Optical transmitter, quantum communication system, and method of operating an optical transmitter

Abstract

To provide an optical transmitter for quantum key delivery, a method for operating the same, and a quantum communication system.SOLUTION: In an optical system 1, an optical transmitter 2 includes a first laser (signal laser 9) and a second laser (decoy laser 11). Each laser emits a phase-randomized pulse. The optical transmitter also includes an optical combiner 10 that combines the pulses emitted from the first and second lasers based on an adjustable combining ratio to make them into a combined stream of the pulses. The optical transmitter further includes an asymmetric Mach-Zehnder interferometer 19 having a short arm 21 and a long arm 23. The interferometer has an input port for receiving the combined stream of the pulses and an output port for providing the output of the interferometer. At least one of the arms includes a phase control element 25 for controlling the phase between the short arm and the long arm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION The embodiments described herein relate to optical transmitters, quantum communication systems, and methods of operating optical transmitters. [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 photon carries one bit of information encoded on a property of the photon, such as its polarization, phase, or energy / time. Quantum communication systems can be used to implement quantum key distribution (QKD), a technique for sharing a cryptographic key between two parties: a sender, often called "Alice," and a receiver, often called "Bob." The appeal of this technique is that it provides an unauthorized eavesdropper, often called "Eve," with a test of whether any part of the key may be known.

[0003] QKD techniques (often referred to as protocols) are often implemented using attenuated laser pulses rather than single-photon states (e.g., by encoding cryptographic keys in the phase of the attenuated laser pulses). This is because suitable single-photon sources are not widely available. It is known that the use of attenuated laser pulses can make QKD protocols susceptible to photon number splitting attacks, thereby reducing the security of communications. Specific QKD protocols have been developed to address this issue. In some of these protocols, the transmitter prepares and transmits a set of additional state pulses (called decoy state pulses) in addition to the standard signal state pulses. The decoy state pulses have an intensity different from that of the signal state pulses (i.e., the average photon number). Decoy state QKD protocols can enhance the technology by ensuring that keys can be shared at high bit rates over long distances with information-theoretic security.

[0004] Decoy-state QKD protocols are often implemented using a single laser in combination with an intensity modulator to adjust the pulse intensity to correspond to either the signal state or the decoy state. In practice, imperfections in commonly used intensity modulators can introduce security threats (known as side channels). Such side channels (known as "pattern effects") are caused by the finite modulation bandwidth of the intensity modulator, which can mean that the intensity of a transmitted pulse depends on the intensity of the previous pulse. This can leak information and reduce the secure key rate. [Brief explanation of the drawings]

[0005] [Figure 1] FIG. 1 is a schematic diagram of an exemplary optical system according to one embodiment. [Figure 2] FIG. 2 shows a table specifying an exemplary configuration of the optical system of FIG. [Figure 3] FIG. 3 illustrates an exemplary pulse sequence during use of the optical system of FIG. [Figure 4] FIG. 4 is a flow diagram of an exemplary process for operating the optical system of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0006] This disclosure aims to provide a new and useful optical device for implementing (decoy-state) QKD protocols that overcomes challenges of conventional devices, such as the "pattern effect" associated with the finite bandwidth of intensity modulators. To this end, this disclosure presents an optical transmitter comprising two independent lasers that generate a signal-state pulse and a decoy-state pulse, respectively (the decoy-state pulse has an intensity different from that of the signal-state pulse, i.e., the average photon number). The outputs of the two lasers are combined using a tunable optical combiner, i.e., a combiner whose combining ratio can be adjusted. Depending on the combining ratio, the combiner can modulate the signal-state pulse differently from the decoy-state pulse, for example, by attenuation or amplification. This makes it possible to control the intensity ratio between the signal-state pulse and the decoy-state pulse. The pulses exiting the combiner are then converted into time-bin qubits by an asymmetric Mach-Zehnder interferometer and transmitted to a receiver. A phase control element in one arm of the Mach-Zehnder interferometer can be used to phase-encode the time-bin qubits. In addition to the signal and decoy pulses described above, the proposed transmitter can also emit a vacuum state (as a second type of decoy state). The proposed transmitter can implement the vacuum state by controlling the average photon number of the emitted pulse to be zero (i.e., by simply switching off both lasers).

[0007] The use of two independent lasers to generate the signal and decoy states means that the splitting ratio of the optical combiner does not need to be switched during transmitter operation, and therefore no (fast) switching of intensity modulators or similar is required (the preferred level of attenuation or amplification can be determined during an initial or periodically repeated calibration process). Therefore, the presented transmitter does not suffer from the "pattern effect" of conventional devices. The presented method of generating the signal and decoy states is compatible with photonic integration, enabling fully integrated QKD transmitters. Furthermore, the signal-to-noise ratio for the vacuum state can be improved by the presented transmitter compared to conventional implementations that prepare the vacuum state by attenuating laser pulses with an intensity modulator (due to the finite extinction ratio of commonly used intensity modulators).

[0008] According to a first aspect, an optical transmitter for quantum key distribution is provided. The optical transmitter includes a first laser and a second laser. Each laser is configured to emit a plurality of optical pulses such that the phase of each pulse among the plurality of optical pulses is randomized. The optical transmitter further includes an adjustable optical combiner configured to combine the plurality of optical pulses emitted by the first laser and the second laser into a combined stream of pulses based on an adjustable combining ratio. The optical transmitter further includes a Mach-Zehnder interferometer having a short arm and a long arm. The Mach-Zehnder interferometer has an input port configured to receive the combined stream of pulses and an output port configured to provide an output of the Mach-Zehnder interferometer. The optical transmitter further includes an output coupler configured to provide the output of the Mach-Zehnder interferometer to a quantum channel. At least one of the short arm and the long arm includes a phase control element configured to control the relative phase between the short arm and the long arm.

