Time bin qubit converter

Abstract

The system includes a first photonic integrated circuit. The circuit includes a qubit encoder configured to receive a spatial mode qubit and convert the spatial mode qubit into a temporal mode qubit, and an optical interconnect configured to receive and transmit the temporal mode qubit. The system further includes a second photonic integrated circuit including a qubit decoder configured to receive the temporal mode qubit and convert the temporal mode qubit back into a spatial mode qubit.

Description

Background Art

[0001] [Incorporation by Reference] The PCT request form is filed herewith, simultaneously with this specification, as part of this application. Each application for which this application claims the benefit of or priority to that identified in the PCT request form filed herewith is hereby incorporated by reference in its entirety for all purposes into this specification.

[0002] Photonic integrated circuits, such as silicon photonic integrated circuits, can be used in many systems, such as quantum communication systems and optical quantum computing systems. These quantum mechanics-based systems are distinguished from "classical" systems by their dependence on their quantum states, such as quantum bits (qubits). To achieve the desired functionality and performance, quantum mechanics-based systems can integrate many passive and active photonic devices, modules, and subsystems into the same system. For example, an optical quantum computer can integrate passive and active photonic integrated circuits, and other optical and electrical components, such as optical fibers or other low-loss optical interconnects, control circuits, and classical processing units, into the same system while achieving the desired functionality and performance, to reliably generate, manipulate (e.g., entangle), and detect hundreds, thousands, or even millions of qubits for computing and error correction.

Summary of the Invention

[0003] The specific embodiments described herein relate to generating, transforming, and transmitting quantum states (e.g., qubits) in systems such as optical quantum computers or quantum communication systems. Various inventive embodiments are described herein, including methods, processes, systems, devices, circuits, packages, modules, units, wafers, dies, networks, and cells.

[0004] According to certain embodiments, the system may include a first photonic integrated circuit including a qubit encoder configured to receive spatial mode qubits and convert the spatial mode qubits to temporal mode qubits. The system may also include an optical interconnect configured to receive and transmit temporal mode qubits, and a second photonic integrated circuit that may include a qubit decoder configured to receive temporal mode qubits and convert the temporal mode qubits back to spatial mode qubits.

[0005] According to certain embodiments, a photonic integrated circuit device may include a dual-rail qubit bus optically coupled to a photonic qubit source, the dual-rail qubit bus including two waveguides, where a first waveguide of the dual-rail qubit bus may include a waveguide delay line disposed in a first layer of the photonic integrated circuit device. The photonic integrated circuit device may also include an optical switch disposed in a second layer of the photonic integrated circuit and optically connected to the two waveguides of the dual-rail qubit bus, and a control circuit configured to switch the optical switch to optically connect output portions of the two waveguides to a single output waveguide of the photonic integrated circuit at different times, where the control circuit is synchronized with the photonic qubit source.

[0006] According to certain embodiments, a method may include converting spatial mode qubits to temporal mode qubits using optical delay and switching in a first photonic integrated circuit; transmitting temporal mode qubits from the first photonic integrated circuit to a second photonic integrated circuit over an optical interconnect, the optical interconnect being coupled to the first photonic integrated circuit and the second photonic integrated circuit; and converting the temporal mode qubits back to spatial mode qubits on the second photonic integrated circuit.

[0007] This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to the entire specification of this disclosure, any or all of the drawings, and the appropriate portions of each claim. The foregoing, together with other features and examples, will be described in more detail hereinafter in the following specification, claims, and accompanying drawings.

Brief Description of the Drawings

[0008] Aspects of the present disclosure are illustrated by way of example. Non-limiting and non-exhaustive aspects are described with reference to the following figures, where, unless otherwise specified, like reference numerals refer to like parts throughout the various figures.

[0009]

Figure 1

[0010]

Figure 2

[0011]

Figure 3

[0012]

Figure 4A

[0013]

Figure 4B

[0014]

Figure 4C

[0015]

Figure 5

[0016]

Figure 6A

[0017]

Figure 6B

[0018]

Figure 7

[0019]

Figure 8A

[0020]

Figure 8B

[0021]

Figure 9A

[0022]

Figure 9B

[0023]

Figure 10

[0024]

Figure 11A

[0025]

Figure 11B

[0026]

Figure 12

[0027]

Figure 13A

[0028]

Figure 13B

[0029]

Figure 14

[0030]

Figure 15A

Figure 15B

Figure 15C

Figure 15D

Figure 15E

Figure 15F

Figure 15G

Figure 15H

Figure 15I

Figure 15J

Figure 15K

Figure 15L

[0031]

Figure 16A

Figure 16B

Figure 16C

Figure 16D

[0032]

Figure 17

[0033]

Figure 18

[0034]

Figure 19

[0035]

Figure 20

[0036]

Figure 21

Embodiments for Carrying Out the Invention

[0037] The techniques disclosed herein are generally related to generating, transforming, and transmitting quantum states (e.g., qubits) in systems such as optical quantum computers or quantum communication systems. Various inventive embodiments are described herein, including methods, processes, systems, devices, circuits, packages, modules, units, wafers, dies, networks, and cells.

[0038] Quantum computing and quantum communication rely on the dynamics of quantum systems, such as photons, electrons, atoms, ions, molecules, and nanostructures, that follow the rules of quantum theory. In quantum theory, the quantum state of a quantum system is described by a set of physical properties, and the complete set is referred to as quantum modes. A quantum mode can be defined, for example, by specifying the value (or distribution of values) of one or more properties of a quantum system. In the case where a quantum system is implemented using photons (referred to as a "photonic quantum system"), quantum modes can be defined by the frequency of the photon, the position of the photon in space (e.g., in which waveguide or superposition of waveguides the photon is propagating), the associated direction of propagation (e.g., the k-vector for a photon in free space), the polarization state of the photon (e.g., the direction of the electric and / or magnetic field of the photon (horizontal or vertical)), the time window during which the photon is propagating, and the orbital angular momentum, among others. In the case where a photon is propagating in a waveguide, the state of the photon can be represented by quantum modes of a set of discrete spatio-temporal modes. For example, the spatial mode of a photon can be determined according to the waveguide in which the photon is propagating among a finite set of discrete waveguides, while the temporal mode of the photon can be determined based on the period during which the photon exists among a set of distinct periods. Other types of quantum modes, such as polarization modes, can also be used to specify the quantum state.

[0039] Many quantum computing or quantum communication systems may use qubits (quantum bits, or qubits) that are each simultaneously in a coherent superposition of two states in order to manipulate information through quantum mechanics. Most of the technologies used to implement qubits have issues such as stability, decoherence, fault tolerance, and scalability challenges. For example, one of the main challenges in realizing quantum computing is that decoherence and other quantum noise can destroy information in the superposition of states in a quantum computer, inaccuracies in the transformation of quantum states accumulate throughout the calculation, and thus can make long calculations difficult. To overcome these challenges, quantum error correction may be needed to achieve fault-tolerant quantum computing that can handle not only noise in stored quantum information but also faulty quantum gates, faulty quantum preparations, and faulty measurements. In some systems, for the purpose of quantum error correction, many physical qubits may be used to generate entities (referred to as logical qubits) that logically act as single qubits in a quantum circuit or algorithm. Some quantum error correction techniques may store the information of one qubit in a highly entangled state of multiple qubits, such as seven, nine, or more physical qubits. If a higher level of encoding greater than one is performed to provide better protection, thousands or more physical qubits may be required for each logical qubit. Thus, logical qubits, such as error-corrected photonic logical qubits or fault-tolerant photonic channels, may include many entangled physical qubits in order to provide the stability, error correction, and fault tolerance necessary to perform useful calculations. In the case of a quantum computer that may use many logical qubits for computing, thousands or millions of physical qubits may need to be generated, entangled, switched, and detected, which may require a large number of passive and active photonic circuits and components, as well as electrical circuits and components to implement. Integrating these circuits and components into a system that can achieve the desired functionality and performance can be very difficult.

[0040] If, for example, the qubits are implemented using dual rails, a pair of optical fibers may need to be used to transmit the qubits between chips or other modules. If a pair of optical fibers is used, the fibers in each pair may experience different environments, such as different temperatures, different stresses, and different coupling conditions, and thus there may be different delays or phase changes for photons propagating on different optical fibers.

[0041] According to certain embodiments, on the transmitter side, a spatial mode qubit may be converted to a time mode qubit using a delay line and a switch controlled by a clock signal. In this way, the qubit may be transmitted in the same optical fiber and thus may experience the same environment in the propagation path. In some embodiments, on the receiver side, the received time mode qubit may be converted to a spatial mode qubit using a delay line and a switch controlled by a clock signal. The transmitter and receiver (and the clock signal) may be synchronized. The clock signal may also be synchronized with the qubit generator. For example, the clock signal may be generated using extra pump photons not used by a single photon generator.

[0042] As used herein, a "qubit" (or quantum bit) refers to a quantum system having associated quantum states that can be used to encode information. The quantum states can be used to encode one bit of information if the quantum state space can be modeled as a (complex) two-dimensional vector space (where one dimension in the vector space maps to the logical value 0 and the other maps to the logical value 1). In contrast to a classical bit, a qubit can have a state that is a superposition of the logical values 0 and 1. More generally, a "higher-dimensional particle" can be any quantum system having a quantum state space that can be modeled as an n-dimensional vector space (for any integer n) that can be used to encode (complex) n bits of information. For clarity of explanation, the term "qubit" is used herein, but in some embodiments, the system can also employ quantum information carriers that encode information in a manner not necessarily associated with binary bits, such as higher-dimensional particles. Qubits (or higher-dimensional particles) can be implemented in various quantum systems. Examples of qubits include: the polarization state of a photon; the presence of a photon in a waveguide; or the energy state of an atom, ion, nucleus, or photon. Other examples can include engineered quantum systems such as flux qubits, phase qubits, or charge qubits (e.g., formed from superconducting Josephson junctions); topological qubits (e.g., Majorana particles), or spin qubits formed from vacancy centers (e.g., nitrogen vacancies in diamond). A physical qubit can be a physical device that acts as a two-state quantum system. In one example, a qubit can be in "dual-rail encoding" such that the logical value of the qubit is encoded by the occupancy of one of two modes of the quantum system.

[0043] As used herein, "resource state" refers to an entangled state of a number of qubits in a non-separable entangled state (an entangled state that cannot be decomposed into smaller separate entangled states). In various embodiments, the number of qubits in the resource state can be a small number (e.g., two or more, or any number up to approximately 20) or a larger number (as large as desired).

[0044] As used herein, "logical qubit" refers to a physical or abstract qubit having a sufficiently long coherence time that can be used by a quantum logic gate. A logical qubit can specify how a single qubit should act in a quantum algorithm, according to the quantum logic operations by the quantum logic gate. Due to issues such as stability, decoherence, fault tolerance, and scalability associated with a physical qubit that includes a single two-state quantum system, a physical qubit may not be usefully used to reliably encode and hold information for a sufficiently long period of time. Thus, quantum error correction may need to be used to generate a scalable quantum computer, where many physical qubits can be used to generate a single error-tolerant logical qubit. Depending on the error correction scheme used and the error rate of each physical qubit, a single logical qubit can be formed using a number (e.g., dozens, hundreds, thousands, or more) of physical qubits. As used in the following sections, the term "qubit" generally refers to a physical qubit, while all references to logical qubits include the modifier "logical."

