A quantum protocol

Abstract

Disclosed herein is a method of quantum communication comprising transmitting photons from at least two different transmitters 202, 204, and receiving the transmitted photons at a receiver 208. Receiving the photons involves interfering the received photons 210 in interference events, registering detections of photons from the interference events, using at least two different detectors 212, 214, and analysing the interference events. Analysing the interference includes distinguishing between a single-photon interference event and a two-photon interference event, the latter describing two single-photon interference events occurring either simultaneously or within a coherence time of the photons. The transmitting step preferably involves encoding a first portion of the transmitted photons with quantum states chosen from a first basis and encoding a second portion of transmitted photons with quantum states chosen from a second basis. The analysing interference step allows the basis to be identified, because single-photon interference events may be associated with the first basis, and two-photon interference events with the second basis.

Description

FIELD OF THE INVENTION Embodiments described herein generally related to quantum communication, including quantum key distribution. BACKGROUND TO THE INVENTION Numerous schemes have been developed for quantum communication. Such schemes are generally implemented on apparatus including a transmitter and a receiver, with the goal that any eavesdropping will be detectable at the receiver. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B show an example transmitter and receiver for use in quantum communication; Fig. 2 shows a quantum communication apparatus in accordance with an embodiment; Fig. 3 shows a further quantum communication apparatus in accordance with an embodiment; Fig. 4 is a flow chart illustrating a method of quantum communication in accordance with an embodiment; and Fig. 5 is a flow chart illustrating a further method of quantum communication in accordance with an embodiment. DETAILED DESCRIPTION In an embodiment, a method of quantum communication is provided, the method comprising: transmitting photons; receiving the transmitted photons; interfering the received photons in interference events; registering detections of photons from the interference events; and analysing the interference events to identify: a single-photon interference event; and a two-photon interference event, the two-photon interference event comprising two single-photon interference events occurring within a coherence time of the photons. This method allows for quantum communication (including quantum key distribution, QKD) to be implemented using a blend of two detection methods, thereby achieving features of both methods. For example, methods based on single photon interference (such as twin-field QKD, TF-QKD) generally offer favourable signal scaling with distance, for example scaling as for transmittivity q. Methods such as TF-QKD can also require global phase locking of the transmitted photons, which is difficult to implement over long distances. The need for global phase locking can be overcome for high-priority information via improved protocols such as send-no-send TF-QKD, though even in such a scheme global phase locking will generally be necessary to send lower-priority information in a secondary basis such as a test basis. Schemes involving coincident two-photon interference generally achieve only linear scaling with q. Correspondingly, methods based on asynchronous two-photon interference (such as mode pairing measurement device independent QKD, MDI-QKD) may have distance scaling dependent on the coherence time. If the coherence time is short, the scaling may tend towards linear scaling with q, while a long coherence time allows scaling to approach The general case will be in between the two limits. The method described above allows for the best / q scaling to be achieved for certain prioritised information (the single-photon interference measurements), while still achieving better than linear scaling for other measurements (the two-photon interference measurements), all while avoiding the need for any global phase coherence. In an embodiment, the method further comprises encoding a first portion of the transmitted photons with quantum states chosen from a first basis, and encoding a second portion of the transmitted photons with quantum states chosen from a second basis. This allows for the separation of high- and low-priority information between bases. For example, the first basis may be a signal basis (e.g. used for transmitting quantum key information) while the second basis may be a test basis (e.g. used for measuring the performance or security of the communication channel, or other such analytics). In this example, it is anticipated that the signal basis will be chosen so as to result in singlephoton interference events with the best scaling, while the less favourable scaling of the two-photon interference events is used for the test basis. In an embodiment, the encoding comprises modulating at least one of an intensity, a relative phase, or