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] FIELD OF THE INVENTION The embodiments described herein relate generally to quantum key distribution. [Background technology]

[0002] In quantum communication systems, information is encoded into a quantum system and sent between a transmitter and a receiver. Quantum effects such as superposition and entanglement can be used to distribute secret keys, perform teleportation, and other exotic tasks.

[0003] Quantum key distribution (QKD) is a technique that results in the sharing of a cryptographic key between two parties: a transmitter, often referred to as "Alice," and a receiver, often referred to as "Bob." The appeal of this technique is that it provides a test for whether any part of the key could be known to an unauthorized eavesdropper, often referred to as "Eve." In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases for encoding bit values. The laws of quantum mechanics dictate that if Eve measures photons without prior knowledge of each encoding base, this will inevitably cause some photons to change state. These photon state changes will introduce errors into the bit values ​​sent between Alice and Bob. Therefore, by comparing portions of their common bit strings, Alice and Bob can determine whether Eve has obtained the 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 interferometric measurement between the photons received from Alice and Bob and publishes the results. Alice and Bob then publish the measurement basis they used, rather than the states they sent. This means that because Alice knows the results of the interferometric measurement and the states she sent to Charlie, when the bases match, Alice knows what state Bob sent. Similarly, Bob can establish the states prepared by Alice.

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

[0006] [Figure 1] Figure 1 shows a schematic diagram of single-photon interferometry. [Figure 2] FIG. 2 illustrates a quantum key distribution (QKD) system according to one embodiment. [Figure 3] 3A-3D show a photonic integrated circuit (PIC) that may be used in the system of FIG. [Figure 4] FIG. 4 shows a further PIC that may be used in the system of FIG. [Figure 5] FIG. 5 shows a further QKD system according to an embodiment. [Figure 6] FIG. 6 shows a flow chart of the QKD method. DETAILED DESCRIPTION OF THE INVENTION

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

[0008] The use of an on-chip secondary laser (i.e., a secondary laser mounted on a PIC) allows the secondary node to be smaller and less expensive than an off-chip laser, while also allowing for higher production yields.

[0009] The use of optical injection locking (OIL) between the primary and secondary lasers enables global phase coherence across the system (i.e., phase coherence between the primary and each secondary laser, and therefore among all secondary lasers), which is generally a prerequisite for implementing twin-field QKD (TF-QKD), as described below.

[0010] Additionally, OIL synchronizes the wavelength of each secondary laser to that of the primary laser, which is particularly beneficial considering that it is often difficult to precisely tune the wavelength of on-chip lasers.

[0011] In cases where the primary laser has a narrower linewidth than the secondary laser, the OIL also allows for a narrower linewidth of the secondary laser. This is advantageous, especially since 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 system cost and complexity.

[0012] On-chip lasers also often have significant side modes that are suppressed by the OIL.

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

[0014] In one embodiment, each PIC may further comprise 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.

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

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

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

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

[0019] This configuration allows the secondary node to be more compact by including the circulator on the PIC and avoiding the need for an external circulator.

[0020] In one embodiment, each PIC may further comprise a port coupled to a port of a respective secondary laser, and each secondary node further comprises a beam combining component coupled to a port of the PIC.

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

[0022] This allows for intensity modulation to encode quantum states into portions (eg, pulses) of light emitted from each secondary laser.

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

[0024] This allows for phase modulation to encode a quantum state into the portion (eg, pulse) of light emitted from each secondary laser.

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

[0026] This allows for a more compact secondary node where the secondary laser and associated encoder are contained on a single PIC.

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

[0028] This allows for general operation (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 providing the additional components separately (i.e., off the PIC).

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

[0030] This allows for the implementation of single-photon interferometry, for example for use in TF-QKD, in a manner generally consistent with FIG. 1 described below.

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

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

[0033] In an embodiment, encoding the plurality of quantum states onto the further light may comprise modulating the intensity of a portion of the further light.

[0034] In an embodiment, encoding the plurality of quantum states onto the further light may comprise modulating the phase of a portion of the further light.

[0035] In one embodiment, performing single-photon interferometry on the further light may comprise receiving light from each of two secondary lasers, combining the received light from each secondary laser in a beam splitter, and registering detection events in a photodetector coupled to an output port of the beam splitter.

