WO2020200449A1

WO2020200449A1 - Optical injection locking in quantum key distribution

Info

Publication number: WO2020200449A1
Application number: PCT/EP2019/058468
Authority: WO
Inventors: Lucian COMANDAR; David Hillerkuss
Original assignee: Huawei Technologies Duesseldorf Gmbh
Priority date: 2019-04-04
Filing date: 2019-04-04
Publication date: 2020-10-08
Also published as: CN113632414B; CN113632414A

Abstract

A receiver for a quantum key distribution system is provided, which receives optical signal(s) including a first signal and a second signal. The first signal is injected into laser which operates as a local oscillator at the receiver. The second signal is quantum signal carrying quantum key data and detected coherently based on the local oscillator provided by the injected laser.

Description

OPTICAL INJECTION LOCKING IN QUANTUM KEY DISTRIBUTION

TECHNICAL FIELD

Embodiments of the present invention relate to the field of quantum key distribution (QKD).

BACKGROUND

QKD is a technique that enables two distant, legitimate parties to establish a common (or shared) secret key in a way that is, considering the laws of quantum mechanics, secure against eavesdropping on the used communication channel(s). To be specific, a shared secret key is a piece of information that is known to both the legitimate parties and unknown to anyone else. Since the shared secret (key) is known only to the legitimate parties, it plays a key role in cryptography where it and has many application, such as secure communication, e.g., encryption, decryption of messages and message authentication. In optical data communications, an eavesdropper, conventionally called Eve, can acquire information about a signal (e.g., a key) transmitted from a sender to a receiver, conventionally called Alice and Bob, respectively, e.g., by splitting of and detecting a fraction of the information carrying light.

In non-QKD systems, the security of the key exchange between two distant parties is typically based on asymmetric cryptography, which relies on the computational complexity of certain mathematical problems, e.g., Diffie-Hellman key exchange or Rivest-Shamir-Adleman public-key cryptosystems. However, as soon as a sufficiently powerful (quantum) computer is available or mathematical progress (e.g., more efficient algorithms) has been made, such key distribution methods may become insecure. Even worse, all data that has been encrypted using keys distributed with these methods can retroactively be broken if the key exchange has been recorded by an eavesdropper.

In QKD, on the other hand, the security of the key distribution is guaranteed by the laws of quantum mechanics, which allow to derive Heisenberg’s uncertainty principle and the no- cloning theorem. The uncertainty principle, which states that certain variables cannot be known simultaneously with arbitrary precision, implies that measuring one variable destroys information about some other variable. Thus, when Eve performs measurements on the transmitted signal, she inevitably leaves a trace by introducing transmission errors. The no cloning theorem states that it is impossible to make a perfect copy of an unknown quantum state, e.g., of a random signal (or a fraction thereof) encoded in an optical mode.

Consequently, it is also impossible to circumvent the uncertainty principle by performing measurements on perfect copies.

Thus, in short, the presence of an eavesdropper spying on communication between the sender and the recipient inevitably leaves a trace that can be detected by way of observing the amount of transmission errors or, equivalently, noise in the transmission channel. In QKD, this is exploited by calculating, based on the observed noise, an upper bound for the information accessible to any eavesdropper. If this upper bound is sufficiently small, a shared secret key can be extracted from the information shared between the sender and the recipient. Under certain conditions, this shared secret key extraction can be proven to be information theoretic secure.

QKD systems can be divided into discrete variable QKD (DV QKD) systems and continuous variable QKD (CV QKD) systems. In DV QKD systems, the information from which the shared secret key is extracted is encoded in a discrete variable, which usually is the polarization/spin degree of freedom of, ideally, single photons, as, e.g., in the BB84 protocol. However, single photon sources and detectors are expensive and difficult to miniaturize. In CV QKD systems, on the other hand, the information from which the shared secret key is extracted is encoded in a continuous variable. Correspondingly, CV QKD protocols are usually based on the transmission of coherent or squeezed states of light, where said information is continuously encoded in the quadratures (phase and amplitude) of the transmitted light/electromagnetic field. At the receiver, the received signal can thus be measured by means of coherent detection (e.g., homodyne, intradyne, or heterodyne detection) using a strong local oscillator (LO). For these reasons, CV QKD is more compatible with standard components and equipment used in current telecommunications systems, and it is even possible to simultaneously use the same optical fiber for QKD and classical signal transmission. SUMMARY

Starting from the above described approaches, the aim of the present disclosure is to further increase the performance of QKD systems, for example, by facilitating precise alignment of transmitter laser and receiver laser.

The foregoing and other objectives are achieved by the subject matter of the independent claims. Further advantageous implementations are apparent from the dependent claims, the description, and the figures.

Embodiments of the present application provide apparatuses and methods for receiving signals according to the independent claims.

In particular, according to a first aspect, the invention relates to a quantum key distribution receiving apparatus for receiving a first optical signal and a second optical signal. The apparatus comprises a laser adapted to be optically injected with the first optical signal, and to thereby generate a third optical signal, and a detector for performing, using the third optical signal as local oscillator, coherent detection on the second optical signal.

By implementing phase and frequency compensation on a physical level using OIL, the receiver laser may inherit the properties of the transmitter laser. Using OIL thus may allow to replace fast digital feedback/compensation with optical locking. Consequently, using OIL in a QKD receiver may enable the elimination or, at least, drastic reduction of frequency feedback and phase compensation loops. Thus, the necessity of digital systems for frequency and phase compensation in DSP, which are computationally rather intensive DSP processes, may be eliminated or, at least, substantially reduced. In particular, it may enable the

reduction/elimination of computing intensive digital signal processing elements during system operation. In consequence, phase/frequency compensation errors may also be

reduced/eliminated. Lurthermore, since using OIL may reduce/relax the laser physical requirements on the receiver side, the use of low quality (poor/cheap) lasers may be enabled on at least the receiver side, thereby possibly further reducing implementation cost and component requirements for CV-QKD. In particular, the necessity for fast frequency stabilization of lasers may be removed/reduced (only thermal control of stability may still be required). Lurthermore, using OIL may enable transmission rate scalability and maximum use, e.g. to make full use of the (entire) Rx detector bandwidth. Moreover, it may allow to select a frequency band with lowest noise and highest ENOB (effective number of bits) for quantum signal and, in a dual polarization detection setup, the removal of polarization tracking.

Advantageously, the apparatus further comprises a polarization controller for controlling, in accordance with a feedback, the polarization of the first optical signal to match a cavity polarization of the laser. Such an apparatus may also comprise a circuitry for providing, based on results of the coherent detection, the feedback to the polarization controller.

Controlling the polarization of the first optical signal to match the cavity polarization of the laser may facilitate optical injection locking.

In some implementations of the first aspect, the apparatus is for receiving the first optical signal and the second optical signal in different optical fibers.

Using a dual fiber configuration, as in classical communications, may allow to use the existing deployed infrastructure.

In some implementations of the first aspect in which the apparatus comprises a polarization controller, the first optical signal and the second optical signal are received with a same polarization in a same optical fiber. In these implementations, the apparatus may further comprise a beam splitter for separating, after the polarization controller has controlled the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal.

