WO2025202884A1 - Generation and transport of a quantum key in a birefringent optical channel - Google Patents

Abstract

The present invention concerns a system for generating quantum keys between two nodes of an optical communication network. The system comprises a transmitting node and a receiving node, operationally connected to each other by at least one birefringent optical communication channel. The transmitting node comprises a photon source, an optical port adapted to transmit and receive photons on the optical communication channel, a detector assembly adapted to detect and count photons received from the receiving node. The receiving node comprises an optical port for receiving and transmitting photons on the optical communication channel, and a photon emitter adapted to emit at least one photon in response to the at least one photon transmitted by the transmitting node. The photon emitter comprises at least one birefringent element. Advantageously, the receiving node comprises control means of the photon emitter. The control means are adapted to adjust a birefringence of the at least one birefringent element as a function of polarization information associated with the at least one photon transmitted by the transmitting node. In particular, the polarization information is extracted from the at least one photon transmitted by the transmitting node or is provided by the transmitting node. Advantageously, the photon emitter comprises an emulator device of a Faraday rotator mirror or comprises a retroreflector consisting of three mirrors orthogonal to each other and rotated by 45° around respective orthogonal axes. The Faraday rotator mirror and the retroreflector are orientable in space to reflect the at least one photon transmitted by the transmitting node uniquely and determined when oriented appropriately.

Description

GENERATION AND TRANSPORT OF A QUANTUM KEY IN A BIREFRANGENT OPTICAL CHANNEL

TECHNICAL FIELD

The present invention concerns the systems for transmitting signals by means of photons. The invention comprises a system for generating and transporting a quantum key (which is based on qu-bits) in a transmission performed in any optical channel provided with birefringence, such as an optical fibre or a free propagation in a medium susceptible to birefringence. The systems according to the embodiments of the present invention exploit the birefringence characteristics of the optical channel between transmitter and receiver to transmit a predetermined key i.e., deterministic - with characteristics of noninterceptability and indecipherability that are typical of transmissions with single qubits, i.e., single polarization encoded photons.

STATE OF THE ART

Known quantum key distribution (QKD) systems or schemes generate a pair of key shared between the transmitter and receiver and are based on a protocol invented by Bennet and Brassard in 1984 in C.H. Bennet and G. Brassard: "Quantum cryptography: public key distribution and coin tossing", Int. Conf. Computer System and Signal Processing, Bangalore, 1984, hence the acronym BB- 84. The non-interceptability of the key is due to the use of a single photon while indecipherability is guaranteed by quantum randomness. The protocol comprises the use of a quantum channel (in free space or in fibre optics) and a classic channel for the exchange of information. The resulting key is shared but random (i.e. wherein the bit sequence is random).

The Applicant proposed quantum key distribution systems and processes also with authentication features, based on a Faraday Rotator Mirror, or FRM. These systems comprise a transmitting node, or TX transmitter, and one or more bidding nodes, or RX receivers, capable of exchanging photons through an optical medium or channel characterized by birefringence - e.g., air, water, optical fibre, etc.

In this type of systems, the receiver comprises an FRM, which is selectively connectable to the optical channel provided with birefringence, alternatively to a mirror. The FRM has the effect of completely compensating for the birefringence due to the optical channel and allows to send back to the transmitter, a signal with state of polarization, or SOP, orthogonal with respect to that sent in accordance with what is described in M. Martinelli: "A universal compensator for polarization changes induced by birefringence in a retracing beam" , Opt. Comm., 72, 341, 1989.

The use of an FRM thus makes it possible to establish a unique connection between transmitter and receiver. This connection is exploited to transmit qubits - in the form of single photons - encoded in polarization.

In particular, if the receiver inserts the FRM in correspondence of a photon being sent by the transmitter through the optical channel, the transmitter will receive from the receiver a photon with known polarization state. Otherwise, if the receiver inserts the mirror in correspondence of a photon being sent by the transmitter through the optical channel, the transmitter will receive a photon with random SOP (not known) from the receiver. Accordingly, by knowing an insertion scheme of FRM and mirror during a photon transmission, it is possible to transmit a secret code between transmitter and receiver.

The system described above has a vulnerability to eavesdropping attacks. In detail, the FRM compensates for the birefringence effect on any input signal. Therefore, when an eavesdropper, also indicated with EVA - observes the receiver - that is, it detects the signals output from the receiver in response to probe signals sent by EVA itself - the effect of the birefringence of the optical channel is also compensated by the observation point of the receiver, a condition that allows to decode the communications between transmitter and receiver.

OBJECTS AND SUMMARY OF THE INVENTION

Aim of the present invention is to overcome the drawbacks of the prior art.

It is also an aim of the present invention to make available a system and method for generating and transporting a quantum type key that makes use of an optical communication channel provided with birefringence, such as optical fibre, air or water and, preferably, uses signals comprising qu-bits.

In particular, the system and method according to the present invention allow to generate a deterministic quantum key - i.e., wherein the bit sequence is predetermined.

Another of the present invention is to provide a system and a method of authentication of a node in an optical communication network based on a deterministic quantum key, which is structurally simple and therefore cost- competitive.

Another object of the present invention is to provide a system for exchanging a quantum key that does not need to relay on the classical channel information relating to the choice of the measurement bases as entailed by BB-84.

