Quantum key distribution device and method suitable for establishing a global quantum key distribution network

Abstract

To provide a quantum key delivery device that establishes a global network for quantum key delivery that does not depend on a trusted third party.SOLUTION: A quantum key delivery device 10 includes an input signal path 12 that receives a plurality of optical input signals from a transmitter device, an output signal path 14 that is connected to the input signal path at a first end 16 and that transmits a plurality of first optical output signals at a second end 20 opposite to the first end, and a detector signal path 22 that is connected to the input signal path at a third end 24 and that includes a photon detector device 26 at a fourth end 28 opposite to the third end. The photon detector device detects the plurality of optical input signals from the transmitter device and establishes a shared quantum encryption key with the transmitter device based on the detected plurality of optical input signals. The input signal path forms an input path of a Mach-Zehnder interferometer 30 equipped with a first beam splitter 18, and the output signal path and the detector signal path form an output path of the Mach-Zehnder interferometer.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to techniques for quantum key distribution (QKD), and in particular to techniques for establishing a global network for quantum key distribution that does not rely on a trusted third party. [Background technology]

[0002] The prospect of future quantum computing threatens the very pillars of security of classical communication networks. The trust and confidence in the security provided by conventional (classical) cryptography is based on the conviction that the computational complexity to break the protection cannot be overcome. Quantum computers, however, have the potential to overcome the existing complexity. Quantum key distribution (QKD) is a technology that overcomes these constraints and makes it possible to establish provably secure classical keys between legitimate parties. Recently, technologies have been established to exchange quantum encryption keys over very long distances, in some cases across the entire globe. See Non-Patent Document 1.

[0003] Quantum key distribution networks that allow provably secure classical encryption keys to be established between three or more authorized users, i.e., in a networked environment, have also been studied. In Non-Patent Document 2, quantum key distribution using polarization coding over fiber links in branched or looped configurations is studied. In Non-Patent Document 3, QKD with a switched multi-node architecture is studied, utilizing optical switching elements such as lithium niobate, microelectromechanical systems (MEMS), and opto-mechanical systems. For further references on lithium niobate optical switches, see Non-Patent Document 4 and Non-Patent Document 5.

[0004] Non-Patent Document 6 discloses a QKD switch-based network that uses a commercially available optical switch, Thorlabs OSW22-2x2.

[0005] In view of the prior art, what is needed is a QKD network that enables reliable sharing of encryption keys at high key rates over long distances, spanning virtually the entire globe. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] N.S. Kirsanov et al., "Long-distance quantum key distribution based on the physical loss control," Cornell Preprint Server, arXiv:2105.00035v1, April 30, 2021. [Non-Patent Document 2] X. Tang et al., "Demonstration of an active quantum key distribution network," Proceedings of SPIE 6305, Quantum Communications and Quantum Imaging IV, 630506, August 29, 2006. [Non-Patent Document 3] P. Toliver et al., "Experimental Investigation of Quantum Key Distribution Through Transparent Optical Switch Elements," IEEE Photonics Technology Letters, Vol. 15, No. 11, November 2003. [Non-Patent Document 4] H. Okayama, "Lithium Niobate Electro-Optic Switching", Springer, Boston, 2006. [Non-Patent Document 5] H. Wang et al., "Broadband 2x2 lithium niobate electrooptic switch based on a Mach-Zehnder interferometer with counter-tapered directional couplers," Appl. Opt, Vol. 56, No. 29, October 2017, pp. 8164-8168 [Non-Patent Document 6] A. Tayduganov et al., "Optimizing the deployment of quantum key distribution switch-based networks," Cornell preprint server, arXiv:2104.04155v2, April 15, 2021. Summary of the Invention [Problem to be solved by the invention]

[0007] These objectives are addressed by a quantum key distribution device according to independent claim 1, a quantum key distribution network according to claim 8 and a quantum key distribution method according to independent claim 11. The dependent claims relate to preferred embodiments. [Means for solving the problem]

[0008] In a first aspect, the present disclosure relates to a quantum key distribution device comprising an input signal path receiving a plurality of optical input signals from a transmitter device, an output signal path connected to the input signal path at a first end and transmitting a plurality of first optical output signals at a second end opposite the first end, and a detector signal path connected to the input signal path at a third end and including a photon detector device at a fourth end opposite the third end. The photon detector device detects the plurality of optical input signals from the transmitter device. The quantum key distribution device establishes a shared quantum encryption key with the transmitter device based on the detected plurality of optical input signals. The input signal path forms an input path of a Mach-Zehnder interferometer, and the output signal path and the detector signal path form an output path of the Mach-Zehnder interferometer.

[0009] Advantageously, Mach-Zehnder interferometers may enable the construction of network nodes in a QKD network that may allow a user to switch between a receive mode, in which a signal is detected, and a transmit mode, in which the signal is transmitted to different nodes in the network. Furthermore, Mach-Zehnder interferometers allow for very fast switching speeds, in the range of a few nanoseconds (ns) or less, with very low losses, and thus may enable fast and reliable exchange of encryption keys on a global scale.

[0010] In this disclosure, an input signal path, an output signal path, and / or a detector signal path may refer to any communication path that transmits an optical signal. For example, the input signal path, the output signal path, and / or the detector signal path may include an optical fiber or a free space optical signal link.

[0011] The first end, second end, third end, and fourth end of each signal path do not represent an order or ranking of the ends of the signal path, but are merely a notational distinction between different ends of the signal path. In some embodiments, the first end, second end, third end, and fourth end may be in different locations. In other embodiments, two or more of the first end, second end, third end, and fourth end may be in the same location or may be coincident.

