Combining unit, transmitter, quantum communication system, and method for combining, transmitting, and quantum communication - Patents.com

Abstract

SOLUTION: To provide a combining unit for combining a first optical input and a second optical input, including a first polarization maintaining fiber (PMF) having a fast optical axis and a slow optical axis, and a second PMF having a fast optical axis and a slow optical axis, wherein the combining unit is configured to combine the first optical input with the second optical input in a free space so that the fast optical axes and the slow optical axes of the first PMF and the second PMF are rotated with respect to one another by a non-zero angle to produce a combined signal.EFFECT: A transmitter can operate a polarization state and use a polarization maintaining fiber to combine inputs into a single output.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] SUMMARY OF THE DISCLOSURE The embodiments described herein relate to a combining unit, a transmitter, a quantum communication system, and methods for combining, transmitting, and quantum communication. [Background technology]

[0002] In quantum communication systems, information is sent between a transmitter and a receiver by means of single encoded quanta, such as single photons. Each photon carries one bit of information, which can be encoded on a property of the photon, such as its polarization.

[0003] Quantum key distribution (QKD) is a technique that results in the sharing of a cryptographic key between two parties, a sender, often called "Alice", and a receiver, often called "Bob". The attraction of this technique is that it provides a test of whether any part of the key may be known to an unauthorized eavesdropper, often called "Eve". In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases for encoding bit values. The laws of quantum mechanics dictate that if Eve measures photons without prior knowledge of each encoding base, this will cause unavoidable changes in the state of some of the photons. These changes in the state of the photons will introduce errors into the bit values ​​sent between Alice and Bob. Thus, by comparing parts of their common bit strings, Alice and Bob can determine whether Eve has obtained the information. [Brief description of the drawings]

[0004] [Figure 1] FIG. 2 is a schematic diagram of a transmitter according to an embodiment. [Diagram 2] 1 is a schematic cross-sectional view of a polarization maintaining fiber. [Diagram 3] FIG. 2 is a schematic diagram of a synthesis unit. [Figure 4] 1 is a flow chart illustrating a method according to an embodiment. [Diagram 5]1 is a schematic diagram of a quantum communication system according to an embodiment; [Figure 6] FIG. 6 is a schematic diagram of a receiver that can be used in the system of FIG. 5. [Figure 7] FIG. 2 is a schematic diagram of a transmitter according to an embodiment capable of generating a decoy state. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] In a first aspect, there is provided a combining unit for combining a first optical input and a second optical input, the combining unit comprising a first polarization maintaining fiber (PMF) having an optical fast axis and an optical slow axis, the combining unit comprising a second PMF having an optical fast axis and an optical slow axis; The combining unit is configured to combine the first optical input with the second optical input in free space such that the optical fast and slow axes of the first and second PMFs are rotated relative to one another by a non-zero angle to generate a combined signal.

[0006] The electric field of light can be described as two perpendicular oscillating waves that may have different amplitudes and phase delays between them. These waves propagate together, defining an overall direction of the electric field known as the state of polarization of the light.

[0007] Information can be encoded by modulating the light polarization state, and in practical applications the state may be selected from the following mutually exclusive basis with two orthogonal basis states: horizontal H and vertical V linear polarization states oriented at angles 0 and 90 degrees, respectively, and diagonal linear polarization states with diagonal D and anti-angle A oriented at 45 and -45 degrees, respectively. It is also possible to encode left-handed L and right-handed R circular polarization states with amplitudes comparable to H- and V-polarized light, but with a relative phase delay between them of π / 2 and -π / 2, respectively.

[0008] The combining unit can combine the outputs from the first and second PMFs in free space and output directly to free space. However, the combination of the first and second PMFs may be output to a single mode fiber. Outputting directly to free space after free space combining avoids compensating for phase drift caused by a single mode output fiber. This makes the combining unit particularly useful in free space communication situations such as satellite communications.

[0009] In one embodiment, the optical axes of the first and second PMFs are mechanically rotated by a fixed angle. In a further embodiment, a quarter wave plate is provided to convert the output from one of the PMFs to be circularly polarized.

[0010] In one embodiment, the synthesis unit further comprises a plurality of hierarchical synthesis stages, each configured to synthesize two inputs into a single output, which is directed to a subsequent synthesis stage via a PMF. Each of the hierarchical synthesis stages may comprise a different type of unit for synthesizing the synthesis units or PMFs.

[0011] The combining unit described above allows the polarization states to be manipulated and combined into a single output using polarization maintaining fiber.

[0012] The combining unit may further comprise a correction stage configured to correct for phase drift (i.e., phase offset) between the output of the first PMF and the output of the second PMF. When the output from the first PMF is rotated relative to the output of the second PMF, the phase delay between the polarization states from the first and second PMFs may result in some elliptical shapes in some of the output states. The correction stage may be provided immediately after the combining or may be provided in a receiver or the like to correct for elliptical shapes after the combined output has traversed free space.

[0013] In a second embodiment, a transmitter is provided for emitting light having a polarization selected from a fixed set of polarizations, the transmitter comprising: a main synthesis stage including a synthesis unit based on the synthesis unit; A plurality of radiation units; wherein an output of each emitting unit is directed into the polarization maintaining fiber such that an output of at least one emitting unit is directed into a first polarization maintaining fiber and an output of at least one other emitting unit is directed into a second polarization maintaining fiber.

