Quantum key distribution network and its multi-function nodes

Abstract

The present invention relates to a network (100) for quantum key distribution and its multifunction nodes. The network (100) comprises a photon source node (10) including a single photon source (12) and a number of multifunction nodes (20, 120) connected between the photon source node (10) and the multifunction nodes by respective optical communication channels (F). Each multifunction node (20, 120) comprises a transmission unit (Tx) including a polarization stabilizer (25) connected to an optical input port (21) for receiving photons transmitted by the photon source node (10), and a first wave plate (22) and a second wave plate (23) arranged downstream of the polarization stabilizer (25) and controlled by a first actuator (24). Each multifunction node (20, 120) further comprises a polarization stabilizer (25), a first wave plate (22) controlled by a second actuator (29), a polarizing beam splitter (26) arranged downstream of the first wave plate (23) and configured to detect the logical state of each photon, at least one photodetector (27a, 27b), and a counting register (28) arranged to receive the logical state of the detected photons. Each multifunction node (20, 120) further comprises an optical switch (30) arranged between the second wave plate (23) and the polarizing beam splitter (26) and which can be activated / deactivated to operate the multifunction node (20) according to a transmitter configuration or a receiver configuration. In a transmitter configuration, a transmitting unit (Tx) modulates photons entering the multifunction node (20) via the optical input port (21) and provides modulated polarized photons to the optical output port (32) of the multifunction node (20, 120). In a receiver configuration, a receiving unit (Rx) demodulates the modulated photons entering the multifunction node (20, 120) via the optical input port (21).

Description

[Technical field]

[0001] The present invention relates to the technical field of quantum cryptography, in particular to a network for quantum key distribution and a multi-function node thereof. [Background technology]

[0002] Quantum Key Distribution (QKD) is a technology that allows sharing a symmetric (i.e., a bit string that simultaneously materializes at the transmitter and receiver) and absolutely secure "key" (i.e., a key that cannot be decrypted by any algorithm and computing power) between two nodes (hereafter 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 measured precisely if it is in a precise reference base (which can be linear, diagonal or circular), otherwise the measurement will produce completely random values. The second quantum property is that the spin state of a particle cannot be replicated. The third quantum property concerns light, as it is possible to generate and propagate single photons in any optical communication channel (free space, optical fiber or integrated optical guides).

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

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

[0006] The original scheme of QKD according to the BB84 protocol is a point-to-point transmission (i.e. a transmitter Tx is connected with only one receiver Rx) and concerns the communication of the quantum key between a transmitter Tx and a receiver Rx through an optical communication channel without birefringence. In a more general case, if there is an optical communication channel that exhibits birefringence, at the end of the transmission, the original basis must be re-established by a device called a polarization stabilizer.

[0007] The transmitter Tx comprises a single photon source, i.e. a photon source whose intensity is attenuated so that it transmits only one photon on average (weak photon source), and first and second waveplates controlled by an actuator to which a single polarized photon from the single photon source is sent. The first waveplate makes it possible to establish a basis for the spin of the photon (the basis can be linear, diagonal or circular) and the second waveplate makes it possible to establish a state or code (for example a logical high state corresponding to 1, a logical low state corresponding to 0) that generates the logical sequence of the bits constituting the key.

[0008] The receiver Rx comprises a polarization stabilizer allowing reorientation of the initial basis in case of a birefringent optical communication channel, and a waveplate controlled by an actuator, located downstream of the polarization stabilizer, allowing the initial basis to be restored (or not restored) by a random process. The receiver further comprises a polarizing beam splitter (PBS), to which each photon leaving the waveplate is assigned. The polarizing beam splitter is configured to distinguish between high and low logic states of the polarized photons and transmit them to a photodetection unit and from there to a counting register, thereby allowing the code transmitted by the transmitter Tx to be reconstructed.

[0009] The transmitter Tx and receiver Rx components are also connected between them by a synchronization channel, which distributes a synchronization signal between the transmitter and receiver, typically generated by a control signal of a shutter located in the transmitter Tx immediately downstream of the single-photon source, or by an identical pulse of the single-photon source.

[0010] In the above quantum key distribution system, the receiver Rx has both basis information and code information, which fully satisfies the integrity of the BB84 protocol. However, the system constructed in this way has some drawbacks.

[0011] First, when a system component is changed or upgraded, the entire transmitter Tx and / or receiver Rx must be upgraded, which is functionally disadvantageous and economically costly.

