Patent Description:
Optical modules are used in communications systems and comprise an electrical interface and an optical interface. The electrical interface connects to an electronic board while the optical interface connects to input and/or output optical links. For example, the optical link comprises fibre optic cable. Optical modules may also be referred to as fibre optic transceivers or optical transceivers. Optical modules act as an interface between electrical signals on the electronic board and optical signals inputted or outputted via the optical links. Optical modules can be connected and disconnected from the electronic board as required. Optical modules may be pluggable.

In communication systems, the exchange of secret cryptographic keys between two parties underpins the security of the communication systems.

A quantum communication system may be used to share secret cryptographic keys between two nodes, a source node and a destination node, often referred to as "Alice" and "Bob", and this technique is known as quantum key distribution (QKD). In a quantum communication network, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarization, phase or frequency/time. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.

The attraction of QKD is that it provides a test of whether any part of the key can be known to an unauthorized eavesdropper "Eve". In many forms of QKD, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information.

<CIT> describes an interposer (support substrate) for an opto-electronic assembly that is formed to include a thermally-isolated region where temperature-sensitive devices (such as, for example, laser diodes) may be positioned and operate independent of temperature fluctuations in other areas of the assembly.

Embodiments will now be described, by way of example only, with reference to the accompanying figures in which:.

In an embodiment, a system with an optical module is provided as recited in claim <NUM>.

Quantum photonic integrated circuits allow the generation, the manipulation or the analysis of quantum properties of optical signals. They typically operate at the quantum level, i.e. down to the single photon level. They may use quantum phenomena to generate photonic qubits or to extract quantum information from quantum fluctuations.

Optical modules comprise a housing that surrounds and protects components. The components comprise a receiver and/or a transmitter, for example. Further, optical modules comprise an electrical interface configured to plug into an electronic board, and an optical interface configured to connect to input and/or output optical links. The electronic board is also referred to as a host electronic board. The host electronic board is external to the optical module. An optical module is configured to be repeatedly connected and disconnected from the host electronic board.

In some examples, an optical module is configured to be plugged and unplugged from the host electronic board. Such optical modules are also referred to as pluggable modules.

Pluggable modules have standard form factors that ensure interoperability. Examples of standard form factors include the Small form-factor pluggable (SFP) transceiver, C form-factor pluggable (CFP), and XFP (<NUM> Gigabit Small Form Factor Pluggable), amongst others.

Pluggable modules may not use photonic integrated circuits (PICs), and, instead, they may comprise individual miniature photonic components (such as light sources, detectors, isolators and/or lenses) that are each packaged into their own optical sub-assemblies.

When pluggable modules are used with photonic integrated circuits (PICs), the PICs are first packaged into an optical sub-assembly which is then assembled inside the pluggable module. The assembly process for a PIC in a pluggable module comprises firstly packaging the PIC into an optical sub-assembly, and then assembling the optical sub-assembly into the optical module.

An example of an optical sub-assembly is a multi-pin package. Multi-pin packages include 'butterfly'-style packages. Multi-pin packages generally comprise a housing into which components can be mounted, an opening for an optical input and/or output, and pins on the exterior of the housing that are configured to provide electrical contact to components mounted inside the housing, from the exterior of the multi-pin package. The pins are soldered to other components or to a printed circuit board (PCB), for example a PCB inside the module. Once fixed, such optical sub-assemblies may not be plugged and unplugged.

The module comprises a circuit board configured to route electric signals to and/or from the photonic integrated circuit.

In an embodiment, the module may comprise an interposer chip, wherein the interposer chip is provided between the photonic integrated circuit and the temperature control element. The interposer chip may comprise a thermally conductive chip carrier.

The interposer chip may be configured to route electric signals from the photonic integrated circuit to the circuit board.

The housing is configured to a dissipate heat. The module may also comprise a temperature sensor, wherein the temperature sensor is configured to monitor the temperature of the photonic integrated circuit. The temperature sensor may be provided on the photonic integrated circuit.

In a further embodiment, an optical component is provided that is configured to collect or inject light into the photonic integrated circuit.

The temperature control element may comprise a thermoelectric cooler or a resistive heater.

As noted above, the module comprises a quantum photonic integrated circuit. The photonic integrated circuit may be configured to operate as:.

In an embodiment, the quantum photonic integrated circuit will use components with low losses. In QKD any loss in the receiver contributes to an excess of noise and significantly reduces the secret key distribution rate and the distance at which a key can be exchanged. In quantum information processors any loss of photon corresponds to a loss of information and reduces the efficiency of the quantum algorithm. In a QKD transmitter the qubits are generated using pulses of light that are attenuated to contain in average less that one photon per pulse. Such attenuation can be done outside of the module. In a QKD transmitter unit, the photon flux (average number of photon per pulse) at the output of the transmitter is typically in the range of <NUM>-<NUM> photon per pulse, for the T12 protocol.

Some quantum photonic integrated circuits will comprise single photon detectors or single photon sources.

The host electronic board may be configured to generate an electric signal for controlling the photonic integrated circuit. The host electronic board may be configured to generate an electric signal for controlling the temperature control element. The host electronic board may also be configured to acquire an electric signal generated by the photonic integrated circuit. The host electronic board may comprise a socket, wherein the circuit board is configured to plug into the socket.

The circuit board may comprise a connector configured to plug into the socket.

In a further embodiment, a sending unit for a quantum communication system is provided, the sending unit comprising:
the system as described above, wherein the photonic integrated circuit comprises:.

In a further embodiment, a receiving unit for a quantum communication system is provided, the receiving unit comprising:
the system as described above, wherein the photonic integrated circuit comprises a quantum bit decoder configured to decode encoded quantum bits generated by a sending unit.

In a further embodiment, a quantum communication system is provided comprising: the above sending and receiving unit and an optical link configured to connect the sending unit to the receiving unit.

<FIG> shows a plan view of an optical module <NUM> according to an embodiment. In an example, the optical module <NUM> is configured for a quantum device. The optical module comprises a housing <NUM> configured to house a plurality of components that will be described next. The optical module <NUM> comprises a temperature control element (TCE) <NUM> which is attached directly onto the housing <NUM>. By directly attaching the TCE <NUM> to the housing <NUM>, it is meant that either the TCE <NUM> is attached to the housing <NUM> without any other part between them, or that the TCE <NUM> is attached to the housing <NUM> with a thermal interface material provided between them. The thermal interface material (TIM) is configured to conduct heat between the TCE <NUM> and the housing <NUM>. The TIM may be a thermally conductive paste or thermally conductive adhesive, for example. The purpose of directly attaching the TCE <NUM> to the housing <NUM> is for the TCE <NUM> and the housing <NUM> to be in direct thermal communication with each other such that heat is efficiently exchanged. In particular, the housing <NUM> is configured to act as a heat sink such that heat generated by the TCE <NUM> is dissipated by the housing <NUM>. Part of the housing is made of metal. In an embodiment, the part of the housing <NUM> that is in contact with the TCE <NUM> is made of metal. For example, the housing comprises aluminium.

The optical module <NUM> further comprises a quantum photonic integrated circuit (PIC) <NUM>. The PIC <NUM> is a semiconductor bare die. The PIC <NUM> comprises a substrate onto which optical components are integrated. In an example, the PIC is based on an InP, Si, and/or silicon nitride standard integration process. In an example, the PIC <NUM> is based on a hybrid InP on Si, or GaAs on Si process. In another example, when PIC <NUM> is configured as a receiver only, the PIC <NUM> may be based on a fully passive Si or silicon nitride process. Different components are optically connected to one another by means of light guiding sections. The PIC <NUM> may further comprise an emitter (<FIG>); a receiver (<FIG>) and/or other components as described below.

The optical module <NUM> further comprises a chip carrier 103b. The chip carrier 103b is also referred to as an interposer. The PIC <NUM> is mounted onto the chip carrier 103b, and the chip carrier 103b is then itself attached to the TCE <NUM>. Thus, the chip carrier 103b is provided between the TCE <NUM> and the PIC <NUM>. The PIC <NUM> is mountable onto the chip carrier 103b using any suitable method such as adhesive bonding, or flip chip bonding. In an example, when adhesive bonding comprises the use of thermos-conductive epoxy. The chip carrier 103b is itself directly attached to the TCE <NUM>. The expression 'directly attached' has the same meaning as described above in relation to the TCE <NUM> and the housing <NUM>.

