Patent Description:
QKD arrangements are expensive and one way of reducing costs is to have a single Alice which transmits pulses to multiple Bobs. Such arrangements have a branched optical path so that each Bob receives a proportion of the pulses. Optical splitters have been used to provide the branched optical path. A problem with splitting the signal is that some of the Bobs may not receive a sufficiently high pulse rate to meet their key generation demands. This is particularly true if, say, one Bob needs to produce a larger number of keys than the others. It may also be true if one Bob is located further from the splitter than the others (as the pulses directed to it will suffer more attenuation), or if there are a large number of Bobs.

It would be desirable to overcome or mitigate some and/or all of the above-mentioned and/or other disadvantages of the prior art. <NPL>, discloses a QKD system with one transmitter and a plurality of quantum receivers. The transmitter is connected to the receivers with a switch.

<CIT> discloses a system in which the incoming signal is split at a splitter so as to separate the quantum signal from the classical signal. The quantum signal is directed to a quantum receiver and the classical signal is directed to a classical receiver. Said document discloses guiding classical signals from the transmitter to the classical receiver via an optical communication path that bypasses the quantum receiver. In said document, it is classical, not quantum signals that bypass the switch.

Embodiments of the invention enable a system to be constructed in which a high-power pulse stream can be provided, via the switch, to a receiver when its key generation need is high, while also continuously providing the other receivers with a sufficiently high-power pulse stream to avoid disconnection and the resulting time-consuming re-initiation process.

According to a second aspect of the invention there is provided a method of performing Quantum Key Distribution between a transmitter and first and second receivers, the method comprising.

A modulation may be applied to the pulses before being transmitted. The step of applying a modulation to the pulses may comprise polarising the pulses using a randomly chosen polarisation basis. Alternatively, the step of applying a modulation to the pulses comprise modulating the phase or the position of the pulse. After being encoded and modulated the pulses may be transmitted in accordance with QKD principles.

The value encoded onto the pulse and/or the modulation basis applied to the pulse and/or the time of transmission of the pulse may be recorded. The modulation basis may be the polarisation basis. Furthermore, the transmitter may be connected for communication with each of the receivers by a respective non-quantum channel. The non-quantum channel may be a metallic wire, or an optical fibre or free space. The transmitter may send some or all of the recorded information to one or more of the receivers using the respective non-quantum channel.

The value that has been encoded onto the pulses (i.e. a one or a zero) may be measured at each of the receivers using randomly chosen modulation basis which may be a randomly chosen polarisation basis. For each received pulse, the measured value and/or the polarisation basis used for the measurement and/or the arrival time of the pulse may be recorded. Each of the receivers may send the measured value and/or the arrival time of the pulse to the transmitter using its respective non-quantum channel.

The optical pulses may be single-photon pulses. Single-photon pulses may be generated by a single-photon generator.

In the first switching position the input may be optically connected to the first receiver only. In the second switching position the input may be optically connected to the second receiver only. The system may comprise one or more further receivers. The input of the optical switch may be in optical communication with the transmitter via a conduit or through the atmosphere. The conduit may be an optical fibre. The switch may have a first output and a second output. When the switch is in the first switching position, the input may be optically connected to the first output. The first output may be optically connected to the first receiver. When the switch is in the second switching position, the input may be in optical communication with the second output. The second output may be optically connected to the second receiver.

The switch may have one or more further switching positions and may have one or more corresponding further outputs. The switch may be adapted such that in each of the one or more further switching positions, the switch input is optically connected to the corresponding further output. Each of the one or more corresponding further outputs may be optically connected to a corresponding one of the one or more further receivers.

The system may comprise a controller adapted to control the optical switch. The controller may be adapted to control the switching position of the switch. The controller may be adapted to send a control signal to the switch to control the switching position of the switch. The controller may instruct the switch to move to the switching position corresponding to the receiver having the highest optical pulse rate requirements. The first and/or second and/or further receivers may be adapted to send to the controller an indication of the optical pulse rate required by that receiver or receivers.

