A system for signal mode-matching for quantum key distribution setups

Abstract

The disclosure regards a signal mode-matching system for coherent optical receivers, the system comprising, an optical hybrid for receiving an optical signal, and comprising a local oscillator, the optical hybrid configured to separate the optical signal to a plurality of orthogonal components, and configured to interfere the plurality of orthogonal components of the optical signal with the local oscillator, a pair of photodiodes per orthogonal component, configured for detecting the outcomes of the interference for each corresponding orthogonal component, each photodiode of the pair generating a primary photocurrent, each pair of photodiodes combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent, a plurality of variable optical attenuators, one for each photodiode, each variable attenuator positioned between the optical hybrid and its corresponding photodiode, and an analog to digital converter configured to digitize each of the plurality of combined photocurrents corresponding to the orthogonal components, wherein the signal mode-matching system is configured to equalize the detection of each orthogonal component by performing frequency dependent gain-balancing in the digital domain using the output of the analog to digital converter, and controlling the variable optical attenuators to maximize the equalization of the optical efficiencies of the plurality of orthogonal components, thereby maximizing mode-matching among the different orthogonal components.

Description

[0001] A system for signal mode-matching for quantum key distribution setups

[0002] The present disclosure relates to a novel system and method for signal mode-matching of orthogonal components of an optical signal by equalizing the detection of each orthogonal component.

[0003] Background

[0004] The state of the art in the field of secure communication relates to cryptographic protocols where encryption of information is performed in order to prevent a third party from accessing sensitive information. A message from a first party can be encrypted, using an encryption key, and it can be decrypted by the second party using a decryption key. However, advances in technology have allowed malicious software to access such information and cause disruptions in governments, companies and institutions around the world. One way of tackling such an ever-growing problem is the implementation of quantum key distribution (QKD) protocols. QKD uses the principles of quantum mechanics to establish cryptographic keys that are impervious to eavesdropping. Two parties can produce a shared random secret key, which can be used to encrypt and decrypt messages. A unique feature of QKD is the ability of the two parties to detect the presence of any third party that is eavesdropping and gaining any information about the key, as the action of eavesdropping will cause noise in the system, thereby warning the two parties that an eavesdropper is present. QKD systems are classified as discrete-variable QKD and continuous-variable QKD (CV-QKD) systems. The present disclosure relates to the field of continuous-variable QKD, where typically linear photodiodes are used in the QKD system to measure quadratures of optical signals.

[0005] Even though CV-QKD and coherent telecommunication systems have some similarities, CV-QKD systems face various challenges, as they can be significantly more sensitive to imperfections compared to coherent telecommunication systems, due to noise stemming from the hardware components or due to the way that the photodiodes detect signals. Therefore, in long distance operations, such noise sources can interfere with quantum states that are processed by the QKD system, and render the QKD protocol impractical as the effect of the noise may reduce the rate at which transmitter and receiver generate secret keys, and it may not be possible to detect whether a third party interferes with the communication. Hence, a new method is required which can minimize the noise contributions from various sources in QKD systems, allowing secure continuous-variable QKD communication over large distances.

[0006] Summary

[0007] One purpose of the present disclosure is to optimize the signals received by a QKD receiver in order to reduce the overall noise in QKD communication. To address the challenge of noise, the present disclosure relates to a signal mode-matching system for coherent optical receivers. In one embodiment the system comprises an optical hybrid for receiving an optical signal, and comprises a local oscillator, wherein the optical hybrid is configured to separate the optical signal to a plurality of orthogonal components, and configured to interfere the plurality of orthogonal components of the optical signal with the corresponding components of the local oscillator. In addition, the system may comprise a pair of photodiodes per orthogonal component, configured for detecting the outcomes of the interference for each / the corresponding orthogonal component, each photodiode of the pair generating a primary photocurrent, wherein each pair of photodiodes are preferably combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent. Moreover, the system may comprise a plurality of optical attenuators, preferably variable attenuators, one for each photodiode, wherein each variable attenuator may be positioned between the optical hybrid and its corresponding photodiode. Furthermore, the system may comprise an analog to digital converter configured to digitize each of the plurality of combined photocurrents corresponding to the orthogonal components. The signal mode-matching system is preferably configured to equalize the detection of each orthogonal component by performing frequency dependent gainbalancing in the digital domain using the output of the analog to digital converter, and controlling the variable optical attenuators to maximize the equalization of the optical efficiencies of the plurality of orthogonal components, thereby maximizing modematching among the different orthogonal components.