a polarisation of the transmitted photons. This allows information to be conveyed in multiple different forms using any of several different kinds of optical equipment for encoding and detection. For example, the signal basis and test basis may be implemented using different polarisation, phase, or intensity encodings. In an embodiment, the single-photon interference event occurs between photons encoded in the first basis and the two-photon interference event occurs between photons encoded in the second basis. As noted above, this allows for the separation of high- and low-priority information between bases, allowing different measurement techniques and / or encoding schemes to be used for each. In an embodiment, the method further comprises announcing the first basis and the second basis after analysing the interference events. This allows encoded information to be correlated between the two or more transmitters, conditioned on the measurement results of the receiver. For example, the operators of the transmitters can compare the announced basis of each measurement with the basis in which the respective photon was originally encoded. This correlation allows information to be conveyed securely over many interference events. In an embodiment, the method further comprises interpreting the identified interference events as transmitted information in a quantum key distribution scheme. This allows for the secure transmission of quantum keys, while achieving the benefits of the method described above. In a further embodiment, a quantum communication system is provided comprising: two transmitters configured to emit photons; a receiver configured to receive photons emitted by the transmitters, the receiver comprising: an interference unit configured to interfere photons from the transmitters; and a detector configured to detect photons from the interference unit and generate detection information; and a processor configured to receive the detection information and analyse the detection information to identify interference events, the interference events comprising: a single-photon interference event occurring in the interference unit; and a two-photon interference event, the two-photon interference event comprising two single-photon interference events occurring in the interference unit within a coherence time of the photons. This system provides the benefits of implementing multiple different transmission and detection schemes for different priorities of information - i.e. a signal channel and a test channel - as described above. In particular, favourable distance scaling may be achieved for the signal channel without the need for global phase locking. In an embodiment, the interference unit comprises a beam splitter. In an embodiment, the beam splitter comprises two input ports and two output ports, the two input ports being respectively coupled to the two transmitters and the two output ports being respectively coupled to two photodetectors. In an embodiment, the system comprises a respective modulation device for each transmitter, each modulation device configured to modulate at least one of phase, polarisation, or intensity of the photons emitted by the respective transmitter. In an embodiment, the transmitters are lasers operating in pulsed mode, and wherein each successive pulse is individually modulated. In an embodiment, each transmitter comprises a laser operating in continuous wave (CW) mode and a pulse carver to convert light emitted from the laser into a series of pulses, and wherein each successive pulse is individually modulated. In a further embodiment, a method of quantum communication is provided, the method comprising: transmitting photons; receiving the transmitted photons; interfering the received photons in interference events; registering detections of photons from the interference events; and analysing the interference events to identify: a single-photon interference event occurring in the interference units and indicated by a detection event at a single detector; and a two-photon interference event, the two-photon interference event comprising two single-photon interference events, wherein the two-photon interference event is indicated by simultaneous detection events at two detectors. This method provides an alternative implementation using coincident two-photon interference (more similar to MDI-QKD) for the two-photon interference events rather than asynchronous (mode-pairing MDI-QKD) of the kind described above. In this implementation the two-photon interference events retain the improved signal-to-noise ratio of the previous method, though this is achieved by reduction of noise rather than enhancement of signal. However, this method generally achieves the same features as the previously-described method. In a further embodiment a quantum communication system is provided comprising: two transmitters configured to emit photons; and a receiver configured to receive photons emitted by the transmitters, the receiver comprising: a first interference unit configured to interfere photons from the transmitters; a second interference unit configured to receive and interfere photons from the first interference