[0012] Secondly, as is well known, the single photon source and the photodetection units in the device may have to be kept very cold, so the transmitter Tx and receiver Rx of the device must be placed in a cryogenically controlled environment, which has negative impacts in terms of space and maintenance, and is logistically disadvantageous. Summary of the Invention

[0013] It is an object of the present invention to overcome the shortcomings of the prior art.

[0014] In particular, it is an object of the invention to present a network for quantum key distribution, and multifunctional nodes thereof, configured to allow greater flexibility in the definition of the network architecture.

[0015] It is also an object of the present invention to provide a network for quantum key distribution, in which each node is configured to exchange quantum keys with any other node in the network while maintaining a share of the photon source node.

[0016] These and other objects of the present invention are achieved by a network for quantum key distribution, and by a multifunctional node thereof, incorporating the features of the appended claims forming an integral part of this specification.

[0017] According to a first aspect, the present invention relates to a network for quantum key distribution comprising a photon source node including a single photon source and a plurality of multifunction nodes connected between the photon source node by respective optical communication channels.

[0018] Each multifunction node comprises a transmitting unit, the transmitting unit including a polarization stabilizer connected to an optical input port for receiving photons transmitted by the photon source node, and a first wave plate and a second wave plate disposed downstream of the polarization stabilizer and controlled by a first actuator.

[0019] Each multifunction node further comprises a receiving unit including a polarization stabilizer, a first wave plate controlled by the second actuator, a polarizing beam splitter disposed downstream of the first wave plate and configured to detect a logical state of each photon, at least one photodetector, and a counting register configured to receive the logical state of the detected photons.

[0020] Each multifunction node further comprises an optical switch disposed between the second waveplate and the polarizing beam splitter and activatable / deactivatable to operate the multifunction node according to a transmitter configuration or a receiver configuration. In the transmitter configuration, the sending unit modulates photons entering the multifunction node via the optical input port and provides modulated polarized photons to the optical output port of the multifunction node. In the receiver configuration, the receiving unit demodulates the modulated photons entering the multifunction node via the optical input port.

[0021] This combination of features allows the single photon source to be shared by all multi-function nodes of the network, which has economic advantages since the cost of the photon source node is shared by all multi-function nodes, but also especially logistical and functional advantages since the single photon source can be installed in a specific room that may use, for example, cryogenic equipment or in a room monitored by specialized technicians.

[0022] Moreover, the photon source nodes can be updated without affecting the multi-function nodes of the network. In a climate of rapid technological innovation, this feature is of great importance in terms of strengthening the investment in system construction.

[0023] Furthermore, since multifunction nodes are structurally the same regardless of their function, the network can achieve economies of scale: each multifunction node can exchange quantum keys with other nodes in the network within the time allocated by the supervisor device.

[0024] In one embodiment, the network comprises a supervisor device and an optical switching array associated with each multi-function node, the optical switching array operable by the supervisor device to select a multi-function node that forms an optical communication channel for distribution of a quantum key outgoing from a photon source node.

[0025] In one embodiment, the optical switching array has a first port and a second port to which the input and output optical communication channels of the respective multi-function node are respectively connected, and the second ports are connected to corresponding second ports of optical switching arrays of the photon source node and / or at least one other multi-function node of the network.

[0026] In one embodiment, each multifunction node further comprises a mirror disposed upstream of the optical output port.

[0027] In one embodiment, each multifunction node comprises two further optical switches, one of which is disposed between the optical input port and the polarization stabilizer and the other of which is disposed upstream of the optical output port, the two further optical switches being activatable to operate the multifunction node according to a pass-through configuration, the optical input port communicating directly with the optical output port of the multifunction node via a pass-through optical communication channel passing through the two further optical switches.

[0028] In one embodiment, the optical switch of each multifunction node is operable by a supervisor device to configure each multifunction node in a transmitter configuration, a receiver configuration or a pass-through configuration.

[0029] In one embodiment, a photon source node is disposed downstream of the single photon source and comprises a shutter configured to generate a synchronization signal, which is distributed along a synchronization line to all nodes in the network.

[0030] In one embodiment, the single photon source of the photon source node is configured to generate a pair of photons, one of which is used within the network to generate a quantum key and the other of which is used to establish a synchronization signal that is distributed along the network along a synchronization channel.

[0031] According to a second aspect, the present invention relates to a multifunction node comprising a transmitting unit, the transmitting unit including a polarization stabilizer connected to an optical input port for receiving photons transmitted by a photon source node, and a first wave plate and a second wave plate arranged downstream of the polarization stabilizer and controlled by a first actuator.

