Quantum bit decoding device, system and method

Abstract

A qubit decoding device (200) is configured to receive a photon having a qubit encoded with a property of the photon, the qubit decoding device (200) comprising a demodulation device (210). The demodulation device (210) comprises an optical modulator (212) configurable to randomly apply one of a plurality of modulation values ​​for decoding the qubit to the property of the photon, and an optical router (214, 414) for routing the photon according to the qubit. The qubit decoding device (200) further comprises an optical delay device (220), the optical delay device (220) comprising an optical combiner (222), a first optical path (224) from the optical router to the optical combiner, and a second optical path (226) from the optical router to the optical combiner. The second optical path (226) has an optical path length different from that of the first optical path to introduce a time delay between the optical paths. The qubit decoding device (200) further includes a detection device (230) configured to detect the photon and operable to determine whether the detected photon is delayed relative to a reference time and to determine the qubit based on the determined time delay.

Description

Technical Field

[0001] The present invention relates to a quantum bit decoding device and to a quantum bit decoding system comprising the quantum bit decoding device. The present invention further relates to a quantum key distribution system and to a communication network node. The present invention further relates to a method for decoding a quantum bit encoded on photons. Background Art

[0002] Quantum communication systems exploit the possibility of transmitting information encoded in quantum states, which are prepared in such a way that an eavesdropper between the two communicating partners cannot avoid introducing detectable interference. In optical communication, quantum information is encoded in the physical properties of photons, such as polarization, phase, or spin.

[0003] Quantum key distribution (QKD) offers a solution to the key distribution problem in symmetric cryptographic systems. In theory, quantum encryption should be applied to the entire message being transmitted using one-time pad encryption. However, this would unacceptably compromise the capacity and latency of the communication channel, as feasible QKD systems can only gradually reach a few Mbit / s and require the processing time required for the sender and receiver to agree on the final key without errors. In practice, QKD is used only to generate and distribute keys, not to transmit message data. The keys are then used together with classical encryption algorithms to encrypt and decrypt messages transmitted over classical high-capacity communication channels.

[0004] In QKD based on the BB84 protocol, as described, for example, by A. Ruiz-Alba et al., “Practical QuantumKey Distribution based on the BB84 protocol” (Waves, 2011, pp. 4-14), the sender Alice generates a random bit, i.e., a “0” or a “1,” and encodes it in one of two different bases using appropriately chosen physical properties of photons. The first basis is then used to encode the “0” bit, and the other basis is used to encode the “1” bit.

[0005] Since the receiver, Bob, is unaware of Alice's choice of basis, he measures the basis of the incoming photons by randomly selecting one of the two possible bases. If he uses the same basis that Alice used, he will deterministically measure the correct bit value. Conversely, if he chooses the wrong basis, his measurement will be a random projection onto the possible values ​​of the encoded basis, which will only give the correct result with a probability of 50%. After having exchanged a long string of photons, Alice and Bob compare the bases they each used for encoding and measurement in order to communicate over the "classical" channel. They retain only the random bits generated and detected using the matching basis, and these bits are said to constitute the "filtered key." In an ideal system without noise, imperfections, and interference, the filtered keys are identical and can be used as private keys.

[0006] Two common implementations of the BB84 protocol use the polarization or phase properties of photons as a basis. For the polarization encoding case, Figure 1 As shown in (a), the sender Alice encodes on two different polarization bases selected by a polarizer, which are rotated 45° relative to each other, namely 0° and 90° for the straight line and 45° and 135° for the diagonal line. At the receiver Bob, the polarization beam splitter PBS transforms the polarization encoding of the photon into spatial decoding, where the photon is received by one of two single photon avalanche photodetectors SPD. For the phase encoding scheme, Figure 1 As shown in (b), and reported for example by Yan Hui et al., “Efficient Phase-Encoding Quantum Key Generation with Narrow-Band SinglePhotons” (Chinese Physics Letters, Vol. 28, No. 7, May 20, 2011), the transmitter uses a Mach-Zehnder interferometer (MZI) to introduce four different phase shifts. (For example, ). The first two phase shifts encode a '0' bit, while the second two phase shifts encode a '1' bit. The receiver then uses a second MZI to randomly introduce two phase shifts (For example, ) one. Here, considering the differential phase Spatial decoding is performed in the case of .

[0007] QKD can also be realized based on photon spin, as reported, for example, by Giuseppe Vallone et al. in “Free-space quantum key distribution by rotation-invariant twisted photons” (Physics Review Letters, Vol. 113, 060503 (2014)).

[0008] In practical implementations of any discrete-variable QKD scheme, a limiting factor that significantly restricts the technology's adoption is the high cost of the optical receiver devices, which represent the most critical component of a practical QKD system. In particular, any implementation of a QKD protocol requires at least two SPDs. Furthermore, the greater the number of qubits encoded per photon, the higher the SPD number, which grows exponentially with the number of qubits. This is why QDK protocols employing multiple qubits per photon have not been extensively studied or proposed. Summary of the Invention

[0009] One object is to provide an improved qubit decoding apparatus. A further object is to provide an improved qubit decoding system. A further object is to provide an improved QKD system. A further object is to provide an improved qubit decoding method.

