Systems and methods for performing quantum key distribution
By introducing optical switches and guides in the quantum key distribution system, some Bobs cannot meet the key generation requirements are solved, and high-power pulse flow supply to the receiver is achieved, avoiding system disconnection and restart.
Patent Information
- Application Number
- CN202080019394.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Priority Date
- 2019-03-22
- Filing Date
- 2020-03-11
- Publication Date
- 2025-05-02
- Estimated Expiration
- 2040-03-11
AI Technical Summary
In existing quantum key distribution systems, some Bobs may not receive a high enough optical pulse rate to meet their key generation needs, especially when the Bob is further away from the divider or there are a large number of Bobs.
A system is designed including a transmitter, a first receiver, a second receiver, an optical switch and a guide. The optical switch is able to switch between a first switching position and a second switching position, and is connected to the first receiver and the second receiver, respectively. The guide bypasses a portion of the multiple optical pulses bypassing the optical switch and directs directly to the first receiver to ensure the supply of high power pulse flow.
The system is able to provide a high power pulse flow to the receiver through an optical switch when the receiver key generation demand is high, while continuously providing a sufficiently high power pulse flow to other receivers to avoid disconnection and time-consuming restarting processes.
Smart Images
Figure CN113544983B_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates to the field of communications, and more particularly to a system and method for transmitting optical pulses from a transmitter to a first receiver and a second receiver for quantum key distribution. Background Art
[0002] QKD (Quantum Key Distribution) is a known high-security communication method that enables two parties to generate a shared key. In the broadest sense, a QKD network consists of a sending unit (called Alice) that generates a single-photon pulse, encodes it with a randomly selected 0 or 1 value, and applies modulation to it. In a known exemplary system, this is done by polarizing the pulse using a randomly selected polarization base. The pulse is then sent to a receiving unit (called Bob) that attempts to measure the encoded value. In the example system, this is done by passing the pulse through a polarizer with a randomly selected polarization base. The process is repeated for multiple pulses. Alice and Bob then exchange information about the pulse, and in doing so, obtain a shared key.
[0003] QKD devices are expensive, and one way to reduce costs is for a single Alice to send pulses to multiple Bobs. Such devices have branched optical paths so that each Bob receives a portion of the pulses. Optical splitters have been used to provide branched optical paths. The problem with splitting the signal is that some Bobs may not receive a high enough pulse rate to meet their key generation needs. This is especially true if, for example, one Bob needs to generate a larger number of keys than the other Bobs. This may also be the case if one Bob is farther from the splitter than the other Bobs (because the pulses going to it will suffer more attenuation), or if there are a large number of Bobs.
[0004] It is desirable to overcome or mitigate some and / or all of the above-mentioned and / or other disadvantages of the prior art. Summary of the invention
[0005] According to a first aspect of the present invention, a system for performing quantum key distribution is provided, the system comprising:
[0006] A transmitter, the transmitter being configured to transmit a plurality of optical pulses;
[0007] a first receiver;
[0008] a second receiver;
[0009] an optical switch having an input end in optical communication with the transmitter, the optical switch being switchable between a first switching position in which the input end is optically connected to the first receiver and a second switching position in which the input end is optically connected to the second receiver,
[0010] A guide is configured to guide a portion of the plurality of optical pulses to the first receiver via an optical path that bypasses the optical switch.
[0011] Embodiments of the invention enable construction of a system in which a high-power pulse stream can be provided to a receiver via a switch when its key generation needs are high, while also continuously providing other receivers with sufficiently high-power pulse streams to avoid disconnection and the resulting time-consuming restart process.
[0012] According to a second aspect of the present invention, there is provided a method for performing quantum key distribution between a transmitter and a first receiver and a second receiver, the method comprising the following steps:
[0013] transmitting a plurality of light pulses at the transmitter;
[0014] directing the plurality of light pulses to an input of an optical switch, the optical switch being switchable between a first switching position in which the input is optically connected to the first receiver and a second switching position in which the input is optically connected to the second receiver;
[0015] A portion of the plurality of pulses is directed from the transmitter to the first receiver via an optical communication path that bypasses the optical switch.
