Modular quantum key distribution system, as well as relative modulation and demodulation modules.

Abstract

The present invention relates to a modular quantum key distribution system (100) and associated modulation and demodulation modules. The system comprises a photon source module (20); a first optical communication channel (F SM a modulation module (30) operably connected to the photon source module (20) by a first optical communication channel and comprising a first polarization stabilizer (36), a first wave plate (32) and a second wave plate (34) disposed downstream of the first polarization stabilizer (36) and controlled by an actuator (35); and a receiving unit (Rx) operably connected to the modulation module by a second optical communication channel and comprising a second polarization stabilizer (42), a third wave plate (44) disposed downstream of the second polarization stabilizer (42) and controlled by an actuator (45), a polarizing beam splitter (46) disposed downstream of the third wave plate (44), and a single-photon photodetector (52) configured to detect a logic state of each photon and transmit the detected logic state to a counting register (48).

Description

[Technical Field]

[0001] This invention relates to the technical field of quantum cryptography. In particular, this invention relates to a modular quantum key distribution system, as well as relative modulation modules and demodulation modules. [Background technology]

[0002] Quantum key distribution (QKD) is a technique that enables the sharing of a symmetric (i.e., bit sequence that materializes simultaneously at both the transmitter and receiver) and absolutely secure (i.e., impossible to decrypt regardless of the algorithm or computational power) "key" between two nodes (hereinafter referred to as transmitter Tx and receiver Rx). The adjective "quantum" refers to the fact that the key is generated using the spin state (classically known as the polarization state) of a single photon.

[0003] In quantum mechanics, it is known that the spin state of a particle can only be accurately measured if it lies within a precise reference basis (which can be a straight line, diagonal, or circular); otherwise, the measurement will produce a completely random value. A second quantum property is that the spin state of a particle cannot be replicated. A third quantum property concerns light, and relates to the fact that it is possible to generate and propagate a single photon in any optical communication channel (free space, optical fiber, or optical integrated guide).

[0004] Based on these properties of photons, CH Bennet and G. Brassard developed a protocol known as BB84, as referred to in this patent application, in 1984.

[0005] In the BB84 protocol, the transmitter Tx sends a sequence of single photons generated in a known basis and a known state (according to a bit logic sequence). The receiver Rx receives the single photons (excluding the single photons lost due to attenuation) and measures the state in a randomly selected basis. This state measurement generates a bit logic sequence. The receiver Rx sends the series of bases used for the measurement to the transmitter Tx via a public communication channel. The transmitter Tx sends a confirmation of its accuracy to the receiver Rx. In this way, the transmitter Tx and the receiver Rx share a subsequence of logical bits that form a quantum key.

[0006] The original scheme of QKD according to the BB84 protocol is point-to-point transmission (i.e., the transmitter Tx is connected to only one receiver Rx), and relates to the communication of a quantum key between the transmitter Tx and the receiver Rx via an optical communication channel without birefringence. In a more general example, when there is an optical communication channel showing birefringence (the case dealt with in this patent application), at the end of the transmission, it is necessary to re-establish the original basis by a device called a polarization stabilizer.

[0007] The transmitter Tx includes a single photon source, i.e., a photon source whose intensity is attenuated so as to transmit only one photon on average (a weak photon source), and the first and second wave plates controlled by an actuator through which a single polarized photon emitted from the single photon source is sent. The first wave plate makes it possible to establish the basis of the photon spin (the basis can be linear, diagonal or circular), and the second wave plate makes it possible to establish the state (for example, a logical high state corresponding to 1, a logical low state corresponding to 0) that generates the bit logic sequence forming the key.

[0008] The receiver Rx includes a polarization stabilizer that enables reorientation of the initial basis in the case of a birefringent optical transmission line, and a waveplate that is controlled by an actuator, disposed downstream of the polarization stabilizer, and enables re-establishing (or not re-establishing) the initial basis in a random process. The receiver further includes a polarization beam splitter (PBS). Each photon emerging from the waveplate is assigned to the polarization beam splitter. The polarization beam splitter is configured to distinguish between the high logic state and the low logic state of the polarized photons and transmit those states to a photodetection unit such as a counting register so as to be able to reconstruct the code transmitted by the transmitter Tx.

[0009] The components of the transmitter Tx and the receiver Rx are also connected therebetween and are adapted to deliver a synchronization signal typically generated by a control signal of a shutter disposed in the transmitter Tx immediately downstream of the single photon source or by the same pulse of the single photon source between the transmitter and the receiver.

[0010] In the aforementioned quantum key distribution system, the receiver Rx has both basis information and code information, whereby the integrity of the BB84 protocol is sufficiently satisfied. However, such a configured system has several drawbacks.

[0011] First, when the components of the system are changed or upgraded, the entire transmitter Tx and / or the entire receiver Rx must be upgraded. This is functionally disadvantageous and costly economically.

[0012] Second, as is well known, the single photon source and the photodetection unit in the device may have to be kept at very low temperatures. Therefore, the transmitter Tx and the receiver Rx of the device must be disposed in an environment controlled by cryogenic technology. This has an adverse effect in terms of space and maintenance and is logically disadvantageous.

