OPTICAL DEVICE, QUANTUM COMMUNICATION SYSTEM, AND QUANTUM COMMUNICATION METHOD

Abstract

To provide a quantum communication system suitable for use with multimode optical fibers.SOLUTION: An optical communication system 1 includes a transmitter device 3 optically coupled to a single mode optical channel and a multimode optical fiber 5, 29 configured to support transmissions of a plurality of guided modes. The plurality of guided modes include a target mode. The transmitter device also includes an optical mode converter 11, 13 that optically couples a mode of the channel to the target mode of the multimode fiber, by gradually modifying a transverse profile of propagating light received from the channel to match a transverse profile of the target mode of the multimode fiber.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] FIELD OF THE DISCLOSURE Embodiments described herein relate to optical devices, quantum communication systems and quantum communication methods. [Background technology]

[0002] In quantum communication systems, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded on a property of the photon, such as its polarization, phase or energy / time. Quantum communication systems can be used to implement quantum key distribution (QKD), a technique for the sharing of cryptographic keys between two parties, a transmitter, often called "Alice", and a receiver, often called "Bob". The appeal of this technique is that it provides a test of whether any part of the key can be known to an unauthorized eavesdropper, often called "Eve".

[0003] QKD protocols generally involve two steps. First, Alice sends quantum information to Bob through a quantum channel, and then, during a post-processing stage, Alice and Bob communicate with each other through a classical channel to establish a shared key. In practice, the quantum and classical channels are often implemented using single-mode optical fibers. An example of a known single QKD communication system is shown in FIG. 1. The system includes a QKD transmitter (Alice) connected through two single-mode fibers to a QKD receiver (Bob). The QKD transmitter includes a "quantum transmitter", i.e., a unit that encodes a quantum state in an optical signal and transmits the encoded signal through the first single-mode fiber to Bob, who performs quantum state measurements on the received signal. Alice and Bob each control a classical communication module connected to the second single-mode fiber to perform classical (two-way) communication during the post-processing stage.

[0004] Although single-mode fiber is widely deployed for optical communications today, multimode fiber networks are not only still in active use but continue to be actively developed and deployed for a variety of use cases. However, conventional QKD technology (such as the QKD transmitter and receiver shown in FIG. 1) is not suitable for use with multimode optical fiber.

[0005] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 illustrates a known QKD communication system. [Diagram 2] FIG. 2 is a schematic diagram of a quantum communication system according to one embodiment. [Diagram 3] FIG. 3 is a diagram showing the modes of a multimode fiber. [Figure 4] FIG. 4 is a diagram showing the modes of a multimode fiber and a single mode fiber. [Diagram 5] FIG. 5 is a schematic diagram of a variation of the quantum communication system of FIG. [Figure 6] FIG. 6 is a schematic diagram of an exemplary implementation of an optical mode converter, according to one embodiment. [Figure 7] FIG. 7 is a schematic diagram of an optical mode converter and multiplexer, according to one embodiment. [Figure 8] FIG. 8 is a schematic diagram of a further quantum communication system according to an embodiment. [Figure 9] FIG. 9 is a schematic diagram of a further quantum communication system according to an embodiment. [Figure 10] FIG. 10 is a schematic diagram of a further quantum communication system according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] To avoid unnecessary repetition, like reference numbers will be used in the figures to denote like features.

[0008] The present disclosure aims to provide new and useful optical devices for implementing quantum communication over multimode optical fibers, i.e., optical fibers that support multiple guided modes (at a certain operating wavelength).

[0009] In one embodiment, an optical communication system is provided. The optical communication system comprises a first transmitter (e.g., a quantum transmitter or a classical communication device) optically coupled to a first single-mode optical channel (e.g., a single-mode fiber) and a multimode fiber configured to support transmission of a plurality of guided modes. The plurality of guided modes comprises a target mode (e.g., a fundamental mode of the multimode fiber). The optical communication system further comprises a first optical mode converter configured to optically couple a mode of the first single-mode optical channel to the target mode of the multimode fiber by gradually modifying a transverse profile of propagating light received from the first single-mode optical channel to match the transverse profile of the target mode of the multimode fiber.

[0010] The disclosed system provides an improved optical communication system. In particular, the disclosed system allows launching an optical signal (by providing a signal to a single mode channel) into a multimode fiber such that the optical signal can be transmitted robustly (i.e., without significant distortion of the signal and / or phase) through the multimode fiber. This is achieved by converting the input mode to match the mode profile of a target mode of the multimode fiber, which ensures coupling of the signal into (substantially) only one mode of the multimode fiber. During propagation through the fiber, the signal may couple into other modes with nearly the same propagation constant, but is suppressed from coupling into modes with different propagation constants, which allows transmission through the fiber with any substantial distortion for the signal.

[0011] In one embodiment, gradually modifying the lateral profile of the light received from the channel may mean that the lateral profile of the light propagating through the optical mode converter has different transverse extents at three or more equidistantly spaced locations along the optical path of the optical converter.

[0012] In one embodiment, the first optical mode converter may be configured to optically couple a mode of the first single mode optical channel to a target mode of the multimode fiber with a selectivity of at least 5 dB.

[0013] In one embodiment, the optical communication system may be configured to transmit a plurality of signals using wavelength division multiplexing. The wavelength division multiplexed plurality of signals may comprise a plurality of quantum signals having a first wavelength and a plurality of classical signals having a different wavelength.

[0014] In one embodiment, the optical communication system further comprises a receiver optically coupled to the second single-mode optical channel and a third optical mode converter configured to optically couple a mode of the second single-mode optical channel to a target mode of the multimode fiber by gradually modifying a transverse profile of propagating light received from the target mode of the multimode fiber to match a transverse profile of the second single-mode optical channel. The first transmitter may be a quantum transmitter configured to provide a plurality of quantum signals, the receiver may be a quantum receiver comprising at least one detector configured to detect the plurality of quantum signals, and the optical communication system may be configured to implement a quantum key distribution protocol.

[0015] In one embodiment, the plurality of guided modes comprises a first group of guided modes and a second group of guided modes. The first group may comprise a target mode. The optical communication system further comprises a second transmitter optically coupled to the second single mode optical channel. The second optical mode converter may be further configured to optically couple a mode of the second single mode optical channel to a mode of the second group of guided modes of the multimode fiber such that the first single mode optical channel and the second single mode optical channel are optically coupled to a plurality of modes of a plurality of different groups of guided modes of the multimode fiber. Coupling between modes of the same mode group may be stronger than coupling between modes of different mode groups.

[0016] The above allows for improved ways of multiplexing optical signals so that they can be robustly transmitted over a multimode fiber (e.g., to efficiently implement a QKD protocol). This can be achieved by grouping the modes of a multimode fiber according to their propagation constants such that the modes within each group are (almost) degenerate (i.e., they have almost the same propagation constant), but the modes in different groups are non-degenerate (i.e., they have different propagation constants), and by coupling optical channels to modes of different mode groups such that each channel is coupled to one or more of the modes of a different group. Since the coupling between the modes of different mode groups is (sufficiently) low, signal and / or phase distortions are strongly reduced. By coupling the signals emitted by a particular transmitter only to the modes of the corresponding mode group, signal and / or phase distortions are strongly reduced and the implementation of the system is improved (e.g., when one transmitter is a quantum transmitter and the system implements a QKD protocol, the distance over which QKD can be implemented is improved). Furthermore, compared to conventional systems that rely on wavelength division multiplexing to implement multiple channels in the same optical fiber, the disclosed system may reduce the utilization of the spectral bandwidth of a multimode fiber by employing mode group division multiplexing (i.e., using different mode groups for different channels). Because the coupling of modes in different mode groups is inherently low, mode group division multiplexing provides a robust method of transmitting multiple channels through a multimode fiber without significant coupling between the channels. Furthermore, when channels are multiplexed by mode group division multiplexing (because the mode profile of a mode carrying a signal of one channel only partially overlaps with the mode profile of a mode carrying a signal of another channel), the noise in such channels (e.g., optical scattering noise caused by another channel) is reduced.

