Quantum communication system, transmitter for quantum communication system, receiver for quantum communication system, and method for controlling quantum communication system

Abstract

To provide a quantum communication system, a transmitter for the quantum communication system, a receiver for the quantum communication system, and a method for controlling the quantum communication system.SOLUTION: In a quantum communication system, a transmitter includes transmitter components, the transmitter components include a pulse radiation source and a modulation unit, and the modulation unit randomly encodes radiation pulses. A receiver includes receiver components, and the receiver components include a demodulator for decoding the randomly encoded pulses and a detector for detecting the decoded pulses. The transmitter and the receiver further include a control unit and an optimization unit, the control unit applies a plurality of control signals defined by a set of control parameters to at least one of the transmitter components and the receiver components, and the optimization unit adjusts the set of control parameters.SELECTED DRAWING: None

Description

[Technical field]

[0001] SUMMARY OF THE DISCLOSURE Embodiments described herein relate to a quantum communication system, a transmitter for a quantum communication system, a receiver for a quantum communication system, and a method for controlling a quantum communication system. [Background technology]

[0002] In quantum communication systems, information is sent between a transmitter and a receiver by single encoded quanta, such as single photons. Each photon carries one bit of information, which can be encoded on a property of the photon, such as its polarization.

[0003] Quantum key distribution (QKD) is a technique that results in the sharing of a cryptographic key between two parties, a transmitter, often called "Alice", and a receiver, often called "Bob". The attraction of this technique is that it provides a test of whether any part of the key may have been learned by 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. 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 ​​transmitted between Alice and Bob. Thus, by comparing parts of their common bit strings, Alice and Bob can determine whether Eve has obtained the information. [Brief description of the drawings]

[0004] [Figure 1] FIG. 1 is a schematic diagram of a point-to-point QKD system. [Diagram 2] FIG. 1 is a schematic diagram of a point-to-point QKD system with control electronics. [Diagram 3] FIG. 1 is a schematic diagram of a QKD emitter with a CW laser. [Figure 4A]FIG. 1 is a schematic diagram of a QKD emitter with a primary laser and a secondary laser seeding arrangement. [Figure 4B] 4 is a schematic diagram of the corresponding drive signals and light outputs. [Figure 5A] FIG. 1 is a schematic diagram of a primary and secondary laser arrangement. [Diagram 5] FIG. 5B is a plot of the optical frequency of the primary laser under small perturbations of the control gain of duration tm, FIG. 5C is a plot of the optical phase trajectories with and without perturbation of the primary laser, and FIG. 5D is a plot of the output pulse of the secondary laser. [Figure 5E] FIG. 2 is a schematic diagram of a gain modulation circuit for driving the primary and secondary lasers. [Figure 5F] 1 is a series of five time-dependent plots, from the top to the bottom: modulation of the primary laser, carrier density of the primary laser, power of the primary laser, modulation of the secondary laser, and power of the secondary laser. [Figure 6] 1 is a flowchart of a method according to an embodiment. [Figure 7] 1 is a point-to-point QKD system according to an embodiment; [Figure 8] FIG. 1 is a diagram for explaining a genetic algorithm. [Figure 9] 1 is a measurement-device independent QKD system according to one embodiment. [Figure 10] 1 is a schematic diagram of an emitter of a QKD system according to one embodiment. [Figure 11] FIG. 2 is a schematic diagram of the drive signals for the primary (master) and secondary (slave) lasers and how they correspond to the optical output of a phase modulator. [Figure 12] FIG. 12A is a map of the experimentally measured QBER of the BB84 QKD protocol as a function of laser detuning frequency and injection ratio, with the box indicating the likely region of optimal operation, and FIG. 12B is an expanded plot of the boxed region. [Figure 13]FIG. 13 is a plot showing the evolution of the optimization over phase coherence for phase-coherent and phase-randomized pulses. The population size for the first three generations is 35 and is reduced to 25 in subsequent generations. The time taken to complete 10 generations is 4 h (50 s per individual evaluation). [Figure 14] Figure 14A is a plot showing the evolution of the optimization for the (top) QBER and the corresponding secure key rate calculated based on the average QBER obtained over five trials, and Figure 14B is the loss function LPR. Figure 14C shows the evolution of the population in the parameter space (only two dimensions are shown). The population size is 60, and the time taken to complete 10 generations is 2.5 hours (14 s per individual evaluation). [Figure 15] 13 is a plot showing the intensity distribution from the interference between two successive pulses seeded by different master laser pulses exhibiting a random relative phase relationship. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] In one embodiment, a quantum communication system is provided, comprising: a transmitter comprising a plurality of transmitter components, the plurality of transmitter components comprising a pulsed radiation source and a modulation unit, the modulation unit configured to randomly encode the plurality of pulses of radiation; a receiver comprising a plurality of receiver components, the plurality of receiver components comprising a demodulator configured to decode the randomly encoded plurality of pulses and a plurality of detectors configured to detect the decoded plurality of pulses; Equipped with The quantum communication system further comprises a control unit and an optimization unit, the control unit configured to apply a plurality of control signals defined by a set of a plurality of control parameters to at least one of the plurality of transmitter components and the plurality of receiver components, and the optimization unit configured to tune the set of a plurality of control parameters; The optimization unit is obtaining a score indicative of a quality of the quantum communication system corresponding to a first set of a plurality of control parameters; estimating a suitable further set of control parameters by an iterative process to obtain an adjusted set of control parameters; The control parameters are set by:

[0006] The above system allows for self-tuning of the hardware of complex QKD systems. Until now, the determination of optimal control parameters and drive signals for all QKD hardware was done manually. While most of this was done during the system design stage based on known hardware specifications, other components need to be adjusted during system operation in response to changes in the environment. The situation is further complicated due to inherent variations between components, i.e., even if one QKD system is manually optimized to perfection, the optimal parameters for a system with similar components may be slightly different. This means that the current manufacturing process for QKD systems involves various manual tasks to fine-tune the settings for each device. Such optimization is a time-consuming task that requires skilled users with expertise in optical elements, electronic circuits, and QKD system design. This requirement is costly in practice and affects manufacturing scalability.

[0007] Furthermore, ongoing research is moving towards the development of new and improved QKD system designs with better performance / fewer components. However, since these benefits are often achieved by precisely exploiting laser dynamics and electrically controlled light-matter interactions, as a result, such "improved" designs (e.g., phase seeding) may actually complicate the manufacturing phase of QKD technology by introducing more manual optimization tasks.

[0008] The task of optimization is extremely complex because the underlying problem involves many coupled variables and nonlinear behavior (e.g., laser dynamics is fundamentally nonlinear). Therefore, optimal performance cannot be found simply by scanning the entire parameter space or by applying a simple feedback loop to each variable in turn (i.e., the problem is not convex). Optimization of the complete system requires a holistic approach that simultaneously considers many variables as well as measures multiple output properties that affect the overall QKD performance.

[0009] In one embodiment, the optimization unit is configured to obtain a score from at least one measurement indicative of the quality of the system. This is a physical measurement of the QKD system. The physical measurement may be derived from measurements normally made in a QKD system, such as the QBER, or may be derived from additional hardware or sensors provided to make additional measurements, such as phase measurements. In one embodiment, the measurement indicative of the quality of the system may be selected from the quantum bit error rate "QBER", the secure bit rate "SBR", phase information of the coded pulses, the count rate of all pulses received at the receiver's detectors, the count rate of the pulses with a predetermined coding, the shape of the received pulses, or the arrival times of the pulses at the detectors.

[0010] In one embodiment, the optimization unit is configured to calculate a score from the quality measures using an objective function.

[0011] In a further embodiment, the plurality of control signals are a plurality of electronic control signals for the pulsed radiation source, comprising at least one of an amplitude of an electronic control signal, a shape of an electronic control signal, and a DC offset of an electronic control signal included in the plurality of electronic control signals. The pulsed radiation source may be provided on a temperature control element, the control unit further providing the control signal to the temperature control element.

