjusted to stabilize and synchronize the length or other characteristics of the channels IO81.2, IO83-2. This adaptability allows receiver units 112 and stabilization feedback loops 114 at the same node 1042 to effectively handle multiplexed signals 146 originating from different transmitter units 110 at different nodes 104i, 1043 and traveling through separate optical channels IO81.2, IO83-2.

[0126] In certain setups, the relative arrival times of the pulsed reference signals from the two transmitter nodes 104i, 1043 are measured, and delay line actuators on each optical channel IO81.2, IO83-2 are then adjusted and synchronized based on this comparison. In this case, there is a single stabilization feedback loop at the receiver node 1042, which uses both reference signals on the two optical channels IO81.2, IO83-2 to determine the correction applied by both actuators. However, the two optical channels IO81.2, IO83-2 may have separate stabilization loops for polarization. Each channel’ s polarization can be stabilized individually, without the need to consider both channels in combination. This setup allows the system to handle different signal properties independently for each channel while still ensuring synchronization of the other characteristics, such as signal timing and phase.

[0127] Referring to Fig. 8, a flow diagram illustrates a method 300 for stabilization of a quantum communication network. This method 300 may be implemented within a quantum communication network 100 or by a network stabilization system 200, as described above with reference to Figs. 1 to 7. The method 300 includes a step 302 of generating a quantum signal and a reference signal at a first node of the network. These signals are generated so that they are distinguishable across at least two degrees of freedom. As mentioned above, the reference signal may be either classical or quantum, and the degrees of freedom may include any two of frequency, time, and polarization, or all three.

[0128] The method 300 continues with a step 304 that involves multiplexing the quantum and reference signals across the at least two degrees of freedom to produce a multiplexed signal. This is followed by a step 306 of transmitting the multiplexed signal from the first node to a second node in the network via a dedicated optical communication channel, such as a fiber or free-space link.

[0129] The method 300 then proceeds with a step 308 of receiving the multiplexed signal at the second node, followed by a step 310 that involves demultiplexing the multiplexed signal into the quantum signal and the reference signal. After demultiplexing, the method 300 includes a network stabilization operation, involving a step 312 of detecting the reference signal; a step 314 of determining error signals associated with the at least two degrees of freedom from the detected reference signal, with the error signals reflecting errors in network parameters associated with the optical communication channel; and a step 316 of controlling at least one optical actuator to apply corrective operations within the quantum communication network, based on the error signals, thereby stabilizing the network parameters.

[0130] Various aspects and features for implementing the method 300 have been described above in relation to network and system implementations.

[0131] In certain embodiments, the disclosed techniques are not employed for error signal determination or network parameter stabilization. Instead, they are used for monitoring purposes, generating monitoring signals that offer insights into the network’s performance. These signals, which correspond to the multiple degrees of freedom across which the quantum and reference signals are multiplexed, serve as status indicators of network parameters associated with the optical communication channel(s).

[0132] Fig. 9 illustrates an embodiment of a quantum communication network 100 designed for monitoring applications. This network 100 includes a node array 102 composed of a plurality of nodes 1041-1044, and an optical communication infrastructure 106 that interconnects these nodes 1041-1044. The nodes 1041-1044 facilitate the exchange of quantum information (e.g., entangled qubits) over pathways established by the optical communication infrastructure 106.

[0133] In the example shown, the node array 102 includes four nodes 1041-1044 for illustrative purposes. However, alternative embodiments may feature any number of nodes, depending on the application requirements. Nodes 104i, 1042 are interconnected by optical communication channel 1081.2 and are used to implement the monitoring functionality. The first node 1041 incorporates a transmitter unit 110 that generates and transmits a multiplexed signal to the second node 1042 over the channel IO81.2. This transmitter unit 110 includes an optical source 116, a multiplexing module 118, and an output port 120. The multiplexed signal combines a quantum signal (the signal of interest) and a reference signal (for network monitoring). The second node 1042 features a receiver unit 112, which receives and separates the multiplexed signal into its quantum and reference components. The receiver unit 112 includes an input port 122 and a demultiplexing module 124. The design and operation of these components of the transmitter unit 110 and receiver unit 112 may be the same or similar to those previously described and need not be detailed again. Although Fig. 9 depicts the first node 104i as a transmitter node and the second node 1042 as a receiver node, any node within the network 100 can function as a transmitter, receiver, or both, depending on operational conditions and application needs.