The chip carrier 103b acts as an electrical interface between the PIC <NUM> and an internal electronic board <NUM> (which will be described later). For example, electrical contacts on the PIC <NUM> may be connected to corresponding contacts on the chip carrier 103b by way of wire bonds (not shown). Other means of electrically connecting the PIC <NUM> to the chip carrier 103b may be also be used. The chip carrier 103b is configured to direct electric signals from the PIC <NUM> to the circuit board <NUM>. The electrical connection from the chip carrier 103b to the internal circuit board <NUM> may be implemented by wire bonding, or any other suitable means (not shown).

Furthermore, the chip carrier 103b is configured to provide a thermally conductive path from the PIC <NUM> to the TCE <NUM>. Additionally and optionally, the chip carrier 103b comprises a thermally conductive material. Additionally and optionally, the chip carrier 103b comprises alumina (Al2O3) or aluminium nitride (AIN) or Rogers laminates (RTM).

The temperature control element (TCE) <NUM> is configured to actively stabilize the temperature of the PIC <NUM>, which is in thermal communication with the TCE <NUM> via the chip carrier 103b. By stabilising the temperature, it is meant that the temperature is kept at or near a set value. In an example, the TCE <NUM> is configured to keep the temperature of the PIC <NUM> to within at least <NUM> of the set value. In another example, the temperature is kept within at least <NUM> of the set value. In another example, the temperature is kept within at least <NUM> of the set value. By actively stabilizing the temperature, it is meant that a control signal is provided to the TCE <NUM> in order to cause it to stabilize the temperature of the PIC <NUM>.

In an example, the TCE <NUM> is configured to electrically stabilize the temperature of the PIC <NUM>. An electrical control signal is input to the TCE to cause a change in temperature.

Stabilising the temperature of the PIC <NUM> reduces drifts in the behaviour of the components of the PIC <NUM>. Therefore, the PIC <NUM> and hence the optical module <NUM> may perform more reliably. In an example, when the PIC is used QKD, temperature changes in the PIC translate into phase changes for the photon at the transmitter or at the receiver, which would be interpreted as noise in the quantum signal and limit the performance of the key exchange.

Additionally and optionally, the temperature control element TCE <NUM> comprises a thermoelectric cooler. The thermoelectric cooler (also referred to as a thermoelectric device) is configured to transfer heat from a cold side to a hot side when a voltage is applied, thereby creating a temperature difference across said cooler. For example, the thermoelectric device may be a Peltier device. A Peltier device comprises a thermocouple (a junction of two different conductors), wherein, on application of an electric current, heat may be generated or removed from the junction. By having the one side thermally coupled to the PIC <NUM> via the chip carrier 103b, and another side being thermally coupled to the housing <NUM>, the thermoelectric cooler may pump or remove heat from the PIC <NUM> in order to adjust its temperature. The thermoelectric device may be controlled by an electrical control signal. The electrical control signal may be routed to the thermoelectric device by way of the internal circuit board <NUM>. The electrical control signal may be provided by a temperature control element (TCE) controller (not shown). In an example, the TCE controller is provided in an electronic host board. In another example, the TCE controller is provided on the internal electronic board <NUM>.

Alternatively, instead of the thermoelectric device, a resistive heater may be used instead. The resistive heater causes heating by the process of Joule heating and is controlled by passing an electrical control signal (e.g. a current) through it. The resistive heater may only cause heating, unlike the thermoelectric device.

The module <NUM> further comprises an internal electronic board <NUM>. For example, the internal electronic board <NUM> may be a printed circuit board (PCB). As shown in <FIG>, the internal circuit board <NUM> comprises an electrical plug connector <NUM>. The purpose of the internal electronic board <NUM> is to route electric signals from chip carrier 103b to the electrical plug connector <NUM>. Optionally, the internal electronic board <NUM> comprises further electric components such as small memories or micro-processor units. The chip carrier 103b connects the internal electronic board <NUM> to the PIC <NUM> as described above. The electrical plug connector <NUM> is configured to enable electrical access from the exterior of the module <NUM>. Thus, circuit board <NUM> and components that are electrically connected to it can be electrically accessed from the exterior of the module <NUM>. By electrical access, it is meant that there is an electrical connection such that an electric signal may be applied or read out. In the embodiment of <FIG>, the electrical plug connector <NUM> is configured to plug into a receptacle connector. The receptacle may be located in an electronic host board, as will be described further below. The circuit board <NUM> is shown in more detail in <FIG> shows that the circuit board <NUM> is isolated from the TCE <NUM>. By isolated, it is meant that the circuit board <NUM> is thermally isolated from TCE <NUM>. In other words, there is no intentional thermal communication path between the circuit board <NUM> and the chip carrier 103b. By intentional, it is meant the circuit board <NUM> is not configured to be in thermal communication with the chip carrier 103b; however, in practice, heat may flow between the two components. Thus, there is also no intentional thermal communication between the circuit board <NUM> and the TCE <NUM>, which is connected to the chip carrier 103b. The effect of this arrangement is to improve the efficiency of the TCE <NUM>. If the PCB <NUM> is in contact with the TCE <NUM> this increases the thermal load to stabilize in temperature.

In an example, the PCB <NUM> is mounted on the housing and the TCE <NUM> is a Peltier device. If the PCB touches the top of the Peltier while the bottom of the Peltier is in contact with the housing <NUM>, this creates an unwanted loop between the top and the bottom of the Peltier, which is detrimental for stabilization. Additionally and optionally, when contact between the PCB and the TCE is unavoidable, thermal insulation trenches are cut into the PCB to limit the heat transfer between the part of the PCB in contact with the TCE and the rest of the PCB.

Returning to <FIG>, the module <NUM> further comprises a coupling element <NUM> and an optical connector <NUM>. The optical connector <NUM> is configured to enable optical access from the exterior of the module <NUM> to components within the module <NUM> via the coupling element <NUM>. By optical access, it is meant that optical signals can be injected into the optical module or that optical signals from the optical module can be read out. The optical connector <NUM> is configured to receive one or more connectorised fibres. For example, the optical connector <NUM> is configured to receive a single fibre. In another example, the optical connector <NUM> is configured to receive a LC or Multiple-Fiber Push-on/Pull-off (MPO) connectorised fibre.

The coupling element <NUM> is configured to optically couple the optical connector <NUM> to the PIC <NUM>. For example the coupling element <NUM> may comprise a single optical fibre. The optical fibre array connects to the optical connector <NUM> at one end, while the other end is optically coupled to the PIC <NUM> by a suitable optical interfacing means (not shown). A suitable optical interfacing means is configured to couple most of the light between the fibre and the PIC <NUM>, avoid reflections back into the PIC (or into the fibres) and avoid uncontrolled scattering of light inside the module <NUM>. In an example, the optical mode of the optical interfacing matches the optical mode of a light collecting element. Examples include lens off chip, grating couplers, spot size converters, evanescent coupling elements. The fibre can be a flat cleaved fibre, a tapered fibre, lensed terminated and with anti-reflection coating.

In use, the optical module <NUM> is plugged into an electronic host board <NUM>, which will be described further below. When plugged, the components of the optical module <NUM> may therefore be electrically connected to the electronic host board. Further, the optical module <NUM> is optically connected to an external optical link by way of optical connector <NUM>, such that the PIC <NUM> is optically coupled to the external optical link. During its operation, the temperature of the PIC may vary, for example, due to changing ambient conditions, or as a result of the electrical and optical signals injected or read out of the PIC <NUM>. The TCE <NUM> is used to actively stabilize the temperature of the PIC <NUM> by injecting and/or removing heat as required. The above described arrangement of the PIC <NUM>, TCE <NUM> and housing <NUM> makes stabilizing the temperature easier.