[0014] This implementation is inherently stable due to the use of only polarization-maintaining (PM) fiber in the composite optics. PM fiber maintains the polarization state of light traveling along either the optical slow or optical fast axis of the fiber, even when the fiber is mechanically or thermally stressed. Also, once the light is multiplexed, the module couples into free space, which suppresses polarization state perturbations. This is especially useful in harsh environments that may have large and fast temperature transients. The transmitter provides a low-cost, low-power, and high-speed platform for generating polarization-encoded pulses.

[0015] The transmitter may further comprise a plurality of hierarchical combining stages, each configured to combine at least two inputs into a single output, which is directed to a subsequent combining stage via a PMF, In some embodiments, the combining stage combines two inputs from the PMF into a single output PMF.

[0016] The transmitter may be configured to selectively output at least one polarization state in one of two non-orthogonal bases from the main combination stage. Such a transmitter may be used for polarization encoding in quantum key distribution (QKD). In polarization encoding for QKD, the polarization states may be encoded to occupy one of at least two non-orthogonal bases. For example, one basis may be defined by horizontally and vertically polarized states, and the second basis may be defined by diagonal and anti-orthogonal states. Thus, in one embodiment, the main combination unit may be considered as a basis combination stage.

[0017] The previous combining stages can be used to combine the states that define each basis, and then the PMFs including the outputs, each PMF including the states that define each basis, are combined by a basis combining stage, which mechanically rotates the optical axis of one PMF relative to the other PMF. Thus, in a further embodiment, the transmitter further comprises at least one further combining stage as a state combining stage, the state combining stage being configured to combine the orthogonal polarization states that form the basis into a single output, and the main combining stage being configured as a basis combining stage for combining the outputs of the state combining stages.

[0018] The state combining stage may be configured to combine polarization states propagating along the same optical axis of two input PMFs, such that a state propagating through one input PMF is coupled to the optical slow axis of the PMF carrying the output of the state combining stage, and a state propagating through the other input PMF is coupled to the optical fast axis of the PMF carrying the output of the state combining stage.

[0019] The state combining stage may comprise a 2x1 beam combiner, for example a fiber combiner that evanescently combines two input PMFs.

[0020] The transmitter may include two state synthesis stages, one configured to output to a first PMF and the other configured to output to a second PMF, the optical axis of the second PMF being rotated 45 degrees relative to the optical axis of the first PMF by the basis synthesis stage.

[0021] The transmitter can also be used in QKD protocols that use pulses of different intensities. This can be achieved by providing inputs with pulses of different intensities to either the basis combining stage or the state combining stage. Therefore, the transmitter may further comprise at least one further combining stage as an intensity combining stage, which is configured to combine polarization states with the same polarization but different intensities into a single output.

[0022] The intensity combining stage may be configured to combine polarization states propagating along the same optical axis of N input PMFs, where N is an integer greater than or equal to 1, and each of the states propagating along each input PMF is coupled to the same optical axis of the PMF carrying the output of the intensity combining stage. The intensity combining stage may comprise an Nx1 fiber combiner.

[0023] The transmitter may comprise at least three emitters (radiation units), where the outputs from two of the emitters are combined in a first combining stage before being directed into a first polarization-maintaining fiber, and the outputs from each emitter reach the first combining stage via a separate polarization-maintaining optical fiber.

[0024] The transmitter may comprise multiple hierarchical combining stages, each configured to combine two inputs into a single output that is directed to a subsequent combining stage via polarization-maintaining fiber.

[0025] the plurality of emitters comprises a first emitter, a second emitter, a third emitter, and a fourth emitter; the outputs of the first emitter and the second emitter are combined in a first combining stage; the outputs of the third emitter and the fourth emitter are combined in a second combining stage; the output of the first combining stage is transmitted to a main combining stage via a first polarization maintaining fiber; The output of the second combining stage is sent to the main combining stage via a second polarization maintaining fiber.

[0026] The transmitter may further include drive electronics connected to each of the plurality of emitters to enable selective excitation of each of the plurality of emitters such that the polarization state output from the main combining stage may be selected by exciting a corresponding emitter, and the transmitter may output a vacuum state at selected times by not selecting an emitter for output at the selected times.

[0027] In a further embodiment, a quantum communication system is provided comprising the transmitter and a receiver configured to receive a polarization state from the transmitter and measure the polarization state in a first polarization basis or a second polarization basis. The receiver may be configured to passively select a polarization basis. The receiver may comprise a polarizing beam splitter and a detector configured to measure an output of the polarizing beam splitter for each polarization basis of the first polarization basis or the second polarization basis.

[0028] In a further embodiment, a method is provided for combining a first optical input and a second optical input, the first optical input being provided through a first polarization maintaining fiber (PMF) having an optical fast axis and an optical slow axis, the second optical input being provided through a second PMF having an optical fast axis and an optical slow axis; The first optical input is combined with the second optical input in free space such that the optical fast and slow axes of the first and second PMFs are rotated relative to one another by a non-zero angle to generate a combined signal.

[0029] The method may further comprise combining the outputs of the first and second PMFs in free space by directing both outputs to impinge on a beam splitter. The method may further comprise correcting for a phase difference induced by the beam splitter in the free space combining unit.

[0030] In a further embodiment, a method is provided for transmitting an optical signal having a polarization selected from a fixed set of polarizations, the method comprising: emitting polarization states from a plurality of radiating units; and directing an output of each radiating unit into the PMF such that an output of at least one radiating unit is directed into a first PMF and an output of at least one other radiating unit is directed into a second PMF; combining an output of the first PMF with an output of the second PMF to generate a combined signal; wherein the optical axis of the second PMF is mechanically rotated relative to the optical axis of the first PMF.