[0045] As used herein, "quantum system" can include particles (such as atoms, ions, nuclei, and / or photons) or engineered quantum systems such as flux qubits, phase qubits, or charge qubits (e.g., formed from superconducting Josephson junctions), topological qubits (e.g., Majorana particles), spin qubits formed from hole centers (e.g., nitrogen vacancies in diamond), qubits encoded differently in multiple quantum systems (e.g., Gottesman-Kitaev-Preskill (GKP) encoded qubits), and entangled states of qubits, etc.

[0046] As used herein, "fusion" (or "fusion operation" or "fusing") refers to a two-qubit entanglement measurement. A "fusion gate" is a structure that receives two input qubits, each of which is typically part of an entangled state of qubits. The fusion gate performs a projective measurement operation on the input qubits such that the two entangled states of the first qubit are fused into a single entangled state of a qubit, generating either one (e.g., "type I fusion") or zero (e.g., "type II fusion") output qubits in a manner that can generate either of the output qubits. The fusion gate is a specific example of a general class of two-qubit entanglement measurements and is particularly suitable for photonic architectures.

[0047] Some exemplary embodiments are described herein with reference to the accompanying drawings. The following description is provided only to present embodiments and is not intended to limit the scope, applicability, or configuration of the present disclosure. Rather, the following description of the embodiments provides those skilled in the art with an explanation that enables the implementation of one or more embodiments. It is understood that various changes can be made to the functions and arrangements of the elements without departing from the spirit and scope of the present disclosure. In the following, for the purpose of explanation, specific details are set forth to provide a complete understanding of certain inventive embodiments. However, it is clear that various embodiments can be implemented without these specific details. The figures and the description are not intended to be limiting. The words "example" or "exemplary" are used herein to mean "serving as an example, instance, or illustration." Any embodiment or design described herein as "exemplary" or "an example" is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

[0048] Quantum computing relies on the dynamics of quantum objects, such as photons, electrons, atoms, ions, molecules, and nanostructures, that follow the rules of quantum theory. In quantum theory, the quantum state of a quantum object is described by a set of physical properties, and the complete set is referred to as modes. In some embodiments, a mode is defined by specifying the value (or distribution of values) of one or more properties of a quantum object. For example, in the case where the quantum object is a photon, the mode can be defined by the frequency of the photon, the position of the photon in space (e.g., in which waveguide or superposition of waveguides the photon is propagating), the associated direction of propagation (e.g., the k-vector for a photon in free space), the polarization state of the photon (e.g., the direction of the electric and / or magnetic field of the photon (horizontal or vertical)), the time window during which the photon is propagating, and the orbital angular momentum, among others.

[0049] In the case of a photon propagating in a waveguide, it is convenient to represent the state of the photon as one of a set of discrete space-time modes. For example, the spatial mode k of the photon iis determined according to one of a finite set of discrete waveguides through which photons propagate, and a time mode t j is determined by which of a set of discrete periods (referred to herein as "bins") the photon is present in. The degree of temporal discretization can be provided by a pulsed laser that serves to generate the photons. In the following examples, the spatial mode is mainly used to avoid the complexity of the description. However, those skilled in the art will understand that the system and method can be applied to any type of mode, such as a time mode, a polarization mode, and any other mode or set of modes that function to specify a quantum state. Further, in the following description, an embodiment employing a photonic waveguide is described to define the spatial mode of a photon. However, those skilled in the art accessing the present disclosure will understand that other types of modes, such as a time mode, and energy states, etc., can be used without departing from the scope of the present disclosure. In addition, those skilled in the art can implement the examples using other types of quantum systems including, but not limited to, other types of photonic systems.

[0050] In the case of a quantum system of a plurality of indistinguishable particles, rather than describing the quantum state of each particle in the system, it is useful to describe the quantum state of the entire many-body system using the formalism of Fock states (sometimes referred to as the occupation number representation). In the description of Fock states, the many-body quantum state is specified by how many particles are present in each mode of the system. For example, a multi-mode, two-particle Fock state

Number

Number

Number

Number

Number

Number

Number

[0051] As used herein, a "qubit" (or quantum bit) is a quantum system having associated quantum states that can be used to encode information. The quantum states can be used to encode one bit of information if the quantum state space can be modeled as a (complex) two-dimensional vector space (where one dimension in the vector space maps to the logical value 0 and the other maps to the logical value 1). In contrast to a classical bit, a qubit can have a state that is a superposition of the logical values 0 and 1. More generally, a "higher-dimensional particle" can be any quantum system having a quantum state space that can be modeled as an n-dimensional vector space (for any integer n) that can be used to encode (complex) n bits of information. For clarity of explanation, the term "qubit" is used herein, but in some embodiments, the system can also employ quantum information carriers that encode information in a manner not necessarily associated with binary bits, such as higher-dimensional particles. Qubits (or higher-dimensional particles) can be implemented in various quantum systems. Examples of qubits include: the polarization state of a photon; the presence of a photon in a waveguide; or the energy state of an atom, ion, nucleus, or photon. Other examples include flux qubits, phase qubits, or charge qubits (e.g., formed from superconducting Josephson junctions); topological qubits (e.g., Majorana particles), or other engineered quantum systems such as spin qubits formed from vacancy centers (e.g., nitrogen vacancies in diamond).

[0052] A qubit can be in "dual-rail encoding" such that the logical value of the qubit is encoded by the occupancy of one of two modes of the quantum system. For example, the logical values of 0 and 1 can be encoded as follows.

Number

Number

Number

Number

[0053] Qubits (and operations on qubits) can be implemented using a variety of physical systems. In some of the examples described herein, a qubit is provided in an integrated photonic system employing waveguides, beam splitters, photonic switches, and single photon detectors, and the modes that can be occupied by photons are spatio-temporal modes corresponding to the presence of photons in the waveguides. Modes can be coupled using a mode coupler, e.g., an optical beam splitter, to implement conversion operations, and measurement operations can be implemented by coupling a single photon detector to a particular waveguide. Those skilled in the art accessing this disclosure will understand that modes defined by any suitable set of degrees of freedom, e.g., polarization modes, time modes, etc., can be used without departing from the scope of this disclosure. For example, in the case of modes that differ only in polarization (e.g., horizontal (H) and vertical (V)), the mode coupler can be any optical element that coherently rotates the polarization, e.g., a birefringent material such as a waveplate. In the case of other systems such as ion trap systems or neutral atom systems, the mode coupler can be any physical mechanism that can couple two modes, e.g., a pulsed electromagnetic field tuned to couple two internal states of an atom / ion.

[0054] In some embodiments of a photonic quantum computing system that uses dual-rail encoding (also referred to herein as spatial mode), a qubit can be implemented using a pair of waveguides. FIG. 1 shows two views (100, 100') of a portion of a pair of waveguides 102, 104 that can be used to provide a dual-rail encoding photonic qubit. In 100, a photon 106 is in waveguide 102 and there is no photon in waveguide 104 (also referred to as the vacuum mode); in some embodiments, this corresponds to the

Number

Number

[0055] The occupancy mode can be generated by using a photon source to generate photons, and then the photons propagate in a desired waveguide. The photon source can be, for example, a resonator-based source that emits photon pairs (also referred to as a heralded single-photon source). In one example of such a source, the source is driven by a pump (e.g., an optical pulse) coupled to a system of optical resonators that can generate pairs of photons by non-linear optical processes (e.g., spontaneous four wave mixing (SFWM), spontaneous parametric down-conversion (SPDC), or second harmonic generation, etc.). Many different types of photon sources can be employed. Examples of photon pair sources can include micro-ring-based spontaneous four wave mixing (SPFW) heralded photon sources (HPS). However, the exact type of photon source used is not critical, and any type of non-linear source that employs any process such as SPFW, SPDC, or any other process can be used. Other classes of sources that do not necessarily require non-linear materials, such as those employing atomic and / or artificial atomic systems, e.g., quantum dot sources, and color centers in crystals, etc., can also be employed. In some cases, the source may or may not be coupled to a photonic cavity, such that it can be an example related to an artificial atomic system such as a quantum dot coupled to a cavity. Other types of photon sources, such as optomechanical systems, also exist for SPWM and SPDC.

[0056] In such cases, the operation of the photon source can be non-deterministic (sometimes also referred to as "probabilistic"), such that a given pump pulse may or may not generate a pair of photons. In some embodiments, coherent spatial and / or temporal multiplexing (referred to herein as "active" multiplexing) of some non-deterministic sources can be used to bring the probability that a single mode is occupied during a given cycle closer to 1. One of ordinary skill in the art will understand that many different active multiplexing architectures incorporating spatial and / or temporal multiplexing are possible. For example, an active multiplexing scheme that employs a log tree, a generalized Mach-Zehnder interferometer, a multimode interferometer, a cascade source, a cascade source using a dump-the-pump, an asymmetric polycrystalline single photon source, or any other type of active multiplexing architecture can be used. In some embodiments, the photon source can employ an active multiplexing scheme involving, for example, quantum feedback control. In some of the embodiments described below, the use of multi-rail encoding enables the probability of the band that a single mode is occupied during a given pulse period to approach 1 without active multiplexing.

[0057] The measurement operation can be implemented by coupling a waveguide to a single photon detector that generates a classical signal (e.g., a digital logic signal) indicating that a photon has been detected by the detector. Any type of photodetector having sensitivity to single photons can be used. In some embodiments, the detection of a photon (e.g., at the output end of the waveguide) indicates an occupied mode, while the absence of a detected photon can indicate an unoccupied mode.

[0058] Figure 2 is a simplified block diagram of an example of a single photon source 200 that may include a set of cascaded HPSs according to some embodiments. In the example shown in Figure 2, the single photon source 200 may include a plurality of HPSs such as 205a and 205b, which may collectively be referred to as HPS 205. Each HPS 205 may include a photon pair source such as photon pair source 210a within HPS 205a or photon pair source 210b within HPS 205b. Each photon pair source 210a or 210b may generate pairs of photons based on, for example, spontaneous four-wave mixing (SFWM) in a third-order passive nonlinear optical material, or spontaneous parametric down-conversion (SPDC) in a second-order passive nonlinear optical material. In some implementations, the photon pair source 210a or 210b may include a ring resonator that may support multiple resonances, as described below.

[0059] Although several embodiments are described in relation to photon pair sources, this is not essential, and it is understood that photon sources other than photon pair sources are included within the scope of the present invention. Thus, by way of example, a micro-ring-based photon source with SPFW heralding (HPS) is described as an example of a photon source. However, the exact type of photon source used is not critical, and any type of nonlinear source that employs any process such as SPFW, spontaneous parametric down-conversion (SPDC), or any other process may be used. Other classes of sources that do not necessarily require a nonlinear material, such as those that employ atomic and / or artificial atomic systems, for example, quantum dot sources, and color centers in crystals, may also be employed.