[0032] The multi-function node further comprises a receiving unit, the receiving unit including a polarization stabilizer, a first wave plate controlled by the second actuator, a polarizing beam splitter disposed downstream of the first wave plate and configured to detect a logical state of each photon, at least one photodetector, and a counting register configured to receive the logical states of the detected photons.

[0033] Each multifunction node further comprises an optical switch disposed between the second waveplate and the polarizing beam splitter and activatable / deactivatable to operate the multifunction node according to a transmitter configuration or a receiver configuration. In the transmitter configuration, the sending unit modulates photons entering the multifunction node via the optical input port and provides modulated polarized photons to the optical output port of the multifunction node. In the receiver configuration, the receiving unit demodulates the modulated photons entering the multifunction node via the optical input port.

[0034] In one embodiment, in a transmitter configuration, the optical switch is switched on and the second actuator for controlling the first waveplate is deactivated to break the connection between the second actuator and the first waveplate, while in a receiver configuration, the optical switch is switched off and the first actuator for controlling the first and second waveplates is deactivated to break the connection between the first actuator and the first waveplate, and the second waveplate is set to a zero birefringence value.

[0035] In one embodiment, the multifunction node further comprises a mirror disposed upstream of the optical output port.

[0036] In one embodiment, the multi-function node comprises two further optical switches, one of which is disposed between the optical input port and the polarization stabilizer and the other of which is disposed upstream of the optical output port, the two further optical switches being activatable to operate the multi-function node according to a pass-through configuration, the optical input port communicating directly with the optical output port of the multi-function node via a pass-through optical communication channel passing through the two further optical switches.

[0037] In one embodiment, the transmitting unit is connected between the optical input port and the optical output port via a transmitting optical communication channel, and the receiving unit is connected to the optical input port via a receiving optical communication channel.

[0038] In one embodiment, in a transmitter configuration, the optical switch is switched on, a further optical switch is switched off and a second actuator for controlling the first waveplate is deactivated to break the connection between the second actuator and the first waveplate, while in a receiver configuration, both optical switches are switched off, the first actuator for controlling the first waveplate and the second waveplate is deactivated to break the connection between the first actuator and the first waveplate, and the second waveplate is set to a zero birefringence value.

[0039] In one embodiment, in the pass-through configuration, two further optical switches are switched on.

[0040] Further features and objects of the present invention will become more apparent from the following description. [Brief description of the drawings]

[0041] The present invention will now be described, by way of non-limiting example, with reference to specific examples illustrated in the accompanying drawings, which illustrate various aspects and embodiments of the invention, and in which, where appropriate, reference numerals designating structures, components, materials and / or similar elements in the various drawings are designated with like reference numerals.

[0042] [Figure 1] 1 illustrates a schematic diagram of a network for quantum key distribution according to a preferred embodiment of the present invention; [Diagram 2] 2 illustrates diagrammatically an optical switching array associated with each multi-function node of the network for quantum key distribution of FIG. 1; [Diagram 3] 2 illustrates a schematic diagram of a single-photon source node of the network for quantum key distribution of FIG. 1; [Figure 4] 1 illustrates a schematic diagram of a multifunction node according to a first embodiment of the present invention; [Diagram 5] 5 illustrates a schematic diagram of a multifunction node according to a second embodiment of the present invention; [Figure 6] 1 shows a schematic diagram of a network for quantum key distribution in possible operational configurations of the multifunction nodes which compose it; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] While the invention is susceptible to various modifications and alternative constructions, certain embodiments are described in detail below for purposes of illustration.

[0044] In any event, it should be understood that there is no intention to limit the invention to the particular embodiments illustrated, but on the contrary, the invention is intended to cover all modifications, alternatives, and equivalent arrangements falling within the scope of the present invention as defined by the appended claims.

[0045] Thus, in the following description, the use of "for example," "e.g.," and "or" indicates open-ended, non-exclusive alternatives unless otherwise stated, the use of "also" means "including, but not limited to," unless otherwise stated, and the use of "including" means "including, but not limited to," unless otherwise stated.

[0046] Referring to FIG. 1, a network for quantum key distribution according to a preferred embodiment of the present invention is shown.

[0047] The network, generally referred to by the reference numeral 100 , includes a photon source node 10 and a number of multi-function nodes 20 .

