Apparatus and method for decoy-state three-state quantum key distribution

Abstract

The invention relates to a Quantum Key Distribution system comprising a transmitter 300 and a receiver 400 for exchanging a quantum key via a quantum channel 600 through a decoy-state three state protocol wherein the transmitter comprises a transmitter processing unit 340 adapted to use random numbers from a quantum random generator to select a quantum state to encode from different states of intensity and basis, a Pulsed light source 310 adapted to generate an optical pulse, a time-bin interferometer 320 through which the generated optical pulse passes and which transforms generated optical pulse into two coherent pulses separated by the time bin duration, a single intensity modulator 360 adapted to change the intensity of the two pulses individually according to the choice made by the transmitter processing unit 340, and a variable optical attenuator 370 adapted to reduce the overall signal intensity to the optimum photon number per pulse.

Claims

1. A Quantum Key distribution system comprising a transmitter and a receiver for exchanging a Quantum key via a quantum channel through a decoy-state three-state protocol wherein the transmitter comprises a transmitter processing unit configured to use random numbers from a quantum random generator to select one quantum state out of nine possible states of intensity and basis, a Pulsed light source configured to generate an optical pulse, a time-bin interferometer through which the generated optical pulse passes and which transforms the generated optical pulse into two coherent pulses separated by a time bin duration, a single modulator configured to encode the two coherent pulses according to the choice made by the transmitter processing unit by changing the intensity of the two pulses individually, and a variable optical attenuator located in series and downstream of the single modulator and configured to reduce the overall signal intensity to the optimum photon number per pulse, wherein the single modulator is an intensity modulator configured to encode a first or a second quantum state by transmitting the pulse in an early or late time-bin, respectively.

2. The Quantum Key distribution system according to claim 1, wherein the Pulsed light source is a gain-switched pulsed laser configured to generate phase randomized optical pulses.

3. The Quantum Key distribution system according to claim 1, wherein the single intensity modulator configured to encode a third state by transmitting the two coherent pulses in both time bins.

4. The Quantum Key distribution system according to claim 1, wherein the single intensity modulator is controlled by a multi-level pulsed generator.

5. The Quantum Key distribution system according to claim 1, wherein the transmitter processing unit is configured to encode a state from seven possible different states.

6. A Quantum Key distribution process comprising exchanging a Quantum key between a transmitter and a receiver via a quantum channel through a decoy-state three-state protocol comprising the steps of selecting an Intensity and a basis of a quantum state to encode from different possible states through the use of a transmitter processing unit using random numbers from a quantum random generator, wherein one of 3 state intensities is first selected such that with a probability P.sub.0 the signal intensity will be sent, with probability P.sub.1<P.sub.0 the decoy intensity is selected, with probability P.sub.2=1−P.sub.0−P.sub.1, the vacuum intensity is selected, generating an optical pulse through a Pulsed light source, transforming the generated pulse into two coherent pulses separated by a time bin duration by passing it through a time-bin interferometer, changing, in an intensity modulator, the intensity of the two coherent pulses individually according to the intensity selected in the first step, and reducing overall signal intensity to the optimum average photon number per pulse, using a variable optical attenuator and sending a signal to the receiver via a quantum channel.

7. The Quantum Key distribution process according to claim 6, wherein vacuum intensity is selected and the vacuum state is sent.

8. The Quantum Key distribution process according to claim 6, wherein a Z basis is selected and a bit value is assigned with equal probabilities.

9. The Quantum Key distribution process according to claim 6, further comprising a splitting step carried out in the receiver which splits a received signal into two paths with an optical splitter with a splitting ratio equal to Pz, which forms the basis choice.

10. The Quantum Key distribution process according to claim 6, further comprising a measurement step, where a Z basis measurement is carried out and is made by measuring the signal directly with a detection unit to measure its arrival time, and a X basis measurement is carried out by passing the signal through a time-bin interferometer and then using either one or two detection units to detect the signal.

11. The Quantum Key distribution process according to claim 6, further comprising a detection announcement step where detection events are announced to the Transmitter over a classical channel.

12. The Quantum Key distribution process according to claim 6, further comprising last steps consisting in the raw key sifting, error reconciliation, privacy amplification and authentication.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Further particular advantages and features of the invention will become more apparent from the following non-limitative description of at least one embodiment of the invention which will refer to the accompanying drawings, wherein

(2) FIGS. 1a and 1b represents a conventional apparatus of the prior art,

(3) FIG. 2 represents an apparatus according to a preferred embodiment of the invention,

(4) FIG. 3 represents protocol states related to the apparatus according to a preferred embodiment of the invention,

(5) FIG. 4 illustrates the behavior of a transmitter of the apparatus according to a preferred embodiment of the invention, and

(6) FIG. 5 represents a functioning method of the apparatus according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) The present detailed description is intended to illustrate the invention in a non-limitative manner since any feature of an embodiment may be combined with any other feature of a different embodiment in an advantageous manner.