It is also an aim of the present invention to provide a system for generating and transporting a quantum key in a fibre optic network built for another purpose, by way of example a PON (Passive Optical Network) type fibre optic telecommunication network or a back-hauling fibre optic network, built for the support of wireless networks, etc.

These and other purposes of the present invention are achieved by a system and method for authenticating a node in an optical communication network, a transmitting node and a receiving node, incorporating the features of the appended claims, which form an integral part of this description.

According to a first aspect, the present invention is directed to a system for exchanging quantum keys between nodes of an optical communication network. The system comprises a transmitting node and a receiving node, operationally connected to each other by at least one birefringent optical communication channel.

The transmitting node comprises a photon source, an optical port adapted to transmit and receive photons on the optical communication channel, a detector assembly adapted to detect and count photons received at the transmitting node. The receiving node comprises an optical port for receiving and transmitting photons on the optical communication channel, and a photon emitter adapted to emit at least one photon in response to the at least one photon transmitted by the transmitting node. The photon emitter comprises at least one birefringent element.

Advantageously, the receiving node comprises control means of the photon emitter. The control means are adapted to adjust a birefringence of the birefringent element as a function of polarization information associated with the at least one photon transmitted by the transmitting node. In particular, the polarization information is extracted from the at least one photon transmitted by the transmitting node or is provided by the transmitting node.

The system according to the present invention allows to generate and transport a quantum key or, more generally, information efficiently and securely. Furthermore, the system according to the present invention guarantees the same characteristics of non-interceptability and indecipherability of the BB-84 protocol, but allows generating a deterministic quantum key - that is, a predetermined bit sequence. In particular, the system according to the invention is mainly based on two characteristics of the system. The first characteristic is the birefringence of the optical communication medium; in fact, the final state of polarization that occurs at the end of any connection through a birefringent optical channel between nodes of the system is unique and unpredictable. The second characteristic is the presence of means able to exactly and unequivocally compensate for the birefringence of the optical channel and therefore to achieve a unique conjugation between transmitter and receiver by means of an appropriate photon emitter at the receiver node. In a first embodiment, detailed below, it comprises using a Faraday rotator mirror virtualization, FRM (active solution), which is realized by means of a birefringence actuator controlled based on the detected polarization of the photons received at the receiver, preferably measured by a Stokes parameters meter. In other words, the photon emitter comprises Faraday virtual mirror rotator FRMV with selectively programmable features. In a second embodiment (passive solution), described below, the photon emitter comprises the use of a retro-reflection device, also called three mirrors, 3S, below. The 3S device is aligned by the system by pilot pulses sent from the transmitter node to the receiver node and this alignment is unique and instantaneous. Once the 3S device is aligned it behaves like an FRM, but only and uniquely when observed by the transmitter node. If an eavesdropper observes the system by sending decoy signals, the decoy signal will normally be retro- reflected to the 3S device but will not have the characteristics of FRM and therefore the eavesdropper will not have any information about the signals conditioned by the receiver.

In this way it is possible to overcome the safety limit posed by the automatic compensation implemented by a common FRM. In fact, automatic compensation does not make the connection between the transmitting node and the receiving node unique.

In one embodiment, the receiving node comprises a measuring element. In this case, the measuring element is adapted to extract the polarization information from the at least one photon transmitted by the transmitting node by measuring Stokes parameters associated with the at least one photon transmitted by the transmitting node, and determining a first polarization state, SOP, of the at least one photon transmitted by the transmitting node.

Further, the control means receives the polarization information from the measuring element and adjusts the birefringence of the birefringent element so that the at least one response photon emitted by the photon emitter selectively presents a second polarization state orthogonal to the first polarization state when received at the transmitting node. Alternatively, the second polarization state is selected from identical to the first polarization state or orthogonal to the first polarization state.

Preferably, the receiving node also comprises a beam splitter (BS) located downstream of the optical port, with respect to the at least one photon transmitted by the transmitting node. The beam splitter is adapted to deflect towards a measuring element first photons received through the optical port and to transmit towards the optical port the at least one response photon emitted by the photon emitter without altering the polarization state of the deflected or transmitted photons.

Preferably, the transmitting node comprises synchronizing means adapted to generate a synchronizing signal when the photon source emits a photon. In this case, the photon emitter comprises a photon source, adapted to generate the at least one response photon as a function of the synchronization signal. In particular, the at least one response photon is transmitted through the birefringent element.

Even more preferably, the birefringent element of the receiver node comprises a half-wave birefringent plate and a quarter-wave birefringent plate. In addition, adjusting means are provided for adjusting, as a function of the polarization information, one or more angular orientations of the half-wave birefringent plate and the quarter-wave birefringent plate.

The system comprising various combinations of the above characteristics implements a controllable virtual FRM in order to adjust a compensation - in terms of polarization state - imposed on the photons passing through it. This together with a photon source allows the receiving node to transmit a deterministic type bit sequence, so the node has the ability to replicate to a photon transmission from the transmitting node with a deterministic (e.g., expected) or random sequence of photons.

In one embodiment, the birefringent element comprises one of a 3S retroreflector, or an 3S retroreflector array. The 3S retroreflector comprises three mirrors, each of which has a main axis parallel to a respective axis of a triplet of Cartesian axes and is rotated 45° around said main axis. In particular, the planes delimited by the triplet of Cartesian axes correspond to the planes of incidence of the photons on the mirrors. The mirrors are adapted to reflect the at least one photon transmitted by the transmitting node, thereby emitting the at least one response photon.