unit; two detectors configured to detect photons from the second interference unit and generate detection information; and a processor configured to receive the detection information and analyse the detection information to identify interference events, the interference events comprising: a single-photon interference event occurring in the interference units and indicated by a detection event at a single detector; and a two-photon interference event, the two-photon interference event comprising a first single-photon interference event in the first interference unit and a second single-photon interference event in the second interference unit, wherein the two-photon interference event is indicated by simultaneous detection events at both detectors. In an embodiment, the first interference unit comprises a beam splitter. In an embodiment, the second interference unit comprises a first beam splitter and a second beam splitter, each beam splitter comprising an input port and two output ports, wherein each input port is coupled to the first interference unit and each output port is coupled to a respective photodetector. In a further embodiment, a receiver is provided for use in a quantum communication system, the receiver configured to receive photons and comprising: a first interference unit configured to interfere photons from the transmitters; a second interference unit configured to receive and interfere photons from the first interference unit; two detectors configured to detect photons from the second interference unit and generate detection information; and a processor configured to receive the detection information and analyse the detection information to identify interference events, the interference events comprising: a single-photon interference event occurring in the interference units and indicated by a detection event at a single detector; and a two-photon interference event, the two-photon interference event comprising a first single-photon interference event in the first interference unit and a second singlephoton interference event in the second interference unit, wherein the two-photon interference event is indicated by simultaneous detection events at both detectors. A basic quantum communication protocol which uses polarisation will now be explained. This can be used for transmission between a transmitter and a receiver. However, it should be noted that this is not meant as limiting and other protocols could also be used. Further, the above system could be used with any QKD system and is not limited to uses with polarisation. For example, phase or energy / time based QKD protocols could also be used. The protocol uses two bases wherein each basis is described by two orthogonal states. For this example the basis of horizontal / vertical (H / V) and Diagonal / Antidiagonal D / A. However, the left circularly polarized / right circularly polarized (L / R) basis could also be selected. The transmitter in the protocol prepares states with one of H, V, D or A polarisation. In other words, the prepared states are selected from two orthogonal states (H and V or D and A) in one of two basis H / V and D / A. This can be thought of as sending a signal of 0 and 1 in one of two basis, for example H=0, V=1 in the H / V basis and D=0, A=1 in the D / A basis. The pulses are attenuated so that they comprise on average, one photon or less. Thus, if a measurement is made on the pulse, the pulse is destroyed. Although the pulse can be split, it is possible to arrange conditions of privacy amplification and error correction that render it very unlikely that an eavesdropper is able to gain any information about the distributed key. The receiver uses a measurement basis for the polarisation of a 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 fixed components, such as a beam splitter. In "active" basis choice, the receiver makes a decision which basis to measure in - e.g. using a modulator with an electrical control signal. If the basis used to measure the pulse at the receiver is the same as the basis used to encode the pulse, then the receiver’s measurement of the pulse is accurate. However, if the receiver selects the other basis to measure the pulse, then there will be a 50% error in the result measured by the receiver. To establish a key, the sender and receiver compare the basis that were used to encoder and measure (decode). If they match, the results are kept, if they do not match the results are discarded. The above method is very secure. If an eavesdropped intercepts the pulses and measures then, the eavesdropper must prepare another pulse to send to the receiver. However, the eavesdropper will not know the correct measurement basis and will therefore only has a 50% chance of correct measuring a pulse. Any pulse recreated by the eavesdropper will cause a larger error rate to the receiver which can be used to evidence the presence of an eavesdropper. The sender and receiver compare a small part of the key to determine the error rate and hence the presence of an eavesdropper. Although the above has been described in relation to polarisation this is as an illustration. Other QKD protocols could be used which