Quantum communication system, transmitter for a quantum communication system, receiver for a quantum communication system, and method for controlling a 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 INVENTION Embodiments described herein relate to quantum communication systems, transmitters for quantum communication systems, receivers for quantum communication systems, and methods of controlling quantum communication systems. [Background technology]

[0002] In quantum communication systems, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons, each carrying one bit of information that can be encoded on a property of the photon, such as 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 appeal of this technique is that it provides a test for whether any part of the key may be known to an unauthorized eavesdropper, often called "Eve." In many forms of QKD, 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. Therefore, by comparing portions of their common bit strings, Alice and Bob can determine whether Eve has obtained the information. [Brief explanation of the drawings]

[0004] [Figure 1] 1 is a schematic diagram of a point-to-point QKD system. [Figure 2] FIG. 1 is a schematic diagram of a point-to-point QKD system with control electronics. [Figure 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 secondary laser seeding arrangement. [Figure 4B] 5 is a schematic diagram of the corresponding drive signal and light output. [Figure 5A] FIG. 1 is a schematic diagram of a primary and secondary laser arrangement. [Figure 5] FIG. 5B is a plot of the optical frequency of the primary laser under a small perturbation of the control gain of duration tm, FIG. 5C is a plot of the optical phase trajectory 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 output of the primary laser, modulation of the secondary laser, and power output 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 one embodiment; [Figure 8] FIG. 1 is a diagram illustrating 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] Figure 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 Figure 12B is a magnified plot of the boxed region. [Figure 13]

[0033] Figure 12 is a plot showing the evolution of optimization for phase coherence for phase-coherent and phase-randomized pulses. The population size is 35 for the first three generations and is reduced to 25 in subsequent generations. The time taken to complete 10 generations is 4 hours (50 seconds 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 parameter space (only two dimensions are shown). The population size was 60, and the time taken to complete 10 generations was 2.5 hours (14 seconds per individual evaluation). [Figure 15] 10 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 INVENTION

[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, wherein the control unit is 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 is configured to tune the set of a plurality of control parameters; The optimization unit obtaining a score indicative of a quality of the quantum communication system corresponding to the first set 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 enables self-tuning of complex QKD system hardware. Until now, determining the optimal control parameters and drive signals for all QKD hardware was done manually. While much of this was done during the system design phase based on known hardware specifications, other components needed to be adjusted during system operation in response to environmental changes. The situation is further complicated by inherent variations between components; even if one QKD system is perfectly optimized manually, 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 requiring skilled users with expertise in optical elements, electronic circuits, and QKD system design. This requirement imposes costs and impacts manufacturing scalability.

[0007] Furthermore, ongoing research is moving toward developing new and improved QKD system designs with better performance and / or fewer components. However, because 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 easily found 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). Optimizing the complete system requires a holistic approach that simultaneously considers many variables and also measures multiple output characteristics that affect the overall QKD performance.

[0009] In one embodiment, the optimization unit is configured to derive a score from at least one measure indicative of the quality of the system. This is a physical measure of the QKD system. The physical measure may be derived from measurements normally made in a QKD system, such as QBER, or may be derived from additional hardware or sensors provided to make further measurements, for example, of phase. In one embodiment, the measure indicative of the quality of the system may be selected from a quantum bit error rate "QBER", a secure bit rate "SBR", phase information of the coded pulses, a count rate of all pulses received at multiple detectors of the receiver, the count rate of multiple pulses with a predetermined coding, the shape of the received pulses, or multiple arrival times of multiple pulses at multiple 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 a 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, and the control unit further supplies 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, and in such a situation, the set of 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 an intensity of the 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 pulses. The control signals for the intensity modulators may also be optimized via the optimization unit. For both the intensity modulator and the phase modulator, the control signals may comprise one or more of an intensity of the 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, and comprise at least one of an intensity 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 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 amplitude, 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 plurality of 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 plurality of 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 circuitry.