Furthermore, the assembly of the optical module <NUM> is simplified because the PIC <NUM> is mounted onto the TCE <NUM>, via the chip carrier 103b, directly into the optical module. The PIC <NUM> is not first mounted into an optical sub-assembly which is then mounted into the optical module. Thus, the assembly of the module <NUM> is simplified.

<FIG> shows a perspective view of the optical module <NUM> of <FIG>. Compared to <FIG>, the PIC <NUM> and the chip carrier 103b are omitted. <FIG> further shows a lid <NUM> (shown as a translucent layer). The lid is removable and its purpose is to protect the components housed in the housing <NUM>. In particular, the lid protects the photonic integrated circuit. In <FIG>, the lid is shown as a translucent layer. Alternatively, the lid is an opaque layer or transparent layer. The lid does not contribute to heat exchange and may be made of any material. In an example, the lid is made of the same material as the housing <NUM>.

<FIG> also shows that the electrical plug connector <NUM> is part of the internal circuit board <NUM>. In other words, the internal board <NUM> and the electrical plug connector <NUM> are unitary such that the internal board <NUM> is pluggable directly into a receptacle connector in the electronic host board. <FIG> further shows an opening 106b. The opening 106b is configured to accommodate optical connector <NUM> (which is not shown in this figure). The optical connector is fixed to the housing in the opening 106b this facilitates alignment of fibres to the circuit. For example, if a fibre is connected to the connector and the connector mounted on the housing aperture 106b prior fibre alignment then the rigidity of the fibre may affect its handling and its precise alignment to the optical couplers on chip.

<FIG> is a close up view of <FIG> showing the PIC <NUM>, the chip carrier 103b, and the internal circuit board <NUM> in more detail. The TCE <NUM>, provided between the chip carrier 103b and the housing <NUM>, is not visible in this view. The internal circuit board <NUM> comprises temperature control solder holes <NUM>. The purpose of the solder holes <NUM> is to connect the wiring of the TCE <NUM> to the top of the internal board <NUM>, for convenience and ease of assembly.

As shown in <FIG>, the chip carrier 103b comprises temperature sensor mounting pads <NUM>. The mounting pads <NUM> are for attaching and electrically connecting a temperature sensor onto the chip carrier 103b. The temperature sensor is not shown. The temperature sensor may then provide an indication of the temperature of the PIC <NUM>. For example, the temperature sensor may be a thermistor.

<FIG> also shows a red arrow that arrow illustrates the optical input and output to the PIC <NUM>. In practice light is coupled in and out of the PIC <NUM> by way of coupling element <NUM>, described above.

<FIG> shows a perspective view of the housing <NUM> of optical module <NUM> from <FIG> and <FIG>. In this figure, all components other that the housing <NUM> are omitted. The housing <NUM> is configured to accommodate components such as the TCE <NUM>, the internal circuit board <NUM>, and the PIC <NUM>, as described above. The housing <NUM> corresponds to the CFP2 form factor. Further, the housing <NUM> comprises a TCE receptacle 102b that is configured to receive the TCE <NUM>. The TCE receptacle 102b is configured to adjust the height of the PIC <NUM> relative to the PCB. The PCB is mounted at a predetermined height inside the housing in order to be able to plug into an electronic host board. The TCE receptacle 102b allows the height of the PIC <NUM> to be adjusted accordingly. <FIG> shows the same view as <FIG> with the TCE <NUM> shown assembled into the receptacle 102b.

Although <FIG> shows a housing corresponding to the CFP2 form factor, other form factors may be used.

<FIG> show side views of the photonic chip <NUM> assembled onto the temperature control element (TCE) <NUM> such that the chip <NUM> is in thermal communication with the TCE <NUM>. <FIG> shows that the chip <NUM> is connected to a chip carrier 103b, which is then connected to the TCE <NUM>. <FIG> corresponds to the arrangement shown in <FIG>.

<FIG> shows an alternative arrangement in which the chip carrier 103b is omitted. In this case, the PIC <NUM> is attached directly to the TCE <NUM>. The expression 'attached directly' has the same meaning described above in relation to the attachment of the TCE <NUM> to the housing <NUM>. In this arrangement, electrical contacts on the PIC <NUM> may be connected to contacts on the internal circuit board <NUM> by way of wire bonds (not shown).

The following features can be optionally and additionally combined with the optical module described above in relation to <FIG>, or as alternatives to some of the features described.

Additionally and optionally, the housing <NUM> comprises a heatsink configured to cool the module <NUM>. In an example, the heatsink is provided on the exterior of the housing <NUM>.

Although the description of the module <NUM> refers to the coupling element <NUM> as comprising a single optical fibre, it will be understood that, alternatively, the coupling element <NUM> may comprise a plurality of individual optical fibres. The optical interfacing means may correspond to that of the single optical fibre above. Yet alternatively, the coupling element <NUM> may comprise an optical fibre array. The optical fibre array may be arranged in a row (1D) or in a rectangular array (2D). The optical interfacing mean may correspond to that of the single optical fibre above. Optionally, the fibre array comprises one or more lensed fibres. The optical interfacing means may correspond to that of the single optical fibre above.

Although, the description of the module <NUM> refers to the temperature sensor pads <NUM> being on the chip carrier 103b (as shown in <FIG>) and hence the temperature sensor being on the chip carrier 103b, it will be understood that alternatively, the temperature sensor is provided on the PIC <NUM> instead. In this alternative arrangement, the temperature sensor may be a thermistor, such as NTC, PT100, or PT1000, or a resistive sensor fabricated in the PIC <NUM>. By providing the temperature sensor on the PIC <NUM>, a more accurate measurement of the temperature of the PIC <NUM> may be obtained and the temperature of the PIC <NUM> may therefore be more easily stabilised.

In an example which is not shown, coupling element <NUM> and optical connector <NUM> comprise a micro-LC to LC adapter. Using micro-LC in the module <NUM> is helpful to reduce the space occupied by the connector. MPO is also useful, when there is high-fibre count.

In an example which is not shown a PCB edge connector is used as an alternative to the unitary electrical plug connector <NUM> and internal circuit board <NUM> described above. For example, the connector is a CFP2 edge connector. One end of the PCB edge connector is soldered to the internal circuit board <NUM>, and the other end is configured to be plugged into the electronic host board.

The chip carrier 103b may further comprise some small electronic components such as resistors, capacitors or trans-impedance amplifiers. It can also be configure to perform first stage of processing of the signal from the chip. For example a small circuit can be implemented on the chip carrier to subtract the signal from two photodiodes in case the PIC is configured to perform homodyne measurements.

<FIG> shows a plan view of an optical module <NUM> according to another embodiment. The arrangement of <FIG> is similar to that shown in <FIG>, except that the module <NUM> and housing <NUM> is configured to house two PICs <NUM>. As shown in <FIG>, each of the PICs <NUM> is mounted onto a TCE <NUM>, as described in relation to <FIG>. Note that, alternatively, each of the PICs may be mounted on a chip carrier which is then attached to the TCE, as described in relation to <FIG>.

<FIG> shows a schematic illustration of system comprising an electronic host board <NUM> and the optical module <NUM> in accordance with any of the previous embodiments.

The electronic host board <NUM> comprises a receptacle <NUM> into which the module <NUM> is plugged in. The receptacle <NUM> comprises a receptacle connector <NUM> that is configured to mate with the electrical plug connector <NUM> on the module <NUM> (or alternatively, with the PCB edge connector of Figure <NUM> (b)). Thus, when plugged, there is electrical connection between the electronic host board <NUM> and the optical module <NUM>, via the receptacle connector <NUM> and the electrical plug connector <NUM> (or the alternative shown in Figure <NUM> (b)). The receptacle connector <NUM> is also referred to as a socket.

The electronic host board <NUM> further comprises one or more controllers configured to generate the electronic signals that are then applied into the optical module <NUM>. For example, the host board <NUM> is configured to generate an RF multilevel signal <NUM> for a coherent source on the module <NUM>; an RF square signal <NUM> for a gain-switched laser; and an RF square signal <NUM> for an intensity modulator. These signals may be applied to components on the PIC <NUM> (via the electrical connection to optical module <NUM>) to generate optical pulses with well-defined properties.

