Quantum key distribution network

Abstract

To provide a Quantum Key Distribution (QKD) system having a primary node and at least two secondary nodes, which enables the secondary nodes to be smaller and less costly than an off-chip laser and to increase production yields.SOLUTION: In a QKD network 200, a primary node (central node) 202 includes a primary laser (master laser) 204 and an interference unit (measurement device) 206. A plurality of secondary nodes (users) 208 include a secondary laser 210 implemented on a photonic integrated circuit (PIC) and an encoder 212 that encodes a plurality of quantum states on light emitted by the secondary laser. The secondary laser is injected by light from the primary laser in a light injection synchronization manner, and the interference unit performs a plurality of single photon interference measurements on the light received from the secondary laser.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The embodiments described in this specification generally relate to quantum key distribution.

Background Art

[0002] In a quantum communication system, information is encoded into a quantum system and sent between a transmitter and a receiver. Using quantum effects such as superposition and entanglement, a secret key can be distributed and teleportation and other novel tasks can be performed.

[0003] Quantum key distribution (QKD) is a technique that typically results in the sharing of a cryptographic key between a transmitter, often called "Alice," and a receiver, often called "Bob." The attraction of this technique is that it typically provides a test as to whether any part of the key could be known to an unauthorized eavesdropper, often called "Eve." In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases for encoding bit values. According to the laws of quantum mechanics, if Eve measures photons without prior knowledge of each encoding basis, it is said to inevitably cause a change in the state of some of the photons. These changes in the state of the photons will result in errors in the bit values sent between Alice and Bob. Therefore, by comparing a portion of their common bit string, Alice and Bob can determine whether Eve has obtained information.

[0004] MDI protocols such as Measurement Device Independent (MDI)-QKD and Twin Field QKD (TF-QKD) have been developed. In these protocols, Alice and Bob prepare photons and send them to a measurement unit hosted by Charlie. Charlie performs an interference measurement between the photons received from Alice and Bob and publishes the results. Alice and Bob then publish the measurement basis they used, not the state they sent. This means that since Alice knows the result of the interference measurement and the state she sent to Charlie, when the basis matches, Alice knows what state Bob sent. Similarly, Bob can establish the state prepared by Alice.

[0005] Optical injection locking (OIL) is a widely used technique in laser systems, including in fields such as quantum communications. OIL typically involves a primary and a secondary laser. For example, light from the primary laser may be used to define the phase between pulses output by the secondary laser. [Brief explanation of the drawing]

[0006] [Figure 1] Figure 1 shows a schematic diagram of a single-photon interference measurement. [Figure 2] Figure 2 shows a quantum key distribution (QKD) system according to one embodiment. [Figure 3] Figures 3A to 3D show photonic integrated circuits (PICs) that may be used in the system shown in Figure 2. [Figure 4] Figure 4 shows additional PICs that may be used in the system shown in Figure 2. [Figure 5] Figure 5 shows a further QKD system according to one embodiment. [Figure 6] Figure 6 shows a flowchart of the QKD method. [Modes for carrying out the invention]

[0007] In one embodiment, a quantum key distribution (QKD) system is provided, comprising a primary node having a primary laser and an interference unit, and two secondary nodes, each having a photonic integrated circuit (PIC), each PIC comprising a secondary laser configured to be injected by light from the primary laser, each secondary node further comprising an encoder configured to encode a plurality of quantum states on the light emitted by the secondary laser, and the interference unit configured to perform a plurality of single-photon interference measurements on the light received from the plurality of secondary lasers.

[0008] By using an on-chip secondary laser (i.e., a secondary laser implemented on the PIC), the secondary node can be made smaller and less expensive compared to an off-chip laser, while also increasing production yield.

[0009] By using optical injection locking (OIL) between the primary and secondary lasers, global phase coherence across the system (i.e., phase coherence between the primary laser and each secondary laser, and ultimately between all secondary lasers) becomes possible. This is generally a requirement for implementing twin-field QKD (TF-QKD), as described below.

[0010] Furthermore, OIL synchronizes the wavelength of each secondary laser to the wavelength of the primary laser. This is particularly beneficial considering that precisely tuning the wavelength of on-chip lasers is often difficult.

[0011] If the primary laser has a narrower linewidth than the secondary laser, OIL can be used to narrow the linewidth of the secondary laser. This is particularly advantageous because on-chip lasers often have wide linewidths. This avoids the drawbacks of a wide secondary laser linewidth and also avoids the need to use a dedicated and / or expensive narrow-linewidth laser for the secondary laser, thereby reducing the cost and complexity of the system.