Transmitting/receiving the data signal (first optical signal) and the injection signal (second optical signal) in a same optical fiber may be more advantageous in QKD systems than in classical systems using OIL. In particular, since the data signal (quantum signal/first signal) is transmitted/received with below single photon/pulse power density, it may be possible to use essentially the entire frequency band (e.g., no reduction of spectral efficiency due to the usage of OIL/transmission of the pilot tone). In other words, in QKD systems using OIL, it may be possible to support almost arbitrarily small or very large frequencies between the classical and quantum signal, which strongly differentiates QKD system from classical implementations.

Advantageously, the beam splitter is then for separating the first optical signal and the second optical signal according to a predetermined intensity ratio of the first optical signal to the second optical signal. In other implementations of the first aspect in which the apparatus comprises a polarization controller, the first optical signal and the second optical signal are received in a same optical fiber with mutually orthogonal polarizations. In such implementations, the polarization controller may further be for controlling, in accordance with the feedback, the polarization of the second optical signal to a polarization that is orthogonal to the cavity polarization of the laser. Furthermore, the apparatus may then comprise a polarization beam splitter for separating, after the polarization controller has controlled the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal, and for changing the polarization of the second optical signal to correspond to the polarization of the first optical signal.

Separating an optical signal into its two orthogonal polarization components may be accomplished with a polarization beam splitter, which is a standard optical component. Such an implementation may thus further reduce the implementation /component costs on the receiver side.

According to some implementations of the first aspect, the first optical signal and the second optical signal are received with a same polarization in a same optical fiber. In these implementations, the apparatus may further comprise a beam splitter for separating the first optical signal and the second optical signal according to a predetermined intensity ratio of the first optical signal to the second optical signal, and a polarization projector for projecting the polarization of the first optical signal to match a cavity polarization of the laser.

Using a polarization projection unit, may remove the need to use a polarization controller at the receiver, which requires feedback from DSP. Thus, it may be possible to replace the polarization control feedback loop with a polarization projection unit, which may further reduce costs/requirements on the receiver side or may improve the systems performance.

In any of the above exemplary implementations of the first aspect, the quantum key distribution system may be a continuous variable quantum key distribution system using light in a coherent state, which is included in the optical signal.

According to a second aspect, the invention relates to a method for receiving a first optical signal and a second optical signal. The method is for quantum key distribution and includes the step of optically injecting a laser with the first optical signal, and thereby generating a third optical signal. The method further includes the step of performing, using the third optical signal as local oscillator, coherent detection on the second optical signal.

reduced/eliminated. Furthermore, since using OIL may reduce/relax the laser physical requirements on the receiver side, the use of low quality (poor/cheap) lasers may be enabled on at least the receiver side, thereby possibly further reducing implementation cost and component requirements for CV-QKD. In particular, the necessity for fast frequency stabilization of lasers may be removed/reduced (only thermal control of stability may still be required). Furthermore, using OIL may enable transmission rate scalability and maximum use, e.g. to make full use of the (entire) Rx detector bandwidth. Moreover, it may allow to select a frequency band with lowest noise and highest ENOB (effective number of bits) for quantum signal and, in a dual polarization detection setup, the removal of polarization tracking.

Advantageously, the method further includes the step of controlling, in accordance with a feedback, the polarization of the first optical signal to match a cavity polarization of the laser; and the step of providing, based on results of the coherent detection, the feedback.

In some implementations of the second aspect, the method further includes the step of receiving the first optical signal and the second optical signal in different optical fibers.

Using a dual fiber configuration, as in classical communications, may allow to use the existing deployed infrastructure. In some implementations of the second aspect in which the method includes a step of controlling the polarization of the first optical signal, the method further includes the step of receiving the first optical signal and the second optical signal with a same polarization in a same optical fiber; and the step of separating, after controlling the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal.

Transmitting/receiving the data signal (first optical signal) and the injection signal (second optical signal) in a same optical fiber may be more advantageous in QKD systems than in classical systems using OIL. In particular, since the data signal (quantum signal/first signal) is transmitted/received with a below single photon/symbol power density it may be possible to use essentially the entire frequency band (e.g., no reduction of spectral efficiency due to the usage of OIL/transmission of the pilot tone). In other words, in QKD systems using OIL, it may be possible to support almost arbitrarily small or very large frequencies between the classical and quantum signal, which strongly differentiates QKD system from classical implementations.

Advantageously, the first optical signal and the second optical signal are then separated according to a predetermined intensity ratio of the first optical signal to the second optical signal.

In other implementations of the second aspect in which the method includes a step of controlling the polarization of the first optical signal, the method further includes the step of receiving the first optical signal and the second optical signal in a same optical fiber with mutually orthogonal polarizations. In such implementations, the method may further comprise the step of controlling, in accordance with the feedback, the polarization of the second optical signal to a polarization that is orthogonal to the cavity polarization of the laser; the step of separating, after controlling the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal, and the step of changing, after controlling the polarization of the first optical signal and the second optical signal, the polarization of the second optical signal to correspond to the polarization of the first optical signal.

According to some implementations of the second aspect, the method further includes the step of receiving the first optical signal and the second optical signal with a same polarization in a same optical fiber, the step of separating the first optical signal and the second optical signal according to a predetermined intensity ratio of the first optical signal to the second optical signal; and the step of projecting the polarization of the first optical signal to match a cavity polarization of the laser.

In any of the above exemplary implementations of the second aspect, the method may be for continuous variable quantum key distribution using light in a coherent state, which is included in the optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following embodiments of the invention are described in more detail with reference to the attached figures and drawings, in which:

Fig. 1 is a block diagram illustrating the principles of optical injection locking;

Fig. 2 is a graph illustrating the frequency noise vs the frequency for a free running laser and for an optically injected laser;

Fig. 3 is a graph illustrating the locking bandwidth vs the optical emission ratio of a laser;

Fig. 4 is a block diagram showing an exemplary receiving device for receiving the first optical signal and the second optical signal in different optical fibers;

Fig. 5 is a block diagram showing an exemplary receiving device for receiving the first optical signal and the second optical signal with the same polarization in the same optical fiber that requires feedback for polarization control; Fig. 6 is a block diagram showing an exemplary receiving device for receiving the first optical signal and the second optical signal with different polarizations in the same optical fiber; and

Fig. 7 is a block diagram showing an exemplary receiving device for receiving the first optical signal and the second optical signal with the same polarization in the same optical fiber that requires no feedback for polarization control.

In the following, identical reference signs refer to identical or at least functionally equivalent features.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the invention or specific aspects in which embodiments of the present invention may be used. It is understood that embodiments of the invention may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

For instance, it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise. Moreover, it is noted that, in general, all numerical values are examples for the sake of a full description.

As explained above, in continuous-variable quantum key distribution (CV-QKD) protocols coherent detection is performed on the received quantum signal. Quantum signal is a signal containing the key information, i.e. carrying the raw key data out of which the key data is then obtained at the transmitter and receiver according to a particular QKD protocol. However, coherent detection requires a so-called local oscillator (LO) as phase/frequency reference. In CV-QKD systems in which the local oscillator is generated locally at the receiver (e.g., by a laser located at the receiver), the receiver side laser is, however, in general, not synchronized with the laser at the transmitter side that has been used to generate the quantum signal.

