Patent Description:
Information may be secured in a number of ways. Information that is confidential in nature may comprise financial, medical, corporate, political or personal information, for example.

Confidential information may be stored in secure premises, preventing accidental or malicious access to the information by placing it in a locked place, such as for example in a safe in an office. Corporate locations may be further, or alternatively, provided with alarm systems, guards, fences and/or other access control functions.

Confidential information may be stored in computers that are not connected to any unsecure networks, to prevent unauthorized network intrusion therein to obtain the information. Such computers may be referred to as "air walled" computers as they have no connection to unsecure networks.

One way to prevent unauthorized access to confidential information is encryption, wherein a plaintext, for example a text in a natural language, such as French, is converted to a ciphertext using an encryption algorithm and a key. Encryption algorithms are designed to render it very difficult to obtain the plaintext from the ciphertext without the key. In general, ciphertext may be known as encrypted information.

In quantum communication, QC, two parties may exchange information encoded in quantum states. The quantum states, or qubits, may comprise specially defined properties of photons such as pairs of polarization states, such as <NUM>° and <NUM>°, or circular basis states such as left-handedness and right-handedness. Through quantum communication, the two parties may produce a shared random series of bits known only to them, which can then be used as secret keys in subsequent encryption and decryption of messages. A third party can, in theory, eavesdrop on the QC between the two parties. Such eavesdropping perturbs the QC, however, introducing anomalies that the two intended parties can detect. The two parties may post-process the results of the QC to remove any partial information acquired by an eavesdropper, and form shared secret keys from the remaining information resulting from the QC.

An eavesdropper intercepting and re-transmitting a photon comprised in a quantum communication can only guess the original sending basis when it re-encodes and re-transmits the photon toward its original destination. The receiver may detect the eavesdropping since for subsets of bit values for which sending basis and measuring basis are found to match, parity values should match exactly, assuming the communication system is well tuned and free from imperfections in transmission and reception. Discrepancies in bit values introduced by eavesdropping enable the transmitter and receiver to detect eavesdropping and correct the secret keys.

Document <CIT> discloses a secure wireless communication solution, wherein dual-rail encoded states of light are output into a time slot, and these states are converted to polarization-encoded states. A transmitter may comprise three laser diodes providing input to a <NUM>-<NUM> converter, which provides input to a polarization rotator-combiner. Document <CIT> discloses a fibre-based communication solution, wherein dual-rail encoded light is converted into polarization encoded light, wherein compensation adjustment information concerning the fibre is obtained and used to control dual-rail encoding. Document <CIT> discloses a method for quantum cryptography, wherein an optical signal is encoded for quantum cryptography at a stage on at least two signal paths. Communication may proceed in a fibre or in free space, and the system may employ more than one light source.

In accordance with a first aspect of the present invention, there is provided an apparatus comprising means for obtaining light from a light source in each of two optical transmitters, encoding, in each of the optical transmitters dual-rail encoded light in a respective dual-rail and converting, in each of the optical transmitters, the dual-rail encoded light into polarization encoded light, obtaining compensation adjustment information concerning two fibres and controlling the dual-rail encoders based at least in part on the compensation adjustment information, wherein the light sources comprise laser sources operating at the same frequency, such that the laser sources are injection locked to the same frequency.

In accordance with a second aspect of the present invention, there is provided an apparatus comprising means for receiving two optical signals from two fibres from two respective optical transmitters, converting the optical signals into dual rail form optical signals and causing the dual rail form optical signals to interfere with each other, measuring the dual rail form optical signals, and obtaining (<NUM>) compensation adjustment information concerning the two fibres, and informing the optical transmitters of the compensation adjustment information.

In accordance with a third aspect of the present invention, there is provided method, comprising in each of two optical transmitters, obtaining light from a light source, encoding, in each of the optical transmitters dual-rail encoded light in a respective dual-rail encoder and converting, in each of the optical transmitters, the dual-rail encoded light into polarization encoded light, obtaining compensation adjustment information concerning two fibres and controlling the dual-rail encoders based at least in part on the compensation adjustment information, wherein the light sources comprise laser sources operating at the same frequency, such that the laser sources are injection locked to the same frequency.

At least some embodiments of the present invention find industrial applicability in improving communication over fibres and/or distribution of encryption keys.

Two field quantum key distribution, QKD, enables increasing a distance between communicating parties. Embodiments of the present disclosure relate to pre-compensating for polarization rotation in fibres used in two field communication, such that the two optical signals arrive at the receiver in a same polarization state. Thus polarization rotations incurred in the fibres used to convey the optical signals can be corrected for.