[0036] This allows for the implementation of single-photon interferometry, for example for use in TF-QKD, in a manner generally consistent with FIG. 1 described below.

[0037] FIG. 1 is a schematic diagram of a single-photon interferometry measurement 100 .

[0038] Now, both Alice 102 and Bob 104 send a single signal pulse to Charlie containing three random items of information: Bit information (α) Base information (β) Random phase (ρ)

[0039] The results of the interference measurements in the interference unit 106 are detected using detectors 108 and 110. If they are twins, the detection results depend on the bit information sent by Alice and Bob, as shown below: As shown, 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 . The classical channel is used to announce the result of which detector detected the twin.

[0041] Then, Alice and Bob use the classical channel to share information about the basis and random phase they chose to prepare their states. They keep the data they determine to be twins and discard all other data. This allows Alice and Bob to obtain each other's undisclosed bits of information.

[0042] [Table 1]

[0043] Thus, Charlie's interference measurements allow Alice and Bob to determine each other's states if their bases match.

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

[0045] The primary node 202 comprises a primary laser 204 (also referred to herein as a master laser) and an interferometry unit 206 (also referred to herein as a measurement device). The interferometry unit 206 is configured to perform single-photon interferometric measurements, which may be performed, for example, as described above with respect to Figure 1. In that case, the interferometry unit may comprise, for example, a beam splitter and two photodetectors in an arrangement functionally similar 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 a quantum state onto light emitted from a secondary laser 210 located in the same secondary node 208 as the encoder 212 (i.e., the respective secondary laser 210). This encoding may involve, for example, phase modulation and / or intensity modulation.

[0047] The secondary lasers 210 are implemented on a photonic integrated circuit (PIC), also referred to herein as a secondary PIC. Each encoder 212 may be implemented on the same secondary PIC as the respective secondary laser 210, or may be separate and not implemented on a PIC.

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

[0049] Each secondary laser 210 is configured to be injected with light from the primary laser 204 via optical injection locking (OIL). This allows for global phase coherence between the primary laser 204 and each secondary laser 210 (and therefore among all secondary lasers 210), which is generally a necessary condition 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. It will be understood that many other laser technologies may be used for the secondary lasers 210, and that there may be a mix of laser technologies among the secondary lasers.

[0051] If the primary laser 204 has a narrower linewidth than the secondary laser 210, the use of an OIL can narrow the secondary laser linewidth as described above. Therefore, to take advantage of this effect, it is envisioned that in many applications the primary laser 204 may be a narrow linewidth laser, and in particular 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 wider linewidths than off-chip lasers.

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

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

[0054] 3A-3D illustrate possible implementations of the secondary PICs described above with reference to FIG. 2. In each case, the PIC 300 comprises a substrate 302 and a secondary laser 210. The substrate may comprise indium phosphide (InP). Additionally or alternatively, the substrate may comprise any other material suitable for mounting a PIC, such as silicon (Si) or silicon nitride (SiN). As shown, the PIC 300 is generally assumed to further comprise a waveguide disposed on the surface of the substrate 302 suitable for carrying light to components included on the PIC 300.

[0055] 3A shows an arrangement sometimes referred to as transmission seeding. In this embodiment, the PIC 300 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 PIC 300 to the input port 310 of the secondary laser 210, and also to connect the output port 312 of the secondary laser 210 to the output port 308 of the PIC 300.

[0056] In operation, light from the primary laser 204 may arrive at the input port 306 of the PIC 300 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 may be injected into the secondary laser 210 by an OIL. The secondary laser 210 may then emit additional light from its output port 312. This additional light may travel to the output port 308 of the PIC 300 and from there to the interferometric unit 206 for measurement.

[0057] In this embodiment, it is generally assumed that the secondary laser 210 can substantially block light arriving at the input port 306 of the PIC 300 from reaching the output port 308 of the PIC 300.

[0058] 3B shows an arrangement sometimes referred to as reflection seeding. In this embodiment, PIC 300 has input port 306 and output port 308. Secondary laser 210 has port 314, which serves as both an input and an output. Waveguides leading from input port 306 and output port 308 of PIC 300 meet at splitter 304, which is further connected to port 314 of secondary laser 210.