In addition, adjusting means are provided adapted to adjust, as a function of the polarization information, an orientation (angular and Cartesian) in space of the 3S retroreflector, or of the array of retroreflectors 3S.

Preferably, the transmitting node comprises processing means adapted to generate the polarization information.

In this case, the detector assembly is adapted to determine a polarization state of the at least one response photon. Furthermore, the polarization information indicates whether the polarization state measured by the detector assembly matches and an expected polarization state.

Preferably, the detector assembly comprises a polarizing beam splitter (PBS) adapted to deflect the at least one response photon towards a photodetector if the polarization state of the response photon corresponds to an expected polarization state. In this case, the response information is determined as a function of the detection of the response photon. Furthermore, the adjusting means are adapted to orient, as a function of polarization information, a 3S retroreflector, or a 3S retroreflector array so that its orientation corresponds to the orientation of the polarization diversity beam splitter.

Preferably, the photon emitter comprises a Faraday rotator of 45° power that is selectively insertable between the 3S retroreflector optical port, or a 3S retroreflector array. In this case, the control means adapted to control the insertion of the Faraday rotator between the optical port and the 3S retroreflector, or a 3S retroreflector array as a function of the polarization information.

Alternatively, the photon emitter comprises a shutter positioned between the optical port and the 3S retroreflector, or a 3S retroreflector array, and control means adapted to control the opening of the shutter as a function of the synchronism signal provided by the transmitter node.

The system comprising a combination of the above features allows to generate and transport quantum keys or, more generally, information substantially corresponding to what is considered above with respect to the other embodiments with a particularly simple and compact structure of the receiving node.

A different aspect of the present invention concerns a method for generating and transporting quantum keys between nodes of an optical communication network of the system described above. The method comprises the following steps.

At the transmitting node, at least one photon is meant to be sent to the receiving node.

At the receiving node, it is meant to: extract from the at least one photon or receiving polarization information associated with the at least one photon, and adjust birefringence of at least one birefringent element as a function of the polarization information.

In one embodiment, the step of transmitting at least one photon comprises transmitting a pilot photon sequence. Further, the step of extracting the polarization information comprises: measuring Stokes parameters associated with the pilot photons sequence, and calculating a polarization state of the photons of the pilot photons sequence. Finally, adjusting a birefringence of the at least one birefringent element comprises adjusting the birefringence element in order to impose on the at least one response photon a polarization state orthogonal to the polarization state of the photons of the pilot photon sequence when the response photon is received at the transmitting node.

Alternatively, when the photon emitter comprises a 3S retroreflector, the method further comprises the following steps.

At the transmitting node: detecting a correspondence between the polarization state of the at least one response photon and an expected polarization state, and transmitting the polarization information to the receiving node, the polarization information comprising an indication of whether the polarization state of the response photon matches the expected polarization state.

In this case, the step of transmitting at least one photon comprises transmitting a pilot photon sequence.

Further, adjusting a birefringence of the at least one birefringent element comprises changing an orientation in space of the 3S retroreflector or of an array of 3S retroreflectors from an initial position to an end position, defined as a function of the polarization information.

The method according to the present invention allows to obtain the same advantages set forth above with respect to the system according to the embodiments of the present invention.

In one embodiment, the photon emitter of the receiver node generates a photon sequence in response to a photon sequence transmitted by the transmitter node. In this case, the birefringence of the at least one birefringent element is adjusted such that each photon of the response photon sequence selectively has an orthogonal polarization state or a polarization state identical to a polarization state of the photons of the transmitted photon sequence. Preferably, the selection between the orthogonal or identical polarization state being based on a code stored in a controller element of the birefringent element.

The method according to the present invention allows two nodes of a network to transmit a deterministic code after having established a unique transmission through a birefringent optical channel.

Further features and purposes of the present invention will become more evident from the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereinbelow with reference to certain examples provided by way of non-limiting example and illustrated in the accompanying drawings. These drawings illustrate different aspects and embodiments of the present invention and reference numerals illustrating structures, components, materials and/ or similar elements in different drawings are indicated by similar reference numerals, where appropriate.

Figure 1 is a block diagram of a system for transmitting the quantum key according to a first embodiment of the present invention;

Figure 2 is a flowchart of a method of operation of the system of Figure 1;

Figures 3A and 3B depict two different operating configurations of a virtual Faraday Rotator Mirror implemented by a receiving node of the system of Figure 1;

Figure 4 is a block diagram of a system for transmitting a quantum key in accordance with a second embodiment of the present invention;

Figure 5 is a qualitative diagram illustrating the interaction between a photon and a three-mirror retroreflector or, 3S, according to one embodiment of the present invention;

Figure 6 is a flowchart of a method of operation of the system of Figure 4, and

Figure 7 is a block diagram of a system for transmitting a quantum key according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Some preferred embodiments will be described in detail below, although the invention is susceptible to various alternative modifications. In any case it must be understood that there is no intention to limit the invention to the specific embodiment illustrated, but, on the contrary, the invention intends to cover all the modifications and equivalents that fall within the scope of the invention as defined in the claims.

Unless otherwise defined, all the terms of the art, notations and other scientific terms used herein are intended to have the meanings commonly understood by those who are skilled in the art to which this description pertains. In some cases, terms with commonly understood meanings are defined herein for the sake of clarity and/or for ease of reference; the inclusion of such definitions in the present description should thus not be interpreted as representing a substantial difference from what is generally understood in the art.