[0012] The transmitter device may refer to any device that transmits optical signals, such as single photons or photon fluxes, to the quantum key distribution device. In particular, the transmitter device may include a photon source.

[0013] In this disclosure, a photon detector device may refer to any device that detects optical signals, such as photons. In some embodiments, a photon detector device may detect single photons. In other embodiments, a photon detector device may detect photon flux or light intensity.

[0014] According to an embodiment, the quantum key distribution device comprises a first beam splitter connecting the input signal path to a first end of the output signal path and to a third end of the detector signal path.

[0015] Alternatively or additionally, the quantum key distribution device may comprise a second beam splitter connecting the output signal path and the detector signal path, particularly downstream of the first beam splitter.

[0016] The first beam splitter and / or the second beam splitter may be a 50 / 50 beam splitter.

[0017] According to an embodiment, the quantum key distribution device may, in a first configuration of the quantum key distribution device, transmit a plurality of optical input signals via the output signal path, and in particular transmit the plurality of optical input signals substantially unaffected via the output signal path.

[0018] The first configuration may correspond to a transmission mode in which the quantum key distribution device acts as a bystander, whereby the optical input signal is transmitted substantially unaffected to another quantum key distribution device in the quantum key distribution network.

[0019] According to an embodiment, the quantum key distribution device may, in a first configuration of the quantum key distribution device, transmit an optical input signal to an output signal path with probability p, where p is greater than or equal to 0.7, in particular greater than or equal to 0.8 or greater than or equal to 0.9.

[0020] Additionally, the quantum key distribution device may transmit a plurality of optical input signals to the detector signal path in a second configuration of the quantum key distribution device that is different from the first configuration.

[0021] The second configuration setting may correspond to a receive mode in which a plurality of optical signals received from the transmitter device are detected by the optical detector device, whereby a quantum encryption key can be established with the transmitter device based on the detected plurality of optical input signals.

[0022] In an embodiment, the quantum key distribution device, in particular the Mach-Zehnder interferometer, may be adapted to switch between the first and second configurations, in particular in response to a received switch signal.

[0023] In this disclosure, a switch signal may represent a classical signal received from another quantum key distribution device in the quantum key distribution network, or from a central management node of the quantum key distribution network.

[0024] It is an advantage of the present disclosure that a Mach-Zehnder interferometer can be quickly and efficiently switched between a first configuration in which an optical input signal is transmitted and a second configuration in which the signal is detected.

[0025] According to an embodiment the quantum key distribution device may comprise a first phase modulator device in the detector signal path, the first phase modulator device configured to modulate the phase of the optical signal in the detector signal path and / or to modulate the intensity of the optical signal in the detector signal path and / or to form an optical signal in the output signal path and / or to modulate the intensity of the optical signal in the output signal path.

[0026] In particular, the first phase modulator device may be located between the first beam splitter and the second beam splitter.

[0027] The first phase modulator device may modulate the phase of the optical signal in the detector signal path before the second beam splitter, which phase modulation may result in a change in intensity in the detector signal path and / or the output signal path after the second beam splitter.

[0028] The first phase modulator device may adjust the signal strength in the detector signal path and / or the signal strength in the output signal path depending on a phase setting of the first phase modulator device.

[0029] According to an embodiment, the first phase modulator device may be adapted to switch between the first and second configuration settings, particularly in response to a received switch signal.

[0030] In some cases, a single phase modulator device may be sufficient in a quantum key distribution device according to the present disclosure, however, as will be described in more detail below, some limitations on the signal strength range may apply in configurations with only one phase modulator device.

[0031] According to an embodiment, the quantum key distribution device comprises a second phase modulator device in the output signal path, which modulates the phase of the optical signal in the output signal path and / or modulates the intensity of the optical signal in the output signal path and / or forms an optical signal in the output signal path and / or modulates the intensity of the optical signal in the detector signal path.

[0032] In particular, the second phase modulator device may be located between the first beam splitter and the second beam splitter.

[0033] The second phase modulator device may relax the restrictions on the signal strength range that may be applied in a quantum key distribution device comprising only one phase modulator device, in particular the second phase modulator device may enable the quantum key distribution device to launch an arbitrarily small strength optical signal into the output signal path.

[0034] The second phase modulator device may correspond in design and function to the first phase modulator device. According to an embodiment, the first phase modulator device and / or the second phase modulator device comprises an optoelectronic phase modulator.

[0035] It is an advantage of the techniques of the present disclosure that the first phase modulator device and / or the second phase modulator device allow very fast switching, on the order of a few nanoseconds or less.

[0036] According to an embodiment the switching speed of the first phase modulator arrangement and / or the second phase modulator arrangement is 5 ns or less, in particular 3 ns or less, 2 ns or less, 1 ns or less, 0.5 ns or less, 0.2 ns or less or 0.1 ns or less.

[0037] According to an embodiment, the quantum key distribution device further comprises a photon source for launching a plurality of optical signals onto an output signal path.

[0038] Depending on the photon source, the quantum key distribution device may act as a transmitter device to transmit optical signals within a quantum key distribution network, such as for establishing quantum encryption keys with receiver devices in the network.

[0039] According to an embodiment, the quantum key distribution device further comprises a random number generator, in particular a quantum random number generator. The random number generator, in particular the quantum random number generator, may be connected to the detector signal path, in particular to the first phase modulator device.

[0040] A random number generator, in particular a quantum random number generator, may control the operation of the first phase modulator device and / or the operation of the second phase modulator device.

[0041] A random number generator, in particular a quantum random number generator, may enable a quantum key distribution device to randomly generate optical signals by methods based on symmetric encryption, such as one-time pad, for example to establish secure encryption keys for secure communication and / or authentication with another device in the quantum key distribution network.

