QKD system with synchronization channel verification

1. In a quantum key distribution (QKD) system having first and second QKD stations Alice and Bob, a method of verifying a synchronization (“sync”) channel that operably connects Alice and Bob, the method comprising:

transmitting a first pseudo-random bit stream (PRBS-A) from Alice to Bob over the sync channel;

verifying at Bob an error-free reception of a first select number of bits in PRBS-A;

transmitting a second pseudo-random bit stream (PRBS-B) from Bob to Alice over the sync channel; and

verifying at Alice an error-free reception of a second select number of bits in PRBS-B.

2. The method of claim 1 , including transmitting PRBS-A and PRBS-B over an optical fiber link that optically connects Alice to Bob.

3. The method of claim 1 , including converting PRBS-A and PRBS-B from an electrical signal to an optical signal.

4. The method of claim 1 , wherein the first select number of bits is the same as the second select number of bits.

5. The method of claim 1 , wherein the first and second select number of bits is in the range between 10 7 and 10 9 bits.

6. The method of claim 1 , wherein Alice and Bob include respective controllers CA and CB, and including:

storing PRBS-B in controller CA and storing PRBS-A in controller CB;

wherein said verifying at Bob includes comparing received PRBS-A bits sent by Alice to corresponding stored PRBS-A bits; and

wherein said verifying at Alice includes comparing received PRBS-B bits sent by Bob to corresponding stored PRBS-B bits.

7. The method of claim 1 , wherein PRBS-A and PRBS-B are generated at Alice and Bob by respective field-programmable gate arrays.

8. The method of claim 1 , wherein prior to transmitting a first pseudo-random bit stream (PRBS-A) from Alice to Bob over the sync channel, including establishing an ability to transmit and receive sync signals over the sync channel by:

generating a first electrical signal at Alice;

converting the first electrical signal to an optical signal at Alice;

receiving the optical signal at Bob; and

converting the optical signal to a second electrical signal at Bob.

9. The method of claim 1 , wherein transmitting the first pseudo-random bit stream (PRBS-A) from Alice to Bob over the sync channel includes:

generating first electrical signals at Alice that carry the bits in PRBS-A;

converting the first electrical signals to optical signals at Alice;

receiving the optical signals at Bob; and

converting the optical signals to second electrical signals at Bob.

10. The method of claim 1 , further comprising establishing a sync channel error rate.

11. The method of claim 10 , further comprising generating an error signal if an acceptable sync channel error rate is not achieved.

12. A quantum key distribution (QKD) system comprising:

first and second QKD stations Bob and Alice operably coupled by a synchronization (“sync”) channel used for transmitting sync signals and by a quantum channel used for transmitting quantum signals;

a controller CA at Alice adapted to generate a first pseudo-random bit stream PRBS-A and send it over to Bob via the sync channel, controller CA having stored therein a second pseudo-random bit stream PRBS-B;

a controller CB at Bob adapted to generate a second pseudo-random bit stream PRBS-B and send it over to Alice via the sync channel, controller CB having stored therein the first pseudo-random bit stream PRBS-A; and

wherein controllers CA and CB are adapted to coordinate the sequential transmission of pseudo-random bit streams PRBS-A and PRBS-B from Alice to Bob and from Bob to Alice, respectively, and compare received bits to the corresponding bits in the stored pseudo-random bit stream in order to establish a sync channel error rate over the sync channel.

13. The QKD system of claim 12 , wherein each controller CA and CB includes a field-programmable gate array (FPGA) adapted to generate the corresponding pseudo-random bit stream PRBS-A and PRBS-B, respectively and that has stored therein the pseudo-random bit stream generated by the other controller.

14. The QKD system of claim 12 , wherein pseudo-random bit streams PRBS-A and PRBS-B are the same.

15. The QKD system of claim 12 , wherein the sync channel includes an optical fiber link optically coupling Alice to Bob.

16. The QKD system of claim 15 , wherein both the sync channel and a quantum channel are transmitted over the optical fiber link.

17. The system of claim 12 , wherein the first pseudo-random bit stream PRBS-A is carried by a first electrical signal formed at Alice, is converted to an optical signal at Alice for transmission over to Bob, and is converted from the optical signal to a second electrical signal after being received at Bob.

18. The system of claim 12 , wherein establishing a sync channel error rate includes transmitting between 10 7 and 10 9 bits for each pseudo-random bit stream PRSB-A and PRBS-B.

19. The system of claim 12 , further comprising generating an error signal at Bob based on the sync channel error rate of transmitted pseudo-random bit stream PRSB-A.

20. In a quantum key distribution (QKD) system having first and second QKD stations Alice and Bob, a method of performing synchronization (“sync”) channel comprising:

establishing between Alice and Bob the sync channel for transmitting sync signals and a quantum channel for transmitting quantum signals;

sending a test signal over the sync channel to establish two-way communication over the sync channel;

generating and transmitting a first pseudo-random bit stream PRBS-A from Alice to Bob over the sync channel and storing a second pseudo-random bit stream PRBS-B at Alice;

generating and transmitting the second pseudo-random bit stream PRBS-B from Bob to Alice over the sync channel and storing the first pseudo-random bit stream PRBS-A at Bob;

comparing at each of Alice and Bob the respective received and stored pseudo-random bit streams to establish a sync channel error rate; and

generating, based upon the sync channel error rate, one of a status signal indicative of sync channel verification and an error signal indicative of sync channel errors.