[0060] In some cases, the source may or may not be coupled to a photonic cavity, such that the source may be an example of an artificial atomic system such as a quantum dot coupled to a cavity. Other types of photon sources, such as optomechanical systems, also exist for SPWM and SPDC.

[0061] By way of example, an example that employs spatial multiplexing of several non-deterministic sources is described as an example of a multiplexed (MUX) photon source. However, many different spatial MUX architectures are possible without departing from the scope of the present disclosure. Temporal MUXing may be implemented instead of or in combination with spatial multiplexing. A MUX scheme that employs a log tree, a generalized Mach-Zehnder interferometer, a multimode interferometer, a chain source, a chain source using a dump-the-pump, an asymmetric polycrystalline single photon source, or any other type of MUX architecture may be used. In some embodiments, the photon source may employ a MUX scheme with quantum feedback control or the like.

[0062] In each photon pair source 210a or 210b, photons can be generated non-deterministically in pairs, where each pair includes a signal photon and an idler photon, and the presence of one photon (e.g., the idler photon) can indicate the presence of the other photon (e.g., the signal photon) in the pair. The two photons in each pair can be separated into two output channels by a splitter, e.g., a wavelength division demultiplexing (WDDM) device 220a or 220b, based on their different frequencies. One photon (e.g., the idler photon) on one output channel of the splitter (e.g., WDDM device 220a or 220b) can be detected by a single photon detector (SPD) 230a or 230b. When a photon is detected by SPD 230a or 230b, the corresponding photon (e.g., the signal photon) generated in the same pair as the detected photon is present on a different output channel of the splitter (e.g., WDDM device 220a or 220b) and can thus be used as the output of the single photon source 200. When an idler photon is detected by SPD 230a or 230b in one of the cascaded HPSs 205a and 205b, SPD 230a or 230b can send an electrical signal (referred to herein as a heralding signal) to the other HPSs, as a result of which those HPSs can be switched off or bypassed. For example, in the example shown in FIG. 2, when a photon is detected by SPD 230a in the first HPS 205a, SPD 230a can send a heralding signal to the second HPS 205b, as a result of which the second HPS 205b can be turned off or bypassed. The signal photons generated by the first HPS 205a can pass through the second HPS 205b as the output of the single photon source 200.

[0063] Figure 3 illustrates an example of a photon pair source 300 according to some embodiments. The photon pair source 300 may include a first waveguide 310, a second waveguide 330, and a ring resonator 320 positioned between the first waveguide 310 and the second waveguide 330. Pump light (e.g., generated by a laser source) may propagate in the first waveguide 310 (as indicated by the arrow) and may be coupled to the ring resonator 320. The ring resonator 320 may include a waveguide loop, such that resonance for light having a specific wavelength may occur when the optical path length of the ring resonator 320 is an integer of the wavelength of the light. The ring resonator 320 may support multiple resonances at a plurality of wavelengths that may satisfy the resonance condition. The spacing between these resonances may be referred to as the free spectral range (FSR) and may depend on the optical path length of the ring resonator 320.

[0064] The ring resonator 320 may include a nonlinear optical material such as a second - order or third - order passive nonlinear optical medium. Spontaneous four - wave mixing (SFWM) or spontaneous parametric down - conversion (SPDC) processes may occur in the ring resonator 320. In the SFWM process, two pump photons may be converted into a pair of daughter photons (e.g., signal photon and idler photon) in the nonlinear optical material. By energy conservation, the signal photon and the idler photon may be at frequencies symmetrically distributed around the pump frequency (e.g., one at frequency f0+Δf and the other at frequency f0 - Δf where f0 is the frequency of the pump photon).

[0065] The signal photon and the idler photon generated within the ring resonator 320 may be coupled out of the ring resonator 320 into the second waveguide 330 with a specific coupling efficiency. The propagation directions of the photons in the first waveguide 310, the ring resonator 320, and the second waveguide 330 may be indicated by the arrows in FIG. 3. In addition to the photon pair generated by the resonator 320, a certain amount of unconverted pump photons may be coupled from the ring resonator 320 into the second waveguide 330.

[0066] The photon pair source 300 can generate photon pairs in a non-deterministic manner. That is, the photon pairs are not generated on demand, but instead are generated probabilistically. The success rate is only from 1% to 5%. For example, the photon pairs can only succeed in being generated once every 20 pump pulses. As a result, a heralded photon source (HPS) that uses the photon pair source 300 to generate a single photon (e.g., HPS 205a or 205b illustrated in FIG. 2) may generate a single photon in a non-deterministic manner. According to some embodiments, multiple HPSs may be cascaded (e.g., as illustrated in FIG. 2) or multiplexed to form a quasi-deterministic single photon source, as described below.

[0067] FIG. 4A shows, as an example, a single photon source 400 including three multiplexed HPSs 410a - 410c. Pump photon pulses (e.g., laser pulses) are distributed to each of the HPSs 410a, 410b, and 410c, for example simultaneously. The outputs of the HPSs 410a to 410c are coupled to a multiplexer (MUX) 420. FIG. 4A shows a single photon source 400 including three HPSs for illustrative purposes, and the single photon source may include 20 or more HPSs in some embodiments.

[0068] Each of HPSs 410a through 410c can generate a single photon in a non-deterministic manner. For example, FIG. 4B illustrates a schematic sequence of pump photon pulses arriving at HPSs 410a, 410b, and 410c at time slots t1, t2, and t3. As illustrated in FIG. 4C, the first HPS 410a does not have to generate any single photons (indicated by x) at the first time slot t1 and the third time slot t3, but may generate a single photon (indicated by a check symbol) at the second time slot t2. Similarly, the second HPS 410b does not have to generate any single photons at the first time slot t1 and the second time slot t2, but may generate a single photon at the third time slot t3; the third HPS 410c does not have to generate any single photons at the second time slot t2 and the third time slot t3, but may generate a single photon at the first time slot t1.

[0069] If an idle photon is detected in HPS 410a, 410b, or 410c (e.g., by a single photon detector), HPS 410a, 410b, or 410c can send a signaling message to the multiplexer 420. The multiplexer 420 can be configured to select the single photon output of one of HPSs 410a, 410b, and 410c as its single photon output and ignore the single photon outputs of all other HPSs. If a sufficient number of HPSs are present, the single photon source 400 can potentially generate single photons of each pump photon pulse in a quasi-deterministic manner. For example, in the example illustrated in FIGS. 4A through 4C, the multiplexer 420 selects the single photon output of the third HPS 410c at the first time slot t1, the output of the first HPS 410a at the second time slot t2, and the output of the second HPS 410b at the third time slot t3, such that the single photon source 400 can output a single photon of each pump photon pulse at time slots t1, t2, and t3.

[0070] For the system to deterministically sequence the multiple operations of various components, such as one or more single photon sources, such as 400a, 400b, 400c, one or more MUXs, such as MUX420, and / or any other downstream components, a shared or master clock signal may be beneficial. Thus, in some embodiments, the clock signal generator 450 may convert a portion of the pump photon pulse into a clock signal and then distribute it to various components to perform the desired adjustments between the components. While a portion of the pump signal used to derive the clock signal shown in FIG. 4A is derived from the output of the single photon source, the portion may be provided from any different number of upstream, or combinations of blocks, without departing from the scope of the present disclosure.

[0071] Single photons generated by a single photon source may be coupled to a qubit entanglement system that may include, for example, a complex linear optical circuit (e.g., that may have up to one million qubits). The linear optical circuit may include, for example, integrated optical components such as switches, beam splitters, phase shifters, and photon detectors that need to cooperate to entangle two or more single photons or two or more entangled cluster states of photons. The linear optical circuit may also include, for example, quantum gates (e.g., fusion gates) at various positions to perform quantum computing. In some architectures, the quantum gates may be formed / defined by a sequence of measurement and / or fusion operations to be performed on individual photons. Therefore, there may be a number of operations inside the quantum computer that need to be accurately timed. An accurate system clock may be necessary to control the timing of the various operations of the quantum computing system.

[0072] In some embodiments, the system clock can be set by the photon generation rate of a single photon source. As described above, the single photon source can be driven by a high power pump laser that generates a train of pump photon pulses at a particular repetition rate (e.g., pulses at approximately 50-ps to 1-ns intervals corresponding to repetition rates from approximately 1 GHz to approximately 20 GHz). The high power pump laser can, in turn, be driven by an electrical signal having a similar repetition rate. Thus, one option for generating a clock signal is to tap off some of the electrical signals that generate the pump photon pulses and send it to a linear optical quantum computer to be used as the master clock. However, there can be many electrical and optical interconnects between the pump photon source and the linear optical quantum computer that are subject to different temperature environments (e.g., from room temperature of approximately 293K to approximately 4K inside a cryostat). As a result, the master clock generated in this way can have random phase errors. Due to the tight timing tolerances required by the linear optical quantum computer, the phase errors can render the master clock, etc., unsuitable for use in the linear optical quantum computer.

[0073] In some embodiments, some clock signals for a photonic quantum computer can be generated using extra pump photons rejected by the single photon source or pump photons not coupled to the single photon source. In some embodiments, some clock or control signals can be generated from heralding signals when idler photons are detected at a heralded photon source (HPS).

[0074] FIG. 5 illustrates a schematic block diagram of an example of a system 500 for generating a clock signal for a photonic quantum computer according to some embodiments. In the illustrated example, the system 500 may include a pump photon source 502 that exists at room temperature outside the cryostat, and a photon pair source 510 that exists inside the cryostat. The photon pair source 510 is coupled to the pump photon source 502 via a waveguide 504. The photon pair source 510 may include, for example, a first waveguide 514, a resonator 512 optically coupled to the first waveguide 514, and a second waveguide 516 optically coupled to the resonator 512, as described above in connection with FIG. 3. A portion of the pump photons may be coupled from the first waveguide 514 to the resonator 512, while the remaining portion of the pump photons may exit the first waveguide 514 as uncoupled pump photons. The resonator 512 may convert some of the pump photons into pairs of signal photons and herald photons. The converted photon pairs, as well as the extra pump photons that have not been converted, may be coupled to the second waveguide 516 as an output.

[0075] The output of the photon pair source 510 may be coupled to a pump rejection mechanism 520. The pump rejection mechanism 520 separates the signal photons (S) and herald photons (H) from the extra pump photons, for example, by spectral filtering. The extra pump photons may be absorbed by a beam dump (not shown). The signal photons and herald photons are then separated by a wavelength division demultiplexing (WDDM) device 530. The herald photons may be detected by a single photon detector 540 that generates an electrical herald signal. The herald signal may be used by a multiplexer 550 that multiplexes the signal photons generated by a plurality of photon pair sources.

[0076] The system may further include a photo-detector 570 and a clock generator 560. The photo-detector 570 may receive uncoupled pump photons from the first waveguide 514 of the photon pair source 510 as an input and generate an electrical pulse to be input to the clock generator 560. In some embodiments, the photo-detector 572 may receive the extra pump photons rejected by the pump rejector 520 as an input and be used to generate an electrical pulse to be input to the clock generator 560. Since the photon pair source 510 is disposed on a chip inside a cryostat, the clock signal thus generated may also be synchronized with the phase of the single photons generated by the single photon source, and unwanted phase errors and drifts may be prevented or reduced.