[0017] In one embodiment, the plurality of guided modes of the multimode fiber may be a plurality of linearly polarized modes characterized by an azimuthal order v and a radial order μ. A first group of guided modes may be associated with a first mode number and a second group of guided modes may be associated with a second mode number different from the first mode number, with each mode number m satisfying the equation m=v+2μ+1.

[0018] In one embodiment, the relative difference between the propagation constants of any two modes of the same mode group may be less than or equal to 0.02%.

[0019] In one embodiment, the first transmitter may be a quantum transmitter configured to provide a plurality of quantum signals and comprising a pulsed laser, and the second transmitter may be a classical communications device.

[0020] In one embodiment, the optical communication system may further comprise one or more transmitters, each of which may be optically coupled to an additional single-mode optical channel. The multimode fiber may be further configured to support transmission of a corresponding one or more groups of guided modes, and the first optical mode converter and the second optical mode converter may be configured to optically couple each additional channel to a mode of a plurality of different groups of guided modes of the multimode fiber.

[0021] In one embodiment, the optical communications system may further include, for one or more transmitters, a corresponding receiver optically coupled to the plurality of single-mode optical channels, and a second optical mode converter configured to optically couple a plurality of modes of the plurality of single-mode optical channels coupled to the one or more transmitters to a plurality of modes of a plurality of different groups of waveguide modes of the multimode fiber such that each receiver may receive a plurality of optical signals from a corresponding transmitter.

[0022] In one embodiment, the optical communication system may be configured to implement a quantum key distribution protocol. A classical communication device and a corresponding receiver may be configured to exchange a plurality of data signals for deriving a shared encryption key. One of the one or more transmitters may be a second classical communication device configured to transmit a reference signal to a corresponding receiver configured to process the received reference signal to adjust a setting of the quantum transmitter. The quantum transmitter may be configured to provide a plurality of quantum states based on the adjusted setting.

[0023] In one embodiment, the target mode may be the fundamental mode of a multimode fiber.

[0024] In one embodiment, the optical mode converter may comprise a tapered waveguide or a tapered waveguide array or a micro-optic lens.

[0025] In one embodiment, the multimode fiber can be one of a step-index multimode fiber, a graded-index multimode fiber, a hollow-core fiber, or a microstructured optical fiber.

[0026] An exemplary quantum communication system 1 is described with reference to Figures 2 to 4. In general, the quantum communication system 1 is suitable for implementing a QKD protocol over a multimode fiber. For example, the quantum communication system can be used to implement the BB84 protocol [CH Bennett and G. Brassard, Proc. of IEEE Int. Conf. on Comp. Sys. Sign. Process. (IEEE, New York, 1984), pp. 175-179] or a distributed phase reference protocol (such as a coherent one-way protocol or a differential phase shift protocol). The quantum communication system 1 can be operated at telecom wavelengths, for example at a wavelength of 1550 nm.

[0027] The communication system 1 comprises a transmitter device 3 configured to transmit a quantum signal to a receiver device 7 via a multimode optical fiber 5. The multimode fiber 5 is generally long, for example, longer than 100 m, longer than 1 km, or longer than 10 km. As an example, the multimode fiber 5 may be part of a fiber optic local area network. In a broad sense, the multimode fiber 5 supports the transmission of several spatial modes in the same waveguide. For example, the multimode fiber 5 may support the transmission of more than 10 or more than 30 guided modes. The multimode fiber 5 may be of any suitable type, for example, the multimode fiber 5 may be a step-index multimode fiber, a graded-index multimode fiber, a hollow-core fiber, or a microstructured optical fiber (e.g., a photonic crystal fiber). Step-index or graded-index multimode fibers generally comprise a large, high-index core surrounded by a cladding region of low index. The cladding may have a diameter of 125 μm and a core diameter of between 50 and 100 μm, but this is just one example.

[0028] Light can only propagate in a multimode fiber in a finite number of possible patterns, commonly called "modes". The physical distribution of the electromagnetic field is different for each mode, and therefore each mode has different optical properties (dispersion, loss, mode field area, nonlinearity, etc.). Figure 3 shows the LP 01 , L.P. 11a , L.P. 11b , L.P. 02 , L.P. 21a , and L.P. 21b Illustrated are the "mode profiles" 31 of the first six of these modes for a typical graded-index multimode fiber, called linearly polarized (LP) mode. νμare generally described by their azimuthal order ν and their radial order μ. As shown in Figure 3, different LP modes have different mode profiles (i.e., different radial electrical distributions), although there may be some overlap.

[0029] In general, LP modes are paraxial solutions (propagating nearly parallel to the fiber axis) of the wave equation in an optical fiber. These solutions exist in the limit of weakly guiding waveguides when the refractive index difference between the core and the cladding is small. Each mode can exist in two orthogonal states of polarization (e.g., LP 01 Horizontal and LP 01 Vertical). LP 01 The mode is also called the fundamental mode of an optical multimode fiber.

[0030] Conventional quantum communication transmitters and receivers (e.g., the devices shown in FIG. 1) are intended for use with single-mode fiber (i.e., fiber that supports only one guided mode at the operating wavelength). These devices, when connected to multimode fiber, typically couple to multiple modes of the multimode fiber. This is because the modes of conventional single-mode fiber have significantly different mode profiles compared to the modes of multimode fiber. In particular, the mode profiles of the modes of single-mode fiber are generally concentrated over a much smaller area than the modes of multimode fiber. This is due to the LP of a typical multimode fiber (with a core diameter of 50 μm). 01 Mode profile 41 and LP of a typical single-mode fiber (with a core diameter of 9 μm) 01 The mode profile 43 of the multimode fiber is shown in FIG. 4. The white dotted line in FIG. 4 indicates the core-cladding boundary. In this example, the LP of a multimode fiber 01The mode field diameter of the mode is about 33 μm, whereas the mode field diameter of the mode in a single mode fiber is only 9.5 μm. In general, the modes in a single mode fiber have (some) overlap with many of the modes in a multimode fiber, allowing a conventional quantum transmitter to excite all of these modes.

[0031] Spreading an optical signal by exciting multiple modes can result in signal distortion. This is because, on average, each mode has different properties (e.g., different dispersion properties), which means that the optical signal is distorted as it propagates in the fiber. Such distortion is problematic for direct detection optical communications (where pulse amplitudes are used to encode information) as well as for quantum communications. For example, pulses spread (also called "intermodal dispersion") and at high bit rates or long distances can begin to spread apart from one another, which makes them indistinguishable (this problem is also known as "intersymbol interference"). This creates errors when the light is measured and decoded by a receiver. Since each mode experiences (on average) a different phase delay, such multimode excitation can further disturb the phase of the optical pulses, and at the output of the multimode fiber, the modes can interfere and result in a noisy signal. During quantum communications (e.g., during QKD), such phase noise is particularly problematic, since quantum communication protocols often rely on well-controlled phases of the optical pulses. The optical devices described below allow conventional QKD transmitters / receivers to send / receive optical signals to / from multimode optical fibers in a distortion-free manner. As described in detail below, the quantum communication system 1 of FIG. 2 achieves this by providing optical mode converters 11, 13, 17, 19.

[0032] Generally, the modes of the multimode fiber 5 may be grouped into "mode groups" based on their respective propagation constants (i.e. based on the effective refractive index experienced by a particular mode) or, equivalently, the modes may be grouped based on their respective group delays (i.e. based on the time delay experienced by an optical pulse when propagating in a particular mode). In particular, the modes may be grouped such that modes belonging to the same group are (substantially) degenerate, i.e. these modes have almost the same propagation constants and group delays. Thus, the modes within a mode group are well phase-matched, i.e. optical pulses propagating in these modes acquire the same optical phase shift. In practice, it has been found that light propagating in a particular mode may (strongly) couple to other modes in the same mode group during propagation. For example, as shown in FIG. 3, the first mode group is composed of LP 01 The second mode group may consist of LP modes (only). 11a Mode and LP 11b modes, and the third mode group can consist of LP 02 Mode and LP 21a Mode and LP 21b modes, and so on (it should be understood that each of these modes has two orthogonal states of polarization).