[0008] Specifically, the optical hybrid may separate the optical signal to a plurality of orthogonal components, such as components having different phase and polarization. The optical hybrid may operate by extracting orthogonal components of an optical signal, such as phase, amplitude and polarization, by interfering the optical signal with a local oscillator. In a preferred embodiment, the plurality of orthogonal components may be detected using a plurality of sets of photodiodes. A plurality of variable optical attenuators can be used to control the optical powers of the interfered outcomes corresponding to each orthogonal component, such that a precise match of the optical power with the electro- optical characteristics of the photodiodes for ideal balanced detection is achieved. As a result, all the orthogonal components of the optical signal can be measured in the same manner, i.e. by maximizing signal mode-matching among the different orthogonal components. After the two generated photocurrents from the pair of photodiodes are subtracted to yield a combined photocurrent, the plurality of combined photocurrents can be converted to digital signals using digitizers, such as an analog to digital converter. Then, digital-signal-processing (DSP) operations can be performed on a processing unit such as a field programmable gate array (FPGA), which then provide feedback to the physical (the variable optical attenuators) and frequency dependent gain-balancing components. Further details regarding the variable optical attenuators and the digital-signal-processing operations are provided in the detailed description of the present disclosure.

[0009] In a preferred embodiment, four pairs of photodiodes may be utilized, generating four combined photocurrents. Each combined photocurrent relates to a certain phase and / or polarization orthogonal component. For example, the four orthogonal components may be an X polarization in-phase component (XI), an X polarization quadrature component (XQ), a Y polarization in-phase component (Yl), and Y polarization quadrature component (YQ). The combined photocurrents that correspond to said four orthogonal components may be inputted to an analog to digital converter, said analog to digital converter can digitize the combined photocurrents. Then, in the preferred embodiment, frequency dependent gain-balancing in the digital domain can be performed, such that each orthogonal component is measured in the same temporal mode matching the temporal mode of the quantum states. The process of performing this maximizes the equalization of the optical efficiencies of the orthogonal components. At the same time, the variable optical attenuators may be controlled, in order to assist in the equalization of the optical efficiencies.

[0010] The presently disclosed approach utilizes joint physical-digital compensation by conducting optimisation of the plurality of variable optical attenuators in combination with the frequency dependent gain-balancing in the digital domain, thereby maximizing mode-matching among the different orthogonal components. As a result, the noise in the coherent optical receiver can be minimized.

[0011] As the above system can be utilized for detection of quantum states the presently disclosed system can be configured, such that the optical signal carries quantum information, and wherein the system is configured to combine the digitized signals to an output signal relaying said quantum information.

[0012] Moreover, the system may further comprise a feedback loop utilized to repeat the frequency dependent gain-balancing and the control of the variable optical attenuators, thereby optimizing the signal mode-matching. Using such a feedback loop can provide optimal signal mode-matching among the different orthogonal components of the optical signal, thereby minimizing the noise in the coherent optical receiver.

[0013] In an embodiment, the separation of the optical signal to a plurality of orthogonal components is made with respect to the polarization and / or the phase of the optical signal. Depending on the type of quantum communication system, different orthogonal components can be used. For example, in a QKD protocol configuration, there may be only separation with respect to the polarization, or there may be only separation with respect to the phase of the optical signal.

[0014] Furthermore, the system can be configured, such that each variable optical attenuator comprises a fiber, wherein attenuation is introduced to a variable optical attenuator by means of bend loss attenuation, such as through bending the fiber.

[0015] The present disclosure also relates to a coherent optical receiver for detection of quantum information in an optical signal, the receiver comprising a signal modematching system according to any one of the paragraphs of the present disclosure, wherein the optical signal is a quantum signal, and the receiver is configured to combine the digitized signals to an output digital quantum signal.

[0016] Moreover, the present disclosure may also relate to a method for signal modematching of an optical signal, comprising the steps of generating a plurality of orthogonal components based on the optical signal, interfering the plurality of orthogonal components with a local oscillator, and obtaining a plurality of pairs of photodiodes for each orthogonal component, configured for detecting the outcomes of the interference for each / the corresponding orthogonal component, each photodiode of the pair generating a primary photocurrent, each pair of photodiodes combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent. Moreover, the method may comprise the steps of obtaining a plurality of variable optical attenuators, each variable optical attenuator positioned prior to a photodiode, digitizing each of the plurality of combined photocurrents corresponding to the orthogonal components, equalizing the optical efficiencies of the plurality of orthogonal components by controlling the plurality of variable optical attenuators, and performing frequency dependent gain-balancing of the digitized orthogonal components, thereby obtaining maximum mode-matching among the different orthogonal components.