[0031] In a further embodiment, a method of quantum communication is provided, the method comprising: emitting an optical signal having a polarization state selected from the set of four polarizations according to the method described above, where the polarization state is randomly selected from states in the two polarization bases; receiving optical signals having different polarization states at a receiver and varying a polarization measurement basis at the receiver; Equipped with.

[0032] In one embodiment, a method is provided for multiplexing optical pulses from multiple light sources such that the pulses are polarization encoded and combining the encoded pulses into free space such that the polarization state is preserved. The system uses only polarization maintaining fiber in the multiplexing phase.

[0033] Additionally, a method is provided for multiplexing optical pulses from multiple light sources and polarization encoding these pulses. Each light source is coupled into a polarization-maintaining fiber along the optical slow axis of the fiber. Steering of the optical pulses from each of the lasers is accomplished by passive polarization-maintaining (PM) fiber-coupled optics.

[0034] Mechanical rotation of polarization-maintaining fibers is exploited to rectify the polarization state into mutually unbiased bases used for polarization encoding protocols in quantum key distribution (QKD).

[0035] In one embodiment, a customized fiber-coupled 50:50 beam splitter is provided, with one of the inputs rotated 45 degrees to correct the polarization state to a mutually unbiased basis.

[0036] The four light sources may be combined into the multiplexing module described above.

[0037] In a further embodiment, eight light sources are coupled into the multiplexing module described above via unbalanced PM fiber beam splitters to generate four signal states (high intensity) and four decoy states (low intensity), forming an optical encoder that generates four polarization states with two intensity levels without the need for active stabilization or manipulation in the generation of the states.

[0038] The polarization dependent phase picked up at the composite beam splitter may be corrected by a) a series of birefringent waveplates, and b) a liquid crystal retarder.

[0039] Figure 1 shows a schematic diagram of an encoding device or transmitter capable of encoding four polarization states. The transmitter comprises multiple emitters: a first emitter 101, a second emitter 103, a third emitter 105, and a fourth emitter 107. In one embodiment, the emitters are emitters of pulsed radiation. For example, the emitters may be polarized laser diodes, such as vertical cavity surface emitting lasers (VCSELs) or distributed feedback (DFB) lasers, or LEDs combined with linear polarization filters.

[0040] A number of emitters 101, 103, 105, 107 are connected to drive electronics 109, with each emitter being individually addressable so that it can be selected via the drive electronics to output a pulse.

[0041] The transmitter outputs a sequence of pulses, where the polarization of the pulses is selected from one of four polarization states. In summary, in the system of Figure 1, each emitter 101, 103, 105, 107 corresponds to one polarization state, so that activating the emitter selected to output a pulse will result in a pulse being output from the encoder in the corresponding polarization state.

[0042] In Figure 1, each emitter 101, 103, 105, 107 outputs radiation with the same polarization. The pulses then pass through a hierarchy of combining stages. At each stage, the two signals from the previous stage are combined and the angle between the polarizations of the two combined signals is set to allow the formation of different polarization states. The pulses are transported from the emitters to and between the various combining stages using polarization maintaining fibers.

[0043] FIG. 2 is a schematic diagram of a cross-section of a polarization-maintaining fiber 151. Polarization-maintaining fibers have birefringence that allows for two well-defined polarization modes to propagate at different speeds along the fiber 151. There are many different types of polarization-maintaining fibers. The fiber may be geometrically asymmetric or may have a refractive index profile that is asymmetric. Alternatively, stress may be permanently induced in the fiber to generate stress birefringence. This can be achieved using rods of another material contained within the cladding. FIG. 2 shows a so-called "PANDA" type, where there are two rods 153 and 155 that generate stress birefringence around a central core 157. However, this is just one example and other types of fibers may be used.

[0044] Returning to Figure 1, each of the emitters 101, 103, 105, and 107 is connected to drive electronics 109. Each emitter is also coupled to a polarization maintaining fiber. In one embodiment, each emitter is coupled to the polarization maintaining fiber such that the output of the emitter travels along the optical slow axis of the polarization maintaining fiber.

[0045] The output of the first emitter 101 is coupled into a polarization maintaining fiber 111 and the output of the second emitter 103 is coupled into a polarization maintaining fiber 113. As mentioned above, the outputs from both emitters 101 and 103 are coupled into the same optical axis of the fiber. The polarization maintaining fibers 111 and 113 are then combined at a first combining station 121.

[0046] The first combining station 121 comprises a 2x1 beam combiner which produces pulses at its output port. The first combining station is called a state combining station because it serves to combine together the states that will form the basis. The optical slow axis of the second polarization maintaining fiber 113 is rotated with respect to the optical slow axis of the first polarization maintaining fiber 111 upon entering the first state combining station 121. This means that at the output of the first state combining station, the pulses from the polarization maintaining fiber 111 are coupled into the optical slow axis of the output polarization maintaining fiber. However, the pulses from the second polarization maintaining fiber 113 are coupled into the optical fast axis of the output polarization maintaining fiber. This is shown in the cross-sectional view of the polarization maintaining output fiber 123 in the figure, which shows that both the slow and fast modes are occupied.