The guide may comprise an optical splitter. The system may be arranged such that the splitter receives an input from the transmitter. In use, the splitter may receive the transmitted plurality of optical pulses through the input and split the plurality of optical pulses such that the portion of the plurality optical pulses is guided onto a different path to the remainder of the plurality of optical pulses. The portion of the plurality optical pulses that are guided onto a different path may constitute less than <NUM>% of the pulse stream, preferably constitute less than <NUM>% of the pulse stream, and most preferably constitute <NUM>% of the pulse stream. The splitter may have a first output for outputting the portion of the plurality of optical pulses and may have a second output for outputting the remainder of the plurality of optical pulses. The second output may be optically connected to the input of the switch.

Although in preferred embodiments the guide comprises a splitter, the skilled person would understand that it would be possible to provide a guide which did not comprise a splitter but which provides substantially the same functionality. A possible example of this would be an arrangement which provided a plurality of pulses to a first output for a short time period, then provided a plurality of pulses to a second output for a short time period, and repeated this cycle over a long period. Over time such an arrangement would direct a proportion of the pulse stream to each of the outputs.

The guide may guide optical pulses to the second receiver. A second splitter may be provided, having an input which is optically connected to the first output of the first splitter. The second splitter may have a first output which is optically connected to the first receiver. The second splitter may further comprise a second output which is optically connected to and second receiver. In use, the portion of the plurality of optical pulses output from the first output of the first splitter may be input at the input of the second splitter and may be split by the second splitter into a first component which is output to the first receiver and a second component which is output to the second receiver.

In embodiments in which there are one or more further receivers, the guide may guide optical pulses to the one or more further receivers. In these embodiments the second splitter may have one or more further outputs, each of which is optically connected to one of the one or more further receivers. In these embodiments, the portion of the plurality of optical pulses input to the second splitter is split between the first, second and one or more further outputs of the second splitter. The components into which the portion of the plurality of optical pulses is split by the second splitter may be equal in power.

The pulse stream output by the switch may be combined with the pulse stream output by the second splitter before arriving at a respective receiver. The system may further comprise a pulse stream combiner. The pulse stream combiner may be adapted to combine two input pulse streams using a combining ratio of <NUM>:<NUM>. The pulse stream combiner may combine the pulses output at the first output of the switch with the pulses output at the first output of the second splitter. The combiner may output the combined pulse stream to the first receiver. There may be a second pulse stream combiner. The pulse stream combiner may combine the pulses output at the second output of the switch with the pulses output at the second output of the second splitter. The combiner may output the combined pulse stream to the second receiver. There may be one or more further pulse stream combiners. The one or more further pulse stream combiners may combine the pulses output at a corresponding one of the one or more further outputs of the switch with the pulses output at corresponding one of the one or more further outputs of the second splitter. The combiner may output the combined pulse stream to a corresponding one of the one or more further receivers.

A specific embodiment of the invention will now be described, for illustration only, and with reference to the accompanying drawings, in which:.

The present invention concerns improvements in Quantum Key Distribution (QKD). QKD is a method of encryption involving distributing an encryption key from a first quantum node (known as Alice) to a second quantum node (known as Bob). <FIG> shows a schematic view of a simplified QKD arrangement in which Alice is shown at <NUM> and Bob at <NUM>. Although multiple QKD protocols are in use, the present explanation relates to the commonly-used BB84 protocol. In particular, this explanation concerns a type of BB84 in which the modulation is applied to the pulses using polarisation. According to that protocol, Alice <NUM> randomly generates a bit (either <NUM> or <NUM>) and also randomly chooses one of two polarisation bases: rectilinear and diagonal. Alice <NUM> then sends a photon that has been encoded with the chosen bit and the chosen polarisation basis to Bob <NUM> via a quantum channel <NUM>, such as an optical fibre. Bob <NUM> randomly selects one of the two polarisation bases and measures the photon using its chosen basis. If Bob <NUM> uses the same basis as Alice <NUM> then the bit value measured by Bob <NUM> will match that applied to the photon by Alice <NUM>. After repeating the process with a large number of photons, Alice <NUM> and Bob <NUM> perform a key agreement stage. In particular, Alice <NUM> informs Bob <NUM>, via a non-quantum communication channel <NUM> (such as a copper cable), which of the two bases Alice <NUM> applied to each photon, along with the time of transmission by Alice <NUM> of each photon. Bob <NUM> then informs Alice <NUM> which of the two bases Bob <NUM> used when measuring each photon along with the time at which Bob <NUM> received each photon. Alice <NUM> and Bob <NUM> then discard their bit values for which Alice <NUM> and Bob <NUM> used different bases, and keep the remaining bit values. The remaining bit values constitute a secret key that Alice <NUM> and Bob <NUM> both have and which they can use to encrypt messages sent between them over the non-quantum channel <NUM>.