[0012] On-chip lasers often also have prominent side modes that are suppressed by oil.

[0013] OIL also avoids the need for phase-locked loops, which would increase the overall cost and size of the system and require additional hardware such as a reference oscillator and control system.

[0014] In one embodiment, each PIC may further include an input port and an output port, and each secondary laser includes an input port connected to the input port of the PIC and an output port connected to the output port of the PIC.

[0015] This configuration has the advantage of blocking most of the light from the primary laser entering the input port with the secondary laser, thereby limiting the amount of primary laser light that reaches the PIC output and is emitted along with the secondary laser output.

[0016] In one embodiment, each PIC may further include an input port, an output port, and a splitter, the splitter being coupled to the input port, the output port, and the respective secondary laser ports.

[0017] This configuration allows for a more compact secondary node by including the splitter on the PIC and avoiding the need for an external splitter.

[0018] In one embodiment, each PIC may further include an input port, an output port, and a circulator having three ports, wherein the first port of the circulator is coupled to the input port, the second port of the circulator is coupled to the port of each secondary laser, and the third port of the circulator is coupled to the output port.

[0019] This configuration includes a circulator on the PIC, enabling the secondary nodes to be more compact by avoiding the need for an external circulator.

[0020] In one embodiment, each PIC may further include ports coupled to the ports of respective secondary lasers, and each secondary node may further include a beam combining component coupled to the ports of the PIC.

[0021] In one embodiment, one of the plurality of encoders may include an intensity modulator.

[0022] This enables intensity modulation to encode the quantum state into portions (e.g., pulses) of the light emitted from each secondary laser.

[0023] In one embodiment, one of the plurality of encoders may include a phase modulator.

[0024] This enables phase modulation to encode the quantum state into portions (e.g., pulses) of the light emitted from each secondary laser.

[0025] In one embodiment, each encoder may be included on the PIC of each secondary node.

[0026] [[ID=²7]] This enables a more compact secondary node in which the secondary laser and associated encoder are included on a single PIC.

[0027] In one embodiment, one of the plurality of PICs may further include a variable optical attenuator, a wavelength division multiplexer or demultiplexer, an optical filter, or a polarizer.

[0028] This allows for general operations (e.g., intensity control, multiplexing, filtering, polarization control, and / or polarization-based beam splitting) while maintaining a compact secondary node, by including additional components on the same PIC as the secondary laser, rather than adding additional components separately (i.e., outside the PIC).

[0029] In one embodiment, the interference unit may include a beam splitter configured to receive light from each of two secondary lasers, with two photodetectors coupled to multiple output ports of the beam splitter, so that a detection event in one of the two photodetectors indicates a single-photon interference measurement.

[0030] This makes it possible to implement single-photon interference measurements in a manner that generally matches Figure 1, as described below, for use in applications such as TF-QKD.

[0031] In a further embodiment, a quantum key distribution (QKD) method is provided, comprising: emitting light from a primary laser; receiving the emitted light in each of two secondary lasers, where each secondary laser is on a photonic integrated circuit (PIC), and using the received light for optical injection synchronization (OIL) of each secondary laser; emitting further light from each secondary laser; encoding a plurality of quantum states onto the further light; and performing a single-photon interference measurement on the further light.

[0032] This method offers the advantages of the on-chip secondary laser and OIL described above.

[0033] In one embodiment, encoding multiple quantum states onto further light may involve modulating the intensity of a portion of that further light.

[0034] In one embodiment, encoding multiple quantum states onto further light may involve modulating the phase of a portion of that further light.

[0035] In one embodiment, performing single-photon interference measurements on additional light may involve receiving light from each of two secondary lasers, combining the light received from each secondary laser in a beam splitter, and recording (registering) the detection events in a photodetector coupled to the output port of the beam splitter.

[0036] This makes it possible to implement single-photon interference measurements in a manner that generally matches Figure 1, as described below, for use in applications such as TF-QKD.

[0037] Figure 1 is a schematic diagram of the single-photon interference measurement 100.

[0038] At this point, both Alice 102 and Bob 104 send Charlie a single signal pulse containing the following three random pieces of information: Bit information (α) Base information (β) Random phase (ρ)

[0039] The results of interference measurements in the interference unit 106 are detected using detectors 108 and 110. If they are twin detectors, the detection results depend on the bit information sent by Alice and Bob and are as shown below. As shown in the figure, the interference unit 106 may include a beam splitter.

