Abstract: A method of transmitting precoded signals can include precoding an optical signal to produce a precoded optical signal, transmitting a precoded optical signal over an optical portion of a first transmission channel, converting the precoded optical signal to a first MIMO signal, transmitting the first MIMO signal over a wireless portion of the first transmission channel, transmitting the precoded optical signal over an optical portion of a second transmission channel, converting the precoded optical signal to a second MIMO signal; transmitting the second MIMO signal over a wireless portion of the second transmission channel; wherein the precoding takes account of both first channel state information associated with the first transmission channel and second channel state information associated with the second transmission channel.

Abstract: There is herein provided a method of performing Quantum Key Distribution, the method including transmitting, in a first basis state, a first photon from a quantum transmitter to a quantum receiver; transmitting, in a second basis state, a second photon from the quantum transmitter to the quantum receiver, the second basis state being non-orthogonal to the first basis state and the transmitter and receiver being optically connected by both a first optical channel and a second optical channel, wherein transmitting the first photon from the quantum transmitter to the quantum receiver in the first basis state comprises: transmitting the first photon from the quantum transmitter to the quantum receiver along either the first optical channel or the second optical channel, wherein transmitting the second photon from the quantum transmitter to the quantum receiver in the second basis state comprises: transmitting a first portion of the probability distribution of the second photon from the transmitter to the receiver

Abstract: A method of configuring an optical network comprising a switching system, a first node, a second node, and an optical link between the first node and the second node. The optical link includes a first optical connection and a second optical connection. The method includes changing a state of the switching system from a first state to a second state. In the first state, the optical network is configured to use the first and second optical connection to transmit first and second optical signals in first and second directions, respectively. In the second state the optical network is configured to use the second and first optical connections to transmit the first and second optical signals in the first and second directions, respectively.

Abstract: There is herein disclosed a computer-implemented method of optimizing optimising a QKD system, the QKD system being adapted to perform a QKD session that involves the propagation of a plurality of photons from a first device to a second device, one of the first and second devices being located aboard a satellite in orbit, the method including running a machine learning algorithm, inputs of the machine learning algorithm comprising one or more values of one or more of the following factors: atmospheric conditions in the vicinity of a path from the first device to the second device; a spatial separation of the first device and the second device; and a measure of the performance of the first and/or the second device, wherein the output of the algorithm is an estimate of the rate at which the first device and the second device generate bits of a quantum key.

Abstract: There is herein provided a method of performing Quantum Key Distribution, the method comprising, transmitting, in a first basis state, a first photon from a quantum transmitter to a quantum receiver; transmitting, in a second basis state, a second photon from the quantum transmitter to the quantum receiver, the second basis state being non-orthogonal to the first basis state and the transmitter and receiver being optically connected by both a first optical channel and a second optical channel, wherein the step of transmitting the first photon from the quantum transmitter to the quantum receiver in the first basis state comprises: transmitting the first photon from the quantum transmitter to the quantum receiver along either the first optical channel or the second optical channel, wherein the step of transmitting the second photon from the quantum transmitter to the quantum receiver in the second basis state comprises: transmitting a first portion of the probability distribution of the second photon from the t

Abstract: There is herein disclosed a method of communication between a client pair, the client pair including an upstream client and a downstream client, the method being performed over a first optical fiber in an optical fiber pair and including: transmitting a downstream DWDM signal over the first optical fiber, wherein the downstream DWDM signal has a first frequency and contains first data originating from the upstream client, transmitting an upstream DWDM signal over the first optical fiber, wherein the upstream DWDM signal has a second frequency and contains second data originating from the downstream client, wherein the first frequency is different to the second frequency, and performing Quantum Key Distribution over the second optical fiber of the optical fiber pair.

Abstract: There is herein disclosed a method of performing Quantum Key Distribution for generating a shared secret key, the method including, at a first node, preparing or measuring a plurality of non-orthogonal quantum states, each of the plurality of non-orthogonal quantum states being prepared or measured using a respective one of a first set of basis states, and, at a second node, preparing or measuring the plurality of non-orthogonal quantum states each, of the plurality of non-orthogonal quantum states being prepared or measured using a respective one of a second set of basis states, and, at a third node, obtaining an indication of the first set of basis states from the first node and performing a key agreement stage with a fourth node to agree the shared secret key, the key agreement stage involving the first and second sets of basis states.

Abstract: There is herein disclosed a system for performing Quantum Key Distribution, the system including a transmitter adapted to transmit a plurality of optical pulses, a first receiver, a second receiver, an optical switch with an input which is in optical communication with the transmitter, the switch being switchable between a first switching position in which the input is optically connected to the first receiver, and a second switching position in which the input is optically connected to the second receiver, the system further including a guide for guiding a portion of the plurality of optical pulses to the first receiver via an optical path that bypasses the optical switch.

Abstract: A system of transmitting optical pulses from a transmitter to first and second receivers for the purposes of quantum key distribution is disclosed. The system can include a transmitter configured to transmit a plurality of optical pulses; first and second receivers configured to receive some or all of the plurality of optical pulses; a guide for guiding the plurality of optical pulses, the guide having first and second output ports, the first output port being in optical communication with the first receiver and the second output port being in optical communication with the second receiver, the guide being configured to output a proportion of the optical pulses at the first output port and a proportion of the optical pulses at the second output port, modification means for modifying the guide so as to change the proportion of the optical pulses output at the first output port.