[0042] According to an embodiment, the quantum key distribution device may further be connected to the sender device by a classical information channel. In particular, the quantum key distribution device may establish a shared bit sequence and perform privacy amplification together with the sender device, in particular via the classical information channel.

[0043] The step of establishing the shared bit string may include post-selection of the measurements and / or error correction processing.

[0044] In the detailed description above, the techniques of the present disclosure have been described primarily with reference to a single quantum key distribution device. However, the present disclosure is not so limited and in a second aspect also refers to a quantum key distribution network comprising a plurality of interconnected quantum key distribution devices having some or all of the above-mentioned characteristics.

[0045] According to a second aspect, the transmitter device as described above with reference to the first aspect may be a first quantum key distribution device of a plurality of quantum key distribution devices, and an output signal path of the first quantum key distribution device of the plurality of quantum key distribution devices may be connected to an input signal path of a second quantum key distribution device of the plurality of quantum key distribution devices. The second quantum key distribution device may establish a shared quantum encryption key with the first quantum key distribution device based on the detected plurality of optical input signals.

[0046] Advantageously, a quantum key distribution network according to the present disclosure may enable reliable exchange of quantum encryption keys at high key rates between multiple entities distributed across different spatially separated geographic locations.

[0047] According to an embodiment, the multiple quantum key distribution devices may be arranged in a ring topology.

[0048] A ring topology may refer to a network topology as it may be defined by connections to adjacent nodes, but may not necessarily correspond to the geographic locations of each quantum key distribution device in the network.

[0049] According to an embodiment, the ring topology may include n quantum key distribution devices (n is any integer equal to or greater than 2), and the n quantum key distribution devices may be arranged in a ring shape in the order of 1 to n. An output signal path of the nth quantum key distribution device in the ring topology may be connected to an input signal path of a first quantum key distribution device in the ring topology.

[0050] According to another embodiment, the multiple quantum key distribution devices may be arranged in a lattice topology.

[0051] Again, the lattice topology may refer to a network topology as it may be defined by connections to adjacent nodes, but may not necessarily correspond to the geographic locations of the quantum key distribution devices within the network.

[0052] The lattice may include a number of lattice nodes, and adjacent lattice nodes may be connected to each other by optical signal lines.

[0053] According to an embodiment, a plurality of lattice nodes, in particular each lattice node, may include a plurality of m Mach-Zehnder interferometers in a ring topology, where m may be any positive integer equal to or greater than 2, in particular m=4.

[0054] Likewise, topologies other than ring or lattice topologies may be utilized in the present disclosure.

[0055] In some embodiments, the ring topology and / or the grid topology may represent the topology of a single user cluster. Several separate clusters may be interconnected by optical highway lines.

[0056] In a third aspect, the present disclosure relates to a quantum key distribution method including receiving a plurality of optical input signals from a transmitter device and selectively transmitting the received plurality of optical input signals to an output signal path via a Mach-Zehnder interferometer or to a detector signal path including a photon detector device.

[0057] In the case of transmitting the received multiple optical input signals to the detector signal path, the method further includes detecting the multiple optical input signals from the transmitter device in the detector signal path by the photon detector device, and establishing a shared quantum encryption key with the transmitter device based on the detected multiple optical input signals.

[0058] According to an embodiment, the Mach-Zehnder interferometer is a Mach-Zehnder interferometer having some or all of the features described above with reference to the first and / or second aspect of the present disclosure.

[0059] In an embodiment, the optical input signal is transmitted substantially unaffected through the Mach-Zehnder interferometer to the output signal path.

[0060] According to an embodiment, an optical input signal may be transmitted through the Mach-Zehnder interferometer to an output signal path with probability p, where p is 0.7 or greater, in particular 0.8 or greater, or 0.9 or greater.

[0061] In an embodiment, a plurality of received optical input signals are transmitted through a Mach-Zehnder interferometer in response to a received switch signal.

[0062] According to an embodiment, the method further comprises modulating the phase of the optical signal in the detector signal path, in particular by a first phase modulator device, such as a first phase modulator device in the detector signal path, and / or modulating the intensity of the optical signal in the detector signal path, and / or forming an optical signal in the output signal path, and / or modulating the intensity of the optical signal in the output signal path.

[0063] According to an embodiment, the method further comprises modulating the phase of the optical signal in the output signal path, in particular by a second phase modulator device, such as a second phase modulator device in the output signal path, and / or modulating the intensity of the optical signal in the output signal path, and / or forming an optical signal in the output signal path, and / or modulating the intensity of the optical signal in the detector signal path.

[0064] The second phase modulator device may correspond in design and function to the first phase modulator device.

[0065] In an embodiment, the method further includes launching a plurality of optical signals, such as by a controlled photon source, onto an output signal path.

[0066] According to an embodiment, the step of establishing a shared quantum encryption key with the transmitter device comprises the steps of establishing a shared bit string and performing privacy amplification together with the transmitter device, in particular via a classical information channel.