<FIG> shows a known QKD architecture. In particular, Alice <NUM> is connected to four different Bobs <NUM>,<NUM>,<NUM>,<NUM>. An optical splitter <NUM> is provided in the optical path from Alice <NUM> to the four Bobs <NUM>,<NUM>,<NUM>,<NUM>. Alice <NUM> is connected to the splitter <NUM> by an optical fibre <NUM>. Each of the four Bobs <NUM>,<NUM>,<NUM>,<NUM> is connected to the splitter <NUM> by a respective optical fibre <NUM>. Each of the Bobs <NUM>,<NUM>,<NUM>,<NUM> is also connected to Alice <NUM> by its own classical (i.e. non-quantum) channel (not shown).

In use, Alice <NUM> sends a series of photons to the splitter <NUM>, each photon having been encoded with a random bit and a random polarisation base as described above. Each photon passes through the splitter <NUM> and on to one of the Bobs <NUM>,<NUM>,<NUM>,<NUM>. The splitter <NUM> diverts each photon to one of the Bobs <NUM>,<NUM>,<NUM>,<NUM> at random. Therefore, on average, <NUM>% are directed towards each of the four Bobs. Each Bob measures the photons using a randomly-chosen polarisation basis, and, once it has received enough photons to enable a secret shared key to be established with Alice <NUM>, it does so using the key agreement stage described above. The key agreement stage involves Alice <NUM> and each one of the Bobs <NUM>,<NUM>,<NUM>,<NUM> exchanging their respective lists of polarisation bases and takes place over the classical channel. Alice <NUM> and each one of the Bobs use the lists to establish a shared secret key which can be used to encrypt communications between them.

As can be seen in <FIG>, Bob <NUM> is located further from the splitter <NUM> than the other Bobs <NUM>,<NUM>,<NUM>. As the photons sent to Bob <NUM> travel further than the photons sent to the other Bobs <NUM>,<NUM>,<NUM>, the photons transmitted to Bob <NUM> suffer a greater attenuation. The key exchange rate between Alice and Bob <NUM> may therefore be lower than the key exchange rate between Alice and the other Bobs <NUM>,<NUM>,<NUM>. This is undesirable as it slows down the establishment of a secure communication link between Bob <NUM> and Alice <NUM>. Furthermore, if the photon receipt rate at bob <NUM> falls below a threshold rate, it is necessary to re-initiate the connection, which is time consuming.

A further disadvantage of the arrangement of <FIG> is where one of the Bobs, say Bob <NUM>, needs to establish more keys with Alice over a given time period than do the other Bobs. As the splitter splits the photons approximately equally, it may take a long time to establish all the keys at Bob <NUM>, while photons are sent unnecessarily to other Bobs after they have finished establishing their keys.

<FIG> shows an arrangement in accordance with the invention. In this arrangement Alice <NUM> is optically connected to a splitter <NUM>. The splitter <NUM> splits the photon stream it receives from Alice <NUM> with a <NUM>/<NUM> split ratio, such that <NUM>% of the incoming photons are directed towards splitter <NUM> and the remaining <NUM>% of the incoming photons are directed towards an optical switch <NUM>.

The splitter <NUM> is optically connected to four optical combiners <NUM>-<NUM> by respective optical fibres. The splitter <NUM> is configured to split the incoming photon stream it receives from the splitter <NUM> equally into four streams and output each of the four streams towards a respective combiner <NUM>-<NUM>. As the splitter <NUM> receives <NUM>/<NUM> of the photons from splitter <NUM> and splitter <NUM> provides each combiner <NUM>-<NUM> with <NUM>/<NUM> of the photons received from splitter <NUM>, splitter <NUM> outputs to each combiner <NUM>-<NUM>, <NUM>/<NUM> x <NUM>/<NUM> = <NUM>/<NUM> of the photon stream received by splitter <NUM> from Alice <NUM>.