[0040] Bit information matches: Detected by "Detector 1" 108. Bit information does not match: Detected by "Detector 2" 110. A classical channel is used to publish the results of which detector detected the twin.

[0041] Next, Alice and Bob use classical channels to share information about the basis and random phases they have chosen to prepare their states. Alice and Bob keep the data they have determined to be twins and discard all other data. This allows Alice and Bob to retrieve each other's undisclosed bit information.

[0042] [Table 1]

[0043] Therefore, Charlie's interference measurement makes it possible for Alice and Bob to determine each other's states when their bases match.

[0044] Figure 2 shows a quantum key distribution (QKD) network 200. The network 200 comprises a primary node 202 (hereinafter referred to as the central node) and two or more secondary nodes 208 (hereinafter referred to as users). Although Figure 2 shows six secondary nodes 208, the network 200 may be implemented using any number of secondary nodes 208, as long as there are at least two.

[0045] The primary node 202 comprises a primary laser 204 (hereinafter referred to as the master laser) and an interference unit 206 (hereinafter referred to as the measurement device). The interference unit 206 is configured to perform a single-photon interference measurement, which can be performed, for example, as described above with respect to Figure 1. In this case, the interference unit may comprise, for example, a beam splitter and two photodetectors in a functionally similar arrangement to that shown in Figure 1.

[0046] Each secondary node 208 comprises a secondary laser 210 and an encoder 212. Each encoder 212 is configured to encode quantum states on light emitted from the secondary laser 210 located in the same secondary node 208 as the encoder 212 (i.e., each secondary laser 210). This encoding may involve, for example, phase modulation and / or intensity modulation.

[0047] The secondary laser 210 is mounted on a PIC, which is referred to herein as a secondary photonic integrated circuit (PIC). Each encoder 212 may be mounted on the same secondary PIC as its respective secondary laser 210, or it may be separate and not mounted on a PIC.

[0048] The primary laser 204 may optionally be implemented on the PIC, also referred to herein as the primary PIC. Alternatively, the primary laser 204 may be off-chip (i.e., not on the PIC).

[0049] Each secondary laser 210 is configured to be injected using light from the primary laser 204 via optical injection locking (OIL). This enables global phase coherence between the primary laser 204 and each secondary laser 210 (and thus between all secondary lasers 210), which is a requirement for implementing twin-field quantum key distribution (TF-QKD). This process is sometimes referred to as phase dissemination.

[0050] Some of the secondary lasers 210 may be, for example, distributed-feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers. Many other laser technologies may be used for the secondary lasers 210, and it will be understood that there may be mixed laser technologies among the secondary lasers.

[0051] If the primary laser 204 has a narrower linewidth than the secondary laser 210, the use of OIL makes it possible to narrow the secondary laser linewidth as described above. Therefore, in order to take advantage of this effect, in many applications, the primary laser 204 may be a narrow-linewidth laser, and in particular, it may have a narrower linewidth than the secondary laser 210. For example, this is often the case when the primary laser 204 is off-chip, because on-chip lasers (such as the secondary laser 210) generally have a wider linewidth than off-chip lasers.

[0052] During operation, the master laser 204 emits light, at least a portion of which is transported to each secondary laser 210 shown in Figure 2. For example, the emitted light can travel in free space or through optical fibers. This light is injected into each secondary laser 210, synchronizing them with the primary laser 204 as described above. Each secondary laser 210 then emits light, which can be modulated by the corresponding encoder 212 to carry quantum information. At least a portion of this light then travels again, for example, in free space or through optical fibers, to the interference unit 206. The interference unit 206 performs single-photon interference measurements on the received light. Generally speaking, these single-photon interference events may include fields emitted by the pair of secondary lasers 210, which result in an entangled state between the secondary nodes 208.

[0053] This operation can generally be seen as implementing the TF-QKD protocol, enabling the sharing of quantum information between secondary nodes 208. Referring to the above description in Figure 1, Alice 102 and Bob 104 can be seen as corresponding to any two secondary nodes 208, and Charlie can be seen as corresponding to a primary node 202.

