Weak pulse detection for discrete-variable quantum key distribution receiver and method

1 . A receiver for receiving weak pulses of light in a quantum key distribution system with discrete-variable encoding, the receiver comprising:

an optical coupler having a first and a second input and a first and second output, the optical coupler being adapted to be connected to a quantum channel to receive the weak pulses at the first input, each weak pulse having duration T and wavelength λ wp , and local oscillator signals at the second input;

a local oscillator laser adapted to generate the local oscillator signals, each local oscillator signal being an impulse of duration T and wavelength λ LO , wherein the difference between λ LO and λ wp is such that:

1

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≤

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GHz

a laser locking system for locking the difference between the λ LO and λ wp wavelengths to a fixed value, so that the difference remains constant in time;

a synchronizer connected to the local oscillator laser so that the local oscillator laser emits a local oscillator signal at a given time for which the weak pulse and the local oscillator signal reach the first and second input, respectively, at the same time;

a first and a second photodetector, the first and second photodetector being connected to the first and second output, respectively, of the optical coupler and emitting a first and a second electric signal, the first and second electric signal being function of the interference between the weak pulse and the local oscillator signal in the optical coupler;

an electronic circuit configured to obtain a difference signal, the difference signal being function of the difference between the first electric signal emitted by the first photodetector and the second electric signal emitted by the second photodetector;

a filter adapted to filter the difference signal, generating a filtered signal which includes a portion of the difference signal having a frequency in a frequency range around a carrier frequency f, where

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a discriminator, the discriminator being configured to determine whether the filtered signal has a value function of the amplitude above a fixed threshold.

2 . The receiver according to claim 1 , wherein the difference between λ LO and λ wp is such that:

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≤

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3 . A receiver for receiving weak pulses in a quantum key distribution system with discrete-variable encoding, the receiver comprising:

an optical coupler having a first and a second input and first and second outputs, the optical coupler being adapted to be connected to a quantum channel to receive the weak pulses at the first input, each weak pulse having duration T and wavelength λ wp , and local oscillator signals at the second input;

a local oscillator laser adapted to generate the local oscillator signals, each local oscillator signal being an impulse of duration T and wavelength λ LO identical to λ wp ;

a phase variator, the phase variator being adapted to modulate, with a modulation frequency, the phase of the weak pulse or of the local oscillator signal before they interfere in the optical coupler;

a synchronizer connected to the local oscillator laser so that the local oscillator laser emits a local oscillator signal at a given time for which the weak pulse and the local oscillator signal reach the first and second input, respectively, at the same time;

a first and a second photodetector, the first and second photodetectors being connected to the first and second outputs, respectively, of the optical coupler and emitting a first and a second electric signal, the first and second electric signal being function of the interference between the weak pulse and the local oscillator signal in the optical coupler;

an electronic circuit configured to obtain a difference signal, the difference signal being function of the difference between the first electric signal emitted by the first photodetector and the second electric signal emitted by the second photodetector;

a filter adapted to filter the difference signal, generating a filtered signal that includes a portion of the difference signal having a frequency in a frequency range around the modulation frequency that was previously applied by the phase variator;

a discriminator, the discriminator being configured to determine whether the filtered signal has a value function of the amplitude above a fixed threshold.

4 . The receiver according to claim 3 , wherein the phase variator is configured to vary the phase of the local oscillator signal.

5 . The receiver according to claim 1 , further comprising an amplifier positioned at an output of the electronic circuit configured to amplify the difference signal.

6 . The receiver according to claim 1 , wherein the first and second photodetectors are a P-I-N photodetectors.

7 . The receiver according to claim 1 , wherein the filter comprises a pass band filter.

8 . The receiver according to one or more of the preceding claims, claim 1 , wherein the optical coupler is an 50% optical coupler.

9 . A method to detect the presence or the absence of weak pulses in a quantum key distribution system with discrete-variable encoding, the method comprising:

receiving a weak pulse from a quantum channel having duration T and wavelength λ wp ,

generating a local oscillator signal having duration T and wavelength λ LO , wherein the difference between λ LO and λ wp is such that:

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T

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λ

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≤

100

⁢

GHz

;

keeping the difference between λ LO and λ wp constant in time;

inputting to a first and second input of an optical coupler at the same time the weak pulse and the local oscillator signal;

creating an interference signal between the weak pulse and the local oscillator signal using the optical coupler;

detecting the interference signal at a first and at a second output of the optical coupler;

emitting a first and a second electric signal function of the detected interference signal;

subtracting the first and the second electric signal obtaining a difference signal;

filtering the difference signal generating a filtered signal which includes a portion of the difference signal having a frequency in a frequency range around a carrier frequency

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comparing the filtered signal with a threshold; and

determining that a weak pulse has been received if a parameter of the filtered signal is above the threshold.

10 . The method according to claim 9 , wherein the difference between λ LO and λ wp is such that:

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≤

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.

11 . A method to detect the presence or the absence of weak pulses in a quantum key distribution system with discrete-variable encoding, the method comprising:

receiving a weak pulse from a quantum channel having duration T and wavelength λ wp ,

generating a local oscillator signal having duration T and wavelength Azo identical to λ wp ;

inputting to a first and second input of an optical coupler at the same time the weak pulse and the local oscillator signal;

creating an interference signal between the weak pulse and the local oscillator signal using the optical coupler;

modulating the phase of the weak pulse or of the local oscillator signal before interference in the optical coupler, resulting in a modulated interference signal;

detecting the interference signal at a first and at a second output of the optical coupler;

emitting a first and a second electric signal function of the detected interference signal;

subtracting the first and the second electric signal obtaining a difference signal;

filtering the difference signal generating a filtered signal which includes a portion of the difference signal having a frequency in a frequency range around the modulation frequency;

comparing the filtered signal with a threshold; and

determining that a weak pulse has been received if a parameter of the filtered signal is above the threshold.

12 . The method according to claim 9 , wherein the duration T is comprised between 100 picoseconds and 10 nanoseconds.

13 . The method according to claim 9 , wherein the carrier frequency or the modulation frequency is bigger greater than 1/T.

14 . The method according to claim 9 , wherein the wavelength of the weak pulse λ cs is comprised between 800 nm and 1625 nm.

15 . The method according to claim 9 , wherein the frequency range is comprised in 0.1 KHz and 100 GHZ.

16 . The method according to claim 9 , wherein the carrier frequency or the modulation frequency f is comprised between 1 GHZ and 100 GHz.

17 . The method according to claim 9 , wherein the optical coupler is an 50% optical coupler.

18 . The method according to claim 9 , wherein the parameter of the filtered signal is the amplitude or the intensity of the filtered signal.