[0009] In one embodiment, the first laser is a gain-switched laser and / or the second laser is a gain-switched laser.

[0010] In one embodiment, the first laser, the second laser, the optical combiner, and the Mach-Zehnder interferometer may be formed on an integrated chip.

[0011] In one embodiment, the optical combiner includes an intensity control element that can be configured to adjustably modulate the intensity of the plurality of optical pulses emitted by the first laser, and a combining ratio of the optical combiner can be adjustable by adjusting the modulation of the intensity control element.

[0012] In one embodiment, the intensity control element may be a first intensity control element, and the optical combiner may further comprise a second intensity control element configured to adjustably modulate the intensity of the plurality of optical pulses emitted by the second laser. A combining ratio of the optical combiner may be adjustable by adjusting the modulation of the first intensity control element and the second intensity control element.

[0013] In an embodiment, at least one of the first intensity control element or the second intensity control element may comprise at least one of an electro-absorption modulator, a Mach-Zehnder modulator, a tunable interferometer, and a semiconductor optical amplifier.

[0014] In one embodiment, the optical combiner may include a Mach-Zehnder interferometer having two arms of substantially equal length and an adjustable phase control element in at least one of the arms, and the combining ratio of the optical combiner may be adjustable by adjusting the phase control element.

[0015] In one embodiment, the combining ratio of the optical combiner can be adjusted so that, at the output coupler, the intensity of the plurality of optical pulses emitted by the first gain-switched laser is equal to a first predetermined intensity and the intensity of the plurality of optical pulses emitted by the second gain-switched laser is equal to a second predetermined intensity, which can be lower than the first predetermined intensity.

[0016] In one embodiment, the quantum cryptography technique may be a decoy state quantum cryptography technique, and the optical transmitter may be configured to encode a plurality of signal states of a plurality of pulses emitted by a first light source and a plurality of decoy states of a plurality of pulses emitted by a second light source.

[0017] In one embodiment, the optical transmitter may further comprise a further phase control element configured to receive the output of the Mach-Zehnder interferometer, control the phase of the output of the Mach-Zehnder interferometer, and provide the phase-controlled output of the Mach-Zehnder interferometer to the output coupler.

[0018] In one embodiment, the optical transmitter may be configured to emit a pulse from at most one of the first gain-switched laser and the second gain-switched laser during each cycle of the clock rate. Further, in this embodiment, the optical transmitter may, for each cycle of the clock rate, based on a random number: emitting a pulse from a first gain-switched laser; emitting a pulse from the second gain-switched laser; emitting a vacuum pulse from each of a first gain-switched laser and a second gain-switched laser of the optical transmitter; The optical transmitter may be configured to perform one of the following: The optical transmitter may further comprise a random number generator configured to generate a random number.

[0019] In one embodiment, the time delay between the short arm and the long arm may be 500 ps or less, 250 ps or less, or 100 ps or less.

[0020] In one embodiment, the clock rate may be 1 GHz or greater.

[0021] According to a second aspect, there is provided a quantum communication system comprising the optical transmitter of the first aspect, an optical receiver, and an optical channel configured to receive a plurality of optical pulses from an output coupler of the optical transmitter and propagate the plurality of optical pulses to the optical receiver. In one embodiment, the optical coupler outputs the plurality of pulses directly to the optical channel. In further embodiments, one or more components may be provided between the optical coupler and the output channel, such as an encoding component, an attenuator, etc. In one embodiment, the transmitter is configured to attenuate the pulses output to the optical channel to a level such that the average number of photons in each pulse is less than one photon, and this attenuation may be provided by the optical coupler or by a further attenuator provided after the optical coupler. In one embodiment, the optical transmitter further comprises a further attenuator provided to further attenuate the output of the optical coupler, such that the pulses output to the optical channel are attenuated to a level such that the average number of photons in each pulse is less than one photon. The further attenuator may be a fixed attenuator.

[0022] In one embodiment, the optical receiver may include a receiver Mach-Zehnder interferometer having a short arm and a long arm. The receiver Mach-Zehnder interferometer may have an input port configured to receive optical pulses from the optical channel and an output port configured to provide an output of the receiver Mach-Zehnder interferometer. The optical receiver may further include a single-photon detector connected to the output port of the receiver Mach-Zehnder interferometer. A time delay between the short arm and the long arm of the receiver Mach-Zehnder interferometer may be equal to a time delay between the short arm and the long arm of the optical transmitter Mach-Zehnder interferometer.

[0023] In one embodiment, at least one of the short arm and the long arm of the receiver Mach-Zehnder interferometer may comprise a receiver phase control element configured to control the relative phase between the short arm and the long arm of the receiver Mach-Zehnder interferometer.

[0024] According to a third aspect, there is provided a method of operating the optical transmitter of the first aspect, the method comprising selecting one of (i) emitting a pulse from a first gain-switched laser of the optical transmitter and (ii) emitting a pulse from a second gain-switched laser of the optical transmitter, the method further comprising emitting the pulse based on the selection.

[0025] According to a fourth aspect, there is provided a method of operating the optical transmitter of the first aspect, the method comprising selecting one of: (i) emitting a pulse from a first gain-switched laser of the optical transmitter; (ii) emitting a pulse from a second gain-switched laser of the optical transmitter; and (iii) emitting a vacuum-state pulse from each of the first gain-switched laser and the second gain-switched laser of the optical transmitter. The method further comprises emitting the pulse based on the selection.