[0033] Modes in different mode groups have different propagation constants and group delays. Therefore, modes in different mode groups are not well phase-matched. Furthermore, the coupling between modes from different groups is low (e.g., significantly lower than the coupling between modes of the same mode group). Therefore, because the coupling between modes belonging to different mode groups is low, it is possible to transmit (classical or quantum) signals through a multimode fiber without signal distortion by exciting only modes belonging to the same mode group. It is further possible to implement "mode group division multiplexing", as described in detail below, i.e. to encode different signals into different mode groups.

[0034] In broad terms, the mode groups described above may be determined by the symmetry of the multimode fiber 5. It should be understood that small deviations from the targeted symmetry (e.g., when the cross-section of the fiber is not perfectly round, but slightly elliptical) may destroy the degeneracy of other degenerate modes. It should be understood that perfect mode degeneracy is not required in the present disclosure, and small differences in the propagation constants of modes of the same mode group may be tolerated.

[0035] For example, all linearly polarized modes LP (described by their azimuthal order ν and their radial order μ) with mode number m that (not necessarily degenerate) satisfies the equation m=ν+2μ+1. νμ It is known that modes with the same mode number have almost the same propagation constant. Therefore, modes with the same mode number can be said to belong to the same mode group. For example, the first mode group is made up of the mode with mode number m=3 (e.g., LP 01 ), and the second mode group may comprise a mode with mode number m=4 (e.g., LP 11a and L.P. 11b ), and the third mode group may comprise a mode with mode number m=2 (e.g., LP 02 , L.P. 21a and L.P. 21b ), and so on.

[0036] Additionally or alternatively, two modes may be said to belong to the same mode group if the relative difference between their propagation constants is 0.02% or less. Referring again to Figure 2, transmitter 3 comprises a quantum transmitter 9 optically coupled to a (first) optical mode converter 17 and a (first) classical communication device 15 optically coupled to a (second) optical mode converter 11. Transmitter 3 may be implemented in a number of ways, for example as an integrated device (e.g. quantum transmitter 9, classical communication device 15 and optical mode converters 11, 13 may be integrated on a (photonic) integrated circuit) or as a system of discrete components (e.g. the conventional "QKD transmitter" of Figure 1 may provide quantum transmitter 9 and classical communication device 15 connected via optical single mode fibre to the first and second mode converters 11, 13, respectively).

[0037] In general, the quantum transmitter 9 is configured to prepare and provide a quantum signal (i.e., to encode quantum information in light pulses). The quantum transmitter 9 may be of any known suitable design (e.g., the quantum transmitter 9 may be (part of) a conventional QKD transmitter device). For example, the quantum transmitter 9 may be configured to perform time bin encoding (i.e., to provide pairs of light pulses with well-defined time intervals and well-controlled phase relationships). In one embodiment, the quantum transmitter 9 may comprise a (pulsed) laser. Furthermore, the quantum transmitter 9 may comprise an attenuator and a coding interferometer suitable for preparing, from light pulses emitted by the laser, a plurality of quantum states suitable for implementing quantum cryptography techniques. The quantum transmitter 9 may comprise a random number generator configured to provide a random number to select which of the quantum states should be prepared next.

[0038] The quantum transmitter 9 is configured to provide a quantum signal to the first mode converter 11 via an optical channel 21. The optical channel 21 may be configured to support transmission of only a single spatial mode (at the operating wavelength). For example, the optical channel 21 may be a single mode fiber. In another example, the optical channel 21 may be an integrated waveguide. In yet another example, the optical channel 21 may be a free-space beam path (in this example, the optical channel 21 may support multiple spatial modes and the quantum transmitter 9 may provide the quantum signal in a single free-propagation mode).

[0039] The first optical mode converter 11 is configured to optically couple the optical channel 21 to the multimode fiber 5. In particular, the first optical mode converter 11 is configured to couple the optical channel 21 to one or more modes of a particular "target" group of modes of the multimode fiber 5. For example, the first optical mode converter 11 may couple the optical channel 21 to one or more modes of a first mode group (i.e., LP 01 Alternatively, the first optical mode converter 11 may couple the optical channel 21 into a mode of another mode group, for example a mode of the second mode group (i.e., LP 11a and L.P. 11b 2. The optical channel 21 may be coupled to the optical fiber 22 in the optical fiber mode.

[0040] The mode converter 11 may couple the optical channel 21 primarily to one mode of the target mode group or to several modes of the target mode group. For example, when the target mode group is the second mode group, the mode converter 11 may couple the optical channel 21 primarily to one mode of the target mode group. 11a Mode, ii) LP 11b mode, or iii) LP 11a and LP 11bThe mode converter 11 may couple the optical channel 21 into a combination (e.g., linear superposition) with the modes. The mode converter 11 may be further configured such that the optical channel 21 is substantially isolated from modes belonging to other mode groups. In other words, the mode converter 11 couples the optical channel 21 only into modes of the target mode group and does not couple (or only to an insignificant extent) into modes of other mode groups. Exemplary implementations of the optical mode converter 11 are described below with reference to Figures 6 and 7.

[0041] Referring again to Figure 2, the quantum transmitter 9 sends a quantum signal to a mode converter 11 which couples the quantum signal into the multimode fibre 5 such that only modes of one mode group are excited. The quantum signal propagates through the multimode 5 without suffering significant amplitude or phase distortion. This is because, when propagating through the multimode fibre 5, the quantum signal can only propagate in modes that have (substantially) the same (or sufficiently similar) propagation constants (i.e. in modes that are well phase matched).

[0042] For example, the mode converter 11 may be configured to convert the second mode group, in particular the LP 11a The multimode fiber 5 may be configured to couple the optical channel 21 into the LP 11a Mode and LP 11b This can cause coupling between the LP 11a Even if you send a quantum signal to the mode, 11b (modes may be excited during the propagation of the quantum signal through the multimode fiber 5). Such coupling between modes of the same mode group may be difficult to prevent, but may nevertheless be tolerated, since coupling between modes in the same group does not introduce phase noise. As mentioned above, coupling between modes in different mode groups (which would introduce phase noise) is strongly reduced, and therefore the multimode fiber 5 can be used to efficiently couple LP modes (belonging in this example to the target mode group) to the quantum signal. 11a Modes and, for example, LPs (belonging to different mode groups)01 This situation can be compared to the case where a conventional (single-mode fiber based) QKD transmitter is connected to a multimode fiber. In this case, the conventional QKD transmitter is coupled to several (many) modes of the multimode fiber, e.g. (at least) the LP 01 Mode and LP 11a The mode converter 11 excites two modes. Since the propagation velocities of these modes are significantly different (i.e. they are modes belonging to different mode groups), the quantum signal acquires phase noise and timing errors during propagation through the fiber. Therefore, the provision of the mode converter 11 reduces the phase noise and timing errors of the quantum signal.

[0043] The quantum signal propagates through the multimode fiber 5 and is received by a receiver device 7. The receiver device 7 comprises a quantum receiver 23 optically coupled to the (third) optical mode converter 17 and a (second) classical communication device 25 optically coupled to the (fourth) optical mode converter 19. The receiver 7 may be implemented in a number of ways, for example as an integrated device (e.g. the quantum receiver 23, the classical communication device 25 and the optical mode converters 17, 19 may be integrated on a (photonic) integrated circuit) or as a system of discrete components (e.g. the conventional "QKD receiver" of Figure 1 may provide the quantum receiver 23 and the classical communication device 25 connected via optical single mode fiber to the third and fourth mode converters 17, 19, respectively).