are based on phase or other systems such as energy / time. A qubit (qudit) is a two (d) -dimensional quantum system. The above BB84-like protocol uses two modes for the encoding each qubit, one horizontal (H) and one vertical (V). These modes can be referred to as “rails”. Protocols like this are called “dual rail”. In its simplest form, each rail contains a photon, i.e., the first rail is denoted |1H) and the second rail is denoted 110. Then, |1H, Oy) and |0H, 1K) form a basis for the qubit. All qubit states can be written in the form a|lH,O0 + p|OH,10. All qubit states in this example have exactly one photon. This makes the protocol very resilient to losses since the transmitter and receiver discard all instances of losses and retain only successful detection events. They can declare that the photon arrived therefore an eavesdropper could not have taken it. Further, dual-rail schemes generally do not require that the transmitter and receiver are locked in phase, since global phases do not matter, only that the phase between the rails are required to be maintained, because, for example, it must be the case that |1H, 00 +|0w, 10 is distinguishable from |1H, 0K> - |0H, 1 v) but here the phase is between rails. No phase locking of each rail is required. However, variants of TF QKD and others based on single single-photon interferences use one mode for the encoding, and the quantum information is encoded into the phase and amplitude of the single mode. Protocols like this are called “single rail”. Continuous variable (CV) QKD is also generally single rail. The single rail generally consists of a superposition of vacuum and photon states. This makes the protocol not resilient to losses. Single-rail schemes generally require that the transmitter and receiver are locked in phase since for the single-rail qubit state a|0) + p11), the phase between the photon numbers in superposition in the rail is vital. TF QKD is generally single-rail while MDI QKD is dual-rail. Implementations described herein use a combination of dual-rail and single-rail. Single-rail is used for the high-priority information based on single-photon interference with optimal rate-distance scaling proportional to Dual-rail is used for the low-priority information based on two post-matched single-photon interferences (asynchronous) or one two-photon interference (coincident). The above QKD requires two channels, a “quantum channel” which is used for the communication of pulses that contain quantum information, and a classical channel which is used for discussion of the basis (“sifting”). Also, the classical channel can be used for further communication once the key has been established on the quantum channel. It should be noted that the term “channel” is used to refer to a logical channel. The quantum and classical channels may be provided within the same physical fibre. An example of a receiver and transmitter will now be described with reference to Figs. 1A and 1B. An example of a possible transmitter is shown as 100 in FIG. 1A. The transmitter can be any type of quantum transmitter which is capable of emitting encoded photons. In this particular example, polarisation encoding will be discussed, but any type of encoding could be used, for example phase or other encoding types such as energy / time. In the example of FIG. 1A, the transmitter 100 comprises four emitters, 102, 104, 106 and 108, each of which emits horizontally polarized light. The output from emitter 102 is provided towards polarisation combining optics 110. The output from emitter 104 is provided towards polarisation combining optics 110 via a half waveplate which is configured to convert the horizontally polarized light to diagonally polarized light. The output from emitter 106 is provided towards polarisation combining optics 110 via a half waveplate which is configured to convert the horizontally polarized light to vertically polarized light. The output from emitter 108 is provided towards polarisation combining optics 110 via a half waveplate which is configured to convert the horizontally polarized light to antidiagonally polarized light. Polarisation combining optics 110 allows the different polarisations to be combined into a stream of pulses with randomly varying polarisations. This may be achieved in many different ways. For example, the lasers may be pulsed lasers and a controller (not shown) is provided to randomly select a laser from emitters 102, 104, 106 and 108 to randomly output a pulse such that one pulse at a time reaches the polarisation combining optics 110. In other embodiments, the polarisation combining optics 110 or a further component may be configured to randomly select the output from one emitter or randomly selectively block the output from three emitters to allow for the pulsed output stream. The pulses may be produced by pulses lasers, or CW lasers may be used with a further component to chop the output into pulses. An attenuator (not shown) is then used to attenuate the output of the pulses so that they contain on average less than one photon. Alternatively, single photon emitters can be used instead of lasers 105, 107, 109 and 111. A simplified form of the