[0134] The network 100 also includes a network monitoring unit 174. This unit 174 incorporates a detection stage 126 that detects the reference signal after demultiplexing, and a processing stage 128 that analyzes the detected reference signal to generate monitoring signals. These monitoring signals are related to the multiple degrees of freedom and provide status indicators of network parameters associated with optical channel 1081. 2, allowing for monitoring of the network’s performance. Depending on the application, monitoring can be conducted in real-time or, alternatively, data can be stored for later analysis. In some embodiments, the monitoring signals are compared against predetermined thresholds or criteria to assess network performance and to generate status updates or alerts.

[0135] In some embodiments, the transmitter unit 110, receiver unit 112, and monitoring unit 174 collectively form a network monitoring system 400, which is designed to integrate with and monitor a quantum communication network. Unlike previously described embodiments involving corrective actions applied through actuators and feedback loops, this system 400 focuses on monitoring and status tracking, enabling oversight of network conditions without necessitating active intervention. It is noted that in some configurations, both monitoring and error correction functionalities may be integrated within the same network.

[0136] Referring to Fig. 10, a flow diagram illustrates a method 500 for monitoring a quantum communication network. This method 500 may be implemented within a quantum communication network 100 or by a network stabilization system 400, as described above with reference to Fig. 8. The method 500 includes a step 502 of generating a quantum signal and a reference signal at a first node of the network. These signals are generated so that they are distinguishable across at least two degrees of freedom. As mentioned above, the reference signal may be either classical or quantum, and the degrees of freedom may include any two of frequency, time, and polarization, or all three.

[0137] The method 500 continues with a step 504 that involves multiplexing the quantum and reference signals across the at least two degrees of freedom to produce a multiplexed signal. This is followed by a step 506 of transmitting the multiplexed signal from the first node to a second node in the network via a dedicated optical communication channel, such as a fiber or free-space link.

[0138] The method 500 then proceeds with a step 508 of receiving the multiplexed signal at the second node, followed by a step 510 that involves demultiplexing the multiplexed signal into the quantum signal and the reference signal. After demultiplexing, the method 500 includes a network monitoring operation, involving a step 512 of detecting the reference signal, followed by a step 514 of deriving monitoring signals associated with the at least two degrees of freedom from the detected reference signal, with the monitoring signals providing status indicators of network parameters associated with the optical communication channel.

[0139] Throughout the present description, similar features in the drawings have been given similar reference numerals. To avoid cluttering certain figures, some elements may not be indicated if previously identified in preceding figures. Elements in the drawings are not necessarily depicted to scale, emphasis being on clearly illustrating elements and structures of disclosed embodiments. Positional descriptors indicating the location or orientation of one element relative to another are used for ease and clarity of description. Unless indicated otherwise, these descriptors should be understood in the context of the figures and should not be considered limiting. Such spatially relative terms are intended to encompass different orientations in use or operation of disclosed embodiments, in addition to orientations exemplified in the figures. Furthermore, when a first element is referred to as being “on”, “above”, “below”, “over”, or “under” a second element, the first element can be directly or indirectly on, above, below, over, or under the second element, respectively, such that one or multiple intervening elements may be disposed between the first element and the second element.

[0140] The terms “a”, “an”, and “one” are defined herein to mean “at least one”, that is, these terms do not exclude a plural number of elements, unless stated otherwise.

[0141] The term “or” is defined as “and / or”, unless stated otherwise.

[0142] Terms such as “substantially”, “generally”, and “about”, which modify a value, condition, or characteristic should be understood to mean that the value, condition, or characteristic falls within acceptable tolerances for the proper functioning of the described embodiment or within an acceptable range of experimental error. In particular, the term “about” generally denotes a range of values that one skilled in the art would consider equivalent to the stated value (e.g., having the same or an equivalent function or result). In some instances, the term “about” means a variation of ±10% of the stated value. It is noted that all numeric values used herein are assumed to be modified by the term “about”, unless stated otherwise. The term “between” refers to a range defined by endpoints, inclusive of both endpoints, unless stated otherwise.