[0044] In general, the quantum receiver 23 is configured to receive a quantum signal and to perform a quantum state measurement on the received quantum signal. To this end, the quantum receiver 23 may comprise at least one detector (e.g., a single-photon detector such as a single photon avalanche diode) configured to detect the quantum signal. The quantum receiver 23 may be of any known suitable design (e.g., the quantum receiver 23 may be (part of) a conventional QKD receiver or transmitter device). In one embodiment, the quantum receiver 23 may be configured to decode the time-bin encoded quantum signal. In this embodiment, the quantum receiver 23 may comprise a decoding interferometer (e.g., an asymmetric Mach-Zehnder interferometer) having a long arm and a short arm. A phase modulator may be provided in one of the arms of the decoding interferometer that allows the phase of the photons passing through the arm to be set to one of at least two values. At least one output of the decoding interferometer may be connected to a detector.

[0045] Quantum receiver 23 is configured to receive the quantum signal from third mode converter 17 via optical channel 27. Similar to optical channel 21 connecting quantum transmitter 9 and first mode converter 11, optical channel 27 may be configured to support transmission of only a single spatial mode (at the operating wavelength). For example, optical channel 27 may be a single mode fiber. In another example, optical channel 27 may be an integrated waveguide. In yet another example, optical channel 27 may be a free-space beam path (in this example, optical channel 27 may support multiple spatial modes and quantum receiver 23 may be configured to receive only the quantum signal in a single free-propagation mode).

[0046] The third optical mode converter 17 is configured to optically couple the multimode fiber 5 to the optical channel 27. In particular, the third optical mode converter 17 is configured to couple one or more modes of a target mode group of the first mode converter 11 (i.e., the mode converter connected to the quantum transmitter 9) to the optical channel 27. Thus, similar to the first mode converter 11, the third mode converter 17 may be configured to couple the optical channel 27 only to the modes of the target mode group (and not to modes of other mode groups). For example, if the first optical mode converter 11 only couples the modes of the first mode group (i.e., the LP 01 When the third optical mode converter 17 is configured to couple optical channel 21 to a mode of the first mode group, the third optical mode converter 17 may also be configured to couple optical channel 27 to a mode of the first mode group. Thus, in some embodiments, the first and third mode converters 11, 17 may be configured in the same manner (e.g., the first and third mode converters 11, 17 may be the same device).

[0047] In other embodiments, the first and third mode converters 11, 17 may be configured to couple the respective channels 21, 27 to different modes of the same mode group. For example, the first optical mode converter 11 may be configured to couple the LP 11a The third mode converter 17 may be configured to couple the optical channel 21 into the LP mode (i.e., in this example, the target mode group is the second mode group). 11bThe first mode converter 11 may be configured to couple the optical channel 27 to a mode (i.e. to a different mode of the target mode group of the first mode converter 11). In this example, the quantum signal (provided by the quantum transmitter 9 and launched through the first mode converter into the multimode fibre 5) is received (without phase noise) even though the receiving mode converter 17 couples to a different mode than the transmitting mode converter 11. This is because both mode converters 11, 17 couple to modes of the same mode group, and the (long) multimode fibre 5 provides coupling between modes of the same group. Thus, in general, all of the modes of the target mode group are approximately equally populated at the receiver end of the multimode fibre 5. In the above example, when the quantum signal reaches the receiver, the LP 11a Mode and LP 11b The modes can be equally populated. Thus, although the transmit and receive mode converters 11, 17 couple to different modes, the quantum receiver 23 receives a significant amount of the signal (nearly 50% in this case).

[0048] Generally, the provision of a mode converter 17 at the receiver end of the multimode fiber 5 may reduce the noise level of the signal received by the quantum receiver 23. This is because the mode converter 17 can act as a mode filter, i.e. the mode converter 17 can effectively filter out light from modes that would normally contribute to phase noise. In particular, the mode converter 17 can effectively filter out light in modes that do not belong to the target mode group and that have been excited, for example, through the (weak) coupling provided by the multimode fiber 5, and thus this light does not couple into the quantum receiver 23.

[0049] For example, the first mode converter 11 is an LP 01 The multimode fiber 5 may be configured to couple the quantum signal into a LP mode (i.e., a first group of modes), the LP mode being induced by (e.g., induced by local mechanical stress, etc.). 11aIf the quantum receiver 23 is directly connected to the multimode fiber 5, the quantum receiver 01 The part of the quantum signal transferred in the mode and the LP 11a Moved in mode (hence LP 01 In contrast, the provision of a mode converter 17 ensures that only light in the mode of the target mode group is presented to the quantum receiver. Thus, in this example, the mode converter 17 is a LP 11a This filters out spurious light in the mode and prevents this light from reaching the quantum receiver 23 .

[0050] In a broader sense, the provision of the mode converter 11, 17 enables the transmission of a quantum signal through a multimode fiber 5 in a manner similar to the transmission through a single mode fiber, where the signal propagates in a limited number of well-defined (well phase-matched) modes, where undesired coupling into other guided modes is (sufficiently) low.

[0051] As mentioned above, quantum communication systems often require a classical data channel in addition to the quantum channel. In the quantum communication system 1 of FIG. 2, the classical channel is implemented by a (second) multimode fiber 29 that allows the classical communication device 15 of the transmitter 3 to exchange (i.e., send and receive) data signals with the classical communication device 25 of the receiver 7. The second multimode fiber 29 may be of a similar type to the first multimode fiber 5, or it may be of a different type. To reduce signal distortions caused by the multimode fiber 29, both classical communication devices 15, 25 are optically coupled to respective mode converters 13, 19, which allow classical communication by transmission through only one mode group of the multimode fiber 29 (in a similar manner as described above for the transmission of quantum signals).

[0052] In particular, the classical communication device 15 is optically coupled to the (second) mode converter 13 via an optical channel 31. Similar to the optical channel 21 coupling the quantum transmitter 9 and the first mode converter 11, the optical channel 31 may be a single mode fiber, an integrated waveguide or a free space beam path. The second optical mode converter 13 is configured to optically couple the optical channel 31 to a second multimode fiber 29. In particular, the second optical mode converter 13 is configured to couple the optical channel 31 to one or more modes of a target group of modes of the multimode fiber 29 (in the same manner as described above with respect to the first mode converter 11).

[0053] In one embodiment, the classical signal provided by the classical communication device 15 is transmitted through the multimode fiber 29 without significant signal distortion, since the second optical mode converter 13 excites modes from only one mode group, and the coupling between modes of different mode groups is (sufficiently) weak.

[0054] The classical signals transmitted via the second multimode fiber 29 are sent to the receiver device 7, where they are received by the (fourth) optical mode converter 19. The fourth optical mode converter 19 is configured to optically couple the second multimode fiber 29 to an optical channel 33 connected to the second classical communication device 25. In particular, the fourth optical mode converter 19 is configured to couple one or more modes of a target mode group of the second mode converter 13 (i.e., the mode converter connected to the first classical communication device 15) to the optical channel 33. Thus, similar to the second mode converter 13, the fourth mode converter 19 may be configured to couple the optical channel 33 only to the modes of the target mode group (and not to the modes of other mode groups).

[0055] It should be appreciated that in the quantum communication system 1 of FIG. 2, the quantum and classical channels are implemented in two independent multimode fibers, so that the target mode groups for transmitting quantum and classical signals can be selected independently.