Dual-rail encoding may be implemented on two waveguides, which may be parallel. Information is encoded on the relative phase and amplitudes of the light in the two waveguides. Operations on the relative phase and amplitudes can be performed by phase shifters on at least one of the two waveguides and by optical couplers between the two waveguides. A light source, dual-rail encoder and polarization rotator-combiner may be fabricated monolithically on the same chip or using substrates of different materials by heterogeneous/hybrid integration, for example. A polarization rotator-combiner may rotate the polarization of the light coming from one waveguide with respect to the polarization of the light coming from the other waveguide and combine the light of the two waveguides in a single spatial optical mode. In general, dual-rail encoding may thus comprise modifying at least one of amplitude and phase of the light in at least one of the two waveguides.

<FIG> illustrates an example system capable of supporting at least some embodiments of the present invention. The figure illustrates two emitters and receiver <NUM> with communication channels arranged therein between. The emitters may be housed in a same device, or they may be physically distinct from each other. A first emitter comprises signal generator <NUM>, phase pre-compensator <NUM>, attenuator <NUM> and polarization pre-compensator <NUM>. Fibre <NUM> conveys an optical signal from the first emitter to receiver <NUM>. A second emitter comprises, likewise, QKD signal generator <NUM>, phase pre-compensator <NUM>, attenuator <NUM> and polarization pre-compensator <NUM>. Fibre <NUM> conveys an optical signal from the second emitter to receiver <NUM>. Classical communication channel <NUM> is arranged to convey information from receiver <NUM> to QKD electronic processors <NUM> and <NUM>, for the first and second emitters, respectively, and electronic controllers <NUM> and <NUM>, again for the first and second emitters, respectively. Classical channel may comprise a wire-like communication interface, for example. Controllers <NUM>, <NUM> may comprise, for example, field-programmable gate arrays, FPGAs, microcontrollers, microprocessors, processors or other controllers.

Signal generators <NUM>, <NUM> may be configured to encode a bit sequence into an optical signal, for example in dual rail. Phase pre-compensators <NUM>, <NUM> may be configured to enforce a phase difference between the optical signals generated by the two emitters. Phase compensators <NUM>, <NUM> may further be configured to compensate for a phase propagation effect of the respective fibres <NUM>, <NUM>, to control a respective phase at which optical signals arrive in receiver <NUM> from the emitters.

Attenuators <NUM>, <NUM> may be configured to attenuate an amplitude of the optical signals generated by the emitters, for example to a single-photon range. By single-photon range it may be meant, for example, that each pulse comprises a single photon, or a few photons. Finally, polarization pre-compensators <NUM>, <NUM> may be configured to compensate for polarization rotation incurred in the fibres <NUM> and <NUM>, respectively, to control the respective polarizations at which optical signals from the emitters arrive at receiver <NUM>. The fibres <NUM>, <NUM>, may exhibit birefringence, wherein imperfections in the fibres, stress and/or bending of the fibres may cause polarization of photons passing through the fibres to rotate. Birefringence of fibres <NUM>, <NUM> may be time-varying as temperatures of sections of the fibres may change, the fibres may be physically rearranged to change the bends it is arranged in, or the number of physical imperfections in the fibre may increase with time. The core of fibres <NUM>, <NUM> may comprise glass or transparent plastic, surrounded by a layer of material with a lower index of refraction, such as a different glass or plastic, for example. Fibres <NUM>, <NUM> may also cause, at least in part, attenuation of light passing through it and/or depolarization of light passing through it. The fibres may further be of different lengths. The fibres may be single-mode fibres.

In order for the two-field communication to succeed, the optical signals from the emitters should arrive at the receiver with the same polarization. As described above, the fibres may exhibit birefringence, causing polarizations to change during transit via the fibres. To correct this, receiver <NUM> may be configured to transmit compensation adjustment information to the emitters, via classical path <NUM>. Classical path <NUM> may comprise a wire-line or at least in part wireless channel the emitters and the receiver may use to communicate information with each other. In some embodiments, classical path <NUM> uses the fibres <NUM>, <NUM> to send optical signals which are not in the single-photon regime.