[0047] Similarly, the output of the third emitter 105 is coupled into a polarization maintaining fiber 115 and the output of the fourth emitter 107 is coupled into a polarization maintaining fiber 117. As mentioned above, the outputs from both emitters 105 and 107 are coupled into the same optical axis of the fiber. The polarization maintaining fibers 115 and 117 are then combined in a second state combining station 125.

[0048] The second state combining station 125 comprises a 2x1 beam combiner which produces pulses at its output port. The optical slow axis of the fourth polarization maintaining fiber 117 is rotated with respect to the optical slow axis of the third polarization maintaining fiber 115 upon entering the second state combining station 125. This means that at the output of the second state combining station, the pulse from the third polarization maintaining fiber 115 is coupled into the optical slow axis of the output polarization maintaining fiber. However, the pulse from the fourth polarization maintaining fiber 117 is coupled into the optical fast axis of the output polarization maintaining fiber. This is shown in the cross-sectional view of the polarization maintaining output fiber 127 in the figure, which shows that both the slow and fast modes are occupied.

[0049] Therefore, the first state combination stage 121 comprises a polarization maintaining fiber whose output contains either horizontally polarized or vertically polarized pulses depending on whether the pulse comes from the emitter 101 or the second emitter 103.

[0050] The polarization-maintaining fiber carrying the output of the second state-combining stage 125 also outputs pulses that are horizontally or vertically polarized depending on whether the pulses are output from the third emitter 105 or the fourth emitter 107.

[0051] The output from the first state combining stage 121 is output to what is referred to as the first primary polarization maintaining fiber, and the output from the second state combining stage is output to what is referred to as the second primary polarization maintaining fiber. These are then combined in the primary combining stage 129.

[0052] The main combining stage is shown in more detail in FIG. 3. In this example, the main combining stage is referred to as a basis combining stage since it combines the bases formed in the state combining stage. Here, an input from a first main polarization maintaining fiber is provided at a first input port 161 and an input from a second main polarization maintaining fiber is input at a second input port 163. The second input port is configured such that the optical slow axis of the second main polarization maintaining fiber is rotated 45° with respect to the optical slow axis of the first polarization maintaining fiber. Although the above description refers to the optical slow axis, one skilled in the art will appreciate that the same applies to the optical fast axis.

[0053] At the first input port 161 and the second input port 163, there are collimating optics that serve to further focus the incoming signals onto a 50-50 beam splitter 165. The 50-50 beam splitter 165 serves as a combining optic that combines the signals from the first and second polarization maintaining fibers. The output from the combining optics 165 is directed towards a first output port 167, which may be a free space output port or a single mode fiber. In one embodiment, a collimator is provided before the single mode fiber. The output corresponds to pulses having a polarization selected from horizontal, vertical, diagonal, or counter-angle polarization. In this example, the horizontally polarized pulses are from the first emitter 101, the vertically polarized pulses are from the second emitter 103, the diagonally polarized pulses are from the third emitter 105, and the counter-angle polarized pulses are from the fourth emitter 107.

[0054] The semi-transparent mirror 165 allows for a free space output port 167 as well as a further output 169 that can be used for monitoring the signal.

[0055] As discussed above in connection with FIG. 2, the birefringence of polarization-maintaining fiber introduces a phase delay between the horizontally and vertically polarized components traveling in the fiber (i.e., the optical slow and fast axes). In addition, the reflecting surface will impart an additional path length to one of the polarization states. When measured in a linear basis, this phase difference will not be picked up since it acts as a global phase. However, in the case of rotated, unbiased bases (D and A), the phase will introduce ellipticity, which is less visible in the polarization measurement. In an embodiment, since the encoder is intended for use with QKD protocols, this relative phase needs to be compensated to minimize the quantum bit error rate.

[0056] In one embodiment, the correction of the polarization state is implemented by a fiber-based or free-space polarization controller. This can be static or electronically controlled. Typically, this is implemented in free space by using a liquid crystal retarder or a series of wave plates (quarter wave plate, half wave plate, quarter wave plate). This allows the phase difference between the H and V components of the A and D polarization states to be compensated without rotating the polarization states of the pure H and V states.

[0057] The same transformation can be achieved in single mode fiber using a 3- or 4-axis (electronic) polarization controller.

[0058] For the free space output port, the phase does not change with time. However, single mode fiber can introduce both relative phase and axial rotation, which can be compensated for using an electronically controllable polarization rotator. In one embodiment, there is a set of polarization rotators and retarders. This may be done using a design with λ / 4-λ / 2-λ / 4 (quarter wave plate-half wave plate-quarter wave plate). Alternatively, a set of birefringent liquid crystals may be used to achieve the same effect.

[0059] Polarisation compensation at the transmitter location is not strictly necessary. Single mode fibre adds relative phase and / or rotation to the polarisation states, but they are still in a basis set that are mutually unbiased (not H, V, A, D but instead have some elliptical shape at any angle). This can be compensated for at the receiver using a polarisation rotator / retarder. The question is how quickly this can be achieved, perhaps it would be useful to have a bright polarisation reference in addition to the quantum signal. Alternatively, if the polarisation state is fixed at the transmitter location, then only a rotation of the polarisation should be needed to compensate for the polarisation rotation due to the orientation of the transmitter relative to the receiver.

[0060] In one embodiment, phase compensation can be achieved by measurements in the receiver module, and information on how to apply the appropriate transformation in the transmitter can be transferred along with the classical data traffic required for QKD.