[0059] In operation, light from the primary laser 204 arrives at the input port 306 of the PIC 300 via free space, optical fiber, or any other means and may be carried across the substrate 302 to the port 314 of the secondary laser 210 by reflection from the splitter 304. This light may be injected into the secondary laser 210 by the OIL. The secondary laser 210 may then emit additional light from its port 314. This additional light may travel to the output port 308 of the PIC 300 by reflection from the splitter 304, and from there to the interferometer unit 206 for measurement.

[0060] In this embodiment, it is generally assumed that some light arriving at input port 306 of PIC 300 may arrive directly at output port 308 of PIC 300 by reflection at splitter 304 without ever reaching secondary laser 210, and thereby be present (perhaps in small amounts) in the further light emitted from output port 308 of PIC 300.

[0061] 3C shows a further reflective seeding arrangement. In this embodiment, PIC 300 includes input port 306 and output port 308. Secondary laser 210 includes port 314 (also referred to herein as the input / output port) that serves as both an input and an output. Waveguides leading from input port 306 and output port 308 of PIC 300 meet at circulator 316 mounted on PIC 300.

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

[0063] 3C, it can be envisioned that a first port of circulator 316 is connected to input port 306 of PIC 300, a second port of circulator 316 is connected to port 314 of secondary laser 210, and a third port of circulator 316 is connected to output port 308 of PIC 300. This allows light from input port 306 of PIC 300 to be conveyed to port 314 of secondary laser 210 for injection into secondary laser 210, and further light emitted from port 314 of secondary laser 210 to be conveyed to output port 308 of PIC 300. Meanwhile, due to the nature of circulator 316, direct passage of light from input port 306 of PIC 300 to output port 308 of PIC 300 is expected to be minimized.

[0064] In operation, light from the primary laser 204 may arrive at the input port 306 of the PIC 300 via free space, optical fiber, or any other means and be transported across the substrate 302 to the port 314 of the secondary laser 210 via the circulator 316. This light may be injected into the secondary laser 210 by an OIL. The secondary laser 210 may then emit additional light from its port 314. This additional light may travel via the circulator 316 to the output port 308 of the PIC 300, from which it may travel to the interferometer unit 206 for measurement.

[0065] 3D shows a further reflective seeding arrangement. In this embodiment, the PIC 300 includes a port 318 (also referred to herein as an input / output port) that functions as both an input and an output, and the secondary laser 210 also includes an input / output port 314. It is further envisioned that the secondary node 208 housing the PIC 300 further includes a device (collectively referred to as a beam combining component) coupled to the port 318 of the PIC 300 that is capable of splitting and / or combining laser beams. For example, the beam combining component may include one or more of a splitter, a circulator, a polarizer, or a polarizing beam splitter.

[0066] In operation, light from the primary laser 204 may arrive at port 318 of the PIC 300 via free space, optical fiber, or any other means, via the beam combining components, and be transported across the substrate 302 to port 314 of the secondary laser 210. This light may be injected into the secondary laser 210 by an OIL. The secondary laser 210 may then emit additional light from its port 314. This additional light may travel to port 318 of the PIC 300, from where it may travel via the beam combining components to the interferometer unit 206 for measurement.

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

[0068] Figure 4 shows a further PIC 400 that can be used as a secondary PIC, comprising a substrate 402, a secondary laser 210, two variable optical attenuators (VOAs) 406, an intensity modulator 408, and a phase modulator 410, all connected in series.

[0069] PIC 400 is intended to be illustrative of various additional components that may be included on a secondary PIC to provide useful functionality. In particular, it should be noted that intensity modulator 408 and phase modulator 410 can function as encoder 212 described above with reference to Figure 2, and as a result, in the embodiment of Figure 4, encoder 212 is included on secondary PIC 400 and need not be provided separately.

