WO2007141353A1

WO2007141353A1 - System, and quantum key parallel distribution method by means of subcarrier multiplexing

Info

Publication number: WO2007141353A1
Application number: PCT/ES2007/000323
Authority: WO
Inventors: José CAPMANY FRANCOY; Arturo Ortigosa Blanch
Original assignee: Universidad Politecnica De Valencia
Priority date: 2006-06-02
Filing date: 2007-05-31
Publication date: 2007-12-13
Also published as: ES2302433A1; ES2302433B2

Abstract

Transmitter, receiver and system formed by the transmitter and receiver, and method for parallel quantum key distribution transmission and reception using subcarrier multiplexing. Known subcarrier multiplexing techniques are adapted to frequency-encoded QKD systems in order to generate and to send, in parallel, qubits that form part of different keys or of one and the same quantum key. Each qubit is encoded in the phase of a radiofrequency subcarrier. The key-sending rate between transmitter and receiver is improved.

Description

SYSTEM AND DISTRIBUTION PROCEDURE IN PARALLEL OF QUANTILE KEY BY MULTIPLEXATION OF SUBPORTERS

D E S C R I P C I Ó N

FIELD OF THE INVENTION

The present invention applies to the field of telecommunications, in particular optical communications, which deals with secure information transmission systems. Its application extends to the industrial sectors that use these networks for the transmission of data, and more specifically, to the industrial sectors that use said networks for the transmission of data in a secure manner, by encrypting the information based on the laws of quantum mechanics

BACKGROUND OF THE INVENTION

The increase in e-commerce has made such common actions as the use of an ATM or the purchase of items online be based on complicated information encryption techniques. Today these techniques are based either on sharing a secret key between transmitter and receiver, or on the so-called public key cryptography, whose security is based on the difficulty of separating large numbers into their prime factors. The first of these alternatives entails the difficulty that said code remains secret and only transmitter and receiver are aware of it. In turn, the second alternative is less and less secure due to the presence of increasingly powerful computers, which speed up the processing mathematical. For all these reasons, as well as for the proliferation of the use of encryption techniques, it is necessary to look for new techniques to encode the information and thus ensure your privacy.

In 1984, Charles Bennet and Gilíes Brassard proposed a new technique, the protocol known as BB84, in which the key is theoretically protected from attacks. Thus, quantum bits or qubits are defined as quantum units of information measurement that can take two determined values that can be identified as "0" and "1". This technique proposed to code the zeros and some of the classic keys in the polarization state of individual photons. Thanks to the laws of quantum mechanics, these quantum bits or qubits cannot be intercepted, since if a spy measured them, their status would change immediately, because a quantum system cannot be measured without disturbing the system itself. Similarly, the cloning impossibility theorem indicates that the person attacking the link cannot keep an exact copy of the key sent by the transmitter and send a copy to the receiver.

Since 1984, several quantum key distribution systems (QKD) have been developed. Photonic technologies have proven to be an adequate means for the implementation of QKD systems for the distribution of keys among users several hundred kilometers away. To date, three main types of systems based on photonic techniques can be considered:

In 1992 Bennet et al. Demonstrated the use of photon polarization in a QKD system. This system faces the problem of preserving the polarization of states through the fiber optic link

The second type of system, proposed by Townsend et al in 1993, was based on the use of delays in the sent photons and their analysis through the use of balanced fiber interferometers. The main problem to work with these systems is to ensure that fiber interferometers do not get out of order due to mechanical and environmental factors.

The third type of QKD system was proposed in 1999 by J. M. Mérolla et al. ["Quantum cryptographic device using single-photon phase modulation", Physical Review A, VoI. 60, Num. 3, September 1999]. Said system encodes the information in the side bands that appear in a light signal modulated by a radiofrequency wave. In this way, and using four different phases to encode the zeros and some of the key, these systems can be used to implement the BB84 protocol.

A common problem with these systems is that they allow a very low bit rate. The key is encoded in quantum states (usually photons) so that, to preserve the security of the system, only one photon can carry the information of each qubit. Due to the difficulty of creating photon sources that guarantee the emission of a single photon at a time, semiclassical sources must be used where the energy is reduced to levels such that the probability that a pulse carries only one photon drops to 20%. Consequently, approximately 80% of the pulses sent are empty of information. If the need to use a key distillation protocol at the destination is added to this, the bit transfer rate is further reduced. - TO -

On the other hand, the subcarrier multiplexing technique is known. Subcarrier multiplexing is a technique used in other fields, such as microwave photonics for the remote control of antennas and the optical processing of radio signals, in which several radiofrequency carriers simultaneously modulate the same energy source, usually a laser diode

OBJECT OF THE INVENTION

The present invention seeks to solve the problem of the low bit transfer rate present in the systems used to date. Thus, the present invention relates to a new frequency coding and transmission / reception system and method, in which the light is modulated not only by a radio frequency signal, but by a set of radio frequency signals or subcarriers, using the subcarrier multiplexing model. Said system is formed by a transmitter and a receiver. In the present invention known subcarrier multiplexing techniques are adapted to frequency coded QKD ["Quantum Key Distribution"] systems to generate in parallel different quantum keys each associated with the phase of a radiofrequency signal, and thus improve the key sending rate between the transmitter and the receiver by combining the information received encoded in the phase of different radio frequency subcarriers.

