Apparatus and method for stablizing quantum cryptography system

1. A method for stabilizing a quantum cryptography system, comprising:

determining whether the quantum cryptography system operates in a stabilized state, based on a bit error rate or a key rate of the quantum cryptography system;

readjusting a phase modulation value of a phase modulator so that every photon detector among photo detectors in the quantum cryptography system measures a single photon with a same probability; and

in response to the quantum cryptography system not operating in the stabilized state,

determining a reference arrival time of a gate pulse, within a first time range, in which a photon count measured at a photon detector is greater than or equal to a threshold count,

determining an optimal arrival time of the gate pulse, within a second time range, in which the photon count measured at the photon detector is maximal, and

readjusting the arrival time of the gate pulse so that an arrival time of a single photon for the photon detector is aligned with the arrival time of the gate pulse, based on the determined optimal arrival time of the gate pulse,

wherein the optimal arrival time of the gate pulse is determined based on a result of comparing a photon count measured after the arrival time of the gate pulse is changed with a photon count measured before the arrival time of the gate pulse is changed, and

wherein the readjusting of the phase modulation value occurs before the readjusting of the arrival time of the gate pulse.

2. The method for stabilizing the quantum cryptography system according to claim 1 , wherein the second time range includes the reference arrival time of the gate pulse and is smaller than or equal to the first time range.

3. The method for stabilizing the quantum cryptography system according to claim 2 , wherein

the reference arrival time of the gate pulse is determined based on a photon count measured as the arrival time of the gate pulse is changed as much as a first time interval within the first time range,

the optimal arrival time of the gate pulse is determined based on a photon count measured as the arrival time of the gate pulse is changed as much as a second time interval within the second time range, and

the second time interval is less than or equal to the first time interval.

4. The method for stabilizing the quantum cryptography system according to claim 1 , wherein the determining of whether the quantum cryptography system operates in the stabilized state comprises determining that the quantum cryptography system does not operate in the stabilized state, in response to the bit error rate being greater than or equal to a first threshold or the key rate being less than or equal to a second threshold.

5. The method for stabilizing the quantum cryptography system according to claim 1 , wherein the determining of whether the quantum cryptography system operates in the stabilized state comprises determining whether the quantum cryptography system operates in the stabilized state based on the key rate of the quantum cryptography system, which is obtained after the phase modulation value is readjusted.

6. The method for stabilizing the quantum cryptography system according to claim 1 , wherein the quantum cryptography system is a two-way quantum cryptography system.

7. The method of claim 1 , wherein the reference arrival time of the gate pulse is determined by iteratively performing either one or both of leading and lagging the arrival time of the gate pulse by a first interval, until determining that the photon count measured at the photon detector is greater than or equal to the threshold count.

8. The method of claim 7 , wherein the optimum arrival time of the gate pulse is determined by iteratively performing either one or both of leading and lagging the arrival time of the gate pulse by a second interval that is smaller than the first interval, until determining that the photon count measured at the photon detector is maximal.

9. An apparatus for stabilizing a quantum cryptography system, comprising:

a hardware processor configured to:

determine whether the quantum cryptography system operates in a stabilized state, based on a bit error rate or a key rate of the quantum cryptography system;

readjust a phase modulation value of a phase modulator so that every photon detector among a plurality of photon detectors measures a single photon with a same probability; and

in response to the quantum cryptography system not operating in the stabilized state,

determine a reference arrival time of a gate pulse, within a first time range, in which a photon count measured at a photon detector is greater than or equal to a threshold count,

determine an optimal arrival time of the gate pulse, within a second time range, in which the photon count measured at the photon detector is maximal, and

readjust the arrival time of the gate pulse for the photon detector so that an arrival time of a single photon for the photon detector is aligned with the arrival time of the gate pulse, based on the determined optimal arrival time of the gate pulse,

wherein the optimal arrival time of the gate pulse is determined based on a result of comparing a photon count measured after the arrival time of the gate pulse is changed with a photon count measured before the arrival time of the gate pulse is changed, and

wherein the readjusting of the phase modulation value occurs before the readjusting of the arrival time of the gate pulse.

10. The apparatus for stabilizing the quantum cryptography system according to claim 9 , wherein the second time range includes the reference arrival time of the gate pulse and is smaller than or equal to the first time range.

11. The apparatus for stabilizing the quantum cryptography system according to claim 10 , wherein the hardware processor is further configured to:

determine the reference arrival time of the gate pulse based on a photon count measured as the arrival time of the gate pulse is changed as much as a first time interval within the first time range; and

determine the optimal arrival time of the gate pulse based on a photon count measured

as the arrival time of the gate pulse is changed as much as a second time interval within the

second time range, the second time interval being smaller than or equal to the first time interval.

12. The apparatus for stabilizing the quantum cryptography system according to claim 9 , wherein the hardware processor is further configured to determine that the quantum cryptography system does not operate in the stabilized state, in response to the bit error rate being greater than or equal to a first threshold or the key rate being less than or equal to second threshold.

13. The apparatus for stabilizing the quantum cryptography system according to claim 9 , wherein the hardware processor is further configured to determine whether the quantum cryptography system operates in the stabilized state, based on the key rate of the quantum cryptography system, which is obtained after the phase modulation value is readjusted.

14. The apparatus for stabilizing the quantum cryptography system according to claim 9 , wherein the quantum cryptography system is a two-way quantum cryptography system.