[0048] Each multifunction node 20 of the network 100 is associated with an optical switching array 40. The optical switching array 40 is controllable by a supervisor device 60 to select a multifunction node 20 that will form a distribution channel for the quantum key 50 emanating from the photon source node 10. The optical switching array 40 of each multifunction node 20 is connected to and between the photon source node 10 by a respective optical communication channel, e.g., optical fiber F. Alternatively, each optical communication channel may be a free space communication channel.

[0049] As shown in more detail in FIG. 2, each optical switching array 40 has a first port E1, E2 and a second port W1, W2, . . . Wn, and the first ports (E1, E2) are connected to the input optical communication channels F of the respective multifunction node (20). in and output optical communication channel F out , Wn are connected to the photon source node 10 and / or to corresponding second ports W1, W2, ... Wn of other optical switching arrays 40 of the network 100 by optical fibers F. in and output optical communication channel F out is an optical fiber or, alternatively, a free space communication channel. The optical switching array 40 may be integrated into the multifunction node 20 to form an integrated multifunction node 70. In this case, the input optical communication channel F of the multifunction node in and output optical communication channel F out is a fiber optic integrated communications channel.

[0050] 1, the optical communication array 40 of each multifunction node 20 is connected to the optical switching arrays of three multifunction nodes 20, except for the optical switching array 40 of the multifunction node 20 located at the top left, which is also connected to the photon source node 10. In the network 100 configured in this way, the optical switching array 40 also connected to the photon source node 10 has four second ports W1-W4. One of the four second ports is for connection to the photon source node 10, and three are for connection to other optical switching arrays 40 in the network. Each of the other optical switching arrays 40 has three second ports W1-W3 connected to a respective optical switching array 40 in the network 100.

[0051] As will be explained in more detail later in this specification, each multifunction node 20 is configured so that it can operate according to two possible configurations: a transmitter configuration and a receiver configuration.

[0052] In particular, in the transmitter configuration, each multifunction node 20 is configured to modulate the photons arriving from the photon source node 10. Specifically, in the transmitter configuration, each multifunction node 20 selects a basis, which may be linear, diagonal, or circular, and state values, or codes, that generate a logical sequence of bits that constitute the quantum key, e.g., a high logic state or 1 (corresponding to a photon spin up), a low logic state or 0 (corresponding to a photon spin down). Furthermore, in the transmitter configuration, each multifunction node 20 is configured to transmit to its output the modulated photons received as input. The supervisor device 60 also configures the optical switching array 40 associated with the multifunction node 20 to send the modulated photons to other multifunction nodes 20.

[0053] In the receiver configuration, each multifunction node 20 is configured to demodulate modulated photons arriving from the multifunction node 20 in the transmitter configuration, i.e., to determine a reception basis and extract the state value or code associated with each photon.

[0054] 3, the photon source node 10 comprises a single photon source 12, i.e. a photon source whose intensity is reduced to such an extent that it transmits only one photon on average per command pulse (weak photon source). Preferably, the photon source node 10 further comprises a shutter 14 located immediately downstream of the single photon source 12 and capable of generating a synchronization signal. The synchronization signal is distributed among all multifunction nodes 20 of the network 100 along a synchronization line Ls.

[0055] Alternatively, the single photon source 12 is configured to be heralded, i.e. to generate pairs of photons, one of which is used in the network 100 to generate a quantum key 50 and the other of which is used to establish a synchronization signal that is distributed along the network 100 via a synchronization signal channel Ls.

[0056] Finally, the photon source node 10 comprises an optical output port 16, represented by a lens (if subsequent optics operate in free space), by a fiber-to-fiber weld, or by a single optical fiber connector if the communication channel is of optical fiber type. In the example described herein, the optical output port 16 consists of a connector to which an optical fiber F is connected, which connects the photon source node 10 to an optical switching array 40 of the network 100.

[0057] FIG. 4 shows in detail the multifunction node 20 according to the first embodiment of the present invention.

[0058] The multifunction node 20 comprises an optical input port 21, for example an optical fiber connector, at which it receives input photons. As mentioned above, these photons may come from the photon source node 10 or from other multifunction nodes 20 in the network.

[0059] The multifunction node 20 has an optical output communication channel F out It further comprises an optical output port 32 , eg, a fiber optic connector, over which photons can be transmitted and transmitted to other multifunction nodes 20 in the network 100 .

[0060] The optical input port 21 and the optical output port 32 of the multifunction node 20 may have the shape of a lens, a fiber-to-fiber weld, or a single mode fiber optic connector if the subsequent optics operate in free space.