[0056] It is often desirable to reduce the number of optical fibers required to implement a QKD protocol. One way to achieve this is by using wavelength division multiplexing (WDM), i.e., by using the same optical fiber to implement the quantum and classical channels. FIG. 5 shows a corresponding variation of the quantum communication system of FIG. 2. Like the embodiment of FIG. 2, the quantum communication system 51 of FIG. 5 is suitable for implementing the QKD protocol over multimode fiber. In particular, the quantum communication system 51 of FIG. 5 uses WDM to implement the quantum and classical channels between a transmitter 53 and a receiver 55 in the same multimode fiber 5. The transmitter 53 comprises a quantum transmitter 9 and a classical communication device 15 as described above with reference to FIG. 2. In the embodiment of FIG. 5, the quantum transmitter 9 and the classical communication device 15 are further configured to operate at (slightly) different wavelengths (to enable WDM).

[0057] The quantum transmitter 9 and the classical communication device 15 are optically coupled (e.g., via single mode fiber, integrated waveguide, or free space beam path) to a wavelength division multiplexer 57. The wavelength division multiplexer 57, configured to combine the received signals into the optical channels 21, is connected to the optical mode converter 11. As is known, any suitable wavelength division multiplexer may be adapted for use in the transmitter 9. In one embodiment, the quantum transmitter 9, the classical communication device 15, and the wavelength division multiplexer 57 may be part of a conventional WDM-QKD transmitter. In either case, the wavelength division multiplexer 57 is generally a bidirectional device, i.e., the wavelength division multiplexer 57 may perform multiplexing when light is transmitted from the quantum transmitter 9 or the classical communication device 15, and may perform multiplexing when light is received from the optical channels 21.

[0058] As will be explained with reference to FIG. 2, the optical mode converter 11 is configured to optically couple an optical channel 21 (carrying a multiplexed signal) into one or more modes of a target group of (degenerate) modes of a multimode fiber 5. In particular, the optical mode converter 11 is configured to couple a quantum signal and a classical signal into the same mode group. For example, the optical mode converter 11 may couple a mode of the mode group (i.e., LP 01 2. The quantum and classical signals may be coupled into the multimode fiber 5 (mode) via the optical channel 21. As explained above, the quantum and classical signals can propagate through the multimode fiber 5 without significant signal or phase noise.

[0059] The receiver 55 comprises a mode converter 17 optically coupled to a wavelength division demultiplexer 63, a quantum receiver 23, and a classical communication device 25. As described with reference to FIG. 2, the optical mode converter 17 is configured to couple one or more modes of a target mode group of the mode converter 11 of the transmitter device 53 to the optical channel 27. In particular, the mode converter 17 is configured to couple quantum and classical signals propagating in the target mode group to the optical channel 27. The wavelength division demultiplexer 63 is configured to receive the multiplexed signals from the optical channel 27 and to spatially separate the signals according to their wavelengths. In particular, the wavelength division demultiplexer 63 is configured to couple the quantum signals to the quantum receiver 23 and the classical signals to the classical communication device 25. The wavelength division demultiplexer 63 may be implemented in the same manner as the wavelength division multiplexer 57 described above.

[0060] In general, the optical mode converters 11, 13, 17, 19 (described above with reference to Figures 2 and 5) may be implemented using a number of known techniques, also collectively referred to as "optical beam shaping" techniques. Optical beam shaping techniques typically involve methods of redistributing energy within an optical beam to adjust the intensity and / or phase profile. In general, the optical mode converters 11, 13, 17, 19 are configured to gradually modify the transverse profile of the received light to match the transverse profile of a respective target mode. "Gradually modifying" may mean that the transverse profile of the light propagating through the optical mode converter has different transverse extents at three or more equidistantly spaced locations along the optical path of the optical converter. For example, the optical mode converter 11 may receive light having a transverse profile with a small transverse extent from the (single mode) optical channel 21, and may be configured such that the transverse extent of the light increases as the light propagates through the mode converter 11 until the transverse profile matches the fundamental mode of the multimode fiber.

[0061] For example, the optical mode converter may comprise a free-space optical system based on refractive or diffractive optical elements. In general, the mode converter is implemented as a bidirectional optical component. Figure 6 shows the LP of a multimode fiber 63 using a beam shaping system 65 based on appropriately selected lenses. 01 Such a possible implementation of the optical mode converter is shown in Fig. 2 and Fig. 5, which maps the modes of a single mode fiber 61 to the modes. In particular, the lens is selected such that the optical mode converter couples the single mode fiber 61 to the modes of the target mode group (in this example, the first mode group) without providing (substantial) coupling to the modes of other mode groups. It should be understood that the embodiment of Fig. 6 is only an example. In another embodiment, the optical mode converter may be implemented using fiber-integrated micro-optical elements. In another embodiment, the optical mode converter may be implemented using (suitably designed) tapered waveguides to, for example, adiabatically tune the waveguide geometry (or refractive index) to transform the guided mode profile in a substantially lossless manner.

[0062] As mentioned above, a mode converter is generally configured to couple propagating light from a (single mode) optical channel into a selected mode (or multiple modes of the same mode group) of a multimode fiber without significant excitation of other modes of the multimode fiber. More specifically, a mode converter may be configured to couple an optical channel into one or more modes of a target mode group with a selectivity of greater than 5 dB, i.e., the power coupled into the target mode group may be at least 5 dB higher than the total power coupled into the modes of the other mode groups. In embodiments, the selectivity of the mode converter may be greater than 10 dB or 20 dB.

[0063] With reference to Figure 7, one embodiment of an optical mode converter and multiplexer 71 (hereafter simply referred to as mode converter 71 for short) is described. The mode converter 71 of Figure 7 is particularly suitable for "mode group division multiplexing" which is described in more detail below with reference to Figures 8 to 10. In a broad sense, the mode converter 71 converts (bidirectionally) N optical channels 731-73 into (one or more) modes in N different mode groups of a multimode fiber 75. N where N is an integer greater than 0. Mode converter 71 is generally bidirectional, so that mode converter 71 can act as a multiplexer as well as a demultiplexer.

[0064] In other words, N optical channels 731 to 73 N Each of the N optical channels 731-73 is coupled (bidirectionally) to a different target mode group. In particular, the optical mode converter 71 couples N optical channels 731-73 to each target mode group without providing (substantial) coupling to modes of other mode groups. N 7 shows four optical channels 731-734, with the optical mode converter 71 configured to couple the optical channels 731-734, respectively, to the first through fourth mode groups of the multimode fiber 75. In other embodiments, the optical mode converter 71 may couple a different number of optical channels to each target mode group of the multimode fiber 75. It should be understood that the mode converter 71 may be adapted to implement the mode converters described above with reference to FIGS. 2 and 5. Additionally, the optical mode converter 71 may comprise an optical beam shaping system, as known, as described above with reference to FIG. 6. In one embodiment, the mode converter 71 is positioned in close proximity to the multimode fiber (optical channels 731-734, respectively) such that light is coupled from the single mode waveguide to the multimode fiber in a desired manner (i.e., such that each optical channel is coupled to a mode of a different mode group without exciting modes of other mode groups). NFor this purpose, the mode converter 71 may comprise a suitably designed photonic lantern type structure.

[0065] Figure 8 shows a variation of the quantum communication system of Figure 2. Like the embodiment of Figure 2, the quantum communication system 81 of Figure 8 is suitable for implementing QKD protocols over multimode fiber. In particular, the quantum communication system 81 of Figure 8 uses "mode group division multiplexing" to multiplex a quantum channel and two different classical channels (all operating at the same wavelength) for transmission over multimode fiber.

[0066] The quantum communication system 81 comprises a transmitter device 83 optically connected to a receiver device 85 via a multimode fibre 5. The transmitter 83 comprises a quantum transmitter 9 and a classical communication device 15 as described above with reference to Figure 2. In the embodiment of Figure 8, the quantum transmitter 9 and the classical communication device 15 are configured to operate at substantially the same wavelength.

[0067] Transmitter device 83 further comprises an optical mode converter and multiplexer 89 (for short, mode converter 89) optically coupled to quantum transmitter 9 (via optical channel 21), to classical communication device 15 (via optical channel 31), and to an external data channel 87 (e.g., conventional telecom fiber carrying classical signals unrelated to the QKD protocol implemented by system 81). Optical channels 21 and 31 may be implemented as single mode fiber, integrated waveguides or free space beam paths. External data channel 87 may generally be single mode fiber.