The switch <NUM> is also optically connected to the four optical combiners <NUM>-<NUM> by respective optical fibres. At any one time, the switch <NUM> connects the transmitter to only one of the combiners <NUM>-<NUM>. The switching position of switch <NUM> can be adjusted to switch to a different one of the combiners <NUM>-<NUM>. As the switch <NUM> receives <NUM>/<NUM> of the photons from splitter <NUM> and switch <NUM> provides each combiner <NUM>-<NUM> with all of the photons received from splitter <NUM>, switch <NUM> outputs to the combiner <NUM>-<NUM> it is connected to, <NUM>/<NUM> of the photon stream received by splitter <NUM> from Alice <NUM>.

Each of the combiners <NUM>-<NUM> combines, using a <NUM>/<NUM> combining ratio, the photon streams it receives. If the position of the switch <NUM> is such that, say, switch <NUM> is connected to combiner <NUM>, combiner <NUM> combines the input streams it receives from splitter <NUM> and switch <NUM> and outputs the combined stream towards Bob <NUM>. Half of the power of each photon stream arriving at each combiner <NUM>-<NUM> is lost in the combining process. Therefore the combined photon stream output by combiner <NUM> comprises:.

This makes a total of <NUM> of the power of the stream received by splitter <NUM>. This power is output by combiner <NUM> to Bob <NUM>. Each of Bobs <NUM>, <NUM> and <NUM> are similarly connected to the outputs of combiners <NUM>, <NUM> and <NUM> respectively. As the switch <NUM> is switched to combiner <NUM>, that means that combiners <NUM>, <NUM> and <NUM> receive zero power from switch <NUM>. Like combiner <NUM>, each of combiners <NUM>-<NUM> receive, from splitter <NUM>, <NUM>/<NUM> of the power of the stream received by splitter <NUM>. Therefore each of combiners <NUM>-<NUM> output to their respective Bobs <NUM>, <NUM> and <NUM>, <NUM>/<NUM> (i.e. <NUM>) of the power of the stream received by splitter <NUM>.

For analogous reasons, when switch <NUM> is switched to connect to each of the other combiners <NUM>-<NUM>, its respective Bob <NUM>-<NUM> also receives <NUM> of the power of the stream received by splitter <NUM> and the remaining combiners each receive <NUM> of the power of the stream received by splitter <NUM>.

This arrangement provides a continuous photon stream to each Bob <NUM>-<NUM>. This means that the sessions between Alice <NUM> and each of the Bobs <NUM>-<NUM> will remain continuously active, avoiding the need for time consuming re-initiation procedures. Furthermore, if one of the Bobs is located remotely from Alice <NUM> or otherwise requires a larger photon stream than the other Bobs (e.g. because it needs to generate more keys), the switch can be adjusted to connect to that particular Bob while the extra demand persists.

Claim 1:
A system for performing Quantum Key Distribution, the system comprising:
a transmitter (<NUM>) adapted to transmit a plurality of optical pulses;
a first receiver (<NUM>);
a second receiver (<NUM>);
an optical switch (<NUM>) having an input which is in optical communication with the transmitter (<NUM>), the optical switch (<NUM>) being switchable between a first switching position in which the input is optically connected to the first receiver (<NUM>) such that, in use, the first receiver (<NUM>) is adapted to measure a quantum bit-value that has been encoded onto the plurality of optical pulses, and a second switching position in which the input is optically connected to the second receiver (<NUM>) such that, in use, the second receiver (<NUM>) is adapted to measure a quantum bit-value that has been encoded onto the plurality of optical pulses,
characterised in that the system further comprises a guide (<NUM>) for guiding a portion of the plurality of optical pulses to the first receiver (<NUM>) via an optical path that bypasses the optical switch (<NUM>) such that, in use, the first receiver (<NUM>) is adapted to measure a quantum bit-value that has been encoded onto the portion of the plurality of optical pulses.