Abstract: A transmitter for a quantum communication system, comprising: a photon source unit comprising a photon source; a first intensity modulator, configured to receive an input light pulse from the photon source unit, the first intensity modulator comprising: a first element, configured to split the input light pulse into two components; a phase modulator, configured to apply a phase shift between the two components; and a second element, configured to interfere the two components; wherein at least one of the first element and the second element is asymmetric.

Abstract: A quantum communication system for distributing a key between first and second units, the system being configured to implement phase-based measurement device independent quantum cryptography, the system comprising first and second units adapted to apply phase shifts to light pulses and a detection unit adapted to cause interference between light pulses received from the first and second units and measure said interference, wherein the first and second units each comprise at least one phase modulator adapted to apply a phase shift, said phase shift comprising a global phase component and a relative phase component, wherein said global phase component represents a phase shift selected randomly in the range from 0° to 360° from a fixed phase reference and said relative phase component is a phase shift selected randomly from 0°, 90°, 180° and 270° from the phase shift introduced by the global phase component.

Abstract: A communication system comprising n transmitters and a receiver, where n is an integer of at least 2, each of said n transmitters comprising a light source and an encoder such that each transmitter is adapted to output an encoded pulse of light, said receiver comprising a first element, the system further comprising a timing circuit, the timing circuit being configured to synchronize the encoded pulses output by the transmitters such that interference between a light pulse sent from the first transmitter and a light pulse from the second transmitter, interfere at the first element, each transmitter further comprising a suppressing element adapted to stop light exiting one of the transmitters such that the system is switchable between a first operation mode where two transmitters output encoded pulses and where both pulses interfere at the interference element and a second mode of operation where just one transmitter transmits light pulses to said receiver, the suppressing element being controlled to stop ligh

Abstract: A transmitter for a quantum communication system, comprising: a photon source unit comprising a photon source; a first intensity modulator, configured to receive an input light pulse from the photon source unit, the first intensity modulator comprising: a first element, configured to split the input light pulse into two components; a phase modulator, configured to apply a phase shift between the two components; and a second element, configured to interfere the two components; wherein at least one of the first element and the second element is asymmetric.

Abstract: A quantum communication system for distributing a key between first and second units, the system being configured to implement phase-based measurement device independent quantum cryptography, the system comprising first and second units adapted to apply phase shifts to light pulses and a detection unit adapted to cause interference between light pulses received from the first and second units and measure said interference, wherein the first and second units each comprise at least one phase modulator adapted to apply a phase shift, said phase shift comprising a global phase component and a relative phase component, wherein said global phase component represents a phase shift selected randomly in the range from 0° to 360° from a fixed phase reference and said relative phase component is a phase shift selected randomly from 0°, 90°, 180° and 270° from the phase shift introduced by the global phase component.

Abstract: A detection system for monitoring the intensity of a stream of modulated pulses, the system comprising a controller configured to provide a control signal to a component outputting modulated pulses the control signal controlling the level of modulation for each pulse exiting the component and a detector configured to measure the intensity of the pulses in the stream of pulses exiting the component outputting modulated pulses, wherein the detector comprises a gated detector, the controller being configured to send a gating signal to said detector, wherein said gating signal varies the gain of the detector with the control signal, such that pulses with a selected modulation level can be detected.

Abstract: A quantum communication system, comprising: a quantum transmitter optically coupled to a first waveguide; a first communication device optically coupled to a second waveguide; a multi-core optical fiber comprising a first core and a second core; a spatial multiplexing unit, configured to optically couple the first waveguide to the first core and the second waveguide to the second core.

Abstract: A communication system comprising n transmitters and a receiver, where n is an integer of at least 2, each of said n transmitters comprising a light source and an encoder such that each transmitter is adapted to output an encoded pulse of light, said receiver comprising a first element, the system further comprising a timing circuit, the timing circuit being configured to synchronise the encoded pulses output by the transmitters such that interference between a light pulse sent from the first transmitter and a light pulse from the second transmitter, interfere at the first element, each transmitter further comprising a suppressing element adapted to stop light exiting one of the transmitters such that the system is switchable between a first operation mode where two transmitters output encoded pulses and where both pulses interfere at the interference element and a second mode of operation where just one transmitter transmits light pulses to said receiver, the suppressing element being controlled to stop ligh

Abstract: An interference system, comprising: an interference apparatus, configured such that input light pulses interfere at an interference component; wherein the input of said interference apparatus is provided by a phase-randomized light source, said phase-randomized light source comprising: at least one slave light source; at least one master light source configured to intermittently generate master light pulses such that the phase of each master light pulse has a random relationship to the phase of each subsequently generated master light pulse, further configured to supply said master light pulses to the slave light source; and a controller, configured to apply a time varying drive signal to said at least one slave light source such that just one slave light pulse is generated during each period of time for which a master light pulse is received, such that the phase of each slave light pulse has a random relationship to the phase of each subsequently generated slave light pulse.

Abstract: A transmitter for a quantum communication system, the transmitter comprising an interferometer, the interferometer having a first path with a phase modulator and a second path configured such that light pulses entering the interferometer follow either the first path or the second path, the output of the first and second paths being combined, the transmitter further comprising an optical filter positioned such that photons exiting the interferometer pass through the optical filter, the optical filter being configured to restrict the frequency range of pulses passing through the optical filter and temporally broaden the pulses.