[0012] The pulsed radiation source may comprise a primary laser and a secondary laser, where the primary laser provides seeding pulses to the secondary laser, In such a situation, the set of multiple control parameters may comprise control parameters for one or both of the primary laser and the secondary laser.

[0013] In one embodiment, the plurality of control signals are a plurality of electronic control signals for a modulation unit, and comprise at least one of the following: an intensity of an electronic control signal, a shape of the electronic control signal, and a DC offset of the electronic control signal included in the plurality of electronic control signals. In one embodiment, the modulator is a phase modulation unit comprising an interferometer. The transmitter may also have one or more intensity modulators in addition to the modulator used to encode the optical pulse. The control signal for the intensity modulator may also be optimized via the optimization unit. For both the intensity modulator and the phase modulator, the control signal may comprise one or more of the following: an intensity of an electronic control signal, a shape of the electronic control signal, and a DC offset of the electronic control signal.

[0014] In one embodiment, the plurality of control signals are a plurality of electronic control signals for a demodulator, comprising at least one of an electronic control signal strength, an electronic control signal shape, and a DC offset of the electronic control signal included in the plurality of electronic control signals. The demodulator may comprise an interferometer with a phase modulator provided in at least one arm of the demodulator.

[0015] In one embodiment, the plurality of control signals are electronic control signals for the plurality of detectors and comprise at least one of an electronic control signal magnitude, an electronic control signal shape, and a DC offset of the electronic control signal included in the plurality of electronic control signals.

[0016] In a further embodiment, the optimization unit is configured to obtain the adjusted parameters using an evolutionary algorithm. For example, the evolutionary algorithm may be a genetic algorithm. In a further embodiment, the optimization unit is configured to obtain the adjusted parameters using reinforcement learning. In reinforcement learning, the score acts as a reward for the reinforcement learning algorithm. The optimization unit may be embodied in software or hardware. The optimization unit may be a standard computer or may be implemented using hardware acceleration, for example using an FPGA or custom electronic circuit.

[0017] The optimization unit may be provided in one or both of the transmitter and receiver. It is also possible for the transmitter and / or receiver to have their own optimization unit. Thus, in a further embodiment, a transmitter for a quantum communication system is provided, the transmitter comprising a plurality of transmitter components, the plurality of transmitter components comprising a pulsed radiation source and a modulation unit, the modulation unit configured to randomly encode a plurality of pulses of radiation, The transmitter further comprises a control unit and an optimization unit, the control unit configured to apply a plurality of control signals defined by a set of a plurality of control parameters to at least one of the plurality of transmitter components, and the optimization unit configured to adjust the set of a plurality of control parameters; The optimization unit is obtaining a score indicative of a quality of the system corresponding to the first set of the plurality of control parameters; estimating a suitable further set of control parameters by an iterative process to obtain an adjusted set of control parameters; The control parameters are set by:

[0018] In a further embodiment, there is provided a receiver for a quantum communication system, the receiver comprising a plurality of receiver components comprising a demodulator configured to decode the randomly encoded plurality of pulses and a detector configured to detect the decoded plurality of pulses; The receiver further comprises a control unit and an optimization unit, the control unit configured to apply a plurality of control signals defined by a set of a plurality of control parameters to at least one of the plurality of receiver components, and the optimization unit configured to adjust the set of a plurality of control parameters; The optimization unit is obtaining a score indicative of a quality of the system corresponding to the first set of the plurality of control parameters; estimating a suitable further set of control parameters by an iterative process to obtain an adjusted set of control parameters; A plurality of control parameters are set by:

[0019] In a further embodiment, there is provided a method of controlling a quantum communication system, the quantum communication system comprising: a transmitter comprising a plurality of transmitter components, the plurality of transmitter components comprising a pulsed radiation source and a modulation unit, the modulation unit configured to randomly encode the plurality of pulses of radiation; a receiver comprising a plurality of receiver components, the plurality of receiver components comprising a demodulator configured to decode the randomly encoded plurality of pulses and a detector configured to detect the decoded plurality of pulses; Equipped with The method comprises: applying a plurality of control signals defined by a plurality of sets of control parameters to at least one of the plurality of transmitter components and the plurality of receiver components; obtaining a score from at least one measurement indicative of a quality of the system corresponding to the first set of the plurality of control parameters; estimating a suitable further set of control parameters by an iterative process to obtain an adjusted set of control parameters; Equipped with.

[0020] A quantum key distribution (QKD) system is shown in Figure 1. The simplified configuration of Figure 1 comprises a transmitter 1 and a receiver 3 connected by an optical communication channel 5 (e.g. optical fiber or free space).

[0021] Transmitter 1 (often referred to as "Alice") comprises a pulsed emitter 7 and a state encoder 9. Transmitter 1 generates quantum states, which in one embodiment are coherent states formed by the pulsed laser emission of the pulsed emitter 7 and the state encoder 9 (which modulates light with both bit values ​​(0 or 1) in a random basis (e.g., X or Y basis)). Although a variety of different encoding schemes exist (e.g., polarization encoding, time bin encoding, etc.), QKD protocols all rely on fast, high-quality state generation (i.e., negligible error between expected and actual encoded values).

[0022] The encoded light is transmitted along a communication channel 5 where it may experience optical phenomena that change some of the pulse characteristics, e.g., dispersion may cause the pulse to broaden, polarization fluctuations may change the polarization state, channel timing delays may change the expected pulse arrival time, etc.

[0023] At the receiver 3 ("Bob"), the quantum state is measured, which involves using a demodulator (not shown) to randomly select a basis for the measurement (e.g., by applying some unitary operation on the quantum state) and then detecting the signal using a single-photon detector (not shown) to form the quantum state measurement 11. Various detection techniques exist, but they typically rely on the ability to accurately determine the encoded bit value while rejecting noise sources such as detector noise or other noise introduced by the channel.

[0024] The generation, transmission, and measurement of the quantum state are followed by a post-processing stage involving authenticated classical communication between Alice and Bob, in which they reveal a subset of their random choices for sifting, information reconciliation, error correction, and privacy amplification. This results in theoretically secure quantum key distribution of information on both remote nodes, which can then be sent to other devices, for example for data encryption.

[0025] Next, a basic quantum communication protocol using polarized light is described. However, it should be noted that this is not meant to be limiting and other protocols may be used. For simplicity, this protocol refers to a polarization-based protocol, although a phase-based protocol may also be used.

[0026] The protocol uses two bases, where each basis is represented by two orthogonal states, in this example the H / V and D / A bases, however the L / R basis may also be chosen.

[0027] The sender in the protocol prepares a state with one of H, V, D or A polarization. In other words, the prepared state is selected from two orthogonal states (H and V or D and A) in one of two bases H / V and D / A. It can be thought of as sending a signal of 0 and 1 in one of the two bases, for example H=0, V=1 in the H / V base, and D=0, A=1 in the D / A base. The pulse is attenuated to comprise less than one photon on average. Thus, if a measurement is made on the pulse, it is destroyed. It is also not possible to split the pulse.

[0028] The receiver uses a measurement basis for the polarization of the pulse selected from the H / V basis or the D / A basis. The selection of the measurement basis can be active or passive. In passive selection, the basis is selected using a fixed component such as a beam splitter. In "active" basis selection, the receiver uses, for example, a modulator with an electrical control signal to determine in which basis to measure. If the basis used for measuring the pulse at the receiver is the same as the basis used to encode the pulse, the receiver's measurement of the pulse will be accurate. However, if the receiver selects the other basis for measuring the pulse, there will be a 50% error in the result measured by the receiver.

[0029] To establish the key, the sender and receiver compare the basis used for encoding and measurement (decoding). If they match, the result is kept, if they do not match, the result is discarded. The above method is very secure. If an eavesdropper intercepts and measures a pulse, the eavesdropper needs to prepare another pulse to send to the receiver. However, the eavesdropper does not know the correct measurement basis and therefore has only a 50% chance of measuring the pulse correctly. Any pulse recreated by an eavesdropper will result in a large error rate for the receiver, which can be used to prove the presence of an eavesdropper. The sender and receiver compare small parts of the key to determine the error rate (QBER) and therefore the presence of an eavesdropper.