[0077] The clock signal may be input to the multiplexer 550 to control the operation of the multiplexer 550. For example, the clock signal may be used to control the timing of multiplexing. In some embodiments, the clock signal may also be input to the single photon detector 540 to control the operation of the single photon detector 540. For example, the clock signal may be used to control when the single photon detector 540 should be turned on and off. In some embodiments, the output of the single photon detector 540 may be used as a clock or another control signal (e.g., a feed-forward control signal) to control the operation of other circuits such as the multiplexer 550.

[0078] FIG. 6A is a simplified schematic diagram illustrating an example of a system 600 including a clock signal generator according to an embodiment of the present invention. Figure 6B is a set of plots illustrating optical and electrical signals associated with various elements of the system illustrated in Figure 6A. The clock signal generator 630 illustrated in Figure 6A can be any of the clock signal generators described in the present application, for example, can be utilized as the clock generator 560. As illustrated in Figure 6A, the system 600 includes a pump photon source 610 and a photon pair source 612, which can be examples of the pump photon source 502 and the photon pair source 510 in Figure 5, respectively. The optical signal supplied by the pump photon source 610, for example, an optical pulse, is illustrated as plot 650 for the pump in Figure 6B.

[0079] Extra pump photons are supplied to a detector 620, such as the photodetector 570 or 572 in Figure 5. The detector 620 converts the extra pump photons into electrical pulses. The electrical pulses output by the detector 620 are illustrated as plot 652 (“a”, that is, Detector Current). The clock signal generator 630 receives the electrical pulses output by the detector 620 and includes a transimpedance amplifier (TIA) 632 and a limiting amplifier 634. The output of the TIA 632 is illustrated as plot 654 (“b”, that is, TIA Output in Figure 6B). The limiting amplifier 634 thus outputs the clock signal 656 (“ck”, that is, Limiting Amp Output) illustrated in Figure 6B. The clock signal 656 (“ck”) is regarded as a recovered electrical clock from the pump photon source and can be provided to the electrical circuit 640 as described herein.

[0080] To achieve a desired function for quantum computing (e.g., generating, manipulating, and detecting qubits), a photonic quantum computing module may include an optical backplane including at least one optical waveguide layer, and a plurality of die stacks on the optical backplane. Each die stack of the plurality of die stacks may include a photonic integrated circuit (PIC) die, and an electronic integrated circuit (EIC) die bonded to the PIC die. The PIC die may include a photonic integrated circuit optically coupled to at least one optical waveguide layer of the optical backplane. The EIC die may include an electronic integrated circuit for controlling the operation of the photonic integrated circuit of the PIC die. The at least one optical waveguide layer may include waveguides optically coupled to at least two of the plurality of die stacks. The waveguides are configured to transport a photonic quantum system including photons in one of two or more quantum modes between at least two of the plurality of die stacks. The two or more quantum modes may include, for example, two or more different frequencies, two or more different positions (e.g., waveguides or superpositions of waveguides in which photons are propagating), two or more different propagation directions, two or more different polarization states, two or more different time windows, or two or more different orbital angular momenta. The photonic quantum system may include, for example, single photons, qubits, high-dimensional particles, entangled states of qubits, or logical qubits. The photonic integrated circuit may include a single photon source configured to generate photons. In some embodiments, the at least one optical waveguide layer may include at least one of a pair of waveguides or waveguides configured to transport photons in two or more quantum modes.

[0081] As described above, in some embodiments, a qubit can be physically realized using a pair of waveguides into which single photons are introduced. The qubit can be operated when using a mode coupler (e.g., a beam splitter), a variable phase shifter, a photon detector, and the like. For example, entanglement between two (or more) qubits can be generated by providing a mode coupler between waveguides associated with different qubits. As also described above, physical qubits can suffer from losses and noise. As a result, relying on a single physical qubit (e.g., a photon propagating in a pair of waveguides) when performing quantum computing can result in an unacceptably high error rate. To provide fault tolerance, a photonic quantum computer can be designed to operate on one or more logical qubits, where a logical qubit is a multiqubit quantum system in an entangled state that enables error correction (also referred to herein as an error-correcting code). For example, in some embodiments, the structure of the error-correcting code can be represented as a three-dimensional graph. In the context of quantum computing, a logical qubit can improve robustness by supporting error detection and error correction. Logical qubits can also be used in other contexts such as quantum communication.

[0082] FIG. 7 illustrates an example of a subsystem 700 for generating an entangled quantum state (e.g., a resource state or a logical qubit) according to a particular embodiment. The subsystem 700 can include a wafer-scale module 710 that includes a plurality of EPIC die stacks 712 bonded to an optical backplane 716. The EPIC die stacks 712 can be fabricated and bonded to the optical backplane 716 as described in detail above and below. The wafer-scale module 710 can also include a plurality of PCBs 714 (e.g., an electrical backplane). The EIC dies in the EPIC die stacks 712 can be electrically connected to the PCBs 714, for example, using wire bonds. The EPIC die stacks 712 can be used to generate, manipulate, and detect qubits or entangled states of qubits for optical quantum computing or optical quantum communication, for example. For example, the EPIC die stacks 712 can include a single photon generator, a mode coupler, a fusion gate, a beam splitter, a switch, a single photon detector or a multi-photon detector, a waveguide, a delay line, a modulator, an optical switch, a ring oscillator, a coupler, and a photodiode-based phase shifter for receiving data and timing signals, as described above and below. The EPIC die stacks 712 can be optically connected together by optical fibers, optical waveguides in the optical backplane 716, free-space optical interconnects, and / or other optical interconnects.

[0083] The wafer scale module 710 can be connected to the power distribution network 720 by an optical fiber 760. The optical fiber 760 can be coupled to the EPIC die stack 712 by a grating coupler (or edge coupler) and / or an optical backplane 716. The power distribution network 720 can be connected to one or more pump laser sources 740 and a control unit 730 (e.g., by an optical transceiver 750). The control unit 730 can include, for example, a classical computing system. In some embodiments, the control unit 730 and / or the power distribution network 720 can be used to control two or more wafer scale modules 710. The optical fiber 760 can be used, for example, to transmit pump laser pulses from the pump laser source 740 to the EPIC die stack 712 for single photon generation, to transmit control data from the control unit 730 and the optical transceiver 750 to the EPIC die stack 712 (e.g., to control a switch), and to transmit measurement data from the EPIC die stack 712 to the optical transceiver 750 and the control unit 730.

[0084] In one example, each EPIC die stack 712 can include a single-photon generator that deterministically generates single photons via nonlinear optical processes (e.g., SFWM, SPDC, second harmonic generation, etc.) by forming a plurality of multiplexed photon pair sources, including waveguides, ring oscillators, interferometers, couplers, optical switches, WDM filters, and single-photon detectors. In one embodiment, each photon pair source can include a micro-ring-based SFWM heralded photon source (HPS), where detection of one photon of a pair of photons generated by a single-photon detector (e.g., SNSPD) during nonlinear processing can herald the presence of the other photon in the pair that can be used to implement a qubit or generate an entangled resource state. Other classes of photon sources that do not use nonlinear materials can also be employed, such as those that employ atomic and / or artificial atomic systems (e.g., quantum dot sources, and color centers in crystals, etc.). The operation of some photon sources can be nondeterministic (sometimes also referred to as "probabilistic"), such that a given pump pulse may or may not generate a photon pair. In such photon sources, coherent spatial and / or temporal multiplexing of some nondeterministic photon sources can be performed to increase the probability of having one photon in any given cycle. When the number of multiplexed nondeterministic photon sources is large, the probability of having one photon in any given cycle can be approximately 100%.

[0085] As illustrated in FIG. 7, the wafer scale module 710 may also be coupled to one or more optical fibers 762. The one or more optical fibers 762 may be used to transmit single photons, qubits, or entangled states of qubits between different wafer scale modules 710, or may be used to loop qubits back to the same wafer scale module after a delay. As described above and below, in some embodiments, the optical fiber 762 may be coupled to a waveguide in the optical backplane 716 by a low-loss coupler. In some embodiments, the optical fiber 762 may also be used for data communication, to transmit pump laser pulses, or as an interconnect for photonic qubits, as will be described in detail below.

[0086] In some embodiments, a quantum computing system may include a plurality of wafer scale modules and a plurality of optical fibers. Each wafer scale module of the plurality of wafer scale modules may include a substrate, and at least one optical waveguide layer on the substrate; and an optical backplane including one or more die stacks on the optical backplane. Each die stack of the one or more die stacks may include a photonic integrated circuit (PIC) die optically coupled to at least one optical waveguide layer of the optical backplane, and an electronic integrated circuit (EIC) die bonded to the PIC die and including an electronic integrated circuit for controlling the operation of the PIC die. Each optical fiber of the plurality of optical fibers may be optically coupled to the optical backplanes of two of the plurality of wafer scale modules. In some embodiments, the system may include an optical fiber delay line, where both ends of the optical fiber delay line are connected to the optical backplanes of the wafer scale modules of the plurality of wafer scale modules. The optical fiber delay line may be characterized by a time delay between approximately 10 ns and approximately 500 ns.

[0087] FIG. 8A is a top view of an example of a wafer scale module 810 including a plurality of EPIC die stacks 814 on a handle wafer 812 according to a particular embodiment. Each of the EPIC die stacks 814 can include a PIC die and an EIC die and can be fabricated and bonded to the handle wafer 812 (e.g., including an optical backplane or another optical interposer) as described above and below. The EPIC die stacks 814 can be optically connected to another EPIC die stack 814 by one or more optical fibers, one or more optical waveguides in an optical interposer, one or more free space optical interconnects, or other optical interconnects. The wafer scale module 810 can also include a plurality of PCBs 816. The EIC die in the EPIC die stack 814 can be electrically connected to the PCB 816, for example, using wire bonding. The EPIC die stacks 814 can be used, for example, to generate, manipulate, and / or detect qubits (e.g., photonic qubits employing one or more photons) or entangled states of qubits for photonic computing.

[0088] FIG. 8B illustrates an example of a system 800 including a plurality of wafer scale modules 810 interconnected using optical fibers according to a particular embodiment. As described above with respect to FIG. 8A, each wafer scale module 810 can include a plurality of EPIC die stacks. The optical fibers can be used to provide optical interconnects between and / or within wafers. For example, the optical fiber 820 can be used to connect EPIC die stacks 814 on the same wafer scale module 810, while the optical fiber 830 can be used as an interconnect between wafer scale modules 810. The system 800 can be used, for example, to perform qubit generation, manipulation, and / or detection on a larger scale.

[0089] FIG. 9A is a cross-sectional view of an example of a wafer-scale module 900 that includes a plurality of EPIC die stacks 910 coupled to an optical backplane 920, according to a particular embodiment. The wafer-scale module 900 can be an example of the wafer-scale module 710 or 810. FIG. 9B is a zoomed-in view of an example of the wafer-scale module 900, according to a particular embodiment. FIG. 9B shows the optical coupling between the PIC die 912 and the optical backplane 920. The optical backplane 920 can be used as an optical interposer for bonding the EPIC die stack 910 to a substrate and optically connecting the EPIC die stack 910.

