Abstract: A QKD system (10) having two QKD stations (Alice and Bob) optically coupled by an optical fiber link (FL), wherein Bob includes a variable timing delay arranged between Bob's controller (CB) and modulator (MB) or detector unit (40). A set-up and calibration procedure is performed wherein delay DL2 is adjusted until the timings for the modulator and detector unit (TSB and TS42, respectively) are established. Delay DL2 is then fixed so that the detector unit and modulator operate in a common timing mode that is not changed if the synchronization signal is changed. The timing TSS of the synchronization (sync) signals (SS) sent from Alice to Bob is adjusted to arrive at optimum system performance. Once the QKD system is in operation, because the sync signal can drift, the sync signal timing TSS is dithered maintain optimum QKD system performance.

Abstract: Systems and methods for multiplexing two or more channels of a quantum key distribution (QKD) system are disclosed. A method includes putting the optical public channel signal (SP1) in return-to-zero (RZ) format in a transmitter (T) in one QKD station (Alice) and amplifying this signal (thereby forming SP1*) just prior to this signal being detected with a detector (30) in a receiver (R) at the other QKD station (Bob). The method further includes precisely gating the detector via a gating element (40) and a coincident signal (PN1?) with pulses that coincide with the expected arrival times of the pulses in the detected (electrical) public channel signal (SP2). This allows for the public channel signal to have much less power, making it more amenable for multiplexing with the other QKD signals.

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 quantum key distribution (QKD) cascaded network with loop-back capability is disclosed. The QKD system network includes a plurality of cascaded QKD relays each having two QKD stations Alice and Bob. Each QKD relay also includes an optical switch optically coupled to each QKD station in the relay, as well as to input ports of the relay. In a first position, the optical switch allows for communication between adjacent relays and in a second position allows for pass-through communication between the QKD relays that are adjacent the relay whose switch is in the first position.

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 (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: Methods for establishing modulator timing for a QKD system (100) having QKD stations (Alice, Bob) with respective modulators (MA, MB) are disclosed. The timing method includes exchanging non-quantum signals (P1, P2) between the two QKD stations and performing respective coarse timing adjustments by scanning the modulator timing domain with relatively coarse timing intervals (?T1C, ?T2C,) and wide (coarse) modulator voltage signals (W1C, W2C). Coarse timings (T1C, T2C) are established by observing a change in detector counts between single-photon detectors (32a, 32b) when modulation occurs in exchanged non-quantum signals. The method also includes performing a fine timing adjustment by scanning the modulator timing domain with respective fine timing intervals (?T1R, ?T2R) and respective relatively narrow modulator voltage signals (W1R, W2R), and again observing a change in detector counts for exchanged non-quantum signals.

Abstract: Systems and methods for exchanging and processing encoded quantum signals in quantum key distribution (QKD) systems in real time. A stream of quantum signals is sent from Alice to Bob. Alice only encodes sets or “frames” of the streamed quantum signals based on receiving a “ready” message from Bob. This allows for Bob to finish processing the previous frame of data by allowing different bit buffers to fill and then be used for data processing. This approach results in gaps in between frames wherein quantum signals in the stream are sent unencoded and ignored by Bob. However, those quantum signals that are encoded for the given frame are efficiently processed, which on the whole is better than missing encoded quantum signals because Bob is not ready to receive and process them.

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: A QKD system (10) having two QKD stations (Alice and Bob) optically coupled by an optical fiber link (FL), wherein Bob includes a variable timing delay arranged between Bob's controller (CB) and modulator (MB) or detector unit (40). A set-up and calibration procedure is performed wherein delay DL2 is adjusted until the timings for the modulator and detector unit (TSB and TS42, respectively) are established. Delay DL2 is then fixed so that the detector unit and modulator operate in a common timing mode that is not changed if the synchronization signal is changed. The timing TSS of the synchronization (sync) signals (SS) sent from Alice to Bob is adjusted to arrive at optimum system performance. Once the QKD system is in operation, because the sync signal can drift, the sync signal timing TSS is dithered maintain optimum QKD system performance.