<FIG> shows a schematic illustration of an optical device <NUM> on the PIC for optical pulse generation. The optical device <NUM> includes phase modulation and intensity modulation. The optical device <NUM> may be referred to as a sending unit. In an embodiment, the gain switched laser <NUM> and the coherent light source <NUM> are integrated on a substrate, as will be described below. Phase control element <NUM> applies a perturbation to the coherent light source <NUM>, such that there is a phase difference between the first half of long light pulse <NUM> and the second half of long light pulse <NUM>. Long light pulse <NUM> enters light distribution device <NUM> through port A. However, in alternative embodiments, light distribution device <NUM> is omitted, and long light pulse <NUM> travels directly from coherent light source <NUM> to gain-switched laser <NUM>.

The phase control element <NUM> applies a perturbation to the coherent light source <NUM> at regular intervals, which are timed to occur halfway through the generation of each long light pulse <NUM>. The perturbation changes the phase of the second half of the light pulse, producing a phase difference between the first half of a light pulse and the second half of the light pulse. The perturbation is controlled, in other words, the same perturbation will always cause the same phase shift. The amplitude of the perturbation that is applied affects the phase shift that is generated. In one embodiment, the perturbation is a short current pulse.

Alternative embodiments comprise an optical amplifier, for example a semiconductor optical amplifier instead of a gain-switched laser <NUM>. Further alternative embodiments comprise an intensity modulator instead of a gain-switched laser <NUM>. An intensity modulator modulates the intensity of incoming light pulses. In an "off" state, the intensity modulator reduces the intensity of the light to a low level. In an "on" state, the intensity modulator allows a larger fraction of the incoming light to exit. The intensity modulator is switched between an "on" state and an "off" state twice when the light pulse <NUM> from the coherent light source is present in order to generate two short light pulses <NUM>. When the intensity modulator is used instead of the laser <NUM>, the intensity control element <NUM> provides the switching signal; in this case, the RF signal intensity <NUM> for the intensity modulator corresponds to the switching signal. The phase difference between the two short light pulses is determined by the phase applied to the coherent light pulse <NUM> by the phase control element <NUM>. An intensity modulator may modulate the intensity of the light by changing the absorption coefficient of the material in the modulator, for example an electro-absorption modulator. An electro-absorption modulator is a semiconductor device for which the voltage applied to the device changes the absorption coefficient, and therefore the intensity of light travelling through the device. In another embodiment the intensity modulator is based on a Mach-Zehnder interferometer. A Mach-Zehnder based intensity modulator changes the phase difference between the two arms of the interferometer to modulate the output intensity.

Long light pulse <NUM> exits light distribution device <NUM> through port B, and is injected into gain-switched laser <NUM>. Intensity control element <NUM> controls the current applied to gain-switched laser <NUM>, in order to modulate the intensity of the double pulses <NUM> emitted from the gain-switched laser <NUM>. The double pulses <NUM> are emitted from the same aperture through which long light pulse <NUM> was injected, and enter light distribution device <NUM> through port B. The double pulses exit light distribution device <NUM> through port C. The first pair of light pulses that exit port C of light distribution device <NUM> have an intensity that has not been modulated by the intensity control element <NUM>, and the phase difference between the pulses is ϕ<NUM>. The second pair of light pulses that exit port C of the light distribution device <NUM> have an intensity that has been reduced by intensity control element <NUM>. The phase difference between the pulses is ϕ<NUM>.

Gain switching will now be described with reference to <FIG> shows a schematic illustration of a gain-switched semiconductor laser. A gain-switched laser generates light when the laser is switched above the lasing threshold and generates almost no light when the laser is switched below the lasing threshold. Laser <NUM> has a controller <NUM> which allows modulation of the gain of the laser by modification of the pump power. The gain can be modulated in a time varying manner. Driving the laser in this manner can generate short laser pulses (of the order of picoseconds in duration) at the laser output <NUM>.

When laser <NUM> is a semiconductor laser then it can be pumped electrically, by applying a current. In order to modulate the gain of a semiconductor laser, the controller <NUM> modulates the current applied to the laser.

<FIG> shows three graphs illustrating a gain modulation of a semiconductor gain-switched laser. The upper graph shows the current applied to the laser on the vertical axis, with time on the horizontal axis. The DC bias is indicated by a horizontal dotted line. The current applied to the laser has the form of a series of current modulation pulses. The wave is a square-type waveform. In this case, the current is not reduced to zero in between the current modulation pulses, but only reduced to a bias value (which is indicated by the dotted line).

The current modulation signal is applied to the laser and switches the gain of the laser above and below the lasing threshold periodically. The second graph shows the carrier density of the laser on the vertical axis, against time on the horizontal axis. The lasing threshold is indicated by a dashed horizontal line. When a current modulation pulse is applied to the laser, the injected carriers increase the carrier density and the photon density increases.

The laser output generated by the modulation signal is shown in the lower graph. The vertical axis shows the laser intensity, with time on the horizontal axis. The laser outputs light when the carrier density is above the lasing threshold. Photons generated by spontaneous emission inside the laser cavity are amplified sufficiently by stimulated emission to generate an output signal. The length of the delay between the application of the current modulation pulse and the generation of the output light depends on several parameters, such as the laser type, cavity length and pumping power.

The rapid increase of the photon density causes a decrease in the carrier density. This in turn decreases the photon density, which increases the carrier density. At this point the current modulation pulse is timed to switch back down to the DC bias level, and the laser emission dies off quickly. The laser output therefore consists of a train of short laser pulses as shown in the lower graph.

<FIG> shows a schematic illustration of an electrical driving circuit for a semiconductor gain-switched laser. The semiconductor gain-switched laser is laser diode <NUM>. The cathode of laser diode <NUM> is connected to bias-T <NUM> comprising inductor <NUM> and resistor or capacitor <NUM>. Via inductor <NUM> a DC bias current is sent through the laser diode. This provides the gain bias (the minimum level of the current indicated by the dotted line in <FIG>). Via resistor or capacitor <NUM> an AC modulation current is sent through the laser diode, providing the gain modulation needed for gain-switching the laser above and below the lasing threshold. In this case, the modulation input to the bias-T <NUM> is provided by controller <NUM>.

Returning to <FIG>, the RF multilevel signal <NUM> for a coherent source on the module <NUM>; the RF square signal <NUM> for a gain-switched laser; and the RF square signal <NUM> for an intensity modulator will now be described, with further reference to <FIG> and <FIG>.

<FIG> shows the current signal <NUM> applied to the coherent light source <NUM> when modified by phase control element <NUM>. The current signal <NUM> comprises a series of square type pulses, where the duration of the periods between the pulses is shorter than the duration of the pulses. The square type signal can be formed by combining an AC current with a DC bias current via a bias-T. The current signal <NUM> further comprises a smaller current pulse that is added to the above square type signal through the AC input of the bias tee. The smaller current pulse is timed such that it coincides with the mid-point of the upper section of one of the square pulses. The phase control element can be a separate element that generates the smaller current pulses, which are then combined with the square pulse AC signal. The square pulse AC signal is configured such that the duration of the periods between the pulses is shorter than the duration of the pulses. The smaller current pulse can be formed by combining an AC current with a DC bias current via a bias-T. The combined signal is then inputted to the AC input of the bias tee. The smaller current pulse of the current signal <NUM> correspond to the perturbation that imparts a controllable phase sift to the light pulse <NUM>, as described in relation to <FIG>.

<FIG> shows the time varying current <NUM> applied to gain-switched laser <NUM> by a controller. The signal comprises a square wave, of a magnitude such that the gain-switched laser <NUM> is switched periodically above the lasing threshold. The first current pulse is applied when the first half of light pulse <NUM> is present. The first current pulse is timed such that the gain-switched laser <NUM> is switched above the lasing threshold during the section of the time when the first half of light pulse <NUM> is present in the gain-switched laser. The second pulse is timed such that the gain-switched laser is switched above the lasing threshold during the section of the time when the second half of the light pulse <NUM> is present in gain-switched laser <NUM>. The time varying current applied to the coherent light source <NUM> and the time varying current applied to the gain-switched laser can be synchronised in order that the timing of the generation of the short pulses corresponds to the time when the correct section of the long light pulse is present. For example, both time varying currents can be synchronised to a master clock signal.