[0143] The term “based on” as used herein is intended to mean “based at least in part on”, whether directly or indirectly, and to encompass both “based solely on” and “based partly on”. In particular, the term “based on” may also be understood as meaning “from”, “depending on”, “representative of’, “indicative of’, “associated with”, “relating to”, and the like.

[0144] The terms “match”, “matching”, and “matched” refer herein to a condition where two elements are either identical or within a predetermined tolerance of each other. These terms encompass not only exact matches but also substantial, approximate, or subjective matches, as well as a best or highest match among various matching possibilities.

[0145] The terms “connected” and “coupled”, along with their derivatives and variants, refer herein to any form of connection or coupling, whether direct or indirect, between two or more elements, unless stated otherwise. This connection or coupling can take various forms, including, but not limited to, mechanical, optical, electrical, magnetic, thermal, chemical, logical, fluidic, operational, or any combination thereof.

[0146] The term “concurrently” refers herein to the simultaneous or overlapping occurrence of two or more processes. The term “concurrently” does not necessarily imply complete synchronicity but encompasses various scenarios. These scenarios include the simultaneous occurrence of two processes; a first process that both begins and ends during the duration of a second process; and a first process that starts during the duration of a second process but ends after the second process is completed.

[0147] The term “measured” when referring to a quantity or parameter is intended to mean that the quantity or parameter can be measured either directly or indirectly. In the case of indirect measurement, the quantity or parameter can be derived, retrieved, inferred or otherwise determined from directly measured data.

[0148] The terms “light” and “optical”, along with their variants and derivatives, encompass radiation across any appropriate region of the electromagnetic spectrum. This includes not only visible light but also extends to invisible regions such as the terahertz (THz), infrared (IR), and ultraviolet (UV) spectral bands. For example, in certain embodiments, the disclosed techniques can be implemented with optical signals having a wavelength lying within a wavelength band ranging from about 700 nm to about 1700 nm, covering relevant telecommunication wavebands. However, it should be noted that this wavelength range is provided for illustrative purposes, and the disclosed techniques may extend beyond this range. Furthermore, all descriptions provided herein as a function of wavelength could also be formulated as a function of frequency, wave number, energy, or other pertinent spectral parameters.

[0149] The term “processor” as used herein broadly refers to any electronic device, circuitry, or component capable of processing, receiving, or transmitting data or instructions, such as computer programs, commands, functions, processes, software codes, executables, applications, and similar entities. The term “processor” ismeant to encompass a single processor or processing unit, multiple processors or processing units, or other suitably configured processing elements. When a processor includes multiple processing elements, these elements may be located at a single site or distributed across multiple sites interconnected by a communication network. Examples of communication networks include local area networks (LANs) and wide area networks (WANs) such as the Internet. Non-limiting examples of processors include general-purpose single- or multicore processors; central processing units (CPUs); microprocessors; controllers; microcontrollers; digital signal processors (DSPs); programmable logic devices; field-programmable gate arrays (FPGAs); applicationspecific integrated circuits (ASICs); digital processors or circuits; analog processors or circuits; state machines; and / or any other device capable of processing information.

[0150] The term “memory” as used herein broadly refers to any electronic device, circuitry, or component capable of storing electronic data or information. In some instances, the term “memory” may be used interchangeably with the term “computer readable storage medium”. The term “memory” is meant to encompass a single memory or memory unit, multiple memories or memory units, or other suitably configured memory elements. When a memory includes multiple memory elements, these elements may be located at a single site or distributed across multiple sites interconnected by a communication network. Non-limiting examples of memories include random-access memories (RAM) of any type; read-only memories (ROM) of any type; magnetic storage devices; optical storage devices; solid-state drive (SSD) devices, such as flash drive memories; and any other tangible and / or non-transitory computer readable medium capable of storing electronic data or information.

[0151] Numerous modifications could be made to the embodiments described above without departing from the scope of the appended claims.

