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, and in particular to a network for quantum key distribution and its multi-function nodes. [Background technology]

[0002] Quantum Key Distribution (QKD) is a technology that allows for the sharing of a symmetric (i.e., a string of bits that materialize simultaneously at the transmitter and receiver) and absolutely secure "key" (i.e., a key that cannot be decrypted by any algorithm or computing power) between two nodes (hereafter referred to as the 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 accurately if it lies in a precise reference base (which can be linear, 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, namely the possibility of generating and propagating 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, a 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). A receiver Rx receives the single photons (excluding any single photons lost due to attenuation) and measures the states in a randomly selected basis. This state measurement generates a 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 sends a confirmation of accuracy to the receiver Rx. In this way, the transmitter Tx and receiver Rx share a logical subsequence of bits that constitutes a 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 to only one receiver Rx) and involves the communication of a quantum key between a transmitter Tx and a receiver Rx over 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 as to transmit only one photon on average (weak photon source), and first and second wave plates controlled by an actuator onto which the single polarized photon from the single-photon source is sent. The first wave plate makes it possible to establish a basis for the spin of the photon (the basis can be linear, diagonal or circular), and the second wave plate makes it possible to establish a state or code (for example a logic high state corresponding to 1, a logic low state corresponding to 0) that generates the logical sequence of bits that constitute the key.

[0008] The receiver Rx includes a polarization stabilizer that allows for reorientation of the initial basis in the case of a birefringent optical communication channel, and a wave plate that is controlled by an actuator and is positioned downstream of the polarization stabilizer and allows for the initial basis to be restored (or not restored) by a random process. The receiver further includes a polarizing beam splitter (PBS). Each photon exiting the wave plate is assigned to the polarizing beam splitter. 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 enabling 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 quantum key distribution system described above, the receiver Rx has both the basis information and the code information, which satisfies the completeness of the BB84 protocol. However, the system constructed in this way has some drawbacks.

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

[0012] Second, as is well known, the single-photon source and photodetector units in the device may need 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 a logistical disadvantage. 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 present invention to present a network for quantum key distribution and its multifunctional nodes, which are 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, where each node is configured to exchange quantum keys with any other node in the network while maintaining a shared photon source node.

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

[0017] According to a first aspect, the present invention relates to a quantum key distribution network 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 includes 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, 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 the 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 operable to activate / deactivate the multifunction node according to a transmitter or receiver configuration. In the transmitter configuration, the transmitting 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 logistical and functional advantages, especially 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] Furthermore, the photon source nodes can be updated without affecting the multi-function nodes of the network, which is crucial in the context of rapid technological change, strengthening the investment in system construction.

[0023] Furthermore, because multifunction nodes are structurally identical 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 multifunction node, the optical switching array operable by the supervisor device to select a multifunction node that forms an optical communication channel for distribution of a quantum key emanating 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 multifunction node are respectively connected, and the second ports are connected to corresponding second ports of the optical switching arrays of the photon source node and / or at least one other multifunction node of the network.

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

[0027] In one embodiment, each multifunction node comprises two further optical switches, one disposed between the optical input port and the polarization stabilizer and the other 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, wherein the optical input port communicates directly with the optical output port of the multifunction node via a pass-through optical communication channel that passes 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, the photon source node is positioned 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 multifunction 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 the logical state of each photon, at least one photodetector, and a counting register configured to receive the logical state of the detected photons.

[0033] Each multifunction node further comprises an optical switch disposed between the second waveplate and the polarizing beam splitter and operable to activate / deactivate the multifunction node according to a transmitter or receiver configuration. In the transmitter configuration, the transmitting 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 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.

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

[0036] In one embodiment, the multifunction node comprises two further optical switches, one disposed between the optical input port and the polarization stabilizer and the other 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, wherein the optical input port communicates directly with the optical output port of the multifunction node via a pass-through optical communication channel that passes 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, an 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 actuators controlling the first and second waveplates are 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 additional optical switches are switched on.

[0040] Further features and objects of the present invention will become more apparent from the following description. [Brief explanation 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; [Figure 2] 2 shows a schematic diagram of an optical switching array associated with each multifunction node of the network for quantum key distribution of FIG. 1; [Figure 3] 2 shows 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; [Figure 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 representation of a network for quantum key distribution in possible operational configurations of the multifunction nodes that make it up; DETAILED DESCRIPTION OF THE INVENTION

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

[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 / comprises" 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 plurality of multifunction nodes 20 .