Therefore, such CV-QKD systems usually rely on additional signals (synchronization pulse(s) or pilot-tone(s)) transmitted from the transmitter/Bob to the receiver/ Alice to serve as a phase and frequency reference for stability purposes (digitally). These additional signals are then used in digital signal processing (DSP) to compensate for the unsynchronized lasers.

In some CV-QKD systems, the pilot and CV-QKD signal have the same polarization. This represents a relatively simple setup (advantage), which, however, focuses heavily on DSP and has two feedback loops (disadvantage).

In other CV-QKD systems, a polarization diversity scheme where the pilot tone has a different polarization and different frequency than the quantum signal is used. This represents a rather complicated/inefficient and more lossy setup. In particular, this setup requires polarization multiplexing without rate gain, and different types of detectors and polarization tracking.

In most CV-QKD systems where there are a frequency multiplexed pilot tone(s) and signal(s) usually the same detector is used for the pilot tone and the quantum signal. However, the pilot tone contains no information and is used only for frequency and phase recovery. Thus, since the pilot tone occupies a part of the detection bandwidth, the bandwidth occupancy efficiency is reduced. Furthermore, these setups also require relatively fast feedback loops to stabilize beat signals within detector bandwidth.

In general, present (pilot assisted) CV-QKD systems that use reference pulses use the reference pulses transmitted from the transmitter to the receiver for impairments estimation and have several disadvantages. For instance, depending on the degree of freedom on which this pulse is sent (polarization, frequency, time, or a combinations of these) they require additional complex discrimination and additional detection equipment to support

compensation of frequency and phase in DSP. In current systems, this is usually done by transmitting and measuring a known classical signal using the same equipment and at the same time as the quantum signal. After both signals are sampled, the classical signal is used for frequency and phase compensation schemes on the quantum signal. This approach has additional system requirements: the use of low linewidth and highly stable lasers and, on the receiver side, higher bandwidth and/or large dynamic range ADCs are needed. Furthermore, computationally intensive DSP is required for phase compensation and feedback frequency stabilization. This makes the implementation unscalable and resource intensive.

Therefore, according to an embodiment, a quantum key distribution receiving apparatus for receiving a first optical signal and a second optical signal is provided. The apparatus comprises a laser adapted to be optically injected with the first optical signal, and to thereby generate a third optical signal; and a detector for performing, using the third optical signal as local oscillator, coherent detection on the second optical signal.

In general, the first optical signal may be referred to as the reference signal (or, alternatively, pilot tone/signal), the second optical signal may be referred to as the QKD signal (or, alternatively, quantum signal), and the third optical signal may be referred to as the local oscillator. It should be noted that these terms refer only as to how these signals are treated and/or used by the receiver, i.e. these terms refer to the signals available at the receiver. That is to say, the first optical signal refers to the signal that is optically injected into the receiver side laser (thus, it is also referred to as the injected optical signal); the second optical refers to the optical signal on which coherent detection is performed by the receiver, and the third optical signal refers to the optical signal emitted by the receiver side laser and used as local oscillator by the receiver’s coherence detector for coherent detection. This, however, implies that the first optical signal may not exactly correspond to the pilot tone sent by the transmitter (the transmitter pilot tone). Likewise, the second optical signal denoted as the QKD signal may not exactly correspond to the quantum signal transmitted by the transmitter (the transmitter quantum signal). In particular, the first optical signal may also contain a part of the transmitter quantum signal, i.e., may contain a part of optical signal that contains key information. Likewise, the second optical signal may also contain a part of the transmitter pilot tone. On the one hand, this may be caused, e.g., by imperfections of the transmission channel (waveguide) and/or receiver components.

In general, optical injection locking (OIL) may be used to align the receiver side laser, which generates the optical signal used to perform coherent detection on the quantum signal, to the transmitter side laser, which is used to generate said quantum signal. More specifically, by (optically) injecting the receiver side laser with an optical signal that contains a part of the transmitter pilot tone (i.e. with the first optical signal mentioned above), the receiver side laser may be locked to the transmitter pilot tone. OIL is a known physical process that enables the synchronization of two lasers given optical mode matching requirements are met. More specifically, one of the two lasers, which is usually referred to as the slave laser, is optically injected with an optical signal generated by the other laser, which is usually referred to as the master laser. In this way, the slave laser (subjected to OIL) is phase and frequency locked to the master signal (or, in other words, is locked to the master laser).

In general, a CV-QKD system (e.g., the receiver) may use the/a part of the transmitted optical signal (e.g., the pilot signal) from the transmitter to optically lock the receiver laser via OIL. More specifically, the receiver may use a part of the transmitter pilot tone to phase and frequency lock the local oscillator and thus reduce/eliminate the necessity of fast feedback mechanisms (either electrical or in digital signal processing) for phase and frequency control.

In this way, a modified pilot assisted CV-QKD system that enables laser stability via OIL can be provided. In such a system, the transmitter prepares, in addition to the quantum signal, a classical (e.g., with a relatively larger optical power) optical signal (referred to as the transmitter pilot tone) that co-propagates to the receiver. At the receiver side, the classical signal (or a part of it) is fed to the slave laser cavity for OIL. Thus, upon successful OIL, the receiver laser output will be frequency and phase locked to the transmitter laser. The remaining steps to perform CV-QKD remain the same as in standard QKD systems or may be drastically simplified.

The general principles of OIL are illustrated in Figure 1, which shows a test setup for obtaining the results shown in Figures 2 and 3. In particular, Figure 1 shows a master laser 100 generating an optical signal passing attenuator 110 modeling the transmission over an optical waveguide, as well as polarization controller 120 and a power meter 130. The power meter 130 may be used to measure the power of the optical signal received from the master laser 100. Eventually, the optical signal generated by the master laser 100 is optically injected into the slave laser 150 via the optical circulator 140. As indicated by the arrows in Figure 1, the optical circulator 140 has three optical ports. In particular, an optical signal input into the “left” port of the optical circulator 140 is output as output signal at the“upper” port of the optical circulator 140, whereas an optical signal input into the“upper” port of the optical circulator 140 is output at the“right” port of the optical circulator 140. By injecting the slave laser 150 with the signal generated by the master laser 100, the slave laser 150 inherits the properties of the master laser 100. More specifically, by exploiting the process of OIL, a stable frequency and phase relationship between two optical sources (in the present example, between the master laser 100 and the slave laser 150) can be established. The optical signal generated by the slave laser 150 passes the circulator 140, and the power meter 160, which may be used to measure its optical power, to be detected/measured by the detector 170.

By implementing phase and frequency compensation on a physical level using OIL, the receiver laser may inherit the properties of the transmitter laser and will thus be phase and frequency locked. These benefits of OIL can be observed in Figure 2. In particular, Figure 2 shows the frequency noise of a typical laser under free running operation and under OIL. As can further be seen, the Rx laser subjected to OIL mimics the properties of the transmitting (Tx) laser improving the performance with respect to free running operation. It is to be noted that there is a fixed relationship between frequency and phase noise.