[0090] The wafer-scale module 900 can be used, for example, in an optical quantum computer, a communication system, a qubit converter (encoder / decoder) system described below, and other electro-optical hybrid systems. In the illustrated example, the wafer-scale module 900 includes a handle wafer 930 (silicon wafer) on which the optical backplane 920 is formed or bonded. The plurality of EPIC die stacks 910 can be bonded to the optical backplane 920, for example, by an oxide-to-oxide bond. Each EPIC die stack 910 includes an EIC die 914 and a PIC die 912 bonded together by bond pads or bond bumps on the EIC die and the PIC die such that the electrical interconnect between the EIC and the PIC can be shortened. An electrical backplane device 940 (e.g., a PCB) can be bonded to the handle wafer 930 or the optical backplane 920. The electrical backplane device 940 can be electrically connected to the EPIC die stack 910, for example, by bond wires 942. The optical fiber 950 can be coupled to the optical backplane 920, for example, by an edge coupler, a tapered structure, and / or an alignment structure (e.g., a V-shaped groove formed on the handle wafer 930).

[0091] The wafer-scale module 900 can be used to generate single photons by non-linear processes such as spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM), for example, using a pump laser pulse, a waveguide, an optical switch, a ring oscillator, a coupler, a wavelength-division multiplexing (WDM) beam splitter, and a single photon detector (e.g., for detecting heralding photons). In some embodiments, coherent spatial and / or temporal multiplexing of some non-deterministic photon sources can be performed to increase the probability of generating one photon in a given cycle. The wafer-scale module 900 can also be used to generate a qubit resource state or other entangled state from a single photon, for example, using a waveguide, a delay line, a coupler, a splitter, a switch, a modulator, and a fusion gate.

[0092] The wafer-scale module 900 can also be used to detect photons or qubits, for example, using a single photon detector, a waveguide, a delay line, etc. A single photon detector, such as a superconducting nanowire single photon detector (SNSPD), can be used to detect heralding photons that signal the generation of single photons in a single photon generator, or to detect single photons in an entangled state (e.g., a resource state) to detect or perform logical operations on a logical qubit. In some embodiments, the wafer-scale module 900 can include an optical separation structure for relaxation of scattering, such that stray light scattered by other circuits in the wafer-scale module 900 may not reach the single photon detector. For example, the PIC die 912 can include an opaque structure surrounding the single photon detector to prevent stray light from reaching the single photon detector. The single photon detector may also need to operate at a very low temperature, such as cryogenic temperature. Accordingly, the wafer-scale module 900 can also include a cooling structure and a thermal insulation structure, such that heat generated in other regions does not reach the regions that need to operate at a low temperature (e.g., cryogenic temperature). For example, the PIC die 912 can include a cooling structure such as a metal conductor or a microfluidic channel. In some embodiments, the PIC die 912 can also include a heating element. In some embodiments, the PIC die 912 can also include a thermal insulation structure to separate photonic circuits that may need to operate at a low temperature or to prevent heat loss of the heating element.

[0093] In some embodiments, the wafer scale module 900 may include a photodetector or an optical transceiver for receiving and / or transmitting optical communication signals such as data and timing signals. In one example, the wafer scale module 900 may include a Ge photodiode-based phase shifter for receiving data and timing signals from a control unit. In another example, the PIC die 912 and the EIC die 914 may include, for example, an optical transceiver for communicating with a control unit. In some embodiments, the PIC die 912 and the EIC die 914 may include an optical modulator.

[0094] FIG. 9B shows the optical coupling between the PIC die 912 and the optical backplane 920. The optical backplane 920 may include one or more waveguide layers including a plurality of waveguides 922. In some embodiments, one waveguide layer may include routing waveguides for optically connecting the PIC die 912, and another waveguide layer may include waveguide delay lines. Light from the optical fiber 950 or the PIC die 912 may be coupled to the waveguide 922. The light may propagate in the waveguide 922 and may be coupled to the PIC die 912 by the waveguide coupler 916. In some embodiments, the optical signal may also be coupled to waveguides in different waveguide layers in the optical backplane 920. In some embodiments, the light may also be coupled from the PIC die 912 to the waveguide 922 in the optical backplane 920 by the waveguide coupler 916, and then may be coupled from the waveguide 922 to another PIC die 912 by another waveguide coupler 916. Thus, the waveguide 922 may be used for optical signal routing and interlayer transition and the like.

[0095] FIG. 10 shows an example of a system 1000 that includes optical fibers for transmitting, delaying, or storing single photons, qubits, high-dimensional particles (qudits), resource states, or other entangled states. In the illustrated example, system 1000 may include a first wafer-scale module 1010, a second wafer-scale module 1020, and an optical fiber 1030 for qubit connection between the first wafer-scale module 1010 and the second wafer-scale module 1020. If the qubits are implemented using dual rails as shown, for example, in FIG. 1, a pair of optical fibers may be required to transmit the qubits between the wafer-scale modules. If a pair of optical fibers is used, the fibers in each pair may experience different environments such as different temperatures, different stresses, and different coupling conditions, and thus there may be different delays or phase changes for photons propagating on different optical fibers.

[0096] According to certain embodiments, on the transmitter side, a spatial-mode qubit (e.g., a dual-rail encoded photonic qubit) may be converted to a time-mode qubit (e.g., a time-bin encoded photonic qubit, e.g., the time-bin encoded qubit 1132 shown in FIG. 11A) using a qubit converter that includes a delay line, e.g., a waveguide delay line, and a switch controlled by a clock signal. In this way, the qubits may be transmitted in the same optical fiber and thus may experience the same environment in the propagation path. In some embodiments, on the receiver side, the received time-mode qubit may be converted to a spatial-mode qubit using a qubit converter that includes a delay line and a switch controlled by a clock signal.

[0097] FIG. 11A illustrates an example of a system 1100 according to a particular embodiment that includes a first photonic integrated circuit 1110 (e.g., a photonic entanglement state encoder, a qubit encoder), a second photonic integrated circuit 1120 (e.g., a photonic entanglement state decoder, a qubit decoder), and an optical interconnect 1130 (e.g., a fiber, a waveguide in an optical transponder connecting the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120) for transmitting an entangled photonic state (e.g., a photonic qubit) between the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120. The first photonic integrated circuit 1110 and the second photonic integrated circuit 1120 can be chip-scale or wafer-scale modules as described above. In some exemplary embodiments, the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120 include the same components and layout, but the inputs and outputs are inverted with respect to the second photonic integrated circuit 1120, such that the first photonic integrated circuit 1110 operates as a qubit encoder for converting a photonic qubit from a spatial format to a temporal format, and the second photonic integrated circuit 1120 operates as a qubit decoder for converting the received photonic qubit from a temporal format to a spatial format.

[0098] In the illustrated example, the first photonic integrated circuit 1110 may include a qubit preparation circuit (e.g., the state preparation unit 1116). The first photonic integrated circuit may further include a qubit encoder section including a pair of waveguides, a delay line 1112 in one of the waveguides, and a switch 1114 (e.g., an MZI including one or more phase shifters - the triangles in FIG. 11A - in one of the arms of the MZI). The second photonic integrated circuit 1120 may include a qubit decoder section including a pair of waveguides including a switch 1122 and a delay line 1124 in one of the arms of the waveguide. The second photonic integrated circuit 1120 may further include a qubit projection circuit 1126 (e.g., a state preparation photonic circuit with inverted input and output) that outputs to a detector (e.g., to perform tomography). The first photonic integrated circuit 1110 and the second photonic integrated circuit 1120 may be synchronized with each other and may be synchronized with incoming photonic entangled states such as photonic qubits, a portion of a photonic resource state, and the qubit portion of a photonic cluster state.

[0099] Delay lines 1112 or 1124 can have a delay from approximately 25 ps to approximately 500 ps, for example, for a time bin separation of approximately 50 ps to approximately 1 ns of the qubits. Delay lines 1112 or 1124 can include waveguides made of silicon nitride, lithium niobate, silicon, or other materials that can provide low loss (e.g., < approximately 50 mdB / m). Delay lines 1112 and 1124 can be formed on the photonic chip or on the interposer as described above. Switches 1114 and 1122 can include Mach-Zehnder interferometers (MZIs) that each can include, for example, an adjustable phase shifter made of any other suitable electro-optic material such as barium titanate (BTO), or lithium niobate. The switching times of switches 1114 and 1122 may need to be much shorter than the time bin separation described above. Switches 1114 and 1122 can be controlled by synchronous or asynchronous control signals. For example, switches 1114 and 1122 can be controlled by a periodic clock signal to switch at a constant switching rate, or can be controlled by a feed-forward signal that can be asserted only when a photon or qubit is generated or detected.

[0100] In some exemplary embodiments, the first photonic integrated circuit 1110 is adjusted with the second photonic integrated circuit 1120 in a calibration phase to compensate for variations that may occur between the components of the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120. For example, delay line 1112 and delay line 1124 may have manufacturing variations that result in non-identical amounts of delay. In some exemplary embodiments, one or more phase shifters in the first photonic integrated circuit 1110 or the second photonic integrated circuit 1120 are adjusted during a calibration step (e.g., using bright light, optical pulses) to ensure that any phase or delay difference between delay line 1112 and delay line 1124 is minimized or removed, such that the same amount of delay is applied to both delay line 1112 and delay line 1124, thereby ensuring high-performance space-time encoding and time-space decoding. In some exemplary embodiments, switch 1114 includes a plurality of phase shifters, where one of the phase shifters is a "slow" phase shifter (e.g., a heater, the second triangle in switch 1114 not shown in FIG. 11A) used during calibration for equalization between the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120. For example, the slow phase shifter may be adjusted to apply a phase offset to compensate for variations between the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120. After adjustment of the slow phase shifter, another phase shifter in switch 1114 (e.g., the triangle shown in switch 1114) operates as a fast high-speed phase shifter to perform high-speed switching within the range of the delay time of delay line 1112 (e.g., 500 picoseconds). In some exemplary embodiments, switch 1122 in the second photonic integrated circuit 1120 also has one or more additional phase shifters to compensate for delay or variations between the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120. In some exemplary embodiments, the phase shifter may be located outside of the switch (e.g., switch 1114, switch 1122) to perform compensation and equalization between PICs.For example, an additional phase shifter may be disposed between the state preparation unit 1116 and the switch 1114 (e.g., above the lower section, where the upper section includes the delay line 1112), where the additional phase shifter is implemented during calibration to add a phase to equalize the paths in the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120.

[0101] In some exemplary embodiments, the propagation of light by a delay component in the PIC, such as the delay line 1112, imparts additional optical loss to the light propagating in the PIC. For example, in the first photonic integrated circuit 1110, more loss occurs to the light traversing the delay line 1112 than to the light traversing the lower section. For example, if the state preparation unit is configured as a 50:50 beam splitter, the light directed to the upper section toward the delay line 1112 exhibits higher attenuation than the light in the lower section. In some exemplary embodiments, the second photonic integrated circuit 1120 is configured to ensure that the light traversing the delay line 1112 in the upper section of the first photonic integrated circuit 1110 traverses the lower section in the second photonic integrated circuit 1124, thereby avoiding the delay line 1124. Further, the light traversing the non - delayed bottom arm in the first photonic integrated circuit 1110, which experiences less loss than the upper arm with delay, is directed toward the upper section of the second photonic integrated circuit 1120 to traverse the delay line 1124 and incur loss, such that the output of the decoder (e.g., to a detector) is balanced and has similar or the same refractive power. As described above, directing light to the delayed or non - delayed arms of the encoder and decoder PICs can be performed using bright light, and once configured, quantum light (e.g., photonic qubits) is understood to be implemented in the first photonic integrated circuit 1110 and the second photonic integrated circuit 1120.