receiver 112 is shown in FIG. 1B. The receiver 112 comprises a 50-50 beam splitter 114 which will direct the incoming pulse either along a first measurement channel 115 or a second measurement channel 116. Since the pulses contain on average less than one photon, the 50-50 beam splitter 114 will direct the pulse randomly along one of the first measurement channel or the second measurement channel. This has the result of selecting a measurement basis to be the X (D / A) basis or the Z (H / V) basis. The non-polarising beam splitter 114 functions to allow random selection of one of the two bases. The first measurement channel 115 is for the X basis which corresponds to the D / A basis. Here, a half waveplate 118 is provided to rotate the polarisation by 45 degrees between the two detection branches, i.e. giving the two measurement bases X and Z. The output of the half waveplate 118 is then directed towards polarising beam splitter 820. Polarising beam splitter 120 directs pulses with antidiagonal polarisation towards antidiagonal detector 124 and pulses with diagonal polarisation towards diagonal detector 126. Detectors 124 and 126 are single photon detectors, for example avalanche photodiodes. Pulses directed along the second measurement channel 116 are measured in the Z basis to determine if they are horizontal or vertical. Here, the pulses directed into the second measurement channel 116 are directed toward polarising beam splitter 122 which directs vertically polarised pulses towards vertical detector 128 and horizontally polarised pulses towards horizontal detector 130. Again, detectors 128 and 130 are single photon detectors. If a photon is received which is polarised in the D / A basis and this is randomly sent to be measured in the Z basis along the second management channel 116, one of detectors 128, 130 is likely to register a count. However, this result cannot be trusted, as a photon received at polarising beam splitter 122 has a 50-50 chance of being directed towards either the vertical or the horizontal detector. In CV-QKD as mentioned above, quantum information may be encoded into the real and imaginary parts of the complex amplitude emitted by a coherent light source such as a laser, for example using amplitude and phase modulators. The real and imaginary parts of this amplitude serve as choices of basis, analogous to the directions of polarisation in BB84. Such states are referred to as “single rail” since they are single mode. The encoded states are then sent to a receiver, where a coherent measurement such as heterodyne detection is performed. Since quantum information is encoded in the phase of a single mode, this generally requires a phase reference between receiver and transmitter. Additionally, unlike BB84 which requires no phase reference and discards all lost photons, in CV-QKD the receiver generally keeps all photons received. After further steps such as sifting bases, privacy amplification, and error correction, receiver and transmitter arrive at a secret random key. In relay-based QKD such as TF-QKD and MDI-QKD, there are two transmitters, both of which correlate their data via e.g. a Bell state measurement in the relay. For TF-QKD this will generally be a single-rail Bell state measurement, while for MDI-QKD it will be a dual-rail Bell state measurement. Protocols described herein combine features of both TF-QKD and MDI-QKD. Fig. 2 shows a quantum communication apparatus 200 according to an embodiment of the present disclosure. The apparatus 200 comprises transmitters 202, 204 and a receiver 208. The receiver 208 comprises a beam splitter 210 and detectors 212, 214. The transmitters 202, 204 are configured to emit coherent light 206 (otherwise generally referred to herein as emitting photons). For example, the transmitters 202, 204 may be lasers, or may be sources of incoherent light filtered to a narrow linewidth to produce coherent light. While the system 200 comprises two transmitters 202, 204, the scheme may be expanded to other numbers of transmitters. For example, three or more transmitters may be used. The receiver 208 is positioned to receive at least part of the coherent light 206. In embodiments, other optical components may be present in between the transmitters 202, 204 and the receiver 208. For example, components may be provided to control the intensity, phase, or polarisation of light emitted by each transmitter 202, 204. Alternatively, some of the light 206 may be redirected (e.g. at a beam splitter) for some other purpose, with only a portion of the light 206 continuing to the receiver 208. In the embodiment of Fig. 2, the optical path from each transmitter 202, 204 is coupled to a respective input port of a beamsplitter 210 within the receiver 208. Each output of the beamsplitter 210 is then coupled to a respective detector 212, 214. As discussed below with reference to Fig. 3, this is not the only possible arrangement for implementing the disclosure. For example, additional beamsplitters and / or detectors may be used to implement other detection schemes. Alternatively, the beamsplitter may be replaced with any other device facilitating interference of photons (referred to herein