[0026] Broadly speaking, the optical system 1 illustrated in FIG. 1 can be used to transmit optical pulses from one node of an optical network to another node over an optical channel. More specifically, the optical system 1 can be a quantum communication system for distributing quantum cryptographic keys between a transmitter node and a receiver node. For example, phase-encoded time-bin qubits can be prepared at the transmitter node and transmitted to the receiver node over an optical (quantum) channel. Generally, the optical system 1 can be operated to implement quantum cryptography techniques, such as decoy-state quantum cryptography techniques such as the decoy-state BB84 protocol. Broadly speaking, the decoy-state BB84 protocol is designed to overcome the security issue of conventional BB84 protocols, which may be susceptible to photon number splitting attacks when implemented using weak coherent pulses. This is because excess photons can be extracted and stored by an eavesdropper, allowing the eavesdropper to access key information without detection. The decoy-state BB84 protocol can rely on phase randomization and weak coherent pulse intensities randomly selected from a set of predetermined intensities. Randomly selecting the pulse intensity effectively mimics preparing a mixture of photon number states using a variable Poisson's ratio, a technique that can estimate the expected yield of a single photon pulse and provide a way to detect potential photon number splitting attacks by an eavesdropper.

[0027] 1, optical system 1 includes an optical transmitter 2 and an optical receiver 4. Transmitter 2 and receiver 4 are connected to each other through a quantum channel 3. Quantum channel 3 may be an optical fiber through which optical pulses (encoding quantum bits) are transmitted.

[0028] In one embodiment, transmitter 2 and receiver 4 may be further connected to each other through a classical channel (not shown in FIG. 1 ). The classical channel may be a control channel through which control information for generating quantum key data is transmitted and received. In an embodiment, quantum channel 3 and the classical channel may be implemented on the same optical fiber. In this case, the respective signals may be multiplexed (i.e., combined) to be transmitted over the same optical fiber. For example, wavelength multiplexing may be used when the signal transmitted over the classical channel has a different wavelength from the signal transmitted over quantum channel 3. The optical signal transmitted over the classical channel typically has a higher intensity than the signal transmitted over quantum channel 3.

[0029] 1 includes first and second gain-switched semiconductor lasers 9 and 11 (hereinafter referred to as the signal laser 9 and the decoy laser 11). The transmitter 2 further includes an adjustable optical combiner 10, an asymmetric Mach-Zehnder interferometer 19 (hereinafter referred to as the coded interferometer 19), and a controller 7.

[0030] The signal laser 9 and the decoy laser 11 may be configured to generate coherent light. For example, the signal laser 9 and the decoy laser 11 may be controlled (e.g., by the controller 7) to emit continuous waves or to emit light pulses of a predetermined duration. In one embodiment, the signal laser 9 and the decoy laser 11 may be configured so that each laser emits a respective plurality of light pulses such that the phase of each pulse in the respective plurality of light pulses is randomized. In other words, the signal laser 9 and the decoy laser 11 may be configured so that the phase difference between consecutive coherent pulses emitted by the same laser is randomized. In one embodiment, the predetermined durations of the pulses emitted by the signal laser 9 and the decoy laser 11 may be substantially the same. For example, the durations of the pulses emitted by the lasers may be 40 ps or less.

[0031] The pulses from each laser are independent of each other, and therefore have random phases relative to each other. The phase difference between two consecutive pulses emitted by the same laser is only truly random if there is no phase coherence between these consecutive pulses. In the case of semiconductor gain-switched lasers such as the signal laser 9 or decoy laser 11, laser emission is initiated by spontaneous emission. This is a random process, which means that the phase of the generated long light pulses will be random. For the random process of spontaneous emission to be responsible for the initiation of laser emission for every pulse, the laser cavity must be completely emptied before each pulse is generated; that is, the applied current must be below the threshold current for a sufficiently long time between pulses.

[0032] In one embodiment, the signal laser 9 and the decoy laser 11 may be gain-switched semiconductor laser diodes. More specifically, the signal laser 9 and the decoy laser 11 may be distributed feedback (DFB) lasers, vertical-cavity surface-emitting lasers, or ridge lasers. Ridge lasers are also called stripe lasers. A Fabry-Perot laser is a type of ridge or stripe laser. The terms stripe and ridge refer to the form of a laser waveguide. Fabry-Perot refers to the form of a laser cavity, i.e., two parallel mirrors formed by the end faces of a waveguide. In one embodiment, the signal laser 9 and the decoy laser 11 may emit light at telecommunication wavelengths.

[0033] In one embodiment, the signal laser 9 and the decoy laser 11 may be controlled (e.g., by the controller 7) to output light at (substantially) the same wavelength. In this case, the optical spectrum of the light emitted by the signal laser 9 and the decoy laser 11 substantially overlap. For example, the central wavelengths of the signal laser 9 and the decoy laser 11 may differ by less than their respective FWHM (full width at half-maximum) linewidths. To this end, the controller 7 may be configured to control the temperature (and / or drive current) of the lasers to adjust the wavelength of the emitted light accordingly (e.g., by a heating or cooling element). In one embodiment, the transmitter 2 may further comprise means for detecting a wavelength difference between the lasers (e.g., a suitable device for interfering signals emitted by the lasers and detecting the corresponding interference signal). In one embodiment, operating the signal laser 9 and the decoy laser 11 at (substantially) the same wavelength increases the security of communications, as a potential eavesdropper cannot determine which laser emitted a particular pulse by measuring the wavelength of the pulse.