[0048] Each multifunction node 20 in 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 the other optical switching arrays 40 of the network 100 by optical fibers F. The input optical communication channels F of each multifunction node in and output optical communication channel F out is an optical fiber, or alternatively, a free-space communication channel. The optical switching array 40 can 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 in the upper left, which is also connected to the photon source node 10. In this configuration of network 100, 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 to be able to 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 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 a state value, or code, that generates a logical sequence of bits that constitute the quantum key, e.g., a high logic state or 1 (corresponding to a photon's spin up) and a low logic state or 0 (corresponding to a photon's spin down). Furthermore, in the transmitter configuration, each multifunction node 20 is configured to transmit modulated photons received as input to its output. The supervisor device 60 also configures the optical switching array 40 associated with the multifunction node 20 to route 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 the received basis and extract the state value or code associated with each photon.

[0054] 3, the photon source node 10 includes a single photon source 12, i.e., a photon source whose intensity is reduced to the extent that it transmits only one photon per command pulse on average (a weak photon source). Preferably, the photon source node 10 further includes 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 generate a herald type, i.e., a pair of photons, one of which is used in the network 100 to generate the quantum key 50 and the other of which is used to establish a synchronization signal that is distributed along the network 100 via the 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 the 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 has 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 sent to other multifunction nodes 20 in the network 100 .

[0060] The optical input port 21 and 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 optical fiber connector if the subsequent optical components operate in free space.

[0061] Multifunction node 20 further comprises, downstream of optical input port 21, a first wave plate 22 and a second wave plate 23 controlled by a first actuator 24. Single-polarized photons, or qubits, emanating from single-photon source 12 of photon source node 10 are fed to first wave plate 22 and second wave plate 23.

[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 23, and a state or code C that generates the logical sequence of bits that make up the key (e.g., a high logic state corresponding to 1, a low logic state corresponding to 0). The basis is initiated from a predefined state by the action of a half-wave or quarter-wave waveplate, as is well known to those skilled in the art. As is well known to those skilled in the art, the value of the (high or low) logic state can be generated by an appropriately oriented half-waveplate according to the selected basis.

[0063] Input optical fiber F of multifunction node 20 in Because λ / 2 generally 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 photons' spin into the B basis. The polarization stabilizer restores the original configuration of the Poincaré sphere used to modulate the qubits in all three axes, 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 the transmitting unit Tx of the multifunctional node 20 and are connected among themselves via an optical transmission communication channel, for example via 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] Multifunction node 20 further comprises, downstream of wave plates 22 and 23, a polarizing beam splitter 26 to which each polarized photon exiting wave plates 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 photon and transmits them to photodetectors 27 a and 27 b, which are configured to transmit them to counting register 28, allowing reconstruction of the message code.

[0066] The multifunction node 20 further includes a second actuator 29 for 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 in a random manner.

[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 with the optical input port 21 via a receiving optical communication channel, for example 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 multifunction node 20 is in the transmitter configuration, optical switch 30 is switched on (ON position in FIG. 4) and photons are directed to mirror 33 which sends them to optical output port 32 .

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

[0071] In particular, photons enter a polarization stabilizer 25 and then travel through a first wave plate 22 and a second wave plate 23. The polarization stabilizer 25 cancels out 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, and when the switch is turned on, towards 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 in network 100 .

[0073] Preferably, the optical path between the polarization stabilizer 25 and the optical switch 30 should be free of birefringence. Additionally, in the transmitter configuration of the multifunction node 20, the second actuator is deactivated, and the connection between the second actuator 29 and the first wave plate 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 transmitted through the input optical communication channel F in and travels along the receive optical communication channel F2.

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

[0078] In both configurations (transmitter and receiver) of the multifunction node 20, the integrity of the BB84 protocol is fully satisfied. 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 in 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 the previously described multifunction node 20 in that it further comprises optical means configured to allow photons received as input to traverse a path towards output port 32. In this manner, 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), incident 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, are correctly processed, and are 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 , 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, 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 a respective optical fiber 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 because it does not perform pass-through functions at this location.