[0030] However, the above description of a QKD system only describes the basic operation. In practice, in addition to the core quantum state generation / encoding / measurement devices, it also includes components to compensate for channel disturbances or non-idealities in real-world components (e.g., polarization adjustment, power adjustment, dispersion compensation, timing compensation, etc.), and also requires precise electronic control of all the optoelectronic hardware to ensure that Alice and Bob (and all their internal hardware) are well synchronized.

[0031] Figure 2 illustrates a QKD system that more explicitly shows these additional components / stages that are important to ensure high performance operation in the presence of real world disturbances. Note that the exact implementation may vary, with compensation occurring at either Alice and / or Bob, and in any order. To avoid unnecessary repetition, the same reference numbers as in Figure 1 have been used where appropriate. Additionally, in Figure 2, a transmitter, control electronics 17, and signal compensation unit 13 are provided. The receiver is provided with control electronics 19 and signal compensation unit 15.

[0032] A wide range of optical / electronic system designs are possible to perform QKD. Figure 3 shows an example of such an approach. Here, the transmitter comprises a continuous wave (CW) laser 21, followed by modulators 23, 25, and 27 to carve out the pulses in time and encode the phase between the pulses. A first intensity modulator 23 is used to chop the CW output into pulses. A phase modulator 25 is then used to encode the pulses. In one embodiment, the phase modulator comprises an asymmetric Mach-Zehnder interferometer (AMZI), with a phase modulation component in an arm of the AMZI. A second intensity modulator 27 is then provided to attenuate the pulses output by the phase modulator 25 into pulses with one photon or less on average. In a further embodiment, the second intensity modulator is also used to implement the decoy states required in many QKD protocols. The decoy states are similar to the other encoding states, but with reduced amplitude (i.e., a smaller average number of photons is used for the decoy pulses). This makes it possible to use decoy states to evade attacks such as photon number splitting attacks.

[0033] In order to achieve well-defined short (e.g. picosecond) pulses, intensity modulators 23 and 27 with high dynamic range are used. From a phase perspective, in one embodiment, QKD requires that each quantum state is phase randomized. If phase encoding is used, where the quantum state is encoded by the phase between two time bins, the phase between the two bins is precisely controlled.

[0034] This places stringent requirements on all modulators 23, 25, and 27, as well as highly coherent and stable emission of light from laser 21. The quality of the output can be improved by precise control of the electronic drive signals applied to modulators 23, 25, 27, and laser 21. For example, the quality of the output can be improved by adjusting the biasing / control of laser 21 and the control of the modulators. The modulators have the additional complication that they are driven by RF signals with the appropriate amplitude and timing to generate a correctly encoded train of quantum states. In some embodiments of the QKD transmitter, the lasers are also driven by RF signals. In the case of a "phase-seeding" arrangement (i.e., a gain-switched primary laser-secondary laser arrangement), both the primary and secondary lasers have RF signals applied that generate light pulses. Additional electrical control signals with many customizable parameters may also need to be adjusted, such as control signals for other signal compensation devices such as polarization controllers, attenuators, etc. (not shown).

[0035] Figure 4A shows an alternative transmitter design using "phase seeding". In a phase-seeded QKD transmitter, multiple modulators replace the primary-secondary laser arrangement (whereby the primary laser is injection-locked to the secondary laser and the current of the primary laser is modulated to impose phase modulation on the secondary laser). This has the advantage of eliminating the modulators, which are typically bulky and expensive and prevent chip-scale integration of the design onto a photonic chip (as lithium niobate-based modulators are not compatible with optical integration). Figure 4B shows the drive scheme for such an arrangement with primary and secondary lasers.

[0036] The above embodiment of Figures 4A and 4B shows an arrangement where one primary pulse seeds two secondary pulses. Figures 5A-5F show some of the physics behind laser seeding. For simplicity, we will describe here the situation where one primary pulse seeds one secondary pulse.

[0037] Figures 5A-5F are used to describe a particular type of optical source that allows for the creation of an ultra-compact, high performance QKD transmitter. This optical source, shown in Figure 5A, uses gain switching and optical injection locking to directly phase modulate the pulses, without the need for an external phase modulator.

[0038] The light source comprises a pulsed secondary laser 103 into which pulses from the primary laser 101 are injected to define the phase between the output pulses of the secondary laser based on optical injection locking. The primary laser 101 performs the task of phase preparation, whereas the secondary laser 103 performs the task of pulse generation. The description of Figures 5A-5D focuses more on the control of the primary laser. The description of Figures 5E and 5F focuses more on the secondary laser and the combination of the two lasers.

[0039] As shown diagrammatically in FIG. 5A, the primary laser diode 101 is connected to the secondary laser diode 103 via an optical circulator 105. Note that a circulator is not required. Further embodiments use different methods to inject light from one laser into another laser depending on the laser type / packaging. For example, FIG. 4A shows direct injection into one side of the laser and the resulting emission output from the other side. Such an arrangement is possible, for example, when the laser has partially reflective facets on both sides of the cavity. Note that the primary laser diode 101 and the secondary laser diode 103 may be identical and the use of the terms "primary" and "secondary" is merely for clarity and does not imply any physical difference between the primary laser diode 101 and the secondary laser diode 103.

[0040] The primary laser 101 is used for phase preparation and is directly modulated to generate long pulses from quasi-steady-state emission. Each of these pulses coherently seeds a block of two or more short secondary optical pulses emitted by gain-switching the secondary or pulse-generating laser 103. The phase-prepared laser 101 is biased to generate quasi-steady-state optical pulses on the nanosecond scale or even shorter with shallow intensity modulation, which also modifies the optical phase. For clock rates above 1 GHz, the pulse width is less than 1 ns. The gain-switched pulse-generating laser 103 emits short optical pulses that take over the optical phase prepared by the phase-prepared laser. The duration of each phase-prepared laser pulse can be varied to seed pulse trains of different lengths.

[0041] The relative phase between the secondary pulses depends on the phase evolution of the primary pulse and can be set to any value by directly modulating the drive current applied to the primary or phase-ready laser 101 .

[0042] For example, the relative phase φ1 between the two secondary pulses can be obtained by introducing a small perturbation in the drive signal of the phase-ready laser in Figure 5A. Similarly, the relative phases between the three secondary pulses can be set to φ1 and φ2 by adding two small perturbations to the drive signal of the primary laser 101.

[0043] In principle, such perturbations in the drive signal would cause harmful variations in the intensity and frequency of the primary pulses. However, this can be avoided by switching off the gain of the secondary laser 103 in response to the perturbation signal. Effectively, the secondary laser 103 also acts as a filter to reject residual modulation.

[0044] To understand how the optical phase is set by perturbing the drive signal applied to a phase-ready laser, it is useful to consider an above-threshold continuous-wave laser emitting at a central frequency ν.

[0045] FIG. 5B shows the duration t m Figure 5C is a plot of the optical frequency of the phase-ready laser under small perturbations of .mu.m. Figure 5D is a plot of the optical phase trajectory with and without perturbation of the phase-ready laser.

[0046] When a small perturbation is applied to the drive signal, the optical frequency shifts by an amount Δν, changing the course of the phase evolution. When the perturbation is switched off, the frequency returns to its initial value ν0. This perturbation will induce a phase difference of Δφ=2πΔνt m Here, t m is the duration of the perturbation. By optical injection, this phase difference is transferred onto the pair of secondary pulses emitted by the pulsed generation laser shown in FIG. 5D.

[0047] Here the perturbation signal is a voltage modulation applied to the phase-prepared laser. The optical frequency change arises from the effect of carrier density on the refractive index of the laser active medium in the primary laser diode 101. The confinement of the laser cavity allows the optical field to oscillate back and forth within the cavity, changing the refractive index for the entire duration of the perturbation. The enhancement due to the laser cavity allows the phase modulation half-wave voltage to be kept below 1V. Conventional phase modulators lack this cavity feature, so in conventional phase modulators the light passes through the electro-optic medium only once, thus limiting the interaction distance to the device length.