[0070] 4 is illustrative only and is not intended as a limitation. Generally speaking, it is contemplated that the secondary PIC 400 may comprise, in any order, one or more of the following: intensity modulators, phase modulators, VOAs, wavelength division multiplexers (WDMs), wavelength division demultiplexers (de-WDMs), optical filters, polarizers, polarization controllers, and / or polarization beam splitters, in addition to the secondary laser 210. In particular applications, any or all of these components may be usefully included on the secondary PIC 400 in various combinations and orders. Additionally or alternatively, one or more of all or any of these components may be included in the secondary node 208, in any order, separate from the secondary PIC 400, and coupled to ports of the secondary node 208.

[0071] Figure 5 shows a QKD network 500 similar to the QKD network 200 of Figure 2, except that the encoder 212 of each secondary node 502 is included on a respective secondary PIC 504 (also referred to in this case as a transmitter photonic chip) rather than being implemented separately. For example, the secondary PIC 504 of 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 serving as the encoder 212, as described above. Alternatively, the secondary PIC 504 of Figure 5 may comprise any other arrangement of components, including a secondary laser 210 and an encoder 212.

[0072] 6 is a flow chart illustrating an embodiment of a QKD method 600. The method may be implemented, for example, within the QKD networks 200 and 500 described above with reference to FIGS.

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

[0074] In step S604, the method may include receiving the emitted light at each of two secondary lasers, each on a PIC. For example, these may be 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 contemplated that method 600 may 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 comprise emitting additional light from each secondary laser. Due to the OIL of 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 comprise encoding the quantum state onto further light. For example, this may be accomplished 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 the respective secondary laser.

[0079] In particular, encoding the quantum state onto the further light may comprise modulating the phase and / or intensity of a portion of the further light. For example, if the secondary laser is operated in pulsed mode, or if the emitted light is continuous wave (CW) but split into multiple pulses by a pulse carver, it may be envisaged 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 encoder 212 of Figure 2. Correspondingly, phase modulation can be achieved, for example, by one or more phase modulators included in encoder 212 of Figure 2.

[0081] In step S612, the method may comprise performing single-photon interferometry on the further light, which may for example be performed in the interferometry unit 206 described above with reference to Figures 2 and 5.

[0082] In particular, single-photon interferometry may be performed using a method broadly consistent with that described above with reference to Figure 1. That is, performing single-photon interferometry may comprise receiving light from each of two secondary lasers, combining the received light from each secondary laser at a beam splitter, and recording detection events at a photodetector coupled to an output port of the beam splitter.

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

[0084] While specific embodiments have been described, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. The novel devices and methods described herein may be embodied in a variety of other forms, and various omissions, substitutions, and changes in the form of the devices, methods, and products described herein may be made without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover any such forms or modifications that come within the scope and spirit of the invention.

Claims

1. 1. A quantum key distribution (QKD) system, comprising: a primary node comprising a primary laser and an interferometer unit; two secondary nodes, each secondary node comprising a photonic integrated circuit (PIC); each PIC comprising a secondary laser configured to be injected with light from the primary laser, and each secondary node further comprising an encoder configured to encode a plurality of quantum states onto light emitted by the secondary laser; A QKD system, wherein the interferometer unit is configured to perform a plurality of single-photon interferometric measurements on light received from a plurality of secondary lasers.

2. 2. The QKD system of 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. 2. The QKD system of 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 a port of each secondary laser.

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

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

6. The QKD system of claim 1 , wherein one of the plurality of encoders comprises an intensity modulator.

7. The QKD system of claim 1 , wherein one of the plurality of encoders comprises a phase modulator.

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

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

10. 2. The QKD system of 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 multiple output ports of the beam splitter, whereby a detection event at one of the two photodetectors indicates a single-photon interferometry measurement.

11. 1. A method of quantum key distribution (QKD), comprising: emitting light from a primary laser; receiving the emitted light at each of two secondary lasers, wherein each secondary laser is on a photonic integrated circuit (PIC); using the received light for optical injection locking (OIL) of each secondary laser; emitting additional light from each secondary laser; encoding a plurality of quantum states onto said further light; performing single photon interferometry on the further light; and A method comprising:

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

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

14. performing single-photon interferometry on the further light, receiving light from each of two secondary lasers; combining the received light from each secondary laser at a beam splitter; recording a detection event at a photodetector coupled to an output port of the beam splitter; The method of claim 11 , comprising:

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