Summary of the Invention

[0013] The objective of this invention is to overcome the shortcomings of the prior art.

[0014] In particular, an objective of the present invention is to present a modular quantum key distribution system configured to allow for a more flexible definition of the network architecture.

[0015] Furthermore, an object of the present invention is to present a modular quantum key distribution system configured to allow for the modification, upgrade, and / or maintenance of the system's components.

[0016] Furthermore, an object of the present invention is to present a multipoint-to-multipoint quantum key distribution system in which multiple transmitting nodes exchange quantum keys with multiple receiving nodes, while simultaneously maintaining the sharing of both the photon source module and the photodetector module.

[0017] These and other objectives of the present invention are achieved by a quantum key distribution system, modulation module and demodulation module incorporating features of the appended claims that form an integral part of this specification.

[0018] According to the first aspect, the present invention is Photon source module and A modulation module that is physically separated from the photon source module and operably connected to the photon source module via a first optical communication channel, The present invention relates to a quantum key distribution system comprising a receiving unit operably connected to a modulation module via a second optical communication channel, The modulation module comprises a first polarization stabilizer and a first waveplate and a second waveplate located downstream of the first polarization stabilizer and controlled by an actuator. The receiving unit comprises a second polarization stabilizer, a third waveplate located downstream of the second polarization stabilizer and controlled by an actuator, a polarization beam splitter located downstream of the third waveplate, and a single-photon photodetector configured to detect the logic state of each photon and transmit the detected logic state to a counting register.

[0019] The physical separation of the modulation module from the photon source module offers significant advantages. Firstly, the physical separation of the two modules allows for a more flexible network architecture, enabling the provision of systems with a single photon source module and multiple modulation modules. Furthermore, the photon source module and modulation module are separately replaceable or maintainable, providing flexibility in maintenance and system development. For example, the system can be upgraded by switching from the first photon source module to a more efficient one that needs to be maintained in a cryogenically controlled environment, without changing the modulation module.

[0020] In one embodiment, the receiving unit includes a demodulation module and a photodetector module, which are physically separated from each other and communicate optically. The demodulation module includes a second polarization stabilizer, a third waveplate, an actuator for controlling the third waveplate, a polarization beam splitter, and a counting register. The photodetector module includes a single-photon photodetector. The demodulation module is operably connected to the modulation module via a second optical communication channel. The photodetector module, on the other hand, is operably connected to the demodulation module via a third optical communication channel.

[0021] In one embodiment, the demodulation module further comprises a fourth waveplate located downstream of the third waveplate and preferably controlled by an actuator associated with a counting register. The fourth waveplate advantageously introduces further randomness into the polarization state of the photons before they are detected by the receiving unit in a known sequence.

[0022] In one embodiment, the photon source module consists of a single photon source.

[0023] In one embodiment, the photon source module further comprises a shutter located downstream of the single photon source, the shutter generating a synchronization signal that is delivered into the system along a synchronization line.

[0024] In one embodiment, the photon source module includes a parametric two-photon source. The use of such a two-photon source that generates first and second photons is particularly advantageous because the emission of the second photon acts as an indicator of the emission of the first photon. In other words, the detection of the second photon ensures that the first photon was guided into the system.

[0025] In one embodiment, the photon source module further comprises a semiconductor or superconducting single-photon detector positioned downstream of the parametric two-photon source. The single-photon detector is configured to detect a second photon emanating from the parametric two-photon source and generate a synchronization signal at its output, which is delivered into the system along a synchronization line.

[0026] In one embodiment, the first, second, and third optical communication channels consist of optical fibers.

[0027] In one embodiment, the third optical communication channel includes a pair of optical fibers.

[0028] In one embodiment, a first waveplate of the modulation module enables the establishment of a spin basis for each photon entering the modulation module through a first optical communication channel. A second waveplate of the modulation module enables the establishment of a state or code that generates a bit logic sequence forming a quantum key.

[0029] In one embodiment, the system comprises a second modulation module and a second demodulation module. The second modulation module and the second demodulation module are connected to each other by second, first, and third optical communication channels, respectively, and are connected to a photon source module and an optical detection module.

[0030] In one embodiment, the system comprises a first switching device associated with a photon source module and / or a second switching device associated with a photodetector module. The first switching device is operably connected to each modulation module by a first optical communication channel. The second switching device is operably connected to each demodulation module by a third optical communication channel.

[0031] In one embodiment, the system comprises a third switching device associated with each modulation module and a fourth switching device associated with each demodulation module. The third switching device is operably connected to each of the fourth switching devices by their respective second optical communication channels.

[0032] In one embodiment, the system comprises at least a second modulation module and a second receiving unit. The second modulation module and the second receiving unit are connected to each other and to a photon source module by second and first optical communication channels.

[0033] In one embodiment, the system further comprises a supervisor device configured to activate first and third optical communication channels of each modulation module and each demodulation module, or first and second optical communication channels of each modulation module and receiving unit.