[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, there is provided 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 modulating unit, the modulating unit configured to randomly encode the plurality of pulses of radiation, The transmitter further comprises a control unit and an optimization unit, wherein the control unit is configured to apply 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 optimization unit is configured to adjust the plurality of sets of control parameters; The optimization unit obtaining a score indicative of a quality of the system corresponding to the first set 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, 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; The receiver further comprises a control unit and an optimization unit, wherein the control unit is configured to apply a plurality of control signals defined by a plurality of sets of control parameters to at least one of the plurality of receiver components, and the optimization unit is configured to adjust the plurality of sets of control parameters; The optimization unit obtaining a score indicative of a quality of the system corresponding to the first set 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 radiation source 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 pulsed radiation source 7 and 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, for example dispersion can cause the pulse to broaden, polarization fluctuations can change the polarization state, channel timing delays can change the expected pulse arrival time, etc.

[0023] At 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 to the quantum state), and then detecting the signal using a single-photon detector (not shown) to form 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 will be described. However, it should be noted that this is not meant to be limiting and other protocols may be used. For simplicity, this protocol will refer 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 basis, or D=0, A=1 in the D / A basis. The pulse is attenuated to have less than one photon on average. Therefore, if a measurement is made on the pulse, it will be destroyed. It is also impossible 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 to measure 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 to measure 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 must prepare another pulse to send to the receiver. However, the eavesdropper does not know the correct measurement basis, and therefore only has 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 portions 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 its 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.). It 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 clearly illustrates these additional components / stages that are important for ensuring high performance operation in the presence of real-world disturbances. Note that the exact implementation may vary, with compensation performed by either Alice and / or Bob, and in any order. To avoid unnecessary repetition, the same reference numerals as in Figure 1 have been used where appropriate. Additionally, Figure 2 includes a transmitter, control electronics 17, and signal compensation unit 13. The receiver includes control electronics 19 and signal compensation unit 15.

[0032] A wide range of optical / electronic system designs are possible for performing QKD. Figure 3 shows one example of such an approach. Here, the transmitter includes a continuous-wave (CW) laser 21, followed by modulators 23, 25, and 27 for carving out pulses in time and encoding 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 includes an asymmetric Mach-Zehnder interferometer (AMZI), with a phase modulation component in one 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 an average of one photon or less. In a further embodiment, the second intensity modulator is also used to implement a decoy state required in many QKD protocols. The decoy state is similar to the other encoding states but with reduced amplitude (i.e., a lower average number of photons is used for the decoy pulse). This allows the use of decoy states to evade attacks such as photon number splitting attacks.

[0033] 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 be phase-randomized. If phase encoding is used, in which the quantum state is encoded by the phase between two time bins, the phase between the two bins can be precisely controlled.

[0034] This places stringent requirements on highly coherent and stable light emission from all modulators 23, 25, and 27, as well as laser 21. The quality of the output can be improved by precise control of the electronic drive signals applied to modulators 23, 25, and 27 and laser 21. For example, the biasing / control of laser 21 and the control of the modulators can be adjusted to improve the quality of the output. The modulator has the additional complication of being driven by an RF signal with the appropriate amplitude and timing to generate a properly encoded train of quantum states. In some embodiments of QKD transmitters, the laser is also driven by an RF signal. In the case of a "phase-seeding" configuration (i.e., a gain-switched primary laser-secondary laser configuration), both the primary and secondary lasers have RF signals applied to them that generate optical pulses. Additional electrical control signals with many customizable parameters may also need to be adjusted, such as those 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 injection-locks the secondary laser, and the primary laser's current is modulated to impose phase modulation on the secondary laser). This has the advantage of eliminating the modulator, which is typically bulky and expensive and prevents chip-scale integration of the design onto a photonic chip (because 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 in which one primary pulse seeds two secondary pulses. Figures 5A-5F show some of the physics behind laser seeding. For simplicity, we will describe the situation where one primary pulse seeds one secondary pulse.