[0048] Small changes to the primary light source's electrical controller signal (less than 1 volt, much less than what is required by conventional lithium niobate phase modulators) can cause a temporary change in the output frequency of the primary light source's output, which in turn changes the output phase of the secondary laser's optical output.

[0049] In this embodiment, the primary laser 101 is configured to output a sequence of optical pulses comprising a sequence of pairs. The phases of the pulses output by the primary laser are controlled such that the phase between pulses in the same pair is randomly selected from one of a set of phase differences, resulting in random phase differences between pulses from different pairs. In one embodiment, the set of phase differences may be selected from one of 0, π / 2, -π / 2, and π.

[0050] The secondary laser 103 , seeded by the primary laser, will output a sequence of pairs of pulses having the same phase difference as the sequence of pulses output by the primary laser 101 .

[0051] Pulse injection seeding occurs whenever the secondary laser 103 is switched above the lasing threshold. In this case, the generated secondary light pulse has a fixed phase relationship to the injected primary light pulse. Since only one secondary light pulse is generated for each injected primary light pulse, the phase relationship between the pulses output by the secondary laser is the same as the relationship between the pulses injected into the secondary laser.

[0052] Under operating conditions described below in connection with Figures 5E and 5F, the secondary laser 103 generates a new sequence of pulses comprising a sequence of pairs. The phase between pulses in the same pair is randomly selected from one of a set of phase differences, and there is a random phase difference between pulses from different pairs. These pulses also have a smaller time jitter τ'<τ with respect to the pulses output by the primary laser 101. This reduced jitter time improves the interference visibility due to the low time jitter of the secondary light pulses.

[0053] For pulsed injection seeding to occur, the frequency of the light pulses from the primary laser 101 must match within a certain range the frequency of the secondary laser 103. In one embodiment, the difference between the frequency of the light provided by the primary laser 101 and the frequency of the secondary laser 103 is less than 30 GHz. In some embodiments, when the secondary laser 103 is a distributed feedback (DFB) laser diode, the frequency difference is less than 100 GHz.

[0054] For pulsed injection seeding to be successful, the relative power of the output light pulse of the primary laser 101 entering the optical cavity of the secondary laser 103 must be within certain limits that depend on the type of light source used. In one embodiment, the optical power of the injected light pulse is at least 1000 times lower than the optical output power of the secondary laser 103. In one embodiment, the optical power of the injected light pulse is at least 100 times lower than the optical output power of the secondary laser 103.

[0055] In one embodiment, the secondary laser 103 and the primary laser 101 are electrically driven gain-switched semiconductor laser diodes. In one embodiment, the secondary light source and the primary light source have the same bandwidth. In one embodiment, both light sources have a bandwidth of 10 GHz. In one embodiment, both light sources have a bandwidth of 2.5 GHz. Here, bandwidth refers to the highest bit rate achievable with a directly modulated gain-switched laser diode. Lasers of a certain bandwidth can be operated at a lower clock rate.

[0056] FIG. 5E is a schematic diagram of a drive scheme for a phase-randomized light source 500 in which both the primary laser 503 and the secondary laser 502 are driven with a single gain modulation unit 509. The gain modulation unit 509 and the delay line 510 are an example of a controller configured to apply a time-varying drive signal to the secondary laser 502 such that only one optical pulse is generated during each time period in which an optical pulse is received. The primary laser 503 is connected to the secondary laser 502 via an optical connection 505. The optical connection 505 can be a waveguide, for example, an optical fiber. Alternatively, the optical pulses may travel between the primary laser 503 and the secondary laser 502 through free space. The optical connection may include further components such as an optical circulator or a beam splitter as provided in the arrangement of FIG. 5A.

[0057] The gain modulation unit 509 drives both the primary laser 503 and the secondary laser 502 to generate optical pulses. The delay line 510 is used to synchronize the devices. The delay line may be, for example, a fixed length cable. The gain modulation unit is directly connected to the primary laser 503. For example, if the primary laser 503 is a semiconductor laser, the gain modulation circuit is electrically connected to the primary laser 503. The gain modulation unit 509 is connected to the secondary laser 502 through the delay line 510.

[0058] FIG. 5F shows the time sequence for a single gain modulation scheme shown in FIG. 5E. The top graph shows the gain modulation applied to the primary light source 503. The current applied to the laser is shown on the vertical axis and time on the horizontal axis. The gain modulation is a time-varying drive signal having the form of a square wave, which when applied to the primary light source varies the carrier density above and below the lasing threshold. In other words, the gain modulation is a series of pulses. Between the pulses, the gain has a minimum value, which is the gain bias, shown by the dotted line. The wave in this case is a square waveform. Different gain modulation signals can be used, for example a sinusoid or a non-periodic time-varying signal. In this case, the current is not reduced to zero between the current modulation pulses, but only to the bias value (shown by the dotted line).

[0059] A current modulation signal is applied to the laser to periodically switch the gain of the laser above and below the lasing threshold. The second graph shows the carrier density of the laser on the vertical axis and time on the horizontal axis. The lasing threshold is shown by the dashed horizontal line. When a current modulation pulse is applied to the laser, the injected carriers increase the carrier density, resulting in an increase in photon density.

[0060] The laser output generated by the modulation signal is shown in the graph below. The vertical axis shows the laser intensity and the horizontal axis shows time. The laser outputs light when the carrier density is above the lasing threshold. Photons generated by spontaneous emission inside the laser cavity are sufficiently amplified by stimulated emission to generate the output signal. The length of the delay between the application of the current modulation pulse and the generation of the output light depends on several parameters, such as the laser type, the cavity length, and the pumping power.

[0061] The sudden increase in photon density causes a decrease in carrier density. This in turn decreases the photon density, which increases the carrier density. At this point, the current modulation pulse is timed to switch back down to the DC bias level and the laser emission quickly stops. The laser output therefore consists of a train of short laser pulses, as shown in the graph below.

[0062] To generate longer pulses, the gain bias is chosen to be closer to the lasing threshold. This means that the carrier density crosses the lasing threshold sooner, which gives the light pulse more time to emit. First the light intensity overshoots, quickly decreasing the carrier density. This in turn decreases the photon density, increases the carrier density, and then increases the light intensity. This competitive process creates oscillations in the light intensity at the beginning of the pulse, which decay strongly and quickly reach a plateau where the intensity is constant. The oscillations are called relaxation oscillations. When the current pulse ends and the current is switched back to the bias value, the laser pulse ends.

[0063] The following graph shows the output of the primary laser 503. One optical pulse is output each time the carrier density increases above the lasing threshold. As mentioned above, there may be a delay between when the gain increases and when the optical pulse is output. The optical pulse output from the primary laser has a large time jitter τ.

[0064] The next graph shows the gain modulation applied to the secondary laser 502. The gain modulation is the same as that applied to the primary laser 503, but with the addition of a time delay, labeled with an arrow. The gain modulation is a time-varying drive signal applied to the secondary laser. In other words, the gain modulation applied to the secondary laser 502 is shifted in time with respect to the gain modulation applied to the primary laser 503. Each periodic increase in gain is applied to the secondary laser 502 later than it is applied to the primary laser 503. The delay in this case is approximately half the period of the gain modulation signal. The delay means that the periodic increase in gain is applied to the secondary laser 502 after the optical pulse is injected. So, the optical pulse from the primary laser 503 is present in the laser cavity of the secondary laser when the gain increase is applied, and the resulting secondary laser 502 generates an optical pulse by stimulated emission from the primary optical pulse. This means that the generated optical pulse from the secondary laser has a fixed phase relationship with respect to the optical pulse injected into the secondary laser from the primary laser.