as an interference unit). The receiver 208 may be used to implement single- and two-photon interference measurements. For example, single-photon interference measurements may be detected by registering a detection event at one of the detectors 212, 214. These single-photon detection events may be interpreted according to a QKD scheme, such as TF-QKD or send-no-send TF-QKD. Send-no-send TF-QKD in particular avoids the need for global phase coherence between the transmitters 202, 204 in the priority (code) basis since the two logical code states of this basis are either vacuum or a phase-randomised coherent state. Both these states are agnostic to single-mode phase rotations, thus, this basis requires no global phase coherence. Additionally or alternatively, two-photon interference measurements may be implemented by registering two individual detection events, one at each detector. These events may be correlated and post-matched to determine an effective two-photon interference event. This may require that the two individual detection events are separated by a time less than the coherence time of the transmitters 202, 204. If the transmitters 202, 204 have different coherence times, the maximum time for postmatching is the shorter of the two coherence times. This may correspond to a modepairing (or post-matched) MDI-QKD scheme. It is generally envisioned that the post-matching of single-photon interference events to identify two-photon interference events based on the coherence time, and thereby to distinguish single- and two-photon interference events, will be performed by a processor somewhere within the quantum communication apparatus 200. For example, the processor may be operated by an operator of one of the sources 202 or 204. In such an embodiment, the receiver 208 may simply announce a sequence of apparent single-photon interference events (i.e. single clicks on detector 212 or detector 214), some of which may then be retrospectively interpreted by the processor as two-photon interference events. Additionally or alternatively, a processor may be present in the receiver 208. A processor performing such correlation of single-photon interference events may otherwise be referred to herein as a correlator. Methods of interpreting interference events are described in more detail below with reference to Figs. 4 and 5. Fig. 3 shows a further quantum communication system 300 according to an embodiment of the present disclosure. The system 300 comprises transmitters 202, 204 corresponding to those described above with regard to Fig. 2. The system 300 further comprises a receiver 308. The receiver 308 comprises three beamsplitters 310, 320, 322, and four detectors, 312, 314, 316, 318. The precise arrangement of components in Fig. 3 is intended to be merely illustrative of a particular way of implementing the system, and it is envisioned that other arrangements may be used. In the embodiment of Fig. 3, light 206 received from the transmitters 202, 204 at the receiver 308 is split at a first beamsplitter 310, then at two subsequent beamsplitters 320, 322, resulting in four output channels. These four output channels are each coupled to a respective detector 312, 314, 316, 318. It is noted that this arrangement of beamsplitters effectively defines two interference units, with a first interference event happening at the first beamsplitter 310 (the first interference unit), the outputs of which may lead to further interference at one or both of the subsequent beamsplitters 320, 322 (the second interference unit). Any alternative apparatus that facilitates interference events may be used instead of beamsplitters. The receiver 308 may be used to implement single- and two-photon interference measurements. For example, single-photon interference measurements may be implemented in a similar manner to that described above with reference to Fig. 2. In a further example, two-photon interference measurements may be implemented by registering simultaneous detections on two of the detectors 312, 314, 316, and 318. For example, a detection on either 312 or 314 may be registered at the same time as a detection on either 316 or 318. This may generally correspond to a coincident MDI-QKD scheme. In this embodiment, it is generally envisioned that the distinguishing of single- and two-photon interference events may be directly accomplished by the receiver 308, since post-matching is not required. Rather, the receiver 308 may announce a single- or two-photon interference event based on registering either a single detection event, or two simultaneous detection events, as described above. Post-matching by a separate processor may therefore not be necessary. Methods of interpreting interference events are described in more detail below with reference to Figs. 4 and 5. Fig. 4 is a flow diagram showing a method 400 for quantum communication. For example, the method 400 may be implemented on the apparatus 200 described above. At step 402, photons are transmitted. This may be implemented using any of the transmitters or emitters described above, such as the transmitters 100 or 202. The transmitted photons may be encoded in a quantum state according to