(8) FIG. 2 represents the general apparatus of the invention. The apparatus comprises a transmitter 300 composed of a Pulsed light source 310, preferably again-switched pulsed laser 310, an interferometer 320 (preferably with a fiber stretcher 330), a Processing Unit 340, a multilevel generator 350, an Intensity Modulator 360 and a Variable Optical Attenuator 370.

(9) At the Transmitter, the gain-switched pulsed laser 310 generates an optical pulse with a random phase.

(10) This pulse then enters the interferometer 320 in order to generate two pulses (a time-bin qubit with two time bins) which are coherent.

(11) Further, the intensity modulator 360 encodes the states |0 and |1 by transmitting the pulse in the early and late time bin, respectively. Also, the Intensity Modulator 360 encodes a third state, |+, by transmitting the pulses in both time bins, whilst reducing the intensity of each one to half, such as to keep the average number of photons per pulse constant, retaining a qubit description. The phase between the time bins within this third qubit remains constant throughout. However, the phase between all qubits remains random.

(12) The intensity Modulator 360 encoding qubit states |0, |1 and |+ is controlled by a Multi-level pulse generator 350. Qubit states |0, |1 and |+ are randomly defined by a Random Number Generator in the Processing Unit 340.

(13) At the receiver 400, a passive basis choice is made through the use of an asymmetric optical coupler, where the splitting ratio is optimized to take into account different system scenarios. The raw key is generated in the time basis (Z), by detecting the time of arrival of the pulse. To achieve the phase basis measurement, the state passes through an Interferometer 420 and a single photon detector 431 is sufficient to detect interference errors by monitoring the dark port.

(14) FIG. 3 represents all states of the protocol related to the apparatus according to a preferred embodiment of the invention. More particularly, FIG. 3 represents an example of the different time-bin qubits encoded for the described protocol. The security proofs do not propose a particular implementation method, they simply consider that the protocol involves three qubit states, where the first two states |0and |1 and can contribute to key generation, and the third state

(15) $.Math.+.Math.=\frac{.Math.0.Math.+.Math.1.Math.}{\sqrt{2}}$
is for the channel estimation. In our protocol, the state |0 and |1 are encoded by transmitting the pulse with an intensity (p) in the early or late time bin. The third state, |+ is a superposition of two pulses in both time bins, with half of the intensity in order to keep the total intensity constant. The phase between the time bins within this third state remains constant.

(16) To implement the decoy state method, the intensity of each of these 3 qubit states is modulated between the one signal (μ.sub.0) and two decoy intensities (μ.sub.1, μ.sub.2), which gives in total nine possibilities three of which being similar therefore seven different states.

(17) Using the signal intensity (μ.sub.0), we generate the signal qubits |0 510 or |1 520 in the Z basis, and |+ 530 (2 times μ.sub.0/2) in the Z basis.

(18) The first decoy intensity (μ.sub.1), is exploited to generate Decoy 1 qubits 540 or 550 in Z the basis, or 560 (2 times μ.sub.1/2) in the X basis. The first decoy intensity (μ.sub.1) can be optimized in order to maximize the secret key rate.

(19) The second decoy intensity (μ.sub.2), is exploited to generate Decoy 2 qubits 570 or 570′ in Z the basis, or 570″ (2 times μ.sub.2/2) in the X basis. The second decoy intensity (μ.sub.2) is best set to zero, i.e. to a vacuum state. This means that the states are 570, 570′ and 570″ are equal and all a vacuum state. However, in practice μ.sub.2 is limited by the extinction ratio of the intensity modulator 360. Note that in our implementation with a single modulator, the states 570, 570′ and 570″ are all the same, even if μ.sub.2 is not equal. This is not necessary the case in other implementations. The protocol is based on seven different states 510, 520, 530, 540, 550, 560 and 570.

(20) As illustrated in FIG. 3 to achieve the simplest implementation, the first decoy (μ1) intensity can be fixed to half that of the signal (μ.sub.1=μ.sub.0/2). In this case, both the state and the decoy intensity are encoded with the same intensity modulator with 4 levels (μ.sub.0, μ.sub.1, μ.sub.1/2, μ.sub.2≃vacuum).

(21) The intensity μ.sub.0 and μ.sub.1 can be optimized independently, which requires 5 intensity modulation levels generated by the Intensity Modulator 360. The multilevel signals required at Transmitter 300 are generated by a high speed DAC located in the Multilevel Pulse Generator 350, with either 2 or 3 bit capability.