The terms "comprising", "having", "comprising" and "containing" are to be understood as open-ended terms (i.e. the meaning of "comprising, but not limited to") and are to be considered as a support also for terms like "consist essentially of", "consisting essentially of", "consist of" or "consisting of".

The use of "for example", "etc.", "or" indicates non-exclusive alternatives without limitation, unless otherwise indicated. The use of "comprises" means "comprises, but not limited to" unless otherwise indicated.

Referring to Figure 1, there is illustrated a system for transmitting a quantum key between two nodes in an optical communication network according to a preferred embodiment of the present invention.

The system, generally indicated with the reference number 100, comprises a transmitting node 10, or also indicated as transmitter node or TX, and a receiving node 20, also indicated as receiving node or RX, between them operationally connected by an optical communication channel F. The communication channel is any transmission medium featuring birefringence, for example air or water or single-mode optical fibre in which it is possible to transmit a single-mode optical beam. In particular, the optical communication channel follows an optical path from the transmitting node 10 to the receiving node 20, passing through one or more other nodes of an optical communication network.

The transmitting node 10 comprises a single-photon source 12, which, in the embodiment illustrated in Figure 1, is a source capable of emitting both strong pulses, pilot signals, and weak pulses (faint source). The source 12, in a manner known per se, can be made using a laser source, an attenuator and a polarizer; the individual components are not illustrated in Figure 1. Preferably, the laser source generates laser pulses with adjustable frequency and intensity. In the case of weak pulses, the individual pulses are attenuated by an attenuator (not illustrated), so as to emit on average much less than one photon, typically 0.1 photon/ pulse. The polarizer, placed downstream of the attenuator, allows the polarization of the emitted photons to be controlled. Preferably, the singlephoton source 12 emits photons on a rectilinear basis with a horizontal polarization state. Alternatively, the single-photon source 12 can also emit photons on a diagonal or circular basis.

In the following example, account is taken of a source 12 configured to emit photons on a rectilinear basis with horizontal polarization and we shall associate the logical value 1 to photons with horizontal polarization and the logical value 0 to photons with vertical polarization.

At the light pulse, i.e. the generation of the photon, the source 12 generates a synchronism signal, which forms the basis for the identification of a sequence of time slots, within each of which the transmission of a respective polarized photon from the transmitting node to the receiving node and vice versa takes place.

The synchronism signal is distributed between the transmitting node 10 and the receiving node 20 along a synchronism line Ls. Alternatively, the synchronism signal can be generated by a shutter (not shown in Figure 1), which is positioned immediately downstream of the single-photon source 12.

The transmitting node 10 further comprises a first polarizing beam splitter (PBS) 16a, to which each photon output from the single photon source 12 is directed. The first PBS 16a is configured to transmit photons with a horizontal state of polarization (SOP) arriving from the source 12, and to reflect, towards a first photodetector 17a, the photons with horizontal SOP made vertical by the elements 14 and 15 described below, returning from the node 20.

In turn, the photodetector 17a transmits the detected signals to a counting register 18, to allow validating the transmission.

The transmitting node 10 further comprises a control unit C configured to measure the state of polarization of each photon returning from the receiving node 20.

In addition, the generic horizontally polarized photon emitted from the source 12 then passes through the first PBS 16a and passes through a first half-wave retarder plate 14 (HWP) oriented at 22.5 degrees. After crossing the first retarder plate 14, the photon enters a Faraday rotator 15 with a rotational power of 22.5 degrees. The Faraday rotator 15 then returns the polarization of the photon to a horizontal polarization and the same photon can then continue its path passing through a second PBS 16b configured to transmit the photons with a horizontal state of polarization arriving from the source 12, and reflect, towards a second photodetector 17b, the photons with a vertical state of polarization, returning from the receiving node 20. The second photodetector 17b also transmits the received signals to a counting register 18, to allow validating the transmission.

The transmitting node 10 further comprises an optical port 19, through which each photon output from the second PBS 16b is sent to the receiving node 20 the optical channel. Through the same optical port 19, or a parallel optical port (not illustrated), the photons returning from the receiving node 20 enter the transmitting node 10.

The receiving node 20 comprises an optical input port 22 of each photon transmitted by the transmitting node 10 and a beam splitter (BS) 23, to which each photon input into the receiving node 20 is directed through the optical port 22. The BS 23 is configured to send the photons transmitted by the transmitting node 10 to a stoke parameters meter 24 and, at the same time, send the photons generated by a source 25 configured to emit photons, similarly to the source 12 of the transmitting node 10.

The receiving node 20 further comprises an adjustable birefringence generator

26, which is crossed by the photons emitted by the source 25 before reaching the BS 23. The birefringence generator 26 is controlled by a birefringence controller

27, which receives the Stokes parameters measured by the meter 24 and generates a state of polarization of the output photons corresponding to the one they would have if they crossed a Faraday rotator mirror FRM.

For example, the birefringence generator 26 comprises at least two retarder plates, which are suitably orientable in accordance with a control signal provided by the birefringence controller 27. Preferably, the birefringence generator 26 comprises at least one half-wave retarder plate and at least one quarter-wave retarder plate. Furthermore, the birefringence generator 26 comprises adjusting means adapted to control an orientation and/ or an angular position of the plates. In this way, a desired birefringence can be imposed between the photons emitted from the source 25 and the surface of the plates.