[0034] According to a second aspect, the present invention relates to a modulation module comprising a polarization stabilizer connected to an input optical port for receiving photons transmitted by a photon source module. The modulation module further comprises a first waveplate and a second waveplate located downstream of the polarization stabilizer, and actuators adapted to control the first waveplate and the second waveplate to modulate the polarization of photons. Finally, the modulation module comprises an optical output port for transmitting modulated photons via an optical communication channel.

[0035] According to a third aspect, the present invention relates to a demodulation module comprising a polarization stabilizer connected to an input optical port for receiving photons transmitted by a modulation module. The demodulation module further comprises a waveplate located downstream of the polarization stabilizer and an actuator adapted to control the waveplate for demodulating the polarization of photons. The demodulation module further comprises a polarization beam splitter located downstream of the waveplate and configured to distinguish between high and low logic states of received photons. Finally, the demodulation module comprises a counting register configured to receive the logic state of photons transmitted by a photodetector module.

[0036] In one embodiment, the demodulation module comprises an additional waveplate located downstream of the waveplate and preferably controlled by an actuator associated with a counting register.

[0037] Further features and advantages of the present invention will become clearer from the description of the accompanying drawings. [Brief explanation of the drawing]

[0038] The present invention is provided as a non-limiting example and is described below with reference to specific examples shown in the accompanying drawings. These drawings illustrate various aspects and embodiments of the present invention, and reference figures indicating structures, components, materials, and / or similar elements in the various drawings are indicated by similar reference figures where appropriate.

[0039] [Figure 1] A schematic diagram of a conventional quantum key distribution system is shown. [Figure 2] A schematic diagram of a modular quantum key distribution system according to the first embodiment of the present invention is shown. [Figure 3] A schematic diagram of a modular quantum key distribution system according to a second embodiment of the present invention is shown. [Figure 4] A schematic diagram of a modular quantum key distribution system according to a third embodiment of the present invention is shown. [Figure 5]A schematic diagram of a modular quantum key distribution system according to a fourth embodiment of the present invention is shown. [Figure 6] A schematic diagram of a modular quantum key distribution system according to a fifth embodiment of the present invention is shown. [Figure 7] A modular quantum key distribution system according to a sixth embodiment of the present invention is shown. [Figure 8] A schematic diagram of a modular quantum key distribution system according to the seventh embodiment of the present invention is shown. [Modes for carrying out the invention]

[0040] While the present invention may be subject to various modifications and alternative configurations, several embodiments provided for illustrative purposes are described in detail below.

[0041] In any case, it should be understood that the present invention is not intended to be limited to the specific embodiments shown in the illustrations, but rather is intended to cover all modifications, substitutions, and equivalent structures that fall within the scope of the present invention as defined in the claims.

[0042] Therefore, in the following explanation, unless otherwise stated, the use of "for example," "etc.," and "or" indicates an unrestricted, non-exclusive alternative; unless otherwise stated, the use of "also" means "includes, but is not limited to"; and unless otherwise stated, the use of "includes / equips" means "includes, but is not limited to"

[0043] Referring to Figure 1, a conventional quantum key distribution system is shown.

[0044] This system, commonly referred to as reference number 1, comprises a transmitting unit Tx and a receiving unit Rx, which are operably connected to each other by an optical communication channel, typically consisting of optical fiber F.

[0045] The transmitting unit Tx comprises a single-photon source 2, that is, a photon source whose intensity is weakened to transmit only one photon on average (a weak photon source), and a first waveplate 3 and a second waveplate 4 controlled by an actuator 5, to which single-polarized photons or qubits emitted from the single-photon source 2 are sent.

[0046] The first waveplate 3 enables the establishment of a photon spin basis B (which can be linear, diagonal, or circular), and the second waveplate 4 enables the establishment of a state or code C that generates a bit logic sequence that forms a key (for example, a logic high state corresponds to 1, and a logic low state corresponds to 0).

[0047] The receiving unit Rx, in the example of a birefringent optical communication channel, comprises a polarization stabilizer 7 that allows the initial basis B to be reoriented, and a waveplate 8 controlled by an actuator 9 and located downstream of the polarization stabilizer 7, which allows the initial basis to be re-established (or not re-established) in a random process. The receiving unit Rx further comprises a polarization beam splitter 10. Each polarization photon emanating from the waveplate 8 is addressed to the polarization beam splitter 10, which is configured to distinguish between a logic high state 1 or a logic low state 0 and transmit them to a counting register 11, enabling the reconstruction of the code C transmitted by the transmitting unit Tx along the optical fiber F.

[0048] Preferably, the components of the transmitting unit Tx and the receiving unit Rx are connected to each other to deliver a synchronization signal between the transmitting unit Tx and the receiving unit Rx, typically generated by a control signal of a shutter 6 located in the transmitting unit Tx immediately downstream of the single-photon source 2, or by a similar pulse of the single-photon source 2 (dotted line in Figure 1).