[0061] FIG. 4 shows a summary of the system. In step S201, an emitter, which is a pulsed light source, is used to emit pulsed radiation. Typically, the pulses will have a fixed polarization. Then, in step S203, they are fed to a polarization-maintaining combining optical element. In one embodiment, the polarization-maintaining combining optical element comprises a polarization-maintaining fiber. The polarization-maintaining fiber is used to rotate the polarization of the pulses emitted from the different emitters relative to each other, so that a pulse from one emitter is combined with a pulse from another emitter with a fixed rotation between the polarizations of the pulses from these two emitters. The polarization-maintaining combining optical element may be arranged hierarchically, which allows pairs of outputs from multiple emitters to be combined hierarchically, combining the outputs of four or more emitters into a single output.

[0062] Due to the phase delay induced by the interaction with the beam splitter of the main combining unit, a polarization state compensating optical element is used to compensate for this phase delay.

[0063] The encoder described above can be used for quantum key distribution (QKD). Next, a basic quantum communication protocol using polarized light is described. However, it should be noted that this is not meant to be limiting and other polarization-based protocols may be used.

[0064] The protocol uses two bases, where each basis is represented by two orthogonal states, in this example the H / V and D / A bases, however the L / R basis may also be chosen.

[0065] The sender in the protocol prepares a state with one of H, V, D or A polarization. In other words, the prepared state is selected from two orthogonal states (H and V or D and A) in one of two bases H / V and D / A. This can be thought of as sending a signal of 0 and 1 in one of two bases, for example H=0, V=1 in the H / V base, and D=0, A=1 in the D / A base. The pulse is attenuated to comprise less than one photon on average. Thus, if a measurement is made on the pulse, it is destroyed. Also, if the pulse contains one photon, it is not possible to split the pulse.

[0066] The receiver uses a measurement basis for the polarization of the pulse selected from the H / V basis or the D / A basis. The selection of the measurement basis can be active or passive. In passive selection, the basis is selected using a fixed component such as a beam splitter. In "active" basis selection, the receiver uses, for example, a modulator with an electrical control signal to determine in which basis to measure. If the basis used for measuring the pulse at the receiver is the same as the basis used to encode the pulse, the receiver's measurement of the pulse will be accurate. However, if the receiver selects the other basis for measuring the pulse, there will be a 50% error in the result measured by the receiver.

[0067] To establish the key, the sender and receiver compare the bases used for encoding and measurement (decoding). If they match, the result is kept, if they do not match, the result is discarded. The above method is very secure. If an eavesdropper intercepts and measures a pulse, the eavesdropper needs to prepare another pulse to send to the receiver. However, the eavesdropper does not know the correct measurement basis and therefore has only a 50% chance of measuring the pulse correctly. Any pulse recreated by an eavesdropper will result in a large error rate for the receiver, which can be used to prove the presence of an eavesdropper. The sender and receiver compare small parts of the key to determine the error rate and therefore the presence of an eavesdropper.

[0068] The encoder described with reference to FIG. 1 can be used to prepare the four states required for the BB84 protocol described above.

[0069] Figure 5 illustrates an embodiment of a QKD system using the transmitter of Figure 1. The transmitter described above is suitable for use in any QKD system, including point-to-point links and more recent protocol deployments such as MDI QKD.

[0070] Figure 5 shows the transmitter of Figure 1 deployed in a point-to-point QKD system. Here, the transmitter 301 is driven by an FPGA 303 connected to a driver 305 that provides a high-speed signal used to selectively excite the emitters. The driver is a suitable laser driver implemented such that the emitters are gain-switched and therefore generate optical pulses with random phases at each clock cycle. In one embodiment, the laser driver takes a digital signal from the FPGA and generates an analog output capable of driving a laser diode or LED in a gain-switched operation, turning the light source completely off between each pulse. Driving the light source in this way means that each pulse has a random phase due to being seeded from vacuum photons (spontaneous emission).

[0071] The emitters 307 are multiplexed via PM combining optics 309 as described above in connection with Figure 1 (where the number of emitters is four). The output is corrected by correction optics 311 to transmit the desired state of the QKD protocol being implemented. In this embodiment, the encoded optical pulses may be sent over a free space link.

[0072] In one embodiment, the system can be used for the BB84 protocol, where the FPGA is used to randomly select an emitter for outputting a pulse. Due to the polarization maintaining fiber and the configuration of the combining stage, the selection of the polarization state is done by selecting the emitter.

[0073] In this embodiment, the prepared state is then sent via free space or single mode fiber to receiver 321. The receiver may be any type of receiver used to measure polarization states in QKD.

[0074] Receiver 321 receives the multiplexed signal and demultiplexes the signal in demultiplexer 323. The demultiplexer directs the classical signal (clock signal) into classical transceiver 325, which is then directed towards the receiver's FPGA 331. The encoded pulses are first directed into polarizing receiver optics 327 and then into single photon detector 329.

[0075] The combination of polarized receiver optics 327 and single photon detector 329 can be the same as that used in any standard QKD system for polarization encoding. An example of a possible arrangement is shown in FIG.

[0076] Here, the polarized receiver optics comprises a 50:50 beam splitter 405 that directs the incoming pulse along either the first measurement channel 407 or the second measurement channel 409. Since the pulse contains less than one photon on average, the 50:50 beam splitter 405 randomly directs the pulse along either the first measurement channel or the second measurement channel. This results in a selection of the measurement basis being either the X(D / A) basis or the Z(H / V) basis. The non-polarizing beam splitter 405 functions to allow a random selection of one of the two bases.