[0054] Figures 3A to 3D show possible implementations of the secondary PIC described above with reference to Figure 2. In each case, the PIC300 comprises a substrate 302 and a secondary laser 210. The substrate may be indium phosphide (InP). Furthermore or alternatively, the substrate may be any other material suitable for mounting the PIC, such as silicon (Si) or silicon nitride (SiN). As shown, the PIC300 is generally assumed to further comprise waveguides placed on the surface of the substrate 302 that are suitable for transporting light to the components contained on the PIC300.

[0055] Figure 3A shows a configuration sometimes called transmission seeding. In this embodiment, the PIC300 has an input port 306 and an output port 308. The secondary laser 210 also has an input port 310 and an output port 312. A waveguide is used to connect the input port 306 of the PIC300 to the input port 310 of the secondary laser 210, and further to connect the output port 312 of the secondary laser 210 to the output port 308 of the PIC300.

[0056] During operation, light from the primary laser 204 can arrive at the input port 306 of the PIC300 via free space, optical fiber, or any other means, and be transported across the substrate 302 to the input port 310 of the secondary laser 210. This light can then be injected into the secondary laser 210 by oil. The secondary laser 210 can then emit further light from its output port 312. This further light can travel to the output port 308 of the PIC300, from where it can travel to the interference unit 206 for measurement.

[0057] In this embodiment, it is generally assumed that the secondary laser 210 can substantially prevent the light arriving at the input port 306 of the PIC300 from reaching the output port 308 of the PIC300.

[0058] Figure 3B shows a configuration sometimes called reflection seeding. In this embodiment, the PIC300 has an input port 306 and an output port 308. The secondary laser 210 has a port 314 that serves as both an input and an output. Waveguides from the input port 306 and output port 308 of the PIC300 converge at a splitter 304, which is further connected to port 314 of the secondary laser 210.

[0059] During operation, light from the primary laser 204 arrives at the input port 306 of the PIC300 via free space, optical fiber, or any other means, and can be transported across the substrate 302 to the port 314 of the secondary laser 210 by reflection from the splitter 304. This light can be injected into the secondary laser 210 by OIL. The secondary laser 210 can then emit further light from its port 314. This further light can travel to the output port 308 of the PIC300 by reflection from the splitter 304, and from there to the interference unit 206 for measurement.

[0060] In this embodiment, it is generally assumed that some of the light arriving at the input port 306 of the PIC300 may arrive directly at the output port 308 of the PIC300 via reflection at the splitter 304 without ever reaching the secondary laser 210, and thus may be present (perhaps in small amounts) in further light emitted from the output port 308 of the PIC300.

[0061] Figure 3C shows a further reflective seeding configuration. In this embodiment, the PIC300 includes an input port 306 and an output port 308. The secondary laser 210 includes a port 314 (referred to herein as the input / output port) that serves as both an input and an output. Waveguides from the input port 306 and output port 308 of the PIC300 converge in a circulator 316 mounted on the PIC300.

[0062] The circulator 316 is generally assumed to have three ports, as shown in Figure 3C. In this case, the operation of the circulator 316 may be such that light entering the first port is transported to the second port, light entering the second port is transported to the third port, and light entering the third port is transported to the first port. The port numbering is arbitrary, and this mode of operation applies regardless of how the ports are numbered.

[0063] In the embodiment shown in Figure 3C, it can be considered that the first port of the circulator 316 is connected to the input port 306 of the PIC300, the second port of the circulator 316 is connected to the port 314 of the secondary laser 210, and the third port of the circulator 316 is connected to the output port 308 of the PIC300. This makes it possible to transport light from the input port 306 of the PIC300 to the port 314 of the secondary laser 210 for injection into the secondary laser 210, and to transport further light emitted from the port 314 of the secondary laser 210 to the output port 308 of the PIC300. On the other hand, due to the properties of the circulator 316, it is expected that the direct passage of light from the input port 306 of the PIC300 to the output port 308 of the PIC300 will be minimized.

[0064] During operation, light from the primary laser 204 arrives at the input port 306 of the PIC300 via free space, optical fiber, or any other means, and can be transported across the substrate 302 to the port 314 of the secondary laser 210 via the circulator 316. This light can be injected into the secondary laser 210 by oil. The secondary laser 210 can then emit further light from its port 314. This further light can travel via the circulator 316 to the output port 308 of the PIC300, from where it can travel to the interference unit 206 for measurement.