[0034] The adjustable combiner 10 has first and second input ports and an output port. The signal laser 9 is configured to provide coherent light to the first input port of the adjustable combiner 10. The decoy laser 11 is configured to provide coherent light to the second input port of the adjustable combiner 10.

[0035] Generally, the adjustable combiner 10 is configured to combine optical signals received at its input ports based on a combining ratio. This means that the adjustable combiner 10 modulates (e.g., amplifies or attenuates) the optical signals received at its first and second input ports according to the combining ratio. For example, the combining ratio may specify the relative amount of intensity modulation (e.g., attenuation or amplification) applied to the signal received at the first input port compared to the modulation applied to the signal received at the second input port. For example, the combining ratio may specify that the optical signal received at the second input port is attenuated twice as much as the optical signal received at the first input port. In other words, the adjustable combiner 10 combines the optical signals received at its input ports and allows for control of their relative intensities.

[0036] The adjustable combiner can be implemented in several ways. In one embodiment, the adjustable combiner may include an intensity modulator (or intensity control element) applied to the signal received at the first input port and an optical combiner that combines these modulated signals provided by the intensity modulator and the signal received at the second input port according to a fixed (i.e., non-adjustable) combining ratio. The intensity modulator may modulate (e.g., attenuate or amplify) the signal received at the first port. Thus, in this case, the relative intensities of the signals received at the first and second ports are controlled by appropriately adjusting the modulation applied by the intensity modulator. Furthermore, the adjustable combiner may also include a second intensity modulator applied to the signal received at the second input port. In this case, the modulation by the second intensity modulator may be adjustable independently of the modulation of the intensity modulator of the first input port. Thus, the relative intensities of the signals received at the first and second ports are controlled by appropriately adjusting the (independent) modulation applied by the intensity modulator. In the above, the intensity modulators may be implemented in any suitable manner, for example, each of the intensity modulators may comprise at least one of an electroabsorption modulator, a Mach-Zehnder modulator, a tunable interferometer, and a semiconductor optical amplifier.

[0037] In another embodiment, the adjustable combiner may be implemented using a symmetric Mach-Zehnder interferometer. In particular, the combiner may include a Mach-Zehnder interferometer having two arms of substantially equal length and an adjustable phase control element in at least one of the arms. In this case, two input ports of the combiner are respectively connected to two input ports of the symmetric Mach-Zehnder interferometer, and an output port of the combiner is connected to an output port of the symmetric Mach-Zehnder interferometer. In this case, the combining ratio of the adjustable combiner is adjustable by adjusting the phase control element that controls the difference in optical path length between the arms.

[0038] The adjustable combiner 10 of transmitter 2 comprises first and second intensity modulators (IM) 13, 15 (hereinafter signal IM13 and decoy IM15) and a non-adjustable optical combiner 17. Although transmitter 2 is described below with reference to the adjustable combiner implementation shown in FIG. 1, it should be understood that embodiments of transmitter 2 may implement the adjustable combiner 10 in any of the ways described above.

[0039] Referring again to FIG. 1 , the amplitude of the light propagating through signal IM13 (or decoy IM15) is modulated (e.g., attenuated or amplified) depending on the adjustable modulation of signal IM13 (or decoy IM15, respectively). In other words, the optical modulation (e.g., attenuation or amplification) of signal IM13 and decoy IM15 may be individually controlled (e.g., by controller 7). In some embodiments, the optical attenuation of IM13, 15 may be adjustable to any one of a set of discrete modulation values. In other embodiments, the optical modulation of IM13, 15 may be (quasi-)continuously adjustable between a minimum modulation value and a maximum modulation value. As mentioned above, several known optical components are suitable for modulating the coherent light provided by signal laser 9 (or decoy laser 11) and may be used to implement IM13, 15. In one embodiment, each of signal IM13 and decoy IM15 may be one of an electroabsorption modulator, a Mach-Zehnder modulator, a tunable interferometer, or a semiconductor optical amplifier.

[0040] The transmitter 2 further includes an asymmetric Mach-Zehnder interferometer 19 (hereinafter, coded interferometer 19). The non-adjustable optical combiner 17 is configured to combine the coherent light transmitted through signal IM13 and the coherent light transmitted through decoy IM15 into a combining optical path (e.g., a waveguide). The combiner is configured to combine the coherent light transmitted through signal IM13 and the coherent light transmitted through decoy IM15 based on a (fixed) combining ratio. In one embodiment, the combining ratio may be selected to be balanced, i.e., such that the relative intensities between the coherent light transmitted through signal IM13 and the coherent light transmitted through decoy IM15 are substantially maintained in the combining optical path. In an alternative embodiment, the combining ratio may be selected to be unbalanced.

[0041] The coding interferometer 19 is configured to receive coherent light from the combined optical path of the combiner 17 at the input port of the beam splitter and split the received light into a long arm 23 and a short arm 21. The long arm 23 and the short arm 21 are recombined via the beam splitter. As shown in FIG. 1 , one output of the beam splitter provides the output of the coding interferometer 19. The optical path length of the long arm 23 is longer than the optical path length of the short arm 21. The optical pulse propagating in the long arm 23 of the coding interferometer 19 is delayed by a delay time t0 with respect to the pulse traveling in the short arm 21. In an embodiment, the delay time t0 is 500 ps or less, or 250 ps or less.