[0010] One aspect of the present invention provides a qubit decoding device configured to receive a photon having a quantum bit (qubit) encoded with a property of the photon. The qubit decoding device includes a demodulation device, an optical delay device, and a detection device. The demodulation device includes an optical modulator and an optical router. The optical modulator is configurable to randomly apply one of a plurality of modulation values ​​to the property of the photon; the modulation value is configured to decode the qubit. The optical router is disposed after the optical modulator and is configured to route the photon according to the qubit. The optical delay device includes an optical combiner, a first optical path from a first output of the optical router to a first input of the optical combiner, and a second optical path from a second output of the optical router to a second input of the optical combiner. The second optical path has an optical path length different from that of the first optical path to introduce a time delay between the first optical path and the second optical path. The detection device is configured to detect the photon received from the optical combiner. The detection device is operable to determine whether the detected photon is delayed relative to a reference time and to determine the qubit based on the determined time delay.

[0011] The qubit decoding device can achieve a reduction in the complexity of the decoder device required to decode the qubit, so that a temporal decoder can be used instead of a decoder such as in Figure 1This may enable lower cost qubit decoding devices relative to prior art decoding schemes, and may enable qubit decoding devices to be provided at a cost comparable to standard (non-quantum) decoders.

[0012] In an embodiment, the detection apparatus is further operable to determine a time delay applied to the detected photon relative to a reference time and to determine the qubit based on the determined time delay.

[0013] In an embodiment, a detection device includes an optical receiver configured to detect photons received from an optical combiner and a processing circuit configured to determine whether the detected photons are delayed relative to a reference time and to determine a qubit based on the determined time delay.

[0014] The qubit decoding device can achieve a reduction in the number of optical receivers (such as single photon avalanche photodetectors) required to decode qubits. Figure 1 The BB84 protocol decoder shown in FIG, the qubit decoding device of the present invention can achieve a reduction from two photodetectors to a single photodetector.

[0015] One aspect of the present invention provides a qubit decoding system configured to receive a photon having multiple quantum bits (qubits) each encoded with multiple properties of the photon. The qubit decoding system includes a first decoding stage and an output decoding stage. The first decoding stage includes a first optical modulator and a first optical router. The first optical modulator is configured to randomly apply one of a plurality of first modulation values ​​to a first property of the plurality of properties of the photon; the first modulation value is configured to decode a first qubit of the plurality of qubits. The first optical router is configured to route the photon based on the first qubit of the plurality of qubits. The output decoding stage includes a plurality of qubit decoding devices each configured to receive a photon from the first optical router. The qubit decoding device includes a demodulation device, an optical delay device, and a detection device. The demodulation device includes an optical modulator and an optical router. The optical modulator is configurable to randomly apply one of a plurality of modulation values ​​to the properties of the photon; the modulation value is configured to decode the qubit. The optical router is disposed after the optical modulator and is configured to route the photon based on the qubit. The optical delay device includes an optical combiner, a first optical path from a first output of the optical router to a first input of the optical combiner, and a second optical path from a second output of the optical router to a second input of the optical combiner. The second optical path has an optical path length different from that of the first optical path to introduce a time delay between the first optical path and the second optical path. A detection device is configured to detect photons received from the optical combiner. The detection device is operable to determine whether the detected photons are delayed relative to a reference time and to determine a qubit based on the determined time delay. A qubit decoding device is configured to decode a second qubit encoded with a second attribute of the plurality of properties of the photon. The first attribute of the photon and the second attribute of the photon are different, non-commutative attributes of the quantum state of the photon.

[0016] The qubit decoding device can achieve a reduction in the complexity of the decoder device required to decode multiple qubits on a single photon, so that a temporal decoder stage can be used to replace the independent spatial decoder of the prior art. The qubit decoding system can achieve a reduction in the number of optical receivers (such as single-photon avalanche photodetectors) required to decode multiple qubits. This can achieve a lower cost multi-qubit decoding system relative to the decoding schemes of the prior art, and can achieve the provision of the qubit decoding system at a cost comparable to that of standard decoding schemes. Therefore, the qubit decoding system can provide a solution to the penalty cost associated with the increase in photodetectors required by the decoder schemes of the prior art for decoding multiple qubits per photon.

[0017] In one embodiment, the system further includes a third decoding stage disposed between the first decoding stage and the output decoding stage. The third decoding stage includes a plurality of third optical modulators. The third optical modulator is configured to receive a photon from the first decoding stage depending on the first qubit among the plurality of qubits, and is configured to output the photon to the output decoding stage. The third optical modulator is configured to randomly apply one of a plurality of third modulation values ​​to a third attribute among the plurality of attributes of the photon; a third modulation state is configured to decode the third qubit among the plurality of qubits. The third attribute is a third different, non-commutative attribute of the quantum state of the photon.

[0018] In one embodiment, the non-commutative properties of the photon quantum state include polarization, phase and spin.