[0077] The first measurement channel is for the X basis, which corresponds to the D / A basis. Here, a half-wave plate 411 is provided to rotate the polarization by 45 degrees between the two detection branches, giving two measurement bases, X and Z. The output of the half-wave plate 411 is then directed towards a polarizing beam splitter 413, which directs pulses with opposite polarities towards an opposite-angle detector 415 and pulses with diagonal polarities towards a diagonal detector 417. Detectors 415 and 417 are single-photon detectors, for example avalanche photodiodes.

[0078] Pulses directed along the second measurement channel are measured in a Z basis to determine whether they are horizontal or vertical. Here, pulses directed into the second measurement channel are directed towards a polarizing beam splitter 419, which directs vertically polarized pulses towards detector 421 and horizontally polarized pulses towards detector 423. Again, detectors 421 and 423 are single photon detectors.

[0079] If a photon polarized in the D / A basis is received and then randomly transmitted and measured in the Z basis along the second measurement channel 409, it is likely that one of the detectors 421, 423 will register a count. However, this result is unreliable as there is a 50 / 50 chance that a photon received at the polarizing beam splitter 419 will be directed towards either the vertical or horizontal detector, so this result is discarded.

[0080] The above description of QKD assumed that each pulse sent to the receiver has a fixed intensity. However, so-called decoy state QKD protocols are also used, in which pulses with different intensities are sent so that there are multiple intensities per state. One possible attack against a QKD system is the so-called photon number splitting (PNS) attack. In this attack, pulses are intercepted between the transmitter and the receiver. The pulses contain less than one photon on average. However, the number of photons in each pulse varies, so that some pulses may contain two photons. This allows a PNS attack to be performed, in which an eavesdropper splits each pulse, and if there are more than two photons, a photon that is not read by the eavesdropper reaches the receiver. In this situation, the presence of an eavesdropper can be determined by using pulses with different intensity levels (so-called decoy state protocols). A PNS attack causes a variation in the bit error rate at different intensities.

[0081] The risk of pulse-splitting attacks in QKD is mitigated firstly by using low flux, i.e. less than one photon per pulse on average, but also by using states with different fluxes, e.g. one in ten (decoy) and one in one thousand (vacuum), and the BB84 protocol makes it possible to prove that communication is secure against this type of attack.

[0082] FIG. 7 shows a variant of the transmitter of FIG. 1 adapted to emit pulses of different intensities. The transmitter of FIG. 7 has eight emitters. Each pair of emitters corresponds to one polarization state, in this example there are two intensity levels. This is achieved by providing an unbalanced combining stage with an unbalanced combiner which serves to attenuate the output from one emitter of the emitter pair it combines. The output from the unbalanced combining stage is thus a sequence of pulses of the same polarization with intensities that randomly vary between two different levels. The output from this stage is then passed to a first combining stage which allows the states defining one basis to be output in the same polarization-maintaining fiber.

[0083] More specifically, the transmitter of Figure 7 is used to generate four polarization states with two intensity levels by multiplexing eight pulsed light sources using PM fiber optics.

[0084] The transmitter of Figure 7 illustrates the generation of polarization states H, V, D, and A. Sources / emitters 503, 505, 507, 509, 511, 513, 515, and 517 are coupled to drive electronics 501 and are selectively excited depending on the desired output state. Each source / emitter corresponds to a particular polarization state and intensity level at the output of the transmitter module. In this example, the output of each source / emitter is coupled into the optical slow axis (nominally the H polarization state) of a PM fiber.

[0085] The pairs of emitters 503, 505, 507, 509, 511, 513, 515, and 517 are then coupled to four unbalanced PM fiber 2x1 beam combiners 521, 523, 525, 527, which generate optical pulses at output ports with intensity ratios set by the combining ratios (X:Y). The four unbalanced 2x1 beam combiners form an unbalanced combining stage called the intensity combining stage. This allows the generation of decoy states in BB84 protocol QKD, where the intensity ratios are selected to optimize the key rate based on certain system parameters (channel loss, repetition rate, quantum bit error rate, etc.).

[0086] The outputs of the four unbalanced beam combiners 521, 523, 525, 527 are coupled to two polarization beam combiners (PBCs) 531, 533, which in this example form a state combining stage where the states forming each basis are combined into a single polarization maintaining fiber. Here, the polarization state of one input is rotated by 90 degrees and its optical pulse propagates along the optical fast axis of the output PM fiber. The other input remains unaffected and continues to propagate along the optical slow axis of the output fiber. The outputs of the two PBCs 531, 533 therefore contain orthogonally propagating optical pulses (nominally H and V polarization states).

[0087] Finally, the output of the PBC is input to a custom 50:50 beam splitter optic 535, which is the same as the main combining unit in Figures 1 and 3, forming the basis combining stage. Here, one of the PM fiber inputs is mechanically rotated 45 degrees to convert the orthogonally polarized states to mutually unpolarized basis, i.e., H, V → A, D. The light is coupled out of the fiber inside the custom optic via a collimating lens and then acts on a 50:50 beam splitter cube. This operates to combine the four coupled-out polarization states into either a single mode fiber (through further collimation optics) or into free space.

[0088] The output from the main combining stage then passes through correction optics 537. This is the same as the correction optics (correction stage) described with reference to FIG.