Abstract: An optical data transmission system and method for, at an optical transmitter, converting to a second wavelength, an optical data signal at a first wavelength; transmitting at the second wavelength, the optical data signal to an optical receiver over an optical path; and transmitting at the first wavelength, to the receiver over the path a single-photon signal comprising a stream of single photons. The optical path is configured to carry optical signals at different wavelengths and the optical path attenuates signals at the first wavelength less than the optical path attenuates signals at the second wavelength. The optical data transmission system and method for, at the receiver, receiving the single-photon signal at the first wavelength and receiving the optical data signal at the second wavelength; and converting the optical data signal to the first wavelength for detection.

Abstract: A method for operating a communications network node, the node including a first amplified optical section, a second non-optical section, and an optical bypass section the method including receiving at the node, a first optical channel at a first wavelength and a second optical channel at a second wavelength; directing the first optical channel to the first amplified optical section; directing the second optical channel to the second non-optical section during a first time period; and directing the second optical channel to the optical bypass section during a second time period.

Abstract: A method for operating a communications network node, the node including a first amplified optical section, a second non-optical section, and an optical bypass section the method including receiving at the node, a first optical channel at a first wavelength and a second optical channel at a second wavelength; directing the first optical channel to the first amplified optical section; directing the second optical channel to the second non-optical section during a first time period; and directing the second optical channel to the optical bypass section during a second time period.

Abstract: An optical data transmission system and method for, at an optical transmitter, converting to a second wavelength, an optical data signal at a first wavelength; transmitting at the second wavelength, the optical data signal to an optical receiver over an optical path; and transmitting at the first wavelength, to the receiver over the path a single-photon signal comprising a stream of single photons. The optical path is configured to carry optical signals at different wavelengths and the optical path attenuates signals at the first wavelength less than the optical path attenuates signals at the second wavelength. The optical data transmission system and method for, at the receiver, receiving the single-photon signal at the first wavelength and receiving the optical data signal at the second wavelength; and converting the optical data signal to the first wavelength for detection.

Abstract: An optical receiver including a mode-coupling receiver; in which the mode-coupling receiver includes a plurality of inputs; in which the mode-coupling receiver is configured to detect receipt of a single-photon signal comprising a stream of single photons on each of the plurality of inputs.

Abstract: A method of allocating spectrum to a signal in a wavelength division multiplex network including a plurality of nodes, wherein the spectrum comprises a grid divided into a plurality of slots, including allocating to the signal a slot immediately adjacent to a second slot occupied by a second signal, wherein the signal and the second signal are both addressed to the same destination node, and wherein the slots allocated to the signal and the second signal form a spectrum block.

Abstract: A method of generating an internally consistent model of the state of a book captured in a video image is provided. The method comprises obtaining a plurality of pieces of evidence relating to the state of a corresponding plurality of aspects of the book in the video image, associating a quality score with each piece of evidence, generating an initial model of the state of the book wherein the state of the book is constrained by physical properties of the book and at least the highest scoring piece of evidence, and sequentially constraining the model in response to one or more successive pieces of evidence whose scores meet a respective predetermined first threshold value.

Abstract: A method of allocating spectrum to a signal in a wavelength division multiplex network including comprising a plurality of nodes, wherein the spectrum comprises a grid divided into a plurality of slots, including comprising allocating to the signal a slot immediately adjacent to a second slot occupied by a second signal, wherein the signal and the second signal are both addressed to the same destination node, and wherein the slots allocated to the signal and the second signal form a spectrum block.

Abstract: A routing and wavelength assignment method for use in an optical fiber system, comprising: (i) identifying a plurality of paths between a source node and a destination node, (ii) selecting one of the plurality of identified paths, (iii) defining within the spectrum band of the selected path one or more blocks of spectral resource, in which each block comprises either: one or more unused wavelength channels, or one or more wavelength channels having the same spectral width, (iv) obtaining an entropy value of the selected path defining the spectrum fragmentation across its spectrum band, based on a logarithm of the ratio of the number of wavelength channels in each of the one or more blocks, to the total number of wavelength channels across the spectrum band, (v) iterating (ii) to (v) until the entropy value of each of the plurality of identified paths has been determined, and (vi) choosing from the plurality of identified paths a path having the lowest entropy value.

Abstract: An optical switch suitable for use in an add/drop of an optical network node having F>1 optical fiber per direction, the F fibers together carrying optical signals comprising up to N independent wavelength channels, is disclosed. The switch includes an Optical Cross-Connect (OXC) having F input ports and N output ports. F optical splitters are connected to the OXC, the input and output ports of the optical splitters defining ports of the OXC. The OXC is controllable to switch optical signals arriving at any of the F switch input ports to the input port of any of the F optical splitters such that each switch input port is switched to an optical splitter having at least as many splitter output ports as the number of independent wavelength channels received at the switch input port. A method for splitting optical signals is also disclosed.

Abstract: A routing and wavelength assignment method for use in an optical fiber network includes (i) identifying a path between each node pair in the network, (ii) identifying a block of spectral resource within the spectrum band of the identified path of a selected node pair, (iii) calculating a spectrum entropy value of the identified path of the selected node pair based on a logarithm of the ratio of the number of wavelength channels in each of the one or more blocks, to the total number of wavelength channels across the spectrum band, (iv) iterating (ii) and (iii) in respect of each of the paths between each other node pair in the network, until a spectrum entropy value of all the paths between all the node pairs has been calculated, (v) summing the spectrum entropy value of all of the paths between all of the node pairs to obtain a network spectrum entropy value in respect of a network configuration based on the paths between the node pairs, and (vi) determining from the network spectrum entropy value whether a s