Apparatus and method for adjusting the phase-modulator's modulating signal time reference in a quantum-key-distribution system

1. An apparatus ( 100 ) for adjusting the phase modulator's modulating signal time reference in a phase-based Quantum Key Distribution (QKD) system, said QKD system comprising a sending end node and a receiving end node,

said sending end node comprising:

a pulse generator ( 10 ) configured to generate a stream of optical pulses with a pre-defined time interval (T) between the adjacent pulses; and

a phase modulator ( 20 ) configured to receive the optical pulses, during normal key exchange operation said phase modulator configured to use a non-return-to-zero (NRZ) modulating signal to modulate the phase of each of the pulses and generate a first stream of modulated pulses, said NRZ modulating signal being generated based on a random bit stream, said phase modulator ( 20 ) further configured to send the first stream of phase modulated pulses to the receiving end node through a quantum channel,

said apparatus ( 100 ) comprising:

a processing device ( 104 ) configured to detect a phase modulator (PM) alignment state and generate a return-to-zero (RZ) modulating signal during said PM alignment state, said processing device ( 104 ) further configured to send the generated RZ modulating signal to said phase modulator ( 20 ) to facilitate modulation of each of the optical pulses by the phase modulator in accordance with the RZ modulating signal leading to the generation of a second stream of modulated pulses, said RZ modulating signal is encoded with consecutive one and zero bits and generated using a time reference;

a beam splitter ( 112 ) located in the quantum channel and connected to said phase modulator ( 20 ), said beam splitter ( 112 ) configured to receive said second stream of phase modulated pulses from the phase modulator ( 20 ) and split the received second stream of phase modulated pulses into two consequent phase modulated pulses for propagation along a first path (C 1 ) of the quantum channel to said receiving end node and along a second path (C 2 ) for phase adjustment;

a two-arm interferometer ( 110 ) having two output legs ( 110 a , 110 b ) and located in the second path (C 2 ) from the beam splitter ( 112 ), said interferometer ( 110 ) configured to receive the modulated pulses through the second path (C 2 ) and provide demodulated pulses having the pre-defined time interval (T) between adjacent pulses at the output legs ( 110 a , 110 b );

a first photodetector ( 106 ) connected to a first output leg ( 110 a ) of said interferometer ( 110 ), and configured to receive said demodulated pulses after constructive interference from said interferometer ( 110 ) and generate a corresponding digital output; and

a second photodetector ( 108 ) connected to a second output leg ( 110 b ) of said interferometer ( 110 ), and configured to receive said demodulated pulses after destructive interference from said interferometer ( 110 ) and generate a corresponding digital output;

wherein said processing device ( 104 ) is configured to receive said digital outputs from said photodetectors ( 106 , 108 ) and measure an average power in each leg of the interferometer ( 110 ) based on said received digital outputs, said processing device ( 104 ) is further configured to implement a logic to detect a delay between the optical pulses generated by the pulse generator ( 10 ) and the modulating signal based on the measured average power and adjust the time reference to compensate the detected delay for aligning the pulse generator ( 10 ) with the phase modulator ( 20 ).

2. The apparatus ( 100 ) as claimed in claim 1 , wherein said processing device ( 104 ) comprises:

a. a signal generator ( 202 ) configured to detect the PM alignment state and generate the alternate one and zero Return-to-zero (RZ) encoded signal of half bit interval pulse-width (i.e. T/2), using the time reference, as the RZ modulating signal of the phase modulator ( 20 ) during the PM alignment state;

b. a power measuring unit ( 204 ) configured to measure the average power in each leg of the interferometer ( 110 ) using the digital outputs from said first and second photodetectors ( 106 , 108 );

c. a delay detection unit ( 206 ) configured to cooperate with said power measuring unit ( 204 ) to compare the average power measured in first leg of the interferometer ( 110 ) with the average power measured in second leg to detect the delay between the optical pulses generated by the pulse generator ( 10 ) and the RZ modulating signal; and

d. a delay adjusting unit ( 208 ) configured to cooperate with said delay detection unit ( 206 ) to compensate the detected delay by shifting the phase modulator's modulating signal time reference until the average power measured in one leg of said interferometer ( 110 ) is greater than the average power measured in the other leg.

3. The apparatus ( 100 ) as claimed in claim 1 , wherein said processing device ( 104 ) is an integral part of said phase modulator ( 20 ).

4. The apparatus ( 100 ) as claimed in claim 1 , wherein said interferometer ( 110 ) comprises:

a. an optical input path for receiving the second stream of modulated pulses from said beam splitter ( 112 );

b. at least two interferometer arms connected to said optical input path, one of the arms including a delay element ( 110 c ) for providing the pre-defined time interval (T) between the received successive modulated pulses; and

c. two output legs ( 110 a , 110 b ) connected to said interferometer arms, said output legs ( 110 a , 110 b ) configured to receive the demodulated pulses from said arms and feed the demodulate pulses to said first and second photodetectors ( 106 , 108 ).