[0061] Downstream of the optical input port 21, the multi-function node 20 further comprises a first waveplate 22 and a second waveplate 23 controlled by a first actuator 24. The first waveplate 22 and the second waveplate 23 are fed with single polarized photons, or qubits, emanating from the single photon source 12 of the photon source node 10.

[0062] The first wave plate 22 is connected to the optical fiber F in and the second waveplate 23 allows to establish a basis B of the spins of the polarized photons entering the multifunctional node 20 via the first waveplate 24, and a state or code C that generates a logical sequence of bits constituting the key (for example a high logic state corresponding to 1, a low logic state corresponding to 0). The basis is started from a predefined state by the action of a half-wave or quarter-wave waveplate, as is well known to the expert in the field. As is well known to the expert in the field, the values ​​of the (high or low) logic states can be generated by a half-waveplate appropriately oriented according to the chosen basis.

[0063] Input optical fiber F of multifunction node 20 in Since q typically exhibits birefringence, multifunction node 20 further comprises a polarization stabilizer 25 upstream of waveplates 22 and 23. Polarization stabilizer 25 is connected between optical input port 21 and first waveplate 22 to receive incoming polarized photons and reorient the spin of the photons into the B basis. The polarization stabilizer restores the original configuration of the Poincaré sphere used to modulate the qubits in all three axes thereof, and is, for example, a model EPS1000 manufactured and sold by Novoptel GmbH of Germany.

[0064] The polarization stabilizer 25, the first wave plate 22, the second wave plate 23 and the first wave plate actuator 24 constitute a transmitting unit Tx of the multi-functional node 20 and are connected between themselves via an optical transmission communication channel, for example an optical fiber F1 or a free space optical communication channel, as well as to the optical input port 21 and the optical output port 32.

[0065] Multifunctional node 20 further comprises, downstream of waveplates 22 and 23, a polarizing beam splitter 26 to which each polarized photon exiting waveplates 22 and 23 is directed. In particular, polarizing beam splitter 26 identifies a high logic state 1 or a low logic state 0 of the photons and transmits them to photodetectors 27a and 27b, which are configured to transmit them to a counting register 28, allowing reconstruction of the message code.

[0066] The multi-function node 20 further includes a second actuator 29 of the first wave plate 22. The actuator 29 is connected to the counting register 28 and the first wave plate 22 via a synchronization line Ls, and controls the first wave plate 22 to restore the initial basis B by a random procedure.

[0067] The polarization stabilizer 25, the first wave plate 22 controlled by the second actuator 29, the polarization beam splitter 26, the photodetectors 27a, 27b and the counting register 28 constitute the receiving unit Rx of the multifunctional node 20 and are connected between themselves and to the optical input port 21 via a receiving optical communication channel, e.g., an optical fiber F2 or a free space optical communication channel.

[0068] The optical switch 30 is provided between the second wave plate 23 and the polarizing beam splitter 26 .

[0069] When the multifunction node 20 is in the transmitter configuration, the optical switch 30 is switched on (ON position in FIG. 4) and the photons are directed to a mirror 33 which sends them to an optical output port 32 .

[0070] In this case, polarized photons from the photon source node 10 are fed into the input optical communication channel F in and travels along the outgoing optical communication channel F1.

[0071] In particular, the photons enter a polarization stabilizer 25 and then pass through a first wave plate 22 and a second wave plate 23. The polarization stabilizer 25 compensates for birefringence present in the optical fiber traversed by the polarized photons, and the first wave plate 22 and the second wave plate 23 are controlled by a first actuator 25 to generate a basis B and a transmission code C according to the BB84 protocol.

[0072] The photon travels along its path towards optical switch 30, which, when switched on, directs it to mirror 33 and from there to optical output port 32. The photon exiting optical output port 32 is transmitted to optical output communication channel F. out and via respective optical switching arrays 40 to other multifunction nodes 20 of network 100 .

[0073] Preferably, the optical path between the polarization stabilizer 25 and the optical switch 30 should be free of birefringence. Furthermore, in the transmitter configuration of the multi-function node 20, the second actuator is deactivated and the connection between the second actuator 29 and the first waveplate 22 is broken.

[0074] When the multifunction node 20 is in the receiver configuration, the optical switch 30 is switched off (OFF position in FIG. 4).

[0075] In this case, polarized photons coming from the multifunction node 20 of the network 100 in the transmitter configuration are fed into the input optical communication channel F in and travels along receive optical communication channel F2.