[0037] Figures 5A-5F illustrate a particular type of light source that enables the creation of an ultra-compact, high-performance QKD transmitter. This light source, shown in Figure 5A, uses gain switching and optical injection locking to directly phase modulate the pulses, eliminating 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, while 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 schematically 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 for injecting light from one laser into another 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; 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 nanosecond or even shorter quasi-steady-state optical pulses with shallow intensity modulation, which also modulates 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 inherit 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 into 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 deleterious 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] Figure 5B shows the duration t m Figure 5C is a plot of the optical frequency of the phase-ready laser under small perturbations of . 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 ν. This perturbation will induce a phase difference of Δφ=2πΔνt m where t m is the duration of the perturbation. By optical injection, this phase difference is transferred onto a pair of secondary pulses emitted by the pulse generation laser shown in Figure 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 within 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 throughout the duration of the perturbation. The enhancement due to the laser cavity allows the phase modulation half-wave voltage to be kept below 1 V. Conventional phase modulators lack this cavity feature, so in conventional phase modulators, light passes through the electro-optic medium only once, thereby 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 conventional lithium niobate phase modulators require) 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 its lasing threshold. In this case, the generated secondary light pulse has a fixed phase relationship to the injected primary light pulse. Because 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 are random phase differences between pulses from different pairs. These pulses also have a smaller time jitter τ'<τ relative to the pulses output by the primary laser 101. This reduced jitter time improves 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 optical pulses of primary laser 101 entering the optical cavity of 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 optical pulses is at least 1000 times less than the optical output power of secondary laser 103. In one embodiment, the optical power of the injected optical pulses is at least 100 times less than the optical output power of 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 using 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 using a single gain modulation unit 509. The gain modulation unit 509 and delay line 510 are an example of a controller configured to apply a time-varying drive signal to the secondary laser 502 so 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 may be a waveguide, e.g., 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 additional components, such as an optical circulator or 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. A 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] Figure 5F shows the time sequence for the single gain modulation scheme shown in Figure 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 is shown on the horizontal axis. The gain modulation is a time-varying drive signal in 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, such as a sinusoidal wave 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 laser's gain above and below the lasing threshold. The second graph shows the laser's carrier density 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 represents laser intensity, and the horizontal axis represents 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 produce 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, including the laser type, cavity length, and pumping power.

[0061] The sudden increase in photon density causes a decrease in carrier density. This in turn decreases photon density, which increases carrier density. At this point, the current modulation pulse is timed to switch back down to the DC bias level and laser emission quickly ceases. 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 closer to the lasing threshold. This means that the carrier density crosses the lasing threshold sooner, giving the light pulse more time to emit. Initially, the light intensity overshoots, rapidly reducing the carrier density. This in turn reduces the photon density, increases the carrier density, and then increases the light intensity. This competitive process causes oscillations in the light intensity at the beginning of the pulse, which decay strongly and quickly reach a steady state 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 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 relative 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. Therefore, 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 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, resulting in a pulse from the secondary laser initiated by stimulated emission caused by the injected optical pulse. The timing of the onset of the gain bias of the secondary laser 502 is controlled via a 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] 5E, a gain modulation unit 509 applies 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 can 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 4 GHz or less. In one embodiment, the frequency is 2.5 GHz. In one embodiment, the frequency is 2 GHz.

[0069] Gain-switched semiconductor lasers have a good extinction ratio between the "off" state and the state when the pulse is emitted. They 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, when the time-varying current or voltage is a square wave current or voltage with a frequency of 2 GHz, the short optical pulses are 500 ps apart.

[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 also be driven by separate gain modulation units 509. By driving the gain modulation with separate units, it is possible to generate longer optical pulses output from the primary laser 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 sooner, giving the optical 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 a 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 the transmitter has been mentioned above. However, complex multi-component optoelectronic designs are also required for QKD receivers, and these receivers work better if the control parameters are adjusted.

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

[0075] Figure 6 shows how a self-tuning optimization system can be used that uses one or more objective functions (also known as 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 at S51, the QKD system generates initial parameters at step S53. These may be "typical" parameters based on prior knowledge of the system, or may be completely random values. At 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 at step S57, one or more measurements are taken to determine the quality of the system operating with these parameters.