In addition to the previous advantage, if different keys are sent simultaneously that meet any relationship between them, so that the receiver can use groups of keys at different radio frequencies or combinations of these, the invention allows to increase the degree of security between transmitter and receiver.

SUMMARY OF THE INVENTION

One of the aspects of the present invention relates to a transmitter for a quantum key distribution system. Said transmitter comprises a light source that provides an optical carrier at an optical frequency; N radio frequency generators each generating a respective radio frequency subcarrier, where N is a natural number and N> 1, and which include means for randomly and independently varying the relative phase of said radio frequency subcarriers between several phase values; means for encoding a qubit in said relative phase of each radio frequency subcarrier; means for combining the generated radio frequency subcarriers, forming a combined signal; and means for modulating the optical carrier to the optical frequency by said combined signal, so that an optical signal multiplexed by N radio frequency subcarriers is obtained.

Another aspect of the present invention relates to a receiver for a quantum key distribution system comprising: means for receiving an optical signal multiplexed by N radio frequency subcarriers, where N is a natural number and N>1; N radiofrequency generators that each generate a respective radiofrequency signal, where N is a natural number and N> 1 and that include means for randomly and independently varying the relative phase of each of said radiofrequency signals between several phase values; means to combine radio frequency signals forming a combined signal; means for modulating said optical signal received by said combined signal, so that the generated radio frequency signals interfere with the radio frequency subcarriers multiplexing the received optical signal; means for filtering and detecting the result of said interference; where each radio frequency subcarrier that multiplexes the received optical signal has a qubit encoded in its relative phase.

In addition, one aspect of the invention relates to a quantum key distribution system comprising the aforementioned transmitter and receiver.

Another aspect of the invention is a quantum key transmission method comprising the steps of: generating an optical carrier at an optical frequency; generate N radio frequency subcarriers, where N is a natural number and N> 1; randomly and independently vary the relative phase of said radio frequency subcarriers; encode a qubit in said relative phase of each radio frequency subcarrier; combine the radio frequency subcarriers and form a combined signal; and modulating the optical carrier to the optical frequency by said combined signal, so that an optical signal multiplexed by N radio frequency subcarriers is obtained.

A subject of the invention is also a quantum key reception method comprising the steps of: receiving an optical signal multiplexed by N radio frequency subcarriers, where N is a natural number and N> 1; generate N radio frequency signals where

N is a natural number and N>1; randomly and independently vary the relative phase of each of said radio frequency signals; combine the signals of radiofrequency generated in the previous stage and form a combined signal; modulate the optical signal received by said combined signal, so that the radio frequency signals interfere with the radio frequency subcarriers multiplexing the received optical signal; filter and detect the result of such interference; where each of the radio frequency subcarriers multiplexing the received optical signal has a qubit encoded in its relative phase.

BRIEF DESCRIPTION OF THE FIGURES

In order to help a better understanding of the features of the invention according to a preferred example of practical realization thereof and to complement this description, a set of drawings is attached as an integral part thereof, the character of which is illustrative and not limiting . In these drawings:

Figure 1 shows a transmitter for a quantum key distribution system according to the present invention.

Figure 2 shows in detail one of the conventional radio frequency subcarrier generators used.

Figure 3 shows a transmitter for a subcarrier multiplexing system with two subcarriers, as well as the frequency response of the output of said transmitter.

Figure 4 represents a receiver for a quantum key distribution system according to the present invention. Figure 5 represents another receiver for a quantum key distribution system according to the present invention.

Figure 6 represents another receiver for a quantum key distribution system according to the present invention.

Figure 7 represents another receiver for a quantum key distribution system according to the present invention.

Figure 8 shows a scheme that explains the concept of overlapping Bragg networks.

Figure 9 shows a quantum key distribution system according to the present invention.

Figures 10, 11 and 12 show an example of quantum key transmission and reception simulation according to the present invention, using the BB84 protocol with two frequencies.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen in Figure 1, the quantum key parallel distribution system based on subcarrier multiplexing that is the object of the The present invention is formed by a transmitter (1) that includes N signal generators or radio frequency subcarriers (Gi, G ₂ , ..., G _N ), where N is a natural number and N> 1. Each of the radio frequency subcarrier generators comprises a voltage controlled oscillator (in English, VCO), which provides independent control of the phase of the subcarrier it generates. Thus, the radio frequency subcarrier Ai generated by the subcarrier generator Gi has a frequency Q ₁ << ω _Q and a phase O ₁ , the radio frequency subcarrier A2 generated by the subcarrier generator G ₂ has a frequency Ω ₂ "ω ₀ and a phase O ₂ , etc.