[0019] In one embodiment, the plurality of qubit decoding devices of the output decoding stage have different respective time delays between respective first and second optical paths. The plurality of qubit decoding devices have a common detection device. The common detection device is configured to detect a photon received from an optical combiner of any qubit decoding device in the plurality of qubit decoding devices. The common detection device is operable to determine whether the detected photon is delayed by any of the respective time delays relative to a reference time and to determine the qubit based on the determined time delay.

[0020] The qubit decoding system enables a reduction in the number of optical receivers (such as single-photon avalanche photodetectors) required to decode multiple qubits.

[0021] In one embodiment, the common detection device includes an optical receiver configured to detect photons received from the optical combiner, and a processing circuit configured to determine whether the detected photons are delayed by any of the corresponding time delays relative to a reference time and determine the qubit based on the determined time delay.

[0022] The qubit decoding system enables a single optical receiver to decode multiple qubits. This can enable a significant reduction in the complexity and cost of qubit decoding systems for decoding multiple qubits. By employing temporal decoding rather than independent spatial decoding, the qubit decoding system can enable a system for decoding multiple qubits. m The number of qubit light receivers is increased from 2 m to 2 k (where generally 0 ≤ k ≤ m).

[0023] The corresponding embodiments and advantages also apply to the communication network node, QKD system and method described below.

[0024] One aspect of the present invention provides a communication network node including a qubit decoding device. The qubit decoding device is configured to receive a photon having a quantum bit (qubit) encoded with a property of the photon. The qubit decoding device includes a demodulation device, an optical delay device, and a detection device. The demodulation device includes an optical modulator and an optical router. The optical modulator is configured to randomly apply one of a plurality of modulation values ​​to the property of the photon; the modulation value is configured to decode the qubit. The optical router is disposed after the optical modulator and is configured to route the photon according to the qubit. The optical delay device includes an optical combiner, a first optical path from a first output of the optical router to a first input of the optical combiner, and a second optical path from a second output of the optical router to a second input of the optical combiner. The second optical path has an optical path length different from that of the first optical path to introduce a time delay between the first optical path and the second optical path. The detection device is configured to detect the photon received from the optical combiner. The detection device is operable to determine whether the detected photon is delayed relative to a reference time and to determine the qubit based on the determined time delay.

[0025] One aspect of the present invention provides a communication network node including a qubit decoding system. The qubit decoding system is configured to receive a photon having a plurality of quantum bits (qubits) each encoded with a plurality of properties of the photon. The qubit decoding system includes a first decoding stage and an output decoding stage. The first decoding stage includes a first optical modulator and a first optical router. The first optical modulator is configured to randomly apply one of a plurality of first modulation values ​​to a first property of the plurality of properties of the photon; the first modulation value is configured to decode a first qubit of the plurality of qubits. The first optical router is configured to route the photon based on the first qubit of the plurality of qubits. The output decoding stage includes a plurality of qubit decoding devices each configured to receive a photon from the first optical router. The qubit decoding device includes a demodulation device, an optical delay device, and a detection device. The demodulation device includes an optical modulator and an optical router. The optical modulator is configurable to randomly apply one of a plurality of modulation values ​​to the properties of the photon; the modulation value is configured to decode the qubit. The optical router is disposed after the optical modulator and is configured to route the photon based on the qubit. The optical delay device includes an optical combiner, a first optical path from a first output of the optical router to a first input of the optical combiner, and a second optical path from a second output of the optical router to a second input of the optical combiner. The second optical path has an optical path length different from that of the first optical path to introduce a time delay between the first optical path and the second optical path. A detection device is configured to detect photons received from the optical combiner. The detection device is operable to determine whether the detected photons are delayed relative to a reference time and to determine a qubit based on the determined time delay. A qubit decoding device is configured to decode a second qubit encoded with a second attribute of the plurality of properties of the photon. The first attribute of the photon and the second attribute of the photon are different, non-commutative attributes of the quantum state of the photon.