[0077] Measurement S57 may be taken after a short delay to allow time for transients to subside. In one embodiment, these measurements are made 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 derived 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] An objective function is then run in step S59 to calculate a "performance score" based on these measurements. This score gives an indication of how good / bad a parameter is and is intended as a means of comparing parameter sets. The objective function may simply be a single measurement, such as secure bit rate, or the score may be calculated based on a carefully selected formula. The function may reward good performance in some measurements and penalize undesirable performance in other measurements. In one embodiment, a single measurement, such as QBER and SER, is collected in S57, and the objective function simply outputs this value or a value with scaling, e.g., normalization. However, when two or more measurements are taken in step S57, the objective function combines these measurements to output a single score. Examples of possible objective functions include normalization, addition, addition of reciprocal values, cost function, 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 can 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 / unexpected changes in hardware characteristics.

[0080] In fact, with a properly designed optimization engine, a QKD system can maintain performance even as hardware gradually degrades. For example, optical components may degrade over time, requiring biasing / setpoint changes to achieve the same light-matter interaction, and 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 additional sensors and optimization engines 71, 75 (which can be implemented in hardware or software). The transmitter 1 is provided with additional sensors 73, which may 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 for measuring phase information. The transmitter is provided with an optimization engine 71 for performing optimization and adjusting control parameters output from the control electronics 17. Some of the measurements used in calculating the objective function may require information from the receiver 3, which is sent to the optimization engine 71. The receiver 3 may also have its own optimization engine 75, which 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 the receiver 3 and then apply parameter changes at the transmitter 1 and / or the receiver 3, which may require communication between the transmitter node 1 and the receiver node 3 and increase 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 the receiver. Furthermore, in one embodiment, there may be a forced settling time between parameter setting and measurement to avoid transients. Thus, performing the optimization may represent a time window during which QKD keys cannot be generated.

[0083] The optimization techniques can actually be applied 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 preset performance level is reached, the parameters can be fixed and the optimization engine can be disabled. Alternatively, the optimization may stop after a preset 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 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 predetermined 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 normal QKD operation with such “performance test” measurements. For example, consider the RF electrical signals that drive QKD hardware. Under normal operating conditions, electrical signals are generated that drive components to modulate light to encode bits / basis as part of the QKD protocol. To test whether the RF signal's amplitude or some other temporal characteristic is optimal, 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 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., 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, portions of the optimization engine 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 automated optimization, this can use a neural network or even a "deep" neural network.

[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 algorithm." The goal of such a global optimization algorithm is to find globally optimal parameters, making it ideal for non-convex problems that may contain many "maxima / minima." QKD system optimization is such a non-convex problem due to the nonlinear / coupled nature of 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 large, multivariate 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" corresponding 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 goal of this step is to distribute individuals as uniformly as possible across the search space, allowing for the rapid identification of promising regions as individuals evolve. The fitness of each individual in each generation is then evaluated using a fitness function defined by the objective. In nature, natural selection favors individuals with traits that lead to more successful reproduction. Similarly, in GAs, 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 two parents. Sufficient genetic diversity prevents convergence toward local optima, which can be achieved through mutation, in which offspring genes are randomly altered with a specific probability. Over successive generations, the population evolves until it converges to an optimal state by inheriting good genes and eliminating bad ones. To speed up convergence, the concept of "elitism" can be applied, where the most fit individuals in the population (elites) are cloned for the next generation without recombination / mutation.

[0092] Note that the probabilities for reproducible genetic crossover, mutation, and elitism can all be configured to give the best possible GA performance (e.g., which may be configurable by the user 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] Theoretical "QKD protocol parameters" are very different from the hardware control parameters mentioned above. QKD protocol parameters can be calculated completely theoretically 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. However, its application to QKD systems is highly beneficial because QKD hardware is complex and often requires precise generation, control, and measurement of quantum states to function.

[0097] The above focuses 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 schematically in Figure 9). While the exact hardware configurations for these other systems vary, the same problem of identifying optimal driving parameters for the various optoelectronic components remains. In all cases, 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 too 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 for realizing fast, robust, and cost-effective quantum transmitters. Such systems have been described above with reference to Figures 4 and 5. The OIL technique not only improves laser characteristics, such as reduced pulse timing jitter, chirping suppression, and modulation bandwidth expansion, but also enables direct phase encoding, in which 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 to many QKD protocols, including BB84, coherent-one-way QKD, measurement-device-independent (MDI) QKD, and twin-field (TF) QKD. OIL also enables the transmitter to be fabricated on a single chip.