[0065] The secondary laser 502 is switched above the lasing threshold after the optical pulse from the primary laser is injected, so that the pulse from the secondary laser is initiated by the stimulated emission caused by the injected optical pulse. The timing of the start of the gain bias of the secondary laser 502 is controlled via the delay line 510. The last graph shows the output of the secondary laser 502. Only one optical pulse is output each time the carrier density increases above the lasing threshold. Again, there may be a delay between the increase in gain modulation and the output optical pulse. The time jitter of the output optical pulse from the secondary laser is lower than that of the optical pulse from the primary laser.

[0066] In the system shown in Figure 5E, a gain modulation unit 509 applies a time-varying gain modulation to the secondary light source 502 such that it is switched above the lasing threshold only once during the time that each light pulse from the primary laser is incident. The switching of the secondary laser 502 is synchronized with the arrival of the light pulse from the primary laser.

[0067] In the system shown in Figure 5F, the time-varying gain modulated signal has a square waveform, however, the time-varying gain modulated signal may comprise a signal having any pulse shape.

[0068] When the light source is a gain-switched semiconductor laser, the gain modulation signal is an applied current or voltage. In one embodiment, the gain modulation signal is an applied current or voltage having a square waveform. In an alternative embodiment, the time-varying current or voltage is an electrical sine wave generated by a frequency synthesizer. In one embodiment, the frequency of the gain modulation signal is less than or equal to 4 GHz. In one embodiment, the frequency is 2.5 GHz. In one embodiment, the frequency is 2 GHz.

[0069] A gain-switched semiconductor laser has a good extinction ratio between the state when the pulse is emitted and the "off" state. It can be used to generate ultrashort pulses. In one embodiment, the duration of each pulse output from the secondary laser is less than 200 ps. In one embodiment, the duration of each pulse output from the secondary laser is less than 50 ps. In one embodiment, the duration of each pulse output from the secondary laser is on the order of a few picoseconds. In one embodiment, the short optical pulses are 500 ps apart when the time-varying current or voltage is a square wave current or voltage with a frequency of 2 GHz.

[0070] In the light sources shown in these figures, the primary and secondary lasers share the same electrical driver for gain modulation. However, the primary and secondary lasers may be driven by separate gain modulation units 509. By driving the gain modulation by separate units, it is possible to generate light pulses output from the primary laser that are longer than those shown in FIG. 5F, because the gain bias value is closer to the lasing threshold. This means that the carrier density crosses the lasing threshold earlier, which gives the light pulse more time to emit. This can also be used to reduce jitter.

[0071] For the self-tuning of control parameters described herein, in one embodiment there is autonomous setting of the control signals driving both lasers (i.e., each laser is set with different optimal DC bias, AC waveform amplitude, waveform shape, and signal timing), and in addition each laser requires precise wavelength setting (e.g., by controlling a temperature controller attached to each laser) to achieve good injection locking.

[0072] Other QKD transmitter designs are possible, and the methods described below can be applied to many other QKD system embodiments.

[0073] Only transmitters have been mentioned above, however complex multi-component optoelectronic designs are also required for QKD receivers, and these work better if the control parameters for the receivers are adjusted.

[0074] 6 is a flow chart illustrating a method for automatically adjusting the control parameters that define the control signal, thereby allowing autonomous adjustment of the electrical control signal.

[0075] FIG. 6 shows how a self-tuning optimization system can be used that uses one or more objective functions (aka fitness functions) to evaluate overall QKD system performance and then uses this scoring metric to intelligently adjust parameters to obtain optimal QKD performance.

[0076] After starting the method in S51, the QKD system generates initial parameters in step S53. These may be "typical" parameters based on prior knowledge of the system, or may be completely random values. In step S55, the parameters are applied to the system (i.e., digital / electrical drive signals are applied to components of the system to implement these settings), and in step S57, one or more measurements are made to determine the quality of the system operating with these parameters.

[0077] Measurements S57 may be taken after a short delay to allow time for transients to subside. In one embodiment, these measurements are taken using dedicated hardware / sensors added to the QKD system (e.g. to directly measure power / polarization / temporal behavior, etc.). However, in further embodiments, measurements may be extracted from normal QKD operation (e.g. by extracting the quantum bit error rate (QBER) or by obtaining the output secure bit rate (SBR), calculated after the steps of sieving, information alignment, error correction, and privacy amplification). It is possible to combine measurements collected from normal QKD operation with measurements from added dedicated hardware, e.g. sensors.

[0078] Then, in step S59, an objective function is run that calculates a "performance score" based on these measurements. This score gives an indication of how good / bad the parameters are and is intended as a means of comparing parameter sets. The objective function may simply be a single measurement, e.g., secure bitrate, or the score may be calculated based on a carefully selected formula. The function may reward good behavior in some measurements and penalize undesirable output in other measurements. In one embodiment, a single measurement is collected in S57, e.g., QBER and SER, and the objective function simply outputs this value, or outputs a value with scaling, e.g., normalization. However, when more than one measurement is taken in step S57, the objective function combines these measurements and outputs a single score. Examples of possible objective functions include normalization, addition, addition of reciprocal values, cost functions, etc. When the objective value combines measurements, the measurements may be weighted. Different measurements used in the objective function may be weighted differently so that more importance may be given to certain measurements in the final score.

[0079] The optimization engine uses this score in step S61 to calculate an improved set of parameters based on some internal policy (which may be updated periodically as new information becomes available) or using an optimization algorithm. The parameters are then applied to the hardware and the process is repeated to ensure that the QKD system is always operating optimally, even in the presence of different channel characteristics / unanticipated changes in hardware characteristics.

[0080] In fact, with a properly designed optimization engine, a QKD system can maintain performance even with gradual hardware degradation: for example, optical components may degrade over time, requiring biasing / setpoint changes to achieve the same light-matter interactions, but the objective-function-based approach to optimization automatically accounts for such intrinsic component changes.

[0081] Figure 7 is a schematic diagram of the QKD system of Figure 2 with the addition of further sensors and optimization engines 71, 75 (which can be implemented in hardware or software). The transmitter 1 is provided with further sensors 73, which can be, for example, photodiodes (for time-resolved measurements of the generated optical signal), optical spectrum analyzers, power meters, or even interferometers, followed by a photodetector to measure phase information. The transmitter is provided with an optimization engine 71 for performing the optimization and adjusting the control parameters output from the control electronics 17. Some of the measurements used in the calculation of the objective function may require information from the receiver 3 sent to the optimization engine 71. The receiver 3 may also have its own optimization engine 75 that adjusts the control parameters output by the control electronics 19 of the receiver 3.

[0082] The optimization technique may perform measurements at the transmitter 1 and / or receiver 3 and then apply parameter changes at the transmitter 1 and / or receiver 3, which may require communication between the transmitter node 1 and the receiver node 3, increasing the time of the optimization process. In a further embodiment, measurements may be performed at the transmitter 1 and only these measurements are used to adjust the control parameters of the transmitter 1. Similarly, measurements may be performed at the receiver 3 and only these measurements are used to adjust the control parameters of the receiver 3. This avoids the need for communication between the transmitter and receiver. Furthermore, in an embodiment, there may be a forced settling time between parameter settings and measurements to avoid transients. Thus, the optimization run may represent a time window during which the QKD keys cannot be generated.

[0083] The optimization techniques can be applied in practice at various stages of QKD system operation. For example, the optimization engine may be run during the system initialization stage, i.e., at turn-on, in which case, once a pre-set performance level is reached, the parameters can be fixed and the optimization engine can be disabled. Alternatively, the optimization may be stopped after a pre-set period of time.

[0084] To avoid the timing of the optimization process affecting the system's key transmission capabilities, in one embodiment, the optimization process is run when the QKD performance falls below a certain level (e.g., SBR falls below what is expected for a given link / QBER rises too high). In a further embodiment, the optimization process is run periodically at some predefined time interval, for example every 10 minutes, or every hour, or every day. In a further embodiment, the trigger condition for the optimization process is a user-selectable setting in the QKD control software.