[0017] Description of Drawings

[0018] Various embodiments are described hereinafter with reference to the drawings. The drawings are examples of embodiments and are intended to illustrate some of the features of the presently disclosed system for signal mode-matching for quantum key distribution setups, and are not limiting to the presently disclosed system and method.

[0019] Fig. 1 shows a schematic of a digital receiver in homodyne detection configuration. Fig. 2 A, B shows an example of frequency dependent gain-balancing.

[0020] Fig. 3 shows the steps of the method for signal mode-matching of an optical signal. Fig. 4 shows an example of a QKD system comprising an optical transmitter and four optical receivers configured to simultaneously generate encryption keys on all four optical receivers.

[0021] Fig. 5 shows an example of a CV-QKD system comprising an optical receiver that comprises a signal mode-matching system.

[0022] Fig. 6 shows the secret key rate in kBit / s as a function of number of measured states in the asymptotic regime and the composable regime.

[0023] Detailed description

[0024] A purpose of the present disclosure is to optimize the signals received by a QKD system in order to reduce the overall noise in QKD communication. Specifically, the present disclosure relates to a signal mode-matching system for coherent optical receivers, the system comprising, an optical hybrid for receiving an optical signal, and comprising a local oscillator, the optical hybrid configured to separate the optical signal to a plurality of orthogonal components, and configured to interfere the plurality of orthogonal components of the optical signal with the local oscillator, a pair of photodiodes per orthogonal component, configured for detecting the outcomes of the interference for each / the corresponding orthogonal component, each photodiode of the pair generating a primary photocurrent, each pair of photodiodes combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent, a plurality of variable optical attenuators, one for each photodiode, each variable attenuator positioned between the optical hybrid and its corresponding photodiode, and an analog to digital converter configured to digitize each of the plurality of combined photocurrents corresponding to the orthogonal components, wherein the signal mode-matching system is configured to equalize the detection of each orthogonal component by performing frequency dependent gain-balancing in the digital domain using the output of the analog to digital converter, and controlling the variable optical attenuators to maximize the equalization of the optical efficiencies of the plurality of orthogonal components, thereby maximizing mode-matching among the different orthogonal components.

[0025] The optical hybrid can enable the extraction of orthogonal components of an optical signal, such as phase, amplitude and polarization, by performing interferences between the optical signal and a local oscillator. For example, the optical hybrid may be a 90° optical hybrid that enables the extraction of orthogonal components such as phase and amplitude, by performing four 90° phase stepped interferences between the optical signal and the local oscillator. An example of an optical hybrid 100 can be seen in Fig. 1 , where an optical signal 101 is interfered with a local oscillator 102, producing orthogonal components 103 of the optical signal. Preferably, the operation condition for the local oscillator power may be chosen such that all pairs of photodiodes that detect one of the orthogonal components have the same amount of shot noise to electronic noise clearance, i.e. same amount of electronic noise contribution.

[0026] After the plurality orthogonal components are separated, a pair of photodiodes 104 can be used per orthogonal component, with the purpose of detecting the outcomes of the interference of each corresponding orthogonal component. When a light signal, such as an orthogonal component, is transmitted to a photodiode, a photocurrent is generated. Per orthogonal component, two photocurrents are generated, one from each photodiode of a pair, and the two photocurrents are subtracted, resulting in a combined photocurrent 105. It can be advantageous if the combined photocurrents have equalized optical efficiencies, as that can maximize the mode-matching among the different orthogonal components, and reduce the noise in the system. Therefore, a plurality of variable optical attenuators 106 can be used, one for each photodiode, in order to optimize the signal of each orthogonal component before each orthogonal component interacts with its corresponding photodiode. Further details about the characteristics of the variable optical attenuators are provided at the later sections of the present disclosure. In this example, four pairs of photodiodes are utilized, generating four combined photocurrents. Each combined photocurrent relates to a certain phase and / or polarization orthogonal component. For example, the four orthogonal components may be an X polarization in-phase component (XI), an X polarization quadrature component (XQ), a Y polarization in-phase component (Yl), and Y polarization quadrature component (YQ).

[0027] An analog to digital converter 111 can be utilized, in order to digitize the generated combined photocurrents. The combined photocurrents may be radio-frequency (RF) electrical signals, which RF electrical signals can be digitized using the analog to digital converter. A variety of analog to digital (ADC) converters may be used, such as successive approximation ADC, delta-sigma ADC, dual slope ADC, pipelined ADC or Flash ADC.