5. The apparatus ( 100 ) as claimed in claim 1 , wherein said photodetectors ( 106 , 108 ) are connected to the output legs ( 110 a , 110 b ) of said interferometer ( 110 ) in a way that, if adjacent pulses are in phase, the demodulated pulse will appear at said first photodetector ( 106 ) and if adjacent pulses are out of phase, the demodulated pulse will appear at said second photodetector ( 108 ).

6. The apparatus ( 100 ) as claimed in claim 1 , wherein said photodetectors ( 106 , 108 ) are connected to the output legs ( 110 a , 110 b ) of said interferometer ( 110 ) in a way that if adjacent pulses are in phase, the demodulated pulse will appear at said second photodetector ( 108 ) and if adjacent pulses are out of phase, the demodulated pulse will appear at said first photodetector ( 106 ).

7. The apparatus ( 100 ) as claimed in claim 2 , wherein said delay detection unit ( 206 ) comprises:

a. a comparator ( 206 a ) configured to compare the average power measured in first leg of the interferometer ( 110 ) with the average power measured in second leg to generate a comparison output; and

b. a detecting module ( 206 b ) configured to cooperate with said comparator ( 206 a ) to detect the presence of the delay between the optical pulses and the modulating signal when said comparison output is below a pre-defined range which is caused by measurement of a non-zero average power at both the first output leg ( 110 a ) and the second output leg ( 110 b ).

8. The apparatus ( 100 ) as claimed in claim 1 , wherein said processing device ( 104 ) is implemented using one or more Programmable logic devices selected from the group consisting of a mask programmable gate array (MPGA), an Application Specific Integrated Circuit (ASIC), a simple programmable logic device (SPLD), a complex programmable logic device (CPLD), and a Field Programmable Gate Array (FPGA).

9. A method ( 300 ) for adjusting the phase modulator's modulating signal time reference in a phase-based Quantum Key Distribution (QKD) system, said QKD system comprising a sending end node and a receiving end node, said sending end node comprising a pulse generator ( 10 ) configured to generate a stream of optical pulses with a pre-defined time interval (T) between the adjacent pulses and a phase modulator ( 20 ) configured to receive the optical pulses, use a non-return-to-zero (NRZ) modulating signal to modulate the phase of each of the pulses thereby generating a first stream of modulated pulses, and send the first stream of phase modulated pulses to the receiving end node through a quantum channel, said NRZ modulating signal generated based on a random bit stream,

said method ( 300 ) comprising the following steps:

detecting (step- 302 ), by a processing device ( 104 ), a phase modulator (PM) alignment state of the QKD system;

generating (step- 304 ), by said processing device ( 104 ), a return-to-zero (RZ) modulating signal during said PM alignment state, said RZ modulating signal being encoded with consecutive one and zero bits and generated using a time reference;

sending (step- 306 ), by said processing device ( 104 ), the generated RZ modulating signal to said phase modulator ( 20 ) to facilitate modulation of each of the optical pulses by the phase modulator in accordance with the RZ modulating signal leading to the generation of a second stream of modulated pulses;

receiving (step- 308 ), by a beam splitter ( 112 ) located in the quantum channel and connected to said phase modulator ( 20 ), said second stream of phase modulated pulses from the phase modulator ( 20 );

splitting (step- 310 ), by said beam splitter ( 112 ), the received second stream of phase modulated pulses into two consequent phase modulated pulses for propagation along a first path (C 1 ) of the quantum channel to said receiving end node and along a second path (C 2 ) for phase adjustment;

receiving (step- 312 ), by a two-arm interferometer ( 110 ) having two output legs ( 110 a , 110 b ) and located in the second path (C 2 ) from the beam splitter ( 112 ), the modulated pulses through the second path (C 2 );

providing (step- 314 ), by said interferometer ( 110 ), demodulated pulses having the pre-defined time interval (T) between adjacent pulses at the output legs ( 110 a , 110 b );

receiving (step- 316 ), by a first photodetector ( 106 ) connected to a first output leg ( 110 a ) of said interferometer ( 110 ), said demodulated pulses after constructive interference from said interferometer ( 110 ) and generating a corresponding digital output;

receiving (step- 318 ), by a second photodetector ( 108 ) connected to a second output leg ( 110 b ) of said interferometer ( 110 ) said demodulated pulses after destructive interference from said interferometer ( 110 ) and generating a corresponding digital output; and

receiving (step- 320 ), by said processing device ( 104 ), said digital outputs from said photo detectors;

measuring (step- 322 ), by said processing device ( 104 ), an average power in each leg of the interferometer ( 110 ) based on said received digital outputs;

implementing (step- 324 ), by said processing device ( 104 ), a logic to detect a delay between the optical pulses generated by the pulse generator ( 10 ) and the modulating signal based on the measured average power; and

adjusting (step- 326 ), by said processing device ( 104 ), the time reference to compensate the detected delay for aligning the pulse generator ( 10 ) with the phase modulator ( 20 ).

10. The method ( 300 ) as claimed in claim 9 , wherein the step ( 320 ) of adjusting, by said processing device ( 104 ), the time reference to compensate the detected delay comprises shifting the phase modulator's modulating signal time reference until the average power measured in one leg of said interferometer ( 110 ) is greater than the average power measured in the other leg.