Finally, the receiving node 20 comprises a source controller 28, which is configured to command the generation of photons from the source 25 as a function of the synchronization signal transmitted by the transmitting node 10.

The system 100 just described operates in accordance with a method 1000, or operation protocol, described below with reference to the flowchart of Figure 2.

Initially, the transmitting node 10 transmits a pulse sequence receiving node 20 through the optical channel F (step 1001). The pilot pulses received at the receiving node 20 are sent from the BS 23 to the stoke parameter meter 24 (step 1002), which determines a corresponding instantaneous SOP associated with the pilot pulse sequence (step 1003). This SOP uniquely identifies the connection between the two nodes 10-20 - since, in general, the SOP is a time-variant parameter with time constant T. Therefore, a possible eavesdropper would measure a completely different SOP from that measured by the receiving node 20 and it is not possible to reconstruct the SOP measured at the receiving node 20 through any known technique.

Based on the SOP provided by the Stokes parameters meter 24, the birefringence controller 27 adjusts the birefringence generator 26 (step 1004) so that the photons emerging from the birefringence generator 26 (emitted from the source 25) possess an SOP corresponding to an SOP of photons transmitted by the transmitting node 10 and reflected by an FRM. In other words, the birefringence configuration - e.g., obtained by appropriately orienting the retarder plates of the birefringence generator 26 - allows to generate a SOP that emulates the behaviour of an FRM-type device - obtaining, in fact, a virtual FRM, FRMV. Advantageously, the birefringence generator 26 is configurable by the birefringence controller in a shorter time interval dT, preferably much shorter than the period T associated with the SOP of the pilot pulses.

The photons output from the birefringence generator 26 pass through the BS 23 and the optical port 22 before being transmitted on the optical channel F (step 1005).

The transmitting node 10 detects the correct configuration of the birefringence generator 26 when it detects the presence of photons transmitted by the receiving node 20 with an expected SOP, i.e. orthogonal to the SOP of the photons transmitted by the transmitting node 10 (step 1006).

For example, the receiving node 20 transmits to the transmitting node single photons exhibiting a vertical SOP - when received at the transmitting node. As illustrated in the diagrams depicting the polarization variation by Poincare spheres (for simplicity illustrated with polar view) of Figure 3A, the transmitting node 10 sends a photon with horizontal SOP H. The photon is received at the receiving node 20 where its Stokes parameters are measured by the meter 24 and are used, together with the synchronization signal, by the controllers 27 and 28 to adjust the birefringence generator 26 and the source 25 so as to emulate an FRM imposing a SOP variation indicated by reference X in Figure 3A, which responds to a vertical photon V when received at the transmitting node 10. In this way it is possible to exchange a first logical value between nodes 10 and 20. With reference to Figure 3B, it is possible to exchange a second logical value between nodes 10 and 20, by configuring the birefringence generator 26 and the source 25 so as to emulate a FRM plus Faraday Rotator FR torque with 45° rotational power so as to impose a SOP variation indicated by reference Y in Figure 3B. In this way, the photon emitted by the receiving node 20 has a horizontal SOP H once received at the transmitting node 10 corresponding to the SOP of a photon received at the transmitting node originated from a photon with horizontal SOP H received at the receiving node 20 that crosses the FR, is reflected by the FRM and crosses the FR again before being transmitted to the receiving node 10.

Once the configuration of the birefringence generator 26 is finished, the receiving node 20 is correctly configured to communicate information with the transmitting node 10.

As will be evident to the person skilled in the art, thanks to this structure of the receiving node 20 it is possible to send a deterministic code to the transmitting node 10. In other words, the nodes 10-20 can exchange between them photons to which it is associated a corresponding logical value or symbol - for example, a binary logical value 0 or 1, by suitably controlling the photon emitter comprising the birefringence generator 26, the source 25 and the relative controllers 27 and 28. For example, the system 100 is configured such that the known transmitter 10 sends a sequence of pulses to the receiver node 20, which generates a corresponding sequence of response pulses. In particular, the controller of the birefringence generator 27 is configured to adjust the birefringence generator so that each photon of the response sequence has a vertical or horizontal SOP according to a predetermined code calculated or stored by the controller of the birefringence generator.

Advantageously, only the transmitting node 10 can correctly determine the SOP of the photons received from the receiving node 10 given that any eavesdropper eavesdropping the communications transmitted through the optical channel F will detect elliptically polarized photons and, therefore, will not be able to assign any logical value to them.

With reference to the block diagram of Figure 4, in an alternative embodiment of the system 100A the transmitting node 10 corresponds to the transmitting node presented above and its description is not repeated for the sake of brevity. Preferably, although not in a limiting manner, the system 100A comprises an optical reception channel Fr other than an optical transmission channel Ft. Furthermore, the receiving node 20A differs from the receiving node 20 described above as follows, wherein elements unchanged from the previous receiving node are not described for the sake of brevity.

The receiving node 20A comprises a retroreflector 23A separated from the optical port 22 by a shutter 24A. Preferably, the retroreflector 23A is a 3S retroreflector (Figure 5), or an 3S retroreflector array.