[0065] Figure 3D shows a further reflective seeding arrangement configuration. In this embodiment, the PIC300 has a port 318 (referred to herein as an input / output port) that functions as both an input and an output, and the secondary laser 210 also has an input / output port 314. Furthermore, it is assumed that the secondary node 208 housing the PIC300 further includes a device (collectively referred to as a beam combining component) capable of splitting and / or combining the laser beam, coupled to the port 318 of the PIC300. For example, the beam combining component may comprise one or more of a splitter, a circulator, a polarizer, or a polarizing beam splitter.

[0066] During operation, light from the primary laser 204 can arrive at port 318 of the PIC300 via a beam combining component through free space, optical fiber, or any other means, and be transported across the substrate 302 to port 314 of the secondary laser 210. This light can then be injected into the secondary laser 210 by OIL. The secondary laser 210 can then emit further light from its port 314. This further light can travel to port 318 of the PIC300 and from there to the interference unit 206 for measurement via the beam combining component.

[0067] It will be understood that not all secondary nodes 208 within network 200 need to have the same design. In particular, the embodiments shown in Figures 3A, 3B, 3C, and 3D can be freely combined within a single network 200.

[0068] Figure 4 shows a further PIC400 that can be used as a secondary PIC. The PIC400 in Figure 4 comprises a circuit board 402, a secondary laser 210, two variable optical attenuators (VOAs) 406, an intensity modulator 408, and a phase modulator 410, all connected in sequence.

[0069] PIC400 is intended to illustrate various further components that may be included on a secondary PIC to provide useful functionality. In particular, it should be noted that the intensity modulator 408 and the phase modulator 410 can function as the encoder 212 described above with reference to Figure 2, and as a result, in the embodiment of Figure 4, the encoder 212 is included on the secondary PIC400 and does not need to be provided separately.

[0070] The exact component arrangement shown in Figure 4 is illustrative and not intended to be limiting. Generally speaking, it is assumed that the secondary PIC400 may, in addition to the secondary laser 210, comprise one or more of the following in any order: intensity modulator, phase modulator, VOA, wavelength division multiplexer (WDM), wavelength division demultiplexer (de-WDM), optical filter, polarizer, polarization controller, and / or polarization beam splitter. For specific applications, any or all of these components may be usefully included on the secondary PIC400 in various combinations and orders. Furthermore or alternatively, all or any one or more of these components may be included in the secondary node 208 in any order, separately from the secondary PIC400, and coupled to the ports of the secondary node 208.

[0071] Figure 5 shows a QKD network 500 similar to the QKD network 200 in Figure 2, but the encoder 212 of each secondary node 502 is not implemented separately, but rather is included on each secondary PIC 504 (also called the transmitter photonic chip in this case). For example, the secondary PIC 504 in Figure 5 may be similar to the secondary PIC 400 shown in Figure 4, which includes an intensity modulator 408 and a phase modulator 410 suitable for acting as the encoder 212, as described above. Alternatively, the secondary PIC 504 in Figure 5 may have an arrangement configuration of any other components including a secondary laser 210 and an encoder 212.

[0072] Figure 6 is a flowchart showing a QKD method 600 according to one embodiment. This method can be implemented, for example, within the QKD networks 200 and 500 described above with reference to Figures 2 and 5.

[0073] In step S602, the method may include emitting light from a primary laser. For example, this could be the primary laser 204 shown in Figures 2 and 5.

[0074] In step S604, the method may include receiving the emitted light in each of two secondary lasers, each of which is located on the PIC. For example, these could be the secondary lasers 210 shown in Figures 2 and 5.

[0075] It is understood that there may be more than two secondary lasers, and in fact, it is assumed that Method 600 can be implemented for any number of secondary lasers, generally at least two, to enable TF-QKD.

[0076] In step S606, the method may include using the received light for the OIL of each secondary laser.

[0077] In step S608, the method may include emitting additional light from each secondary laser. Due to the OIL in step S606, the additional light may generally be coherent with and have the same wavelength as the light received from the primary laser.

[0078] In step S610, the method may include encoding the quantum state onto further light. For example, this can be achieved using an encoder 212 of the type described above, with reference to Figure 2. There may be one encoder for each secondary laser, which may or may not be implemented on the same secondary PIC as each secondary laser.

[0079] In particular, encoding a quantum state onto further light may involve modulating the phase and / or intensity of the further light portion. For example, if a secondary laser is operating in pulse mode, or if it is a continuous wave (CW) but the emitted light is split into multiple pulses by a pulse carver, it can be assumed that each successive pulse is modulated separately in intensity and / or phase.