[0076] In particular, the photon enters the polarization stabilizer 25 and travels through the first wave plate 22 and the second wave plate 23. The photon continues its path towards the optical switch 30 and, when the optical switch 30 is switched off, towards the polarizing beam splitter 26. The polarizing beam splitter 26 identifies the high logic state 1 or low logic state 0 of the photon and transmits it via photodetectors 27a and 27b to a counting register 28, allowing reconstruction of the received code C.

[0077] The optical path between the polarization stabilizer 25, the optical switch 30 and the polarizing beam splitter 26 must be free of birefringence. Furthermore, in the receiver configuration of the multifunction node 20, the first actuator 24 is deactivated and the second waveplate 23 is set to a zero birefringence value (i.e., the waveplate is oriented according to its principal axis with zero retardation) such that the connection between the first actuator 24 and the first waveplate 22 is interrupted.

[0078] In both configurations of the multifunction node 20 (transmitter configuration and receiver configuration), the integrity of the BB84 protocol is fully fulfilled: indeed, when the multifunction node 20 is in the transmitter configuration, only the node knows both the basis B and the code C of the transmitted quantum key 50, whereas when the multifunction node 20 is in the receiver configuration, knowledge of both the basis B and the code C is only possessed by the receiving unit Rx.

[0079] FIG. 5 shows in detail a multifunction node 120 according to a second embodiment of the present invention.

[0080] Multifunction node 120 differs from previously described multifunction node 20 in that it further comprises optical means configured to allow photons received as an input to follow a path towards output port 32. In this way, multifunction node 120 can also operate as a pass-through node, which does not modulate or demodulate photons but simply passes them through itself. Such a configuration may be useful in certain network configurations.

[0081] Immediately after the input port 21, the multifunction node 120 includes two optical switches 131 and 133, which can assume two positions when the switches are turned on or off. In particular, the optical switch 131 is disposed between the optical input port 21 and the polarization stabilizer 25, while the other optical switch 133 is disposed upstream of the optical output port 32. The two optical switches 131 and 133 are aligned along a straight optical path connecting the input port 21 to the output port 32. In the example of FIG. 5, when the optical switches 131 and 133 are switched on (ON position in FIG. 5), the incoming photons pass through them and proceed until they reach the output port 32.

[0082] On the other hand, when optical switches 131, 133 are switched off (OFF position in FIG. 5 ), the photons are sent to polarization stabilizer 25 where they proceed and are correctly processed and received (if the node is in a receiver configuration) or modulated (if the node is in a transmitter configuration) as previously described for multifunction node 20. In the example of FIG. 5 , the photons redirected by optical switch 131 are directed to polarization stabilizer 25 by mirror 134, which is used to define an optical path. When multifunction node 120 is in a transmitter configuration, the modulated photons exiting waveplate 23 are directed to optical switch 133, which, because it is in the OFF position, redirects the modulated photons to output port 32.

[0083] 6 shows a possible configuration of a network 100 for quantum key distribution according to an embodiment of the present invention. The configuration comprises a photon source node 10 and respective optical fibers F AB , F BC , F CD In the example of FIG. 6, the optical input port 21 of the multifunction node 120A is connected to an optical fiber F. SA is connected to the photon source node 10 by

[0084] In the network configuration of Figure 6, the multi-function nodes are nodes of the three configuration types (pass-through configuration, transmitter configuration and receiver configuration) described above with reference to Figure 5, but node 120D may be of the two configuration type (receiver configuration and transmitter configuration) of Figure 4, since it does not perform a pass-through function at this location.

[0085] Now assume that it is desired to distribute quantum key 50 between a third multifunction node 120C and a fourth multifunction node 120D in network 100.

[0086] To this end, supervisor device 60 places first and second multifunction nodes 120A and 120B in a pass-through configuration, i.e., switches on their optical switches 131 and 133, allowing them to retransmit to the output the photons received as input. Supervisor device 60 instead places third multifunction node 120C in a transmitter configuration and fourth multifunction node 120D in a receiver configuration.

[0087] Photons generated by the photon source node 10 then enter the multifunction node 120A in a pass-through configuration and are directed from there to the optical output port 32 and travel along the pass-through optical communication channel F3 directly, i.e. without entering a transmitting unit Tx or a receiving unit Rx, as shown in detail in FIG. 5 .