Claims

CLAIMS1. A quantum communication network, comprising: a node array having a plurality of nodes comprising a first node and a second node; an optical communication infrastructure interconnecting the plurality of nodes and comprising an optical communication channel linking the first and second nodes; a transmitter unit located at the first node and comprising: an optical source module configured to generate a quantum signal and a reference signal that are distinguishable across at least two degrees of freedom; a multiplexing module configured to combine the quantum and reference signals across the at least two degrees of freedom, thereby generating a multiplexed signal; and an output port configured to transmit the multiplexed signal for propagation through the optical communication channel; a receiver unit located at the second node and comprising: an input port configured to receive the multiplexed signal; and a demultiplexing module configured to separate the multiplexed signal into the quantum signal and the reference signal; and a stabilization feedback loop for network error compensation, comprising: a detection stage located at the second node and configured to detect the reference signal following demultiplexing; a processing stage configured to determine error signals from the detected reference signal, the error signals being related to the at least two degrees of freedom and indicative of errors in network parameters associated with the optical communication channel; and an actuation stage comprising at least one optical actuator configured to apply corrective operations within the quantum communication network, based on the error signals to stabilize the network parameters.

2. The quantum communication network of claim 1, wherein the optical communication channel comprises an optical fiber link, an optical waveguide, or a free-space link.

3. The quantum communication network of claim 1 or 2, wherein the reference signal is a classical signal.

4. The quantum communication network of claim 1 to 2, wherein the reference signal is a quantum signal.

5. The quantum communication network of any one of claims 1 to 4, wherein the quantum signal and the reference signal originate from a same light source within the optical source module.

6. The quantum communication network of any one of claims 1 to 4, wherein the quantum signal and the reference signal originate from different light sources within the optical source module.

7. The quantum communication network of any one of claims 1 to 6, wherein the optical source module is configured to emit the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

8. The quantum communication network of any one of claims 1 to 7, wherein the at least two degrees of freedom comprise (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

9. The quantum communication network of any one of claims 1 to 8, wherein the receiver unit further comprises (i) a quantum processor configured to perform operations on the quantum signal, and / or (ii) a quantum memory configured to store the quantum signal.

10. The quantum communication network of any one of claims 1 to 9, wherein the network parameters comprise optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

11. The quantum communication network of any one of claims 1 to 10, wherein the actuation stage is configured to apply the corrective operations in real-time or near real-time.

12. The quantum communication network of any one of claims 1 to 11, wherein the processing stage is configured to compare the error signals against predetermined threshold levels, and wherein the actuation stage is configured to apply the corrective operations upon determination that the error signals exceed the predetermined threshold levels.

13. The quantum communication network of any one of claims 1 to 12, wherein the actuation stage is configured to apply the corrective operations upstream of the demultiplexing module.

14. The quantum communication network of claim 13, wherein the actuation stage is configured to apply the corrective operations within the receiver unit.

15. The quantum communication network of any one of claims 1 to 14, wherein the at least one actuator comprises an optical delay line for controlling a physical channel length, an electronically controlled polarization module for managing channel polarization, an optical interferometer for addressing phase errors, or any combination thereof.

16. The quantum communication network of any one of claims 1 to 15, wherein: the second node is one of a plurality of second nodes;the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes; the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes; and the stabilization feedback loop is one of a plurality of stabilization feedback loops, each stabilization feedback loop being associated with a respective receiver unit.

17. The quantum communication network of claim 16, wherein the multiplexed signals sent by the transmitter unit are identical across all the receiver units.

18. The quantum communication network of claim 16, wherein the multiplexed signals sent by the transmitter unit are not identical across all the receiver units, differing in their quantum signals, their reference signals, or both.

19. The quantum communication network of any one of claims 1 to 15, wherein: the first node is one of a plurality of first nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node; the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes; the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node; and the stabilization feedback loop is one of a plurality of stabilization feedback loops, each stabilization feedback loop being associated with a respective receiver unit.

20. The quantum communication network of claim 19, wherein the multiplexed signals are identical across all the transmitter units.

21. The quantum communication network of claim 19, wherein the multiplexed signals are not identical across all the transmitter units, differing in their quantum signals, their reference signals, or both.