Abstract: A quantum communication system, comprising: a quantum transmitter optically coupled to a first waveguide; a first communication device optically coupled to a second waveguide; a multi-core optical fibre comprising a first core and a second core; a spatial multiplexing unit, configured to optically couple the first waveguide to the first core and the second waveguide to the second core.

Abstract: A quantum communication system comprising a receiver and a plurality of transmitters, said receiver comprising a detector sub-system, each of said transmitters being configured to emit pulses of radiation, said detector subsystem comprising at least one detector, said detector being configured to detect said light pulses, the system comprising a timing control module, said timing control module being configured to control the number of light pulses received by the detector sub-system, such that just one light pulse from one transmitter reaches the detector sub-system at any one time, the timing control module also allowing the transmitter which sent said pulse to be identified.

Abstract: A detection system for monitoring the intensity of a stream of modulated pulses, the system comprising a controller configured to provide a control signal to a component outputting modulated pulses the control signal controlling the level of modulation for each pulse exiting the component and a detector configured to measure the intensity of the pulses in the stream of pulses exiting the component outputting modulated pulses, wherein the detector comprises a gated detector, the controller being configured to send a gating signal to said detector, wherein said gating signal varies the gain of the detector with the control signal, such that pulses with a selected modulation level can be detected.

Abstract: A detection system for monitoring the intensity of a stream of modulated pulses, the system comprising a controller configured to provide a control signal to a component outputting modulated pulses the control signal controlling the level of modulation for each pulse exiting the component and a detector configured to measure the intensity of the pulses in the stream of pulses exiting the component outputting modulated pulses, wherein the detector comprises a gated detector, the controller being configured to send a gating signal to said detector, wherein said gating signal varies the gain of the detector with the control signal, such that pulses with a selected modulation level can be detected.

Abstract: An interference system, comprising: an interference apparatus, configured such that input light pulses interfere at an interference component; wherein the input of said interference apparatus is provided by a phase-randomised light source, said phase-randomised light source comprising: at least one slave light source; at least one master light source configured to intermittently generate master light pulses such that the phase of each master light pulse has a random relationship to the phase of each subsequently generated master light pulse, further configured to supply said master light pulses to the slave light source; and a controller, configured to apply a time varying drive signal to said at least one slave light source such that just one slave light pulse is generated during each period of time for which a master light pulse is received, such that the phase of each slave light pulse has a random relationship to the phase of each subsequently generated slave light pulse.

Abstract: A transmitter for a quantum communication system, the transmitter comprising an interferometer, the interferometer having a first path with a phase modulator and a second path configured such that light pulses entering the interferometer follow either the first path or the second path, the output of the first and second paths being combined, the transmitter further comprising an optical filter positioned such that photons exiting the interferometer pass through the optical filter, the optical filter being configured to restrict the frequency range of pulses passing through the optical filter and temporally broaden the pulses.

Abstract: A photon detection system is provided comprising a photon detector, configured to detect photons during intervals when in a receiving state and to output a signal when a photon is received, a controller, configured to generate a time varying gating signal wherein said gating signal switches said detector between the receiving state and a non-receiving state, said controller being configured to receive and process information relating to the times photons are expected to arrive at said detector, the controller being configured to generate the gating signal such that the photon detector is in the receiving state for intervals when photons are expected and also in the receiving state for additional intervals between the intervals when the photons are expected; a detection module, configured to distinguish between when the output signal from the photon detector corresponds to an interval when photons are expected and said additional intervals.

Abstract: A communication system comprising an emitter of weak light pulses, a detector which is capable of detecting single photons, and a source of a clock signal, wherein said emitter and detector are synchronized using said clock signal, the system further comprising a frequency divider for said clock signal to produce a reduced frequency clock signal and a clock regenerator for regenerating the original clock signal from the reduced frequency clock signal, the system further comprising a communication channel configured to communicate the clock signal between the emitter and detector, the clock signal being reduced before sending through said channel and reconstructed after it has exited said channel.

Abstract: A detection system for monitoring the intensity of a stream of modulated pulses, the system comprising a controller configured to provide a control signal to a component outputting modulated pulses the control signal controlling the level of modulation for each pulse exiting the component and a detector configured to measure the intensity of the pulses in the stream of pulses exiting the component outputting modulated pulses, wherein the detector comprises a gated detector, the controller being configured to send a gating signal to said detector, wherein said gating signal varies the gain of the detector with the control signal, such that pulses with a selected modulation level can be detected.

Abstract: A quantum communication system, said system comprising: an emitter configured to send signal light pulses having a first intensity and decoy light pulses having a second intensity to a receiver, wherein information is encoded on said signal pulses; a controller configured to set the distribution of signal pulses and decoy pulses and the intensity of the signal pulse such that the maximum average secure bit rate is achieved over the range of potential drift in the decoy pulse intensity during use of the system, while maintaining a non-zero secure bit rate over the range of potential drift in the decoy pulse intensity during use of the system.