[0026] One aspect of the present invention provides a quantum key distribution system, comprising a qubit encoding device and one of a qubit decoding device and a qubit decoding system. The qubit encoding device is configured to encode at least one qubit using at least one corresponding property of a photon. The qubit decoding device is configured to receive a photon having a quantum bit (qubit) encoded using the photon's property. The qubit decoding device includes a demodulation device, an optical delay device, and a detection device. The demodulation device includes an optical modulator and an optical router. The optical modulator is configured to randomly apply one of a plurality of modulation values ​​to the photon's property; the modulation value is configured to decode the qubit. The optical router is disposed after the optical modulator and is configured to route the photons according to the qubit. The optical delay device includes an optical combiner, a first optical path from a first output of the optical router to a first input of the optical combiner, and a second optical path from a second output of the optical router to a second input of the optical combiner. The second optical path has an optical path length different from that of the first optical path to introduce a time delay between the first and second optical paths. The detection device is configured to detect the photons received from the optical combiner. The detection device is operable to determine whether the detected photon is delayed relative to a reference time and to determine a qubit based on the determined time delay. A qubit decoding system is configured to receive a photon having a plurality of quantum bits (qubits) each encoded with a plurality of properties of the photon. The qubit decoding system includes a first decoding stage and an output decoding stage. The first decoding stage includes a first optical modulator and a first optical router. The first optical modulator is configured to randomly apply one of a plurality of first modulation values ​​to a first property of the plurality of properties of the photon; the first modulation value is configured to decode a first qubit of the plurality of qubits. The first optical router is configured to route the photon based on the first qubit of the plurality of qubits. The output decoding stage includes a plurality of qubit decoding devices, each configured to receive a photon from the first optical router. The qubit decoding device includes a demodulation device, an optical delay device, and a detection device. The demodulation device includes an optical modulator and an optical router. The optical modulator is configured to randomly apply one of a plurality of modulation values ​​to the property of the photon; the modulation value is configured to decode the qubit. The optical router is disposed after the optical modulator and is configured to route the photon based on the qubit. The optical delay device includes an optical combiner, a first optical path from a first output of the optical router to a first input of the optical combiner, and a second optical path from a second output of the optical router to a second input of the optical combiner. The second optical path has an optical path length different from that of the first optical path to introduce a time delay between the first optical path and the second optical path. A detection device is configured to detect photons received from the optical combiner. The detection device is operable to determine whether the detected photons are delayed relative to a reference time and to determine a qubit based on the determined time delay. A qubit decoding device is configured to decode a second qubit encoded with a second attribute of the plurality of attributes of the photon.The first property of a photon and the second property of a photon are different, non-commutative properties of the photon's quantum state.

[0027] One aspect of the present invention provides a method for qubit decoding. The method includes receiving a photon having a quantum bit (qubit) encoded with a property of the photon. The method also includes randomly applying one of a plurality of modulation values ​​to the property of the photon; the modulation value is configured to decode the qubit. The method also includes applying a time delay to the photon depending on the qubit. The method also includes detecting the photon. The method also includes determining whether the detected photon is delayed relative to a reference time. The method also includes determining the qubit based on the determined time delay.

[0028] In one embodiment, a photon has a plurality of qubits each encoded with a plurality of properties of the photon. The method comprises the steps of randomly applying one of a plurality of first modulation values ​​to a first property of the plurality of properties of the photon; the first modulation value being configured to decode the first qubit of the plurality of qubits. The method comprises the steps of randomly applying one of a plurality of second modulation values ​​to a second property of the plurality of properties of the photon; the second modulation value being configured to decode the second qubit of the plurality of qubits. The method comprises the steps of applying a time delay to the photon depending on at least one of the first qubit and the second qubit. The method comprises the steps of detecting the photon. The method comprises the steps of determining whether the detected photon is delayed relative to a reference time. The method comprises the steps of determining the qubit based on the determined time delay. The first property of the photon and the second property of the photon are different, non-commutative properties of the quantum state of the photon.

[0029] In one embodiment, the method further comprises the step of randomly applying one of a plurality of third modulation values ​​to a third attribute of the plurality of attributes of the photon; the third modulation value being configured to decode a third qubit of the plurality of qubits. The third attribute is a third, distinct, non-commutative attribute of the quantum state of the photon. The time delay is dependent on at least one of the first qubit, the second qubit, and the third qubit.

[0030] In one embodiment, the time delay depends on each qubit in the qubit.

[0031] In one embodiment, the non-commutative properties of the photon quantum state include polarization, phase and spin.

[0032] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1A block diagram of a prior art QKD scheme is shown;

[0034] Figures 2 to 4 is a block diagram illustrating an embodiment of a quantum key decoding device;

[0035] Figures 5 to 8 is a block diagram illustrating an embodiment of a quantum key decoding system and an embodiment of a communication network node;

[0036] Figures 9 to 11 is a block diagram illustrating an embodiment of a quantum key distribution system; and

[0037] Figure 12 and Figure 13 is a flow chart illustrating an embodiment of the method steps. DETAILED DESCRIPTION

[0038] The same reference numerals will be used for corresponding features in the different embodiments.

[0039] refer to Figure 2 , an embodiment of the present invention provides a qubit decoding device 200. The device is configured to receive a photon having a qubit encoded with a property of the photon, such as polarization, phase or spin.

[0040] The qubit decoding device includes a demodulation device 210 , an optical delay device 220 , and a detection device 230 .

[0041] Demodulation device 210 includes an optical modulator 212 and an optical router 214, which in this example includes an optical splitter. The optical modulator is configurable to randomly apply one of multiple modulation values ​​to the properties of a photon. The modulation value is configured to decode a qubit. Optical router 214 receives the photon modulated by the optical modulator and is configured to route the photon according to the qubit.

[0042] The optical delay device 220 includes an optical combiner 222, a first optical path 224, and a second optical path 226. The first optical path 224 runs from a first output of the optical router to a first input of the optical combiner. The second optical path 226 runs from a second output of the optical router to a second input of the optical combiner. The first and second optical paths have different optical path lengths such that a differential time delay exists between the first and second paths; that is, a photon traveling along the first path will be delayed by the time delay compared to a photon traveling along the second path, or vice versa. This can be achieved by including a delay line in one of the optical paths 224 and not including a delay line in the other optical path 226 (e.g., Figure 2 ) or by including delay lines of different lengths in the two optical paths (as in Figure 3 and Figure 4).