[0042] One arm of the coded interferometer 19 includes a phase control element 25. The phase control element 25 is configured to control the relative phase between the long arm 23 and the short arm 21. For example, the phase control element 25 can be a phase modulator configured to provide an adjustable phase shift to an optical pulse propagating through the phase modulator. In the embodiment shown in FIG. 1 , the coded interferometer 19 includes the phase control element 25 in the short arm 21. In an alternative embodiment, the coded interferometer 19 includes the phase control element 25 in the long arm 23. In another embodiment, both the long arm 23 and the short arm 21 include respective phase control elements configured to cooperatively control the relative phase between the long arm 23 and the short arm 21. In one embodiment, the phase control element 25 can be a thermal phase shifter or an electro-optic phase modulator.

[0043] The coding interferometer 19 is suitable for preparing, from the optical pulses emitted by the lasers 9, 11, a plurality of quantum states suitable for implementing quantum cryptography techniques, such as the decoy state BB84 protocol described above. In particular, the coding interferometer 19 may be used for time-bin encoding of quantum information, i.e., converting an optical pulse emitted by one of the lasers 9, 11 into a time-bin qubit, i.e., two pulses with a well-defined temporal separation (given by the interferometer delay time t0) and a well-controlled phase relationship (controlled by the phase control element 25). For example, a pulse emitted by the signal laser 9 may be converted by the coding interferometer 19 into one of the four signal states of the BB84 protocol, where the four states correspond to a choice between two bases (e.g., either Z or X) and a choice between bit values (either 0 or 1). The choice of basis is determined by the phase value α A =0 or α A =π / 2. The selection of the bit value may correspond to selecting the phase value β A =0 or β A = π. Then, the phase difference between the long arm 23 and the short arm 21 is determined as α A +βA By controlling the phase control element 25 so as to adjust the phase to be equal to the desired signal state, a desired signal state can be prepared.

[0044] 1, the output of coded interferometer 19 is provided to quantum channel 3, for example, via an output coupler. In an embodiment, in addition to or instead of phase control element 25, transmitter 2 may also include a further phase control element (not shown in FIG. 1) configured to apply a phase shift to the output of coded interferometer 19 and provide the phase-shifted output of coded interferometer 19 to quantum channel 3.

[0045] The controller 7 of the transmitter 2 is configured to control the operation of the lasers 9, 11, IMs 13, 15, and the phase control element 25. To this end, the controller 7 may apply (time-varying) electrical signals to the lasers 9, 11, IMs 13, 15, and the phase control element 25, for example, as described below with reference to Figures 2 to 4. The controller 7 may comprise electrical circuitry for generating and applying appropriate electrical signals to control the lasers 9, 11, IMs 13, 15, and the phase control element 25.

[0046] In one embodiment, controller 7 further comprises a random number generator configured to generate random numbers. In another embodiment, controller 7 is configured to receive random numbers from an external device and store the received random numbers for use during operation of transmitter 2. As described below with reference to FIGS. 2-4, the transmitter may typically be operated at a particular clock rate, and during each cycle of the clock rate, transmitter 2 may randomly select (from a set of states) a state to prepare and next emit. The state selection may be based on a random number (whether generated or received by controller 7).

[0047] In one embodiment, the transmitter 2 may comprise a photonic integrated circuit (PIC). The PIC may comprise a semiconductor substrate, on which the lasers 9, 11, IMs 13, 15, and interferometer 19 may be integrated. The integrated components may be suitably connected to one another by integrated optical channels (such as waveguides) that enable light propagation between the components. In some embodiments, the controller 7 may be fully or partially integrated into the PIC. The PIC may be formed from InP. The components integrated into the PIC and any ports for providing optical signals may be formed together. For this purpose, patterns may be defined by photolithography and / or E-beam lithography, and corresponding structures may be formed by plasma dry etching methods. For example, RIE methods such as inductively coupled plasma (ICP) reactive ion etching (RIE) and deep reactive ion etching (DRIE) may be used to form the structures. Optical waveguides for connecting components integrated into the PIC may be formed using (optical and / or electronic) lithography, (plasma and / or chemical) etching, direct laser writing, ion exchange, nanoimprinting, and the like. In other embodiments, the PIC may be formed of another suitable semiconductor material system, such as Si, SOI, SiN, SiO2, SiON, or GaAs. Alternatively, the PIC may be formed of glass or a polymer. Hybrid integration and heterogeneous integration techniques can be used to form a PIC using two or more of the material systems. In embodiments where the signal laser 9 and the decoy laser 11 are formed on the same semiconductor substrate and both lasers are of the same type (e.g., both lasers are either distributed feedback (DFB) lasers, vertical cavity surface emitting lasers, or ridge lasers), both lasers may be (substantially) identical and may operate at (substantially) the same wavelength for (substantially) the same operating conditions (e.g., temperature and drive current). In this case, the task of controlling the lasers to emit at (substantially) the same wavelength may therefore be easier.

[0048] Receiver 4 is configured to receive, from quantum channel 3, the optical signal emitted by transmitter 2. More specifically, receiver 4 may be configured to receive optical pulses (encoding time bin qubits) from transmitter 2 and to decode the received optical pulses (i.e., decode the time bin qubits) using an asymmetric Mach-Zehnder interferometer and two (or more) single-photon detectors.