[0016] Modulation may be applied to the pulse prior to transmission. The step of applying modulation to the pulse may include polarizing the pulse using a randomly selected polarization basis. Alternatively, the step of applying modulation to the pulse includes modulating the phase or position of the pulse. After being encoded and modulated, the pulse may be transmitted according to the QKD principle.
[0017] The value encoded on the pulse and / or the modulation basis applied to the pulse and / or the transmission time of the pulse can be recorded. The modulation basis can be a polarization basis. In addition, the transmitter can be connected to communicate with each receiver through a corresponding non-quantum channel. The non-quantum channel can be a metal wire, or an optical fiber or free space. The transmitter can use the corresponding non-quantum channel to send some or all of the recorded information to one or more receivers.
[0018] The value (i.e., 1 or 0) encoded onto the pulse can be measured at each receiver using a randomly selected modulation basis, which can be a randomly selected polarization basis. For each pulse received, the measured value and / or the polarization basis used for the measurement and / or the arrival time of the pulse can be recorded. Each receiver can send the measured value and / or arrival time of the pulse to the transmitter using its corresponding non-quantum channel.
[0019] The optical pulse may be a single photon pulse. The single photon pulse may be generated by a single photon generator.
[0020] In a first switching position, the input can only be optically connected to the first receiver. In a second switching position, the input can only be optically connected to the second receiver. The system can include one or more additional receivers. The input of the optical switch can be in optical communication with the transmitter through a conduit or through the atmosphere. The conduit can be an optical fiber. The switch can have a first output and a second output. When the switch is in the first switching position, the input can be optically connected to the first output. The first output can be optically connected to the first receiver. When the switch is in the second switching position, the input can be in optical communication with the second output. The second output can be optically connected to the second receiver.
[0021] The switch may have one or more additional switching positions and may have one or more corresponding additional outputs. The switch may be configured such that, in each of the one or more additional switching positions, the switch input is optically connected to the corresponding additional output. Each of the one or more corresponding additional outputs may be optically connected to a corresponding receiver of the one or more additional receivers.
[0022] The system may include a controller for controlling the optical switch. The controller may be used to control a switch position of the switch. The controller may be used to send a control signal to the switch to control the switch position of the switch. The controller may command the switch to move to a switch position corresponding to a receiver with a highest optical pulse rate requirement. The first receiver and / or the second receiver and / or the further receiver may be used to send an indication of the optical pulse ratio required by the one or more receivers to the controller.
[0023] The director may include a divider. The system may be arranged so that the divider receives an input from the transmitter. In use, the divider may receive the transmitted plurality of optical pulses via an input terminal and divide the plurality of optical pulses so that a portion of the plurality of optical pulses is directed to a different path from the remainder of the plurality of optical pulses. The portion of the plurality of optical pulses directed to the different path may constitute less than 50% of the pulse stream, preferably less than 25% of the pulse stream, and most preferably 10% of the pulse stream. The divider may have a first output terminal for outputting a portion of the plurality of optical pulses, and may have a second output terminal for outputting the remainder of the plurality of optical pulses. The second output terminal may be optically connected to the input terminal of the switch.
[0024] Although in the preferred embodiment the director comprises a divider, those skilled in the art will appreciate that a director may be provided that does not include a divider but provides substantially the same functionality. A possible example of this is a device that provides a plurality of pulses to a first output for a short period of time, then provides a plurality of pulses to a second output for a short period of time, and repeats this cycle over a longer period of time. Over time, such a device will direct a portion of the pulse stream to the respective output.
[0025] The guide may guide the optical pulses to a second receiver. A second divider may be provided, the second divider having an input end optically connected to the first output end of the first divider. The second divider may have a first output end optically connected to the first receiver. The second divider may also include a second output end optically connected to the second receiver. In use, a portion of the plurality of optical pulses output from the first output end of the first divider may be input at the input end of the second divider and may be divided by the second divider into a first component output to the first receiver and a second component output to the second receiver.
[0026] In embodiments where one or more additional receivers are present, the director may direct the optical pulses to the one or more additional receivers. In these embodiments, the second divider may have one or more additional outputs, each output being optically connected to one of the one or more additional receivers. In these embodiments, a portion of the plurality of optical pulses input to the second divider is divided between a first output, a second, and one or more additional outputs of the second divider. The power of the components into which the portion of the plurality of optical pulses is divided by the second divider may be equal.