[0089] In the above description, two embodiments are described, the first embodiment being used to prepare four states of the same intensity, and the second embodiment being used to prepare four states with two intensity levels. However, it will be understood that the system may be used to prepare fewer or more states than those described above. Also, more than two intensity levels may be implemented. In a further embodiment, a vacuum state may also be sent, in which case no laser pulses are emitted.

[0090] For example, the system may be implemented to use a simplified B92 protocol in which there are only two states, one in the H / V basis and the other in the D / A basis. In this arrangement, each of the two emitters would output a pulse via polarization maintaining fiber to the main combining stage 129 of FIG.

[0091] Also, more intensity levels can be introduced, for example by using four emitters to generate states with four different intensity levels. The four emitters are combined in pairs in a first unbalanced combining stage to output two polarization-maintaining fibers, each fiber containing pulses of different intensity, and the outputs from the first unbalanced combining stage are combined in a second unbalanced combining stage. The second unbalanced combining stage may also comprise an unbalanced combiner. However, if the unbalanced combiners in the first unbalanced combining stage are different from each other, the second unbalanced combining stage may have a balanced combiner.

[0092] Similarly, fewer or more polarization states may be prepared. In the above examples, two states per basis were used, but it is possible that one basis has two states and the other basis has only one state. Here, a basis with only one state may be directed to the main combination stage without having to go through the first combination stage.

[0093] The above has been described primarily in terms of polarization states output by pulsed optics, however, CW light sources with intensity modulators may also be used.

[0094] The combined basis is also H / V and D / A. However, it is possible to switch one of the bases to be circularly polarized. This is achieved by using a quarter wave plate prior to the free space combination.

[0095] In the above embodiment, the setup uses polarization maintaining fiber to multiplex different polarization states utilizing mechanical rotation of the fiber to achieve polarization rotation, ensuring that no on-the-fly manipulation of the polarization states is required.

[0096] This also reduces the need to monitor the device at each stage of the polarization state, since no polarization alignment is required. This reduces the size, cost, and weight of the system. Corrections to the output state due to imperfections in the setup can be easily accounted for using common optical polarization components and do not have to be reworked once set.

[0097] The system does not require active alignment procedures, making it an ideal candidate for use in harsh or inaccessible environments. The polarized beacon / reference light can be easily coupled and maintains its relative orientation to the encoded channel.

[0098] The system is light weight, primarily due to the lack of need for stabilized opto-mechanical components, the weight is reduced due to the fiber-coupled optics, the system also has low power consumption, allowing the use of low power laser drivers in conjunction with highly efficient light sources, and no optical modulators requiring high drive voltages are required.

[0099] It also has a small form factor because the optical fiber can be coiled into a small area. No bulky polarizing optics are required for state generation. Fiber coupling allows additional lasers for other intensity levels to be easily coupled into the module.

[0100] An all-fiber coupled optical design makes the system more robust against vibrations or other mechanical stresses that can cause misalignment of free space coupling alternatives.

[0101] Since there is no need for dynamic control of the polarization state, the optical assembly can be made compact and cost effective.

[0102] The optical module can be used as the transmitter side for free-space quantum key distribution.

[0103] The generation of optical pulses with encoded information at high clock rates is a key building block for optical communications. The state of polarization of light is one of the most common optical properties that can be modulated to encode information (e.g., polarization shift keying). For practical communication system applications, optical transmitters should be simple, small, low-cost, and low-power. The above embodiments provide these advantages and can be used in many communication areas.

[0104] As mentioned above, the transmitter can be used in quantum communication systems. The transmitter design is suitable for polarization coding quantum key distribution (QKD) and polarization coding measurement device independent quantum key distribution (MDI QKD) devices. Polarization coding is particularly important in free-space QKD applications such as satellite QKD.

[0105] Although specific embodiments have been described, these embodiments are presented merely as examples and are not intended to limit the scope of the invention. The novel devices and methods described herein may be embodied in other various forms, and various omissions, substitutions, and changes may be made in the form of the devices, methods, and products described herein without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover any such forms or modifications that fall within the scope and spirit of the invention.

Claims

1. a combining unit for combining a first optical input and a second optical input, a first polarization maintaining fiber (PMF) having an optical fast axis and an optical slow axis; a second PMF having an optical fast axis and an optical slow axis; Equipped with the first optical input is an input in which a plurality of lights having a polarization state of a first polarization including a plurality of different polarizations and a plurality of different first intensities are combined and input to the first PMF; the second optical input is an input in which a plurality of optical signals having the first polarization and the plurality of different first intensities are combined and input to the second PMF; the plurality of different first intensities have randomly different intensities; a combining unit configured such that the optical fast axis of the first PMF is rotated by a non-zero angle relative to the optical fast axis of the second PMF and the optical slow axis of the first PMF is rotated by the non-zero angle relative to the optical slow axis of the second PMF to generate a polarization state in which a basis of a first optical output of the first PMF and a basis of a second optical output of the second PMF are rotated by the non-zero angle, and at least the angle is set such that the basis of the first optical output and the basis of the second optical output are not biased relative to each other, to generate a combined signal.

2. The combining unit of claim 1 , further comprising an output port configured to output the combined signal into free space.

3. The combining unit of claim 1 , further comprising a correction stage configured to correct a phase offset between an output of the first PMF and an output of the second PMF.