<FIG> shows the time varying current <NUM> after modification of the signal <NUM> shown in <FIG> by the intensity control element <NUM>. The modified signal is then inputted to the AC input of a bias tee and the output current of the bias-T is applied to the gain-switched laser <NUM>. For a decoy-state BB84 protocol, it may be required that <NUM>% of the coherent double pulses are vacuum pulses, <NUM>% of the coherent double pulses are decoy pulses and <NUM>% of the coherent double pulses are signal pulses. The combined signal is generated such that each pair of electrical pulses applied to the gain-switched laser has a <NUM>% probability of having zero amplitude (i.e. such that no short pulses are generated), a <NUM>% probability of having a reduced amplitude, and a <NUM>% probability of having an unmodified amplitude. A portion of the combined signal is shown in <FIG>. The signal of <FIG> has been modified such that one pair of electrical pulses has a reduced amplitude. When this pair of electrical pulses is applied to the gain-switched laser <NUM> a coherent double pulse with a reduced intensity is generated. <FIG> shows the light pulses emitted from the gain-switched laser <NUM>.

Note that the bias T circuit <NUM> described above in relation to <FIG> may be provided in the electronic host board <NUM>, or alternatively on the internal circuit board <NUM> of the optical module <NUM>.

Returning to <FIG>, an RF square signal <NUM> is read out from a photodetector integrated on the PIC <NUM> provided in the module <NUM>. The RF square signal <NUM> may correspond to signals read out by a receiver (containing the photodetector) on the PIC <NUM>.

Alternatively, the RF square signal <NUM> read out from the photodetector is coupled to an analog-to-digital converter (ADC). Optionally, the ADC is further coupled to a post processor. The ADC and, optionally the post processor, may be used to extract information relating to photons received at a photodetector on the PIC <NUM>. For example, the post processor may convert raw data from the ADC in order to extract a sequence of random numbers, when optical pulses arriving at the photodetector have random intensities.

The electronic host board <NUM> further comprises a DC source and a monitor <NUM>. The DC source <NUM> outputs one or more DC output signals. In an example, the DC source has between <NUM> and <NUM> output channels. In an example, the DC signals are high stability voltages or currents. The DC signals may be used to as dc bias in the bias-T <NUM>, as described above.

The electronic host board <NUM> may further comprise a controller for the temperature control element (not shown). In an example, the controller comprises a feedback loop for servo control of the TCE via electric signals. The controller monitors the resistance of the thermistor and provides feedback parameter such that the current through the TCE is adjusted accordingly. In an example the electric signal generated is a high precision signal. In an alternative example, the controller is provided in the optical module instead.

<FIG> shows a schematic illustration of a Quantum Key Distribution (QKD) system. QKD allows two parties to create and share a random secret key, or cipher, in a secure manner using quantum bits, or qubits. QKD theoretically allows the sender (often referred to as "Alice") and receiver (often referred to as "Bob") of the key to tell if an eavesdropper (often referred to as "Eve") has intercepted the communication, compromising the key's security. This relies on the fact that a qubit cannot be measured without affecting the measured property. As such, any such alteration of the received qubits due to Eve's interference can be detected by Alice and Bob.

A quantum bit may be encoded in a light pulse or in a single-photon pulse. A quantum bit source may be a source of light pulses or of single photons. A source of light pulses may be implemented according to the embodiments described herein.

The QKD system may use light. Embodiments disclosed herein may control the light intensity values. Embodiments disclosed herein may control the encoding and/or decoding bases prescribed by the QKD protocol. A particular example may be the BB84 protocol with decoy states, wherein an embodiment according to the present disclosure may output three values: u, v and w, which may correspond to a "signal", "decoy" and "vacuum" state, respectively. It may be desirable for each of these output values to occur with a different output frequency. As such, a string processor as described herein may be used in such a QKD system. Another example is the so-called "efficient BB84 protocol" where in, an embodiment, two bases, Z and X, may be used. These bases can be selected with different occurrence probabilities. As such, a RNG as described below may be used. It is also possible to have jointly an efficient BB84 protocol with decoy states. In this case, multiple output symbols (Zu, Zv, Zw, Xu, Xv, Xw) have to be selected with different probabilities and the RNG described below may be used.

The QKD system comprises two field programmable gate array (FPGA) string processors <NUM> and <NUM> in the sending unit (Alice) and receiving unit (Bob).

In <FIG>, a QKD setup to implement the standard or the efficient version of the decoy-state BB84 protocol is depicted. The embodiment of <FIG> illustrates a sender, Alice, sending an encrypted key to a receiver, Bob. A light source <NUM>, or photon source, generates a light pulse which is then passed through an intensity modulator <NUM>. The intensity modulator <NUM> implements the decoy-state method, wherein each light pulse or photon is randomly modulated to one of a number of predetermined intensities {u, v, w}, corresponding to "signal", "decoy" or "vacuum", respectively; and these predetermined intensities occur with an occurrence frequency determined by the user {fu, fv, fw}. A string processor, or FPGA string processor <NUM> according to an embodiment is used to control the intensity modulator <NUM>, providing three outputs (each corresponding to a certain intensity) with a quality-assured level of randomness and with a predetermined bias.

The intensity-modulated pulses are then split by an input beam splitter <NUM>. One path - the "first path" - goes through a phase modulator <NUM> after the beam splitter <NUM>. The phase modulator <NUM> randomly modulates the photons by a specific phase, thus outputting the photons with one of (in this embodiment) two bases {Z, X}. The occurrence ratio between the two bases may be biased or unbiased, and predetermined by a user. In the former case, the efficient version of the BB84 protocol is realized; in the latter case, the standard version of the BB84 protocol is realized. The invention disclosed herein can cover both cases, with the unbiased case as a trivial particular case. A string processor, or FPGA string processor <NUM> according to an embodiment is used to control the phase modulator <NUM> for the basis selection. The FPGA <NUM> controlling the basis selection in the phase modulator <NUM> of <FIG> is equivalent to the FPGA <NUM> that controls the intensity modulator <NUM>. Alternatively, a different string processor or FPGA according to an embodiment may be used to control the phase modulator <NUM>.

The second path from the input beam splitter <NUM> is sent through an optical delay <NUM>.

Light pulses or photons from the first and second path are then sent to the receiver, Bob, via an optical transmission line <NUM> and two polarising beam splitters <NUM> and <NUM>. Alice's polarising beam splitter <NUM> rotates and combines polarised pulses or photons from the two different paths and send them through the optical transmission line <NUM>. As the pulses or photons are polarised, Bob's polarising beam splitter <NUM> separates them and directs them onto the two paths of his interferometer, this time sending the pulses or photons sent through Alice's phase modulator <NUM> through an optical delay <NUM> and the pulses or photons not sent through Alice's phase modulator <NUM> through phase modulator <NUM>. This way, the pulses or photons can reach the final beam splitter <NUM> at the same time and can interfere.

Bob's phase modulator <NUM> randomly selects one of two bases in which to measure the received pulses or photons, by selecting a phase modulation value. Similar to Alice's phase modulator <NUM>, Bob's phase modulator <NUM> randomly modulates the photons by a specific phase, thus effectively measuring through the detectors <NUM> and <NUM> the photons with one of (in this embodiment) two bases {Z, X}. The occurrence ratio between the two bases is biased, and predetermined by a user to be equal to that of Alice's phase modulator <NUM>. A further string processor or FPGA string processor <NUM> according to an embodiment may be used to control this second phase modulator <NUM> for the basis selection.

The two paths in the receiver are then again combined at the output beam splitter <NUM>. To ensure the two optical delays <NUM> and <NUM> combine to ensure that both overall paths experience the same delay, a variable delay line <NUM> fine tunes the delay in the receiver.