Base on the compensation adjustment information, electronic controllers <NUM>, <NUM> may cause adjustment in the phase and amplitude of the optical signals produced in the emitters, as illustrated in <FIG>. Further, electronic QKD processing units <NUM> and <NUM> may, respectively, adjust the encoded signal generated in signal generators <NUM>, <NUM>. In the illustrated two-field QKD system both emitters pre-compensate their polarization, and optionally phase, according to information provided by receiver <NUM> so that their respective signals arrive at receiver <NUM> with the same polarization, enabling the two-field concept. This polarisation control in the emitters is achieved by performing transformations in dual-rail, that is, adjusting the phase difference and the relative amplitude of the two rails, and combining the two rails with a polarization rotator combiner to obtain the desired polarization of the optical signal. This polarization is chosen in anticipation of the rotations that will happen in the fibres <NUM>, <NUM>. The dual rail encoded signal is transformed into a polarization encoded signal with a polarization rotator combiner before emitting it to the fibre. In some embodiments, only one of the emitters is configured to adjust the polarization. This may be sufficient to cause the optical signals from the emitters to arrive at the receiver with the same polarization, provided that the receiver can receive using any polarization.

Emitters and the receiver communicate with each other through the classical channel <NUM> for several purposes: firstly, the QKD protocol may comprise the receiver making its results public, emitters may exchange information to sift their key and calculate error correction and privacy amplification, secondly, control or monitoring of the phase difference introduced by the two transmission fibers, and thirdly, to monitor the polarization error at the receiver so that the transmitters can apply the right polarization pre-compensation. The information exchanged in this communication may be related to a subset of the detection events chosen randomly. The emitters and the receiver may agree, for example, on a random list of timeslots from which they publicly exchange information about the encoding and the resulting detection events, using classical channel <NUM>. This allows them to evaluate the polarisation alignment, the phase drift, the error rate and the required error correction and privacy amplification, for example. Alternatively, test patterns may be emitted from the emitters for measurement, that is, detection at the receiver, to generate compensation adjustment information.

In the receiver end, two benefits are obtained from having the optical signals arrive in the receiver with the same polarization, firstly, the two optical signals can only interfere if they have the same polarization, and secondly, the receiver itself may be polarisation dependent, making it simpler to manufacture.

Each one of the two emitters may thus perform the following steps: generating a QKD signal, splitting the QKD signal in two rails, applying the adjustments on the phase difference and relative amplitudes of the two rails, optionally polarization multiplexing a reference signal, converting the dual-rail state of light into a polarization state with a polarization rotator combiner, PRC, and transmitting the optical signal through the optical fiber towards the receiver.

Benefits of embodiments disclosed herein include that the system may be implemented on photonic chips and the receiver may be made completely passive, without polarization control, phase shifter, a laser, having only waveguides and detectors. Polarisation pre-compensation in the emitter, rather than in the receiver, allows full integration of the QKD emitter on a photonic chip and requires a slow and bulky polarization rotator in neither the emitters nor the receiver. The fact that the receiver is passive, lacking polarization control, opens more possibilities in the choice of fabrication platform and makes multiplexing easier. One possible implementation for the emitters is based on an integrated optical platform such as indium phosphide, InP, or silicon on insulator, SoI. The receiver can be made on Silicon chips or free space optics, for example.

<FIG> illustrates a polarization rotator combiner, PRC, based on InP. The PRC comprises polarization rotator <NUM>, a birefringent waveguide <NUM>, and a coupler <NUM>. Polarization of the light in one of the two rails/waveguides is rotated and the two signals are combined with a Mach Zehnder interferometer, MZI.

<FIG> illustrates another PRC, this time based on SoI. On the silicon platform, output/input couplers are usually gratings deflecting the light perpendicularly to the chip. In this case the polarization rotation combination is done with a two-dimensional grating. In general, a building block in the receiver end is the polarization splitter rotator, PSR. The latter is physically the same component as the PRC but it is used in reverse.

<FIG> illustrates an example emitter in accordance with at least some embodiments. A light source <NUM> may comprise, for example, a laser light source. To enforce a same frequency between light sources used by the emitters, the light sources of the emitters may be slave lasers of the same laser, or, alternatively, one of the light sources may be a master laser for the other light source. Either way, the light sources may be in this sense injection locked to the same frequency.

Amplitude and phase modulation take place in modulator <NUM>, and attenuator <NUM> reduces the amplitude of the optical signal, for example to the single-photon range or to a range slightly above the single-photon range. Dual rail pre-compensation <NUM> comprises a 1x2 coupler leading the optical signal to two rails, or waveguides, from the attenuator <NUM>, first phase shifter <NUM>, a 2x2 coupler and a second phase shifter <NUM>. The phase shifters are used to adjust a phase difference between the two rails. The phase shifters may be controlled by electronic controller <NUM>, which receives information over the classical channel <NUM>, as described herein above in connection with <FIG>. The encoding performed by the dual-rail encoder therefore may be a combination of the information to be communicated and the compensation adjustment. PRC <NUM> converts the dual rail form optical signal to a single rail optical signal with the desired polarization.