[0085] In further embodiments, it may be possible to interleave such "performance test" measurements with normal QKD operation. For example, consider the RF electrical signals that drive the QKD hardware. Under normal operating conditions, electrical signals are generated that drive components to modulate light to encode bits / bases as part of the QKD protocol. To test whether the amplitude of the RF signal is optimal, or some other temporal feature of it, the optimization engine can drive the control electronics to exhibit a different amplitude every N pulses (N can be any integer depending on the desired frequency at which this optimization is performed). The optimization engine then performs measurements on these modified pulses and scores them, while leaving other pulses unaffected so that they can be used for standard QKD to distil the QKD key. Using this approach, the optimization engine can identify (and test to find) optimal waveform parameters without significantly affecting QKD operation (if N is large, e.g., if only every 100th pulse is used for optimization).

[0086] The process can use various techniques in step S61 to select an improved parameter set based on objective function scoring of past parameters.

[0087] In one embodiment, the optimization engine can implement "reinforcement learning", a machine learning technique. In this case, the optimization engine portion can be viewed as an "agent" whose goal is to maximize its reward function (objective function output score) by applying actions to the QKD system and observing the effects. Depending on the number of variables involved for automatic optimization, this can use neural networks or even "deep" neural networks.

[0088] In another embodiment, the optimization engine implements a global optimization algorithm such as an evolutionary algorithm. This may include "particle swarm optimization" or "genetic algorithms." The goal of such a global optimization algorithm is to find globally optimal parameters, making such algorithms ideal for non-convex problems that may contain many "maxima / minima." QKD system optimization is such a non-convex problem due to the non-linear / coupled nature of the underlying physical phenomena such as laser dynamics.

[0089] In one embodiment, a genetic algorithm (GA) is used. GAs are particularly attractive here due to their relative simplicity and well-known ability to find global maxima / minima in multivariate large parameter space problems. More specifically, GAs are heuristic search algorithms inspired by Darwin's theory of natural selection, which mimic the process of biological evolution to determine the "fittest" individuals to perform a given task.

[0090] The central concept of GA is illustrated in Figure 8. In GA, each possible solution (i.e., parameter set) is represented by an "individual". Each individual has a set of "genes" that correspond to the values ​​of each parameter to be optimized (e.g., parameters A, B, C, and D in Figure 8). The goal is to determine the "fittest" individual that gives the best performance.

[0091] First, the population is initialized by randomly assigning genes to individuals. The purpose of this step is to distribute the individuals as uniformly as possible in the search space, so that promising regions can be quickly identified as the individuals evolve. The fitness of each individual in a generation is then evaluated by a fitness function prescribed by the objective. In nature, natural selection favors individuals with traits that make them more successful in reproduction. Similarly in GA, individuals are selected as parents with a probability based on their fitness score, the higher the score, the more likely they are to be selected as parents. Offspring are then created by randomly crossing genes from the two parents. Sufficient genetic diversity prevents convergence towards a local optimum, which can be achieved by mutation, where the offspring's genes change randomly with a certain probability. Over successive generations, the population evolves until it converges to an optimum by inheriting good genes and eliminating bad ones. To speed up the convergence, the concept of "elitism" can also be applied, where clones of the most fit individuals (elites) in the population are created for the next generation without recombination / mutation.

[0092] Note that probabilities for reproducible genetic crossover, mutation, and elitism can all be configured to give the best possible GA performance (which may be configurable by the user, for example, or are themselves set by some optimization routine).

[0093] The above method is applied to hardware control parameters, which are not the same as the theoretical QKD protocol parameters, which are quantities such as the photon flux of the decoy states / signals and the probability of selecting each state / basis.

[0094] The theoretical "QKD protocol parameters" are very different from the hardware control parameters mentioned above. The QKD protocol parameters can be completely theoretically calculated based on the QKD security proof.

[0095] The above process focuses on the hardware, on experimental hardware optimization, and illustrates how the method can be applied to a fully self-tuning QKD system (i.e., without even a pre-training step).

[0096] The optimization engine described above can be applied to advanced optoelectronic systems that require multivariate optimization of difficult problems to achieve optimal performance, but is highly beneficial for QKD systems because QKD hardware is complex and often requires precise generation, control, and measurement of quantum states to function.

[0097] Above we focused on point-to-point QKD systems. However, it should be noted that the present techniques can be applied to other QKD protocol / system designs. This includes CV QKD and three-node architectures such as measurement device independent (MDI) QKD and twin field (TF) QKD (shown diagrammatically in FIG. 9). The exact hardware configurations for these other systems vary, but the same problem of identifying optimal driving parameters for the various optoelectronic components remains. In all cases, the QBER or SBR can be used as part of the objective function (as well as other protocol-specific measurements).

[0098] As the adoption of QKD technology continues to grow, so does the need for more robust and reliable systems. One of the key components of a QKD system is the transmitter where the quantum state is prepared. Optical injection locking (OIL) with gain-switched laser diodes has emerged as a promising technique to realize fast, robust, and cost-effective quantum transmitters. Such a system has been described above with reference to Figs. 4 and 5. The OIL technique improves laser characteristics such as reduced pulse timing jitter, chirping suppression, and modulation bandwidth expansion, and also enables direct phase encoding, where phase information can be directly encoded by varying the electrical waveform applied to the laser, thereby eliminating the need for conventional bulky and expensive modulators. OIL has been widely applied in many QKD protocols, including BB84, coherent-one-way QKD, measurement device independent (MDI) QKD, and twin-field (TF) QKD. OIL also allows the transmitter to be formed on a single chip.

[0099] However, the underlying laser dynamics of OIL systems are highly complex and involve interactions between multiple control parameters. To achieve stable synchronization conditions for low-noise and high-coherence output, it is inevitable that multiple parameters must be optimized simultaneously. Moreover, even with the same model of laser, every laser has slightly different characteristics resulting from natural variations during manufacturing and component tolerances. Therefore, the optimal parameters determined for one system are very often not directly applicable to another system, and each system must be optimized separately.

[0100] Figure 10 shows a GA-based self-adjusting QKD transmitter, which in this embodiment enables autonomous optimization of the OIL laser system using machine intelligence. In one embodiment, the method is applied to optimize pulse-to-pulse phase coherence and QBER for the BB84 protocol.

[0101] The core concept of GA is illustrated and explained above in connection with Figure 8. In one embodiment, the crossover rate and mutation rate for reproduction are set to typical values ​​of 50% and 30%, respectively. In a further embodiment, there is a 30% chance that a mutated gene will change to a value close to its elite counterpart in a generation. This provides additional degrees of freedom to control the exploration and exploitation of the search space.

[0102] The experimental setup is shown in Figure 10. The transmitter comprises two distributed feedback (DFB) lasers in an OIL configuration, where light from the primary laser is injected into the cavity of the secondary laser via an optical circulator. A variable optical attenuator (VOA) is used to control the injection power. Each laser wavelength is temperature stabilized with an integrated thermoelectric cooler that can be adjusted via a controller. RF signals and DC bias from a current source are combined using a bias tee to drive the two lasers. The primary laser is gain-switched to generate a pulse train at 1 GHz. Between pulses, the primary laser is driven below the lasing threshold to ensure that each pulse generated has a random phase. The primary laser pulses are injected into the secondary laser, which is gain-switched at 2 GHz to generate short pulses of approximately 70 ps duration. The RF signals of the two lasers are shown in Figure 11. The two lasers are aligned in time such that each primary pulse seeds two secondary laser pulses that form early and late time bins of a single clock cycle (i.e., a single quantum bit) that share the same globally random phase.

[0103] To encode the relative phase between two secondary laser pulses, the RF signal of the primary laser is modulated by adding a small amplitude perturbation during the time interval between the secondary laser pulses. The perturbation changes the carrier density of the primary laser cavity, which in turn changes its emission frequency and its phase evolution. Since the secondary laser pulse is seeded by the injected primary photons, it inherits the phase of the primary pulse. The induced phase difference in the primary laser pulse is subsequently transferred onto the phase between successive secondary laser pulses, thereby achieving direct phase encoding. The applied phase shift can be precisely controlled by changing the amplitude of the electrical perturbation signal. A VOA is used to attenuate the pulse before sending it into the quantum channel.