[0102] FIG. 11B is a flowchart of an example of process 1140 according to some exemplary embodiments. In some implementations, one or more of the process blocks of FIG. 11B may be performed by a first photonic integrated circuit and a second photonic integrated circuit. As shown in FIG. 11B, process 1140 may include using a qubit source (block 1150) to generate spatial mode qubits. For example, the first photonic integrated circuit may use the qubit source to generate spatial mode qubits as described above. As also shown in FIG. 11B, process 1140 may include using the optical delay and switch of the first photonic integrated circuit (block 1155) to convert spatial mode qubits to temporal mode qubits. For example, the first photonic integrated circuit may use the optical delay and switch of the first photonic integrated circuit to convert spatial mode qubits to temporal mode qubits as described above. As further shown in FIG. 11B, process 1140 may include transmitting temporal mode qubits from the first photonic integrated circuit to the second photonic integrated circuit in an optical interconnect, where the optical interconnect is a fiber or waveguide that couples the first photonic integrated circuit and the second photonic integrated circuit together (block 1160). As also shown in FIG. 11B, process 1140 may include a procedure (block 1165) to convert the temporal mode qubits back to spatial mode qubits in the second photonic integrated circuit. For example, as described above, the second photonic integrated circuit may convert the temporal mode qubits back to spatial mode qubits.

[0103] Although FIG. 11B shows exemplary blocks of process 1140, in some implementations, process 1140 may include additional blocks, fewer blocks, different blocks, or blocks arranged differently than those shown in FIG. 11B. Additionally or alternatively, two or more of the blocks of process 1140 may be performed in parallel.

[0104] FIG. 12 illustrates an example of a system 1200 according to a particular embodiment. System 1200 can be an example of system 1100. System 1200 can include a clock circuit 1201 that can provide timing signals (e.g., clock signals) to controllers such as controller 1203 for chip 1 and controller 1205 for chip 2. In some embodiments, the clock signal can be an optical clock signal. As described above, in some embodiments, the clock signal can be derived from a photonic qubit source 1207 such as the single photon generator shown in FIGS. 1 through 6B. Photonic qubit 1207 may be integrated on chip 1 or may be on a different chip. A qubit generation circuit such as that shown in FIG. 11A may be on the same chip (e.g., chip 1) as the qubit converter circuit or may be on a different chip. In the example illustrated, the qubit converter circuit can be a photonic integrated circuit and can include a waveguide delay line 1215 on one rail 1219 (e.g., a waveguide) of a dual rail qubit bus 1211 (including a pair of waveguides), and a switch 1213 controlled by controller 1203 to connect each of the two rails to an output waveguide 1221 of the photonic integrated circuit at different times (based on the state of the switch). For example, the switch can be turned off during a first period (e.g., the first half of the clock cycle), and as a result, the output portion of the lower rail 1217 of the dual rail qubit bus can be connected to the output waveguide 1221 during the first period. The switch can be turned on during a second period (e.g., the second half of the clock cycle), and as a result, the output portion of the upper rail 1219 of the dual rail qubit bus (where photons can be delayed for half of the clock cycle) can be connected to the output waveguide 1221 during the second period. Thus, in output waveguide 1221, the qubit can be temporarily encoded, where different quantum states of the qubit can be represented by photons during the first half of the clock cycle or during the second half of the clock cycle. Such temporary encoding is advantageous for transmitting the qubit over long distances, for example, by optical fiber 1223.

[0105] In chip 2, the switch can be similarly controlled by controller 1205 to selectively connect input waveguide 1225 to two rails of the dual-rail qubit bus. In the illustrated example, in the first half of the clock cycle, the switch is turned on to connect the optical fiber to the upper rail that may include an on-chip integrated delay line, and any incoming photons in the first half of the clock cycle can be sent to the upper rail and the delay line. In the second half of the clock cycle, the switch can be turned off so that the optical fiber can be connected to the lower rail that does not include an additional delay line. As a result, any incoming photons during the second half of the clock cycle can be sent to the lower rail. Therefore, after passing through the delay line, photons that can be received earlier (in the first half of the clock cycle) by chip 2 can be delayed by the delay line (e.g., by half of the clock cycle), and as a result, can be spatially aligned with photons received later (e.g., in the second half of the clock cycle) by chip 2.

[0106] FIG. 13A shows an example of system 1300 and an example of control signals for controlling the operation of system 1300, according to a particular embodiment. FIG. 13B shows the relationship between some of the control signals. System 1300 can be an example of system 1200. In the illustrated example, switch 1 (SW1) on the first chip can be controlled by a control circuit controlled by a clock signal (tb CLK) and a switch control signal (SW ctrl). In an exemplary example, the switch control signal can be a feed-forward signal from a photon source (e.g., from a heralded photon detector), and thus can be asserted only when photons are generated and detected. In this way, the switch can switch only when there is a valid qubit present, and thus can reduce power consumption. Switch 2 (SW1) on the second chip can be controlled by a control circuit controlled by a clock signal (tb CLK) that can be a periodic clock signal and a switch control signal that can be from a photon source (e.g., generated using the extra pump photons described above with respect to FIG. 5). In some embodiments where the feed-forward signal can be passed to the second chip, the feed-forward signal can be used as the switch control signal for the control circuit of switch 2. Thus, the switch control signal can be synchronized with the pump pulse or control clock of the photon pump source and have the same frequency as it (e.g., having a clock period of approximately 1 ns or a pulse repetition rate of approximately 1 GHz). In some embodiments, qubits from a qubit generator may need to be delayed by the feed-forward signal latency to align the center of the optical mode (quantum mode) with the center of each switch state, such that the switch can reach a steady state (rather than a transition period) when a photon or optical mode reaches the switch.

[0107] As illustrated in FIGS. 13A and 13B, tb CLK can have approximately half or less than half the period of the switch control signal (or approximately twice or more than twice the frequency of the switch control signal). The delay line in one rail of the dual-rail qubit bus can have a time delay δT that is approximately the period of tb CLK or half the period of the switch control signal tb As a result, the optical mode of the qubit can be temporarily shifted for the period of tb CLK. In some embodiments, the time delay δT tb can be shorter than half the period of tb CLK or the period of the switch control signal. In some embodiments, a delay Δt tb can be added to the control signal for controlling switch 1 or switch 2 to compensate for the interconnect delay of the control signal. In some embodiments, the qubit from the qubit generator may need to be delayed by the feedforward signal latency to align the center of the optical mode at the center of each switch state, so that the switch can reach a stable state (rather than the transition period) when a photon or optical mode reaches the switch. For example, as shown in FIG. 13B, the optical mode can be aligned with the center of the high state of the switch control signal, where the switch is turned on, so that the switch can be in the "CROSS" state, and another optical mode can be aligned with the center of the low state of the switch control signal, where the switch is turned off, so that the switch can be in the "STRAIGHT" or "THROUGH" state.

[0108] FIG. 14 includes 1400, which is a diagram showing an example of the timing of some control signals in FIGS. 13A and 13B according to a particular embodiment. FIG. 14 shows a switch control signal that may have a return-to-zero (RZ) format and may have the same frequency as the pump pulse (e.g., synchronized with the pump pulse). tb CLK may have a frequency that is approximately twice the frequency of the switch control signal, where the rising edge of tb CLK may be aligned with the center of the high / low state of the switch control signal. The retimed switch voltage signal may have rising / falling edges aligned with the rising edge of tb CLK.

[0109] FIGS. 15A through 15L show an example of the propagation of optical modes (indicated by yellow and red dots) in a system including a qubit converter according to a particular embodiment. FIGS. 15A through 15L show the positions of the optical modes at different times. For example, in FIG. 15A, the two optical modes of the qubit may be in the photonic qubit. FIG. 15B shows that the optical modes may enter the dual-rail qubit bus and may be temporally and spatially aligned. FIG. 15C shows that the optical mode in the upper rail may be delayed, such that when the optical mode in the lower rail reaches the switch, the optical mode in the upper rail is not yet at the switch. The switch may be turned off (in the "STRAIGHT" or "THROUGH" state), and thus the optical mode in the lower rail may pass through the switch and enter the optical fiber. FIG. 15D shows that after the optical mode in the lower rail has passed through the switch (and entered the optical fiber), the switch may be turned on (in the "CROSS" state), and thus the optical mode in the upper rail may cross the switch and enter the optical fiber at a later time. FIGS. 15E and 15F show that a spatial dual-rail qubit may become a temporal single-rail qubit (temporal mode qubit) in the optical fiber, where one optical mode may be after another optical mode of the qubit.

[0110] FIG. 15G shows that the first optical mode of the time-mode qubit can reach a switch on the second chip, while the second optical mode of the time-mode qubit can still be in the optical fiber. FIG. 15H shows that the switch in the second chip can be turned off (in the "STRAIGHT" or "THROUGH" state), and thus the first optical mode can pass through the switch and enter the lower rail of the dual-rail qubit bus on the second chip. FIG. 15I shows that after the first optical mode passes through the switch, the switch can be turned on and can be in a stable "CROSS" state when the second optical mode arrives. Therefore, the second optical mode can be transmitted by the switch to the upper rail of the dual-rail qubit bus on the second chip as shown in FIG. 15J. FIG. 15K shows that while the second optical mode propagates in the upper rail, the first optical mode in the lower rail can be delayed by a delay line. FIG. 15L shows that after the first optical mode in the lower rail passes through the delay line, the first optical mode and the second optical mode can be spatially and temporally aligned.

[0111] Figures 16A through 16D show various structures of an MZI that can be used as a switch in the qubit converter disclosed herein according to a particular embodiment. Each MZI can include two 50 / 50 splitters / combiners and two arms fabricated with optical waveguides. The phase shift in the arms of the MZI can be achieved by applying an electric field by an electro-optic material that affects the optical mode propagating in the waveguide. The electrodes can be designed and terminated as traveling wave electrodes. In the example shown in FIG. 16A, a voltage signal can be applied to one arm of the MZI to adjust the phase delay in one arm. In the example shown in FIG. 16B, a common VDD can be applied to the center of the two arms, and opposite voltage signals can be applied to both arms of the MZI to adjust the phase delay in both waveguides. As a result, the electric field can increase in one arm and decrease in the other arm, thereby changing the phase in both arms simultaneously. The voltage across each arm needs to be shifted by 90 degrees. In the example shown in FIG. 16C, the common VDD can be on both sides of the MZI, and opposite voltage signals can be applied to the arms of the MZI from the center of the two arms. In the example shown in FIG. 16D, the VDD signal can be applied from one side of the MZI, and the ground can be on the opposite side of the MZI. A common signal can be applied to the center of the two arms.