Preferably, the 3S retroreflector 23 A consists of 3 mirrors orthogonal to each other that reflect the individual photons causing a variation in the direction and SOP thereof. The action on the photons of these mirrors is illustrated in the qualitative example of Figure 5 where the faces of the cube represent the planes of incidence and reflection of the photons on the mirrors. Preferably, each mirror of the 3S retroreflector 23A has a main axis xl, z2 and y3 parallel to a respective axis X, Y, Z of a triplet of Cartesian axes delimiting three corresponding incidence surfaces XY, XZ and YZ. In addition, each mirror is rotated by 45° around said main axis, so as to create a reflection path P (indicated by arrows in Figure 5).

Furthermore, the retroreflector 23A comprises adjusting means adapted to vary the angular and Cartesian orientation of the 3S retroreflector 23A in space.

In the system 100A according to the embodiment of Figure 4, when a Cartesian reference system aligned with the incidence planes of the 3S retroreflector - abbreviated in the Cartesian reference system of the 3S retroreflector below - is oriented in space substantially corresponding to a Cartesian reference system aligned with the faces of the PBS 16a and 16b of the transmitting node 10 - which have a cubic shape -, the 3S retroreflector 23A has a behaviour corresponding to an FRM and the return photons acquire a state of polarization that, as in an FRM, retrace the outward path and arrive at the transmitter with a determined and orthogonal SOP. Otherwise, i.e. when there is no correspondence between the orientation of the 3S and PBS reference systems, the SOP of the reflected photons to the transmitting node 10 may be any.

Consequently, by appropriately controlling the orientation of the retroreflector 23A with respect to the orientation of the PBSs 16a and 16b it is possible to realize an element equivalent to an FRM, but not automatic.

For this purpose the orientation of the 3S retroreflector 23A is controlled by a retroreflector controller 25A. Similarly, the shutter 24A is governed by a controller of the shutter 26A.

The system 100A implements a method 2000, or protocol, of operation illustrated in the flowchart of Figure 6, described below. Initially, the transmitting node 10 transmits a pilot pulse sequence to the receiving node 20 through the optical channel F (step 2001). The pilot pulses received at the receiving node 20 are reflected by the 3S retroreflector 23 A, while the orientation thereof has varied, preferably in discrete steps (step 2002).

For example, the retroreflector controller 25A is adapted to vary the orientation of the 3S retroreflector 23A in discrete increments starting from a known starting position.

Under these conditions, the PBSs 16a and 16b will transfer signals transmitted from the receiving node 20 to the counting register 18 - through the photodetectors 17a and 17b -, as a function of the orientation in space of the Cartesian reference systems of the 3S retroreflector 23A and the PBSs 16a and 16b. In particular, the signals (with horizontal SOP) will be transmitted to the counting register 18 when the orientation of the Cartesian reference systems is substantially corresponding.

Thus, when the counting register 18 reaches a signal threshold count number with correct SOP in response to the output of the pilot pulse sequence, the orientation in space of the Cartesian reference systems of the 3S retroreflector 23 A and the PBSs 16a and 16b is considered to correspond - i.e. the 3S retroreflector 23A and the PBSs 16a and 16b are "docked" (decision step 2003). In other words, the 3S retroreflector 23A and the PBSs 16a and 16b have a substantially corresponding orientation in space, unless rigid translations.

Once this docking condition is detected, the transmitting node 10 transmits a coupling signal to the receiving node 20 (step 2004) which is received by the retroreflector controller 25A which interrupts the movement of the retroreflector 23A.

At this point, the transmitting node 10 enables the receiving node 20 to transmit a quantum key in the following manner.

The transmitting node 10 transmits single-photon pulses through the optical channel F and, at the same time, the shutter controller 26A acts on the shutter 24A as a function of the synchronization signal received from the transmitter node 10 (decision step 2005). As will be apparent, the opening or closing of the shutter 24A allows or prevents, respectively, the reflection of the photons by the retroreflector 25A.

Consequently, if the transmitting node 10 transmits a sequence of photons with horizontal SOP and in response receives a corresponding number of photons with vertical SOP (identified by means of the PBS 16a, 16b, photodetectors 17a, 17b, and counting register 18), the transmitting node 10 determines an open shutter condition 24A to which it attributes a logical value, for example the logical value 1 (step 2006). Otherwise, an absence of photons transmitted from the receiving node 20A to the transmitting node 10 is interpreted as an indeterminate condition, since it is not uniquely interpretable as a closed shutter condition 24A, as the photons transmitted from the receiving node 20A can be absorbed into the optical communication channel (step 2007). Finally, if the receiving node 10 receives one or more photons with non-vertical SOP, the receiving node 10 detects a possible eavesdropping (step 2008).

As will be apparent to the person skilled in the art, the receiving node 20A of the system 100A may only generate a single-value code.

A different embodiment of the system 100B, illustrated in Figure 7, differs from the alternative embodiment 100A just described in that the receiving node 20B comprises a Faraday Rotator 24B with power equal to 45° located between the optical port 22 and the retroreflector 23A that can be activated or deactivated with the controller 26B. Consequently, the receiving node 20B also provides a controller 24B, that is, the 26B configured to control the operation of the FR 24B as a function of the synchronization signal received from the transmitting node 10.

By controlling the insertion and removal of the FR 24B, in addition to the orientation of the 3S, it is possible to emulate the behaviour of a system provided with an FRM plus Faraday rotator 45° and replicate the SOP variations described above for the system 100 and depicted in Figures 3A and 3B, mutatis mutandis.