[0028] Using the digitized signals, the system may perform frequency dependent gainbalancing 107 in the digital domain by utilizing digital signal processing tools. In addition, the attenuation of the variable optical attenuators 106 may be controlled, in order to maximize the equalization of the optical efficiencies of the plurality of orthogonal components. Combining the control of the variable optical attenuators, and the frequency dependent gain-balancing, the present disclosure can equalize the detection of each orthogonal component, and ensure vacuum-noise limited detection. After the optimization is complete, the orthogonal components can be combined 108, and the system can output 109 an optical signal.

[0029] In an embodiment, each variable optical attenuator can be implemented as a fiberbased component where attenuation is introduced by various means, such as by bend- induced loss, with bending radii adjusted via piezoelectric actuators controlled by a feedback loop. For example, during an initial calibration phase, the direct current (DC) output from each photodiode pair can be monitored and compared to a target reference level derived from the expected shot noise clearance for each orthogonal component. The feedback loop, implemented in a processing unit such as a field programmable gate array (FPGA), may dynamically adjusts the bend radius of each fiber until the photocurrent outputs of all channels are matched within a predetermined tolerance, e.g. less than 0.05 dB. During live operation, the same feedback loop may continue to adjust the variable optical attenuators in response to slow drifts due to environmental conditions or system instability. This adaptive control can ensure that the optical powers incident on the photodiodes are balanced, thereby equalizing the optical efficiencies across the orthogonal components. The result is enhanced mode-matching and reduced excess noise, which can support secure and high-rate key generation in continuous-variable quantum key distribution systems.

[0030] Further means of introducing attenuation on the VOAs can be via utilizing thermo-optic effects, electro-optic attenuations, or tunable directional couplers.

[0031] Fig. 2 A, B shows an example of frequency dependent gain-balancing, where the power 200 shown in arbitrary units of an orthogonal component is measured as a function of frequency 201 in MHz. Fig. 2A shows a measurement of the power of an orthogonal component 202 before performing frequency dependent gain-balancing, where the power of the orthogonal component is not variable as a function of the frequency. Fig. 2A, B also show a measurement of the electronic noise 203 of the system as a function of frequency. Fig. 2B shows a measurement after the frequency dependent gain-balancing is performed, where the power of the orthogonal component 204 is stable for a frequency up to approximately 300 MHz. In order to minimize the noise in the system, one goal is to achieve constant power for a range of frequencies. For example, such a constant power is shown in Fig. 2B, where the power of the vacuum plus electronic noise in stable in a range from 10 MHz to approximately 300 MHz. The security protocol for CV-QKD systems typically assumes that the vacuum states detected alongside the quantum states are independent and identically distributed. If this is not the case, then potentially the correlations may result in a higher amount of noise in the system upon executing the protocol, leading to decreased system performance. Therefore, achieving a relatively constant power of the orthogonal component 204 as a function of frequency is related to a reduction of noise in the system.

[0032] Moreover, the system can be configured, such that the optical signal carries quantum information, and wherein the system is configured to combine the digitized signals to an output signal relaying said quantum information. As the present disclosure can be directed to an optical receiver for QKD, the signals that are processed may be quantum signals carrying quantum information. Such quantum signals can also be separated into orthogonal components.

[0033] In addition, the system can be configured to comprise a feedback loop utilized to repeat the frequency dependent gain-balancing and the control of the variable optical attenuators, thereby optimizing the signal mode-matching. In certain scenarios, the first iteration of frequency dependent gain-balancing and of the control of the variable optical attenuators may not provide an optimal result. Therefore, a feedback loop process may be utilized, which can use as input the output optical signal. That information can be processed by a processing unit 110, which can provide feedback to the frequency dependent gain-balancing process and to the control of the variable optical attenuators. Such a loop can be repeated a plurality of times, unit the resulting optical signal fulfils certain requirements, such as maximized mode-matching among the different orthogonal components.

[0034] In an embodiment, the feedback loop process can be configured, such that the feedback loop process comprises a threshold value, selected to trigger the termination of the feedback loop when the mode-matching among the different orthogonal components meets the provided threshold value. For example, depending on the setting that the system is applied, there may be different tolerance on the noise level. Therefore, a threshold value can be used, which can be tuned depending on the application and on the type of optical signal, in order to optimize the feedback loop process.

[0035] The system can be configured, such that the separation of the optical signal to a plurality of orthogonal components is made with respect to the polarization and / or the phase of the optical signal. As described in the earlier parts of the present disclosure, depending on the type of application, the optical signal may be separated to different orthogonal components. In the schematic shown in Fig. 1, the optical signal 101 is separated to four orthogonal components, using four pairs of photodiodes. The four orthogonal components may be an X polarization in-phase component (XI), an X polarization quadrature component (XQ), a Y polarization in-phase component (Yl), and Y polarization quadrature component (YQ).