[0027] The pulse stream output by the switch can be combined with the pulse stream output by the second divider before reaching the corresponding receiver. The system can also include a pulse stream combiner. The pulse stream combiner can be used to combine two input pulse streams using a 50:50 combination ratio. The pulse stream combiner can combine the pulse output at the first output of the switch with the pulse output at the first output of the second divider. The combiner can output the combined pulse stream to the first receiver. There can be a second pulse stream combiner. The pulse stream combiner can combine the pulse output at the second output of the switch with the pulse output at the second output of the second divider. The combiner can output the combined pulse stream to the second receiver. There can be one or more additional pulse stream combiners. One or more additional pulse stream combiners can combine the pulse output at a corresponding one of the one or more additional outputs of the switch with the pulse output at a corresponding one of the one or more additional outputs of the second divider. The combiner can output the combined pulse stream to a corresponding one of the one or more additional receivers. BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Specific embodiments of the present invention will now be described for illustration only and with reference to the accompanying drawings, in which:
[0029] Figure 1 is a schematic diagram of a basic QKD architecture including a single Alice and a single Bob;
[0030] Figure 2 is a schematic diagram of a known QKD architecture including a single Alice and multiple Bobs;
[0031] Figure 3 is a schematic diagram of an embodiment of the present invention. DETAILED DESCRIPTION
[0032] The present invention relates to improvements in quantum key distribution (QKD).QKD is an encryption method that involves distributing an encryption key from a first quantum node (called Alice) to a second quantum node (called Bob). Figure 1A schematic diagram of a simplified QKD arrangement is shown, wherein Alice is shown as 100 and Bob is shown as 200. Although multiple QKD protocols are used, the present description relates to the commonly used BB84 protocol. In particular, the present description relates to the following type of BB84: in which modulation is applied to pulses using polarization. According to the protocol, Alice 100 randomly generates a bit (0 or 1) and also randomly selects one of two polarization bases, a rectilinear basis and a diagonal basis. Alice 100 then sends a photon encoded with the selected bit and the selected polarization basis to Bob 200 via a quantum channel 300 such as an optical fiber. Bob 200 randomly selects one of the two polarization bases and measures the photon using the base he selected. If Bob 200 uses the same base as Alice 100, the bit value measured by Bob 200 will match the bit value applied to the photon by Alice 100. After repeating the process with a large number of photons, Alice 100 and Bob 200 perform a key agreement phase. Specifically, Alice 100 informs Bob 200 through a non-quantum communication channel 400 (e.g., a copper cable) which of the two bases is applied to the corresponding photon in Alice 100, and the time when Alice 100 sends the corresponding photon. Then Bob 200 informs Alice 100 which of the two bases Bob 200 uses when measuring each photon and the time when Bob 200 receives each photon. Then Alice 100 and Bob 200 discard the bit values that Alice 100 and Bob 200 use different bases, and retain the remaining bit values. The remaining bit values constitute a secret key that both Alice 100 and Bob 200 have, and can be used by Alice 100 and Bob 200 to encrypt messages transmitted between the two through the non-quantum channel 400.
[0033] Figure 2 A known QKD architecture is shown. Specifically, Alice 2 is connected to four different Bobs 3, 4, 5, 6. A splitter 7 is provided in the optical path from Alice 2 to the four Bobs 3, 4, 5, 6. Alice 2 is connected to the splitter 7 via an optical fiber 10. The four Bobs 3, 4, 5, 6 are each connected to the splitter 7 via a corresponding optical fiber 50. Bobs 3, 4, 5, 6 are each also connected to Alice 2 via their own conventional (i.e. non-quantum) channels (not shown).