In the relative phase of each of said radio frequency subcarriers (Ai, A ₂ , ..., A _N ), by means of the active control of their respective phase (O ₁ , Φ ₂ , ...), the information is encoded of a qubit or quantum bit, which as already explained, is defined as a quantum unit of information measurement that can take two determined values that can be identified as "0" and "1". Thus, each radio frequency subcarrier (Ai, A ₂ , ..., A _N ) has a qubit encoded. A non-limiting example of the coding of qubits in the subcarrier phase is given below.

The transmitter (1) randomly and independently varies the phase (O ₁ , O ₂ , ...) of the electrical signals used to direct the modulator between several phase values. In a particular embodiment, four phase values (0, π / 2, π and 3π / 2) are chosen, which form a pair of conjugate bases (0, π) and (π / 2, 3π / 2). However, in other particular embodiments, they may choose another number of phase values, such as two, eight or others. This is because the transmission of the quantum key is controlled by a protocol that can be associated with schemes of two states, four states or more. There are many protocols that control and impose the patterns of transmission and reception of quantum key. Among the best known are the protocol BB84 and B92. A non-limiting example of the transmission and reception of quantum key according to one of these protocols is provided below.

Figure 2 shows in detail one of the conventional radio frequency (Gi) subcarrier generators used and referred to in Figure 1 as "VCOs". Each of said radio frequency subcarrier generators (Gi, G ₂ , ..., G _N ) comprises at least one radio frequency oscillator (3), a random bit generator (4) and a voltage controlled radio frequency offset ( 5). The radio frequency oscillator (3) oscillates around a radio frequency Ω. The voltage-controlled radio frequency phase shifter (5), controlled by the random bit generator (4), varies the phase of the oscillator radio frequency signal (3). The random bit generator (4) allows the phase Φ of the radio frequency subcarrier produced by the subcarrier generator (VCO) to be randomly varied between a certain number of phase values. This number of phase values is four in a particular embodiment of the present invention: 0, π / 2, π and 3π / 2, but may be different in other embodiments (for example, but not limited to, two: 0 and π) . Thus, the radio frequency subcarrier generated by the VCO has a frequency Ω <<a> ₀ and a phase Φ. This radio frequency subcarrier it subsequently contributes to the subcarrier-modulated optical signal with two side bands centered on ω _or ~ Ω, and on ω _or + Ω. and a phase Φ present in each of the two lateral bands.

Thus, each subcarrier generator at a different radio frequency (Gi, G ₂ , ..., G _N ), comprising a radio frequency oscillator (3), a random bit generator (4) and a voltage controlled radiofrequency phase shifter (5), create a quantum key. Said quantum key will be transmitted around the oscillator radiofrequency frequency (3), and the qubits will be given by the set of phase values generated by the voltage controlled radio frequency offset (5), which is controlled by the generator random bits (4). In this way, and depending on the protocol used between sender and receiver, a set of phase values will be identified with the qubit value "0" and different ones will be identified with the qubit value "1". In a particular, but not limiting, embodiment of the present invention, in which sender and receiver implement their communication based on the BB84 protocol, the random phase values are four: 0, π / 2, π and 3π / 2. In addition, and by virtue of what is stipulated in said protocol, the values of phase 0 and π / 2 correspond to qubits of value "0", and the values of phase π and 3π / 2 correspond to qubits of value "1".

Returning to figure 1, the signals or subcarriers

(Ai, A ₂ , ..., A _N ) generated by each radio frequency signal generator (Gi, G ₂ , ..., G _N ) are combined by a signal combiner (6) without generating intermodulation terms. The output signal (A _τ ) of the combiner Signal (6) is connected to a light modulator (8).

Between the signal combiner (6) and the modulator

(8) A linearizer (7) can be placed, which ensures that the response of the modulator (8) is always within a linear area and that the modulator (8) does not generate higher order bands.

The light modulator (8) modulates an optical carrier ω ₀ provided by a light source (9). To modulate the optical carrier ω ₀ any type of known modulator can be used, as well as any type of constellation. The system supports phase, amplitude, or more general light modulators in which phase and amplitude components exist. In addition, the modulation scheme is not limited to one input and one output, but the system supports configurations with more than one output and / or input.

Said optical carrier ω ₀ is, in principle, any wavelength belonging to the commonly used transmission windows, that is, the first transmission window (around 850 nm), the second transmission window (around 1310 nm ) or the third transmission window (around 1550 nm). The optical carrier ύ) ₀ is provided from a light source that can be, for example, a laser diode emitting at the corresponding wavelength, but the light source is not limited to said laser diode, but can Be any other.