[0036] The range of frequencies where the frequency dependent gain-balancing is performed may be different depending on the application of the system, and / or depending on the type of the optical signal. Hence, the system can be configured, such that the frequency dependent gain-balancing in the digital domain is performed over a range of frequencies, preferably from 1 MHz to 25 GHz, more preferably from 1 MHz to 50 GHz, even more preferably from 1 MHz to 100 GHz, most preferably from 1 kHz to 100 GHz. The bandwidth of the frequency dependent gain-balancing may be chosen depending on various parameters, such as the type of the optical signal, the physical characteristics of the electro-optic components used, and the bandwidth of the photodiodes.

[0037] Furthermore, the system can be configured, such that each variable optical attenuator comprises a fiber, wherein attenuation is introduced to a variable optical attenuator by means of bend loss attenuation, such as through bending the fiber. Introducing bending on a fiber leads to an increase of the attenuation. Such a process can be performed automatically, by connecting the feedback loop process to the fiber of each variable optical attenuator. Alternatively, attenuation may be introduced by using insertion loss attenuation. For example, that can be achieved by including certain components in communication with the variable optical attenuators, said component can attenuate a signal.

[0038] Moreover, the system can be configured, such that the bend loss attenuation is associated with a bending radius. For example, the larger the radius of a bent part of a fiber, the larger the resulting attenuation. Such a relation can be utilized, in order to tune the attenuation on each orthogonal component, thereby achieving maximum mode-matching among the different orthogonal components.

[0039] In order to fine-tune the attenuation and to provide optimal orthogonal components, each variable optical attenuator can be configured, such that each bending radius may modify the attenuation in steps, preferably in steps of less than 2.5 dB, more preferably less than 0.5 dB, even more preferably less than 0.1 dB, most preferably less than 0.05 dB. The lower the attenuation step, the higher the resolution of tuning each orthogonal component.

[0040] In an embodiment, the signal mode-matching system is configured to process the digitized combined photocurrents to equalize a gain associated with each orthogonal component, such that the vacuum noise level corresponding to each orthogonal component is symmetrized. The signal mode-matching system may apply a digital scaling factor to each digitized combined photocurrent to correct gain imbalances between orthogonal components such as XI, XQ, Yl, and YQ. These gain imbalances may arise from variations in optical paths, photodiode characteristics, or electronic amplification. The scaling factors may be chosen such that the variance of vacuum noise measured on each component matches a common reference level. The processing may be implemented in a digital signal processor, for example in an FPGA or microcontroller, and may operate in real time or during calibration. This gain equalization may allow consistent sensitivity across components and improve the symmetry of detection, which may be beneficial for quantum communication protocols relying on assumptions of uniform measurement statistics. Symmetrization of the vacuum noise level may improve the detection balance between channels, such that identical quantum states are detected with statistically indistinguishable noise characteristics regardless of the orthogonal component in which they are encoded. In certain implementations, the gain equalization may be combined with whitening filters, frequency domain corrections, or calibration routines based on reference measurements.

[0041] In one embodiment of the present disclosure, the system comprises a calibration scheme configured to measure the variance of vacuum noise in each orthogonal component of the plurality of orthogonal components, and apply a scaling factor to equalize the measured vacuum noise among the plurality of orthogonal components. The calibration scheme may be executed during a setup phase or intermittently during operation, by measuring the response of the detection system when only vacuum noise is present. The variances of the digitized combined photocurrents may be compared across components, and component-specific scaling factors may be calculated and applied in the digital domain. This may ensure consistent vacuum noise levels across all channels, improving measurement balance and reducing the impact of technical imperfections. The calibration may be performed automatically using internal switching or control logic, and may be repeated to compensate for environmental or hardware changes. An advantage of such a calibration scheme is that it can enable the system to achieve more uniform and predictable measurement behaviour across orthogonal components, reducing asymmetry and lowering the contribution of noise. This may in turn increase the effective signal-to-noise ratio for quantum state measurements and improve the robustness of the quantum key distribution protocol. The calibration may also be used in conjunction with other routines, such as electronic noise calibration, common-mode rejection optimization, or optical alignment procedures. By incorporating a dedicated calibration scheme, the system may provide an automated and scalable approach to maintaining performance across varying conditions and deployment scenarios.