[0034] In use, Alice 2 sends a series of photons to the splitter 7, each photon having been encoded with a random bit and a random polarization basis as described above. Each photon passes through the splitter 7 and reaches one of Bob 3, 4, 5, 6. The splitter 7 randomly passes each photon to one of Bob 3, 4, 5, 6. Therefore, on average, 25% is directed to each of the four Bobs. Each Bob measures the photons using a randomly selected polarization basis, and when it has received enough photons to enable a secret shared key to be established with Alice 2, it does so using the key agreement phase described above. The key agreement phase involves Alice 2 exchanging its respective polarization basis lists with each of Bob 3, 4, 5, 6, and is conducted over a conventional channel. Alice 2 and each Bob use the lists to establish a shared secret key, which can be used to encrypt communications between them.
[0035] As in Figure 2 As can be seen in FIG, Bob 6 is farther from divider 7 than the other Bobs 3, 4, and 5. In the case where the photons sent to Bob 6 travel farther than the photons sent to the other Bobs 3, 4, and 5, the photons sent to Bob 6 suffer greater attenuation. Therefore, the key exchange rate between Alice and Bob 6 will be lower than the key exchange rate between Alice and the other Bobs 3, 4, and 5. This is undesirable because it slows down the establishment of a secure communication link between Bob 6 and Alice 2. In addition, if the photon reception rate at Bob 6 drops below the threshold rate, the connection must be restarted, which is time consuming.
[0036] Figure 2 Another disadvantage of the arrangement is that one of the Bobs, for example Bob 4, needs to contact Alice to establish more keys in a given period of time than the other Bobs. Since the divider divides the photons approximately equally, it takes a long time to establish all the keys at Bob 4, while the other Bobs are sent photons unnecessarily after they have finished establishing their keys.
[0037] Figure 3 An arrangement according to the invention is shown. In the arrangement, Alice 2 is optically connected to a divider 12. The divider 12 divides the photon stream it receives from Alice 2 with a division ratio of 10 / 90, so that 10% of the incoming photons are directed to the divider 13, while the remaining 90% of the incoming photons are directed to the optical switch 14.
[0038] The divider 13 is optically connected to the four optical combiners 23-26 through respective optical fibers. The divider 13 is configured to equally divide the input photon stream received from the divider 12 into four streams and output each of the four streams to the corresponding combiner 23-26. When the divider 13 receives 1 / 10 of the photons from the divider 12 and the divider 13 provides 1 / 4 of the photons received from the divider 12 to each combiner 23-26, the divider 13 outputs 1 / 10×1 / 4=1 / 40 of the photon stream received by the divider 12 from Alice 2 to each combiner 23-26.
[0039] The switch 14 is also optically connected to the four optical combiners 23-26 through respective optical fibers. At any one time, the switch 14 connects the transmitter to only one of the combiners 23-26. The switching position of the switch 14 can be adjusted to switch to a different one of the combiners 23-26. When the switch 14 receives 9 / 10 of the photons from the divider 12 and the switch 14 provides all the photons received from the divider 12 to each combiner 23-26, the switch 14 outputs 9 / 10 of the photon stream received by the divider 12 from Alice 2 to the combiner 23-26 to which it is connected.
[0040] Each combiner 23-26 combines the photon streams it receives using a 50 / 50 combination ratio. If the position of switch 14 is assumed to be connected to combiner 23, then combiner 23 combines the input streams it receives from divider 13 and switch 14 and outputs the combined stream to Bob 3. Half of the power of each photon stream reaching each combiner 23-26 is lost in the combination process. The combined photon stream output by combiner 23 therefore includes:
[0041] (i) from divider 13, 1 / 80 of the power of the photon stream received by divider 12; and
[0042] (ii) From the switch 14 , 9 / 20 of the power of the photon stream received by the divider 12 .
[0043] This results in 0.4625 of the total power of the stream received by divider 12. This power is output by combiner 23 to Bob 3. Each of Bob 4, 5 and 6 is similarly connected to the output of combiners 24, 25 and 26, respectively. When switch 14 switches to combiner 23, this means that combiners 24, 25 and 26 receive zero power from switch 14. Similar to combiner 23, each of combiners 24-26 receives 1 / 40 of the power of the stream received by divider 12 from divider 13. Therefore, each combiner 24-26 outputs 1 / 80 (i.e. 0.0125) of the power of the stream received by divider 12 to its respective Bob 4, 5 and 6.