In other words, thanks to this structure of the receiving node 20B it is possible to send a deterministic code to the transmitting node 10. In other words, the nodes 10-20 can exchange between them photons to which it is associated a corresponding logical value or symbol - for example, a binary logical value 0 or 1, by suitably controlling the photon emitter comprising the retroreflector 23A, the FR 24B and the relative controllers 25A and 26B. Furthermore, also in this case, only the transmitting node 10 can correctly determine the SOP of the photons received from the receiving node 10 given that any eavesdropper eavesdropping the communications transmitted through the optical channel F will detect elliptically polarized photons and, therefore, will not be able to assign any logical value to them.

However, it is clear that the above examples must not be interpreted in a limiting sense and the invention thus conceived is susceptible of numerous modifications and variations. As will be apparent the birefringence generator 26, the source 25 of the first presented embodiment, as well as the 3S retroreflector 23A of the second presented embodiment and the 3S retroreflector 23A and the Faraday rotator 24B of the third presented embodiment, form an emitter element adapted to emit photons with a desired SOP, in particular capable of emulating the emission of photons reflected by an FRM or a mirror. Similarly, the controllers 27 and 28 of the first embodiment, and the 3S retroreflector controller 23A of the second embodiment, and the controllers 25A and 26B of the third embodiment form respective control means that control the operation of the photon emitter in order to impose a desired SOP on the photons transmitted by the receiver node 20, 20A, 20B in response to receiving a high energy pulse train or a low energy pulse sequence transmitted by the transmitter node.

For example, in one embodiment (not illustrated), the receiver node comprises a single controller that integrates the birefringence controller and the source controller, the Faraday rotator controller, and/ or the shutter controller.

In an alternative embodiment (not illustrated), the source 25 is adapted to transmit the selectively generated photons directly to the BS or by traversing the birefringence generator, thereby obtaining a predetermined SOP or a random SOP for the photons received at the transmitting node.

As will be clear to the person skilled in the art, it is possible to implement different transmission protocols by means of the systems 100A and 100B according to the conditions of use thereof.

In one embodiment (not illustrated), the photon source is a two-photon parametric source, rather than a single-photon source.

As will be apparent to the person skilled in the art, one or more steps of the abovedescribed methods may be performed in parallel with each other or in an order different from that presented above. Similarly, one or more optional steps can be added or removed from one or more of the methods described above.

In an embodiment (not illustrated), a switching device is interposed between the transmitting node and the receiving node, which can be controlled by a supervisory device. Such a solution can be used in a network with a multitude of receiving nodes, in which case the switching device allows photons to be diverted toward a selected node of the optical communication network in order to direct them to the receiving node.

Naturally, all the details can be replaced with other technically-equivalent elements. In general, the system according to the various embodiments of the present invention is usable with any optical channel likely to become birefringent - e.g., preferably optical fibre, air, water, glass, etc.

In conclusion, the materials used, as well as the shapes and contingent dimensions of the devices, apparatus and terminals mentioned above, may be any according to the specific implementation needs without thereby departing from the scope of protection of the following claims.

Claims

1. System (100; 100A; 100B) for generating and transporting quantum keys between two nodes of an optical communication network, the system comprising a transmitting node (10) and a receiving node (20; 20A; 20B), operationally connected one with the other by at least one birefringent optical communication channel (F; Fl, F2), wherein the transmitting node (10) comprises a source of photons (12), an optical port (19) adapted to transmit and receive photons on the optical communication channel (F; Fl, F2), a detector assembly (16A, 16b, 17A, 17b, 18) adapted to detect and count photons received at the transmitting node (10), and wherein the receiving node comprises an optical port (22) adapted to receive and transmit photons on the optical communication channel (F; Fl, F2), and a photon emitter (25, 26; 23A, 24A; 23A, 24B) adapted to emit at least one photon in response to the at least one photon transmitted by the transmitting node (10), the photon emitter comprising at least one birefringent element (26; 23 A), characterized by the fact that the receiving node comprises control means (27, 28; 25A, 26A; 25A, 26B) of the photon emitter adapted to adjust a birefringence of the at least one birefringent element (26; 23A) as a function of polarization information associated with the at least one photon transmitted by the transmitting node, the polarization information being extracted from the at least one photon transmitted by the transmitting node or being provided by the transmitting node.

2. System (100; 100A; 100B) according to claim 1, wherein the receiving node (20; 20A; 20B) comprises a measuring element (24), wherein the measuring element (24) is adapted to extract the polarization information from the at least one photon transmitted by the transmitting node (10) by measuring Stokes parameters associated with the at least one photon transmitted by the transmitting node (10), and determining a first state of polarization, SOP, of the at least one photon transmitted by the node transmitter (10), and wherein the control means (27, 28) receives the polarization information from the measuring element (24) and adjusts the birefringence of the birefringent element (26) so that the at least one response photon emitted by the emitter photons (25, 26) selectively presents a second state of polarization orthogonal or identical to the first state of polarization when received at the transmitting node (10).

3. System (100; 100A; 100B) according to claim 2, wherein the transmitting node (10) comprises synchronization means (C) adapted to generate a synchronization signal when the photon source (12) emits a photon, and wherein the photon emitter (25, 26) comprises a photon source (25), adapted to generate the at least one response photon as a function of the synchronization signal, wherein the at least one response photon is transmitted through the birefringent element (26).