a predetermined basis, or one of several predetermined bases. This encoding may be achieved through modulation of, for example, one or more of polarisation, phase, and intensity. For example, if the photons are emitted by a pulsed laser, it may be that each successive pulse is separately encoded by modulating the polarisation, phase, and / or intensity of that pulse. At step 404, the transmitted photons are received. This may be implemented using any of the detectors or receivers described above, such as the receivers 112 or 204. At step 406, the received photons are interfered in interference events, for example by the beam splitter 210 described above with reference to Fig. 2. At step 408, photons resulting from the interference events are detected, for example by the detectors 212, 214 described above with reference to Fig. 2. At step 410, the interference events are analysed. For example, an operator and / or processor may review the detection events that have been recorded at each detector 212, 214. In particular, they may determine, based on knowledge of the coherence times of the transmitters, whether there are any pairs of interference events occurring within one coherence time of each other. If the two transmitters have differing coherence times, the relevant time is the shorter of the two. This analysis may lead to identifying two types of interference events, as described below. At step 412, the operator or processor identifies single-photon interference events. For example, these may be associated with a detection event at a single one of the detectors 212, 214, with no other detection events occurring within one coherence time. At step 414, the operator or processor identifies pairs of detection events, one at each detector, that are separated by a time interval that is less than the shortest coherence time among all the transmitters being used (e.g. the transmitters 212, 214). These pairs of events may be flagged as potential two-photon interference events. Following this, there may be an additional step wherein the detection results are shared with the operators transmitting the photons (e.g. the operators of the transmitters 212, 214). The operators of the transmitters may then in exchange disclose which basis each photon was encoded in. The analysis of the interference events may then be continued. For example, for each of the identified single-photon interference events, it may be checked whether the two photons involved were encoded in the same basis, and which basis that was. For example, if both photons were encoded in e.g. a signal basis, this event may be correlated with other signal basis events to e.g. extract information relating to a quantum key as part of a QKD scheme. Additionally or alternatively, each of the flagged potential two-photon interference events may be checked to determine whether each of the two events was an interference event between photons encoded in the same basis (e.g. a test basis). If two single-photon interference events occur within the coherence time, both between photons encoded in the test basis, this may be interpreted as a two-photon interference event in the test basis and used accordingly to determine information about the communication channel. The above method allows for the benefits of single-photon interference methods (improved distance scaling) to be achieved for the signal basis while simultaneously avoiding the need for global phase coherence. The test basis distance scaling may be less favourable since it relies on two-photon interference, but this is of less importance since the test basis is not used for sending high-priority information. Alternatively, the test and signal bases may be assigned the other way around to the way suggested above. Fig. 5 is a flow diagram showing a method 500 for quantum communication. For example, the method 500 may be implemented on the apparatus 300 described above. The above discussion of the method 400 applies equally to method 500, including the corresponding steps 402-412 and the potential additional steps of encoding photons and / or announcing and correlating results, with references to Fig. 2 altered to references to Fig. 3. However, at step 514, the identified two-photon interference event may be a coincident event rather than asynchronous. This means that, in order to identify a two-photon interference event, the operator and / or processor may require there to be simultaneous detection events on two detectors at once (for example, two of the detectors 312, 314, 316, and 318). If further analysis is conducted after optionally announcing detection results, a successful two-photon interference event will then require that the received photons resulting in the simultaneous detection events were both (or all) encoded in the same basis, e.g. either both in a test basis or both in a signal basis. Whilst certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel devices, and methods described herein may be embodied in a variety of other forms; furthermore, 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 inventions. The accompanying claims and their 5 equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