[0049] The receiver 4 includes an asymmetric Mach-Zehnder interferometer 31 (hereinafter, "decoding interferometer 31") for decoding the received time bin quantum bits. The decoding interferometer 31 is configured to receive coherent light from the quantum channel 3 at an input port of a beam splitter and split the received light into a long arm 33 and a short arm 35. The long arm 33 and the short arm 35 are recombined via the beam splitter. As shown in FIG. 1, the beam splitter provides two output ports of the decoding interferometer 31, which are connected to single-photon detectors 39 and 41, respectively. The optical path length of the long arm 33 is longer than the optical path length of the short arm 35. The decoding interferometer 31 is configured such that the delay time of the decoding interferometer 31 is substantially equal to the delay time of the interferometer 19. The single-photon detectors 39 and 41 are configured to receive and detect the coherent light from the decoding interferometer 31. The single-photon detectors 39, 41 may have a bandwidth greater than 1 / t (i.e., the reciprocal of the delay time t of the encoding / decoding interferometers 19, 31). In one embodiment, the single-photon detectors 39, 41 may have a bandwidth greater than 20 GHz.

[0050] The decoding interferometer 31 includes a decoding phase control element 37 in one of the arms. The decoding phase control element 37 is configured to control the relative phase between the long arm 33 and the short arm 35. For example, the decoding phase control element 37 may be a phase modulator configured to provide an adjustable phase shift to an optical pulse propagating through the phase modulator. In the embodiment shown in FIG. 1 , the decoding interferometer 31 includes a phase control element 37 in the short arm 35. In an alternative embodiment, the decoding interferometer 31 includes a phase control element 37 in the long arm 33. In another embodiment, both the long arm 33 and the short arm 35 include respective phase control elements configured to cooperatively control the relative phase between the long arm 33 and the short arm 35. In one embodiment, the decoding phase control element 37 may be a thermal phase shifter or an electro-optic phase modulator. For example, the phase control element may be a phase modulator configured to adjust the α phase shift to decode the received time bin qubit in either the Z basis or the X basis. A =0 or α A =π / 2.

[0051] The controller 29 of the receiver 4 is configured to control the operation of the decoding phase control element 37 and the single-photon detectors 39, 41. To this end, the controller 29 may apply (time-varying) electrical signals to the decoding phase control element 37 and the single-photon detectors 39, 41. The controller 29 may comprise electrical circuitry for generating and applying appropriate electrical signals to control the decoding phase control element 37 and the single-photon detectors 39, 41.

[0052] In one embodiment, receiver 4 may comprise a photonic integrated circuit (PIC). The PIC of receiver 4 may comprise a semiconductor substrate, and decoding interferometer 31 and detectors 39, 41 may be integrated on the semiconductor substrate. In some embodiments, controller 29 of receiver 4 may be fully or partially integrated in the PIC. The PIC of receiver 4 may be formed from InP. In other embodiments, the PIC of receiver 4 may be formed from another suitable semiconductor material system, such as, for example, Si, SOI, SiN, SiO2, SiON, or GaAs. Alternatively, the PIC may be formed from glass or a polymer. Hybrid and heterogeneous integration techniques can be used to form the PIC of receiver 4 using two or more of the material systems.

[0053] The optical system 1 can be used to implement a decoy state QKD protocol. In one embodiment, the decoy state QKD protocol may require the transmitter to prepare a pulse at one of three intensities (at each clock cycle). A signal state pulse may be prepared at a first pulse intensity, a first decoy state pulse may be prepared at a second pulse intensity, and a second decoy state pulse may be prepared at a third intensity. The transmitter 2 of the optical system 1 can implement such a decoy state-based QKD protocol while avoiding the “pattern effect” problem of conventional devices. To this end, the signal state may be encoded by a pulse emitted from the signal laser 9. The intensity of the signal state pulse may be adjusted using IM 13. A first decoy state (hereinafter simply referred to as the decoy state) may be encoded by a pulse emitted from the decoy laser 11. The intensity of the decoy state pulse may be adjusted using IM 15 to be different from (higher or lower than) the intensity of the signal state pulse. A second decoy state may be encoded by a vacuum state pulse (i.e., by a pulse having an average photon number of zero). Thus, emitting a vacuum state pulse during a particular clock cycle means that the average photon number provided at the output coupler of transmitter 2 is (substantially) zero. The desired intensities of the signal state pulse and decoy state pulse (and thus the corresponding modulation of IM 13, 15) may be selected (and found by performing an appropriate calibration process) to optimize the secure key rate of optical system 1.

[0054] 2 shows a table of exemplary configurations of the transmitter 2 for implementing the above-described signal pulse, decoy pulse, or vacuum pulse preparation. To transmit a signal state pulse from the transmitter 2 to the receiver 4 during a particular clock cycle, the signal laser 9 may be operated to emit a pulse while the decoy laser 11 is switched “off” (e.g., the drive current of the decoy laser 11 is switched off or at least switched below the laser threshold of the decoy laser 11). To transmit a decoy state pulse from the transmitter 2 to the receiver 4 during a particular clock cycle, the decoy laser 11 may be operated to emit a pulse while the signal laser 9 is switched “off” (e.g., the drive current of the signal laser 9 is switched off or at least switched below the laser threshold of the signal laser 9). To transmit a vacuum state pulse from the transmitter 2 to the receiver 4 during a particular clock cycle, both the signal laser 9 and the decoy laser 11 may be switched “off” (e.g., their respective drive currents are switched off or at least below their laser thresholds).

[0055] In the above, the vacuum state pulse is prepared by switching off both lasers, which is advantageous over conventional devices that prepare the vacuum state by attenuating the laser pulse, as this allows for a better signal-to-noise ratio when estimating, for example, the dark count rate and secure key rate.

[0056] Once the desired intensity values for the signal and decoy states are found, there is no need to switch the modulation of IMs 13 and 15, thus preventing the "pattern effect" problem of conventional devices. Thus, transmitter 2 provides an elegant solution to the problem of generating decoy states without introducing information-leaking side channels.