As described above, using OIL may allow to replace fast digital feedback/compensation with optical locking. Using OIL in a QKD receiver thus may enable the elimination or, at least, drastic reduction of frequency feedback and phase compensation loops. Thus, the necessity of digital systems for frequency and phase compensation in DSP, which are computationally rather intensive DSP processes, may be eliminated or substantially reduced. In particular, it may enable the reduction/elimination of computing intensive digital signal processing elements during system operation. In consequence, phase/frequency compensation errors may also be reduced/eliminated. Furthermore, since using OIL may reduce/relax the laser physical requirements on the receiver side, the use of low quality (poor/cheap) lasers may be enabled on, at least, the receiver side, thereby possibly further reducing implementation cost and component requirements for CV-QKD. In particular, the necessity for fast frequency stabilization of lasers may be removed/reduced (only thermal control of stability may still be required). Furthermore, using OIL may enable transmission rate scalability and maximum use, e.g. to make full use of the (entire) Rx detector bandwidth. Moreover, it may allow to select a frequency band with lowest noise and highest ENOB (effective number of bits) for quantum signal and, in a dual polarization detection setup, the removal of polarization tracking.

In general the receiver laser may be adapted to be optically injected with the first optical signal by having a simple fiber connection to an optical circulator (e.g., a device that allows spatial division between the input and the output). For instance, as shown in Figure 1, the optical circulator 140 may have three optical ports (inputs/outputs), one of which (e.g., the first) is used as input for the first optical signal. The first optical signal may then leave the optical circulator 140 through the second optical port (input/output), which may be connected via, e.g., a simple optical fiber to the receiver laser 150. The third optical signal generated by the receiver laser 150 then enters the optical circulator 140 through the second part (input/output) and leaves the optical circulator 140 through the third port.

In general, when using OIF the optical signal injected into the slave laser may be chosen to fulfil certain optical mode matching requirements (wavelength similarity, optical power ration between injected optical power and slave laser emission, polarization alignment of the incoming light). All of these would be aligned by the system (transmitter and/or receiver).

In particular, firstly, the optical power of the injected optical signal may be chosen in the injection power range of the slave laser (in general, the power of the injected optical signal is lower than the emission power of the slave laser). This injection power range depends on the lasers used but is typically in the range of 30-40 dB lower than the emitted power by the controlled (slave) laser (literature reports have shown values as low as -65 dB). The optical power of the injected optical signal may, for instance, be adapted by adjusting the amount of optical power on the pilot tone. This may be done at the receiver or transmitter side, e.g., using an optical attenuator.

Secondly, the injected optical signal may be within a certain bandwidth, the so-called locking bandwidth, which is the range of frequencies within which OIF can occur for a given optical power. In other words, locking bandwidth generally depends on the optical power of the injected signal. Furthermore, the transmitter and receiver laser may be chosen to emit naturally within the locking bandwidth. The locking bandwidth can typically be determined up to a few GHz for a given laser, and, thus, the values are high enough to enable independent thermal stabilization of lasers and remove signal processing feedback. In general, the temperature of a laser (which in turn will determine variations of the laser cavity) will determine frequency drifts. It is expected that, if the laser is temperature stable, the emission frequency center is also stable. In reality, the temperature stabilization units have some drift range which determines some laser frequency drift (10s of MHz). Thus, a locking range of a few GHz is a lot higher than the frequency drift induced by the temperature tolerance of the lasers. The injected optical signal may, for instance, be adjusted to the locking bandwidth of the receiver laser (or vice versa) by aligning the transmitter laser and receiver laser to have similar emission frequencies, e.g., by temperature tuning.. Typical values for said locking bandwidth of low quality lasers are shown in Figure 3.

Thirdly, the injected mode (the injected optical signal) may match a certain polarization, the so-called polarization of the laser cavity (or cavity polarization, which is the (single) linear polarization emitted by the laser). It is to be noted that the cavity polarization is also the polarization of the light emitted by the injected laser.

Therefore, according to some embodiments, the apparatus further comprises a polarization controller for controlling, in accordance with a feedback, the polarization of the first optical signal to match a cavity polarization of the laser; and a circuitry for providing, based on results of the coherent detection, the feedback to the polarization controller.

According to an embodiment, the apparatus is for receiving the first optical signal and the second optical signal in different optical fibers.

In general, in some embodiments, a dual fiber configuration as in classical communications may be employed. Such a configuration, which represents the majority of currently deployed optical systems, may allow to use the existing deployed infrastructure. More specifically, the first optical signal and the second signal may be transmitted and/or received in different (e.g., two) optical fibers. In this case, the first optical signal and the second optical signal or, equivalently, the pilot tone and the quantum signal, may be located (e.g., received) at the same frequency or at different frequencies. Likewise, the first optical signal in the second optical signal may be located with the same polarization or with different polarizations.

An exemplary description of a transmitter as well as a receiver according to the present embodiment is now given below using Figure 4 as reference. In particular, Figure 4 shows, at the transmitter side, the laser 400 that generates an optical signal that is split into two optical signals, the transmitter reference signal (transmitter pilot tone) and the transmitter QKD signal (transmitter quantum signal). Each of the two optical signals is then fed into an optical modulator 410. Such an optical modulator 410 may perform intensity modulation and/or phase modulation and/or frequency modulation and/or attenuation on the respective optical signal. After the two optical signals have been modulated accordingly, each of the transmitter QKD signal and the transmitter reference signal are then fed into a different optical fiber 420.

At the receiver side, the first optical signal and the second optical signal are received in different (separate) optical fibers (e.g., at different optical inputs). The optical signal received in one of the optical fibers 420 (advantageously, the reference signal is received in this fiber 420) is fed into a polarization controller 430. The polarization controller 430 compensates the polarization drift of the reference signal (polarization drift/difference of the received reference signal with respect to the transmitter reference signal) and aligns the reference signal (first optical signal) to the laser cavity mode (cavity polarization of the receiver laser 450).

After being aligned to the cavity polarization of the receiver laser 450, the first optical signal is injected to the receiver laser 450 via an optical circulator 140. The receiver laser 450 may then lock onto the first optical signal and generate a third optical signal that follows frequency/phase drifts of the first optical signal (and thus follows phase/frequency drifts of the transmitter laser). The receiver (Rx) laser 450 (e.g., the third optical signal generated by the laser 450) is then used as a local oscillator for a shot noise limited dual polarization balanced detector 460. The optical components (e.g., the optical fibers, and circulator 140) between the laser 450 and the shot noise limited dual polarization balanced detector 460 should maintain polarization stability. The laser 450 may include a phase modulation section (integrated to the laser or external) that is used for selecting the measurement quadrature.

The QKD/second optical signal, which is the optical signal received in the other optical fiber (at the other optical input) than the first optical signal, is fed to the dual polarization balanced detector 460. The dual polarization balanced detector 460 performs coherent detection on the second optical signal using the third optical signal as local oscillator. More specifically, the dual polarization balanced detector 460 performs balanced detection on two (e.g., orthogonal) polarizations of the second signal (the QKD signal). Such a dual polarization balanced detector 460 may be implemented, e.g., as a polarization beam splitter that splits the second input signal into two orthogonal states of polarization (SOP), each of which is then directed into a (different/separate) shot noise limited balanced detector. From the digital signal processing chain 470 (e.g., performed by an electric circuitry such as a digital signal processor, DSP, or by another hardware and/or software) a slow feedback loop is required for the polarization controller 430. It is noted that the two signals (QKD and reference) may suffer from slightly different phase evolution due to different transmission channels (the separate optical fibers 420), which, however, can be compensated in DSP.