Photon detectors <NUM> and <NUM> are then used to measure the result of the interference between the pulses or photons on the two paths; and from these results the key may be derived. For example, the key bit value <NUM> can be assigned if detector <NUM> clicks while the key bit value <NUM> can be assigned if detector <NUM> clicks.

In the above, Alice's phase modulator <NUM> and Bob's phase modulator <NUM> are configured to encode and decode quantum bits respectively.

Random numbers can be produced from Random Number Generators (RNG). In particular, Quantum Random Number Generators (QRNG) may be used. In QRNG, the source of randomness is physical and relies on the unpredictability of a measurement, and, in particular, the unpredictability relies on a quantum mechanical property. QRNGs can be implemented using gained-switched diode lasers. In gain-switched diode lasers, the lasing threshold is governed by spontaneous emission, which is a quantum mechanical process, such that the phase of the emitted pulse is random. By repeatedly switching the diode laser on and off, a stream of optical pulses, each having a random phase, can be generated. By measuring the random phase of each optical pulse in the stream of optical pulses, a sequence of random numbers can be obtained.

At least some of the components of the QKD system of <FIG> are implemented on the PIC <NUM> provided in the optical module <NUM>.

In the sending unit (Alice), the light source <NUM> may be implemented on PIC <NUM> using the emitter described in <FIG> below.

Phase modulator <NUM> is implemented as an electro-optic modulator, wherein the refractive index of the material is a function of applied electric field. Changes in refractive index result in changes in optical path length and results in changes in the phase shift applied by phase modulator. Different voltages are applied to the phase modulator so as to impart a different phase shift. A phase modulator such as described can comprise a crystal, such as a LiNbO3 crystal, in which the refractive index is a function of electric field strength, and an electric field may be applied by applying a voltage to electrodes positioned around the LiNbO3 crystal. Alternatively, the phase modulator <NUM> is implemented as a traveling-wave modulator, or using piezo electric actuator.

The intensity modulator <NUM> may be implemented as a Mach Zehnder Interferometer MZI switch. The MZI switch is configured to operate as a variable optical attenuator. The MZI switch has two inputs and two outputs. However a single input and/or a single output may be used. At the input side, the inputs are evanescently coupled together and then split into two arms of the interferometer. One arm contains a phase shifter. The phase shifter is configured to add a phase to the input light and the amount of phase added may be controlled. The phase shifter may be implemented in the same manner as for phase modulator <NUM>. The light passing through the phase shifter interferes with the light that has not passed through the phase shifter and the amplitude of the light at each output of the MZI depends on the relative phase shift. By dynamically adjusting the phase shift of the phase shifter, the power splitting ratio may be controlled and the power transferred to each output may be controlled. Thus, the intensity of light output by the intensity modulator <NUM> may be controlled. Alternatively the intensity modulator <NUM> may be implemented as an electro-absorption modulator (EAM).

The beam splitter <NUM> is implemented as a 2x2 directional coupler; and the delay <NUM> is implemented as a delay line, comprising a light guiding section. In an example, the light guiding section is a few centimetres long. In another example, when the light guiding section is based on InP, the line is approximately <NUM> long and provides a delay of approximately <NUM> ps. The different components are optically coupled together by means of light guiding sections integrated on the PIC <NUM>. The polarising beam splitter <NUM> may be implemented off chip.

In a further embodiment, the transmitter <NUM> of <FIG> is used as the sending unit. This can be driven unit the driving scheme of <FIG> and this removes the need of electro optic modulator. Removing the electro-optic modulators is a way to reduce the power consumption of the chip and it also allows to make the chip more compact. The removal of the electro-optic modulator also simplifies the electronics on the host board.

In the receiving unit (Bob), the photo detectors <NUM> and <NUM> is may be implemented on PIC <NUM> using the receiver described in <FIG> below. Polarising beam splitter <NUM>, delay <NUM>, beam splitter <NUM> and phase modulator <NUM> are implemented in a similar manner as for the sending unit. The variable delay line <NUM> may be implemented off chip.

The FPGA <NUM> and <NUM> are connected to the electronic host board <NUM> and provide the RF signals for the intensity modulator and the phase modulators. QRNGs provide strings to the string processor (FPGA board) to generate the qubit states.

<FIG> shows a schematic illustration of a Quantum Key Distribution (QKD) system. The QKD system comprises an optical module <NUM>, configured as a transmitter, plugged into a host board (Alice), and another optical module <NUM>, configured as a receiver, plugged into a host board (Bob). The module <NUM> is compatible with different designs and different QKD protocols. In an example, the same host electric board may drive different quantum transmitters. The same transmitter can be used to generate qubits for one or more protocols (BB84, DPS, COW, SARG, RFI, MDI-QKD, TF-QKD.

The receiver may also be compatible with multiple protocols if for example it is implemented as in <CIT>.

<FIG> is a schematic illustration of a differential-phase shift quantum communication system (DPS-QKD) in accordance with an example, where the Quantum Transmitter comprises an optical device <NUM> with a phase control element such as that shown in <FIG>.

A schematic illustration of optical device <NUM> is shown in <FIG>. Coherent light source <NUM> is a semiconductor laser. Coherent light source <NUM> is configured to generate CW light. In alternative embodiments, the coherent light source generates long light pulses of duration greater than or equal to 10ns. In order to encode information, phase control element <NUM> may apply a short current pulse to coherent light source <NUM>. This results in a phase difference between a first part and a second part of the CW light. The CW light enters gain-switched laser <NUM>, which generates a first short pulse when the first part of the light is incident, and generates a second short pulse when the second part of the light is incident. There is a phase difference between the generated short light pulses equal to the phase difference between the first part and second part. The phase control element <NUM> encodes information between consecutive pulses by controlling the phase difference between parts of the generated CW light. In this DPS-QKD example, the intensity control element <NUM> of <FIG> is not used.

The phase control element <NUM> is configured to introduce a phase shift of either <NUM> or π between subsequent pulses to generate the sequence of light pulses <NUM>. For the pulses shown in <FIG>, phase control element <NUM> applies no short current pulse to coherent light source <NUM> initially, resulting in no phase difference between the first part and the second part of the CW light, and thus no phase difference between the first and second short light pulses. At the next regular interval, phase control element <NUM> then applies a short current pulse to coherent light source <NUM>, resulting in a phase difference of π between the second part and the third part of the CW light, and thus a phase difference of π between pulse <NUM> and <NUM>. At the next regular interval, phase control element <NUM> applies a short current pulse of a particular amplitude to coherent light source <NUM>, resulting in a phase difference of π between the third and fourth part of the CW light, and thus a phase difference of π between pulses <NUM> and <NUM> in the sequence. A phase difference of <NUM> between subsequent pulses is associated with a bit value of <NUM>. A phase difference of π between subsequent pulses is associated with a bit value of <NUM>.

In other words, the phase control element <NUM> may set the differential phase between subsequent light pulses. For example, a phase difference of <NUM> between subsequent pulses may be associated with a bit value of <NUM>, while a phase difference of π between subsequent pulses may be associated with a bit value of <NUM>. The light pulses are transmitted to a quantum receiver (Bob) which detects the phase difference between the coherent pulses received and decodes a bit value of <NUM> or <NUM>. The security of DPS stems from the fact that if an eavesdropper Eve ties to measure one pulse, she destroys the coherence between that pulse and its neighbour and this can be detected by Alice and Bob.

In an embodiment, the coherent light source <NUM> is turned off periodically to randomise the phase after each qubit emission cycle which is required by some quantum communication protocols, for example, the BB84 protocol. An example operation is shown in <FIG>. The coherent light source <NUM> emits long coherent light pulses. The gain switched laser <NUM> is driven with two subsequent pulses during the time that a long coherent pulse is emitted by light source <NUM>, generating a time-bin encoded qubit. The relative phase ϕ1 of the two time-bins encode a desired qubit state, for example, according to the DPS-QKD protocol described above. The system described herein can be configured as required for the different protocols. For example, <NUM> phase states are used in the DPS protocol, and <NUM> phase states for the BB84 protocol. For the DPS protocol phase randomisation is not needed. For decoy states protocols the further intensity modulator is needed. The encoded qubits are then sent to Bob.

