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: 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 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 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: 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 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 phase modulator circuit and method for generating an output signal having individually positionable edges is described. The phase modulator includes a programmable pulse generator, such as an interval counter, or a delay for producing the output signal, and a control value source, such as a memory, for delivering a sequence of control values to the generator. The control values determine the time between successive output pulses.