According to the present embodiment, the first optical signal and the second optical signal are received in different optical fibers. However, the present invention is not limited thereto as, according to other embodiments, the first optical signal and the second optical signal may be received in (transmitted via) the same optical fiber.

According to some embodiments, the first optical signal and the second optical signal are received with a same polarization in a same optical fiber; and the apparatus further comprises a beam splitter for separating, after the polarization controller has controlled the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal.

At this point, with respect to (frequency or temporal) multiplexing of the first optical signal and the second optical signal, it should be noted that the first optical signal and the second optical signal (or the transmitter pilot tone and the transmitter quantum signal) may, in general, be transmitted/received at the same time without need for a guard band, i.e., without a frequency band between the frequency bands of the two signals that is not used for any transmission (this holds for all embodiments). This is due to the fact that the transmitter quantum signal is, in general, too weak (Pquantum«Ppilot) to have an impact when injected into the receiver laser. That is to say, the power of the transmitter quantum signal is, in general, not in the injection power range of the receiver laser. More specifically, even if the first optical signal, which is injected into the receiver laser, contains a part of the transmitter quantum signal (as received), the receiver laser will lock onto the much stronger (classical) component/part of the first optical signal that corresponds to (a part of) the transmitter pilot tone and not lock onto said part of the transmitter quantum signal even if said part of the transmitter quantum signal is within the locking bandwidth of the receiver laser.

This ability to support almost arbitrarily small or very large frequencies between the classical and quantum signal strongly differentiates this system from classical implementations. It makes OIL in QKD systems particularly advantageous since, in contrast to classical implementations, essentially the entire frequency band can still be used (e.g., no reduction of spectral efficiency due to the usage of OIL/transmission of the pilot tone).

According to some embodiments, the beam splitter is for separating the first optical signal and the second optical signal according to a predetermined intensity ratio of the first optical signal to the second optical signal.

In general, when the first optical signal and the second optical signal are received in the same transmission medium such as the same optical fiber, they may be separated at the receiver. This may be done using a beam splitter. More specifically, the optical signal received by the receiver (in a single fiber/at a single optical input of the receiver) includes the first optical signal and the second optical signal. This received optical signal may be fed into a beam splitter that has one input and two outputs. The beam splitter may be a passive component that splits the optical signal that enters input (the input signal) into two optical signals each of which leaving the beamsplitter (BS) at one of its outputs (the output signals). The input signal may be split by the BS into two signals (the two output signals) according to a

predefined/predetermined intensity ratio (in case of a passive BS this ratio may be a fixed property of the BS). More specifically, this intensity ratio may be the ratio h/(1— h) of the intensities of the two output signals. In general, h% of the input signal (e.g., of the intensity of the input signal) may leave the BS at the first output, and (1-h)% of the input signal (e.g., of the intensity of the input signal) may leave the BS at the second output (the separation is done in intensity with respect to the optical outputs of the BS). For example, it may be that 90% of the input signal leave the BS at the one output, and 10% of the input signal leave the BS at the other output.

It is to be noted that the received optical signal (receiver input signal) carries the transmitter reference signal and the transmitter QKD signal. For instance, using the exemplary numbers above, 90% of the transmitter reference signal as well as 90% of the transmitter QKD signal may leave the BS at one of its outputs, and 10% of the transmitter reference signal as well as 10% on the transmitter QKD signal may leave the BS at the other output.

In other words, the intensity BS does not separate the transmitter reference signal from the transmitter QKD signal (as received by the receiver). The BS separates the first optical signal (which is the signal injected into the receiver laser) from the second optical signal (which is the signal on which coherent detection is performed). The beam splitter thus separates the signal used for laser injection (first signal) from the QKD signal used for quantum key derivation (second signal).

It is further noted that, according to the present embodiments, the transmitter pilot signal may be transmitted at a different frequency (than the transmitter quantum signal), co-polarized with (e.g., having the same polarization as) the transmitter quantum signal and sent along the same channel as the transmitter quantum signal. This is also how the standard transmitter operates in state-of-the-art CV QKD systems. However, the present embodiments are not limited thereto. For instance, the transmitter pilot signal may be transmitted at the same frequency/within the same frequency band as the transmitter quantum signal, which may allow to increase the spectral efficiency. In general, the pilot signal and the quantum signal may also be transmitted and/or received at different polarizations.

As mentioned above, in order to match the polarization of the first optical signal to the cavity polarization of the receiver laser, the first optical signal is fed into a polarization controller. In case the first optical signal is fed into said polarization controller before the first optical signal and the second optical signal are separated from each other by the BS, the polarization controller may further be for controlling, in accordance with the feedback, the polarization of the second optical signal to match the cavity polarization of the laser. In other words, the (single) received optical signal, which comprises the first optical signal and the second optical signal, may be (first) be fed/input to the polarization controller, which aligns the received optical signal to the cavity polarization of the receiver laser. The polarization controller may thus align the first optical signal and the second optical signal to the cavity polarization of the receiver laser. After being subjected to the polarization controller, the aligned received optical signal may then be fed to the BS that separates the first from the second optical signal (according to some intensity ratio).

An exemplary description of a transmitter as well as a receiver according to the present embodiment is now given below using Figure 5 as reference.

In particular, Figure 5 shows, at the transmitter side, (denoted as Alice / Transmitter in Figure 5) a laser 500. This laser 500 may be a continuous wave laser 500. The output of the continuous wave laser 500 may then be modulated in both amplitude and phase using intensity and phase modulators 410 (denoted as Mod in the figure). Two or more signals are generated on the same polarization and include at least the transmitter QKD signal fQKD and the classical reference signal fc (i.e., the transmitter pilot tone). In general, the pilot tone is a classical reference signal with a much higher amplitude than the QKD signal. The optical signal generated by the laser 500 is then modulated by the modulator 510 (phase/frequency modulated). An attenuator (not shown in Figure 5) may further be used to lower the QKD signal to the required photon flux at the output of the transmitter. In this way, at the transmitter, an optical signal that includes both the transmitter pilot signal as well as the transmitter quantum signal is generated. An exemplary frequency spectrum at the output of the transmitter according to the present example for a minimum of two signals (the classical reference signal (fc) and the quantum QKD signal (fQKD)) is plotted for reference in Figure 5 as well.

The two signals (the transmitter pilot signal and the transmitter quantum signal) then propagate through the transmission channel 520 (e.g., an optical fiber) until reaching

Bob/receiver.

At the receiver/Bob, the received optical signal (e.g., at the input) is first fed to a polarization controller (Pol Ctrl) 530, which compensates polarization changes which the optical signal may have received in the transmission from Alice to Bob. The polarization controller 530 aligns, using feedback based on results of the coherent detection performed by the balanced detector 560, the polarization of the received optical signal to the cavity polarization of the receiver laser 550. Since the received optical signal before splitting carries the first optical signal as well as the second optical signal, the optical controller 530 may align the first optical signal and the second optical signal to the cavity polarization of the laser 550 (e.g., at the same time, i.e. by the same operation).