4. System (100; 100A; 100B) according to claim 3, wherein the birefringent element (26) comprises a half-wave birefringent plate, a quarter-wave birefringent plate, and adjusting means adapted to adjust, as a function of the polarization information, an angular orientation of at least one between the half-wave birefringent plate and the quarter-wave birefringent plate.

5. System (100; 100A; 100B) according to claim 1, wherein the birefringent element (23A, 24A; 23A, 24B) comprises one between: a retroreflector consisting of three mirrors, or 3S retroreflector (23A), where each mirror has a main axis parallel to a respective axis of a triplet of Cartesian axes and is rotated 45° around said main axis, and a matrix of 3S retroreflectors, adapted to reflect the at least one photon transmitted by the transmitting node (10), thereby emitting the at least one response photon, and adjusting means adapted to adjust, as a function of the polarization information, an orientation in space of the retroreflector 3S (23A) or of the array of retroreflectors 3S.

6. System (100; 100A; 100B) according to claim 5, wherein the transmitting node (10) comprises processing means (C) adapted to generate the polarization information, and wherein the detector assembly (16a, 16b, 17a, 17b, 18) is adapted to determine a state of polarization of the at least one response photon, and wherein the polarization information indicates whether the state of polarization measured by the detector assembly matches and an expected state of polarization.

7. System (100; 100A; 100B) according to claim 6, wherein the photon emitter (23A, 24B) comprises a Faraday rotator (24B) selectively insertable between the optical port and the 3S retroreflector (23A) or the 3S retroreflector array, and Control means adapted to control the insertion of the Faraday rotator between the optical port and the 3S retroreflector or the 3S retroreflector array as a function of the polarization information.

8. System (100; 100A; 100B) according to claim 6, wherein the transmitting node (10) comprises: synchronization means (C) adapted to generate a synchronization signal when the photon source (12) emits a photon, and wherein the photon emitter (23A, 24A) comprises: a shutter (24A) positioned between the optical port and the 3S retroreflector (23A) or the 3S retroreflector array, and control means adapted to control the opening of the shutter as a function of the synchronization signal.

9. Method (1000, 2000) for generating and transporting quantum keys between two nodes of an optical communication network, comprising a transmitting node (10) and a receiving node (20; 20A; 20B), operationally connected to each other by at least one birefringent optical communication channel (F; Fl, F2), wherein the transmitting node (10) comprises a source of photons (12), an optical port (19) adapted to transmit and receive photons on the optical communication channel (F; Fl, F2), a detector assembly (16A, 16b, 17A, 17b, 18) adapted to detect and count photons received from the transmitting node (10), and wherein the receiving node (20; 20A; 20B) comprises an optical port (22) adapted to receive and transmit photons on the optical communication channel (F; Fl, F2), and a photon emitter (25, 26; 23A, 24A; 23A, 24B) adapted to emit at least one response photon to the at least one photon transmitted by the transmitting node (10), the photon emitter (25, 26; 23 A, 24A; 23 A, 24B) comprising at least one birefringent element (26, 23A), the method comprising the steps of at the transmitting node (10), transmitting (1001; 2001) at least one photon towards the receiving node (20; 20A; 20B), at the receiving node (20; 20A; 20B): extracting (1003) from the at least one photon or receiving (2004) a polarization information associated with the at least one photon, and regular (1004; 2002, 2003) birefringence of at least one birefringent element as a function of polarization information.

10. Method (1000) according to claim 9, wherein the step transmitting (1001) at least one photon comprises transmitting a sequence of pilot photon pulses, and wherein the step of extracting (1003) the polarization information comprises: measuring Stokes parameters associated with the pilot photons sequence, calculating a state of polarization of the photons of the pilot photons sequence, and wherein adjusting (1004) a birefringence of the at least one birefringent element comprises adjusting the birefringence element in order to impose on the at least one response photon a state of polarization orthogonal to the state of polarization of the photons of the driving photon sequence when the response photon is received at the transmitting node.

11. Method (2000) according to claim 9, wherein the birefringence element comprises one between a reflector formed by three mirrors, or 3S reflector (23A), wherein each mirror has a principal axis parallel to a respective one axis of three Cartesian axes and is rotated 45° about said principal axis, and a 3S reflector array, wherein the method further comprises the steps of: at the transmitting node, detecting (2002, 2003) a correspondence between the state of polarization of the at least one response photon and an expected state of polarization, and transmitting (2004) the polarization information to the receiving node, the polarization information comprising an indication of whether the state of polarization of the response photon matches the expected state of polarization, and wherein the step of transmitting (2001) at least one photon comprises transmitting a sequence of pilot photons, and wherein adjusting a birefringence (2002, 2003) of the at least one birefringent element comprises changing an orientation in space of the 3S retroreflector (23 A) or an array of 3S retroreflectors from an initial position to an end position, the end position being defined as a function of the polarization information.

12. Method (1000; 2000) according to any one of the preceding claims 9-11, the step of transmitting (1001) at least one photon comprises transmitting a sequence of pilot photon pulses, and wherein the photon emitter of the receiver node generates a corresponding response photons sequence to the transmitted photon sequence, adjusting the birefringence of the at least one birefringent element such that each photon of the response photons sequence selectively has a state of polarization when received at the transmitter node orthogonal or identical to a state of polarization of photons of the transmitted photons sequence, the selection between the orthogonal or identical state of polarization being based on a code stored in a controller element of the birefringent element.