Abstract: A method of autocalibrating a quantum key distribution (QKD) system (200) is disclosed. The QKD system includes a laser ((202) that generates photon signals in response to a laser gating signal (S0) from a controller (248). The method includes first performing a laser gate scan (304) to establish the optimum arrival time (TMAX) of the laser gating signal corresponding to an optimum- e.g., a maximum number of photon counts (NMAX)—from a single-photon detector (SPD) unit (216) in the QKD system when exchanging photon signals between encoding stations (Alice and Bob) of the QKD system. Once the optimal laser gating signal arrival time (TMAX) is determined, the laser gate scan is terminated and a laser gate dither process (308) is initiated. The laser dither involves varying the arrival time (T) of the laser gating signal around the optimum value of the arrival time TMAX. The laser gate dither provides minor adjustments to the laser gating signal arrival time to ensure that the SPD unit produces an optimum (e.g.

Abstract: Methods for calibrating the modulators in a QKD system (100) are disclosed. The methods include setting the voltage (VB) of Bob's modulator (MB) to a positive value and then adjusting the voltage (VA) of Alice's modulator (MA) in both the positive and negative direction to obtain overall relative phase modulations that result in maximum and minimum photon counts (N) in the two single-photon detectors (32a, 32b). Bob's modulator voltage is then set to a negative value and the process repeated. When the basis voltages (VB(1), VB(2), VA(1), VA(2), VA(3) and VA(4)) are established, the QKD system is operated with intentionally selected incorrect bases at Bob and Alice to assess orthogonality of the basis voltages by assessing whether or not the probability of photon detection at the detectors is 50:50. If not, the modulator voltages are adjusted to be orthogonal. This involves changing Bob's basis voltage (VB(1) and/or VB(2)) and repeating the process until a 50:50 detector count distribution is obtained.

Abstract: A method of improving the security of a QKD system is disclosed. The QKD system exchanges qubits between QKD stations, wherein the brief period of time surrounding the expected arrival time of a qubit at a modulator in a QKD station defines a gating interval. The method includes randomly activating the modulator in a QKD station both within the gating interval and outside of the gating interval, while recording those modulations made during the gating interval. Such continuous or near-continuous modulation prevents an eavesdropper from assuming that the modulations correspond directly to the modulation of a qubit. Thus, an eavesdropper (Eve) has the additional and daunting task of determining which modulations correspond to actual qubit modulations before she can begin to extract any information from detected modulation states of the modulator.

Abstract: A pulse generator electrical circuit capable of operating as both a clock-based pulse generator and a delay-based pulse generator while minimizing the limitations of these two types of pulse generators is disclosed. When the pulse generator operates in “delay mode,” the smallest output pulse width possible corresponds to the minimum set point delay between the two delay circuits. The largest possible output pulse width corresponds to the difference between the maximum and minimum of the delay circuits. When the pulse generator operates in “clock mode,” the output of one of the delay circuits is blocked so that the output of the gate depends solely on the output of other delay circuit. This limits the lower pulse width interval to that of the retimer clock, but allows for an arbitrarily long (wide) pulse.

Abstract: A method (300) of performing photon detector autocalibration in quantum key distribution (QKD) system (200) is disclosed. The method (300) includes a first act (302) of performing a detector gate scan to establish the optimum arrival time of a detector gate pulse (S3) that corresponds with a maximum number of photon counts (NMAX) from a single-photon detector (216) in the QKD system (200). Once the optimal detector gate pulse arrival time is determined, then in an act (306), the detector gate scan is terminated and in an act (308) a detector gate dither process is initiated. The detector gate dither act (308) involves varying the arrival time (T) of the detector gate pulse (S3) around the optimal value of the arrival time established during the detector gate scan process. The detector gate dither provides minor adjustments to the arrival time to ensure that the detector (216) produces maximum number of photon counts (NMAX).

Abstract: A single-photon “watch dog” detector for a two-way quantum key distribution (QKD) system. The detector can detect weak probe signals associated with a Trojan horse attack, or weak substitute signals associated with a “man in the middle” attacks. The detector provides for a significant increase in security for a two-way QKD system over the prior art that employs a conventional detector such as a photodiode. By counting the number of weak pulses entering and/or leaving the reflecting QKD station (Alice), an eavesdropper that attempts to add weak pulses to the quantum channel in order to gain phase information from the phase modulator at Alice can be detected.

Abstract: Systems and methods for verifying error-free transmission of the synchronization (“sync”) channel of a QKD system are disclosed. The method includes sending a first pseudo-random bit stream (PRBS) over the sync channel from Alice to Bob, and verifying at Bob the accurate transmission of the first PRBS. The method also includes sending a second pseudo-random bit stream (PRBS) over the sync channel from Bob to Alice, and verifying at Alice the accurate transmission of the first PRBS. If the transmissions of a select number of bits in the first and second PRBSs are error-free, then the sync channel is verified and the QKD system can commence operation.

Abstract: A method of improving the security of a QKD system is disclosed. The method includes sending synchronization (“sync”) signals from a first QKD station to the second QKD station over a sync signal channel and recording data relating to the arrival times of the sync signals at the second QKD station. The method also includes processing the arrival time data to discern between extra signals in the sync signal channel that were not sent by the first QKD station over the sync channel, and sync signals that were sent by the first QKD station over the sync channel. The method also includes sending an alarm signal when it is determined that extra signals in the sync channel could be due to an attack on the QKD system.