In general, the signal for the polarization feedback loop (e.g., the feedback for the polarization controller) may be obtained from the DSP and has slow speed requirements (in some instances this requirement is removed altogether). It is noted that the DSP uses the results of the coherent detection as input/feedback. It is further noted that the optical components at the receiver may here be composed of only polarization maintaining components aligned to the laser cavity polarization.

In the present example, the aligned received optical signal is then fed/input to BS 540 that separates/divides the amplitude of the incoming signal. More specifically, the BS 540 separates the aligned received optical signal into two optical signals according to the intensity ratio h/(1— h) and outputs at each of its two outputs one of the two optical signals obtained by separating the aligned received optical signal (one with intensity h, the other with intensity 1- h). The beam splitter may be an optical coupler as illustratively shown in Figure 5. The optical coupler may be a fixed coupler (with a fixed ration) or a variable (controllable) coupler.

It is to be noted that the choice for the BS ratio h/(1— h) at the receiver input has a defining role in the system performance. This is due to the fact that one of the output signals of the beam splitter is used as the quantum signal by the receiver. That is to say, coherent detection is performed on one of the output signals of the beam splitter. However, any loss experienced by the quantum signal, e.g., the h% power loss due to the intensity beam splitter, cannot be compensated for and should therefore kept at a minimum in order to minimize the reduction in the key rate.

Thus, for the quantum signal path point of view the BS should be biased towards the quantum/second signal with a large value (1-h)% to reduce losses and increase key rate.

However, due to complementarity, the other output port of the BS, corresponding to the first optical signal, will than only transmit h% of the optical power, which would still have to meet the minimum requirement for the optical power in the pilot tone for OIL to occur. The minimum optical power for OIL to occur has been to shown to be as low as -65 dBm in state of the art publications. One may thus generally assume that OIL can be obtained for a ratio of

-40dB between the injected optical power and the emission power of the slave laser.

Assuming the local oscillator power in the receiver is 0 dBm, the power budget for the system is then mathematically described by,

Ppilot - Llink - Lq > -40 dBm, where Ppilot is the pilot optical power at the transmitter in dBm, Llink represents the losses of the transmission channel - typically a maximum of around 100 km or 20dB, and Lq represents the losses due to the beamsplitter in dB. Thus, for a typical maximum transmission link losses of 20dB the formula becomes:

Ppilot - Lq > -20 dBm An upper bound for Ppilot is given by the maximum power that can be transmitter in optical fibers before non-linear effects occur. Typically, this is in the few dBm regime. It is therefore conceivable that the h% ratio can be anywhere between (1 and 10%) to achieve losses between 20dB and lOdB for Ppilot. Variations on these numbers can be achieved by considering lower transmission distances which would require lower transmission Ppilot, or higher Lp.

Here, it is to be noted that, in general, a small part of the received optical signal is used to optically inject the slave laser 550. Of course, h should be large enough to obtain a first optical signal with an optical power/intensity in the injection power range of the laser 550. At the same time, h should not be chosen close to unity to reduce losses on the quantum path.

In the present example, the h% part (of the aligned received optical signal) is used for optical injection locking Bob’s laser 550 (in other words, it is injected into the receiver laser 550) and, thus, corresponds to the first optical signal. Conversely, the (1-h)% part of the aligned received optical signal corresponds to the second optical signal.

As can be seen, after leaving the BS 540, the first optical signal first enters an optical circulator 140. The first optical signal leaves the optical circulator 140 through the port of the optical circulator 140 that is (optically) connected to the receiver laser 550. The first optical signal is thus injected to the receiver laser 550. Given that the OIL requirements are met (e.g., large enough amplitude for fc at the receiver side to enable locking), the receiver laser 550 output (the third optical signal) mimics the optical signal fc (and, thus, mimics the optical signal generated by the transceiver laser 500) behavior in terms of phase and frequency, albeit with a much higher optical power. In other words, the optically injected slave laser 550 will inherits the frequency and phase properties of the master laser 500. Here, it noted again that the existence of the QKD signal does not affect the OIL due to its negligible (quantum) optical power and the QKD signal is no longer present in the receiver laser emission (as also illustrated in Figure 5).

The optically injected slave laser 550 is used as a local oscillator, LO, and mixed with the quantum signal (the second optical signal) at the balanced detector 560.

More specifically, the balanced detector 560 uses the output of Bob’s laser 550 (the third optical signal) as a local oscillator and mixes it with the second optical signal (in order to detect/measure it). The output of the balanced detector 560 is then sampled by an ADC and the normal CV-QKD digital signal processing steps 570 follow.

The optical components (e.g., the optical fibers, BS 540, and circulator 140) between the polarization controller 530 and the balanced detector 560 should maintain polarization stability. The laser 550 may include a phase modulation section (integrated in the laser or external) that is used for selecting the measurement quadrature for the balanced detector 560.

It is to be noted that, in the present embodiments, polarization control (Pol Ctrl) and slow thermal control of the lasers is still required. However, as slow thermal stabilization is a common feature of laser, the feedback required for polarization control may be the only feedback required.

Moreover, it should be mentioned that a receiver according to the present embodiment may act as an interferometer whose stability should either be compensated or maintained.

Instabilities may be determined by temperature fluctuations that happen on relatively large timescales and thus compensation systems are easily implementable. Another particularity expected from this implementation is the presence of a strong DC component. This strong DC component may be filtered to take into account the existence of signal components (fc) on both the LO and the signal path (fc).

According to the present embodiment, the first optical signal and the second optical signal may be received with a same polarization in the same optical fiber. However, the present invention is not limited thereto.

According to some embodiments, the first optical signal and the second optical signal are received in a same optical fiber with mutually orthogonal polarizations; the polarization controller is further for controlling, in accordance with the feedback, the polarization of the second optical signal to a polarization that is orthogonal to the cavity polarization of the laser; and the apparatus further comprises a polarization beam splitter for separating, after the polarization controller has controlled the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal, and for changing the polarization of the second optical signal to correspond to the polarization of the first optical signal. In general, it is possible to transmit the reference signal and the QKD signal on orthogonal polarizations (strictly speaking, the transmitter transmits the transmitter reference signal and the transmitter quantum signal on orthogonal polarizations) on the same optical fiber as shown in Figure 6 to which further reference will be made. As shown in Figure 6, on the transmitter side this may be realized using a dual polarization modulator 610 to encode the transmitter reference and transmitter QKD signal on orthogonal polarizations (for simplicity here denoted as H and V). The optical signals then propagate through the channel 620 (e.g., an optical fiber) until reaching the receiver.

At the receiver/Bob, the received optical signal received (e.g., at the input) is first fed to a polarization controller (Pol Ctrl) 630, which compensates polarization changes the optical signals may have received in the transmission from Alice to Bob and may adjust/align, using feedback based on results of the coherent detection performed by the balanced detector 660, the polarization of the received optical signals (e.g., the polarization of the transmitter reference signal and the polarization of the transmitter quantum signal, both as received by the receiver) to those of the polarization beam splitter 600 (PBS 600). In short, the transmitter reference signal (first optical signal) is aligned, by the polarization controller 630, to the mode cavity of the slave laser 650; and the transmitter quantum signal (second optical signal) is adjusted to a polarization orthogonal to the mode cavity of the slave laser 650. The feedback signal for the polarization controller is based on reducing any DC components detected after the balanced detector 660 in digital signal processing 670.