<FIG> illustrates an example receiver in accordance with at least some embodiments. The receiver may correspond to receiver <NUM> of <FIG>, for example. Fibre <NUM> conveys an optical signal from the first emitter, and fibre <NUM> conveys an optical signal from the second emitter. PSRs <NUM> and <NUM> convert the received optical signals to dual rail format, an interference between the optical signals is allowed to occur before receiving the conveyed information at two-field receiver stage <NUM>. The detectors <NUM> may comprise single-photon detectors, SPDs.

The receiver may also perform polarization monitoring <NUM>, with detectors <NUM>, which may comprise, for example, SPDs or photon detectors, PDs. A phase modulator <NUM> may be provided to correct a phase difference between the signals in case the emitter side does not perform phase correction.

<FIG> illustrates an example receiver in accordance with at least some embodiments. Fibres <NUM> and <NUM> convey the optical signals from the first and second emitter. As in <FIG>, also here PSRs <NUM>, <NUM> convert the incoming signals to dual rail format. Phase modulators <NUM>, <NUM> are provided for both incoming optical signals. Measurements can here be conducted with any polarisation as long as the two channels have the same polarisation. To do that, the one output of each PSR <NUM>, <NUM> is directed to an interferometer, while the two other outputs are directed towards another interferometer for two-field QKD. Polarisation optimisation may be performed, based on public feedback of an events subset, by maximising the interference visibility in the two interferometers.

<FIG> illustrates an example receiver in accordance with at least some embodiments. The implementation of <FIG> is similar to that of <FIG>, except that it requires no waveguide crossing, as was the case for <FIG>. The 4x4 multimode interferometer, MMI, is twice as long as a <NUM>-degree hybrid 4x4 MMI. In this figure the preferred implementation is silicon photonics with vertical fibre couplers and superconducting nanowire detectors, SNSPDs, deposited on the silicon waveguides for readout.

<FIG> illustrates an example fixed polarization receiver in accordance with at least some embodiments. In this implementation, polarization monitoring interferometers enable a full characterisation of the input polarisations. This information may be provided to the emitters to allow them to correct their polarisations in one step, rather than using trial and error optimisation. The interferometer may be a simple beam splitter as in previous figures, but it may also be a more complex interferometer allowing unambiguous measurement of the phase differences. Fibres <NUM> and <NUM> convey the optical signals from the first and second emitter. As in <FIG> and <FIG>, also here PSRs <NUM>, <NUM> convert the incoming signals to dual rail format. A more accurate measurement of the polarization and therefore a faster and more accurate adjustment of the polarization can be done by measuring the phase between the two outputs of the PSRs. Polarization monitoring <NUM> and two-field QKD <NUM> are performed, as in implementations described above. This can be done by redirecting part of the light towards an interferometer I/F, for example a <NUM>-degree hybrid followed by single photon detectors or photodiodes, in case the transmitters occasionally emit intense pulses. <FIG> illustrates three examples of interferometers I/F usable in the architecture of <FIG>.

<FIG> illustrates an example fixed polarization receiver in accordance with at least some embodiments. As before, optical signals are received from the two emitters via fibres <NUM> and <NUM>. In the <FIG> implementation, a polarization-independent tap <NUM> is used to split the incoming optical signal, rather than a polarization beam splitter. Part of the light is directed to the QKD interferometer, and the rest to polarization analysers <NUM>, <NUM>. Polarization analysis is performed in polarisation analysers <NUM>, <NUM> to accomplish polarization monitoring <NUM>. A phase modulator <NUM> may be provided in case the phase is not adjusted in the emitter side, and SPDs <NUM> are employed to perform the two-field QKD.

<FIG> illustrates an example of a polarization analyser, usable with the implementation of <FIG>. Here a PSR <NUM> is used to split the light, and a <NUM> degree hybrid is placed before detectors, which may comprise, for example, SPDs or photodiodes.

<FIG> illustrated a device in accordance with at least some embodiments. Illustrated is device <NUM>, which may comprise, for example, an emitter device such as the emitter of FIGURE 1A or <FIG>. Comprised in device <NUM> is processor <NUM>, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multi-core processor comprises more than one processing core. Processor <NUM> may comprise a Qualcomm Snapdragon <NUM> processor, for example. Processor <NUM> may comprise more than one processor. A processing core may comprise, for example, a Cortex-A8 processing core manufactured by Intel Corporation or a Brisbane processing core produced by Advanced Micro Devices Corporation. Processor <NUM> may comprise at least one application-specific integrated circuit, ASIC. Processor <NUM> may comprise at least one field-programmable gate array, FPGA. Processor <NUM> may be means for performing method steps in device <NUM>. Processor <NUM> may be means for performing method steps in device <NUM>. Processor <NUM> may be configured, at least in part by computer instructions, to perform actions.