Figure 3 shows an example transmitter for a system with multiplexing of subcarrier, as well as the frequency response of the output of said transmitter, in which two radio frequency subcarriers (Ai, A ₂ ) modulate an optical carrier of co ₀ optical frequency. The spectrum of the multiplexed subcarrier signal (10) is represented on the right side of Figure 3, and comprises a central band, corresponding to the frequency of the optical carrier CO ₀ and lateral bands on both sides of the carrier, corresponding to the two radio frequency subcarriers. Each radio frequency subcarrier Ω frequency _; it contributes to the signal modulated with two components at the frequencies (D ₀ -Q ₁ and in y _or + Ω ₍ . As can be seen, each of the subcarriers is defined by its frequency (which in the spectrum of Figure 3 is represented by Ω ₁₅ Ω ₂ ) and its relative phase, in which the information of each qubit or quantum bit is encoded.

Finally, returning to Figure 1, said optical signal ω ₀ multiplexed by the radio frequency subcarriers (Ai, A ₂ , ..., A _N ) is emitted (10) so that it travels along an optical transmission medium , which is preferably optical fiber, but in a particular embodiment it may be, for example, free space or any other means of optical transmission.

This transmitter (1) therefore provides two possibilities:

On the one hand, this transmitter (1) provides the possibility of simultaneously transmitting several totally independent quantum keys associated with each subcarrier (Ai, A ₂ , ..., A _N ). In this case, each qubit forms an independent quantum key. In other words, the transmitter (1) allows at least one of the qubits encoded in the radio frequency signals (Ai, A ₂ , ..., A _N ) to correspond to a quantum key independent of the other quantum keys associated with the rest of qubits encoded in the rest of radio frequency signals (Ai, A ₂ , ..., A _N ).

In a particular embodiment, each subcarrier is associated with a different quantum key, and that respective subcarrier associated with each respective quantum key is used to send, one by one, each of the qubits that form said quantum key. That is, at each instant of time a qubit is transmitted in each of the subcarriers, and after a certain number of transmissions, the sending of each of the quantum keys is completed.

On the other hand, this transmitter (1) provides the possibility of transmitting a single larger quantum key. In this case, the quantum key is formed by several qubits that are transmitted in parallel, each encoding in a subcarrier, so that the entire quantum key is transmitted faster, increasing the transmission speed of said key. That is, the transmitter (1) allows sending simultaneously

(in parallel) different qubits belonging to the same quantum key, so that the receiver adds them later. In this way the limitation of the bit send rate inherent in the systems that conform the state of the art is overcome. In other words, the transmitter (1) allows all or part of the qubits encoded in each of the subcarriers to be part of the same quantum key. In the event that the quantum key is formed by a number of qubits greater than the number of subcarriers, this process is repeated until the complete transmission of the quantum key is completed.

In addition, the system allows the different keys that are sent simultaneously to fulfill some previously determined relationship between them, so that the receiver uses groups. of keys to different radio frequencies or combinations thereof, thus increasing the security between transmitter and receiver.

Figure 4 represents a receiver (2) for a quantum key distribution system according to the present invention. This receiver (2) receives an optical signal (10 ') modulated by N radio frequency subcarriers, where N is a natural number and N> 1. The receiver (2) therefore has means to receive this optical signal (10 '). Each of the radio frequency subcarriers multiplexing the optical carrier has a quantum bit or qubit encoded in its relative phase. As explained above, each of these qubits can belong to a quantum key independent of the other quantum keys associated with the rest of qubits encoded in the rest of radio frequency subcarriers, or correspond to the same quantum key transmitted simultaneously (in parallel) .

The receiver (2) also comprises N radio frequency generators (Gi ', G ₂ ', -, G _N '), where N is a natural number and N> 1, similar to those used in the transmitter (1), which generate radio frequency signals or subcarriers (B ₁ , B ₂ , ..., B _N ) similar to those generated in the transmitter (1). The phases of the signals or radio frequency subcarriers generated at the receiver

(2) they are also varied randomly and independently between different phase values. In particular embodiments of the invention, these values between which the phase is varied can be, without this being a limitation, two phase values (0, π) or four phase values (0, π / 2, π and 3π / 2), may be other phase values.

These radio frequency signals (Bi, B ₂ , ..., B _N ) generated in the receiver (2) are combined in a signal combiner (6 '), obtaining a combined signal (B _τ ) and used to modulate a the optical signal received (10 '). As in the transmitter (1), in the receiver (2) a linearizer (V) can be placed between the signal combiner (6 ') and the modulator (8'), which ensures that the response of the modulator (8 ') is always within a linear zone and that the modulator (8') does not generate higher order bands. The modulator (8 ') modulates said optical signal received (10') by said combined signal (B _τ ), so that the radiofrequency signals (Bi, B ₂ , ..., B _N ) generated interfere with the subcarriers of radio frequency multiplexing the received optical signal (10 '). The result of interference (H ') at different frequencies is filtered and detected independently.