[0067] The step of establishing the shared bit string may include post-selection of the measurements and / or error correction processing. [Brief description of the drawings]

[0068] The techniques and associated advantages of the present disclosure are best understood from the following detailed description of illustrative embodiments in connection with the accompanying drawings. [Figure 1] FIG. 1 is a schematic diagram of a quantum key distribution device according to an embodiment. [Diagram 2] FIG. 2 is a schematic diagram of a Mach-Zehnder interferometer architecture in which one phase modulator is utilized in a quantum key distribution device according to an embodiment. [Diagram 3] FIG. 3 is a schematic diagram of another Mach-Zehnder interferometer architecture in which two phase modulators are utilized in a quantum key distribution device according to an embodiment. [Figure 4] FIG. 4 is a flow diagram of a quantum key distribution method according to an embodiment. [Diagram 5] FIG. 5 is a schematic diagram of a quantum key distribution network according to an embodiment. [Figure 6] FIG. 6 is a diagram illustrating a schematic configuration of a ring topology quantum key distribution network according to an embodiment. [Figure 7] FIG. 7 is a schematic diagram illustrating a network operation method for quantum key distribution in a quantum key distribution network having a ring topology according to an embodiment. [Figure 8a] FIG. 8a is a schematic diagram of a lattice topology quantum key distribution network according to an embodiment. [Figure 8b]FIG. 8b is a schematic diagram of a lattice node according to an embodiment in a quantum key distribution network having a lattice topology. [Figure 9] FIG. 9 is a diagram illustrating a schematic diagram of a cluster topology quantum key distribution network according to an embodiment. [Figure 10] FIG. 10 is a schematic diagram of a global quantum key distribution network constructed from multiple clusters according to an embodiment. [Figure 11] FIG. 11 illustrates the key generation rate that can be achieved over global distances in a quantum key distribution network according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0069] 1 is a schematic diagram of a quantum key distribution device 10 according to an embodiment. The quantum key distribution device 10 comprises an input signal path 12 that receives a plurality of optical input signals from a transmitter device (not shown), which may be connected to the quantum key distribution device 10 by an optical fiber channel. The quantum key distribution device 10 further comprises an output signal path 14 that is connected to the input signal path 12 at a first end 16, e.g., a first beam splitter 18. The output signal path 14 may further emit a plurality of first optical output signals at a second end 20 opposite the first end 16.

[0070] As can be further seen from Figure 1, the quantum key distribution device 10 further comprises a detector signal path 22 connected to the input signal path 12 at a third end 24. In the configuration shown in Figure 1, the third end 24 of the detector signal path 22 corresponds to the first end 16 of the output signal path 14 and thus to the location of the first beam splitter 18. The first beam splitter 18 splits the input signal path 12 into the output signal path 14 and the detector signal path 22 at the first end 16 of the output signal path 14 / the third end 24 of the detector signal path 22.

[0071] In the architecture shown in FIG. 1 , input signal path 12 forms the input path of an apparatus that can be functionally thought of as a Mach-Zehnder (MZ) interferometer 30, and output signal path 14 and detector signal path 22 form the output path of the Mach-Zehnder interferometer 30.

[0072] Located at a fourth end 28 of the detector signal path 22 opposite the third end 24 is a photon detector device 26. The photon detector device 26 detects a plurality of optical input signals from the transmitter device, which may reach the photon detector device 26 via the input signal path 12, the first beam splitter 18, and the detector signal path 22 depending on the switch settings of the quantum key distribution device 10, or in particular the switch settings of the Mach-Zehnder interferometer 30.

[0073] Input signal path 12, output signal path 14, and detector signal path 22 may include optical fibers, although in general any channel for transmitting optical signals may be applied in the present invention, including free space transmission links.

[0074] The designations of first end 16, second end 20, third end 24, and fourth end 28 of the signal paths are for notational distinction purposes only and do not necessarily imply any sequence or order.

[0075] A quantum key distribution device 10, such as that shown in FIG. 1, establishes a shared quantum encryption key, such as in the form of a one-time pad, with a transmitter device based on a plurality of detected optical input signals.

[0076] Advantageously, the first beam splitter 18 may switch the quantum key distribution device 10 between a transmit configuration, in which the optical input signals received on the input signal path 12 are transmitted substantially unaffected to the output signal path 14, and a receive configuration, in which the optical signals received on the input signal path 12 are transmitted to the detector signal path 22 and received and measured at the photodetector device 26. In the transmit configuration, the quantum key distribution device 10 acts as a bystander device that does not (substantially) interfere with the input signals. In contrast, in the receive configuration of the quantum key distribution device 10, the optical signals received on the input signal path 12 are intercepted and measured at the photodetector device 26, as will be described in more detail below, enabling the quantum key distribution device 10 to establish a shared quantum encryption key with the transmitter device based on the detected optical input signals.

[0077] Figure 2 is a schematic diagram of a quantum key distribution device 10' implemented as a Mach-Zehnder interferometer 30, according to an embodiment. The functionality of the quantum key distribution device 10' of Figure 2 corresponds generally to the quantum key distribution device 10 of Figure 1, and corresponding components share the same reference numbers.

[0078] At a first beam splitter 18, which may be a 50 / 50 beam splitter, the input signal path 12 splits into an output signal path 14 and a detector signal path 22. The Mach-Zehnder interferometer 30 further includes a second beam splitter 32 downstream of the first beam splitter 18, which may be another 50 / 50 beam splitter that splits the output signal path 14 back into the detector signal path 22. A first phase modulator 34, such as an optoelectronic phase modulator, may be located in the detector signal path 22 between the first beam splitter 18 and the second beam splitter 32. In response to a switch signal, the first phase modulator 34 may modulate the phase of the optical signal in the detector signal path 22, thereby changing the optical signal strength in the detector signal path 22 and / or the output signal path 14. By this mechanism, the first phase modulator 34 may effectively switch the Mach-Zehnder interferometer 30 between a transmit configuration and a receive configuration.

[0079] The optoelectronic phase modulator 34 has the advantage that it allows very short switching times, on the order of a few nanoseconds (ns), e.g., 0.5 ns or 0.1 ns. These switching times are preferably comparable to those of commercially available microelectromechanical switches (MEMS), which are typically in the range of 0.1 ms to 30 ms, or to opto-mechanical switching elements, which have typical switching times of about 1 ms.