[0057] An exemplary operation of the optical system 1 will be further described with reference to Figures 3-4. Figure 3 shows in graphs 51 and 53 the intensities of the light emitted by the signal laser 9 and the decoy laser 11, respectively, after modulation by the signal IM13 and the decoy IM15. In this example, the transmitter 2 is operated at a clock rate (e.g., 1 GHz or higher), and the graph in Figure 3 illustrates the operation of the transmitter 2 for five clock cycles (shown as t1-t5 in Figure 3). In each of these clock cycles, steps S101 and S102 of the exemplary process in Figure 4 are performed. The duration of a clock cycle period is shown as t in Figure 3. clock It is expressed as:

[0058] In step S101, one of emitting a signal state pulse, emitting a decoy state pulse, and emitting a vacuum state pulse is selected. In one embodiment, this means selecting one of (i) emitting a pulse from the first gain-switched laser of the transmitter, (ii) emitting a pulse from the second gain-switched laser of the transmitter, and (iii) emitting a vacuum state pulse from each of the first gain-switched laser and the second gain-switched laser of the transmitter.

[0059] In one embodiment, the selection in step S101 may be based on a random number. The random number may be provided by the controller 7. During each clock cycle, a different random number may be used for the selection in step S101.

[0060] In one embodiment, step S101 further comprises randomly selecting a basis and bit values and adjusting the coding interferometer to encode the selected basis and bit values (e.g., adjusting a phase value applied by phase control element 25). The selection of the basis and bit values may be based on the same random numbers used to select the signal / decoy / vacuum states, or may be different random numbers provided by controller 7.

[0061] For example, in FIG. 3, during the first clock cycle (time t=t1), the emission of a signal state pulse is selected, resulting in a pulse being emitted from the signal laser 9 and modulated by the signal IM13 according to a predetermined modulation. The decoy laser 11 does not emit a pulse during this clock cycle. During the second clock cycle (time t=t2), the emission of a decoy state pulse is selected, resulting in a pulse being emitted from the decoy laser 11 and modulated by the decoy IM15 according to a predetermined modulation. The signal laser 9 does not emit a pulse during this clock cycle. As shown in FIG. 4, in this example, the intensity of the decoy state pulse after IM15 (at time t2) is lower than the intensity of the signal state pulse after IM13 (e.g., at time t1). Furthermore, in this example, another signal state pulse occurs during the third clock cycle (time t=t3). During the fourth cycle time (time t=t4), the emission of a vacuum state pulse is selected, and neither the signal laser 9 nor the decoy laser 11 is switched on.

[0062] 2-4, the operation of transmitter 2 has been described in the context of a decoy-state-based QKD protocol in which transmitter 2 chooses (at every clock cycle) to emit a signal-state pulse, a decoy-state pulse, or a vacuum-state pulse. It should be understood that transmitter 2 may be used to implement other (decoy-state) QKD protocols. For example, in one embodiment, transmitter 2 may choose (at every clock cycle) to either emit a signal-state pulse or a decoy-state pulse (i.e., in this example, the transmitter does not choose to emit a vacuum-state pulse).

[0063] As noted above, while the embodiment of FIG. 1 includes a transmitter with two IMs (one for each laser), variations of transmitter 2 may have only one IM (before the combiner). In such an embodiment, the output of one laser (e.g., the decoy laser) passes through an intensity modulator (shown in FIG. 1 ), while the output of the other laser (the signal laser) is directed directly into the combiner (i.e., without passing through an intensity modulator). In this example, the intensity of the signal-state pulse, when provided to the quantum channel, is determined by the initial intensity of the pulse and the (static) optical loss of the transmitter. Furthermore, in this example, the intensity modulator may be used to adjust the intensity of the decoy pulse (so that the intensity of the decoy-state pulse is sufficiently different from the intensity of the signal-state pulse when provided to the quantum channel).

[0064] In a further embodiment, the transmitter 2 may further (i.e., in addition to the adjustable combiner 10) comprise first and second high-speed intensity modulators. In this embodiment, the light emitted by the signal laser 9 is modulated by the first high-speed modulator, and the light emitted by the decoy laser 11 is modulated by the second high-speed modulator. The first and second high-speed modulators may be switchable between (at least) two states: an on state and an off state. The intensity of the modulated pulse is higher when the high-speed modulator is in the on state than when it is in the off state (e.g., the attenuation is lower in the on state than when it is in the off state, or the amplification is higher in the on state than when it is in the off state). In particular, the pulse modulated by one of the high-speed modulators in the off state has a negligibly low intensity. The advantage of providing a high-speed modulator is that the signal laser 9 and the decoy laser 11 can be operated at a fixed (e.g., the same) pulse repetition rate, and each high-speed modulator can be controlled to select, on each clock cycle, whether a signal state pulse or a decoy state pulse should be sent to the adjustable combiner 10.

[0065] While the above embodiments primarily refer to the use of optical transmitters to generate signal and decoy states, optical transmitters can be used for any type of multiplexing of two or more laser outputs. In further embodiments, the first and second lasers have different wavelengths, and an optical combiner is used to enable wavelength multiplexing while independently adjusting the average photon number.

[0066] While specific embodiments have been described, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. The novel devices and methods described herein may be embodied in a variety of other forms, and various omissions, substitutions, and changes in the form of the devices, methods, and products described herein may be made without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover any such forms or modifications that come within the scope and spirit of the invention.