[0088] The photons exiting the optical output port 32 of the multifunction node 120A are then transmitted to the optical communication channel F AB to the second pass-through multifunction node 120B, where it is directed by the optical switch 133 directly to the optical output port 32 and travels along the pass-through optical communication channel F3, in the same manner as described above with respect to the first pass-through multifunction node 120A.

[0089] The photons leaving the optical output port 32 of the pass-through multifunction node 120B are then transmitted to the optical communication channel F BC to the third multifunction node 120C of the transmitter arrangement, where the photons are modulated.

[0090] In particular, referring to FIG. 5, photons enter a polarization stabilizer 25, from which they pass to a first wave plate 22 and a second wave plate 23, controlled by a first actuator 24, to generate a basis B and a transmission code C according to the BB84 protocol.

[0091] The photons are directed by optical switch 133 to optical output port 32, from where they are transmitted through optical communication channel F CD and is received as an input to multifunction node 120D, which acts as a receiver.

[0092] 5, the photons enter the polarization stabilizer 25 and from there to the first waveplate 22 and the second waveplate 23 to generate a receive basis B according to the BB84 protocol. To this end, as explained above, the first waveplate 22 is controlled by the second actuator 29, the polarization stabilizer 25 compensates for any birefringence present in the optical fiber traversed by the polarized photons, and the second waveplate 23 is set to a birefringence value of zero.

[0093] The photons are then directed to a polarizing beam splitter 26, which distinguishes between the high logic state 1 or the low logic state 0 of the photons and transmits them to photodetectors 27a and 27b, from where they are transmitted to a counting register 28, allowing the reconstruction of the received code C.

[0094] From the above description, it is evident that the above described system and network for quantum key distribution can achieve the proposed objectives, in particular, the above described multi-function node can obviously be used very flexibly to generate different types of networks.

[0095] Thus, it is obvious to a person skilled in the art that modifications and variations are possible to the solution described with reference to the drawings without going beyond the scope of protection of the invention as defined by the appended claims.

[0096] For example, it will be apparent that in the network of FIG.

Claims

1. A network (100) for distribution of a quantum key (50), comprising: a photon source node (10) including a single photon source (12); A plurality of multifunction nodes (20, 120); the plurality of multi-function nodes (20, 120) are connected to the photon source node (10) and between the plurality of multi-function nodes by respective optical communication channels (F); Each multifunction node (20, 120) a transmitting unit (Tx) including a polarization stabilizer (25) connected to an optical input port (21) for receiving photons transmitted by said photon source node (10), and a first wave plate (22) and a second wave plate (23) arranged downstream of said polarization stabilizer (25) and controlled by a first actuator (24); a receiving unit (Rx) including the polarization stabilizer (25), the first wave plate (22) controlled by a second actuator (29), a polarizing beam splitter (26) arranged downstream of the first wave plate (22) and configured to detect a logical state of each photon, at least one photodetector (27a, 27b), and a counting register (28) configured to receive the logical state of the detected photons; an optical switch (30) disposed between the second wave plate (23) and the polarizing beam splitter (26), the optical switch (30) being activatable / deactivatable to operate the multi-function node (20) according to one of a transmitter configuration and a receiver configuration; In the transmitter configuration, the transmitting unit (Tx) modulates the photons entering the multifunction node (20) via the optical input port (21) and provides modulated polarized photons to an optical output port (32) of the multifunction node (20, 120); In the receiver configuration, the receiving unit (Rx) demodulates the modulated photons entering the multifunction node (20, 120) via the optical input port (21). Network (100).

2. The network comprises a supervisor device (60); an optical switching array (40) associated with each said multifunction node (20); the optical switching array (40) is operable by the supervisor device (60) to select the multi-function node (20) that forms the optical communication channel for distribution of the quantum key (50) out of the photon source node (10); The network (100) of claim 1.

3. The optical switching array (40) has first ports (E1, E2) and second ports (W1, W2, ... Wn); The first ports (E1, E2) are connected to the input optical communication channels (F) of the respective multifunction nodes (20). ｉｎ ) and the output optical communication channel (F ｏｕｔ ) are connected to each other, said second ports (W1, W2, ... Wn) are connected to corresponding second ports (W1, W2, ... Wn) of an optical switching array (40) of said photon source node (10) and / or at least one other multifunction node (20) of said network (100); The network (100) of claim 2.

4. The network (100) of claim 1, wherein each multifunction node (20) further comprises a mirror (33) disposed upstream of said optical output port (32).