[0049] Referring to Figure 2, a modular quantum key distribution system according to the first embodiment of the present invention is shown.

[0050] The system, generally referred to by reference number 100, comprises a photon source module 20, a modulation module 30, a demodulation module 40, and a photodetection module 50. These are operably connected to each other by their respective optical communication channels, for example, by their respective optical fibers. Alternatively, the optical communication channels may be free-space channels or integrated optical fiber channels.

[0051] The photon source module 20 and the modulation module 30 form the transmitting unit Tx of the system 100, and the demodulation module 40 and the photodetection module 50 form the receiving unit Rx of the system 100.

[0052] The photon source module 20 comprises a single photon source 22 and, preferably, a shutter 24 located immediately downstream of the single photon source 22 and having the function of generating a synchronization signal. The synchronization signal is delivered between the transmitting unit Tx and the receiving unit Rx of the system 100 along the synchronization line Ls. Alternatively, the synchronization signal may be generated from the same pulse as the single photon source 22. The photon source module 20 is physically isolated from the modulation module 30 and connected by a first optical communication channel, for example, an optical fiber F SM It is connected to the modulation module 30.

[0053] The modulation module 30 comprises a first waveplate 32 and a second waveplate 34 controlled by an actuator 35, to which polarized single photons or qubits emitted from the single photon source 22 of the photon source module 20 are sent.

[0054] The first waveplate 32 is made of standard optical fiber F SMThe second waveplate 34 enables the establishment of a base B (which can be linear, diagonal, or circular) for the spin of polarized photons entering the modulation module 30 via the waveplate, and enables the establishment of a state or code C that generates a bit logic sequence that forms a key (for example, a logic high state corresponds to 1, and a logic low state corresponds to 0). The base is started from a predetermined state by the action of a half-wave or quarter-wave waveplate, as is well known to experts in the field. The state value (high or low) can be generated by a half-waveplate appropriately oriented according to the selected base, as is well known to experts in the field.

[0055] Optical fiber F for connection between photon source module 20 and modulation module 30 SM Since light generally exhibits birefringence, the modulation module 30 further comprises a first polarization stabilizer 36 upstream of the waveplates 32 and 34. The first polarization stabilizer 36 is connected to the input optical port 31, represented by a lens, of the modulation module 30 for receiving photons transmitted by the photon source module 20 and for reorienting the basis B of the photon spins.

[0056] The demodulation module 40 is physically separated from the modulation module 30 and connected by a second optical communication channel, for example, an optical fiber F MD This enables the modulation module 30 to be operationally connected.

[0057] The demodulation module 40 is connected to an input optical port 41 of the demodulation module 40, which is represented by a lens, and includes a second polarization stabilizer 42 for receiving modulated photons coming out of the output optical port 37 of the modulation module 30, which is represented by a lens, and for reorienting the initial basis B of the photon spins.

[0058] The demodulation module 40 further comprises a third waveplate 44, which is controlled by an actuator 45 and located downstream of the second polarization stabilizer 42, allowing for the random re-establishment (or non-re-establishment) of the initial basis B of the photon spin. Finally, downstream of the third waveplate 44 is a polarization beam splitter 46, to which each polarized photon exiting the third waveplate 44 is directed.

[0059] The photodetection module 50 is physically separated from the demodulation module 40 and includes a single-photon photodetector 52, preferably a superconducting nanowire single-photon photodetector. The photodetection module 50 is connected by a third optical communication channel, specifically a pair of optical fibers F DF The units are operably connected to the demodulation module 40, each receiving a single photon from the polarizing beam splitter 46 of the demodulation module 40 at its respective input optical port 51, which is represented by a lens. The logic state detected by the single-photon photodetector 52 of the photodetection module 50, high 1 or low 0, is sent to the counting register 48 of the demodulation module 40 via its respective standard communication channel 54 to reconstruct the information for the receiving unit Rx.

[0060] Generally, a pair of optical fibers F DF Each optical fiber exhibits birefringence, but in this case, no problems arise because the photodetector of the photodetector module 50 is generally insensitive to the polarization state.

[0061] In the aforementioned system 100, the integrity condition of the BB84 protocol is missing because, in the receiving unit Rx, and particularly in its demodulation module 40, the basis information B is separated from the information of code C, which travels on a reserved but unconditionally insecure channel. However, the pair of optical fibers F connecting the demodulation module and the optical detection module DF Furthermore, if the standard communication channel 54 between the two modules is properly protected, for example, if they are in the same building, the integrity condition of the BB84 protocol is restored.

[0062] Referring to Figure 3, a quantum key distribution system according to a second alternative embodiment of the present invention is shown.

[0063] This system, generally referred to by reference number 1100, is generally similar to system 100 described above with reference to Figure 2, but differs in that it includes a fourth waveplate within the demodulation module.

[0064] Thus, the system 1100 comprises a photon source module 120, a modulation module 130, a demodulation module 140, and a photodetection module 150, and these modules are operablely connected to each other by their respective optical communication channels, for example, by optical fibers.