Advantageously, one of the polarizations of the PBS 600 (described below) corresponds to the cavity polarization of the receiver laser 650. Thus, advantageously, the polarization controller 630 may align/adjust the polarization of the received reference signal (received within the optical signal) to the polarization of the BS 600 that corresponds to the cavity polarization of the receiver laser 650. Conversely, the polarization controller 630 may align/adjust the polarization of the transmitter quantum signal (as received by the receiver) to the respective other polarization of the PBS 600. The polarization controller 630 may do this simultaneously for the received optical signal includes transmitter reference signal as well as the transmitter quantum signal. It is noted that, in general, when a polarization controller is said to control the polarization of a particular signal to correspond to a certain polarization it is meant that the polarization controller adjusts the polarization of the particular signal to that certain polarization.

More specifically, the PBS 600 may have one input (ports) and two outputs (ports) and may split a signal (henceforth referred to as the input signal) input into its input into two signals (henceforth referred to as the output signals) which are output at its outputs. The two output signals may correspond to two orthogonal (polarization) components of the input signal. These two orthogonal polarizations are here referred to as the polarizations of the PBS 600. The PBS 600 may further (e.g., in case, the PBS 600 is a glass plate with a reflective dielectric coating on one side) give a phase shift of 180° to one of the orthogonal polarization components. In consequence, the two output signals may have the same polarization.

For instance, in reference to Figure 6, the cavity polarization of the laser 650 may be the H polarization. The polarization controller 630 may then adjust the first optical signal to the H polarization and the second optical signal to the V polarization. Thus, the QKD signal may initially be aligned to the V polarization before entering the PBS 600 and is then (when leaving the PBS 600) aligned to the H polarization. This may enable maximum interference the first and the third optical signal at the balanced detector 660.

The optical components (e.g., the optical fibers, PBS 600, and circulator 140) between the polarization controller 630 and the balanced detector 660 should maintain polarization stability. The laser 650 may include a phase modulation section (integrated to the laser or external) that is used for selecting the measurement quadrature for the balanced detector 660.

It is noted that, in the present example, the first optical signal may correspond to the transmitter reference signal as received by the receiver, and the second optical signal may correspond to the transmitter quantum signal as received by the receiver.

According to some embodiments, the first optical signal and the second optical signal are received with a same polarization in a same optical fiber, further comprising: a beam splitter for separating the first optical signal and the second optical signal according to a

predetermined intensity ratio of the first optical signal to the second optical signal; and a polarization projector for projecting the polarization of the first optical signal to match a cavity polarization of the laser. In some implementations, the polarization controller at the receiver can be replaced by a polarization projection unit. For instance, the polarization controller 430 in Figure 4, may be replaced by a polarization projection unit. Such a polarization controller, or State of

Polarization (SOP) projection setup projects the incoming light at least partially to the desired polarization (e.g., to the cavity polarization of the receiver laser). In other words, the (usual) polarization controller as shown in Figure 6, which requires feedback from DSP, is no longer required, and the polarization control feedback loop is replaced with a state of polarization projection.

Such a setup is illustrated in Figure 7. As can be seen, the transmitter may be a standard CV- QKD transmitter, such as the transmitter shown in Figure 5, that transmits the reference signal and the pilot signal in the same optical fiber 720. However, the SOP projection setup is not limited thereto and may also be used in a dual fiber implementation, such as the one shown in Figure 4.

It is further note that, in the present implementation, the first optical signal and the second optical may be sent, by the transmitter, with a same polarization. However, the present implementation is not limited thereto, and the first optical signal and the second optical signal may be sent and/or received with different polarizations, e.g., with mutually orthogonal polarizations.

As can be seen, at the receiver, the received signal is immediately fed into an intensity beam splitter 710, that works similarly as the intensity beam splitter 550 described with respect to Figure 5. In particular, the beam splitter 710 divides the incoming signals into two output signals. As shown, in general, the beam splitter 710 may divide (ignoring power losses in the bean splitter 710) the transmitter pilot signal and the transmitter quantum signal with ratio h/(1-h). It is noted that the intensity beam splitter 710 does not separate the transmitter pilot signal from the transmitter quantum signal.

One of the outputs of the beam splitter 710 is used as signal input and directly fed into the dual polarization balanced detector 760, which may look similar to the dual polarization balanced detector 460. The other output (in general, the smaller output signal) of the beam splitter 710 is directed to the state of polarization projection (SOP) unit 700, the output of which is used for optical injecting the receiver laser 750. This SOP unit 700 ensures that for any random polarization input there is light present at the output and, thus, is useable to OIL the slave laser 750. The polarization projection requires no feedback, may be made of polarization maintaining fiber, and may work as follows: First, a PBS 725 splits the incoming light in its components parts (denoted in Figure 7 as H and V) and at the output of one of the ports the polarization state is rotated to match the other (in Figure 7, indicated by the small arrow from V to H). Thus, the PBS 725 may work similarly to the PBS 600 described above.

On one of the outputs a phase modulator 730 (denoted Ph Mod in Figure 7) is also inserted. Thus, one of the output signals of the PBS 725 is subjected to phase modulation. The two output paths of the polarization beam splitter 725 are coupled to a 50:50 polarization maintaining beam splitter 745. Most specifically, after one of the output signals of the polarization beam splitter 725 is been subjected the phase modulator 730, it is joined with the other output of the polarization beam splitter 725.

The SOP unit 700 resembles an interferometer where the two outputs are complementary depending on the phase difference between the two paths. Only one of the outputs is used to then transmit the projected light via a circulator to the laser cavity. The role of the phase modulator 730 is to modulate at relatively low frequencies (for example sinusoid>100kHz) the phase difference between the two arms as to avoid long term destructive interference at the output of the interferometer. The signal generator 740 drives the phase modulator 730 with electrical signals that should facilitate a minimum 2p phase swing. The interferometer will translate the phase modulation introduced by the phase modulator 730 into optical intensity modulation which however, is reduced by the OIL. Any influence from this can be removed in DSP 770. In other words, the phase modulator 730, which is similar to the modulator 510, is a device that modulates the phase of passing light proportional to an applied voltage. The voltage for this is provided by the Signal generator 740 (Sig Gen). The voltage amplitude should be large enough to facilitate a minimum 2pi phase swing.

The optical components between the BS 710 (e.g., the optical fibers, SOP Projection 700, and circulator 140) and the dual polarization shot noise limited balanced detector 760 () should maintain polarization stability. The laser 550 may include a phase modulation section (integrated to the laser or external) that is used for selecting the measurement quadrature for the dual polarization shot noise balanced detector 760. As described above, the OIL Slave laser (i.e., the third optical signal) is used as a LO and mixed with the second optical signal at the dual Pol balanced detector to perform coherent detection thereon.

It is noted that the SOP Projection setup can also be used to test the LO OIL for QKD security purposes.

According to some embodiments, the quantum key distribution system is a continuous variable quantum key distribution system using light in a coherent state; and the optical signal includes the light.