The fact of taking some phase values or others, both in the transmitter and in the receiver, depends on the quantum key transmission protocol chosen. For example, the BB84 protocol uses four phase values (0, π / 2, π and 3π / 2) in the transmitter and two phase values (0, π) in the receiver, while the B92 protocol uses two values of phase (0, π) in the transmission and two (0, π) in the reception. Other protocols may use, for example, six phase values in the transmission and / or reception, or a number of different phase values.

Below is an example of a quantum key transmission and reception simulation in which the sender and receiver use the BB84 protocol with only two RF (radio frequency) subcarriers:

The transmitter modulates the optical carrier with two RF signals, Q ₁ and Ω ₂ , with electrical phases φ _A1 and φ _A2 respectively. The emitter sends the qubits using the relative phase of the bands chosen randomly between 4 phase values (0, π) and (π / 2, 3π / 2), which form a pair of conjugate bases. When the signal reaches the receiver, it modulates the signal at the same frequencies that the transmitter has used, but with different phases φ _BΪ and φ _B2 respectively, which vary randomly between the values (0, π).

Figure 10 shows the simulation of the optical spectrum (frequencies relative to ω ₀ in GHz) the simplest case, which is the one in which the emitter decides to encode both RF tones with the same value. The simulation corresponds to the case where Q ₁ = 2 GHz and Ω ₂ = 3 GHz. In this case, φ _Ál = φ _A1 and both side bands behave in the same way. When the relative phase Aφ =

(<P _A ~ Ψ _B ) used by the sender and receiver is 0 or π, the carrier and, either the upper two bands or the lower two bands, are detected. When {φ _A - φ _B ) = ± π / 2, the carrier and the four bands are detected.

However, one of the advantages of the system The present invention lies in the possibility of sending independent keys in each frequency. This is shown in Figure 11, where the relative phase difference

{φ _Aι - φ _Bι ) is 0 or π and, therefore, only two of the four bands are present at the detection in the receiver. Thus, it is clear how the two keys can be sent independently without one interfering with the other.

Finally, Figure 12 shows the case in which the relative phase difference (φ _Al - φ _Bι ) is 0 or π for one of the frequencies and (φ _Aι - φ _Bι ) = ± π / 2 for the other. Once again the independence between keys is appreciated.

Turning to Figure 4, as filtering and detection means, the present invention provides various alternatives.

As a first alternative, as shown in Figure 4, the filtering stage comprises a circulator (12) followed by 2N + 1 commercial generic bandpass filters (where N is the number of radio frequency signals generated by N radio frequency generators where N a natural number and N> 1, and where the 2N + 1 bandpass filters (13) are designed to allow the passage of the 2N + 1 frequency components that form the received optical signal (10 '). Commercial generic bandpass filters they are, for example, filters

Fabry-Pérot (13).

Alternatively, as shown in Figure 5, the filtering step comprises a circulator (12) followed by 2N + 1 Bragg networks recorded in fiber and arranged in parallel (14), where N is the number of radiofrequency signals generated by N radio frequency generators and where the 2N + 1 bandpass filters (13) are designed to allow the passage of the 2N + 1 frequency components that form the received optical signal (10 '). Each of the Bragg networks is centered on each of the frequencies corresponding to the generated sidebands. Bragg fiber optic networks act as a pass-band optical filter and can be manufactured in the region of interest. In addition, like conventional filters, it has tuning capability.

Alternatively, as shown in Figure 6, the filtering stage comprises a circulator followed by different Bragg networks recorded in fiber and arranged in series (15), where N is the number of radiofrequency signals generated by N radiofrequency generators and where the 2N + 1 bandpass filters (13) are designed to allow the 2N + 1 frequency components that form the received optical signal (10 ') to pass - The signal reflected by each of the Bragg networks is recovered thanks to the circulator ( 12). Each of the Bragg networks is centered on each of the frequencies corresponding to the generated sidebands.

Alternatively, as shown in Figure 7, the filtering stage comprises a circulator (12) followed by different superimposed Bragg networks recorded in fiber and arranged in series (16). The signal reflected by each of the Bragg networks is recovered thanks to the circulator. Each of the overlapping Bragg networks is centered on each of the frequencies corresponding to the generated sidebands.

Figure 8 illustrates a scheme explaining the Bragg networks concept superimposed. The resulting superimposed Bragg network (16) is the equivalent of the sum of the different Bragg networks.

The detection stage (18) used is based on avalanche photodiodes and / or photon counters.

To know the information sent by the transmitter (1), the detection pattern is observed, which is nothing more than the relation of detection events that has occurred in each of the detectors. From the detection pattern (the photons detected by the detection means (D)) and by exchanging certain information through an unsecured public channel, transmitter (1) and receiver (2) are able to distill a quantum key, formed by one or more qubits, valid and secure to apply to your communication. This key distillation stage, carried out by a key distillation software.