22. A method for stabilization of a quantum communication network comprising a node array having a plurality of nodes comprising a first node and a second node, and an optical communication infrastructure interconnecting the plurality of nodes and comprising an optical communication channel linking the first and second nodes, the method comprising: generating a quantum signal and a reference signal at the first node, wherein the quantum signal and the reference signal are distinguishable across at least two degrees of freedom; multiplexing the quantum and reference signals across the at least two degrees of freedom, thereby generating a multiplexed signal;transmitting the multiplexed signal from the first node to the second node through the optical communication channel; receiving the multiplexed signal at the second node; demultiplexing the multiplexed signal into the quantum signal and the reference signal; detecting the reference signal following demultiplexing; determining error signals from the detected reference signal, wherein the error signals are associated with the at least two degrees of freedom and are indicative of errors in network parameters associated with the optical communication channel; and controlling at least one optical actuator to apply corrective operations within the quantum communication network, based on the error signals to stabilize the network parameters.

23. The method of claim 22, wherein the reference signal is a classical signal.

24. The method of claim 22, wherein the reference signal is a quantum signal.

25. The method of any one of claims 22 to 24, wherein generating the quantum signal and the reference signal comprises emitting the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

26. The method of any one of claims 22 to 25, wherein the at least two degrees of freedom comprise (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

27. The method of any one of claims 22 to 26, wherein the network parameters comprise optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

28. The method of any one of claims 22 to 27, wherein controlling the at least one optical actuator comprises applying the corrective operations in real-time or near real-time.

29. The method of any one of claims 22 to 28, further comprising comparing the error signals against predetermined threshold levels, and wherein controlling the at least one optical actuator comprises applying the corrective operations upon determination that the error signals exceed the predetermined threshold levels.

30. The method of any one of claims 22 to 29, wherein controlling the at least one optical actuator comprises applying the corrective operations upstream of the demultiplexing module.

31. The method of claim 30, wherein controlling the at least one optical actuator comprises applying the corrective operations within the receiver unit.

32. The method of any one of claims 22 to 31, wherein the at least one actuator comprises an optical delay line for controlling a physical channel length, an electronically controlled polarization module for managing channel polarization, an optical interferometer for addressing phase errors, or any combination thereof.

33. The method of any one of claims 22 to 32, wherein the second node is one of a plurality of second nodes, the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes, and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes, and the error signals are indicative of errors in network parameters associated with the plurality of optical communication channels.

34. The method of any one of claims 22 to 32, wherein the first node is one of a plurality of first nodes, the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node, the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes, the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node, and the error signals are indicative of errors in network parameters associated with the plurality of optical communication channels.

35. A network stabilization system for a quantum communication network comprising a node array having a plurality of nodes comprising a first node and a second node, and an optical communication infrastructure interconnecting the plurality of nodes and comprising an optical communication channel linking the first and second nodes, the network stabilization system comprising: a transmitter unit at the first node, comprising: an optical source module configured to generate a quantum signal and a reference signal that are distinguishable across at least two degrees of freedom; a multiplexing module configured to combine the quantum and reference signals across the at least two degrees of freedom, thereby generating a multiplexed signal; and an output port configured to transmit the multiplexed signal from the first node to the second node through the optical communication channel; a receiver unit at the second node, comprising: an input port configured to receive the multiplexed signal; and a demultiplexing module configured to separate the multiplexed signal into the quantum signal and the reference signal; and a stabilization feedback loop for network error compensation, comprising: a detection stage configured to detect the reference signal after demultiplexing;a processing stage configured to determine error signals from the detected reference signal, the error signals being associated with the at least two degrees of freedom and indicative of errors in network parameters associated with the optical communication channel; and an actuation stage comprising at least one optical actuator configured to apply corrective operations within the quantum communication network, based on the error signals to stabilize the network parameters.

36. The network stabilization system of claim 35, wherein the reference signal is a classical signal.

37. The network stabilization system of claim 35, wherein the reference signal is a quantum signal.

38. The network stabilization system of any one of claims 35 to 37, wherein the optical source module is configured to emit the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

39. The network stabilization system of any one of claims 35 to 38, wherein the at least two degrees of freedom comprise (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

40. The network stabilization system of any one of claims 35 to 39, wherein the receiver unit further comprises (i) a quantum processor configured to perform operations on the quantum signal, and / or (ii) a quantum memory configured to store the quantum signal.

41. The network stabilization system of any one of claims 35 to 40, wherein the network parameters comprise optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

42. The network stabilization system of any one of claims 35 to 41, wherein the actuation stage is configured to apply the corrective operations in real-time or near real-time.