Claims

1. A method of quantum communication, the method comprising:transmitting photons;receiving the transmitted photons;interfering the received photons in interference events;registering detections of photons from the interference events; andanalysing the interference events to identify:a single-photon interference event; anda two-photon interference event, the two-photon interference event comprising two single-photon interference events occurring within a coherence time of the photons.

2. The method of claim 1, further comprising encoding a first portion of the transmitted photons with quantum states chosen from a first basis, and encoding a second portion of the transmitted photons with quantum states chosen from a second basis.

3. The method of claim 2, wherein the encoding comprises modulating at least one of an intensity, a relative phase, or a polarisation of the transmitted photons.

4. The method of claim 2 or 3, wherein the single-photon interference event occurs between photons encoded in the first basis and the two-photon interference event occurs between photons encoded in the second basis.

5. The method of any preceding claim, further comprising announcing the first basis and the second basis after analysing the interference events.

6. The method of any preceding claim, further comprising interpreting the identified interference events as transmitted information in a quantum key distribution scheme.

7. A quantum communication system comprising:two transmitters configured to emit photons;a receiver configured to receive photons emitted by the transmitters, the receiver comprising:an interference unit configured to interfere photons from the transmitters; anda detector configured to detect photons from the interference unit and generate detection information; anda processor configured to receive the detection information and analyse the detection information to identify interference events, the interference events comprising:a single-photon interference event occurring in the interference unit; anda two-photon interference event, the two-photon interference event comprising two single-photon interference events occurring in the interference unit within a coherence time of the photons.

8. The system of claim 7, wherein the interference unit comprises a beam splitter.

9. The system of claim 7 or 8, wherein the beam splitter comprises two input ports and two output ports, the two input ports being respectively coupled to the two transmitters and the two output ports being respectively coupled to two photodetectors.

10. The system of any of claims 7 to 9, further comprising a respective modulation device for each transmitter, each modulation device configured to modulate at least one of phase, polarisation, or intensity of the photons emitted by the respective transmitter.

11. The system of claim 10, wherein the transmitters are lasers operating in pulsed mode, and wherein each successive pulse is individually modulated.

12. The system of claim 10, wherein each transmitter comprises a laser operating in continuous wave (CW) mode and a pulse carver to convert light emitted from the laser into a series of pulses, and wherein each successive pulse is individually modulated.

13. A method of quantum communication, the method comprising:transmitting photons;receiving the transmitted photons;interfering the received photons in interference events;registering detections of photons from the interference events; andanalysing the interference events to identify:a single-photon interference event occurring in the interference units and indicated by a detection event at a single detector; anda two-photon interference event, the two-photon interference event comprising two single-photon interference events, wherein the two-photon interference event is indicated by simultaneous detection events at two detectors.

14. A quantum communication system comprising:two transmitters configured to emit photons; anda receiver configured to receive photons emitted by the transmitters, the receiver comprising:a first interference unit configured to interfere photons from the transmitters;a second interference unit configured to receive and interfere photons from the first interference unit;two detectors configured to detect photons from the second interference unit and generate detection information; anda processor configured to receive the detection information and analyse the detection information to identify interference events, the interference events comprising:a single-photon interference event occurring in the interference units and indicated by a detection event at a single detector; anda two-photon interference event, the two-photon interference event comprising a first single-photon interference event in the first interference unit and a second single-photon interference event in the second interference unit, wherein the two-photon interference event is indicated by simultaneous detection events at both detectors.

15. The system of claim 14, wherein the first interference unit comprises a beam splitter.

16. The system of claim 14 or 15, wherein the second interference unit comprises a first beam splitter and a second beam splitter, each beam splitter comprising an inputport and two output ports, wherein each input port is coupled to the first interference unit and each output port is coupled to a respective photodetector.

17. A receiver for use in a quantum communication system, the receiver configured to receive photons and comprising:a first interference unit configured to interfere photons from the transmitters;a second interference unit configured to receive and interfere photons from the first interference unit;two detectors configured to detect photons from the second interference unit and generate detection information; anda processor configured to receive the detection information and analyse the detection information to identify interference events, the interference events comprising:a single-photon interference event occurring in the interference units and indicated by a detection event at a single detector; anda two-photon interference event, the two-photon interference event comprising a first single-photon interference event in the first interference unit and a second single-photon interference event in the second interference unit, wherein the two-photon interference event is indicated by simultaneous detection events at both detectors.

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