Below is an example that illustrates the practical sending of qubits using the BB84 protocol, as well as the distillation of said quantum key:

Emitter Em 0 0 1 1 0 0 0 0 1 1 1 0 0 1 0 1 0

Phase Em 0 0 180 180 90 90 90 90 180 180 270 0 90 270 0 270 0 180

Rcpt phase 180 180 90 180 180 180 180 180 180 90 180 180 90 90 90 180 180 180

Difference -180 -180 90 0 -90 -90 -90 -90 0 90 90 -180 0 180 -90 90 -180 0

Measure -180 -180? 0 -? -? -? -? 0? ? -180 0 180 -? ? -180 0

Comparison 0 0 180 180 0 90 270 0 180

Password recovered 0 0 1 1 0 0 1

The transmitter sends ^Nλ 0 "and" 1 "encoded in the RF phase. Thus the values of phase 0 and π / 2 encode the ^λλ 0" and the phase values π and 3π / 2 encode the "1".

When the signal reaches the receiver, it modulates the signal at the same frequency and with random phase values, such as π / 2 and π.

As explained in the example of quantum key transmission and reception simulation in which the sender and receiver use the BB84 protocol with only two RF tones (figures 10 to 12), depending on the value of the phase difference between transmitter and receiver Áφ, the band is detected or not. Since the protocol is known, and also because the receiver knows the phase values that he has used, the key sent by the sender can be calculated.

Next, the cases of the second and third qubit are analyzed as an example. In the second qubit, the transmitter randomly sends a zero "0" encoded in the phase 0 value. Upon reaching the receiver, it randomly chooses a phase value π. Therefore, the phase difference is π, which means that the receiver has detected only the minor band of the carrier. With this information, and also given that the receiver knows that he has used the π phase, he can conclude that the sender has used phase 0 and, therefore, that the sender has sent a zero "0".

In the third qubit, the sender randomly sends a "1" encoded in the phase value π. Upon reaching the receiver, it randomly chooses a phase value π / 2. Therefore, the phase difference is π / 2, which means that the receiver may have detected both modulation bands. With this information, the receiver discards said qubit, since it cannot identify with certainty what value the sender has sent.

Figure 9 shows a parallel quantum key distribution system based on subcarrier multiplexing formed by a transmitter (1), an optical transmission medium (17) and a receiver (2).

As indicated in the transmission stage, each of the transmitted and received encoded qubits can be part of a quantum key independent of the rest of the quantum keys associated with the rest of the encoded qubits, or each of the transmitted and received encoded qubits can be part of the same quantum key, transmitting each qubit in a different subcarrier, in parallel.

As already indicated above, when generating the keys and associating them with each radiofrequency subcarrier, it is possible to impose that they fulfill some relationship between them, thus creating groups of keys to different radio frequencies or combinations thereof. The receiver (2) can thus use these groups of keys at different radio frequencies or combinations thereof, to increase the security between transmitter and receiver. That is, this system allows the addition of an extra security element: the transmitter (1) sends qubits that form independent keys for the different channels associated to each radio frequency subcarrier. The receiver (2) receives the different keys and uses a protocol agreed with the transmitter (1) previously to, by comparing quantum keys, evaluate the degree of privacy of the communication. This protocol by which bases between sender and receiver are agreed can consist, for example, of operations of logical sum, use of control qubits, or other modes determined by the protocol in question.

In the described system, only the key is securely exchanged, while in the non-secure channel the message to be protected is exchanged, encoded with the key that is exchanged securely in the quantum key distribution system. In the unsecured system, information on the bases used by the transmitter (1) is also exchanged to carry out the process of extending privacy between transmitter and receiver and distilling the password. This information is shared once the password has been sent through the secure channel. The unsecured channel may be a telephone line, an optical fiber, free space, or any means of transmission or communication between transmitter and receiver known to those skilled in the art.

In view of this description and set of figures, the person skilled in the art may understand that the invention has been described according to some preferred embodiments thereof, but that multiple variations can be introduced in said preferred embodiments, without departing from the object of the invention as claimed.

Claims

RE IVINDICATIONS

1.- Transmitter (1) for a quantum key distribution system comprising:

- a light source (9) that provides an optical carrier at a frequency ω ₀ ;

- N radio frequency generators (Gi, G ₂ , ..., G _N ) each generating a respective radio frequency subcarrier (Ai, A ₂ , ..., A _N ), where N is a natural number and N> 1, and which include means (4, 5) for randomly and independently varying the relative phase of said radio frequency subcarriers (Ai, A ₂ , ..., A _N ) between various phase values; characterized by:

- means (3, 4, 5) for encoding a qubit in said relative phase of each radio frequency subcarrier (Ai, A ₂ , -, A _N ); means (6) for combining the generated radio frequency subcarriers (Ai, A ₂ , ..., A _N ), forming a combined signal (A _τ );

- means (8) for modulating the optical carrier at the frequency ω ₀ by said combined signal (A _τ ), so that an optical signal multiplexed by N radio frequency subcarriers is obtained.