43. The network stabilization system of any one of claims 35 to 42, wherein the processing stage is configured to compare the error signals against predetermined threshold levels, and wherein the actuation stage is configured to apply the corrective operations upon determination that the error signals exceed the predetermined threshold levels.

44. The network stabilization system of any one of claims 35 to 43, wherein the actuation stage is configured to apply the corrective operations upstream of the demultiplexing module.

45. The network stabilization system of claim 44, wherein the actuation stage is configured to apply the corrective operations within the receiver unit.

46. The network stabilization system of any one of claims 35 to 45, wherein the at least one actuator comprises an optical delay line for controlling a physical channel length, an electronically controlled polarization module for managing channel polarization, an optical interferometer for addressing phase errors, or any combination thereof.

47. The network stabilization system of any one of claims 35 to 46, wherein: the second node is one of a plurality of second nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes; the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes; and the stabilization feedback loop is one of a plurality of stabilization feedback loops, each stabilization feedback loop being associated with a respective receiver unit.

48. The network stabilization system of claim 47, wherein the multiplexed signals sent by the transmitter unit are identical across all the receiver units.

49. The network stabilization system of claim 47, wherein the multiplexed signals sent by the transmitter unit are not identical across all the receiver units, differing in their quantum signals, their reference signals, or both.

50. The network stabilization system of any one of claims 35 to 46, wherein: the first node is one of a plurality of first nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node; the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes; the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node; and the stabilization feedback loop is one of a plurality of stabilization feedback loops, each stabilization feedback loop being associated with a respective receiver unit.

51. The network stabilization system of claim 50, wherein the multiplexed signals are identical across all the transmitter units.

52. The network stabilization system of claim 50, wherein the multiplexed signals are not identical across all the transmitter units, differing in their quantum signals, their reference signals, or both.

53. A quantum communication network, comprising: a node array with a plurality of nodes, the plurality of nodes comprising a first node and a second node;an optical communication infrastructure interconnecting the plurality of nodes, the optical communication infrastructure comprising an optical communication channel linking the first and second nodes; a transmitter unit at the first node, comprising: an optical source module configured to generate a quantum signal and a reference signal, the quantum and reference signals being distinguishable across at least two degrees of freedom; a multiplexing module configured to combine the quantum and reference signals across the at least two degrees of freedom to produce a multiplexed signal; and an output port configured to transmit the multiplexed signal through the optical communication channel; a receiver unit at the second node, comprising: an input port configured to receive the multiplexed signal; and a demultiplexing module configured to separate the multiplexed signal into the quantum signal and the reference signal; and a network monitoring unit, comprising: a detection stage located at the second node and configured to detect the reference signal after demultiplexing; and a processing stage configured to derive monitoring signals from the detected reference signal, the monitoring signals being related to the at least two degrees of freedom and providing status indicators of network parameters associated with the optical communication channel.

54. The quantum communication network of claim 53, wherein the optical communication channel comprises an optical fiber link, an optical waveguide, or a free-space link.

55. The quantum communication network of claim 53 or 54, wherein the reference signal is a classical signal.

56. The quantum communication network of claim 53 or 54, wherein the reference signal is a quantum signal.

57. The quantum communication network of any one of claims 53 to 56, wherein the optical source module is configured to emit the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

58. The quantum communication network of any one of claims 53 to 57, wherein the at least two degrees of freedom comprise (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

61. The quantum communication network of any one of claims 53 to 60, wherein the network parameters comprise optical channel polarization, optical channel length, optical propagation time, node clock frequencyand phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

62. The quantum communication network of any one of claims 53 to 61, wherein: the second node is one of a plurality of second nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes; and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes.

63. The quantum communication network of any one of claims 53 to 61, wherein: the first node is one of a plurality of first nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node; the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes; and the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node.

64. A method for monitoring a quantum communication network comprising a node array having a plurality of nodes comprising a first node and a second node, and an optical communication infrastructure interconnecting the plurality of nodes and comprising an optical communication channel linking the first and second nodes, the method comprising: generating a quantum signal and a reference signal at the first node, the quantum and reference signals being distinguishable across at least two degrees of freedom; multiplexing the quantum and reference signals across the at least two degrees of freedom to produce a multiplexed signal; transmitting the multiplexed signal from the first node to the second node through the optical communication channel; receiving the multiplexed signal at the second node; demultiplexing the multiplexed signal into the quantum signal and the reference signal; detecting the reference signal following demultiplexing; and deriving monitoring signals from the detected reference signal, the monitoring signals being related to the at least two degrees of freedom and providing status indicators of network parameters associated with the optical communication channel.