[0104] In the receiver, an Asymmetric Mach-Zehnder Interferometer (AMZI) is used to decode the relative phase between the secondary laser pulses. The long arm of the AMZI has a delay of 500 ps, ​​which matches the temporal separation of successive secondary laser pulses. Successive secondary laser pulses can interfere constructively or destructively depending on their relative phase, which makes it possible to assign the bits "0" and "1" to the two output ports. The AMZI output is measured using a photodiode or single-photon detector. The GA controls all of the laser electronics and can remotely set the value of each parameter. The output of each parameter set is then measured and acts as feedback to the algorithm for evaluation.

[0105] An example of laser parameters that can be optimized by GA is listed in Table 1. In general, to achieve a stable OIL, the frequency detuning between the primary and free-running secondary lasers, which depends on temperature and bias current, and the injection power from the primary laser need to be carefully selected. The dynamics of the OIL become more complicated under gain-switching operation. The injection power from the primary laser needs to be strong enough to overcome the effect of spontaneous emission noise on the phase of the secondary laser, however, excessive injection light may produce undesired parasitic effects and degrade the performance. In addition, the bias current of the primary laser should be set at a level that allows the laser to be driven below threshold between each pulse for phase randomization, while also affecting other important output characteristics such as the phase and duration of the pulse. To shift the phase, the two lasers need to be aligned in time, and the duration of the primary laser pulse should be long enough to seed the generation of two consecutive secondary laser pulses. When phase modulation is considered, the implemented phase depends on the amplitude modulation applied to the driving signal of the primary laser. Therefore, all of these parameters need to be adjusted to take advantage of the OIL.

[0106] To explore the complexity of the laser dynamics, we measured the QBER for the BB84 QKD protocol as a function of the frequency detuning between the two lasers and the injection ratio (defined as the ratio between the injected primary power and the free-running secondary power) with all other parameters fixed at a given optimum value, as shown in Figure 12A and Figure 12B. A promising region of operation is shown in the box in Figure 12A and further expanded in Figure 12B. While the QBER is affected by many factors, the interference fringes observable in Figure 12A are likely due to changes in the phase relationship between the primary and secondary lasers, which results in an encoding error in the relative phase between the secondary laser pulses. From Figure 12B, the sparseness of the optimum region can be clearly observed. Mapping the QBER can take more than 8 hours to complete, even when limited to two parameters. This therefore highlights the need for efficient methods to determine the optimum region of operation, especially in a large parameter space.

[0107] Phase encoding, where the secret bit is encoded in the relative phase between successive pulses, is widely used in QKD protocols, and therefore high phase coherence between pulses contributes to the quality of the system. Furthermore, a key requirement for secure quantum communication is that the phase of each qubit, consisting of pre- and post-time bins (Figure 11), is uniformly random. This allows us to treat the coherent states of the damped pulses as photon number states, which allows us to obtain security proofs against the most common attacks.

[0108] OIL combined with gain switching represents a very efficient way to generate pulses that meet these requirements. As mentioned above, gain switching allows each primary pulse to carry a random phase, while optical injection seeding allows the phase manipulation on the primary pulse to be coherently transferred to the relative phase between successive secondary laser pulses.

[0109] To investigate phase coherence, the primary laser is pulsed without additional modulation. As a result, two secondary laser pulses seeded by the same primary pulse are in phase and constructive and destructive interference can be obtained. In contrast, secondary laser pulses seeded by different primary laser pulses do not have a constant phase relationship and therefore should have minimal visibility due to interference. To satisfy these conditions, the following fitness function is used, where the algorithm aims to maximize by optimizing the parameters shown in Table 1: [Table 1]

number

[0110] The results of the optimization are shown in FIG. 13, where the performance of the best individual in the population over successive generations is plotted. The optimization is initialized by assigning random values ​​to the parameters from a given range (i.e., within a safe operating range). Due to the stochastic nature of evolution and the random initial conditions, each optimization has a different trajectory. Therefore, in this embodiment, the optimization is repeated five times to capture all the features and verify its repeatability. As expected, through evolution, the algorithm learns to make the system work, and the individuals in each new generation become increasingly competent as the quality of their genes improves. As the visibility of phase-coherent pulses improves, the visibility of phase-randomized pulses is simultaneously suppressed over generations. Interestingly, sometimes the visibility increases steadily over generations (trials 2 and 4), and sometimes it remains at a local maximum for several generations (trials 3 and 5). However, due to mutations and crossovers, the algorithm suddenly improves after a plateau and eventually finds a better operating region. As a result, all trials converged towards visibility of approximately 97% for phase-coherent pulses and <2% for phase-randomized pulses, matching performance that could otherwise be achieved by manually adjusting the transmitter. This shows that the algorithm can optimize the laser to generate highly phase-coherent pulse pairs suitable for QKD encoding, while at the same time ensuring that each qubit has a globally random phase.

[0111] QBER is the main measure of the performance of practical QKD systems. Minimizing QBER has been an essential task for QKD operation so far. Here, the above mentioned direct phase modulation scheme (Fig. 11) is implemented, which encodes random bits into secondary laser pulses. After traveling through an optical channel with 16 dB loss (emulated by a VOA), the encoded pulses are decoded by the receiver AMZI and measured by a single-photon detector. The proof-of-principle BB84 QKD protocol is then implemented and the QBER is optimized. As a coherence optimization, it is important to take into account the phase randomization so that the QBER can be suppressed while ensuring that the phases of the qubits are randomized simultaneously. To achieve this, it is exploited that the intensity resulting from the interference between two phase-randomized pulses (called side peaks) is exactly half the intensity of the constructive interference between two phase-coherent pulses (called signal peaks), as exemplified in Fig. 11. Based on this, a phase randomization "cost function" L PR is defined and minimized.

number

number

number

[0112] The input parameters are listed in Table 1. The evolution of QBER and the corresponding secure key rate, as well as L for five iterative optimization trials. PRare plotted in Fig. 14A and Fig. 14B. As with the phase coherence optimization, all optimization trials eventually locate optimal parameters and converge towards a QBER of about 2.5%. Fig. 14C further illustrates how the population evolves during optimization. Starting with a random distribution in the parameter space, individuals gradually move towards the optimal region over generations, while some scatter in the parameter space to continue searching. To verify the phase randomization, we eliminated the channel attenuation and measured the intensity probability distribution of the output with an oscilloscope. As shown in Fig. 15, the distribution follows the typical profile expected from the interference between two phase randomization pulses.

[0113] In terms of practical implementation, the above-mentioned GA-based optimization technique can be seamlessly integrated into the software layer of the QKD system without the need for additional hardware modifications. In particular, the QBER optimization is designed to run within the QKD transmitter and receiver without other diagnostic tools. Therefore, the optimization procedure is self-contained, allowing multiple QKD systems to be automatically optimized in parallel. This feature is useful for scaling up the manufacturing of QKD systems, especially for chip-based QKD systems, which are often more difficult to optimize. For QKD systems that are deployed in real-world environments where test equipment and QKD experts are not readily available, the above process allows the QKD system to self-optimize in the field, ensuring continuous optimal performance, for example, when system parameters are detuned from the optimal solution due to various unexpected changes in the field environment. This makes the system more robust overall and reduces the chance of downtime.

[0114] Regarding optimization performance, the speed of convergence depends on the complexity of the problem at hand and the control parameter configuration of the genetic algorithm. To efficiently locate a global optimum, there needs to be enough genetic diversity in the population, especially in the first generation, so that the search for the optimum does not rely too heavily on random mutations. So, when the number of good solutions is very small compared to the size of the search space, a large population size is necessary to maintain diversity and avoid convergence to local optima. However, the larger the population, the greater the convergence time will necessarily be. Control parameter configurations are known to be problem-dependent. In this particular embodiment, the population size for phase coherence optimization was chosen to be 35, but was expanded to 60 for QBER optimization due to its larger parameter space. These values ​​were empirically determined to give repeatable convergence within a reasonable time, and further optimization is possible but beyond the scope of this work.

