Systems and methods of enhancing QKD security using a heralded photon source
View Patent ↗Systems and method of enhancing the security of a QKD system having operably coupled QKD stations (Alice, Bob) using correlated photon pulses (P 1 , P 2 ) are disclosed. The method includes generating the correlated photon pulses at Alice and detecting one of the pulses (P 2 ) to determine the number of photons in the other pulse (P 1 ). Pulse P 1 is then randomly modulated to form a modulated pulse P 1 ′, which is transmitted to Bob. Bob then randomly modulates pulses P 1 ′ to form twice-modulated pulses P 1 ″. Bob then detects pulses P 1 ″ at select timing slots that correspond to the expected arrival times of pulses P 1 ″, as well as to the number of photons in pulse P 1 (and thus in P 1 ″). Bob then communicates with Alice to determine the number N 1 of single-photon pulses P 1 ″ detected and the number N 2 of multi-photon pulses P 1 ″ detected. A security parameter (SP) is defined based on the probabilities of detecting single-photon and multi-photon pulses. Actual numbers of detected single-photon and multi-photon pulses are then compared to the security parameter to assess whether an eavesdropper is or could be interfering with the QKD process.
1. A method of forming a key in a quantum key distribution (QKD) system having optically coupled first and second QKD stations Alice and Bob, comprising:
generating at Alice, via a single light source that operates via spontaneous parametric down conversion using a non-linear medium, correlated pairs of photon pulses, with one of the pulses in each pair serving as an outgoing pulse having either a single-photon or multiple photons;
at Alice, identifying time slots and a number of photons in each outgoing pulse by detecting the other of the outgoing pulse for each pulse pair using a photon-number-resolving (PNR) detector;
communicating the time slot identities from Alice to Bob;
altering the state of each outgoing pulse;
transmitting a block of altered-state outgoing pulses from Alice to Bob;
performing at Bob a second state alteration of each outgoing pulse received at Bob and then detecting each twice-altered outgoing pulse;
determining at Bob a first number of detected single-photon pulses and a second number of detected multi-photon pulses using said time slot identities; and
comparing the first and second numbers to respective probabilities of detecting said first and second numbers.
2. The method of claim 1 , wherein the first and second altering of the state of the outgoing pulses at Alice and Bob, respectively, includes applying a random phase modulation to the outgoing pulses based on a select set of possible phase modulations.
3. The method of claim 1 , further comprising:
defining a security parameter based on probabilities of detecting single-photon and multi-photon pulses;
comparing the security parameter to data that includes the number of detected single-photon pulses and the number of detected multi-photon pulses; and
generating an alarm if the security parameter indicates a possible breach of security of the QKD system.
4. The method of claim 1 , further comprising:
generating timing signals from said detecting of the others of the outgoing pulses.
5. The method of claim 4 , wherein generating the timing signals includes generating synchronization signals and then sending the synchronization signals from Alice to Bob.
6. The method of claim 5 , wherein generating the timing signals includes:
generating a trigger signal and then causing the modulation of the outgoing pulse using the trigger signal.
7. The method of claim 1 , wherein altering the state of each outgoing pulse includes imparting one of a first random phase and a first random polarization.
8. The method of claim 1 , wherein performing a second state alteration of each outgoing pulse received at Bob includes imparting one of a second random phase and a second random polarization, wherein the first and second alterations are either both phase or both polarization alterations.
9. The method of claim 1 , wherein the non-linear medium comprises a non-linear crystal.
10. The method of claim 9 , including forming the non-linear medium as a bulk crystal or a waveguide-based crystal.
11. The method of claim 1 , further comprising optically connecting the non-linear medium to the PNR detector using an optical fiber.
12. The method of claim 1 , further comprising optically connecting the non-linear medium to a first modulator at Alice with an optical fiber that includes a delay line for delaying the arrival of the outgoing pulse at the first modulator.
13. The method of claim 12 , further including optically coupling Alice and Bob with an optical fiber link.
14. The method of claim 1 , including:
generating a trigger signal upon detecting the other of the outgoing pulse; and
modulating the outgoing pulse using a modulator activated by the trigger signal.