[0042] The present disclosure further relates to a coherent optical receiver for detection of quantum information in an optical signal, the receiver comprising a signal modematching system according to any one of the embodiments described in the present disclosure, wherein the optical signal is a quantum signal, and the receiver is configured to combine the digitized signals to an output digital quantum signal. For example, such a coherent optical receiver may be used in a QKD system.

[0043] In addition, the present disclosure relates to a method for signal mode-matching of an optical signal, comprising the steps of generating a plurality of orthogonal components based on the optical signal, interfering the plurality of orthogonal components with a local oscillator, obtaining a plurality of pairs of photodiodes for each orthogonal component, configured for detecting the outcomes of the interference for each / the corresponding orthogonal component, each photodiode of the pair generating a primary photocurrent, each pair of photodiodes combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent, obtaining a plurality of variable optical attenuators, each variable optical attenuator positioned prior to a photodiode, digitizing each of the plurality of combined photocurrents corresponding to the orthogonal components, equalizing the optical efficiencies of the plurality of orthogonal components by controlling the plurality of variable optical attenuators, and performing frequency dependent gain-balancing of the digitized orthogonal components, thereby obtaining maximum mode-matching among the different orthogonal components. Furthermore, the method may comprise the step measuring a variance of vacuum noise in each orthogonal component of the plurality of orthogonal components, and applying a scaling factor to equalize the measured vacuum noise among the plurality of orthogonal components. Such steps can be performed according to the calibration scheme described herein.

[0044] The steps of the method are described in Fig. 3, where the method comprises the steps of generating a plurality of orthogonal components based on an optical signal 300, interfering the plurality of orthogonal components with a local oscillator 301, obtaining a plurality of pairs of photodiodes for each orthogonal component, configured for detecting the outcomes of the interference for each / the corresponding orthogonal component, each photodiode of the pair generating a primary photocurrent, each pair of photodiodes combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent 302, obtaining a plurality of variable optical attenuators, each variable optical attenuator positioned prior to a photodiode 303, digitizing each of the plurality of combined photocurrents corresponding to the orthogonal components 304, equalizing the optical efficiencies of the plurality of variable optical attenuators 305, and performing frequency dependent gain-balancing of the digitized orthogonal components 306, thereby obtaining maximum modematching among the different orthogonal components 307.

[0045] Moreover, the method can be executed by means of the system according to any one of the embodiments described in the present disclosure.

[0046] The present disclosure further relates to a method for detection of quantum information in an optical signal, comprising the steps of performing signal mode-matching of the optical signal according to any one of the embodiments of the present disclosure, and obtaining a coherent optical receiver for detection of quantum information in the optical signal.

[0047] Examples

[0048] This system can be used for the detection of continuous variable quantum states creating correlations between two parties, one transmitting the states (Alice) and one detecting the states (Bob). The receiving party, in order to implement most properly the QKD protocol must detect the transmitted quantum states, congruent with the security assumptions of the protocol which e.g., are that the frequency response of all devices in the detector are maximally flat over the frequency range in which the quantum states are transmitted. The physical efficiency of the detection is equalized such that each component of the quantum states are detected equally. This ensures secure implementation of the CV-QKD protocol, and that after the signal processing procedures are performed maximum correlation between both parties is preserved. The described system is a two party communications system.

[0049] The present disclosure may be also generalized to a multi-partite system to which the above assumptions still apply, the first of which is the measurement device independent (MDI) protocol in QKD. Two spatially separate parties (Alice and Bob) will generate quantum states of light which are transmitted over respective communications links to an untrusted third party (Charlie) who can measure the quantum states transmitted from Alice and Bob after they are combined on an optical beam combiner using the optimal process mentioned above. The third party will announce faithfully the result of this measurement with which Alice and Bob will generate a key via the measured correlations with each other.

[0050] A multi-partite use case for the present disclosure (extending to three or more parties) is in a 1 x N configuration where N is greater than 1, forming a network configuration where one Alice is generating a key with many Bobs via using an optical splitter. In the simplest case this is a multitude of the system described above, however with additional loss from the optical splitter.

[0051] The feedback process for the VOAs may comprise a computing unit (CU) either field programmable gate array (FPGA), application-specific integrated circuit (ASIC), central processing unit (CPU), graphics processing unit (GPU) or microcontroller. The CU will take a DC output (read out from the pairs of photodiodes) in either digital or analog format and use it as a feedback signal to perform balancing of the VOAs.