Abstract: A QKD cascaded network (5) with loop-back capability is disclosed. The QKD system network includes a plurality of cascaded QKD relays (10, 20, 30) each having two QKD stations Alice (A) and Bob (B) therein. Each QKD relay also includes an optical switch (50). The optical switch is optically coupled to each QKD station in the relay, as well as to the input ports (PI) of the relay. In a first position, the optical switch allows for communication between adjacent relays. In a second position, the optical switch allows for pass-through communication between the QKD relays (10 and 30) that are adjacent the relay whose switch is in the first position. Also in the second position, the optical switch allows for communication between the QKD stations A and B within the relay. This, in turn, allows for diagnostic measurements to be made of one or both of the QKD stations via an optical path (90) that is entirely within the relay station enclosure (12, 22, 32).

Abstract: A cascaded modulator system (20) and method for a QKD system (10) is disclosed. The modulator system includes to modulators (M1 and M2) optically coupled in series. A parallel shift register (50) generates two-bit (i.e., binary) voltages (L1, L2). These voltage levels are adjusted by respective voltage adjusters (30-1 and 30-2) to generate weighted voltages (V1, V2) that drive the respective modulators. An electronic delay element (40) that matches the optical delay between modulators provides for modulator timing (gating). The net modulation (MNET) imparted to an optical signal (60) is the sum of the modulations imparted by the modulators. The modulator system provides four possible net modulations based only on binary voltage signals. This makes for faster and more efficient modulation in QKD systems and related optical systems when compared to using quad-level voltage signals to drive a single modulator.

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: 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: 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: Systems and methods for exchanging and processing encoded quantum signals in quantum key distribution (QKD) systems in real time. A stream of quantum signals is sent from Alice to Bob. Alice only encodes sets or “frames” of the streamed quantum signals based on receiving a “ready” message from Bob. This allows for Bob to finish processing the previous frame of data by allowing different bit buffers to fill and then be used for data processing. This approach results in gaps in between frames wherein quantum signals in the stream are sent unencoded and ignored by Bob. However, those quantum signals that are encoded for the given frame are efficiently processed, which on the whole is better than missing encoded quantum signals because Bob is not ready to receive and process them.

Abstract: Systems and methods for multiplexing two or more channels of a quantum key distribution (QKD) system are disclosed. A method includes putting the optical public channel signal (SP1) in return-to-zero (RZ) format in a transmitter (T) in one QKD station (Alice) and amplifying this signal (thereby forming SP1*) just prior to this signal being detected with a detector (30) in a receiver (R) at the other QKD station (Bob). The method further includes precisely gating the detector via a gating element (40) and a coincident signal (PN1?) with pulses that coincide with the expected arrival times of the pulses in the detected (electrical) public channel signal (SP2). This allows for the public channel signal to have much less power, making it more amenable for multiplexing with the other QKD signals.

Abstract: A quantum noise random number generator system that employs quantum noise from an optical homodyne detection apparatus is disclosed. The system utilizes the quantum noise generated by splitting a laser light signal using a beamsplitter having four ports, one of which receives one of which is receives the laser light signal, one of which is connected to vacuum, and two of which are optically coupled to photodetectors. Processing electronics process the difference signal derived from subtracting the two photodetector signals to create a random number sequence. Because the difference signal associated with the two photodetectors is truly random, the system is a true random number generator.

Abstract: A QKD system that includes a dual-mode pulse generator 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. The delay mode is used to send relatively narrow optical pulses between the QKD stations to establish a quantum key. When the dual-mode 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 arbitrarily long (wide) optical pulses.

Abstract: A cascaded modulator system (20) and method for a QKD system (10) is disclosed. The modulator system includes to modulators (M1 and M2) optically coupled in series. A parallel shift register (50) generates two-bit (i.e., binary) voltages (L1, L2). These voltage levels are adjusted by respective voltage adjusters (30-1 and 30-2) to generate weighted voltages (V1, V2) that drive the respective modulators. An electronic delay element (40) that matches the optical delay between modulators provides for modulator timing (gating). The net modulation (MNET) imparted to an optical signal (60) is the sum of the modulations imparted by the modulators. The modulator system provides four possible net modulations based only on binary voltage signals. This makes for faster and more efficient modulation in QKD systems and related optical systems when compared to using quad-level voltage signals to drive a single modulator.