[0112] FIG. 17 shows an example of a system 1700 in which a feedforward signal generated on a first chip can be transmitted to a second chip. As described above, the feedforward signal can be used, for example, to control the switch of a qubit decoder on the second chip to reduce power consumption. The feedforward signal can be transmitted optically or electrically.

[0113] FIG. 18 shows an example of a system 1800 in which an optical signal (e.g., a qubit and / or a feed-forward optical signal) can be transmitted between photonic chips using waveguides, optical fibers, and / or free-space optical interconnects within an optical interposer. For example, a feed-forward signal can be converted to an optical signal using a modulator in a first photonic chip, transmitted by an optical fiber or a free-space optical interconnect, and received by a photodetector in a second photonic chip. Electrical signals can be transmitted between EIC / PIC die stacks using an electrical redistribution layer that can be bonded to the EIC / PIC die stacks. For example, a clock signal or other control signal (e.g., a feed-forward signal) can be transmitted through the electrical redistribution layer.

[0114] FIG. 19 illustrates an example of an integrated chip 1900 that includes a qubit encoder and a qubit decoder connected by an optical fiber delay line connected to the integrated chip 1900 by a Fiber Array Unit (FAU). In the example illustrated, photons can be received in the integrated chip 1900 and used to generate qubits. Qubits in a spatial mode can be converted to qubits in a time mode by a qubit encoding circuit, and the qubits in the time mode can be transmitted by the FAU to an optical fiber delay line. The delayed qubits in the time mode can be transmitted to a qubit decoding circuit and converted back to the spatial mode, as described above.

[0115] FIG. 20 is a simplified block diagram of an example of a quantum computing system 2000 according to some embodiments. The quantum computing system 2000 may implement, for example, measurement-based quantum computing (MBQC) or fusion-based quantum computing (FBQC). Some embodiments of the quantum computing system 2000 may use photonic physical qubits to generate a fault-tolerant cluster state that can be used to represent logical qubits for MBQC, while other embodiments of the quantum computing system 2000 may generate a measurement data reflection entanglement structure for fault-tolerant FBQC. In the illustrated example, the quantum computing system 2000 may include a resource state generator 2010, a delay circuit 2020, a switch circuit 2030, a detector 2040, and one or more classical processing units 2050.

[0116] The resource state generator 2010 may include one or more resource state generators (RSGs). The RSGs may operate autonomously and may not require an input of data. Each RSG may generate one resource state per clock cycle (e.g., which may be shorter than approximately 1 ns, approximately 1 ns, or longer than approximately 1 ns). Each resource state may include a plurality of (e.g., seven or nine) entangled physical qubits. The resource state may be output to the delay circuit 2020 at a rate of approximately n*N photons per clock cycle, where n is the number of qubits in each resource state and N is the number of RSGs. The resource state generator 2010 may also transmit a classical data output (e.g., indicating the success or failure of various elements of the resource state generation process) to the classical processing unit 2050 via a data path 2022. In some embodiments, the resource state generator 2010 may be maintained at cryogenic temperatures (e.g., 4K). The delay circuit 2020 may have a delay of one clock cycle, L clock cycles, and L2 To delay or store photons corresponding to specific qubits by an appropriate delay time such as a number of clock cycles, it may include an optical fiber, other waveguides, an optical memory, or other components, where L can be any integer. The delay circuit 2020 does not necessarily need to operate at extremely low temperatures. The photons exiting the delay circuit 2020 can be supplied to the switch circuit 2030 via, for example, an optical fiber, an on-chip waveguide, or any other type of waveguide or optical interconnect.

[0117] The switch circuit 2030 may include active switches and waveguides to perform mode coupling, mode swapping, phase shifting, and other operations on the qubits. In various embodiments, the switch circuit 2030 can perform a mode coupling operation associated with a fusion operation as described below and / or a basis selection operation associated with the measurement of individual qubits. In some embodiments, the switch circuit 2030 can be dynamically reconfigurable in response to control signals from the classical processing unit 2050, and thus, the quantum computing system 2000 can perform different calculations by reconfiguring the switches in the switch circuit 2030. The switching circuit 2030 can supply output photons to the detector 2040 via, for example, an optical fiber, an on-chip waveguide, or any other type of optical interconnect.

[0118] The detector 2040 may include a photon detector capable of detecting single or multiple photons. Each photon detector can be coupled to one waveguide and generate an output (classical) signal indicating whether a photon has been detected. In some embodiments, some or all of the detectors 2040 may be capable of counting photons, and the output signal from each detector 2040 can indicate the number of photons detected by the detector 2040. In some embodiments, the detector 2040 can operate at extremely low temperatures. The detector 2040 can provide a classical output signal indicating the number of photons or a binary signal indicating whether a photon has been detected to the classical processing unit 2050 via a signal path 2024 such as an optical fiber.

[0119] The classical processing unit 2050 can be a classical computer system that can communicate with the resource state generator 2010, the switch circuit 2030, and the detector 2040 using classical digital logic signals. In some embodiments, the classical processing unit 2050 can determine appropriate settings for the switch circuit 2030 based on a particular quantum computation (or program) to be executed. The classical processing unit 2050 can receive feedback signals (e.g., measurement results) from the resource state generator 2010 and the detector 2040 and can determine the result of the computation based on the feedback signals. In some embodiments, the classical processing unit 2050 can use the feedback signals to modify subsequent control signals sent to the switch circuit 2030. The operation of the classical processing unit 2050 can incorporate error correction algorithms and other techniques.

[0120] The quantum computing system 2000 of FIG. 20 is illustrative and can be modified and varied. The separately shown blocks can be combined, or a single block can be implemented using multiple separate components. The resource state generator 2010, the delay circuit 2020, the switch circuit 2030, and the detector 2040 can implement the circuits described above and below to generate an entanglement structure. For example, the delay circuit 2020 can implement a delay line for the fusion of resource states, while the switch circuit 2030 can implement a reconfigurable switch and a mode coupler associated with reconfigurable fusion, and the detector 2040 can implement a destructive measurement associated with the fusion operation. The quantum computing system 2000 can be just one example of a quantum computing system or another photonic system that can use the wafer scale modules described herein. One of ordinary skill in the art will understand that many different systems can be implemented using wafer scale modules each including a PIC or EPIC die bonded and optically coupled to an optical backplane having low loss waveguides.

[0121] FIG. 21 is a simplified system block diagram of an example of a hybrid QC system 2100 that includes an electro-optical device (e.g., a switch) according to a particular embodiment. To operate at low temperatures, such as liquid helium temperature, embodiments of the present disclosure integrate the electro-optical switches described herein into systems that include a cooling system. Accordingly, embodiments of the present disclosure provide a hybrid computing system such as that illustrated in FIG. 21, for example. Hybrid quantum computing (QC) system 2100 includes a user interface device 2104 communicatively coupled to a hybrid quantum computing subsystem 2106. User interface device 2104 can be any type of user interface device, such as a terminal including, for example, a display, keyboard, mouse, and touch screen. Additionally, the user interface device can itself be a computer such as a personal computer (PC), laptop, and tablet computer. In some embodiments, user interface device 2104 provides an interface through which a user can interact with hybrid QC subsystem 2106. For example, user interface device 2104 can execute software such as a text editor, interactive development environment (IDE), command prompt, and graphical user interface, and as a result, the user can program or otherwise interact with the QC subsystem to execute one or more quantum algorithms. In other embodiments, hybrid QC subsystem 2106 can be pre-programmed and user interface device 2104 can simply be an interface through which a user can initiate quantum computing, monitor progress, and receive results from hybrid QC subsystem 2106. Hybrid QC subsystem 2106 further includes a classical computing system 2108 coupled to one or more quantum computing chips 2110.In some examples, the classical computing system 2108 and the quantum computing chip 2110 can be coupled to other electronic components 2112 such as, for example, a pulsed pump laser, a microwave oscillator, a power supply, network hardware, and the like.

[0122] In some embodiments that utilize cryogenic operation, the quantum computing system 2100 can be housed, for example, inside a cryostat such as cryostat 2114. In some embodiments, the quantum computing chip 2110 can include one or more constituent chips that can include various waveguide structures and / or EO devices disclosed herein, for example, a hybrid electronic chip 2116 and an integrated photonic chip 2118. Signals can be routed on and off chip in any number of ways, for example, by optical interconnects 2120 and by other electronic interconnects 2122. Additionally, the hybrid quantum computing system 2100 can employ quantum computing processing, such as measurement-based quantum computing (MBQC) that employs one or more cluster states of qubits.

[0123] The following are exemplary embodiments.

[0124] Example 1: A system comprising a first photonic integrated circuit having a qubit encoder configured to receive a spatial mode qubit and convert the spatial mode qubit to a temporal mode qubit, and an optical interconnect configured to receive and transmit the temporal mode qubit; and a second photonic integrated circuit having a qubit decoder configured to receive the temporal mode qubit and convert the temporal mode qubit back to the spatial mode qubit.

[0125] Example 2: A first control circuit configured to control the qubit encoder; and a second control circuit configured to control the qubit decoder, wherein the second control circuit is synchronized with the first control circuit, the system of Example 1.

[0126] Example 3: The qubit encoder includes: a dual-rail qubit bus optically coupled to a qubit source, the dual-rail qubit bus having two waveguides, wherein a first waveguide of the dual-rail qubit bus includes a delay line; and an optical switch optically connected to the two waveguides of the dual-rail qubit bus, the system of Example 1 or Example 2.

[0127] Example 4: The spatial mode qubit is generated by a qubit source, wherein the qubit source includes a clock generator configured to generate a clock signal using a pump laser pulse, the system of any one of Examples 1 to 3.

[0128] Example 5: Further comprising an electrical interconnect channel or an optical interconnect channel configured to transmit the clock signal to the second photonic integrated circuit, the system of any one of Examples 1 to 4.

[0129] Example 6: The qubit encoder includes a first dual-rail qubit bus including a first set of waveguides, one of the first set of waveguides including a first delay, the qubit decoder includes a second dual-rail qubit bus including a second set of waveguides, one of the second set of waveguides including a second delay, the system of any one of Examples 1 to 5.

[0130] Example 7: Further comprising a phase shifter to compensate for a phase difference between the first delay and the second delay, the system of any one of Examples 1 to 6.

[0131] Example 8: The phase shifter is any one of the systems of Examples 1 to 7 that is within the switch of the qubit encoder in the first photonic integrated circuit.

[0132] Example 9: The spatial mode qubit is any one of the systems of Examples 1 to 8 that corresponds to a dual-rail encoded photonic qubit.

[0133] Example 10: The optical interconnect has an optical fiber and is any one of the systems of Examples 1 to 9.

[0134] Example 11: The optical interconnect is a waveguide of an optical interposer coupled to the first photonic integrated circuit and the second photonic integrated circuit, and is any one of the systems of Examples 1 to 10.

