Abstract: Systems and methods for enhanced quantum key distribution (QKD) using an actively compensated QKD system. The method includes exchanging quantum signals between first and second QKD stations and measuring the quantum signal error. An error signal SE representative of the system visibility error is then generated. An error-signal threshold STH that defines a system visibility error limit is then selected. Those qubits measured with the condition SE>STH are called “above-threshold” qubits, while those qubits measured with the condition SE?STH are called “below-threshold” qubits. Only below-threshold qubits are stored and used to form the final quantum key. This is accomplished by sending a blanking signal SB to the memory unit where the qubits are stored. The blanking signal prevents above-threshold qubits from being stored therein. The raw quantum key so formed has few errors and thus forms a longer final quantum key for a given number of exchanged quantum signals.

Abstract: Systems and methods for generating RF pulses that have a reduced phase error are disclosed. The systems are optical based and thus are highly linear, so that phase errors, including jitter, are significantly reduced as compared to electrical RF pulse generation systems and methods. The optical-based RF pulse generation methods includes generating laser light, imparting an envelope modulation to the laser light, imparting a carrier modulation to the laser light, and detecting the envelope-modulated and carrier-modulated light to form the electrical RF pulse. The electrical RF pulse can then be carried by a cable to an external device.

Abstract: Systems and methods for transmitting quantum and classical signals over an optical network are disclosed, wherein the quantum signal wavelength either falls within the classical signal wavelength band, or is very close to one of the classical signal wavelengths. The system includes a deep-notch optical filter with a blocking bandwidth that includes the quantum signal wavelength but not any of the classical signal wavelengths. The deep-notch optical filtering is applied to the classical signals prior to their being multiplexed with the quantum signals to prevent noise generated by the classical signals from adversely affecting transmission of quantum signals in the transmission optical fiber. Narrow-band filtering is also applied to the quantum signals prior to their detection in order to substantially exclude spurious non-quantum-signal wavelengths that arise from non-linear effects in the optical fiber.

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: Systems and methods for enhanced quantum key distribution (QKD) using an actively compensated QKD system. The method includes exchanging quantum signals between first and second QKD stations and measuring the quantum signal error. An error signal SE representative of the system visibility error is then generated. An error-signal threshold STH that defines a system visibility error limit is then selected. Those qubits measured with the condition SE>STH are called “above-threshold” qubits, while those qubits measured with the condition SE?STH are called “below-threshold” qubits. Only below-threshold qubits are stored and used to form the final quantum key. This is accomplished by sending a blanking signal SB to the memory unit where the qubits are stored. The blanking signal prevents above-threshold qubits from being stored therein. The raw quantum key so formed has few errors and thus forms a longer final quantum key for a given number of exchanged quantum signals.

Abstract: Systems and methods for transmitting quantum and classical signals over an optical network are disclosed, wherein the quantum signal wavelength either falls within the classical signal wavelength band, or is very close to one of the classical signal wavelengths. The system includes a deep-notch optical filter with a blocking bandwidth that includes the quantum signal wavelength but not any of the classical signal wavelengths. The deep-notch optical filtering is applied to the classical signals prior to their being multiplexed with the quantum signals to prevent noise generated by the classical signals from adversely affecting transmission of quantum signals in the transmission optical fiber. Narrow-band filtering is also applied to the quantum signals prior to their detection in order to substantially exclude spurious non-quantum-signal wavelengths that arise from non-linear effects in the optical fiber.

Abstract: A method of synchronizing the operation of a two-way QKD system by sending a sync signal (SC) in only one direction, namely from one QKD station (ALICE) to the other QKD station (BOB). The one-way transmission greatly reduces the amount of light scattering as compared to two-way sync signal transmission. The method includes phase-locking the sync signal at BOB and dithering the timing of the quantum signals so as to operate the QKD system in three different operating states. The number of detected quantum signals is counted for each state for a given number of detector gating signals. The QKD system is then operated in the state associated with the greatest number of detected quantum signals. This method is rapidly repeated during the operation of the QKD system to compensate for timing errors to maintain the system at or near its optimum operating state.

Abstract: Systems and methods of incorporating a QKD system (Q) into a WDM network (2) are disclosed. The methods include electrically gating the single-photon detectors (SPDs) (30, 30?) as well as optically gating the SPDs with optical gates (28, 28?). The electronic gating width (TSPD) and the optical gating width (TOG) are selected to significantly reduce noise from scattered photons. The combined optical and electronic gating of the SPDs provides for Fourier-transform-limited detection of the quantum signal (SQ) that is not otherwise possible in a WDM-QKD system that employs only electronic SPD gating.

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: Systems and methods for transmitting quantum and classical signals over an optical network are disclosed, wherein the quantum signal wavelength either falls within the classical signal wavelength band, or is very close to one of the classical signal wavelengths. The system includes a deep-notch optical filter with a blocking bandwidth that includes the quantum signal wavelength but not any of the classical signal wavelengths. The deep-notch optical filtering is applied to the classical signals prior to their being multiplexed with the quantum signals to prevent noise generated by the classical signals from adversely affecting transmission of quantum signals in the transmission optical fiber. Narrow-band filtering is also applied to the quantum signals prior to their detection in order to substantially exclude spurious non-quantum-signal wavelengths that arise from non-linear effects in the optical fiber.

Abstract: A cosite interference rejection system allows cancellation of large interfering signals with an optical cancellation subsystem. The rejection system includes an interference subsystem coupled to a transmit system, where the interference subsystem weights a sampled transmit signal based on a feedback signal such that the weighted signal is out of phase with the sampled transmit signal. The optical cancellation subsystem is coupled to the interference subsystem and a receive antenna. The optical cancellation subsystem converts an optical signal into a desired receive signal based on an interfering coupled signal and the weighted signal. The weighted signal is therefore used to drive the optical cancellation subsystem. The rejection system further includes a feedback loop for providing the feedback signal to the interference subsystem based on the desired receive signal.