Quantum receiver <NUM> (Bob) is described below. Quantum Transmitter <NUM> and Quantum Receiver <NUM> are connected by an optical transmission line <NUM>.

In the embodiment of <FIG>, the quantum receiver <NUM> comprises an asymmetric MZI <NUM> with an optical delay <NUM> which is equal to the time delay Δt between two subsequent pulses of coherent pulse sequence <NUM>. However, it should be noted that the delay line is optional. When a transmitter <NUM> of the type shown in <FIG> is used, there is no delay line in the transmitter <NUM> and hence the delay between the two pulses can be freely adjusted to match the delay of the receiver. Therefore a variable delay line is not needed in the receiver when using a transmitter <NUM>.

The pulse sequence <NUM> enters one input of beam splitter <NUM>. A first output of input beam splitter <NUM> is connected to the long arm <NUM> of the interferometer <NUM>, and a second output is connected to the short arm <NUM> of the interferometer <NUM>. A first fraction of each pulse of the pulse sequence <NUM> is sent along the short arm <NUM> of interferometer <NUM> and a second fraction is sent along the long arm <NUM> of interferometer <NUM>. The long arm <NUM> is connected to a first input of output beam splitter <NUM> and the short arm is connected to a second input of output beam splitter <NUM>. At the output beam splitter <NUM> subsequent pulses of pulse sequence <NUM> are overlapped in time. For example, a second fraction of the first light pulse (which has travelled the long arm <NUM>) will arrive at the output beam splitter <NUM> at the same time as a first fraction of the second light pulse (which has travelled the short arm <NUM>).

The pulses are coherent and therefore they interfere at the beam splitter. The output depends on the phase difference. If the phase difference is zero there is a detection at detector <NUM>. If, on the other hand, the phase difference is π, there is a detection at detector <NUM>. For any other value of difference between the phase modulation applied at the phase modulator, there will be a finite probability that a photon may output at detector <NUM> or detector <NUM>.

The phase difference between the second fraction of the first light pulse and the first fraction of the second light pulse is zero, therefore detector <NUM> registers a detection. This corresponds to a bit value of <NUM>. The phase difference between the second fraction of the second light pulse and the first fraction of the third light pulse is π, therefore the detector <NUM> registers a detection. Both the fractions of each light pulse interfere and can give rise to a detection. Specifically, the second fraction of the nth pulse can always interfere with the first fraction of the (n+<NUM>)th pulse and result in a detection. This happens because each pulse is coherent with the following one. Which detector registers a detection depends on whether the phase value is <NUM> or π. At the receiver the temperature of the PIC is also stabilised to maximize the interference visibility.

Although the explanation refers to fractions of the light pulses, for an application in which the pulses have on average less than one photon per pulse, each photon will either go along the long arm or the short arm. In these cases, a photon detected at any detector cannot lead to a detection event in any other detector.

In some embodiments in which the optical device is used in a quantum communication system, an attenuator reduces the intensity of the light pulses emitted from the optical device. In some embodiments, the intensity is reduced such that the light pulses comprise <NUM> or fewer photons. In some embodiments, the average number of photons per pulse is less than <NUM>.

Alternatively, the PIC <NUM> is configured as other quantum devices. <FIG> and <FIG> show examples of the PIC <NUM> configured as a quantum random number generator (QRNG).

<FIG> shows a PIC <NUM> that comprises: a gain-switched laser <NUM>; a time delay interferometer <NUM>, which is also referred to as an asymmetric Mach Zehnder interferometer (AMZI), and a photodetector <NUM>. The gain-switched laser <NUM> is configured to produce pulses that each have a random phase in relation to one another, as described herein. The pulses are directed to the AMZI <NUM> via light guiding sections (not shown). The AMZI is similar to the MZI described above except that the long arm comprises a delay element. The delayed and non-delayed pulses are caused to interfere in a 2x2 directional coupler in the AMZI and the interfered pulses are directed to the photodetector <NUM> where the intensities of the interfered pulses are converted to a signal. The signal corresponding to the intensities of the interfered pulses has a random value because the phases of the reference and delayed pulses are random. Random numbers may be generated from the random intensities of the interfered pulses. Here the photodetector may be on chip (integrated in the PIC <NUM>) as shown in <FIG> or off chip as shown in <FIG>. If off chip, the optical interface <NUM>, <NUM> of the module <NUM> is used to couple light from the PCI <NUM> to the photodetector provided off the chip. If on chip, the electrical interface of the module is used to read out the measured intensities. The gain switched laser may be on chip (integrated in the PIC <NUM>) as shown in <FIG> or off chip as shown in <FIG>. If off chip, the optical interface <NUM>, <NUM> of the module <NUM> is used to couple light from the gain switched laser provided off the chip to the PIC.

<FIG> show further examples of PICs <NUM>. However, here instead of the AMZI of <FIG>, a multimode interferometer is used. In <FIG> two independent pulsed lasers, laser <NUM><NUM> and laser <NUM><NUM>, are driven at equal repetition rates. The two lasers <NUM>, <NUM> output light at the same intensity and wavelengths. When the repetition rate is low enough, each pulse from the stream of pulses may have a random phase. Furthermore, the streams of pulses from each laser are independent from each other and therefore the pulses from each laser have random phases relative to each other. The pulses from the two lasers temporally overlap and are interfered in multimode interferometer <NUM>, which may be, for example, a 2x2 directional coupler or a beam splitter. The interfered signal is sent to a photodetector <NUM> where the intensity of the interfered pulse is converted to a signal. The signal corresponding to the intensity of the interfered pulse has a random value because the phases of the pulses from laser <NUM><NUM> and laser <NUM><NUM> are random. Random numbers may be generated from the random intensity of the interfered pulses.

Here the photodetector <NUM> may be on chip (integrated in the PIC <NUM>) as shown in <FIG> or off chip as shown in <FIG>. If off chip, the optical interface <NUM>, <NUM> of the module <NUM> is used to couple light from the PCI <NUM> to the photodetector provided off the chip. If on chip, the electrical interface of the module is used to read out the measured intensities. The gain switched lasers <NUM> and <NUM> may be on chip (integrated in the PIC <NUM>) as shown in <FIG> or off chip as shown in <FIG>. If off chip, the optical interface <NUM>, <NUM> of the module <NUM> is used to couple light from the gain switched lasers provided off the chip to the PIC <NUM>.

In another embodiment, the PIC <NUM> is configured for use in quantum computing. <FIG> show such a module <NUM>. The module <NUM> comprises a photonic integrated quantum processor <NUM> which can process single photons produced externally or within the module. Control of the processor <NUM> is possible with the host electronic board. In a photonic quantum processor, a waveguide circuit is fed with single photons. The waveguide circuit is used to route the photons, generate interferences, entanglement and perform computation according to quantum algorithms. Examples of such processors are discussed in <CIT>.

<FIG> shows a diagram where the single photon input to be processed by the processing waveguide circuit <NUM> is generated off the chip and detected after processing via detector array <NUM>. The detector array <NUM> outputs electronic signals which are output from the chip via plug <NUM> of <FIG>.

In <FIG>, the optical input to the quantum processor <NUM> is generated on the chip via photon array <NUM>. Here the processor processes the optical signals and outputs them using optical interface <NUM>, <NUM> (<FIG>) to an external detector.

It is also possible for the light to be processed to be generated off the chip and for the light signals output from the processor to be detected off the chip. Such an arrangement is shown in <FIG>. Here, the light enters the chip and the light exits the chip from the same side. <FIG> shows an example of a fabrication sequence for a device that may be used in the receiver. In this example, the receiver is the type of device described for example in <CIT>. For completeness a description of this device is included below.

In this example, the device is an avalanche photodiode which is part of a photon detection device. The device comprises detection regions with an avalanche multiplication region integrated on a semiconductor substrate.

The device may be fabricated from one or more semiconductor materials, depending on the wavelength of the light which it is designed to detect.