[0135] Example 12: A photonic integrated circuit device includes a dual-rail qubit bus optically coupled to a photonic qubit source. The dual-rail qubit bus has two waveguides. Here, the first waveguide of the dual-rail qubit bus includes a waveguide delay line disposed in a first layer of the photonic integrated circuit device; an optical switch disposed in a second layer of the photonic integrated circuit and optically connected to the two waveguides of the dual-rail qubit bus; and a control circuit configured to switch the optical switch so as to optically connect output portions of the two waveguides to a single output waveguide of the photonic integrated circuit at different times. Here, the control circuit is synchronized with the photonic qubit source.

[0136] Example 13: The photonic qubit source of the photonic integrated circuit device of Example 12 includes a clock generator configured to generate a clock signal using a pump laser pulse.

[0137] Example 14: The waveguide delay line is a photonic integrated circuit device according to Example 12 or Example 13, characterized by a delay equal to or less than half of the clock period of the clock signal.

[0138] Example 15: The waveguide delay line is a photonic integrated circuit device according to any one of Examples 12 to 14, characterized by a delay between 25 ps and 500 ps.

[0139] Example 16: The control circuit is a photonic integrated circuit device according to any one of Examples 12 to 15, which is controlled by a feedforward control signal generated by the photonic qubit source when a pair of photons is generated.

[0140] Example 17: The optical switch includes a Mach-Zehnder interferometer, and is a photonic integrated circuit device according to any one of Examples 12 to 16.

[0141] Example 18: The optical switch is a photonic integrated circuit device according to any one of Examples 12 to 17, and is controlled by a clock signal characterized by a frequency that is at least twice the clock frequency of the photonic qubit source.

[0142] Example 19: A method comprising: converting a spatial mode qubit to a temporal mode qubit using the optical delay and switch of a first photonic integrated circuit; transmitting the temporal mode qubit from the first photonic integrated circuit to a second photonic integrated circuit on an optical interconnect, the optical interconnect being coupled to the first photonic integrated circuit and the second photonic integrated circuit; and converting the temporal mode qubit back to the spatial mode qubit on the second photonic integrated circuit.

[0143] Example 20: The method according to Example 19, wherein the optical interconnect includes an optical fiber.

[0144] It will be apparent to those skilled in the art that substantial variations may be made in accordance with a particular implementation. For example, customized hardware may also be used, and / or certain elements may be implemented in hardware, software (including portable software such as applets), or both. Further, connections to other computing devices such as network input / output devices may be employed.

[0145] Referring to the accompanying drawings, components that may include memory may include non-transitory machine-readable media. As used herein, the terms “machine-readable media” and “computer-readable media” refer to any storage media that participates in providing data that causes a machine to operate in a particular fashion. In the embodiments provided above, various machine-readable media may be involved in providing instructions / codes to a processor and / or other devices for execution. Additionally or alternatively, machine-readable media may be used to store and / or carry such instructions / codes. In many implementations, computer-readable media are physical and / or tangible storage media. Such media may take many forms including, but not limited to, non-volatile media, volatile media, and transmission media. Common forms of computer-readable media include, for example, magnetic and / or optical media, punch cards, paper tape, any other physical media with patterns of holes, RAM, programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), flash EPROM, any other memory chip or cartridge, carrier waves as described below, or any other media from which a computer can read instructions and / or codes.

[0146] The methods, systems, and devices described herein are examples. Various embodiments may appropriately omit, substitute, or add various procedures or components. For example, the features described with respect to a particular embodiment may be combined in various other embodiments. Different aspects and elements of embodiments may be combined in a similar manner. The various components in the figures provided herein may be embodied in hardware and / or software. Also, technology evolves, and thus many of the elements are examples that do not limit the scope of the present disclosure to their specific examples.

[0147] For the reasons of mainly general use, it has sometimes been found to be convenient to refer to such signals as bits, information, values, elements, symbols, characters, variables, terms, numbers, or codes, etc. However, it should be understood that all of these or similar terms should be associated with appropriate physical quantities and are merely convenient labels. As is apparent from the above description, unless specifically described otherwise, the descriptions throughout this specification using terms such as "processing", "calculating", "computing", "judging", "confirming", "identifying", "associating", "measuring", "executing", etc. are understood to refer to actions or processes of a specific device such as a dedicated computer or a similar dedicated electronic computing device. In the context of this specification, therefore, a dedicated computer or a similar dedicated electronic computing device can operate or transform signals, which are usually represented by physical electronic, electrical, or magnetic quantities, inside the memory, registers, or other information storage devices, transmission devices, or display devices of the dedicated computer or a similar dedicated electronic computing device.

[0148] One skilled in the art will understand that the information and signals used to communicate the messages described herein may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referred to throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or magnetic particles, optical fields or optical particles, or any combination thereof.

[0149] The terms "and", "or", and "and / or" as used herein may include various meanings depending at least in part on the context in which such terms are used and as expected. Usually, when used to associate a list such as A, B, or C, "or" is intended to mean A, B, and C (here used in an inclusive sense), as well as, A, B, or C (here used in an exclusive sense). In addition, the term "one or more" as used herein may be used to describe any single feature, structure, or property, or may be used to describe some combination of features, structures, or properties. However, it should be noted that this is merely an exemplary example and the claimed subject matter is not limited to this example. Further, when used to associate a list such as A, B, or C, the term "at least one of" may be construed to mean any combination of A, B, C, or any combination of A, B, and / or C such as AB, AC, BC, AA, AAB, ABC, AABBCCC, etc.

[0150] References to "one example", "an example", "a specific example", or "an exemplary implementation" throughout this specification mean that a particular feature, structure, or characteristic described in connection with the feature and / or example may be included in at least one feature and / or example of the claimed subject matter. Thus, the appearances of the phrases "in one example", "an example", "in a specific example", "in a particular implementation", or other similar phrases in various places throughout this specification are not necessarily all referring to the same feature, example, and / or limitation. Further, the particular features, structures, or characteristics may be combined in one or more examples and / or features.

[0151] In some implementations, an operation or process may involve physical manipulation of physical quantities. Usually, though not necessarily, such quantities may take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, or otherwise manipulated. For reasons of common usage, it has sometimes proven convenient to refer to such signals as bits, data, values, elements, symbols, characters, terms, numbers, or codes, among others. However, it should be understood that all of these or similar terms should be associated with appropriate physical quantities and are merely convenient labels. As is apparent from the description herein, unless specifically stated otherwise, descriptions throughout this specification using terms such as "processing", "computing", "calculating", "determining", etc., refer to actions or processes of a specific apparatus, such as a special purpose computer, a special purpose computing device, or similar special purpose electronic computing device. In the context of this specification, therefore, a special purpose computer or similar special purpose electronic computing device is capable of operating or transforming signals, typically represented as physical electronic or magnetic quantities, within the memory, registers, or other information storage devices, transmission devices, or display devices of the special purpose computer or similar special purpose electronic computing device.

[0152] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the claimed subject matter. However, it will be understood by those skilled in the art that the claimed subject matter may be practiced without these specific details. In other instances, well-known methods and devices have not been described in detail so as not to obscure the claimed subject matter. Accordingly, the claimed subject matter is not limited to the specific examples disclosed, but is intended to cover all aspects included within the scope of the appended claims and their equivalents.

Claims

1. A first photonic integrated circuit, comprising: A qubit encoder configured to receive a spatial mode qubit and convert the spatial mode qubit into a temporal mode qubit; An optical interconnect configured to receive and transmit the temporal mode qubit A first photonic integrated circuit having; and A second photonic integrated circuit, comprising: A qubit decoder configured to receive the temporal mode qubit and convert the temporal mode qubit back into the spatial mode qubit A second photonic integrated circuit having A system comprising.

2. A first control circuit configured to control the qubit encoder; and A second control circuit configured to control the qubit decoder Further comprising, Wherein the second control circuit is synchronized with the first control circuit, the system according to claim 1.

3. The qubit encoder comprises: A dual-rail qubit bus optically coupled to a qubit source, the dual-rail qubit bus having two waveguides, wherein the first waveguide of the dual-rail qubit bus includes a delay line; and An optical switch optically connected to the two waveguides of the dual-rail qubit bus The system according to claim 1 having.

4. The spatial mode qubit is generated by a qubit source, wherein the qubit source includes a clock generator configured to generate a clock signal using a pump laser pulse, the system according to claim 1.

5. The system according to claim 4, further comprising an electrical interconnect channel or an optical interconnect channel configured to transmit the clock signal to the second photonic integrated circuit.

6. The qubit encoder includes a first dual-rail qubit bus including a first set of waveguides, one of the first set of waveguides including a first delay, the qubit decoder includes a second dual-rail qubit bus including a second set of waveguides, one of the second set of waveguides including a second delay, the system according to claim 5.

7. The system according to claim 6, further comprising a phase shifter to compensate for a phase difference between the first delay and the second delay.

8. The phase shifter is within the switch of the qubit encoder in the first photonic integrated circuit, the system according to claim 7. **Claim 9** The spatial mode qubit corresponds to a dual-rail encoded photonic qubit, the system according to any one of claims 1 to 8. **Claim 10** The optical interconnect has an optical fiber, the system according to any one of claims 1 to 8. **Claim 11** The optical interconnect is a waveguide of an optical interposer coupled to the first photonic integrated circuit and the second photonic integrated circuit, the system according to any one of claims 1 to 8. **Claim 12** A photonic integrated circuit device, A dual-rail qubit bus optically coupled to a photonic qubit source, the dual-rail qubit bus having two waveguides, wherein a first waveguide of the dual-rail qubit bus includes a waveguide delay line disposed in a first layer of the photonic integrated circuit device; An optical switch disposed in a second layer of the photonic integrated circuit device and optically connected to the two waveguides of the dual-rail qubit bus; and A control circuit configured to switch the optical switch to optically connect output portions of the two waveguides to a single output waveguide of the photonic integrated circuit device at different times, wherein the control circuit is synchronized with the photonic qubit source A photonic integrated circuit device comprising. **Claim 13** The photonic qubit source includes a clock generator configured to generate a clock signal using a pump laser pulse, the photonic integrated circuit device according to claim 12. **Claim 14** The waveguide delay line is characterized by a delay equal to or less than half of the clock period of the clock signal, the photonic integrated circuit device according to claim 13. **Claim 15** The waveguide delay line is characterized by a delay between 25 ps and 500 ps, the photonic integrated circuit device according to claim 12. **Claim 16** The control circuit is controlled by a feedforward control signal generated by the photonic qubit source when a pair of photons is generated, the photonic integrated circuit device according to claim 12. **Claim 17** The photonic integrated circuit device according to any one of claims 12 to 16, wherein the optical switch includes a Mach-Zehnder interferometer.

18. The photonic integrated circuit device according to any one of claims 12 to 16, wherein the optical switch is controlled by a clock signal characterized by a frequency that is at least twice the clock frequency of the photonic qubit source.

19. Converting a spatial mode qubit to a temporal mode qubit using the optical delay and switch of a first photonic integrated circuit; Transmitting the temporal mode qubit from the first photonic integrated circuit to a second photonic integrated circuit over an optical interconnect, the optical interconnect being coupled to the first photonic integrated circuit and the second photonic integrated circuit; and Converting the temporal mode qubit back to the spatial mode qubit on the second photonic integrated circuit A method comprising:

20. The method according to claim 19, wherein the optical interconnect includes an optical fiber.

Images