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

[0100] Figure 10 shows a GA-based self-tuning QKD transmitter, which in this embodiment enables autonomous optimization of an 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 using an integrated thermoelectric cooler, which 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 its lasing threshold to ensure that each generated pulse has a random phase. The primary laser pulse is injected into the secondary laser, which is gain-switched at 2 GHz to generate short pulses with a duration of approximately 70 ps. 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 the 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. Because 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 then 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 pulses before sending them to 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 500 ps delay, which matches the temporal separation of successive secondary laser pulses. Successive secondary laser pulses can interfere constructively or destructively depending on their relative phase, allowing bits "0" and "1" to be assigned 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. Generally, 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 must be carefully selected. The OIL dynamics become more complex with gain switching. 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 power can produce undesirable parasitic effects and degrade performance. Furthermore, the bias current of the primary laser should be set to a level that allows the laser to be driven below threshold between pulses for phase randomization, while also affecting other important output characteristics, such as pulse phase and duration. To shift the phase, the two lasers must 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 primary laser's drive signal. Therefore, all of these parameters need to be adjusted to take advantage of the benefits of OIL.

[0106] To investigate the complexity of 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 of injected primary power to free-running secondary power), with all other parameters fixed at predetermined optimal values, as shown in Figures 12A and 12B. The promising operating region is indicated by a box in Figure 12A and further expanded in Figure 12B. While 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. The sparseness of the optimal region can be clearly observed in Figure 12B. Mapping QBER, even when limited to two parameters, can take more than eight hours to complete. This therefore highlights the need for efficient methods for determining the optimal operating region, especially in large parameter spaces.

[0107] Phase encoding, in which a 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 the pre- and post-time bins (Figure 11), is uniformly random. This allows us to treat the coherent states of the attenuated pulses as photon number states, providing security proofs against the most common attacks.

[0108] OIL combined with gain switching represents a highly 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 any additional modulation. As a result, two secondary laser pulses seeded by the same primary pulse are in phase, resulting in constructive and destructive interference. In contrast, secondary laser pulses seeded by different primary laser pulses do not have a constant phase relationship and therefore should minimize interference visibility. To satisfy these conditions, the following fitness function is used, which the algorithm aims to maximize by optimizing the parameters shown in Table 1: [Table 1]

number

[0110] The optimization results are shown in Figure 13, where the performance of the best individuals in the population over successive generations is plotted. The optimization is initialized by assigning random values ​​to parameters from a given range (i.e., within a safe operating range). Due to the stochastic nature of evolution and random initial conditions, each optimization has a different trajectory. Therefore, in this embodiment, the optimization is repeated five times to capture all features and verify its repeatability. As expected, through evolution, the algorithm learns to operate the system, and individuals in each new generation become increasingly competent as their genetic quality improves. As the visibility of phase-coherent pulses improves, the visibility of phase-randomized pulses is simultaneously suppressed over generations. Interestingly, sometimes the visibility steadily increases over generations (trials 2 and 4), and sometimes it remains at a local maximum for several generations (trials 3 and 5). However, due to mutation and crossover, the algorithm plateaus and then suddenly improves, eventually finding a better operating region. As a result, all trials converged toward a 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 demonstrates that the algorithm can optimize the laser to generate highly phase-coherent pulse pairs suitable for QKD encoding, while simultaneously ensuring that each qubit has a globally random phase.