[0068] The mode converter 89 is configured to optically couple each optical channel 21, 31, 87 to a mode(s) of a different mode group. The mode converter 71 (described above with reference to FIG. 7 when N≧3) may be used to implement the mode converter 89 of FIG. 8. In the embodiment of FIG. 8, the mode converter 89 may be configured to couple the quantum emitter 9, the classical communication device 15, and the external data channel 87 to modes of the first, second, and third mode groups, respectively, of the multimode fiber 5. Because each signal propagates only within its respective target mode group and is only weakly (if at all) coupled to modes of other mode groups, the mode group multiplexed signals propagate through the fiber 5 without acquiring substantial signal or phase distortion.

[0069] The receiver device 85 comprises a quantum receiver 23 and a classical communication device 25 as described above with reference to FIG. 2. The receiver 85 further comprises an optical mode converter and multiplexer 91 (short, mode converter 91) for demultiplexing signals received from the multimode fiber 5 and directing the demultiplexed signals to corresponding receivers. The mode converter 91 is coupled to the quantum receiver 23 (via optical channel 27), to the classical communication device 25 (via optical channel 33), and to a further external data channel 93. The optical channels 27 and 33 may be implemented as single mode fibers, integrated waveguides or free space beam paths. Like the data channel 87, the external data channel 93 may generally be a single mode fiber.

[0070] The mode converter 91 is configured to perform mode group division demultiplexing on the signals received from the multimode fiber 5, i.e., each channel 27, 33, 93 is optically coupled to one or more modes of a target mode group of the mode converter 89 of the transmitter device 83. In the embodiment of Figure 8, the mode converter 91 may be configured to couple the quantum receiver 23, the classical communication device 25, and the external data channel 93 to modes of the first, second and third mode groups of the multimode fiber 5, respectively.

[0071] Thus, the quantum communication system 81 of Figure 8 uses less spectral bandwidth than conventional QKD using WDM (because the quantum channel and the two data channels operate at the same wavelength), which can free up more bandwidth on the fiber network for other applications.

[0072] In one embodiment, the quantum communication system 81 described above can also improve QKD performance beyond that of conventional single-mode fiber-based WDM-QKD systems. This is because by employing mode group division multiplexing, noise in the quantum channel caused by Raman scattering of classical light can be reduced. In conventional single-mode fiber-based systems, Raman scattering of classical signals when multiplexed with quantum signals is a fundamental source of noise that may not be filtered out. In particular, spontaneous Raman emission from a fiber core (e.g., silica) when "pumped" by a classical light field can result in emission coupled into the same single mode. In contrast, in quantum communication system 81, Raman scattering of classical fields into mode groups implementing quantum channels is reduced because classical and quantum signals propagate in different modes with little overlap. Thus, using mode group division multiplexing for quantum and classical channels allows for lowering the effects of spontaneous Raman scattering compared to WDM.

[0073] While the embodiment of FIG. 8 employed mode group division multiplexing of one quantum channel and two classical channels, other embodiments may multiplex more or fewer channels (an exemplary embodiment in which four channels are multiplexed is described below with reference to FIG. 9). Furthermore, it should be understood that the embodiments of FIG. 5 and FIG. 8 may be combined, which may lead to a lower noise level compared to conventional WDM-QKD systems. In other words, the above-described mode group division multiplexing technique and WDM may be combined, for example, such that each of the implemented channels propagates in a different mode group and is implemented at a different wavelength.

[0074] Figure 9 shows a variation of the quantum communication system of Figure 2. Like the embodiment of Figure 2, the quantum communication system 101 of Figure 9 is suitable for implementing a QKD protocol over multimode fiber. In particular, the quantum communication system 101 of Figure 9 uses mode group division multiplexing to multiplex multiple quantum channels (all operating at the same wavelength) and one classical channel for transmission over multimode fiber. By using mode group division multiplexing of multiple quantum channels, the key rate of the QKD system can be increased (without having to implement multiple instances of the entire QKD system).

[0075] The quantum communication system 101 comprises a transmitter device 103 optically connected to a receiver device 105 via a multimode fiber 5. The transmitter 103 comprises three quantum transmitters 9, 107, 109 and a classical communication device 15. The quantum transmitters 107, 109 may be different instances of the quantum transmitter 9 as described above with reference to FIG. 2 and may be configured to operate independently of each other (so that each quantum transmitter may generate a quantum signal that is independent of the quantum states generated by the other quantum transmitters). In the embodiment of FIG. 9, the quantum transmitters 9, 107, 109 and the classical communication device 15 are configured to operate at substantially the same wavelength. The transmitter device 103 further comprises a post-processing module 115 configured to extract a secure key by processing data specifying the quantum states prepared by the quantum transmitters 9, 107, 109 and data specifying the measurement results obtained by the receiver device 105 (this data is received by the classical communication device 15).

[0076] The transmitter device 103 further comprises an optical mode converter and multiplexer 114 (or, for short, mode converter 114) coupled to the quantum transmitters 9, 107, 109 and to the classical communication device 15 by optical channels 21, 111, 113, 31. The optical channels 21, 111, 113, 31 may be implemented as single mode fibers, integrated waveguides or free space beam paths.

[0077] The mode converter 114 is configured to optically couple each optical channel 21, 111, 113, 31 to a mode(s) of a different mode group. The mode converter 71 (described above with reference to FIG. 7 when N≧4) may be used to implement the mode converter 114 of FIG. 9. In the embodiment of FIG. 9, the mode converter 114 may be configured to couple the quantum emitters 9, 107, 109 and the classical communication device 15 to modes of the first, second, third and fourth mode groups, respectively, of the multimode fiber 5. The mode group multiplexed signals propagate through the fiber 5 without acquiring substantial signal or phase distortion, since each signal propagates only within its respective target mode group and is only weakly (if at all) coupled to modes of other mode groups.

[0078] The receiver device 105 comprises three quantum receivers 23, 119, 121 and a classical communication device 25. The quantum receivers 119, 121 may be different instances of the quantum receiver 23 described above with reference to FIG. 2. The receiver device 105 further comprises an optical mode converter and multiplexer 116 (short, mode converter 116) for demultiplexing signals received from the multimode fiber 5 and guiding the demultiplexed signals to corresponding receivers. In particular, the mode converter 116 is coupled to the quantum receivers 23, 119, 121 and the classical communication device 25 via optical channels 27, 123, 125, 33. The optical channels 27, 123, 125, 33 may be implemented as single mode fibers, integrated waveguides or free space beam paths.

[0079] The mode converter 116 is configured to perform mode group division demultiplexing on the signals received from the multimode fiber 5, i.e., each channel 27, 123, 125, 33 is optically coupled to one or more modes of a target mode group of the mode converter 114 of the transmitter device 103. In the embodiment of Figure 8, the mode converter 91 may be configured to couple the quantum receivers 23, 119, 121 and the classical communications device 25 to modes of the first, second, third and fourth mode groups of the multimode fiber 5, respectively.

[0080] The receiver device 105 further comprises a post-processing module 127 configured to extract a secure key by processing data specifying the measurement results obtained by the quantum receiver 23, 119, 121 and data specifying the basis on which the quantum state was prepared by the quantum transmitter 9, 107, 109 (this data is transmitted via a classical channel, i.e. transmitted by the classical communication device 15 and received by the classical communication device 25).

