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 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: An optical fiber interferometer (10) with relaxed loop tolerance, and a quantum key distribution (QKD) system (200) using same is disclosed. The interferometer includes two optical fiber loops (LP1 and LP2). The loops have an optical path length (OPL) difference between them. A polarization-maintaining (PM) optical fiber section (60) of length (L60) and having fast and slow optical axes (AF and AS) optically couples the two loops. The length and fast-slow axis orientation is selected to introduce a time delay (?T1-2) between orthogonally polarized optical pulses traveling therethrough that compensates for the OPL difference. This allows for drastically relaxed tolerances when making the loops, leading to easier and more cost-effective manufacturing of the interferometer as well as related devices such as a optical-fiber-based QKD system.

Abstract: A narrow-band single-photon source (10) is disclosed, along with a QKD system (200) using same. The single-photon source is based on spontaneous parametric downconversion that generates signal and idler photons (PS and PI) as an entangled photon pair. Narrow-band signal photons are generated by selectively narrow-band-filtering the idler photons. This results in a non-local filtering of the signal photons due to the time-energy entanglement of the photon pair. Subsequent detection of the filtered idler photon establishes the narrow-band signal photon. The narrow-band single-photon source is particularly useful in a QKD system, wherein the narrow-band signal photons are used as quantum signals to mitigate the adverse effect of chromatic dispersion on QKD system performance.

Abstract: Key banking methods and systems for quantum key distribution (QKD) are disclosed. A method of the invention includes establishing a primary key bank that stores perfectly secure keys associated with exchanging true quantum pulses between two QKD stations Bob and Alice. The method also Includes establishing a secondary key bank that stores less-than-perfectly secure keys associated with exchanging relatively strong quantum pulses between Bob and Alice. The primary keys are used for select applications such as authentication that are deemed to require the highest security, while the secondary keys are used for applications, such as encrypted bit sifting, that are deemed to require less-than-perfect security. A benefit of the two-key-bank architecture is that exchanging primary and secondary keys actually allows for an increase in the distance over which the primary keys can be securely distributed.

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: A method of encrypting information using an encryption pad based on keys exchanged between quantum key distribution (QKD) stations is disclosed. The method includes establishing raw keys between two stations using QKD, processing the keys to establish a plurality of matching privacy amplified keys at each station and buffering the keys in a shared key schedule. The method also includes the option of expanding one or more of the keys in the shared key schedule using a stream cipher to create a supply of expanded keys that serve as pads for one-time-pad encryption.

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: Methods and systems for suppressing the electromagnetic interference (EMI) signature generated by a QKD station are disclosed. One of the methods includes generating two or more modulator drive signals corresponding to two or more of the n possible modulator states of the particular QKD protocol. The modulator drive signals are sent to a random number generation (RNG) unit, which randomly selects one of the two or more modulator drive signals and passes it to the modulator. Another method involves generating two modulator drive signals, wherein the voltage sum is constant. One signal is sent to the modulator while the other is sent to a circuit-terminating element, which can be a second modulator. The method suppresses the EMI signature associated with individual modulation states. This prevents an eavesdropper from gaining information about the modulator states via the EMI signature, which information could otherwise yield information about the exchanged key.

Abstract: Systems and methods for suppressing the unwanted detection of backscattered light in a two-way quantum key distribution (QKD) system is disclosed. The system includes a first QKD station that has two or more laser sources that emit light at different wavelengths, and corresponding two or more sets of detectors. In a two-way QKD system, backscattered light is typically generated in an optical fiber link connecting the first and second QKD stations by the relatively strong outgoing optical pulses. To prevent the backscattered light from interfering with the detection of the weak optical pulses returned from the second QKD station to the first station, a controller sequentially activates different light sources, and also sequentially activates the different sets of detectors.

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: Key banking methods and systems for quantum key distribution (QKD) are disclosed. A method of the invention includes establishing a primary key bank that stores perfectly secure keys associated with exchanging true quantum pulses between two QKD stations Bob and Alice. The method also includes establishing a secondary key bank that stores less-than-perfectly secure keys associated with exchanging relatively strong quantum pulses between Bob and Alice. The primary keys are used for select applications such as authentication that are deemed to require the highest security, while the secondary keys are used for applications, such as encrypted bit sifting, that are deemed to require less-than-perfect security.

Abstract: A two-way actively stabilized QKD system that utilizes control signals and quantum signals is disclosed. Because the quantum signals do not traverse the same optical path through the system, signal collisions in the phase modulator are avoided. This allows the system to have a higher transmission rate than a two-way system in which the quantum signals traverse the same optical path. Also, the active stabilization process, which is based on maintaining a fixed relationship between an intensity ratio of interfered control signals, is greatly simplified by having the interferometer loops located all in one QKD station.

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.

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.