5. Each multifunction node (120) comprises two further optical switches (131, 133); one of the two further optical switches is disposed between the optical input port (21) and the polarization stabilizer (25); the other of the two further optical switches is located upstream of the optical output port (32); the two further optical switches (131, 133) are activatable to operate the multifunction node (120) according to a pass-through configuration; said optical input port (21) communicating directly with said optical output port (32) of said multifunction node (120) via a pass-through optical communication channel (F3) passing through said two further optical switches (131, 133); The network (100) of claim 1.

6. 6. The network (100) of claim 5, wherein the optical switches (30, 131, 133) of each multifunction node (20, 120) are operable by a supervisor device (60) to configure each multifunction node into the transmitter configuration, the receiver configuration or the pass-through configuration.

7. 2. The network (100) of claim 1, wherein the photon source node (10) comprises a shutter (14) arranged downstream of the single photon source (12) and configured to generate a synchronization signal, the synchronization signal being distributed along a synchronization line (Ls) to all nodes (10, 20, 120) of the network.

8. The single photon source (12) of the photon source node (10) is configured to generate a pair of photons; one of the pair of photons is used within the network to generate the quantum key (50) and the other is used to establish a synchronization signal distributed along the network via a synchronization channel (Ls), The network (100) of claim 1.

9. a transmitting unit (Tx) including a polarization stabilizer (25) connected to an optical input port (21) for receiving photons transmitted by a photon source node (10), and a first wave plate (22) and a second wave plate (23) arranged downstream of the polarization stabilizer (25) and controlled by a first actuator (24); a receiving unit (Rx) including the polarization stabilizer (25), the first wave plate (22) controlled by a second actuator (29), a polarizing beam splitter (26) arranged downstream of the first wave plate (22) and configured to detect a logical state of each photon, at least one photodetector (27a, 27b), and a counting register (28) configured to receive the logical state of the detected photons; an optical switch (30) disposed between the second wave plate (23) and the polarizing beam splitter (26), the optical switch (30) being activatable / deactivatable to operate the multi-function node (20, 120) according to one of a transmitter configuration and a receiver configuration; In the transmitter configuration, the transmitting unit (Tx) modulates the photons entering the multifunction node (20, 120) via the optical input port (21) and provides modulated polarized photons to an optical output port (32) of the multifunction node (20, 120); In the receiver configuration, the receiving unit (Rx) demodulates the modulated photons entering the multifunction node (20, 120) via the optical input port (21). A multi-function node (20, 120).

10. 10. The multi-function node (20, 120) of claim 9, wherein in the transmitter configuration, the optical switch (30) is switched on and the second actuator (29) for controlling the first wave plate (22) is deactivated so as to break the connection between the second actuator (29) and the first wave plate (22), and in the receiver configuration, the optical switch (30) is switched off and the first actuator (24) for controlling the first wave plate (22) and the second wave plate (23) is deactivated so as to break the connection between the first actuator (24) and the first wave plate (22), and the second wave plate (23) is set to a zero birefringence value.

11. A multifunction node (20) according to claim 9 or 10, further comprising a mirror (33) arranged upstream of said optical output port (32).

12. two further optical switches (131, 133), one of the two further optical switches is disposed between the optical input port (21) and the polarization stabilizer (25); the other of the two further optical switches is located upstream of the optical output port (32); the two further optical switches (131, 133) are activatable to operate the multifunction node (120) according to a pass-through configuration; said optical input port (21) communicating directly with said optical output port (32) of said multifunction node (120) via a pass-through optical communication channel (F3) passing through said two further optical switches (131, 133); A multifunction node (120) according to claim 9 or 10.

13. 10. The multi-function node (20, 120) of claim 9, wherein the transmitting unit (Tx) is connected between the optical input port (21) and the optical output port (32) via a transmitting optical communication channel (F1), and the receiving unit (Rx) is connected to the optical input port (21) via a receiving optical communication channel.

14. 13. The multi-function node (120) of claim 12, wherein in the transmitter configuration, the optical switch (30) is switched on, two further optical switches (131, 133) are switched off and the second actuator (29) for controlling the first wave plate (22) is deactivated so as to break the connection between the second actuator (29) and the first wave plate (22), and in the receiver configuration, the further optical switch (131) is switched off, the first actuator (24) for controlling the first wave plate (22) and the second wave plate (23) is deactivated so as to break the connection between the first actuator (24) and the first wave plate (22), and the second wave plate (23) is set to a zero birefringence value.

15. 13. The multifunction node (120) of claim 12, wherein in the pass-through configuration, the two further optical switches (131, 133) are switched on.

Images