[0065] The photon source module 120 and the modulation module 130 form the transmitting unit Tx of the system 1100, and the demodulation module 140 and the photodetection module 150 form the receiving unit Rx of the system 1100.

[0066] The photon source module 120 comprises a single photon source 122 and, preferably, a shutter 124 located immediately downstream of the single photon source 122 and having the function of generating a synchronization signal. The synchronization signal is delivered between the transmitting unit Tx and the receiving unit Rx of the system 100 along the synchronization line Ls. The photon source module 120 is physically isolated from the modulation module 130 and connected by a first optical communication channel, for example, an optical fiber F SM It is connected to the modulation module 130.

[0067] The modulation module 130 is composed of a first waveplate 132 that enables establishing the spin basis B of each polarized photon entering the modulation module 130 (which can be linear, diagonal, or circular), and a second waveplate 134 that enables establishing a state or code C that generates a bit logic sequence forming a key. The first waveplate 132 and the second waveplate 134 are controlled by an actuator 135. The modulation module 130 further includes a first polarization stabilizer 136 upstream of the waveplates 132 and 134. The first polarization stabilizer 136 is connected to the input optical port 131 of the modulation module 30 to receive the photons transmitted by the photon source module 120 and to reorient the spin basis B of the photons.

[0068] The demodulation module 140 is physically separated from the modulation module 130 and is connected to the modulation module 130 by a second optical communication channel, for example, by an optical fiber F MD by.

[0069] The demodulation module 140 includes a second polarization stabilizer 142 connected to the input optical port 141 of the demodulation module 40 for receiving the modulated photons exiting the output optical port 137 of the modulation module 130 and for reorienting the initial spin basis B of the photons. The demodulation module 140 further includes a third waveplate 144, which is controlled by an actuator 145 and is arranged downstream of the second polarization stabilizer 142 and enables re - establishing (or not re - establishing) the initial spin basis B of the photons in a random process.

[0070] The demodulation module 140 further includes a polarization beam splitter 146 downstream of the third waveplate 144. Each polarized photon exiting the third waveplate 144 is directed to the polarization beam splitter 146.

[0071] The photodetector module 150 is physically separated from the demodulation module 140 and includes a single-photon photodetector 152, preferably a superconducting nanowire single-photon photodetector. The photodetector module 150 is connected by a third optical communication channel, particularly a pair of optical fibers F DF The units are operably connected to the demodulation module 140, each receiving a single photon from the polarization beam splitter 146 of the demodulation module 140 at its respective input optical port 151. The logic state detected by the single-photon photodetector 152 of the photodetection module 150, high 1 or low 0, is sent to the counting register 148 of the demodulation module 140 via its respective standard communication channel 154 to reconstruct the information for the receiving unit Rx.

[0072] The demodulation module 140 further comprises a fourth waveplate 149. The fourth waveplate 149 is located downstream of the third waveplate 144, controlled by an actuator 143, and preferably associated with a counting register 148. The fourth waveplate 149 introduces further randomness into the polarization state of the photons before they are detected by the receiving unit Rx in a known sequence. In this way, the receiving unit Rx is the only entity that knows the logical state of the information transmitted by the photodetector module 150. This advantageously makes it possible to restore the integrity of the BB84 protocol.

[0073] Referring to Figure 4, a quantum key distribution system according to a third embodiment of the present invention is shown.

[0074] The system, generally referred to by reference number 2100, comprises at least two sets of modulation and demodulation modules, and a photon source module 20 and an optical detection module 50 shared between the sets of modulation and demodulation modules. These modules are physically separated from each other and connected by their respective optical communication channels, preferably by their respective optical fibers.

[0075] In the illustrated example, system 2100 comprises three sets of modulation and demodulation modules, 30a, 40b, 30f, 40c, and 30e, 40d, respectively, each module advantageously configured to generate its own quantum key that is not shared by the other sets of modulation and demodulation modules in the system.

[0076] The photon source module 20 communicates through each first optical communication channel, for example, each optical fiber F SMa F SMf F SMe The modulation modules 30a, 30f, and 30e of each set of modulation and demodulation modules are connected by the respective second optical communication channels, for example, each optical fiber F MDab F MDfc F MDed The respective modulation modules 30a, 30f, and 30e are connected to the optical detection module 50, each of which is connected to the respective third optical communication channel, for example, each optical fiber F DFb F DFc F DFd They are connected to the respective demodulation modules 40b, 40c, and 40d of the modulation module and demodulation module pair.

[0077] The photon source module 20 and the photodetector module 50 of system 2100 are operably connected to first switching devices 260 and 262, respectively. The first switching devices 260 and 262 are controlled by a supervisor device 270, respectively, to establish an optical communication channel, namely the first optical communication channel F between the photon source module 20 and the modulation modules 30a, 30f, and 30e. SMa F SMf F SMe , and the third optical communication channel F between the demodulation modules 40b, 40c, 40d and the optical detection module 50 DFb F DFc F DFd Both are assigned to the modulation module and demodulation module sets.