[0080] The Mach-Zehnder interferometer 30 with the optoelectronic phase modulator 34 has several further advantages that make it particularly suitable for use in a quantum key distribution network: in transmit mode it is characterized by a relatively small signal loss (about 10% or less), and in receive mode the transmission further down the output signal path 14 can be completely blocked.

[0081] 1, detector signal path 22 may include an optical isolator element 36 downstream of second beam splitter 32. Optical isolator element 36 may act as a photodiode or a one-way element (as indicated by the arrow) and may prevent the optical signal from photodetector device 26 from reaching output signal path 14.

[0082] Optimal parameters are calculated for the Mach-Zehnder interferometer 30 with only one phase modulator 34. The passive operation mode of the Mach-Zehnder interferometer 30 depends on the phase φ provided by the phase modulator 34, e.g., φ=π for receiving and φ=0 for transmitting. In general, the optical state interacting with the Mach-Zehnder interferometer 30 can be represented as a two-dimensional vector, with the input state having a vacuum 0 at one port and an input signal with complex amplitude α at another port corresponding to the input signal path 12. When the MZ type switch is in the input state (0,α) T The transformation to

number

number

number

[0083]

number

number

[0084] where α′ and α″ are the complex amplitudes of the light emerging from detector signal path 22 and output signal path 14, respectively. Rewriting these equations gives:

number

number

number

number

[0085] The same configuration may be used for transmitting pulses. In transmit mode, the input state is (β,0) T and the output is

number

number

number

[0086] The quality of the signal in the optical fiber can be degraded by polarization and phase fluctuations. For proper operation of the MZ switch, these fluctuations may be compensated for by continuous signal tuning. The input signal can be tuned, for example, with a high-speed three-paddle polarization controller. Such tuning can be performed during the line control subroutine of the QKD protocol, which is performed with high-intensity test pulses. To increase effectiveness, the switch itself may be assembled on a polarization-maintaining fiber instead of a normal single-mode fiber. The phase difference fluctuations of the MZ switch itself can be controlled by a phase modulator 34 at a frequency much lower than the operating frequency for information transmission.

[0087] The Mach-Zehnder architecture described above with reference to Fig. 2 allows the operation mode of the quantum key distribution device 10, 10' to be switched between a receive mode and a transmit mode by the first phase modulator 34, and is therefore entirely sufficient to operate the quantum key distribution device 10, 10' whose main role is to receive signals, and not necessarily to transmit signals to the quantum key distribution network. However, in some configurations it may be advantageous to provide a Mach-Zehnder architecture which can also transmit particularly optical signals to the quantum key distribution network, and thus act as a combined receiver-transmitter device within the quantum key distribution network.

[0088] A quantum key distribution network may include one or more quantum key distribution devices that operate primarily or exclusively as receiver devices and one or more quantum key distribution devices that act as combined receiver-transmitter devices.

[0089] In some configurations, a central control or management node (not shown) in the quantum key distribution network may specify the functionality of different quantum key distribution devices in the quantum key distribution network. In particular, the central control or management node may be connected to multiple different quantum key distribution devices in the network and may configure the operational settings of the multiple different quantum key distribution devices so that they selectively act as receiver devices or combined receiver-transmitter devices.

[0090] FIG. 3 is a schematic diagram of another quantum key distribution apparatus 10″ that generally corresponds to the quantum key distribution apparatus 10′ described above with reference to FIG. 2, where corresponding elements share the same reference numbers. However, the quantum key distribution apparatus 10″ further comprises a second phase modulator 38 in the output signal path 14 between the first beam splitter 18 and the second beam splitter 32. The second phase modulator 38 may generally correspond in design and function to the first phase modulator 34 described above with reference to FIG. 2. When used together, the first phase modulator 34 and the second phase modulator 38 may adjust the signal strength of the output signal path 14 and the detector signal path 22 to any signal level without limiting the minimum signal strength.

[0091] As further shown in FIG. 3, the quantum key distribution device 10″ further comprises a random number generator, such as a quantum random number generator 40, operably coupled to the first phase modulator 34 and / or the second phase modulator 38. Compared to the configuration of FIG. 2, the architecture shown in FIG. 3 provides more flexibility in generating the optical signal and providing it to the output signal path 14, but this may come at the expense of increased signal loss.

[0092] The quantum key distribution devices 10, 10', 10" shown above with reference to Figures 1-3 enable rapid and reliable selective switching between a transmit mode, in which a plurality of optical signals received on an input signal path 12 from a transmitter device (not shown) are transmitted substantially undistorted to an output signal path 14, and a receive mode, in which a plurality of optical signals received on the input signal path 12 are redirected to a detector signal path 22 and detected by a photon detector device 26. In the receive mode, the quantum key distribution devices 10, 10', 10" enable establishing a shared quantum encryption key with the transmitter device based on the detected plurality of optical input signals. To this end, each quantum key distribution device 10, 10', 10" may be further connected to the transmitter device via a classical information channel to provide communication with information matching and privacy amplification.

[0093] The photon detector device 26 may perform intensity measurements of the received optical signal, although in other examples homodyne detection or phase encryption may be used as well. In general, in the present disclosure, any quantum key distribution protocol may be used to establish encryption keys between the transmitter device and the respective quantum key distribution device 10, 10', 10". In particular, the protocol may take into account physical loss control along the transmission line as described in non-patent document 1, which may advantageously result in particularly high key generation rates even over very large distances in the range of thousands of kilometers.

[0094] FIG. 4 is a flow diagram showing a quantum key distribution method according to an embodiment.

[0095] In a first step S10, a plurality of optical input signals are received from a transmitter device.

[0096] In a second step S12, the received optical input signals are sent through a Mach-Zehnder interferometer and selectively routed to either an output signal path or a detector signal path which includes a photon detector device.