[0044] For similar reasons, when switch 14 is switched to connect to each of the other combiners 24-26, its corresponding Bob 4-6 also receives 0.4625 of the power of the stream received by divider 12, and the remaining combiners each receive 0.0125 of the power of the stream received by divider 12.
[0045] This arrangement provides a continuous stream of photons to each of Bob 3-6. This means that the session between Alice 2 and each of Bob 3-6 will remain continuously active, avoiding the need for a time-consuming restart process. In addition, if one of the multiple Bobs is located far away from Alice 2 or requires a larger stream of photons than the other Bobs (for example, because it needs to generate more keys), the switch can be adjusted to connect to that specific Bob while the additional demand persists.
[0046] The position of switch 14 is electronically controlled by controller 60. Controller 60 is connected to each Bob 3-6 and switch 14. Each Bob 3-6 sends an indication to controller 60 whether the photon power currently received by it is sufficient. If one of the multiple Bobs sends an indication to controller 60 that the power is insufficient, controller 60 electronically adjusts switch 14 to connect it to the combiner associated with that Bob. Controller 60 continuously monitors the suitability of the photon power source for the Bob in Bob 3-6 and adjusts the position of switch 14 accordingly.
Claims
1. A system for performing quantum key distribution, the system comprising: A transmitter, the transmitter being configured to transmit a plurality of optical pulses; The first quantum receiver; a second quantum receiver; an optical switch having a first input terminal in optical communication with the transmitter, the optical switch being switchable between a first switching position in which the first input terminal is optically connected to the first quantum receiver and a second switching position in which the first input terminal is optically connected to the second quantum receiver, and A guide is used to guide a portion of the plurality of light pulses to the first quantum receiver via an optical path that bypasses the optical switch.
2. The system according to claim 1, wherein: The system comprises one or more further quantum receivers.
3. The system according to claim 2, wherein: The optical switch has one or more output terminals, each of which is optically connected to a corresponding one of the one or more additional quantum receivers.
4. The system according to claim 1, wherein: The system includes a controller for controlling the optical switch.
5. The system according to claim 4, wherein: The controller is used to control the switching position of the optical switch.
6. The system according to claim 5, wherein: The controller is used to send a control signal to the optical switch to control the switching position of the optical switch.
7. The system according to claim 1, wherein: The guide includes a first light splitter.
8. The system according to claim 7, wherein: In use, the first optical splitter receives the transmitted plurality of optical pulses through the second input terminal, and splits the plurality of optical pulses so that a portion of the plurality of optical pulses and the remaining optical pulses of the plurality of optical pulses are directed onto different paths.
9. The system according to claim 8, wherein: The portion of the plurality of light pulses directed onto different paths constitutes less than 25% of the plurality of light pulses.
10. The system according to claim 9, wherein: The portion of the plurality of light pulses directed onto different paths constitutes 10% of the plurality of light pulses.
11. The system according to claim 1, wherein: The director directs one or more of the plurality of light pulses to the second quantum receiver.
12. The system according to claim 7, wherein: The guide includes a second light splitter having a third input end optically connected to the first output end of the first light splitter.
13. The system according to claim 12, wherein: The first pulse stream output by the optical switch is combined with the second pulse stream output by the second optical splitter before reaching the corresponding quantum receiver.
14. The system according to claim 13, wherein: The combining is performed by a pulse stream combiner, and the pulse stream combiner is different from the first quantum receiver and the second quantum receiver.
15. A method for performing quantum key distribution between a transmitter and a first quantum receiver and a second quantum receiver, the method comprising the following steps: transmitting a plurality of light pulses at the transmitter; directing the plurality of light pulses to an input of an optical switch, the optical switch being switchable between a first switching position, in which the input is optically connected to the first quantum receiver, and a second switching position, in which the input is optically connected to the second quantum receiver; as well as A portion of the plurality of light pulses is directed from the transmitter to the first quantum receiver via an optical communication path that bypasses the optical switch.
16. The method according to claim 15, wherein: Directing a portion of the plurality of light pulses from the transmitter to the first quantum receiver via an optical communication path that bypasses the optical switch includes splitting the plurality of light pulses.
Citation Information
Patent Citations
Network having quantum key distribution
US20100299526A1