[0081] Thus, by using the same classical channel for three (independent) quantum channels, the quantum communication system 101 can achieve a three-fold increase in secure key rate with a simple setup (i.e., much simpler than operating three independent conventional systems in parallel). Advantageously, mode group multiplexing allows for the implementation of multiple quantum channels over a single optical fiber without relying on WDM (thus conserving spectral bandwidth). Furthermore, the efficiency and implementation of QKD post-processing generally depends on the sample size. In one embodiment, the processing is performed in blocks of a fixed size. In this case, by using three quantum channels to collect data for a single post-processing transaction, blocks of data can be collected faster than a conventional system with three separate complete QKD systems, where each QKD system collects and processes their own blocks of data. Thus, the quantum communication system 101 of FIG. 9 allows for faster processing of data to generate keys more periodically (by employing multiple multiplexed quantum channels).

[0082] Mode group division multiplexing may be particularly advantageous for implementing measurement device independent (MDI) QKD protocols such as "twin field quantum key distribution" (TF-QKD). In general, MDI QKD protocols often consider a network with three nodes, i.e., two transmitters (Alice and Bob) each optically connected to a central node ("Charlie"). Charlie may be untrustworthy. MDI QKD systems often have different (or additional) technical requirements than two-node QKD protocols (such as the BB84 protocol). In particular, MDI QKD protocols generally require sharing additional stable signals between Alice and / or Bob and / or Charlie, which may be efficiently implemented using mode group division multiplexing, as described below.

[0083] For example, MDI QKD may require that Alice and Bob emit indistinguishable photons, meaning that their lasers must emit at the same wavelength. Because the emission spectrum of a laser can drift over time due to thermal changes, a reference signal between Alice and Bob is often required. This may involve Alice and Bob sharing some continuous wave laser light to use as a wavelength reference (e.g., provided by a central node). This requires an additional stabilization subsystem and a "servo channel" connected to Alice and Bob. In one embodiment, this servo channel may be mode group division multiplexed into higher order mode groups of a multimode fiber (similar to the system described above with reference to Figures 8 and 9), thereby avoiding the need for a separate servo fiber.

[0084] TF-QKD is a specific MDI QKD protocol. In general, in TF-QKD, both Alice and Bob send signals to a receiver Charlie, which performs first-order optical interference between signals from Alice and Bob. By disclosing the results of the interference, Alice and Bob can establish a secret key. TF-QKD has specific servo channel requirements, e.g., since the protocol relies on first-order interference measurements, TF-QKD may require wavelength spreading and a phase reference between Alice and Bob. Conventionally, signals for TF-QKD stabilization feedback are often sent by wavelength division multiplexing the quantum channel and the phase reference, thereby using additional spectral bandwidth for the phase reference (and thus blocking this bandwidth for other applications).

[0085] A specific example of mode group division multiplexing applied to a TF-QKD system will now be described with reference to Fig. 10. Broadly speaking, Fig. 10 shows a quantum communication system 131 suitable for implementing a TF-QKD scheme, where Alice and Bob each encode information in the phase of an optical pulse, which is then transmitted via a multimode fiber to Charlie, who sends a phase reference light to both Alice and Bob via their respective multimode fibers. This is achieved by applying mode group division multiplexing to multiplex the encoded quantum signal and the phase reference light into dedicated (higher order) mode groups of the multimode fiber. As the quantum signal and the phase reference light propagate in the same fiber, phase shifts experienced, for example, due to fiber bending, thermal contraction, etc., will be corrected. Thus, phase measurements of the phase reference light can be used to correct phase shifts in the quantum channel.

[0086] Compared to conventional approaches to implementing TF-QKD, the embodiment of Figure 10 reduces bandwidth usage and / or the need for a dedicated servo fiber. In the embodiment of Figure 10, the classical service channel is also mode-group division multiplexed into the multimode fiber that carries the quantum signal and the phase reference light. In other embodiments, the classical service channel may be implemented in a separate fiber.

[0087] We now turn to a detailed description of the embodiment of Fig. 10. A first transmitter device 133 (Alice) is connected to a central receiver device 137 (Charlie) via a multimode fiber 139. A second transmitter device 135 (Bob) is connected to the central receiver device 137 via a multimode fiber 141. The multimode fibers 139, 141 are similar to the multimode fiber 5 described above with reference to Fig. 2. Alice and Bob may be symmetric users of the quantum communication system 131, i.e., the transmitters 133, 137 may be the same. In particular, the transmitters 133, 137 may be configured to apply mode group division multiplexing to implement one quantum channel and two classical channels in the connected multimode fibers 139, 141.

[0088] The transmitter device 133 comprises a quantum transmitter 138 configured to provide a phase-encoded quantum signal to an optical mode converter and multiplexer 146 (mode converter 146 for short) via an optical channel 140. The quantum transmitter 138 may comprise a laser configured to emit coherent light and an encoding unit for preparing a phase-encoded quantum state from the coherent light emitted by the laser. The encoding unit may comprise a phase modulator and an intensity modulator. In one embodiment, the quantum transmitter 138 transmits a phase value α A =0 or α A = π / 2, we can find a random "basis" Z or X and a phase value β A =0 or β A = π. In this case, the optical pulses provided by the quantum transmitter 138 then have a total electromagnetic phase, α A +β A The phase of the optical pulse may be related to the optical phase of a phase reference light received from Charlie 137 as described below.

[0089] The transmitter device 133 further comprises a stabilization subsystem 143 optically coupled to the classical communication device 15 (as described above) and to the mode converter 89 via the optical channel 31, 145. The stabilization subsystem 143 may be configured to stabilize the optical phase and / or wavelength of the laser of the quantum transmitter 138 to ensure that the quantum signals prepared by Alice and the quantum signals prepared by Bob have a well-controlled phase relationship. In particular, the stabilization subsystem 143 may be configured to adjust the optical phase and / or wavelength of the laser of the quantum transmitter 138 based on the phase reference light received from Charlie 137, e.g., the stabilization subsystem 143 may be configured to phase-lock the laser of the quantum transmitter 138 to the received phase reference light.

[0090] The optical mode converter and multiplexer 146 (or, for short, the mode converter 146) is configured to optically couple each of the optical channels 140, 145, 31 to a mode(s) of a different mode group of the multimode fiber 139. The mode converter 71 (described above with reference to FIG. 7 when N≧3) may be used to implement the mode converter 146 of FIG. 10. In the embodiment of FIG. 10, the mode converter 146 may be configured to couple the quantum emitter 138, the stabilization subsystem 143, and the classical communication device 15 to modes of the first, second, and third mode groups of the multimode fiber 139, respectively.

[0091] As described above, the second transmitter device 135 (Bob) is connected to the multimode fiber 141 and comprises the same components as the first transmitter 135, i.e., the second transmitter device 135 comprises corresponding instances of a quantum transmitter 138, a stabilization subsystem 143, a classical communication device 15, and a mode converter 146.

[0092] The central receiver device 137 (Charlie) comprises two optical mode converters and multiplexers 147, 148 (mode converters 147, 148 for short), a quantum receiver 149, a stabilization subsystem 151, and a classical communication device 25. The quantum receiver 149 is connected to both mode converters 147, 148 via respective optical channels 152, 153. The stabilization subsystem 151 is connected to both mode converters 147, 148 via respective optical channels 154, 155. The classical communication device 25 is connected to both mode converters 147, 148 via respective optical channels 156, 157. The optical channels 140, 145, 31, 152-157 may be implemented as single mode fibers, integrated waveguides or free space beam paths.

[0093] The mode converter 147 is configured to optically couple each of the channels 152, 154, 156 connected to one or more modes of the target mode group of Alice's mode converter 146. In particular, the mode converter 147 may be configured to couple the channels 152, 154, 156 to modes of the first, second, and third mode groups, respectively, of the multimode fiber 139. The mode converter 148 is configured to optically couple each of the channels 153, 155, 157 connected to one or more modes of the target mode group of Bob's mode converter 146. In particular, the mode converter 148 may be configured to couple the channels 153, 155, 157 to modes of the first, second, and third mode groups, respectively, of the multimode fiber 141.