[0097] If the received optical input signals are transmitted to a detector signal path, the optical input signals from the transmitter device are detected in the detector signal path by the photon detector device in a third step S14.

[0098] In a fourth step S16, a shared quantum encryption key is established with the transmitter device based on the detected plurality of optical input signals.

[0099] The techniques of the present disclosure have been described above with reference to Figures 1-4 in relation to a single quantum key distribution device 10, 10', 10". However, a particular advantage of the techniques of the present disclosure is that they may be advantageously used to build larger quantum key distribution networks from individual building blocks, such that a shared encryption key may be established between any two such building blocks in the quantum key distribution network by any-to-any communication.

[0100] FIG. 5 is a schematic diagram of a quantum key distribution network 42 in which the techniques of this disclosure may be employed.

[0101] The quantum key distribution network 42 of Fig. 5 includes a plurality of quantum key distribution devices 10a, 10b, 10c, each of which may correspond to one of the configurations described above with reference to Figs. 1 to 4. Corresponding reference numerals are therefore used to indicate corresponding elements. In particular, reference numerals 12a, 12b, 12c indicate respective input signal paths, and reference numerals 14a, 14b, 14c indicate respective output signal paths of the quantum key distribution devices 10a, 10b, 10c.

[0102] 5, within the quantum key distribution network 42, the output signal path 14a of the first quantum key distribution device 10a is connected to or coincides with the input signal path 12b of the second quantum key distribution device 10b. Similarly, the output signal path 14b of the second quantum key distribution device 10b is connected to or coincides with the input signal path 12c of the third quantum key distribution device 10c.

[0103] For example, in the operating mode of the quantum key distribution network 42 shown in Fig. 5, the first quantum key distribution device 10a may act as a transmitter device and may emit an optical signal to the output signal path 14a. When a secure encryption key is established with the second quantum key distribution device 10b, the second quantum key distribution device 10b may be instructed to operate in a receive mode, such as by a switch signal that triggers a corresponding setting of a phase modulator of the second quantum key distribution device 10b, as described above with reference to Figs. 1-4. The second quantum key distribution device 10b may receive the switch signal from a user of the second quantum key distribution device 10b or from a central control or management node (not shown) in the quantum key distribution network 42.

[0104] In the receive mode of the second quantum key distribution device 10b, all optical signals received on the input signal path 12b reach the detector signal path of the second quantum key distribution device 10b, so that the third quantum key distribution device 10c learns nothing about the established keys.

[0105] Alternatively, if a secure encryption key is established between the first quantum key distribution apparatus 10a and the third quantum key distribution apparatus 10c, the second quantum key distribution apparatus 10b may be triggered by a switch signal to operate in a transmit mode, such as by changing the operating phase of the phase modulator 34. In this configuration, the second quantum key distribution apparatus 10b may transmit the optical signal received from the first quantum key distribution apparatus 10a on the second input signal path 12b to the second output signal path 14b (substantially) undistorted towards the third quantum key distribution apparatus 10c operating in a receive mode. In this configuration, the second quantum key distribution apparatus 10b acts as an intermediate node or an innocent bystander and does not learn anything about the encryption key established between the first quantum key distribution apparatus 10a and the third quantum key distribution apparatus 10c. Should the second quantum key distribution apparatus 10b eavesdrop on the communication, the quantum key distribution protocol would allow this to be detected in the first quantum key distribution apparatus 10a and the third quantum key distribution apparatus 10c.

[0106] Although Fig. 5 shows three quantum key distribution devices 10a, 10b, 10c connected in series, this is for ease of explanation only, and in general, a quantum key distribution network may utilize any integer number of quantum key distribution devices, which may be connected in different topologies. In some embodiments, the signal lines connecting the quantum key distribution devices in the network may be equipped with a cascade of optical amplifiers acting as quantum repeaters to increase the transmission distance and / or increase the quantum key generation rate. Furthermore, all quantum key distribution devices in the network may be connected by a classical authentication network, such as to facilitate privacy amplification and information integrity.

[0107] Estimate the number of users (N) with MZ type switches that can be placed sequentially in the quantum key distribution network. The first and last users can still communicate. The analysis in [1] shows that the QKD protocol can work with a transmission distance D = 40,000 km when there are only two users. However, adding more users to the network attenuates the signal due to losses through those switches. Thus, the maximum transmission distance decreases. To estimate the number of possible users, consider the distance between two neighbors a = 1 km. Let us assume that the transmission probability of one switch is T s = 0.8 (which is realistic for an MZ switch with one phase modulator, such as the configuration in Figure 2), the distance between neighbors is effectively

number

number

number

[0108]

number

[0109] Substituting this number, we find that N ≈ 10,000. Using a network structure with higher performance than the linear one in Figure 5, this number can be further increased.

[0110] Fig. 6 shows a schematic representation of a quantum key distribution network 42' which generally corresponds in function and design to the quantum key distribution network 42 described above with reference to Fig. 5, and corresponding components share the same reference numbers. However, in the configuration of Fig. 6, the quantum key distribution devices 10a, 10b, 10c are arranged in a ring topology, so that the output signal path 14c of the last (in this case the third) quantum key distribution device 10c coincides with or is connected to the input signal path 12a of the first quantum key distribution device 10a. As further shown in Fig. 6, the different rings may be interconnected by an optical switch 44. This allows the scope of the quantum key distribution network 42' to be further expanded.