[0085] Now assume that it is desired to distribute quantum key 50 between third multifunction node 120C and fourth multifunction node 120D of 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 photons received as input to their output. Supervisor device 60 instead places third multifunction node 120C in a transmitter configuration and fourth multifunction node 120D in a receiver configuration.

[0087] The 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, directly to the optical switch 133, i.e., without entering the transmitting unit Tx or the receiving unit Rx, as shown in detail in Figure 5, and travel along the pass-through optical communication channel F3.

[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 optical switch 133 directly to optical output port 32 and travels along 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 through 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, generating 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 which they are transmitted through optical communication channel F CD and is received as input to multifunction node 120D, which acts as a receiver.

[0092] 5, photons enter polarization stabilizer 25 and from there to first waveplate 22 and second waveplate 23 to generate a received basis B according to the BB84 protocol. To this end, as explained above, first waveplate 22 is controlled by second actuator 29, polarization stabilizer 25 cancels out any birefringence present in the optical fiber traversed by the polarized photons, and 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 a high logic state 1 or a 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 clear that the above described system and network for quantum key distribution can achieve the proposed objectives, and in particular the above described multi-function node can clearly be used very flexibly to create different types of networks.

[0095] It is therefore obvious to a person skilled in the art that changes and modifications can be made 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. 1, node 120 may be used instead of node 20.

Claims

1. A network (100) for distribution of quantum keys (50), comprising: a photon source node (10) including a single photon source (12); a plurality of functional nodes (20, 120) having transmitter functionality in a transmitter configuration and receiver functionality in a receiver configuration; the plurality of functional nodes (20, 120) are connected to the photon source node (10) and between the plurality of functional nodes by respective optical communication channels (F); Each function node (20, 120) a transmitting 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); 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 the 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 waveplate (23) and the polarizing beam splitter (26), the optical switch (30) being activatable / deactivatable to operate the functional node (20) according to one of the transmitter configuration and the receiver configuration; In the transmitter configuration, the transmitting unit (Tx) modulates the photons entering the functional node (20) via the optical input port (21) and provides modulated polarized photons to an optical output port (32) of the functional node (20, 120); In the receiver configuration, the receiving unit (Rx) demodulates the modulated photons entering the functional 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 of said functional nodes (20); the optical switching array (40) is operable by the supervisor device (60) to select the functional 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 functional nodes (20). ｉｎ ) and the output optical communication channel (F ｏｕｔ ) are connected to each other, the second ports (W1, W2, ... Wn) are connected to corresponding second ports (W1, W2, ... Wn) of an optical switching array (40) of the photon source node (10) and / or at least one other functional node (20) of the network (100); The network (100) of claim 2.

4. A network (100) as described in claim 1, wherein each functional node (20) further comprises a mirror (33) positioned upstream of the optical output port (32).

5. Each function 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 functional node (120) according to a pass-through configuration; the optical input port (21) communicates directly with the optical output port (32) of the functional node (120) via a pass-through optical communication channel (F3) passing through the two further optical switches (131, 133); The network (100) of claim 1.

6. A network (100) as described in claim 5, wherein the optical switches (30, 131, 133) of each functional node (20, 120) are operable by a supervisor device (60) to configure each functional 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 that is distributed along the network via a synchronization channel (Ls); The network (100) of claim 1.

9. A functional node (20, 120) having a transmitter function in a transmitter configuration and a receiver function in a receiver configuration, 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 the 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 functional node (20, 120) according to one of the transmitter configuration and the receiver configuration; In the transmitter configuration, the transmitting unit (Tx) modulates the photons entering the functional node (20, 120) via the optical input port (21) and provides modulated polarized photons to an optical output port (32) of the functional node (20, 120); In the receiver configuration, the receiving unit (Rx) demodulates the modulated photons entering the functional node (20, 120) via the optical input port (21). Function node (20, 120).

10. 10. The functional 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 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 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. 11. The functional node (20) of 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 functional node (120) according to a pass-through configuration; the optical input port (21) communicates directly with the optical output port (32) of the functional node (120) via a pass-through optical communication channel (F3) passing through the two further optical switches (131, 133); A functional node (120) according to claim 9 or 10.

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

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