[0043] Detection device 230 is configured to detect photons received from optical combiner 222. Detection device 230 is operable to determine whether the detected photons are delayed relative to a reference time and determine a qubit based on the determined time delay. For example, if the photons are delayed, the qubit is determined to be a "1" bit, and if the photons are not delayed, the qubit is determined to be a "0" bit.

[0044] In one embodiment, detection device 230 includes an optical receiver D1 (such as a single-photon avalanche photodetector (SPD)) and processing circuitry configured to determine whether a detected photon is delayed relative to a reference time and to determine a qubit based on the determined time delay. In one embodiment, the processing circuitry is configured to detect the presence or absence of a time delay in the received photon relative to a reference clock signal.

[0045] In an embodiment, the processing circuit is operable to determine a time delay applied to the detected photon relative to a reference time. The processing circuit is configured to determine the qubit based on the determined time delay.

[0046] As described above with reference to the BB84 protocol decoder, in the case where the optical modulator is set to the wrong modulation value (i.e., the wrong basis is chosen for measurement), there will be a quantum state superposition between the two optical paths, which will collapse with equal chance with or without time delay after the optical receiver.

[0047] The qubit decoding device 200 implements a change from independent spatial decoding used in the known BB84 decoder scheme to temporal decoding. By introducing a relative time delay between the first and second optical paths and introducing an optical combiner to recombine the optical paths, the detection device 230 including a single optical receiver D1 can be replaced. Figure 1 The two photodetectors D1 and D2 in the BB84 decoder scheme shown in FIG are used to receive and detect photons.

[0048] Thus, the total number of photodetectors can be greatly reduced, as described further below.The apparatus 200 can be implemented for any kind of encoding used for QKD, such as, for example, polarization, spin, or phase.

[0049] refer to Figure 3 , an embodiment of the present invention provides a qubit decoding device 300 configured to receive a photon having a qubit encoded with the phase of the photon.

[0050] The optical modulator 310 of this embodiment is a Mach-Zehnder interferometer (MZI). One arm of the MZI extends from the input beam splitter BS 314 to the optical router 214 (here, the second BS), while the other arm extends from the input BS 314 via the mirror 316 and the phase modulator PM 312 to the optical router 214. Figure 1 As in the BB84 decoder shown in (a), the phase modulator 312 can be configured to randomly apply one of two modulation values ​​(phase shifts in this implementation) to decode a "0" bit encoded with one of the two phase shifts or a "1" bit encoded with one of the other two phase shifts. Therefore, if the sender uses a Mach-Zehnder interferometer (MZI) to apply four different phase shifts (e.g., ), where the first two phase shifts encode a '0' bit and the second two phase shifts encode a '1' bit, the phase modulator 312 is configured to randomly apply two phase shifts configured to decode the qubit (e.g., ) one of them. Figure 1 In the BB84 protocol MZI-based decoder of (a), photons are routed from the output BS to D1 or D2, i.e., different spatial locations, while in the qubit decoding device 300, photons are routed from the output BS / optical router 214 to the first optical path 224 or the second optical path 226, and the presence or absence of a time delay is used to determine whether the qubit is a "0" bit or a "1" bit.

[0051] refer to Figure 4 , embodiments of the present invention provide a qubit decoding device 400 configured to receive a photon having a qubit encoded with the photon's polarization.

[0052] The optical modulator 412 of this embodiment is a polarization rotator, and the optical router 414 is a polarization beam splitter PBS. Figure 1 As in the BB84 decoder shown in (b), the polarization rotator 412 can be configured to randomly apply one of two modulation values ​​(polarization angles in this implementation) to decode a "0" bit encoded with one of two phase shifts or a "1" bit encoded with one of the other two phase shifts. Therefore, if the sender uses a polarization rotator to apply one of four polarization angles to encode on one of the rectilinear basis 0° and 90° or the diagonal basis 45° and 135°, the polarization rotator 412 is configured to randomly apply 0° or 45° to resolve the qubit, and the PBS routes the photon according to the qubit. Figure 1In the BB84 protocol polarization-based decoder of (b), photons are routed from the PBS to D1 or D2, i.e., different spatial locations, whereas in the qubit decoding device 400, photons are routed from the PBS / optical router 414 to the first optical path 224 or the second optical path 226, and the presence or absence of a time delay is used to determine whether the qubit is a "0" bit or a "1" bit.

[0053] refer to Figure 5 Embodiments of the present invention provide a qubit decoding system 500 configured to receive a photon having a plurality of qubits respectively encoded with a plurality of properties of the photon. The system 500 includes a first decoding stage 510 and an output decoding stage 520.

[0054] The first decoding stage includes a first optical modulator 512 and a first optical router 514. The first optical modulator 512 is configured to randomly apply one of a plurality of first modulation values ​​to a first attribute of the plurality of attributes of the photon. The first modulation value is configured to decode a first qubit of the plurality of qubits. The first optical router is configured to route the photon according to the first qubit of the plurality of qubits.