[0111] Based on the techniques described above with reference to Figs. 1-5, quantum encryption keys can be exchanged between any two quantum key distribution devices in the quantum key distribution network 42', as long as the length of the ring sector between the sending node and the receiving node does not exceed the transmission distance limit of the QKD protocol. The (unavoidable to a certain extent) power losses may be compensated with a cascade of optical amplifiers (not shown) acting as quantum repeaters. Although ring structures are relatively simple in construction, the average communication in such a cluster involves a large number of users and is characterized by a relatively large transmission distance. This may increase the risk of unexpected losses by the communication line.

[0112] The quantum key distribution between different pairs of quantum key distribution devices in the quantum key distribution network 42, 42′ may require some logical organization, such as in the form of a central control or manager node. However, within the protocol, the quantum key generation can be fully decentralized. Since the intended senders and receivers can control the transmission themselves, they do not need any third party help for quantum key generation, which is an advantage over quantum key distribution networks that rely on trusted intermediaries.

[0113] As will now be explained in more detail with reference to the quantum key distribution network 42′ in FIG. 7, a possible operating protocol may be based on a first-in, first-out (FIFO) scheme, with FIG. 7 showing another ring topology with six quantum key distribution devices 10a-10f. It is assumed that a quantum encryption key is established between the quantum key distribution devices 10a and 10c on the one hand and between the quantum key distribution devices 10d and 10e on the other hand (as indicated by the arrows in FIG. 7). The respective communication paths do not overlap, so that the quantum key generation can proceed in parallel without interference. However, if a quantum encryption key is established between the quantum key distribution devices 10a and 10c on the one hand and between the quantum key distribution devices 10f and 10b on the other hand, the respective signal paths will overlap and time multiplexing techniques may be used to avoid jamming.

[0114] FIG. 8a illustrates a quantum key distribution network 42″ having a two-dimensional lattice topology, where each of the lattice nodes 46a-46p may include at least one quantum key distribution device, such as any of the quantum key distribution devices 10, 10′, 10″ described above with reference to FIGS. 1-7. Although FIG. 8a illustrates a lattice having 16 lattice nodes 46a-46p, in general the quantum key distribution network 42″ may include any number of lattice nodes.

[0115] Compared with the ring topology of Figures 6 and 7, the quantum key distribution network 42" with a two-dimensional lattice topology is characterized by more signal links between any two quantum key distribution devices and may further allow establishing alternative paths between different quantum key distribution devices, thereby reducing the average transmission distance and at the same time increasing the reliability and resilience of the quantum key distribution network 42". Again, time multiplexing techniques may be utilized in case the signal paths between any of the lattice nodes 46a-46p overlap.

[0116] FIG. 8b is a schematic diagram of a lattice node of a quantum key distribution network having a lattice topology, such as lattice node 46g of quantum key distribution network 42'' described above with reference to FIG. 8a.

[0117] In the example shown in FIG. 8b, a lattice node 46g includes four quantum key distribution devices 10a-10d arranged in a ring topology, any of which may correspond to one of the quantum key distribution devices 10, 10′, 10″ described above with reference to FIGS. 1-5. In this configuration, a high degree of flexibility and redundancy can be realized, and optical signals for quantum key distribution can be exchanged with any of the adjacent nodes, possibly in either direction.

[0118] To further expand the geographical scope of key generation, several quantum key distribution networks may be combined into a larger cluster network. FIG. 9 shows a cluster network 48 with multiple clusters 50a-50f, each of which may include one or more of the quantum key distribution networks 42, 42′, 42″ described above with reference to FIGS. 6-8b. The different clusters 50a-50f may be connected by optical highway lines 52a-52h, which may be optical fiber links containing no or only a few nodes and may span distances of up to 5,000 km or more while still providing high signal rates. In some cases, almost any two of the clusters 50a-50f may be directly connected, in which case the number of optical highway lines 52a-52h grows quadratically with the number of clusters 50a-50f.

[0119] These techniques may be used to create a globe-spanning quantum key distribution network 54, as shown diagrammatically in FIG.

[0120] The quantum key rate that can be obtained in a quantum key distribution network according to the present disclosure may be estimated by utilizing the technique of Non-Patent Document 1. Figure 11 shows the key generation rate L over a global communication distance of 40,000 km for different values ​​of the distance d between quantum repeaters in the signal path (from bottom to top: d=50 km, d=40 km, d=20 km, d=10 km). f / L is the signal leakage along the transmission path, r E In particular, the more amplifiers that are installed, the higher the key generation rate that can be obtained. -5 The leakage value r in the practically relevant range above E We show that it is still possible to achieve high quantum key generation rates for

[0121] It should be understood that the description and drawings of the specific embodiments are merely intended to illustrate the technology of the present disclosure and are not intended to be limiting in any way, the scope of the present disclosure being determined from the appended claims.

Claims

1. A quantum key distribution device (10, 10', 10"; 10a to 10f), an input signal path (12; 12a-12f) for receiving a plurality of optical input signals from a transmitter device; an output signal path (14; 14a-14f) connected at a first end (16) to said input signal path (12; 12a-12f) and emitting a plurality of first optical output signals at a second end (20) opposite said first end; a detector signal path (22) connected to the input signal path (12) at a third end (24) and including a photon detector device (26) at a fourth end (28) opposite the third end (24); the photon detector device (26) detects the plurality of optical input signals from the transmitter device; the quantum key distribution device (10, 10', 10"; 10a-10f) establishing a shared quantum encryption key with the transmitter device based on the detected optical input signals; the input signal path (12; 12a-12f) forms an input path of a Mach-Zehnder interferometer (30), and the output signal path (14; 14a-14f) and the detector signal path (22) form an output path of the Mach-Zehnder interferometer (30); the quantum key distribution device (10, 10', 10"; 10a to 10f) transmits the plurality of optical input signals via the output signal path (14; 14a to 14f) in a first configuration setting of the quantum key distribution device (10, 10', 10"; 10a to 10f); the quantum key distribution device (10, 10', 10"; 10a-10f) transmits the plurality of optical input signals to the detector signal path (22) in a second configuration of the quantum key distribution device (10, 10', 10"; 10a-10f) that is different from the first configuration; The plurality of optical input signals received on the input signal paths (12; 12a-12f) are intercepted and measured by the photon detector device (26) and are detected by the photon detector device (26). Quantum key distribution device (10, 10', 10''; 10a to 10f).