4. 1. A transmitter for emitting light having a polarization selected from a set of fixed polarizations, comprising: A main synthesis stage comprising a synthesis unit according to claim 1; A plurality of emitting units emitting light having the same polarization; an output of each radiating unit is directed into a PMF such that an output of at least one radiating unit is directed into the first PMF and an output of at least one other radiating unit is directed into the second PMF, the radiating units being arranged based on a number of states included in the first optical output basis and the second optical output basis and a number of the plurality of different first intensities.

5. 5. The transmitter of claim 4, further comprising a plurality of hierarchical combining stages, each configured to combine two inputs into a single output, the single output being directed to a subsequent combining stage via a PMF.

6. 5. The transmitter of claim 4, configured to selectively output from the main combination stage at least one state of polarization in one of two non-orthogonal bases.

7. 7. The transmitter of claim 6, further comprising at least one further combining stage as a state combining stage configured to combine orthogonal polarization states forming a basis into a single output, and the main combining stage configured as a basis combining stage for combining outputs of the state combining stages.

8. 8. The transmitter of claim 7, wherein the state combining stage is configured to combine polarization states propagating along the same optical axis of two input PMFs, a state propagating along one input PMF being coupled to the optical slow axis of the PMF carrying the output of the state combining stage, and a state propagating along the other input PMF being coupled to the optical fast axis of the PMF carrying the output of the state combining stage.

9. The transmitter of claim 7 , wherein the state combining stage comprises a 2×1 fiber combiner.

10. 8. The transmitter of claim 7, comprising two state combining stages, one configured to output to the first PMF and the other configured to output to the second PMF, the optical axis of the second PMF being rotated 45 degrees with respect to the optical axis of the first PMF by the basis combining stages.

11. 7. The transmitter of claim 6, further comprising at least one further combining stage as an intensity combining stage, the intensity combining stage being configured to combine polarization states with the same polarization but different intensities into a single output.

12. 12. The transmitter of claim 11, wherein the intensity combining stage is configured to combine polarization states propagating along the same optical axis of N input PMFs, where N is an integer greater than or equal to 2, and each of the states propagating along each input PMF is coupled to the same optical axis of a PMF carrying the output of the intensity combining stage.

13. The transmitter of claim 12 , wherein the intensity combining stage comprises an N×1 fiber combiner.

14. 5. The transmitter of claim 4, further comprising drive electronics connected to each of the plurality of radiating units to enable selective excitation of each of the plurality of radiating units, such that the polarization state output from the main combining stage can be selected by exciting a corresponding radiating unit.

15. 15. The transmitter of claim 14, further configured to output a vacuum condition at a selected time by not selecting the radiating unit for output at the selected time.

16. 5. A quantum communication system comprising a transmitter and a receiver as claimed in claim 4, wherein the receiver is configured to receive a polarization state from the transmitter and to measure the polarization state in a first polarization basis or a second polarization basis.

17. 17. The quantum communication system of claim 16, wherein the receiver is configured to passively select the first polarization basis or the second polarization basis.

18. 1. A method for combining a first optical input and a second optical input, the first optical input being input to a first polarization maintaining fiber (PMF) having an optical fast axis and an optical slow axis, and the second optical input being input to a second PMF having an optical fast axis and an optical slow axis; the first optical input is an input in which a plurality of lights having a polarization state of a first polarization including a plurality of different polarizations and a plurality of different first intensities are combined and input to the first PMF; the second optical input is an input in which a plurality of optical signals having the first polarization and the plurality of different first intensities are combined and input to the second PMF; the plurality of different first intensities have randomly different intensities; a first PMF and a second PMF are configured to generate a polarization state in which the optical fast axis of the first PMF is rotated by a non-zero angle relative to the optical fast axis of the second PMF and the optical slow axis of the first PMF is rotated by the non-zero angle relative to the optical slow axis of the second PMF to generate a combined signal, the first PMF and the second PMF generating polarization states in which a basis of a first optical output of the first PMF and a basis of a second optical output of the second PMF are rotated by the non-zero angle, and at least the angle is set such that the basis of the first optical output and the basis of the second optical output are not biased relative to one another.

19. 1. A method for transmitting an optical signal having a polarization selected from a fixed set of polarizations, comprising the steps of: emitting polarization states from a plurality of radiating units; and directing an output of each radiating unit into the PMF such that an output of at least one radiating unit is directed into a first PMF and an output of at least one other radiating unit is directed into a second PMF; combining an output of the first PMF with an output of the second PMF in free space to generate a combined signal; an optical fast axis of the second PMF is rotated by a non-zero angle with respect to an optical fast axis of the first PMF, and an optical slow axis of the second PMF is rotated by the non-zero angle with respect to an optical slow axis of the first PMF; The input of the first PMF is an input in which a plurality of lights having a polarization state of a first polarization including a plurality of different polarizations and a plurality of different first intensities are combined, the input of the second PMF is an input in which a plurality of lights having the first polarization and the plurality of different first intensities are combined; the plurality of different first intensities have randomly different intensities; a basis for the output of the first PMF and a basis for the output of the second PMF generate polarization states rotated by the non-zero angle, and at least the angle is set such that the basis for the output of the first PMF and the basis for the output of the second PMF are not biased relative to one another.

20. 20. A method according to claim 19, comprising emitting an optical signal having a polarization state selected from a set of four polarizations, wherein the polarization state is randomly selected from states in two polarization bases; receiving the optical signals having different polarization states at a receiver and varying a polarimetry basis at the receiver; A quantum communication method comprising:

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