[0078] Referring to Figure 5, a quantum key distribution system according to the fourth embodiment of the present invention is shown. This system differs from the system 2100 in Figure 4 in that the photodetector module 50 is located in the same place as the photon source module 20.

[0079] Thus, the system, generally referred to by reference number 3100, comprises at least two sets of modulation and demodulation modules, in this example three sets of modulation and demodulation modules, namely 30a, 40b, 30f, 40c, and 30e, 40d, respectively, and a photon source module 20 and an optical detection module 50 shared between the sets of modulation and demodulation modules.

[0080] The photon source module 20 is physically separated from the modulation modules 30a, 30f, and 30e of each set of modulation and demodulation modules, and is connected by their respective first optical communication channels, for example, each optical fiber F SMa F SMf F SMe The modulation modules 30a, 30f, and 30e of each set of modulation and demodulation modules are connected by a second optical communication channel, for example, each optical fiber F MDab F MDfc F MDed The optical detection module 50 is physically separated from the respective demodulation modules 40b, 40c, and 40d, and connected to their respective third optical communication channels, for example, their respective optical fibers F DFb F DFc F DFd It is connected to demodulation modules 40b, 40c, and 40d.

[0081] The photon source module 20 and the photodetector module 50 of system 2100 are operably connected to the first switching devices 360 and 362, respectively. The first switching devices 360 and 362 are controlled by the supervisor device 370, respectively, to establish a single-photon communication channel, i.e., the first optical communication channel F between the photon source module 120 and the modulation modules 30a, 30f, and 30e. SMa F SMf F SMe , and the third optical communication channel F between the demodulation modules 40b, 40c, 40d and the optical detection module 50 DFb F DFc F DFd Both are assigned to the modulation module and demodulation module sets.

[0082] Figure 6 shows a quantum key distribution system according to a fifth embodiment of the present invention. This system differs from systems 2100 and 3100 in Figures 4 and 5 in that only the photon source module 20 is shared by a set of modulation and demodulation modules, while instead of providing sets of demodulation modules operably connected between them by a second optical communication channel, for example, each optical fiber F MRxab F MRxfc F MRxed The difference lies in the existence of receiving units Rxb, RXc, and Rxd that are operablely connected to each of the modulation modules 30a, 30f, and 30e.

[0083] Therefore, each receiving unit Rxb, Rxc, and Rxd comprises a second polarization stabilizer 42, 142, a third waveplate 44, 144 located downstream of the second polarization stabilizer and controlled by actuators 45, 145, a polarization beam splitter 46, 146 located downstream of the waveplate 44, 144, and a single-photon photodetector 52, 152 configured to detect the logic state of each photon and transmit the detected logic state to counting registers 48, 148.

[0084] The photon source module 20 communicates, for example, each optical fiber F, through the first optical communication channel. SMa F SMf F Sme Each modulation module 30a, 30f, and 30e is connected to the respective optical fiber F MRxab F MRxfc F MRxed Each receiving unit, Rxb, Rxc, and Rxd, is mounted accordingly.

[0085] The photon source module 20 of system 4100 is operably connected to a first switching device 460. The first switching device 460 is controlled by a supervisor device 470 and establishes a single-photon communication channel, i.e., a first optical communication channel F between the photon source module 20 and the modulation modules 30a, 30f, and 30e. SMa F SMf F Sme , and the second optical communication channel F between the modulation modules 30a, 30f, 30e and the receiving units Rxb, Rxc, Rxd MRxab F MRxfc F MRxed Both are assigned to each set of modulation modules and receiving units.

[0086] Referring to Figure 7, a quantum key distribution system according to a sixth preferred embodiment of the present invention is shown. This system differs from systems 2100 and 3100 in that switching devices are provided in both the modulation module and the demodulation module. In this way, each module of the system can exchange quantum keys with multiple modules.

[0087] The system, commonly referred to by reference number 5100, comprises at least two sets of modulation and demodulation modules, and a photon source module 20 and an optical detection module 50 shared between the sets of modulation and demodulation modules. These modules are physically separated from one another.

[0088] In the illustrated example, the system comprises three sets of modulation and demodulation modules, 30a, 40b, 30f, 40c, and 30e, 40d, respectively, each module being advantageously configured to generate its own quantum key that is not shared by the other sets of modulation and demodulation modules in the system.

[0089] The photon source module 20 provides a first optical communication channel, for example, each optical fiber F SMa F SMf This is connected to the modulation modules 30a and 30f of the modulation module and demodulation module set.

[0090] The demodulation module 40b uses each second optical communication channel to, for example, each optical fiber F MDab F MDfb It is connected to the modulation modules 30a and 30f by the respective second optical communication channels, for example, each optical fiber F MDac F MDfc and F MDec It is connected to the modulation modules 30a, 30f, and 30e. The demodulation module 40d is connected to each second optical communication channel, for example, each optical fiber F MDfd and F MDed It is connected to the modulation modules 30f and 30e.

[0091] The optical detection module 50 communicates through each third optical communication channel, for example, each optical fiber F DFb F DFc F DFd It is connected to demodulation modules 40b, 40c, and 40d.