Memory <NUM> may comprise magnetic, optical and/or holographic memory, for example.

Device <NUM> may comprise a transmitter <NUM>. Device <NUM> may comprise a receiver <NUM>. Transmitter <NUM> and receiver <NUM> may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. Transmitter <NUM> may comprise more than one transmitter. Receiver <NUM> may comprise more than one receiver. Transmitter <NUM> and/or receiver <NUM> may be configured to operate with an optic fibre.

Device <NUM> may comprise user interface, UI, <NUM>. UI <NUM> may comprise at least one of a display, a keyboard or a touchscreen.

Processor <NUM>, memory <NUM>, transmitter <NUM>, receiver <NUM> and/or UI <NUM> may be interconnected by electrical leads internal to device <NUM> in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to device <NUM>, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the embodiment various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the present invention.

<FIG> is a signalling diagram in accordance with at least some embodiments. On the vertical axes are, from the left, first emitter E1, second emitter E2, and on the right, receiver <NUM>. In phases <NUM> and <NUM> the first and second emitters, respectively, transmit information over their fibres to receiver <NUM>. This information need not be in the single-photon range, as described herein above. Receiver <NUM> measures the information it receives and provides, via the classical channel, compensation adjustment information to first and second emitters E1 and E2, in phase <NUM>.

In phases <NUM> and <NUM>, emitters E1 and E2 transmit in two-field mode to receiver <NUM>, such that the optical signals the emitters provide via their respective fibres are pre-corrected to account for the effect of the fibres on polarization and, optionally, phase. Thus receiver <NUM> can receive the optical signals via the fibres with the same polarization, enabling successful interference between the optical signals.

<FIG> is a first flow graph of a first method in accordance with at least some embodiments of the present invention. The phases of the illustrated first method may be performed in an emitter, for example. Phase <NUM> comprises, in each of two optical transmitters, obtaining light from a light source, encoding dual-rail encoded light in a dual-rail encoder and converting the dual-rail encoded light into polarization encoded light. Phase <NUM> comprises obtaining compensation adjustment information concerning two fibres and controlling the dual-rail encoders based at least in part on the compensation adjustment information, wherein the light sources comprise laser sources operating at the same frequency, such that the laser sources are injection locked to the same frequency, phase <NUM>.

<FIG> is a second flow graph of a second method in accordance with at least some embodiments of the present invention. The phases of the illustrated first method may be performed in a receiver, for example. Optional phase <NUM> comprises converting incoming polarization encoded light to dual-rail encoded light. Phase <NUM> comprises measuring encoded light in at least two different bases. In embodiments comprising optional phase <NUM>, the measured encoded light is dual-rail encoded light. Phase <NUM> comprises obtaining compensation adjustment information concerning a fibre. Finally, phase <NUM> comprises adjusting at least one of the encoded light and an output of at least one detector based at least in part on the compensation adjustment information. Obtaining compensation adjustment information concerning the fibre may comprise deriving the compensation adjustment information concerning the fibre.

An advantage of pre-compensating for rotations incurred in the fibres in a dual-rail encoded phase, rather than in a polarization encoded phase, is that a compact, integrated implementation on a chip for at least one of the emitter and the receiver is possible. Truly efficient compensation in polarization encoded phase typically requires controlling polarization of light in the receiver, using fibre straining or free space, which is bulky.

Claim 1:
An apparatus comprising two optical transmitters each comprising a light source (<NUM>) and means configured to obtain light from the light source, respectively, the two optical transmitters further each comprising a dual-rail encoder (<NUM>, <NUM>, <NUM>, <NUM>), configured to encode in each of the optical transmitters dual-rail encoded light and the two optical transmitters each further comprising means configured to convert the dual-rail encoded light into polarization encoded light; the apparatus further comprising means configured to obtain compensation adjustment information concerning two fibres (<NUM>, <NUM>) and means configured to control the dual-rail encoders (<NUM>, <NUM>, <NUM>, <NUM>) based at least in part on the compensation adjustment information, wherein
the light sources (<NUM>) comprise laser sources operating at the same frequency, such that the laser sources are injection locked to the same frequency.