65. The method of claim 64, wherein the reference signal is a classical signal.

66. The method of claim 64, wherein the reference signal is a quantum signal.

67. The method of any one of claims 64 to 66, wherein generating the quantum signal and the reference signal comprises emitting the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

68. The method of any one of claims 64 to 67, wherein the at least two degrees of freedom comprise (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

69. The method of any one of claims 64 to 68, wherein the network parameters comprise optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

70. The method of any one of claims 64 to 69, wherein the second node is one of a plurality of second nodes, the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes, and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes, and the monitoring signals provide status indicators of network parameters associated with the plurality of optical communication channels.

71. The method of any one of claims 64 to 69, wherein the first node is one of a plurality of first nodes, the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node, the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes, the receiver unit is one of a plurality of receiver units, each receiver unit located at the second node, and the monitoring signals provide status indicators of network parameters associated with the plurality of optical communication channels.

72. A network monitoring system for a quantum communication network comprising a node array having a plurality of nodes comprising a first node and a second node, and an optical communication infrastructure interconnecting the plurality of nodes and comprising an optical communication channel linking the first and second nodes, the network monitoring system comprising: a transmitter unit at the first node, comprising: an optical source module configured to generate a quantum signal and a reference signal that are distinguishable across at least two degrees of freedom; a multiplexing module configured to combine the quantum and reference signals across the at least two degrees of freedom, thereby generating a multiplexed signal; and an output port configured to transmit the multiplexed signal from the first node to the second node through the optical communication channel;a receiver unit at the second node, comprising: an input port configured to receive the multiplexed signal; and a demultiplexing module configured to separate the multiplexed signal into the quantum signal and the reference signal; and a network monitoring unit, comprising: a detection stage located at the second node and configured to detect the reference signal after demultiplexing; and a processing stage configured to derive monitoring signals from the detected reference signal, the monitoring signals being related to the at least two degrees of freedom and providing status indicators of network parameters associated with the optical communication channel.

73. The network stabilization system of claim 72, wherein the reference signal is a classical signal.

74. The network stabilization system of claim 72, wherein the reference signal is a quantum signal.

75. The network stabilization system of any one of claims 72 to 74, wherein the optical source module is configured to emit the quantum signal and the reference signal within a waveband ranging from about 700 nm to about 1700 nm.

76. The network stabilization system of any one of claims 72 to 75, wherein the at least two degrees of freedom comprise (i) frequency and time, or (ii) frequency and polarization, or (iii) time and polarization, or (iv) frequency, time, and polarization.

77. The network stabilization system of any one of claims 72 to 76, wherein the network parameters comprise optical channel polarization, optical channel length, optical propagation time, node clock frequency and phase, node optical frequency and phase, alignment between quantum state analyzers, or any combination thereof.

78. The network stabilization system of any one of claims 72 to 77, wherein: the second node is one of a plurality of second nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking the first node to a respective one of the second nodes; and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes.

79. The network stabilization system of claim 78, wherein the multiplexed signals sent by the transmitter unit are identical across all the receiver units.

80. The network stabilization system of claim 78, wherein the multiplexed signals sent by the transmitter unit are not identical across all the receiver units, differing in their quantum signals, their reference signals, or both.

81. The network stabilization system of any one of claims 72 to 77, wherein: the first node is one of a plurality of first nodes; the optical communication channel is one of a plurality of optical communication channels, each optical communication channel linking a respective one of the first nodes to the second node; the transmitter unit is one of a plurality of transmitter units, each transmitter unit located at a respective one of the first nodes; and the receiver unit is one of a plurality of receiver units, each receiver unit located at a respective one of the second nodes.

82. The network stabilization system of claim 81, wherein the multiplexed signals are identical across all the transmitter units .

83. The network stabilization system of claim 81, wherein the multiplexed signals are not identical across all the transmitter units, differing in their quantum signals, their reference signals, or both.

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