[0080] Intensity modulation can be achieved, for example, by one or more intensity modulators included in the encoder 212 in Figure 2. Correspondingly, phase modulation can be achieved, for example, by one or more phase modulators included in the encoder 212 in Figure 2.

[0081] In step S612, the method may include performing a single-photon interference measurement with respect to further light. For example, this may be done in the interference unit 206 described above with reference to Figures 2 and 5.

[0082] In particular, single-photon interference measurements can be performed using a method that broadly matches that described above with reference to Figure 1. That is, performing a single-photon interference measurement may involve receiving light from each of two secondary lasers, combining the light received from each secondary laser in a beam splitter, and recording the detected event in a photodetector coupled to the output port of the beam splitter.

[0083] In embodiments with more than two secondary lasers, it is generally assumed that single-photon interference events are recorded as a result of the synthesis of fields emitted by various pairs of secondary lasers, thereby spreading entanglement across the QKD network. This can generally be considered as an implementation of the TF-QKD described above.

[0084] While specific embodiments have been described, these embodiments are presented only as examples and are not intended to limit the scope of the invention. The novel devices and methods described herein can be embodied in a variety of other forms, and furthermore, various omissions, substitutions, and modifications of the forms of devices, methods, and products described herein can be made without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover forms or variations included in the scope and spirit of the invention.

Claims

1. A quantum key distribution (QKD) system, A primary node equipped with a primary laser and an interference unit, Two secondary nodes, each of which is equipped with a photonic integrated circuit (PIC), Each PIC comprises a secondary laser configured to be injected by light from the primary laser, and each secondary node further comprises an encoder configured to encode a plurality of quantum states on the light emitted by the secondary laser. The aforementioned interference unit is configured to perform multiple single-photon interference measurements on light received from multiple secondary lasers, and is a QKD system.

2. The QKD system according to claim 1, wherein each PIC further comprises an input port and an output port, and each secondary laser comprises an input port connected to the input port of the PIC and an output port connected to the output port of the PIC.

3. The QKD system according to claim 1, wherein each PIC further comprises an input port, an output port, and a splitter, the splitter being coupled to the input port, the output port, and the ports of the respective secondary lasers.

4. Each PIC further includes an input port, an output port, and a circulator with three ports, and further, The first port of the circulator is coupled to the input port, The second port of the circulator is coupled to the ports of each secondary laser, The third port of the circulator is coupled to the output port. The QKD system according to claim 1.

5. The QKD system according to claim 1, wherein each PIC further comprises a port coupled to the port of its respective secondary laser, and each secondary node further comprises a beam combining component coupled to the port of the PIC.

6. The QKD system according to claim 1, wherein one of the multiple encoders is equipped with an intensity modulator.

7. The QKD system according to claim 1, wherein one of the multiple encoders is a phase modulator.

8. The QKD system according to claim 1, wherein each encoder is included on the PIC of each secondary node.

9. The QKD system according to claim 1, wherein one of the multiple PICs further comprises a variable optical attenuator, a wavelength division multiplexer or demultiplexer, an optical filter, or a polarizer.

10. The QKD system according to claim 1, wherein the interference unit comprises a beam splitter configured to receive light from each of two secondary lasers, and two photodetectors are coupled to a plurality of output ports of the beam splitter, so that a detection event in one of the two photodetectors indicates a single-photon interference measurement.

11. A method of quantum key distribution (QKD), Emitting light from the primary laser, The emitted light is received in each of the two secondary lasers, where each secondary laser is located on a photonic integrated circuit (PIC). The received light is used for optical injection synchronization (OIL) of each secondary laser, Further light is emitted from each secondary laser, Encoding multiple quantum states onto the further light, Performing single-photon interference measurements on the aforementioned additional light, A method that includes [a certain feature].

12. The method according to claim 11, wherein encoding a plurality of quantum states onto the further light comprises modulating the intensity of a portion of the further light.

13. The method according to claim 11, wherein encoding a plurality of quantum states onto the further light comprises modulating the phase of a portion of the further light.

14. Performing single-photon interference measurements on the aforementioned additional light means that Receiving light from each of the two secondary lasers, The light received from each secondary laser is combined in a beam splitter, The detection event is recorded in the photodetector coupled to the output port of the beam splitter, The method according to claim 11, comprising:

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