Claims

1. An optical transmitter for quantum key distribution according to quantum cryptography, comprising: a first laser and a second laser, each laser configured to emit a respective one of a plurality of optical pulses such that a phase of each pulse of the plurality of optical pulses is randomized, and the pulses are emitted based on selecting one of emitting a pulse from the first laser or emitting a pulse from the second laser; an adjustable optical combiner configured to combine the plurality of optical pulses emitted by the first laser and the second laser into a combined stream of pulses based on an adjustable combining ratio; a Mach-Zehnder interferometer having a short arm and a long arm, the Mach-Zehnder interferometer having an input port configured to receive the combined stream of pulses and an output port configured to provide an output of the Mach-Zehnder interferometer; an output coupler configured to provide the output of the Mach-Zehnder interferometer to a quantum channel; Equipped with an optical transmitter, wherein at least one of the short arm and the long arm comprises a phase control element configured to control the relative phase between the short arm and the long arm;

2. 10. The optical transmitter of claim 1, wherein the first laser, the second laser, the optical combiner, and the Mach-Zehnder interferometer are formed on an integrated chip.

3. 2. The optical transmitter of claim 1, wherein the optical combiner comprises an intensity control element configured to adjustably modulate the intensity of the plurality of optical pulses emitted by the first laser, and the combining ratio of the optical combiner is adjustable by adjusting the modulation of the intensity control element.

4. 4. The optical transmitter of claim 3, wherein the intensity control element is a first intensity control element, and the optical combiner further comprises a second intensity control element configured to adjustably modulate the intensity of the plurality of optical pulses emitted by the second laser, and the combining ratio of the optical combiner is adjustable by adjusting modulation of the first intensity control element and the second intensity control element.

5. 5. The optical transmitter of claim 4, wherein at least one of the first intensity control element or the second intensity control element comprises at least one of an electroabsorption modulator, a Mach-Zehnder modulator, a tunable interferometer, and a semiconductor optical amplifier.

6. 2. The optical transmitter of claim 1, wherein the optical combiner comprises a Mach-Zehnder interferometer having two arms of substantially equal length and an adjustable phase control element in at least one of the arms, and the combining ratio of the optical combiner is adjustable by adjusting the phase control element.

7. 5. The optical transmitter of claim 4, wherein the combining ratio of the optical combiner is adjusted so that, at the output coupler, the intensities of the plurality of optical pulses emitted by the first laser are equal to a first predetermined intensity and the intensities of the plurality of optical pulses emitted by the second laser are equal to a second predetermined intensity, the second predetermined intensity being different from the first predetermined intensity.

8. 2. The optical transmitter of claim 1, wherein the quantum cryptography technique is a decoy state quantum cryptography technique, and the optical transmitter is configured to encode one of a plurality of signal states or a plurality of decoy states of a plurality of pulses emitted by the first laser and another of a plurality of signal states or a plurality of decoy states of a plurality of pulses emitted by the second laser.

9. 10. The optical transmitter of claim 1, further comprising a further phase control element configured to receive the output of the Mach-Zehnder interferometer, control the phase of the output of the Mach-Zehnder interferometer, and provide a phase-controlled output of the Mach-Zehnder interferometer to the output coupler.

10. 10. The optical transmitter of claim 1, wherein the optical transmitter is configured such that at most one of the first laser and the second laser emits a pulse during each cycle of a clock rate.

11. The optical transmitter, for each cycle of the clock rate, based on a random number: emitting a pulse from the first laser; emitting a pulse from the second laser; emitting a vacuum pulse from each of the first laser and the second laser of the optical transmitter; 11. The optical transmitter of claim 10, configured to perform one of the following:

12. The optical transmitter of claim 11 , further comprising a random number generator configured to generate the random number.

13. 10. The optical transmitter of claim 1, wherein the time delay between the short arm and the long arm is 500 ps or less, 250 ps or less, or 100 ps or less.

14. 11. The optical transmitter of claim 10, wherein the clock rate is 1 GHz or greater.

15. 10. The optical transmitter of claim 1, wherein the first laser and the second laser are gain-switched lasers.

16. The optical transmitter according to claim 1; an optical receiver; an optical channel configured to receive a plurality of optical pulses from the output coupler of the optical transmitter and to propagate the plurality of optical pulses to the optical receiver; A quantum communication system comprising:

17. The optical receiver includes: a receiver Mach-Zehnder interferometer having a short arm and a long arm, wherein the receiver Mach-Zehnder interferometer has an input port configured to receive the plurality of optical pulses from the optical channel and an output port configured to provide an output of the receiver Mach-Zehnder interferometer; a single-photon detector connected to the output port of the receiver Mach-Zehnder interferometer; Equipped with 17. The quantum communication system of claim 16, wherein a time delay between the short arm and the long arm of the receiver Mach-Zehnder interferometer is equal to a time delay between the short arm and the long arm of the optical transmitter Mach-Zehnder interferometer.

18. 18. The quantum communication system of claim 17, wherein at least one of the short arm and the long arm of the receiver Mach-Zehnder interferometer comprises a receiver phase control element configured to control the relative phase between the short arm and the long arm of the receiver Mach-Zehnder interferometer.

19. 10. A method of operating the optical transmitter of claim 1, comprising: (i) emitting a pulse from a first laser of the optical transmitter; (ii) emitting a pulse from a second laser of the optical transmitter; and selecting one of emitting a pulse based on the selection; and A method comprising:

20. 10. A method of operating the optical transmitter of claim 1, comprising: (i) emitting a pulse from a first laser of the optical transmitter; (ii) emitting a pulse from a second laser of the optical transmitter; (iii) emitting a vacuum state pulse from each of the first laser and the second laser of the optical transmitter; and selecting one of emitting a pulse based on the selection; and A method comprising:

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