[0052] Between pairs of photodiodes, the splitting ratio of the optical hybrid between respective components of light is initially characterized, this characterization is then used to set initial parameters for the VOAs in accordance with the requirements described in the present disclosure. Measurements taken during the on-line calibration phase of the CV-QKD protocol can be used as a further feedback to adjust the VOA settings as needed, with reference to initial calibration. Fig. 4 shows an example of a CV-QKD system that comprises the signal modematching system for coherent optical receivers as disclosed herein. An optical transmitter 400 can generate signals that can be fed into a 1:4 splitter 401. Therefore, the signal can be transmitted to four different optical receivers 402, where each optical receiver has a difference distance to the 1 :4 splitter. Each optical receiver comprises the signal mode-matching system as described herein. By performing such an experiment, encryption keys in the four optical receivers can be simultaneously generated by utilizing the signal mode-matching system. Specifically, each optical receiver is configured to perform frequency dependent gain-balancing, and each optical receiver is configured to control the variable optical attenuators on each optical receiver to maximize the equalization of the optical efficiencies of the plurality of orthogonal components, thereby maximizing mode-matching among the different orthogonal components.

[0053] Fig. 5 shows an example of a CV-QKD system 500 comprising an optical receiver 501 that comprises a signal mode-matching system 502 as described in the present disclosure. The CV-QKD system also comprises a transmitter 503 for transmitting optical signals and a 40 km channel 504 where the optical signal is transmitted to the optical receiver. As described herein, the signal-mode matching system is configured to separate the optical signal to a plurality of orthogonal components, such as polarization in-phase component (XI), polarization quadrature component (XQ), polarization in- phase component (Yl), polarization quadrature component (YQ). In the illustrated configuration of Fig. 5, two orthogonal components are separated, but depending on the type of application more orthogonal components can be detected. An optical hybrid may be used for such a task. As described herein, the system comprises a pair of photodiodes per orthogonal component, configured for detecting the outcomes of the interference for each corresponding orthogonal component. Each photodiode of the pair can generate a primary photocurrent, and each pair of photodiodes are combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent. The system may also comprise a plurality of variable optical attenuators, one for each photodiode, each variable attenuator positioned between the optical hybrid and its corresponding photodiode. Then, signal mode-matching system can equalize the detection of each orthogonal component by performing frequency dependent gain-balancing in the digital domain using the output of an analog to digital converter, and by controlling the variable optical attenuators to maximize the equalization of the optical efficiencies of the plurality of orthogonal components, thereby maximizing mode-matching among the different orthogonal components. After digitizing the combined photocurrents corresponding to each orthogonal component, the system performs digital equalization of the gain per polarization component in order to symmetrize the vacuum noise levels. Specifically, a shot-noise calibration is performed wherein the optical signal is suppressed and only vacuum fluctuations are detected. The digitized outputs corresponding to each orthogonal component are statistically analyzed to determine their respective noise variances. Based on these measurements, a digital gain correction factor can be calculated and applied to each component such that the variance of the vacuum noise is equal across all channels. This process can ensure that the detection of each orthogonal component occurs with equal sensitivity to vacuum fluctuations. As a result, excess noise is minimized and the symmetry of the receiver’s quantum efficiency across polarizations is maintained. In the example shown in Fig. 5, frequency dependent gain-balancing is performed by first digitizing the combined photocurrents corresponding to the orthogonal components using a high-speed analog to digital converter. The digitized signals are inputted to a digital signal processing unit, wherein a whitening filter is applied to decorrelate the detector response and flatten the baseline response of each photodiode channel. Following this, the gain profiles of each orthogonal component are equalized by calculating the power spectral density of the received signal and identifying deviations from flatness across the target bandwidth, for example from 1 MHz to 500 MHz. A gainequalization filter is then dynamically constructed and applied per channel to achieve uniform gain over frequency. Hence, frequency dependent gain-balancing is achieved. In an embodiment, a signal, such as a pilot signal or a beat signal, can be derived from the local oscillator and can be isolated and tracked, and its frequency location can then be estimated through linear phase fitting. Such a process may enhance phase coherence in a QKD setup.

[0054] Utilizing the system of the present disclosure, it is possible to generate key rates of 42 kBit / s for 1.8 x 1O10states. Such a result is achieved by utilizing the frequency dependent gain-balancing and the to maximize the equalization of the optical efficiencies of the plurality of orthogonal components by controlling the variable optical attenuators. Fig. 6 shows the secret key rate 600 in kBit / s as a function of number of measured states 601 in the asymptotic regime 602 and the composable regime 603. A secret key generation with at least 1010measured states is achieved, over the total set of states (» 1.8x1 o10). A maximum throughput of 42 kBit / s is demonstrated, equivalent to 44% of the asymptotic limit of 96 kBit / s. As a result, utilizing the system of the present disclosure enables the generation of higher key rates in CV-QKD setups.