Each detection region comprises an avalanche multiplication region. For each detection region, there is a corresponding contact. In this case the contact is an anode contact, however it will be appreciated that this could alternatively be a cathode contact. Each anode contact is connected to a metal contact region <NUM>.

The basis for the heterostructure is a substrate <NUM>, on which the subsequent layer structure is fabricated. The substrate may be an InP substrate for example.

A uniform heterolayer, the second layer <NUM>, is deposited on said substrate <NUM>. The second layer <NUM> may be an un-doped or lightly doped n-type InGaAs layer for example.

A uniform n+ type heterolayer, the highly doped layer <NUM>, is deposited on said second layer <NUM>. This layer may be a highly doped n-type InP layer for example.

A uniform layer, the first layer <NUM> is deposited on said highly doped layer <NUM>. The first layer <NUM> may be un-doped or lightly doped n-type InP for example.

A cross-sectional view of the device at this stage in fabrication is shown in i.

Areas of highly-doped p-type material <NUM> are incorporated into the first layer <NUM>. The areas may be incorporated by Zn diffusion, or alternatively by gas immersion laser doping or ion implantation for example.

In an embodiment, further areas of highly doped material, forming the guard ring regions <NUM>, are also incorporated into the first layer <NUM>. The guard ring regions may be formed in the same step as the highly doped regions <NUM>, or in a separate step, and by the same method or by a different method.

A cross-sectional view of the device at this stage in fabrication is shown in ii.

In an alternative embodiment, the first <NUM> and second <NUM> layers may be silicon, in which p-type and n-type doping may be achieved using Boron or Phosphorous impurities respectively. The device may alternatively be based on a Silicon - Germanium heterostructure or based on any of the Ill-V class of semiconductors.

In an alternative embodiment, the device comprises highly n-doped regions <NUM> which are incorporated into a moderately doped n-type heterolayer <NUM>, for example by gas immersion laser doping, implantation or diffusion.

The passivation layer <NUM> is deposited on the surface of the device, except for a portion of the surface above each highly doped region <NUM>. The passivation layer <NUM> may be a dielectric, for example silicon nitride or silicon oxide.

A cross-sectional view of the device at this stage in fabrication is shown in iii.

The metal contact region <NUM> corresponding to each detection region is then deposited on the edge portion of the passivation layer and the outer portion of the inner portion of the highly doped region <NUM>. For example, the metal contact region <NUM> may be a Chromium/Gold double layer where the highly doped p-type regions are InP. The metal contact region on the opposite surface of the substrate to the fabricated layers may be a different metal or semiconductor.

A cross-sectional view of the device at this stage in fabrication is shown in iv.

An anti-reflective coating <NUM> may be deposited on the remaining portion of the highly doped regions <NUM>. The material of the anti-reflective coating <NUM> may depend on the wavelength of light intended for the detector. For example, for an InP based detector, silicon nitride with a selected thickness may be used so that the reflection at the surface is minimal.

A cross-sectional view of the device at this stage in fabrication is shown in v.

In the above example, two detectors are fabricated as there are two exposed portions of highly doped regions <NUM>. However, the fabrication can also be used for just one detector.

<FIG> show a schematic illustration of a distributed Bragg reflector (DBR) laser that can be provided on the PIC <NUM> in order to implement the coherent source <NUM> or the gain-switched laser <NUM>. Although <FIG> show a DBR laser, it will be understood that a distributed feedback (DFB) lasers or ridge lasers may alternatively be used.

The lasers may comprise a grating region. The grating region may be separate from the active region or the active region may comprise the grating. A laser where the active region and grating are separate is referred to as DBR (distributed Bragg reflector) laser. A DBR is shown in <FIG>. A laser where the active region comprises the grating is a DFB laser.

A DBR is shown in <FIG> shows a side view of the DBR, and <FIG> shows a cross-section front view. The active area comprises a multi quantum well region (MQW). The MQW region comprises a plurality of quantum well layers. Where the laser is configured for <NUM> operation, the MQW region comprises alternating layers of materials such as, for example: AlInGaAs/InP, AlInGaAs/AlInAs, InGaAsP/InP, InGaAsP/AllnAs or InGaAs/AlInGaAs. All these layers are lattice matched to an InP substrate.

The device comprises a substrate <NUM>. On one surface of the substrate is an n-contact <NUM>. Overlying and in contact with the opposite surface to the substrate <NUM> is the buffer layer <NUM>. Both the substrate <NUM> and the buffer layer <NUM> are n-type layers. Alternatively, the structure can be reversed, such that the substrate <NUM> is a p-type layer. The layers may be n-doped InP. Overlying and in contact with the buffer layer <NUM> is the n-type layer <NUM>. The n-type layer <NUM> may be n-doped InP. Overlying and in contact with a strip of the n-type layer <NUM> is a first waveguide material 410a. Overlying and in contact with the first waveguide material 410a is a MQW layer <NUM>. Overlying and in contact with the MQW layer <NUM> is a second waveguide material 410b. On either side of the strip and overlying and in contact with the n-type material <NUM> is a p-type material <NUM>, which may be p-doped InP. An n-type layer <NUM> is overlying and in contact with the p-type layer <NUM>, and may be n-doped InP. The p-type layer <NUM> is overlying and in contact with the second waveguide layer 410b and the n-type layer <NUM>, and may be p-doped InP. A p-type contact layer <NUM> is overlying and in contact with the p-type layer <NUM>. In one embodiment, the p-type contact layer <NUM> is heavily doped InP, i.e. having a dopant concentration higher than that of layer <NUM>. A p-contact metal <NUM> is overlying and in contact with part of the p-contact layer <NUM>.

As shown in the side view in <FIG> the MQW strip runs along the length of the device. There is a first p-type contact layer <NUM> over part of the MQW strip. On either side of the part of the strip under the first p-contact <NUM> along the direction in which light is emitted, there is a diffraction grating <NUM> in the second waveguide material.

A current is applied between the first p-contact <NUM> and the n-contact <NUM> in order to generate light in the MQW strip of the coherent light source <NUM>. Light generated in the MQW strip of the laser is emitted along the MQW layer. The light is laterally confined by the p-type layer <NUM> and vertically confined by the waveguide layers 410a and b. The light exits the MQW layer through an aperture of the laser.

In all of the above embodiment, techniques of integrating waveguides onto chips and combining such waveguides to form optical components such as those described in <CIT> can be used. Also, the fabrication techniques taught in <CIT> can be used to fabricate any of the above devices.

Claim 1:
A system comprising:
an optical module (<NUM>); and
a host electronic board (<NUM>),
wherein the optical module (<NUM>) is configured to be plugged into the host electronic board (<NUM>), the optical module comprising:
a quantum photonic integrated circuit (<NUM>);
a temperature control element, TCE (<NUM>);
a housing (<NUM>) configured to house said quantum photonic integrated circuit (<NUM>) and said temperature control element (<NUM>); and
a circuit board (<NUM>) configured to route electric signals to and/or from the quantum photonic integrated circuit (<NUM>),
wherein the quantum photonic integrated circuit (<NUM>) is attached to the temperature control element (<NUM>), such that said quantum photonic integrated circuit (<NUM>) is in thermal communication with said temperature control element (<NUM>); and
the temperature control element (<NUM>) is attached directly to the housing (<NUM>), wherein the temperature control element (<NUM>) is attached to the housing (<NUM>) without any other part between them or the temperature control element (<NUM>) is attached to the housing (<NUM>) with a thermal interface material between them, such that said temperature control element (<NUM>) is in direct thermal communication with the housing (<NUM>) such that the housing is configured to dissipate heat generated by the temperature control element,
the host electronic board (<NUM>) comprises a socket (<NUM>), and, wherein the circuit board (<NUM>) comprises a connector (<NUM>) configured to plug into the socket (<NUM>),
the housing further comprising a temperature control element receptacle (102b) that is configured to receive the TCE (<NUM>), the TCE receptacle (102b) being configured to adjust the height of the quantum photonic integrated circuit (<NUM>) relative to the circuit board (<NUM>).