Abstract: A system and method for providing two-way communication of quantum signals, timing signals, and public data is provided. Generally, the system contains a first public data transceiver capable of transmitting and receiving public data in accordance with a predefined timing sequence, a first optical modulator/demodulator capable of transmitting and receiving timing signals in accordance with the predefined timing sequence, a first quantum transceiver capable of transmitting and receiving quantum signals in accordance with the predefined timing sequence, and a first controller connected to the first public data transceiver, the first optical modulator/demodulator, and the first quantum transceiver. The first controller is capable of controlling the transmission of the public data, the timing signals, and the quantum signals in accordance with the predefined timing sequence.

Abstract: A method of improving the security of a QKD system is disclosed. The method includes randomly modulating the modulator in a QKD station both within the gating interval and outside of the gating interval, while recording those modulations made during the gating interval. Such continuous modulation prevents an eavesdropper from assuming that the modulations correspond directly to the modulation of a qubit. Thus, an eavesdropper (Eve) has the additional and daunting task of determining which modulations correspond to actual qubit modulations before she can begin to extract any information from detected modulation states of the modulator.

Abstract: Systems and methods for reducing or eliminating timing errors in a quantum key distribution (QKD) system (100) are disclosed. The QKD system has a pulse generator with retimer (PGRT) that includes a field-programmable gate array (FPGA) (or FPGA output) which is used as a timing generator (TG). While an FPGA has the desired degree of programmability for use in a QKD system, it also suffers from undue amounts of jitter in the digital output. The present invention utilizes emitter-coupled logic (ECL) to reduce the timing jitter from the FPGA by coupling two ECL delays (ECL delay 1 and ECL delay 2) to the FPGA and to retiming block, and by using an ECL logical AND gate to set the pulse width of the various synchronization signals. An embodiment of the present invention includes multiple clock domains having individual clocks (CLK), phase-lock loops (PLLs), retiming circuits (RT) and timing generators (TG) for robust jitter reduction and hence highly accurate QKD system timing.

Abstract: A pulse generator electrical circuit capable of operating as both a clock-based pulse generator and a delay-based pulse generator while minimizing the limitations of these two types of pulse generators is disclosed. When the pulse generator operates in “delay mode,” the smallest output pulse width possible corresponds to the minimum set point delay between the two delay circuits. The largest possible output pulse width corresponds to the difference between the maximum and minimum of the delay circuits. When the pulse generator operates in “clock mode,” the output of one of the delay circuits is blocked so that the output of the gate depends solely on the output of other delay circuit. This limits the lower pulse width interval to that of the retimer clock, but allows for an arbitrarily long (wide) pulse.

Abstract: QKD systems having timing systems and timing method that allow for QKD to be performed in actual field conditions associated with practical commercial applications of quantum cryptography. The QKD system includes optical modems in each QKD station. Each modem has a circulator with an optical receiver and an optical transmitter coupled to it. One of the optical modems includes two phase lock loops and the other optical modem includes a phase lock loop and a transmit clock. Synchronization pulses are exchanged between the optical modems over a timing channel to synchronize the operation of the QKD system. The phase lock loops serve to lock a receive timing domain to a transmit time domain to ensure proper encoding and detection of weak quantum signals exchanged between the QKD stations.

Abstract: QKD system networks (50, 200, 300) and methods of communicating between end-users (P1, P2) over same are disclosed. An example QKD system network (50) includes a first QKD station (A1) and a second QKD station (A2) with a relay station (58) in between. The relay station includes a single third QKD station (B) and an optical switch (55). The optical switch allows the third QKD station to alternately communicate with the first and second QKD stations so as to establish a common key between the first and second QKD stations. The end-users are coupled to respective QKD stations A1 and A2. A secret key (S) is shared between P1 and P2 by QKD station B being able to independently form keys with A1 and A2. This basic system, represented as P1-A1-B-A2-P2, can be expanded into more complex linear networks, such as P1-A1-B1-A2-B2-P2 with B1 and A2 making up the relays. The basic QKD system network can also be expanded into multi-dimensions.

Abstract: Systems and methods for graphically displaying statistical information relating to the operation of a quantum key distribution (QKD) system. The method includes exchanging quantum photons between first and second QKD stations for each combination of modulator states, collecting data on the number of quantum photon counts obtained in each of two detectors for each modulator state combination, defining a statistical region for each modulator state combination based on the collected data, and displaying the statistical regions on a graph having indicia indicating ideal locations for the statistical regions. The method also optionally includes adjusting the QKD system based on the graphically displayed information to optimize system performance.

Abstract: Methods and systems for generating calibrated optical pulses in a QKD system. The method includes calibrating a variable optical attenuator (VOA) by first passing radiation pulses of a given intensity and pulse width through the VOA for a variety of VOA settings. The method further includes resetting the VOA to maximum attenuation and sending through the VOA optical pulses having varying pulse widths. The method also includes determining the power needed at the receiver in the QKD system, and setting the VOA so that optical pulses generated by the optical radiation source are calibrated to provide the needed average power. Such calibration is critical in a QKD system, where the average number of photons per pulse needs to be very small—i.e., on the order of 0.1 photons per pulse—in order to ensure quantum security of the system.