According to some embodiments, a method for receiving a first optical signal and a second optical signal is provided. The method is for quantum key distribution and includes the step of optically injecting a laser with the first optical signal, and thereby generating a third optical signal; and the step of performing, using the third optical signal as local oscillator, coherent detection on the second optical signal.

According to some embodiments, the method further includes the step of controlling, in accordance with a feedback, the polarization of the first optical signal to match a cavity polarization of the laser; and the step of providing, based on results of the coherent detection, the feedback.

According to some embodiments, the method further includes the step of receiving the first optical signal and the second optical signal in different optical fibers. According to some embodiments, the method further includes the steps of receiving the first optical signal and the second optical signal with a same polarization in a same optical fiber; and the step of separating, after controlling the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal.

According to some embodiments, the first optical signal and the second optical signal are separated according to a predetermined intensity ratio of the first optical signal to the second optical signal.

According to some embodiments, the method further includes the step of receiving the first optical signal and the second optical signal in a same optical fiber with mutually orthogonal polarizations; the step of controlling, in accordance with the feedback, the polarization of the second optical signal to a polarization that is orthogonal to the cavity polarization of the laser; and the step of separating, after controlling the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal, and changing, after controlling the polarization of the first optical signal and the second optical signal, the polarization of the second optical signal to correspond to the polarization of the first optical signal.

According to some embodiments, the method further includes the step of receiving the first optical signal and the second optical signal with a same polarization in a same optical fiber; the step of separating the first optical signal and the second optical signal according to a predetermined intensity ratio of the first optical signal to the second optical signal; and the step of projecting the polarization of the first optical signal to match a cavity polarization of the laser.

Variations Variations to this embodiment could include one or a combination of the following:

To facilitate OIL, the laser on the Rx side may not have an input isolator.

Two large linewidth lasers (cheap and easy to manufacture) one on the Tx and one on the Rx side may be used; alternatively, a small linewidth laser on the Tx side and a large linewidth laser on the Rx side may be used. Phase modulation on the LO may be used for some CV-QKD protocols. For this, the laser on the Rx side can be laser that enables the phase modulation of the output (for example a gain- switched laser on which applying a voltage bias on the pins results in phase shift).

Alternatively, phase change can be applied using external modules.

It is noted that the operating principle is independent of laser wavelengths. The balanced detector refers to the coherent detection class of devices, which for example would include (but would not be limited to) balanced detectors, 90° hybrid detectors, dual polarization balanced detectors or dual polarization 90 ⁰ hybrid detectors. In other words, the balanced detector in any of the described embodiments may be any detector in the coherent detection class of devices (in particular, any device that performs a form of coherent detection and/or uses a local oscillator for detection).

Even when sent in the same optical fibre, the frequency difference between the pilot signal and the quantum signal can be arbitrarily small. As further modification for dual fiber setups, depending on the modulation scheme chosen, the modulator on the reference signal side could be replaced with a simple attenuator.

The dual fibre configuration could also be easily combined with existing dual fiber infrastructure carrying classical communications by using frequency selective components to multiplex signals at the transmitter side and demultiplex at the receiver side the reference and QKD signal.

Furthermore, in case of multiplexing, wavelengths different than the traditional

communications bands can be used.

Moreover, the feedback loop for the polarization controller can be replaced with a physical feedback one based on a PBS.

Claims

1. A quantum key distribution receiving apparatus for receiving a first optical signal and a second optical signal, the apparatus comprising: a laser (450, 550, 650, 750) adapted to be optically injected with the first optical signal, and to thereby generate a third optical signal; and a detector (460, 560, 660, 760) for performing, using the third optical signal as local oscillator, coherent detection on the second optical signal.

2. The apparatus according to claim 1, further comprising: a polarization controller (430, 530, 630) for controlling, in accordance with a feedback, the polarization of the first optical signal to match a cavity polarization of the laser (450, 550, 650); and a circuitry (470, 570, 670) for providing, based on results of the coherent detection, the feedback to the polarization controller (430, 530, 630).

3. The apparatus according to claim 1 or 2, wherein the apparatus is for receiving the first optical signal and the second optical signal in different optical fibers (420).

4. The apparatus according to claim 2, wherein the first optical signal and the second optical signal are received with a same polarization in a same optical fiber (520); and the apparatus further comprises: a beam splitter (540) for separating, after the polarization controller (430, 530, 630) has controlled the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal.

5. The apparatus according to claim 4, wherein the beam splitter (540) is for separating the first optical signal and the second optical signal according to a predetermined intensity ratio of the first optical signal to the second optical signal.

6. The apparatus according to claim 2, wherein the first optical signal and the second optical signal are received in a same optical fiber (620) with mutually orthogonal polarizations; the polarization controller (630) is further for controlling, in accordance with the feedback, the polarization of the second optical signal to a polarization that is orthogonal to the cavity polarization of the laser (650); and the apparatus further comprises a polarization beam splitter (600) for separating, after the polarization controller (630) has controlled the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal, and for changing the polarization of the second optical signal to correspond to the polarization of the first optical signal.

7. The apparatus according to claim f , wherein the first optical signal and the second optical signal are received with a same polarization in a same optical fiber (720), further comprising: a beam splitter (710) for separating the first optical signal and the second optical signal according to a predetermined intensity ratio of the first optical signal to the second optical signal; and a polarization projector (700) for projecting the polarization of the first optical signal to match a cavity polarization of the laser (750).

8. The apparatus according to any of the claims 1 to 7, wherein the quantum key distribution system is a continuous variable quantum key distribution system using light in a coherent state; and the optical signal includes the light.

9. A method for receiving a first optical signal and a second optical signal, the method being for quantum key distribution and including the steps of: optically injecting a laser (450, 550, 650, 750) with the first optical signal, and thereby generating a third optical signal; and performing, using the third optical signal as local oscillator, coherent detection on the second optical signal.

10. The method according to claim 9, further including the steps of: controlling, in accordance with a feedback, the polarization of the first optical signal to match a cavity polarization of the laser (430, 530, 630); and providing, based on results of the coherent detection, the feedback.

11. The method according to claim 9 or 10, further including the step of: receiving the first optical signal and the second optical signal in different optical fibers (420).

12. The method according to claim 10, further including the steps of: receiving the first optical signal and the second optical signal with a same polarization in a same optical fiber (520); and separating, after controlling the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal.

13. The method according to claim 12, wherein the first optical signal and the second optical signal are separated according to a predetermined intensity ratio of the first optical signal to the second optical signal.

14. The method according to claim 10, further including the steps of: receiving the first optical signal and the second optical signal in a same optical fiber (620) with mutually orthogonal polarizations; controlling, in accordance with the feedback, the polarization of the second optical signal to a polarization that is orthogonal to the cavity polarization of the laser (650); and separating, after controlling the polarization of the first optical signal and the second optical signal, the first optical signal and the second optical signal, and changing, after controlling the polarization of the first optical signal and the second optical signal, the polarization of the second optical signal to correspond to the polarization of the first optical signal.

15. The method according to claim 9, further including the steps of: receiving the first optical signal and the second optical signal with a same polarization in a same optical fiber (720); separating the first optical signal and the second optical signal according to a predetermined intensity ratio of the first optical signal to the second optical signal; and projecting the polarization of the first optical signal to match a cavity polarization of the laser (750).