[0094] Quantum receiver 149 is configured to perform first order optical interference between the encoded quantum signals from Alice and Bob. To this end, quantum receiver 149 may comprise a beam splitter with which the signals received from Alice and Bob may interfere and at least one detector for detecting the results of the interference. The output of the detector may be output by Charlie (via classical communication module 25) to Alice and Bob (i.e. to classical communication devices 15 with Alice and Bob) so that Alice and Bob can extract the private keys, respectively.

[0095] Charlie's stabilization subsystem 151 is configured to provide phase reference light to Alice and Bob so that they can tune the wavelength and phase of their respective quantum emitter lasers (e.g., by implementing phase-locking techniques), which means that the encoded quantum signals provided by Alice and Bob have a well-controlled phase relationship.

[0096] The phase reference signal is generally a bright signal compared to the encoded quantum signal, but using mode group division multiplexing makes it possible to send the phase reference light and the quantum signal through the same fiber (the signal phase reference can be orders of magnitude larger than the encoded quantum signal, which may have less than one photon on average). Because the phase reference light and the quantum signal propagate in different mode groups and coupling between modes of different mode groups is low, noise in the quantum channel caused by the reference light is strongly suppressed.

[0097] Thus, the quantum communication system 131 may implement a TF-QKD scheme in which the quantum channel, the QKD service channel, and the stabilization channel are all implemented in the same optical fiber, which means that the quantum communication system 131 may implement the TF-QKD scheme with less fiber than conventional systems and / or without relying on WDM.

[0098] In one embodiment, WDM may be applied to quantum communication system 131, for example, to further reduce coupling between different channels.

[0099] In one embodiment, Charlie's stabilization subsystem 151 is further configured to provide a second phase reference signal (at a different wavelength than the first reference signal). In this case, Charlie's stabilization subsystem 151 comprises a WDM to multiplex the first and second phase reference light. Alice and Bob's stabilization subsystem 143 may be further configured to send the received second phase reference light back to Charlie. Charlie's stabilization subsystem 151 is further configured to perform a phase measurement on the received second phase reference light to determine the phase shift caused by the multimode fiber 139, 141. Charlie sends data specifying the determined phase shift (via classical communication module 25) to Alice and Bob (i.e., to Alice and Bob's classical communication device 15) so that Alice and Bob can adjust their quantum transmitters 138 to correct the phase shift.

[0100] Although the embodiments of Figures 2 to 10 have been described in the context of QKD, it should be understood that the techniques described above may be applied to other types of quantum communications. For example, the techniques described above for mode group division multiplexing may be applied to connect quantum memories and / or quantum computers. Connecting quantum memories and / or quantum computers generally requires various channels (carrying quantum signals, classical signals, optical references, etc.) between separate geographic locations. Mode group division multiplexing using the optical mode converters and multiplexers described above (with or without the addition of wavelength division multiplexing) can reduce the number of separate fibers required to connect multiple locations.

[0101] Although several embodiments have been described, these embodiments are presented merely as examples and are not intended to limit the scope of the present invention. Indeed, the apparatus according to the present invention and the methods described herein may be embodied in various other forms, and further, various omissions, substitutions, and changes in the form of the devices, methods, and products described herein may be made without departing from the spirit of the present invention. The appended claims and their equivalents are intended to cover such forms or modifications that would fall within the scope and spirit of the present invention.

Claims

1. 1. An optical communication system, comprising: a first transmitter optically coupled to the first single mode optical channel; a multimode fiber configured to support transmission of a plurality of guided modes, the plurality of guided modes comprising a target mode; a first optical mode converter configured to optically couple a mode of the first single mode optical channel to the target mode of the multimode fiber by gradually modifying a transverse profile of propagating light received from the first single mode optical channel to match a transverse profile of the target mode of the multimode fiber; An optical communication system comprising:

2. 2. The optical communication system of claim 1, wherein the first optical mode converter is configured to optically couple the mode of the first single mode optical channel to the target mode of the multimode fiber with a selectivity of at least 5 dB.

3. The optical communication system of claim 1 , wherein the first transmitter is configured to transmit multiple signals using wavelength division multiplexing.

4. 4. The optical communication system of claim 3, wherein the plurality of wavelength division multiplexed signals comprises a plurality of quantum signals having a first wavelength and a plurality of classical signals having a different wavelength.

5. a receiver optically coupled to the second single mode optical channel; a third optical mode converter configured to optically couple a mode of the second single mode optical channel to the target mode of the multimode fiber by gradually modifying a transverse profile of propagating light received from the target mode of the multimode fiber to match a transverse profile of the second single mode optical channel; The optical communication system of claim 1 further comprising:

6. 6. The optical communication system of claim 5, wherein the first transmitter is a quantum transmitter configured to provide a plurality of quantum signals, the receiver is a quantum receiver comprising at least one detector configured to detect the plurality of quantum signals, and the optical communication system is configured to implement a quantum key distribution protocol.

7. the plurality of guided modes comprises a first group of guided modes and a second group of guided modes, the first group comprising the target mode, and the optical communication system further comprises a second transmitter optically coupled to a second single-mode optical channel; the first optical mode converter is further configured to optically couple a mode of the second single mode optical channel to a mode of the second group of guided modes of the multimode fiber such that the first single mode optical channel and the second single mode optical channel are optically coupled to a plurality of modes of a plurality of different groups of guided modes of the multimode fiber; Coupling between modes in the same mode group is stronger than coupling between modes in different mode groups.

2. The optical communication system according to claim 1.

8. 8. The optical communication system of claim 7, wherein the plurality of guided modes of the multimode fiber are a plurality of linearly polarized modes characterized by an azimuthal order v and a radial order μ, the first group of the guided modes being associated with a first mode number and the second group of the guided modes being associated with a second mode number different from the first mode number, and each of the mode numbers m satisfying the equation m=v+2μ+1.

9. 8. The optical communication system of claim 7, wherein the relative difference between the propagation constants of any two modes of the same mode group is less than or equal to 0.02%.

10. 8. The optical communication system of claim 7, wherein the first transmitter is a quantum transmitter configured to provide a plurality of quantum signals and comprising a pulsed laser, and the second transmitter is a classical communication device.

11. further comprising one or more transmitters each optically coupled to the additional single mode optical channel; 11. The optical communication system of claim 10, wherein the multimode fiber is further configured to support transmission of a corresponding group or groups of guided modes, and the first optical mode converter is configured to optically couple each further channel to a plurality of modes of a plurality of different groups of guided modes of the multimode fiber.

12. for one or more transmitters, a corresponding receiver optically coupled to a plurality of single mode optical channels; a second optical mode converter configured to optically couple a plurality of modes of the plurality of single mode optical channels coupled to the one or more transmitters to a plurality of modes of a plurality of different groups of waveguide modes of the multimode fiber such that each receiver may receive a plurality of optical signals from a corresponding transmitter; The optical communication system of claim 10 further comprising:

13. 13. The optical communication system of claim 12, wherein the optical communication system is configured to implement a quantum key distribution protocol, and the classical communication device and a corresponding receiver are configured to exchange a plurality of data signals to derive a shared encryption key.

14. 14. The optical communication system of claim 13, wherein one of the one or more further transmitters is a second classical communications device configured to transmit a reference signal to the corresponding receiver configured to process the received reference signal to adjust a setting of the quantum transmitter, the quantum transmitter configured to provide a plurality of quantum states based on the adjusted setting.

15. The optical communication system of claim 1 , wherein the target mode is a fundamental mode of the multimode fiber.

16. The optical communication system of claim 1 , wherein the optical mode converter comprises a tapered waveguide or a tapered waveguide array.

17. The optical communication system of claim 1 , wherein the optical mode converter comprises a micro-optical lens.

18. 10. The optical communication system of claim 1, wherein the multimode fiber is one of a step-index multimode fiber, a graded-index multimode fiber, a hollow-core fiber, or a microstructured optical fiber.

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