[0111] The QBER is a key measure of the performance of practical QKD systems. Minimizing the QBER has been an essential task for QKD operation to date. Here, the direct phase modulation scheme described above (Figure 11) is implemented, encoding random bits into secondary laser pulses. After traveling through an optical channel with a 16 dB loss (emulated by a VOA), the encoded pulses are decoded by a receiver AMZI and measured by a single-photon detector. The proof-of-principle BB84 QKD protocol is then implemented to optimize the QBER. As a coherence optimization, it is important to take phase randomization into account so that the QBER can be suppressed while ensuring that the phases of the qubits are simultaneously randomized. To achieve this, we exploit the fact 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 Figure 11. Based on this, we define 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 Figures 14A and 14B. As with the phase coherence optimization, all optimization trials eventually locate optimal parameters and converge toward a QBER of approximately 2.5%. Figure 14C further illustrates how the population evolves during optimization. Starting with a random distribution in parameter space, individuals gradually move toward the optimal region over generations, while others scatter in parameter space to continue searching. To verify the phase randomization, we eliminated channel attenuation and measured the output intensity probability distribution using an oscilloscope. As shown in Figure 15, the distribution follows the typical profile expected from interference between two phase-randomized pulses.

[0113] In terms of practical implementation, the above-described GA-based optimization technique can be seamlessly integrated into the software layer of a QKD system without requiring additional hardware modifications. In particular, the QBER optimization is designed to run within the QKD transmitter and receiver without the need for additional 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 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 possibility of downtime.

[0114] Regarding optimization performance, the speed of convergence depends on the complexity of the problem at hand and the genetic algorithm's control parameter configuration. To efficiently locate a global optimum, the population, especially in the first generation, must have sufficient genetic diversity so that the search for the optimum does not rely excessively on random mutations. Therefore, 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, as the population size increases, convergence time inevitably increases. Control parameter configuration is known to be problem-dependent. In this particular embodiment, the population size for phase coherence optimization was selected to be 35, but was increased to 60 for QBER optimization due to its larger parameter space. These values ​​were empirically determined to provide repeatable convergence within a reasonable time; further optimization is possible but beyond the scope of this study.

[0115] Furthermore, GA-based self-tuning techniques are goal-oriented: such optimization methods allow achieving optimal behavior without a priori knowledge of the underlying complex dynamics.

[0116] 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 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 come within the scope and spirit of the invention.

Claims

1. a transmitter comprising a plurality of transmitter components, said plurality of transmitter components comprising a pulsed radiation source and a modulation unit, said 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, wherein the control unit is 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 is 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, and comprise 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 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 using an objective function, 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 in a reinforcement learning algorithm that applies actions to the quantum communication system with the goal of maximizing the score, which is a reward; and and adjusting the plurality of control parameters using a reinforcement learning algorithm.

2. 2. The quantum communication system of claim 1, 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 pulses at the detectors.

3. 10. 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.

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

5. 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.

6. 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.

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 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.

8. 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.

9. 9. The quantum communication system of claim 8, wherein the set of control parameters comprises control parameters for both the primary laser and the secondary laser.

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

11. 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.

12. 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 plurality of sets 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, and comprise 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 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 using an objective function, 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 in a reinforcement learning algorithm that applies actions to the quantum communication system with the goal of maximizing the score, which is a reward; and and adjusting the plurality of control parameters using a reinforcement learning algorithm.

13. 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, wherein the control unit is configured to apply a plurality of control signals defined by a plurality of sets of control parameters to at least one of the plurality of receiver components, and the optimization unit is configured to obtain an adjustment set by adjusting the plurality of sets of control parameters; the plurality of control signals are a plurality of electronic control signals for a pulsed radiation source, and comprise at least one of an amplitude, a shape, and a DC offset of the electronic control signals included in the plurality of electronic control signals; The optimization unit 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 using an objective function, 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 in a reinforcement learning algorithm that applies actions to the quantum communication system with the goal of maximizing the score, which is a reward; and and adjusting the plurality of control parameters using a reinforcement learning algorithm.

14. 1. A method of controlling a quantum communication system, the quantum communication system comprising: a transmitter comprising a plurality of transmitter components, said plurality of transmitter components comprising a pulsed radiation source and a modulation unit, said 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, wherein the control unit is 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 is 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 using an objective function, 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 in a reinforcement learning algorithm that applies actions to the quantum communication system with the goal of maximizing the score, which is a reward, and adjusting the control parameters using a reinforcement learning algorithm; A method comprising:

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