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: This invention provides a quantum key distribution (QKD) system and method for determining initial quantum keys (QKs), including an initial QKA (220) and an initial QKB (230), determining an initial QKA value of a first function applied to said initial QKA, wherein a value of said first function depends upon values of specified information unit of a QK, including bit i (210), determining an initial QKB value of said first function applied to said initial QKB; and forming a revised QKA by depending a value of an information unit of said revised QKA on a value of information unit i of said initial QKA, if said initial QKA value equals said initial QKB value.

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: 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: Apparatus and methods for high fidelity quantum communication over long communication channels even in the presence of significant loss in the channels are disclosed. The invention employs laser manipulation of quantum correlated atomic ensembles using linear optic components, optical sources of low intensity pulses, beam splitters, and single-photon detectors requiring only moderate efficiencies. The invention provides fault-tolerant entanglement generation and connection, using a sequence of steps that each provide built-in entanglement purification and that are each resilient to the realistic noise. The invention relies upon collective excitation in atomic ensembles rather than single particle excitations in atomic ensembles so that communication efficiency scales polynomially with the total length of a communication channel.

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: 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: 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: 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 minimum 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 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 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: 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: Apparatus for and methods of random number generation are disclosed, wherein a detector receives a group of n light pulses having single-photon intensity levels. Each of the n light pulses has a probability of less than one to produce a successful detection event at its time of arrival at the detector, and the detector is adapted to detect only one of the n pulses in the group. This single detection per group is thus a discrete random event that occurs only during one of n fixed time slots. The random event occurring during one of n timeslots is converted into a corresponding random integer from 1 to n. A series of such random numbers is generated by using a plurality of groups of n light pulses.

Abstract: The invention includes systems for and methods of performing quantum computation. The method of quantum computation includes preparing a set of one or more qubits capable of storing quantum information in 2n possible states, wherein the number of qubits n?1. The qubit set is subject to a decoherence mechanism that could cause a loss of quantum information stored in some but not all of the qubit states. The method also includes determining, via a quantum measurement of the qubit system or just by analyzing the decoherence of the qubit states, which of the 2n states or their superposition is/are not susceptible to decoherence. The method further includes encoding and processing quantum information in one or more of the decoherence-free states by controlling qubit-qubit interactions or via an electromagnetic interaction with the set of qubits.

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 method of performing a quantum Fourier transform in a quantum computing circuit is disclosed. The method includes forming a quantum computing circuit as a collection of two-qubit gates operating on a sequence of input qubits. Auxiliary qubits are then interacted with the original input qubits to place the auxiliary qubits in a state corresponding to an output of a discrete Fourier transform of a classical state of the input qubits. The original input qubits are then re-set to their ground state by physically interacting the input qubits with the auxiliary qubits. The auxiliary qubits are then transformed to a state representative of a quantum Fourier transform of the sequence of input qubits.

Abstract: Systems and methods for compensating a QKD system for variations in temperature are disclosed. One of the methods includes identifying an optimum detector gating signal timing as a function of temperature for a single-photon detector (SPD) control board in one of the QKD stations. The detector gating signal timing versus temperature information is stored in a look-up table in a memory unit. The QKD system's temperature is monitored during operation and the timing of the detector gating signal is adjusted based on the operating temperature and the corresponding timing value adjustment in the look-up table. The result is a compensated detector gating timing signal provided to the SPD that yields an optimum number of photon counts even as the temperature of the QKD station varies.

Abstract: The present invention is directed to systems and methods of providing universal quantum computation that avoid certain external control fields that either are hard or impossible to implement, or are serious sources of decoherence (errors). The systems and methods extend the set of scalable physical platforms suitable for implementing quantum computation in solid state, condensed matter and atomic and molecular physics systems. The invention includes identifying of suitable encodings of logical qubits into three physical qubits—i.e. three quantum mechanical systems of two levels—and performing quantum computing operations by changing the quantum states of physical qubits making up one or more logical qubits using only generalized anisotropic exchange interactions. This includes performing a quantum unitary operation over a single logical qubit or a non-local (entangling) two-qubit unitary operation.