(22) FIG. 4 represents the behavior of a transmitter of the apparatus according to a preferred embodiment of the invention. At the transmitter, the pulsed light source 310 is triggered generating an optical pulse. If the pulsed light source is implemented using a laser, between each pulse it is brought below the lasing threshold in order to randomize the optical phase 670 between each pulse. The optical pulse passes through the time-bin interferometer. The Michelson interferometer 320 used Faraday mirrors in order to make it insensitive to polarization transformations. The interferometer 320 has two arms of different length. Due to this arm length difference two coherent pulses are generated. The two pulses pass through an intensity modulator 360 which can apply a different attenuation for each pulse (in the two time bins), depending on the desired state. All of the possible states are outlined in the previous FIG. 3.

(23) Two bases are used in the protocol, where the Z basis is used to encode the bit value by transmitting either the first or the second optical pulse. The X basis is used to check for the influence of an eavesdropper on the system. In this basis a pulse is sent in both time bins, whilst reducing the intensity by half, in order to maintain a constant average photon number per qubit. The bit values are chosen with uniform probability, whilst the basis choice can be biased towards the Z basis (p.sub.z>p.sub.x) in order to increase the secret key rate. A variable optical attenuator regulates the global photon number per pulse. As an implementation, the qubit encoding operation is done through an RNG 610 located in the Processing Unit 340. RNG 610 output defines choice basis in a further step 640 (either Z or X) and the Intensity Choice 630 (either μ.sub.0, μ.sub.1, μ.sub.2). The probabilities to generate these possible combinations are optimized in order to achieve the highest secret key rate as a function of the experimental parameters, like e.g. the loss in the quantum channel.

(24) As an output of the Digital part 600, one of the seven quantum states is encoded on the optical pulses 680 through the Intensity Modulator 360.

(25) At the Receiver 400, the measurement basis choice is made by an optical beam splitter 410, which can be asymmetric. In general, the splitting ratio matches the probability of sending the Z (p.sub.z) basis at the transmitter. The Z basis measurement is carried out by detecting the time of arrival of the optical pulse with Detection Unit 430. The X basis measurement is carried out through the use of an interferometer 420 with an equal arm length difference as the interferometer 320 at the transmitter, followed by either one detector 431 on the dark port or two detectors, one on each port. The relative phase between the two interferometers 320 and 420 is kept constant through the use of a fiber stretcher 330 in the transmitter interferometer.

(26) FIG. 5 represents a functioning method of the apparatus according to a preferred embodiment of the invention

(27) In a first step 720, the transmitter processing unit 340 uses random numbers from a quantum random generator to select the quantum state to encode from a total of seven possibilities shown in FIG. 3.

(28) Preferably one of 3 state intensities is first selected. With probability P.sub.0 the signal intensity will be sent. With Probability P.sub.1<P.sub.0 the decoy intensity is selected. With Probability P.sub.2=1−P.sub.0−P.sub.1, the vacuum intensity is selected. In this case, the vacuum state is sent

(29) If the selected intensity is not vacuum, the basis choice is made with probabilities P.sub.z>P.sub.x.

(30) If the Z basis is selected, a bit value is assigned with equal probabilities.

(31) In a second step 700, the Pulsed light source 310 generates an optical pulse.

(32) In a third step 710, the pulse passes through a time-bin interferometer 320 which generates two coherent pulses separated by the time bin duration.

(33) In a fourth step 730, an intensity modulator 360 changes the intensity of the two pulses individually according to the choice made in step 1, as pictured in FIG. 3.

(34) In a fifth step 740 the overall signal intensity is reduced to the optimum average photon number per pulse, using a variable optical attenuator 370.

(35) The signal is then sent to the receiver 400 via a quantum channel 600.

(36) In a seventh step 750, the receiver splits the signal into two paths with an optical splitter 410 with a splitting ratio equal to P.sub.z, which forms the basis choice.

(37) In the represented eighth step 760, the Z basis measurement is carried out and is made by measuring the signal directly with a detection unit 430 to measure its arrival time. Further, the X basis measurement is carried out by passing the signal through a time-bin interferometer 420 and then using either one or two detection units 431, 432 to detect the signal.

(38) The Subsequent steps are classical operations done by QKD systems in order to generate a shared secret key between a Transmitter 300 and a Receiver 400 and are therefore not essential to the present invention.

(39) Then in ninth step 770, the detection events are announced to Transmitter over a classical channel 500.

(40) Last steps 780 consists in the raw key sifting, error reconciliation, privacy amplification and authentication.

(41) While the above embodiments have been described in conjunction with a number of example, it is evident that many alternatives, modifications and variations would be or are apparent to those of ordinary skill in the applicable arts. Accordingly, this disclosure is intended to embrace all such alternatives, modifications, equivalents and variations that are within the scope of this disclosure. This for example particularly the case regarding the different apparatuses which can be used.