Claims

Claims1. A signal mode-matching system for coherent optical receivers, the system comprising,• an optical hybrid for receiving an optical signal, and comprising a local oscillator, the optical hybrid configured to separate the optical signal to a plurality of orthogonal components, and configured to interfere the plurality of orthogonal components of the optical signal with the local oscillator,• a pair of photodiodes per orthogonal component, configured for detecting the outcomes of the interference for each corresponding orthogonal component, each photodiode of the pair generating a primary photocurrent, each pair of photodiodes combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent,• a plurality of variable optical attenuators, one for each photodiode, each variable attenuator positioned between the optical hybrid and its corresponding photodiode, and• an analog to digital converter configured to digitize each of the plurality of combined photocurrents corresponding to the orthogonal components, wherein the signal mode-matching system is configured to equalize the detection of each orthogonal component by o performing frequency dependent gain-balancing in the digital domain using the output of the analog to digital converter, and o controlling the variable optical attenuators to maximize the equalization of the optical efficiencies of the plurality of orthogonal components, thereby maximizing mode-matching among the different orthogonal components.

2. The system according to claim 1, wherein the optical signal carries quantum information, and wherein the system is configured to combine the digitized signals to an output signal relaying said quantum information.

3. The system according to any one of the preceding claims, comprising a feedback loop utilized to repeat the frequency dependent gain-balancing andthe control of the variable optical attenuators, thereby optimizing the signal mode-matching.

4. The system according to claim 3, comprising a threshold value, selected to trigger the termination of the feedback loop when the mode-matching among the different orthogonal components meets the provided threshold value.

5. The system according to any one of the preceding claims, wherein the separation of the optical signal to a plurality of orthogonal components is made with respect to the polarization and / or the phase of the optical signal.

6. The system according to any one of the preceding claims, wherein the frequency dependent gain-balancing in the digital domain is performed over a range of frequencies, preferably from 1 MHz to 25 GHz, more preferably from 1 MHz to 50 GHz, even more preferably from 1 MHz to 100 GHz, most preferably from 1 kHz to 100 GHz.

7. The system according to any one of the preceding claims, wherein each variable optical attenuator comprises a fiber, wherein attenuation is introduced to a variable optical attenuator by means of bend loss attenuation, such as through bending the fiber.

8. The system according to claim 7, wherein the bend loss attenuation is associated with a bending radius.

9. The system according to claim 8, wherein each bending radius is configured to modify the attenuation in steps, preferably in steps of less than 2.5 dB, more preferably less than 0.5 dB, even more preferably less than 0.1 dB, most preferably less than 0.05 dB.

10. The system according to any one of the preceding claims, wherein the signal mode-matching system is configured to process the digitized combined photocurrents to equalize a gain associated with each orthogonal component, such that the vacuum noise level corresponding to each orthogonal component is symmetrized.

11. A coherent optical receiver for detection of quantum information in an optical signal, the receiver comprising a signal mode-matching system according to any one of the claims 1-10, wherein the optical signal is a quantum signal, and the receiver is configured to combine the digitized signals to an output digital quantum signal.

12. A method for signal mode-matching of an optical signal, comprising the steps of• generating a plurality of orthogonal components based on the optical signal,• interfering the plurality of orthogonal components with a local oscillator,• obtaining a plurality of pairs of photodiodes for each orthogonal component, configured for detecting the outcomes of the interference for each corresponding orthogonal component, each photodiode of the pair generating a primary photocurrent, each pair of photodiodes combined such that the two primary photocurrents of each pair of photodiodes generate a combined photocurrent,• obtaining a plurality of variable optical attenuators, each variable optical attenuator positioned prior to a photodiode,• digitizing each of the plurality of combined photocurrents corresponding to the orthogonal components,• equalizing the optical efficiencies of the plurality of orthogonal components by controlling the plurality of variable optical attenuators, and• performing frequency dependent gain-balancing of the digitized orthogonal components, thereby obtaining maximum mode-matching among the different orthogonal components.

13. The method according to claim 12, further comprising the step of measuring a variance of vacuum noise in each orthogonal component of the plurality of orthogonal components, and applying a scaling factor to equalize the measuredvacuum noise among the plurality of orthogonal components.

14. The method according to any one of the claims 12-13, wherein the method is executed by means of the system according to any one of the claims 1-11.

15. A method for detection of quantum information in an optical signal, comprising the steps of• performing signal mode-matching of the optical signal according to any one of the preceding claims 12-13, and • obtaining a coherent optical receiver for detection of quantum information in the optical signal.

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