2. Transmitter (1) according to claim 1, characterized by means (7) to linearize the response of the modulator (8) and to prevent the modulator (8) from generating higher order bands.

3. Transmitter (1) according to any of the preceding claims, characterized in that at least one of the qubits encoded in the radio frequency subcarriers (Ai, A ₂ , ..., A _N ) is part of a key quantum independent of the others quantum keys of which the rest of qubits encoded in the rest of radio frequency subcarriers (Ai, A2, ..., A _N ) are part.

4. Transmitter (1) according to claim 3, characterized in that the N qubits encoded in the N radio frequency subcarriers (Ai, A ₂ , ..., A _N ) are part of N independent quantum keys .

5. Transmitter (1) according to any one of claims 1 or 2, characterized in that at least two of the qubits encoded in the radio frequency subcarriers (Ai, A ₂ , ..., A _N ) are part of The same quantum key.

6. Transmitter (1) according to claim 5, characterized in that the N qubits encoded in the N radio frequency subcarriers (Ai, A ₂ , ..., A _N ) are part of a single quantum key.

7. Transmitter (1) according to any of the preceding claims, characterized in that the different quantum keys that are sent through qubits encoded in the radio frequency subcarriers (Ai, A ₂ , ..., A _N ) they fulfill some previously determined relationship between them.

8. Transmitter (1) according to any of the preceding claims, characterized by means for traveling the optical signal (10) through an optical transmission means.

9. Transmitter (1) according to any of the preceding claims, characterized in that each generator of radio frequency subcarriers (Gi, G ₂ , -, G _N ) comprises at least one radio frequency oscillator (3), a random bit generator (4) and a voltage controlled radiofrequency phase shifter

(5) .

10.- Receiver (2) for a quantum key distribution system comprising:

- means for receiving an optical signal (10 ') multiplexed by N radio frequency subcarriers, where N is a natural number and N> 1;

- N radio frequency generators (Gi ', G ₂ ', ..., G _N ') each generating a respective radio frequency subcarrier (Bi, B ₂ , ..., B _N ), where N is a natural number and N> 1 and which include means for randomly and independently varying the relative phase of each of said radio frequency subcarriers (Bi, B ₂ , ..., B _N ) between several phase values; means (6 ') for combining the radio frequency subcarriers (Bi, B ₂ , ..., B _N ) forming a combined signal (B _τ );

- means (8 ') for modulating said optical signal received (10') by said combined signal (B _τ ), so that the radio frequency subcarriers (Bi, B ₂ , ..., B _N ) generated interfere with the subcarriers radio frequency multiplexing the received optical signal (10 ');

- means for filtering (12, 13, 14, 15, 16) and for detecting (D) the result of said interference; characterized in that each radio frequency subcarrier multiplexing the received optical signal (10 ') has a qubit encoded in its relative phase.

11. Receiver (2) according to claim 10, characterized in that the filtering means (12, 13, 14, 15, 16) comprise a circulator (2) and 2N + 1 band pass filters (13), where N is the number of radio frequency subcarriers generated by N Radio frequency generators, N being a natural number and N> 1, and where the 2N + 1 bandpass filters (13) are designed to allow the 2N + 1 frequency components that form the received optical signal (10 ') to pass through.

12. Receiver (2) according to claim 11, characterized in that the 2N + 1 band pass filters (13) are Fabry-Perot filters.

13. - Receiver (2) according to claim 11, characterized in that the 2N + 1 bandpass filters (13) comprise 2N + 1 fiber-recorded Bragg networks arranged in parallel (14).

14. Receiver (2) according to claim 11, characterized in that the 2N + 1 bandpass filters (13) comprise 2N + 1 fiber-recorded Bragg networks and arranged in series (15).

15. Receiver (2) according to claim 11, characterized in that the 2N + 1 band pass filters (13) comprise a plurality of superimposed Bragg networks recorded in fiber and arranged in series

(16).

16. - Receiver ^• (2) according to any of claims 10 to 15, characterized in that the detection stage (18) comprises avalanche photodiodes and / or photon counters.

17. Receiver (2) according to any of claims 10 to 16, characterized in that at least one of the qubits encoded in the relative phase of a radio frequency subcarrier that multiplexed to the received optical signal (10 ') is part of a quantum key independent of the other quantum keys of which the rest of qubits encoded in the relative phases of the remaining radio frequency subcarriers multiplexing the received optical signal are part.

18. Receiver (2) according to claim 17, characterized in that the N qubits encoded in the relative phases of the radio frequency subcarriers multiplexing the received optical signal (10 ') form part of N independent quantum keys between yes.