[0055] The output decoding stage 520 includes a plurality of qubit decoding devices 200, as described above with reference to Figure 2 As described, they are each configured to receive a photon from the first optical router. Which qubit decoding device receives the photon depends on the decoding of the first qubit performed by the first decoding stage; if the first qubit is determined to be a "0" bit, the first optical router routes it to the first output, and the first qubit decoding device in the qubit decoding devices will receive the photon; and if the first qubit is determined to be a "1" bit, the first optical router routes it to the second output, and the second qubit decoding device in the qubit decoding devices will receive the photon. Each qubit decoding device 200 is configured to decode a second qubit encoded with a second attribute of the plurality of attributes of the photon.

[0056] The primary and secondary properties of a photon are different, non-commutative properties of the photon's quantum state. Two physical properties of a quantum system are considered non-commutative (independent) if they can be measured without affecting each other—that is, if they are not subject to the Heisenberg uncertainty principle. Examples include phase, polarization, and spin.

[0057] refer to Figure 6 , embodiments of the present invention provide a qubit decoding system 600 configured to receive a photon having two qubits respectively encoded with the polarization and phase of the photon.

[0058] In this embodiment, the first decoding stage includes a polarization rotator 612 and a PBS 614, as described above with reference to Figure 4 Just as described.

[0059] Output decoding stage 620 includes two qubit decoding devices 300, as described above with reference to Figure 3 As depicted, each qubit decoding device 300 is separately configured to receive a photon from PBS 614 .

[0060] In one embodiment, detection device 230 includes an optical receiver D1 (such as a single-photon avalanche photodetector (SPD)) and processing circuitry configured to determine whether a detected photon is delayed relative to a reference time and to determine a qubit based on the determined time delay. In one embodiment, the processing circuitry is configured to detect the presence or absence of a time delay in the received photon relative to a reference clock signal.

[0061] In an embodiment, the processing circuit is operable to determine a time delay applied to the detected photon relative to a reference time. The processing circuit is configured to determine the qubit based on the determined time delay.

[0062] and use Figure 1 Compared to the BB84 decoder shown in (a), qubit decoding system 600 enables the number of required optical receivers to be reduced by half.

[0063] refer to Figure 7 , embodiments of the present invention provide a qubit decoding system 700 configured to receive a photon having two qubits respectively encoded with the polarization and phase of the photon.

[0064] In this embodiment, the two qubit decoding devices 300 of the output decoding stage 720 have different time delays between their respective first and second optical paths. This can be achieved by each optical path including a delay line, the delay lines having different lengths, such as Figure 7 This means that four different time delays can be applied to the photon, depending on the route it takes to reach combiner 222. Alternatively, this can be achieved by having three of the optical paths contain delay lines, while the remaining optical paths do not; thus, one optical path has no delay, while the other three optical paths have three different time delays.

[0065] The two qubit decoding devices 300 share a common detection device 230 comprising a single optical receiver D1 and processing circuitry.A further optical combiner 702 is provided which routes the outputs of the optical combiners 222 of the two qubit decoding devices 300 to the optical receiver D1.

[0066] The processing circuit is configured to determine whether the detected photon is delayed by any of the corresponding time delays relative to a reference time and to determine the qubit based on the determined time delay.

[0067] In an embodiment, the processing circuit is configured to determine whether the detected photon is delayed by any of the corresponding time delays relative to the reference clock signal and to determine the qubit based on the determined time delay.

[0068] and use Figure 1 Compared to the BB84 decoder shown in (a), qubit decoding system 700 reduces the number of required optical receivers from four to one.

[0069] In summary, using Figure 1 The BB84 phase-based decoder shown in (a) would require 2 m optical receivers, of which m is the number of qubits to be decoded (in this case, m = 2), and the number of optical receivers will increase exponentially with the number of qubits. In contrast, the described qubit decoding systems 600, 700 enable the number of optical receivers to be reduced to 2 k , where 0 ≤ k ≤ m , that is, for m = 2, reducing the number from 4 to 1. This can be very useful to employ the optimal number of photoreceivers, thereby optimizing the trade-off between cost reduction and the required temporal resolution after photodetection.

[0070] refer to Figure 8 , embodiments of the present invention provide a qubit decoding system 800 configured to receive a photon having three qubits respectively encoded with three non-commutative properties of the photon (eg, polarization, phase, and spin).

[0071] The qubit decoding system 800 further includes a third decoding stage 810 disposed between the first decoding stage 510 and the output decoding stage 520. The third decoding stage includes a plurality of third optical modulators configured to receive a photon from the first decoding stage and output the photon to the output decoding stage depending on the first qubit in the plurality of qubits. The third optical modulator is configured to randomly apply one of a plurality of third modulation values ​​to a third property in the plurality of properties of the photon. The third modulation value is configured to decode the third qubit in the plurality of qubits. The third property is a third, different, non-commutative property of the quantum state of the photon.

[0072] An embodiment of the present invention provides a communication network node 250, 350, 450, which includes the above reference Figures 2 to 4 The qubit decoding devices 200, 300, 400 are described.

[0073] The embodiment of the present invention provides a communication network node 550, 650, 750, 850, which includes the above reference Figures 5 to 8 The qubit decoding systems 500, 600, 700, 800 are described.