[0092] The photon source module 20 and the photodetector module 50 of system 5100 are operably connected to the first switching devices 560 and 562, respectively. The third switching devices 561a, 561f, 561e and the fourth switching devices 563b, 563c, 563d are also operably connected to the modulation modules 30a, 30f, 30e and the demodulation modules 40b, 40c, 40d.

[0093] Each switching device 560, 562, 561a, 561f, 561e and 563b, 563c, 563d is controlled by the supervisor device 570 so that each module of the system can exchange quantum keys with multiple modules.

[0094] Referring to Figure 8, a quantum key distribution system according to a seventh alternative embodiment of the present invention is shown.

[0095] This system, commonly referred to as reference number 6100, is quite similar to system 100 described above with reference to Figure 2, but differs from system 100 in that its photon source module includes a parametric two-photon source instead of a single-photon source.

[0096] Thus, the system 6100 includes a photon source module 220. The photon source module 220 includes a parametric two-photon source 222, for example, an SPDC (Spontaneous Parametric Down Conversion) type parametric two-photon source 222, and preferably a semiconductor or superconducting type single-photon detector 224 located immediately downstream of the parametric two-photon source 222 and having the function of generating a synchronization signal. The synchronization signal is delivered along the synchronization line Ls between the transmitting unit Tx and the receiving unit Rx of the system 2100. In particular, the parametric two-photon source 222 generates a first photon and a second photon, of which the first photon is transmitted into a first optical communication channel, for example, an optical fiber F SMThe photon is guided inward, while the second photon is preferably detected by a single-photon detector 224. The single-photon detector 224 generates a synchronization signal at its output, which is distributed within the system along the synchronization line Ls.

[0097] Naturally, the photon source module 220 with a parametric two-photon source, shown in Figure 8 and applied to system 100 in Figure 2, can be used in place of the single-photon source photon source modules (20, 120) in all other systems 1100, 2100, 3100, 4100, and 5100.

[0098] In all of the aforementioned systems 100, 1100, 2100, 3100, 4100, 5100, and 6100, synchronization is delivered via the synchronization line Ls. Furthermore, each system 100, 1100, 2100, 3100, 4100, and 5100 also shares a classic communication channel (not shown).

[0099] From the above explanation, it is clear that the objectives of this presentation can be achieved by the aforementioned quantum key distribution system, modulation module, and demodulation module.

[0100] Therefore, it will be apparent to those skilled in the art that modifications and variations can be made to the solution described with reference to the drawings without departing from the scope of protection of the present invention as defined by the attached claims.

Claims

1. Photon source modules (20, 120, 220) and Physically separated from the aforementioned photon source modules (20, 120), the first optical communication channel (F ＳＭ A modulation module (30, 130) operably connected to the photon source module via ) A quantum key distribution system (100, 1100) comprising a receiving unit (Rx) operably connected to the modulation module (30, 130) via a second optical communication channel, The modulation module (30, 130) comprises a first polarization stabilizer (36, 136), and a first waveplate (32, 132) and a second waveplate (34, 134) located downstream of the first polarization stabilizer (36, 136) and controlled by actuators (35, 135). The receiving unit (Rx) comprises a second polarization stabilizer (42, 142), a third waveplate (44, 144) located downstream of the second polarization stabilizer (42, 142) and controlled by an actuator (45, 145), a polarization beam splitter (46, 146) located downstream of the third waveplate (44, 144), and a single-photon photodetector (52, 152) configured to detect the logic state of each photon and transmit the detected logic state to a counting register (48, 148). Quantum key distribution system (100, 1100).

2. The receiving unit comprises demodulation modules (40, 140) and optical detection modules (50, 150), The demodulation module (40, 140) includes the second polarization stabilizer (42, 142), the waveplate (44, 144), the actuator (45, 145) for controlling the waveplate (44, 144), the polarization beam splitter (46, 146), and the counting register (48, 148). The photodetection module (50, 150) includes the single-photon photodetector (52, 152), The demodulation modules (40, 140) are connected to the second optical communication channel (F ＭＤ The modulation modules (30, 130) are operably connected via the third optical communication channel (F ＤＦ ) operably connected to the demodulation module (40, 140) via The system according to claim 1 (100, 1100).

3. The system (1100) according to claim 2, wherein the demodulation module (140) further comprises a fourth waveplate (149) located downstream of the waveplate (144), controlled by an actuator (143), and preferably associated with the counting register (148).

4. The system (100, 1100) according to any one of claims 1 to 3, wherein the photon source module (20, 120) comprises a single photon source (22, 122).

5. The system (100, 1100) according to claim 4, wherein the photon source module (20, 120) further comprises a shutter (24, 124) located downstream of the single photon source (22, 122) and configured to generate a synchronization signal that is delivered into the system along a synchronization line (Ls).

6. The system (6100) according to any one of claims 1 to 3, wherein the photon source module (200) includes a parametric two-photon source (222).