[0115] Moreover, GA-based self-tuning techniques are goal-oriented: such optimization methods make it possible to achieve optimal behavior without a priori knowledge of the underlying complex dynamics.

[0116] Although specific embodiments have been described, these embodiments are presented merely as examples and are not intended to limit the scope of the invention. The novel devices and methods described herein may be embodied in other various forms, and various omissions, substitutions, and changes may be made in the form of the devices, methods, and products described herein without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover any such forms or modifications that fall within the scope and spirit of the invention.

Claims

1. a transmitter comprising a plurality of transmitter components, the plurality of transmitter components comprising a pulsed radiation source and a modulation unit, the modulation unit configured to randomly encode a plurality of pulses of radiation; a receiver comprising a plurality of receiver components, the plurality of receiver components comprising a demodulator configured to decode the randomly encoded plurality of pulses and a plurality of detectors configured to detect the decoded plurality of pulses; Equipped with The quantum communication system further comprises a control unit and an optimization unit, the control unit configured to apply a plurality of control signals defined by a set of a plurality of control parameters to at least one of the plurality of transmitter components, and the optimization unit configured to adjust the set of the plurality of control parameters to obtain an adjustment set; the plurality of control signals are a plurality of electronic control signals for the pulsed radiation source, the plurality of control signals comprising at least one of a magnitude, a shape, and a DC offset of electronic control signals included in the plurality of electronic control signals; The optimization unit comprises: obtaining a score indicative of a quality of the quantum communication system corresponding to a first set of control parameters, the score being a reward for a reinforcement learning algorithm; estimating, by an iterative process, a second set of a plurality of control parameters to obtain the adjustment set by using the optimization unit as an agent whose goal is to maximize a reward function by applying actions to the quantum communication system; and adjusting the plurality of control parameters using a reinforcement learning algorithm.

2. The quantum communication system of claim 1 , wherein the optimization unit is configured to derive the score from at least one measure indicative of a quality of the quantum communication system.

3. 3. The quantum communication system of claim 2, wherein the at least one measurement is selected from a quantum bit error rate (QBER), a secure bit rate (SBR), phase information of the encoded pulses, a count rate of each of all pulses received at the detectors of the receiver, the count rate of each of a plurality of pulses with a predetermined encoding, a shape of the received pulses, or an arrival time of the plurality of pulses at the detectors.

4. The quantum communication system of claim 3 , wherein the optimization unit is configured to calculate the score from the at least one measurement using an objective function.

5. The quantum communication system of claim 1 , further comprising a plurality of sensors arranged to obtain a plurality of measurements indicative of a quality of the quantum communication system.

6. The quantum communication system of claim 1 , wherein the pulsed radiation source is provided on a temperature control element, and the control unit further provides a control signal to the temperature control element.

7. 2. The quantum communication system of claim 1, wherein the plurality of control signals further comprises a plurality of electronic control signals for the modulation unit, the plurality of electronic control signals for the modulation unit comprising at least one of a magnitude, a shape, and a DC offset of an electronic control signal included in the plurality of electronic control signals for the modulation unit.

8. 2. The quantum communication system of claim 1, wherein the plurality of control signals further comprises a plurality of electronic control signals for the demodulator, the plurality of electronic control signals for the demodulator comprising at least one of a magnitude, a shape, and a DC offset of an electronic control signal included in the plurality of electronic control signals for the demodulator.

9. 2. The quantum communication system of claim 1 , wherein the plurality of control signals further comprises a plurality of electronic control signals for the plurality of detectors, the plurality of electronic control signals for the plurality of detectors comprising at least one of a magnitude, a shape, and a DC offset of electronic control signals included in the plurality of electronic control signals for the plurality of detectors.

10. 10. The quantum communication system of claim 1, wherein the pulsed radiation source comprises a primary laser and a secondary laser, the primary laser configured to apply a seeding pulse to the secondary laser.

11. 11. The quantum communication system of claim 10, wherein the set of control parameters comprises control parameters for both the primary laser and the secondary laser.

12. The quantum communication system of claim 1 , wherein the modulation unit is a phase modulation unit comprising an interferometer.

13. The quantum communication system of claim 1 , wherein the quantum communication system is selected from a point-to-point quantum communication system, a measurement device independent quantum communication system, and a twin-field quantum communication system.

14. 1. A transmitter for a quantum communication system, the transmitter comprising a plurality of transmitter components, the plurality of transmitter components comprising a pulsed radiation source and a modulation unit, the modulation unit configured to randomly encode a plurality of pulses of radiation; The transmitter further comprises a control unit and an optimization unit, the control unit configured to apply a plurality of control signals defined by a set of a plurality of control parameters to at least one of the plurality of transmitter components, and the optimization unit configured to adjust the set of the plurality of control parameters to obtain an adjustment set; the plurality of control signals are a plurality of electronic control signals for the pulsed radiation source, the plurality of control signals comprising at least one of a magnitude, a shape, and a DC offset of electronic control signals included in the plurality of electronic control signals; The optimization unit comprises: obtaining a score indicative of a quality of the quantum communication system corresponding to a first set of control parameters, the score being a reward for a reinforcement learning algorithm; estimating, by an iterative process, a second set of a plurality of control parameters to obtain the adjustment set by using the optimization unit as an agent whose goal is to maximize a reward function by applying actions to the quantum communication system; and adjusting the plurality of control parameters using a reinforcement learning algorithm.

15. 1. A receiver for a quantum communication system, the receiver comprising a plurality of receiver components, the plurality of receiver components comprising a demodulator configured to decode a plurality of randomly encoded pulses, and a plurality of detectors configured to detect the decoded plurality of pulses; The receiver further comprises a control unit and an optimization unit, the control unit configured to apply a plurality of control signals defined by a set of a plurality of control parameters to at least one of the plurality of receiver components, and the optimization unit configured to adjust the set of the plurality of control parameters to obtain an adjustment set; the plurality of control signals are a plurality of electronic control signals for a pulsed radiation source, comprising at least one of a magnitude, a shape, and a DC offset of the electronic control signals included in the plurality of electronic control signals; The optimization unit comprises: obtaining a score indicative of a quality of the quantum communication system corresponding to a first set of control parameters, the score being a reward for a reinforcement learning algorithm; estimating, by an iterative process, a second set of a plurality of control parameters to obtain the adjustment set by using the optimization unit as an agent whose goal is to maximize a reward function by applying actions to the quantum communication system; and adjusting the plurality of control parameters using a reinforcement learning algorithm.

16. 1. A method of controlling a quantum communication system, the quantum communication system comprising: a transmitter comprising a plurality of transmitter components, the plurality of transmitter components comprising a pulsed radiation source and a modulation unit, the modulation unit configured to randomly encode a plurality of pulses of radiation; a receiver comprising a plurality of receiver components, the plurality of receiver components comprising a demodulator configured to decode the randomly encoded plurality of pulses and a plurality of detectors configured to detect the decoded plurality of pulses; Equipped with The quantum communication system further comprises a control unit and an optimization unit, the control unit configured to apply a plurality of control signals defined by a set of a plurality of control parameters to at least one of the plurality of transmitter components, and the optimization unit configured to adjust the set of the plurality of control parameters to obtain an adjustment set; The method comprises: applying a plurality of control signals defined by a plurality of sets of control parameters to at least one of the plurality of transmitter components, the plurality of control signals being a plurality of electronic control signals for the pulsed radiation source, the plurality of control signals comprising at least one of a magnitude, a shape, and a DC offset of an electronic control signal included in the plurality of electronic control signals; obtaining a score from at least one measure indicative of a quality of the quantum communication system corresponding to a first set of control parameters, the score being a reward for a reinforcement learning algorithm; estimating a second set of control parameters by an iterative process to obtain the adjustment set by using the optimization unit as an agent whose goal is to maximize a reward function by applying actions to the quantum communication system, and adjusting the control parameters using a reinforcement learning algorithm; A method comprising:

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