19. Receiver (2) according to any of claims 10 to 16, characterized in that at least two of the qubits encoded in the relative phases of the respective radio frequency subcarriers multiplexing the received optical signal (10 ') They are part of the same quantum key.

20. Receiver (2) according to claim 19, characterized in that the N qubits encoded in the relative phases of the radio frequency subcarriers multiplexing the received optical signal (10 ') form part of a single quantum key.

21. Receiver (2) according to any of claims 10 to 20, characterized by means for distilling, from the photons detected by the detection means (D), the quantum key or keys that form the received qubits.

22.- Receiver (2) according to claim 21, characterized in that the receiver (2) uses a previously determined protocol to compare said quantum keys and evaluate the degree of communication privacy.

23. A quantum key distribution system comprising a transmitter (1) according to any one of claims 1 to 9 and a receiver (2) according to any of claims 10 to 22.

24.- Quantum key transmission procedure that includes the steps of:

- generate an optical carrier at a frequency ω ₀ ;

- generate N radio frequency subcarriers (Ai, A ₂ , ..., A _N ), where N is a natural number and N>1;

- vary (4, 5) randomly and independently the relative phase of said radio frequency subcarriers (Ai,

A ₂ , ..., A _N ); characterized by the stages of:

- encoding (3, 4, 5) a qubit in said relative phase of each radio frequency subcarrier (Ai, A2, ..., A _N ); - combine (6) the radio frequency subcarriers (Ai, A ₂ , ..., A _N ) and form a combined signal (A _τ );

- modulate (8) the optical carrier at the frequency ω ₀ by said combined signal (A _τ ), so that an optical signal multiplexed by N radio frequency subcarriers is obtained.

25. A quantum key transmission method according to claim 24, characterized in that at least one of the qubits encoded in the radio frequency subcarriers (Ai, A ₂ , ..., A _N ) is part of a key quantum independent of the other quantum keys of which the rest of qubits encoded in the rest of radio frequency subcarriers (Ai, A ₂ , ..., A _N ) are part.

26.- Quantum key transmission method according to claim 25, characterized in that the N qubits encoded in the N radio frequency subcarriers (Ai, A ₂ , ..., A _N ) are part of N independent quantum keys each.

27.- Quantum key transmission method according to claim 24, characterized in that at least two of the qubits encoded in the radio frequency subcarriers (Ai, A ₂ , ..., A _N ) are part of the same quantum key

28.- Quantum key transmission method according to claim 27, characterized in that the N qubits encoded in the N radio frequency subcarriers (Ai, A ₂ , ..., A _N ) form part of a single quantum key .

29.- Method of quantum key transmission according to any of claims 24 to 28, characterized by the step of traveling the resulting modulated signal (10) through an optical transmission means.

30. Quantum key reception method comprising the steps of: receiving an optical signal (10 ') multiplexed by N radio frequency subcarriers, where N is a natural number and N>1; - generate N radio frequency subcarriers (Bi, B ₂ , ...,

B _N ), where N is a natural number and N>1;

- randomly and independently varying the relative phase of each of said radio frequency subcarriers (Bi, B ₂ , ..., B _N ); - combine the radio frequency subcarriers (Bi, B ₂ ,

..., B _N ) generated in the previous stage and form a signal combined (B _T );

- modulate the received optical signal (10 ') by said combined signal (B _T ), so that the radio frequency subcarriers (Bi, B2, ..., B _N ) interfere with the radio frequency subcarriers multiplexing the optical signal received (10 ');

- filter (12, 13, 14, 15, 16) and detect (18) the result of said interference; characterized in that each of the radio frequency subcarriers multiplexing the received optical signal (10 ') has a qubit encoded in its relative phase.

31. A quantum key reception method according to claim 30, characterized in that at least one of the qubits encoded in the relative phase of a radio frequency subcarrier multiplexing the received optical signal (10 ') is part of a quantum key independent of the other quantum keys of which the rest of qubits encoded in the relative phases of the rest of radio frequency subcarriers multiplexing the received optical signal are part.

32. Quantum key reception method according to claim 31, characterized in that the N qubits encoded in the relative phases of the radio frequency subcarriers multiplexing the received optical signal (10 ') form part of N independent quantum keys each.

33. A quantum key reception method according to claim 30, characterized in that at least two of the qubits encoded in the relative phases of the respective radio frequency subcarriers multiplexing the received optical signal (10 ') form part of The same quantum key.

34. Quantum key reception method according to claim 33, characterized in that the N qubits encoded in the relative phases of the radio frequency subcarriers multiplexing the received optical signal (10 ') form part of a single quantum key .

35. Method of receiving quantum key according to any of claims 30 to 34, characterized by the step of distilling the quantum key or keys formed by the qubits.

36. The procedure for receiving a quantum key according to claim 35, characterized by the step of using a previously determined protocol to compare said quantum keys and assess the degree of privacy of the communication.