[0074] refer to Figure 9 , an embodiment of the present invention provides a quantum key distribution QKD system 900, the system 900 includes a qubit encoding device 910 and as described above with reference Figure 2 The qubit decoding device 200 is described.

[0075] The qubit encoding device is configured to encode a qubit using properties of a photon. The qubit decoding device 200 is configured to receive a photon and determine a qubit, as described above.

[0076] In one embodiment, detection device 230 includes an optical receiver D1 (such as a single-photon avalanche photodetector (SPD)) and processing circuitry configured to determine whether a detected photon is delayed relative to a reference time and to determine a qubit based on the determined time delay. In one embodiment, the processing circuitry is configured to detect the presence or absence of a time delay in the received photon relative to a reference clock signal.

[0077] In an embodiment, the processing circuit is operable to determine a time delay applied to the detected photon relative to a reference time. The processing circuit is configured to determine the qubit based on the determined time delay.

[0078] refer to Figure 10 , an embodiment of the present invention provides a QKD system 1000, which includes a qubit encoding device 1010 and a Figure 6 The qubit decoding system 600 is described.

[0079] The qubit encoding device 1010 includes a single photon source 1012, a first optical modulator 1020 configured to encode a first qubit with a first property of the photon, and a second optical modulator 1030 configured to encode a second qubit with a second property of the photon. The first property of the photon and the second property of the photon are different, non-commutative properties of the quantum state of the photon.

[0080] For example, the first optical modulator 1020 is a Mach-Zehnder interferometer MZI, which includes a plurality of optical modulators configured to apply four different phase shifts (eg, ), where the first two phase shifts encode a '0' bit and the second two phase shifts encode a '1' bit. The second optical modulator 1030 is a polarization rotator that applies one of four polarization angles, for example, to encode a '0' bit on a rectilinear basis of 0° and 90° or to encode a '1' bit on a diagonal basis of -45° and 45°.

[0081] In one embodiment, the detection device 230 includes a light receiver D1 (such as a single photon avalanche photodetector SPD) and processing circuitry configured to determine whether the detected photon is delayed relative to a reference time and determine the qubit based on the determined time delay.

[0082] In one embodiment, the processing circuit is configured to detect the presence or absence of a time delay of the received photon relative to a reference clock signal. The reference clock signal may be provided by a clock synchronized between the qubit encoding device 1010 and the qubit decoding system 600.

[0083] refer to Figure 11 , an embodiment of the present invention provides a QKD system 1100, which includes a qubit encoding device 1010 and a qubit encoding device 1010 as described above. Figure 7 The qubit decoding system 700 is described.

[0084] In one embodiment, the processing circuit is configured to detect the presence or absence of a time delay of the received photon relative to a reference clock signal. The reference clock signal may be provided by a clock synchronized between the qubit encoding device 1010 and the qubit decoding system 600.

[0085] refer to Figure 12 , an embodiment of the present invention provides a method 1200 for quantum bit decoding. The method includes the following steps:

[0086] Receiving 1210 a photon having a quantum bit (qubit) encoded with properties of the photon;

[0087] randomly applying 1212 one of a plurality of modulation values ​​to the property of the photon, the modulation value being configured to decode the qubit;

[0088] Depending on the qubit, a 1214 time delay is applied to the photon;

[0089] Detected 1216 photons;

[0090] determining 1218 whether the detected photon is delayed relative to a reference time; and

[0091] 1220 qubits are determined based on a certain time delay.

[0092] Method 1200 may be used as described above. Figures 2 to 4 The invention can be implemented by any of the described qubit decoding devices 200, 300, 400.

[0093] refer to Figure 13 , an embodiment of the present invention provides a qubit decoding method 1300. The method of this embodiment is applicable to decoding a photon having two qubits respectively encoded with two properties of the photon.

[0094] The method 1300 includes the following steps:

[0095] randomly applying 1312 one of a plurality of first modulation values ​​to a first attribute of the plurality of attributes of the photon, the first modulation value being configured to decode a first qubit of the plurality of qubits;

[0096] randomly applying 1314 one of a plurality of second modulation values ​​to a second attribute of the plurality of attributes of the photon, the second modulation value being configured to decode a second qubit of the plurality of qubits;

[0097] applying 1316 a time delay to the photon depending on at least one of the first qubit and the second qubit;

[0098] Detecting 1216 photons; and

[0099] determining 1218 whether the detected photon is delayed relative to a reference time; and

[0100] 1220 qubits are determined based on a certain time delay.

[0101] The first property of the photon and the second property of the photon are different non-commutative properties of the quantum state of the photon, as described above.

[0102] Method 1300 may be used as described above. Figures 5 to 7 The invention can be implemented with any of the described qubit decoding systems 500, 600, 700.

[0103] In one embodiment, method 1300 is adapted to decode a photon having three qubits, each encoded with three properties of the photon. Method 1300 includes the additional step of randomly applying one of a plurality of third modulation values ​​to the third property of the photon. The third modulation value is configured to decode a third qubit. The third property is a third, distinct, non-commutative property of the photon's quantum state. The time delay depends on at least one of the first qubit, the second qubit, and the third qubit.