2. The quantum key distribution device (10, 10', 10"; 10a-10f) transmits the plurality of optical input signals substantially unaffected via the output signal path (14) in the first configuration. Quantum key distribution device (10, 10', 10''; 10a to 10f) according to claim 1.

3. the quantum key distribution device (10, 10', 10"; 10a-10f) comprises a first phase modulator device (34) in the detector signal path (22); The first phase modulator device (34) modulates the phase of the optical signal on the detector signal path (22) and / or modulates the intensity of the optical signal on the detector signal path (22) and / or forms an optical signal on the output signal path (14; 14a-14f) and / or modulates the intensity of the optical signal on the output signal path (14; 14a-14f). Quantum key distribution device (10, 10', 10''; 10a to 10f) according to claim 1.

4. the quantum key distribution device (10, 10', 10"; 10a-10f) further comprises a second phase modulator device (38) in the output signal path (14; 14a-14f); The second phase modulator device (38) modulates the phase of the optical signal on the output signal path (14; 14a-14f) and / or modulates the intensity of the optical signal on the output signal path (14; 14a-14f) and / or forms an optical signal on the output signal path (14; 14a-14f) and / or modulates the intensity of the optical signal on the detector signal path (22). Quantum key distribution device (10, 10', 10''; 10a to 10f) according to claim 1.

5. The first phase modulator device (34) includes an optoelectronic phase modulator. A quantum key distribution device (10, 10', 10''; 10a to 10f) according to claim 3.

6. The second phase modulator device (38) includes an optoelectronic phase modulator. A quantum key distribution device (10, 10', 10''; 10a to 10f) according to claim 4.

7. The switching speed of the first phase modulator device (34) is less than 5ns. A quantum key distribution device (10, 10', 10''; 10a to 10f) according to claim 3.

8. The switching speed of the second phase modulator device (38) is 5ns or less. A quantum key distribution device (10, 10', 10''; 10a to 10f) according to claim 4.

9. Further, a photon source is provided for transmitting a plurality of optical signals to said output signal paths (14; 14a-14f). Quantum key distribution device (10, 10', 10''; 10a to 10f) according to claim 1.

10. A quantum key distribution network (42, 42', 42'') comprising a plurality of interconnected quantum key distribution devices (10, 10', 10''; 10a to 10f) according to any one of claims 1 to 9, the transmitter device is a first quantum key distribution device (10, 10', 10"; 10a to 10f) among the plurality of quantum key distribution devices (10, 10', 10"; 10a to 10f), and an output signal path (14; 14a to 14f) of the first quantum key distribution device (10, 10', 10"; 10a to 10f) among the plurality of quantum key distribution devices (10, 10', 10"; 10a to 10f) is connected to an input signal path (12; 12a to 12f) of a second quantum key distribution device (10, 10', 10"; 10a to 10f) among the plurality of quantum key distribution devices (10, 10', 10"; 10a to 10f); The second quantum key distribution device (10, 10', 10"; 10a to 10f) establishes a shared quantum encryption key with the first quantum key distribution device (10, 10', 10"; 10a to 10f) based on the detected optical input signals. A quantum key distribution network (42, 42', 42'').

11. The quantum key distribution devices (10, 10', 10"; 10a to 10f) are arranged in a ring topology. A quantum key distribution network (42') according to claim 10.

12. The quantum key distribution devices (10, 10', 10"; 10a to 10f) are arranged in a lattice topology. The quantum key distribution network (42'') according to claim 10.

13. 1. A quantum key distribution method, comprising: receiving a plurality of optical input signals from a transmitter device; selectively transmitting the received optical input signals via a Mach-Zehnder interferometer (30) to an output signal path (14; 14a-14f) in a first configuration of the quantum key distribution apparatus (10, 10', 10"; 10a-10f) in which the optical input signals are conveyed to different nodes in a network, or to a detector signal path (22) including a photon detector device (26) in a second configuration of the quantum key distribution apparatus (10, 10', 10"; 10a-10f) different from the first configuration; When transmitting the received optical input signals to the detector signal path (22), the quantum key distribution method further comprises: detecting the plurality of optical input signals from the transmitter device in the detector signal path (22) by the photon detector device (26); and establishing a shared quantum encryption key with the transmitter device based on the plurality of optical input signals detected at the photon detector device (26). Quantum key distribution method.

14. The received optical input signals are transmitted through the Mach-Zehnder interferometer (30) in response to a received switch signal. The quantum key distribution method according to claim 13.

15. The method further comprises the steps of modulating the phase of the optical signal in the detector signal path (22), in particular by a first phase modulator device (34), and / or modulating the intensity of the optical signal in the detector signal path (22), and / or forming an optical signal in the output signal path (14; 14a-14f), and / or modulating the intensity of the optical signal in the output signal path (14; 14a-14f). The quantum key distribution method according to claim 13.

16. Further comprising the step of transmitting a plurality of optical signals to said output signal paths (14; 14a-14f). The quantum key distribution method according to claim 13.

17. Establishing the shared quantum encryption key with the transmitter device includes establishing a shared bit string and performing privacy amplification with the transmitter device, in particular over a classical information channel. The quantum key distribution method according to any one of claims 13 to 16.

Images