Abstract: A bulk-optics assembly for a transmitting/receiving QKD station (BOB1) in a two-way autocompensating QKD system (101) is disclosed. The assembly consists of a first beamsplitter (104) having a high (e.g., 90:10) beamsplitting ratio, a 50:50 beamsplitter (106) and a polarizing beamsplitter (108). The assembly also optionally includes a polarizer (102), and/or a fixed attenuator (FOA), and/or an optional blocking filter (110) downstream of the polarizing beamsplitter. The compact bulk-optics assembly is easier to manufacture than a fiber-based optical system, and is simpler and more compact than prior art bulk-optics assemblies for QKD systems.

Abstract: A cosite interference rejection system allows cancellation of large interfering signals with an optical cancellation subsystem. The rejection system includes an interference subsystem coupled to a transmit system, where the interference subsystem weights a sampled transmit signal based on a feedback signal such that the weighted signal is out of phase with the sampled transmit signal. The optical cancellation subsystem is coupled to the interference subsystem and a receive antenna. The optical cancellation subsystem converts an optical signal into a desired receive signal based on an interfering coupled signal and the weighted signal. The weighted signal is therefore used to drive the optical cancellation subsystem. The rejection system further includes a feedback loop for providing the feedback signal to the interference subsystem based on the desired receive signal.

Abstract: An automatic polarization controller is described that includes an optical input that receives a polarization multiplexed optical pulse train that comprises a first and a second polarized optical pulse train. A dither modulation signal is superimposed on at least one of the first and the second polarized optical pulse trains. A polarization transformer transforms an input polarization state of the polarization multiplexed optical pulse train to an output polarization state in response to a control signal that is applied to a control input of the polarization transformer. A polarization selective element receives the transformed polarization multiplexed optical pulse train and passes a polarized optical pulse train including the dither modulation signal. A detector receives the polarized optical pulse train including the superimposed dither modulation signal and generates a signal that is proportional to the amplitude of the dither modulation signal.

Abstract: An apparatus and method for cleaving optical waveguides to precise differential length are described. A first end of a waveguide is coupled to an input port of a reflectometer. A reference mirror is then positioned in a path of radiation propagating through the second end of the waveguide. A waveguide cutting tool is then positioned proximate to the waveguide and at a distance relative to a reference mirror. A first reflectometry measurement is performed on the waveguide to a second end of the waveguide. A second reflectometry measurement is performed on the waveguide to the reference mirror. The waveguide is then positioned relative to the reference mirror and waveguide cutting tool so that the first reflectometery measurement is a measurement increment apart from the second reflectometry measurement. The waveguide is then cut with the cutting tool positioned at the distance relative to the reference mirror.

Abstract: A cosite interference rejection system allows cancellation of large interfering signals with an optical cancellation subsystem. The rejection system includes an interference subsystem coupled to a transmit system, where the interference subsystem weights a sampled transmit signal based on a feedback signal such that the weighted signal is out of phase with the sampled transmit signal. The optical cancellation subsystem is coupled to the interference subsystem and a receive antenna. The optical cancellation subsystem converts an optical signal into a desired receive signal based on an interfering coupled signal and the weighted signal. The weighted signal is therefore used to drive the optical cancellation subsystem. The rejection system further includes a feedback loop for providing the feedback signal to the interference subsystem based on the desired receive signal.

Abstract: Electrostatic levitation and positioning of a charged, spherical micro-lens to steer an optical beam from a transmit fiber to a receive lens/fiber pair. Bundled arrays of N fibers and lenses provide a switch having a switch count that scales linearly in the port count N.

Abstract: A demultiplexer is described that includes an optical splitter that receives an optical data signal having a plurality of data channels and that generates a plurality of identical optical data signals. An electrical clock recovery circuit receives the optical data signal and generates an electrical clock signal that is synchronized to the optical data signal and that has a frequency that is an integer multiple of a bit rate of one of the plurality of data channels. One of a plurality of phase shifters receives the electrical clock signal and generates a phase-shifted electrical clock signal in response to a control signal applied to one of the plurality of phase shifters. A respective one of a plurality of sampling circuits receives one of the identical optical data signals and the phase-shifted electrical clock signal and generates an electrical signal representing a data channel of the optical data signal.

Abstract: An apparatus and method for cleaving optical waveguides to precise differential length are described. A first end of a waveguide is coupled to an input port of a reflectometer. A reference mirror is then positioned in a path of radiation propagating through the second end of the waveguide. A waveguide cutting tool is then positioned proximate to the waveguide and at a distance relative to a reference mirror. A first reflectometry measurement is performed on the waveguide to a second end of the waveguide. A second reflectometry measurement is performed on the waveguide to the reference mirror. The waveguide is then positioned relative to the reference mirror and waveguide cutting tool so that the first reflectometery measurement is a measurement increment apart from the second reflectometry measurement. The waveguide is then cut with the cutting tool positioned at the distance relative to the reference mirror.

Abstract: An apparatus and method for dynamically optimizing performance in a communication channel are described. The communication channel can be part of a high-speed digital network such as the Internet and can be a dense wavelength-division multiplexed (DWDM) optical communication channel. The DWDM signal includes forward error correction (FEC) information which is added to the DWDM signal and is used within the DWDM optical layer to monitor transfer errors in the data. Error correction statistics generated as a result of analysis of the FEC information in the data in accordance with the invention are used in generating an adjustment to and/or optimizing performance of the system. An adjust signal used in generating and making the adjustment is generated using FEC error correction statistics. In one embodiment, the adjust signal is a feedback signal transferred back to the transmission end of the channel.