[0104] The method of this embodiment can be used with reference to Figure 8 The qubit decoding system 800 described is implemented.

[0105] In one embodiment, the time delay depends on each of the first qubit, the second qubit, and the third qubit.

Claims

1. A qubit decoding device configured to receive a photon having a qubit (qubit) encoded with a property of the photon, the device comprising: A demodulation device, the demodulation device comprising: an optical modulator configurable to randomly apply one of a plurality of modulation values ​​to the property of the photon, the modulation value configured to decode the qubit; and an optical router disposed after the optical modulator and configured to route the photons according to the qubits; An optical delay device, the optical delay device comprising: optical combiners; a first optical path from a first output port of the optical router to a first input port of the optical combiner; a second optical path from a second output terminal of the optical router to a second input terminal of the optical combiner, the second optical path having an optical path length different from that of the first optical path so as to introduce a time delay between the first optical path and the second optical path; and A detection device is configured to detect photons received from the optical combiner and is operable to determine whether the detected photons are delayed relative to a reference time and to determine the qubit based on the determined time delay.

2. A qubit decoding system configured to receive a photon having a plurality of qubits (qubits) each encoded with a plurality of properties of the photon, the system comprising: A first decoding stage, the first decoding stage comprising: a first optical modulator configured to randomly apply one of a plurality of first modulation values ​​to a first attribute of the plurality of attributes of the photon, the first modulation value configured to decode a first qubit of the plurality of qubits; and a first optical router configured to route the photon according to the first qubit of the plurality of qubits; an output decoding stage, the output decoding stage comprising a plurality of qubit decoding devices as claimed in claim 1, the plurality of qubit decoding devices being respectively configured to receive the photons from the first optical router, the plurality of qubit decoding devices being configured to decode a second qubit encoded with a second property of the plurality of properties of the photons, The first property of the photon and the second property of the photon are different non-commutative properties of the photon quantum state.

3. The system of claim 2 further comprises a third decoding stage disposed between the first decoding stage and the output decoding stage, the third decoding stage comprising a plurality of third optical modulators, the plurality of third optical modulators being configured to: receive the photon from the first decoding stage depending on the first qubit among the plurality of qubits; and output the photon to the output decoding stage, the third optical modulator being configured to randomly apply one of a plurality of third modulation values ​​to a third attribute among the plurality of attributes of the photon, the third modulation value being configured to decode the third qubit among the plurality of qubits, the third attribute being a third different non-commutative attribute of the quantum state of the photon.

4. A system as claimed in claim 2 or claim 3, wherein: The non-commutative properties of the photon quantum state include polarization, phase and spin.

5. The system of claim 2, wherein: The plurality of qubit decoding devices of the output decoding stage have different respective time delays between respective first and second optical paths, and wherein the plurality of qubit decoding devices have a common detection device configured to detect a photon received from an optical combiner of any qubit decoding device of the plurality of qubit decoding devices, and wherein the common detection device is operable to determine whether the detected photon is delayed by any of the respective time delays relative to a reference time and to determine the qubit based on the determined time delay.

6. A communication network node, comprising the quantum bit decoding device according to claim 1 or the quantum bit decoding system according to any one of claims 2 to 5.

7. A quantum key distribution system, comprising: a qubit encoding device configured to encode at least one qubit with at least one corresponding property of a photon; as well as The qubit decoding device of claim 1 or the qubit decoding system of any one of claims 2 to 5.

8. A method for quantum bit decoding, the method comprising the following steps: receiving a photon having a quantum bit (qubit) encoded with a property of the photon; randomly applying one of a plurality of modulation values ​​to the property of the photon, the modulation value configured to decode the qubit; applying a time delay to the photon depending on the qubit; detecting the photons; determining whether the detected photons are delayed relative to a reference time; as well as The qubit is determined according to the determined time delay.

9. The method of claim 8, wherein: The photon has a plurality of qubits respectively encoded with a plurality of properties of the photon, and wherein the method comprises the steps of: randomly applying one of a plurality of first modulation values ​​to a first property of the plurality of properties of the photon, the first modulation value being configured to decode a first qubit of the plurality of qubits; randomly applying one of a plurality of second modulation values ​​to a second attribute of the plurality of attributes of the photon, the second modulation value being configured to decode a second qubit of the plurality of qubits; applying a time delay to the photon depending on at least one of the first qubit and the second qubit; detecting the photons; and determining whether the detected photon is delayed relative to a reference time; and determining the qubit according to the determined time delay, The first property of the photon and the second property of the photon are different non-commutative properties of the photon quantum state.

10. The method of claim 9, further comprising the step of randomly applying one of a plurality of third modulation values ​​to a third property of the plurality of properties of the photon, the third modulation value being configured to decode a third qubit of the plurality of qubits, the third property being a third different non-commutative property of the photon quantum state, wherein The time delay depends on at least one of the first qubit, the second qubit, and the third qubit.

11. The method of claim 9 or claim 10, wherein: The time delay depends on each of the qubits.

12. The method of claim 9, wherein: The non-commutative properties of the photon quantum state include polarization, phase and spin.

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