7. The system (6100) according to claim 6, wherein the photon source module (200) further comprises a semiconductor or superconducting single-photon detector (224) located downstream of the parametric two-photon source (222), the single-photon detector being configured to detect a second photon from the parametric two-photon source (222) and to generate a synchronization signal at its output that is delivered into the system along a synchronization line (Ls).

8. The first, second, and third optical communication channels (F ＳＭ , F ＭＤ , F ＤＦ The system according to claim 2 or 3 (100, 1100), wherein the optical fiber is made of optical fiber.

9. The third optical communication channel is a pair of optical fibers (F ＤＦ The system (100, 1100) according to any one of claims 2, 3, and 8, including ).

10. The first waveplate (32, 132) of the modulation module (30, 130) has the first optical communication channel (F ＳＭ The system (100, 1100) according to any one of claims 1 to 9, wherein the second waveplate (34, 134) enables the establishment of a spin basis (B) for each photon entering the modulation module (30, 130) via the waveplate, and enables the establishment of a state or code (C) that generates a bit logic sequence forming a quantum key.

11. A second modulation module (30a, 30f, 30e) and a second demodulation module (40b, 40c, 40d), the second modulation module (30a, 30f, 30e) and the second demodulation module (40b, 40c, 40d) being connected to each other by way of second, first, and third optical communication channels (F ＭＤａｂ , F ＭＤｆｃ , F ＭＤｅｄ , F ＳＭａ , F ＳＭｆ , F ＳＭｅ , F ＤＦｂ , F ＤＦｃ , F ＤＦｄ ), and being connected to the photon source module (20) and the photodetection module (50), the system (2100, 3100, 5100) according to any one of claims 2, 3, 8, and 9.

12. The photon source module (20) is associated with a first switching device (260, 360, 560) and / or the photodetector module (50) is associated with a second switching device (262, 362, 562), The first switching devices (260, 360, 560) each have a first optical communication channel (F ＳＭａ , F ＳＭｆ , F ＳＭｅ Each modulation module (30a, 30f, 30e) is operably connected by ) The second switching devices (262, 362, 562) each have a third optical communication channel (F ＤＦｂ , F ＤＦｃ , F ＤＦｄ Each demodulation module (40b, 40c, 40d) is operationally connected by ) The system according to claim 11 (2100, 3100, 5100).

13. Each modulation module (30a, 30f, 30e) is associated with a third switching device (561a, 561f, 561e), and each demodulation module (40b, 40c, 40d) is associated with a fourth switching device (563b, 563c, 563d), The third switching devices (561a, 561f, 561e) each have a second optical communication channel (F ＭＤａｂ , F ＭＤｆｃ , F ＭＤｅｄ ) are operably connected to each of the fourth switching devices (563b, 563c, 563d) The system (5100) according to claim 12.

14. The system comprises a second modulation module (30a, 30f, 30e) and a second receiving unit (Rxb, Rxc, Rxd), wherein the second modulation module (30a, 30f, 30e) and the second receiving unit (Rxb, Rxc, Rxd) are connected to a second and first optical communication channel (F ＭＲｘａｂ F ＭＲｘｆｃ F ＭＲｘｅｄ , F ＳＭａ , F ＳＭｆ , F ＳＭｅ The system (4100) according to any one of claims 1 to 10, wherein each of the ) is connected between them and connected to the photon source module (20).

15. The system according to any one of claims 11 to 13 (2100, 3100, 4100, 5100), further comprising a supervisor device (270, 370, 470, 570), wherein the supervisor device (270, 370, 470, 570) is configured to activate the first and third optical communication channels of each modulation module (30a, 30f, 30e) and each demodulation module (40b, 40c, 40d), or the first and second optical communication channels of each modulation module (30a, 30f, 30e) and receiving units (Rxb, Rxc, Rxd).

16. Polarization stabilizers (36, 136) connected to input optical ports (31, 131) for receiving photons transmitted by photon source modules (20, 120, 220), A first waveplate (32, 132) and a second waveplate (34, 134) are arranged downstream of the polarization stabilizer (36, 136), In order to modulate the polarization of the photons, actuators (35, 135) are configured to control the first waveplates (32, 132) and the second waveplates (34, 134), Optical communication channel (F ＭＤ ) via optical output ports (37, 137) for transmitting the modulated photons, A modulation module (30, 130) equipped with the following.

17. A polarization stabilizer (42, 142) connected to an input optical port (41, 141) for receiving photons modulated by a modulation module (30, 130), Waveplates (44, 144) arranged downstream of the polarization stabilizers (42, 142), In order to demodulate the polarization of the photons, actuators (45, 145) are configured to control the waveplates (44, 144), A polarizing beam splitter (46, 146) is positioned downstream of the waveplate (44, 144) and configured to distinguish between high and low logic states associated with the received photons, A counting register (48, 148) configured to receive the logic state of the photons transmitted by the photodetector module (50, 150), Demodulation modules (40, 140) equipped with these.

18. The demodulation module (140) according to claim 17, further comprising a further waveplate (149) positioned downstream of the waveplate (144) and controlled by an actuator (143).

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