A system and method for polarisation encoding

Abstract

A polarisation encoding system comprising: a photon source configured to output a first pulse with a first phase and a second pulse with a second phase, wherein: the first pulse is outputted before the second pulse; and a polarisation state of the first pulse and a polarisation state of the second pulse are the same; a polarisation adjuster configured to adjust the polarisation state of the first pulse and the polarisation state the second pulse to respectively generate a first polarisation adjusted pulse and a second polarisation adjusted pulse; and a phase interferometer configured to delay the first polarisation adjusted pulse to cause an interference between the first polarisation adjusted pulse and the second polarisation adjusted pulse and generate an interference pulse, wherein a polarisation state of the interference pulse is dependent on a phase difference between the first phase of the first pulse and the second phase of the second pulse.

Description

A SYSTEM AND METHOD FOR POLARISATION ENCODINGField of Invention

[0001] The present application relates to a system and method for polarisation encoding.

[0002] Quantum key distribution (QKD) allows two distant parties to share a key in an information theoretic secure way that is guaranteed by the laws of physics. QKD can be carried out over optical fibres. However, the loss experienced over terrestrial links via optical fibres can limit the range of QKD. By utilising the negligible loss experienced by photons travelling through most of the atmosphere, satellite QKD can overcome the limitations of terrestrial links via optical fibres and enable inter-continental QKD.

[0003] To achieve satellite QKD, a number of techniques have been proposed and demonstrated for producing the required polarisation states for BB84 at sufficiently high speeds i.e. on the order of GHz. Each technique has its own advantages and drawbacks.

[0004] One technique involves polarisation states that can be generated by utilising separate discrete laser diodes corresponding to each polarisation state. Separate discrete laser diodes are simple to implement and has been demonstrated. However, separate discrete laser diodes introduce additional distinguishability between polarisation states, thus resulting in a side-channel that an eavesdropper can exploit.

[0005] In another technique, phase changes from fast phase modulators are used to implement polarisation encoding at required speeds. However, fast phase modulators rely on expensive and bulky components that require high modulation voltages.

[0006] Accordingly, there is a desire for a compact and efficient system for encoding polarisation states that cannot be exploited by an eavesdropper.

[0007] The embodiments described below are not limited to implementations which solve any or all of the disadvantages of the known approaches described above.

[0008] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to determine the scope of the claimed subject matter; variants and alternative features which facilitate the working of the invention and / or serve to achieve a substantiallysimilar technical effect should be considered as falling into the scope of the invention disclosed herein.

[0009] According to a first aspect there is a polarisation encoding system comprising: a photon source configured to output a first pulse with a first phase and a second pulse with a second phase, wherein: the first pulse is outputted before the second pulse; and a polarisation state of the first pulse and a polarisation state of the second pulse are the same; a polarisation adjuster configured to adjust the polarisation state of the first pulse and the polarisation state the second pulse to respectively generate a first polarisation adjusted pulse and a second polarisation adjusted pulse; and a phase interferometer configured to delay the first polarisation adjusted pulse to cause an interference between the first polarisation adjusted pulse and the second polarisation adjusted pulse and generate an interference pulse, wherein a polarisation state of the interference pulse is dependent on a phase difference between the first phase of the first pulse and the second phase of the second pulse.

[0010] According to an embodiment, the photon source comprises: a first laser diode configured to output a phase pulse to set the first phase and the second phase; and a second laser diode configured to output a reference pulse to set an intensity of the first pulse and the second pulse.

[0011] According to an embodiment, the first laser diode and the second laser diode are in an optical locking configuration.

[0012] According to an embodiment, the optical locking configuration comprises a circulator, wherein: a first port of the circulator is connected to the first laser diode; a second port of the circulator is connected to the second laser diode; and the third port of the circulator is an output of the photon source.

[0013] According to an embodiment, the phase pulse is configured to be outputted at a first repetition rate and the reference pulse is configured to be outputted at a second repetition rate, wherein the first repetition rate is a continuous wave with small perturbations in between the second repetition rate.

[0014] According to an embodiment, the photon source comprises a single laser diode configured to output the first pulse and the second pulse.

[0015] According to an embodiment, the polarisation adjuster is a polarisation controller.

[0016] According to an embodiment, the polarisation adjuster configured to adjust the first polarisation adjusted pulse and the second polarisation adjusted pulse such that both the firstpolarisation adjusted pulse and the second polarisation adjusted pulse are superposition of a first polarisation component and a second polarisation component, wherein the two polarisation components are orthogonal pairs.

[0017] According to an embodiment, the phase interferometer comprises: a first leg and a second leg; an input terminal and an output terminal, wherein: the first leg and the second leg are connected in parallel between by the input terminal and the output terminal, wherein the first leg is longer than the second leg and the phase interferometer is configured to transmit the first polarisation adjusted pulse through the first leg to delay the first polarisation adjusted pulse; and the interference of the first polarisation adjusted pulse and the second polarisation adjusted pulse is at the output terminal.

[0018] According to an embodiment, the phase interferometer is configured to transmit, for both polarisation adjusted pulses, the first polarisation component to the first leg and transmit the second polarisation component to the second leg, as a result cause an interference between the first polarisation component of the first polarisation adjusted pulse and the second polarisation components of the second polarisation adjusted pulse.

[0019] According to an embodiment, the phase interferometer is an asymmetric Mach Zehnder interferometer, AMZI.

[0020] According to a second aspect there is a method of using a polarisation encoding system, the method comprising: outputting a first pulse with a first phase and a second pulse with a second phase from a photon source, wherein: the first pulse is outputted before the second pulse; and a polarisation state of the first pulse and a polarisation state of the second pulse are the same; adjusting the polarisation state of the first pulse and the polarisation state the second pulse using a polarisation adjuster to respectively generate a first polarisation adjusted pulse and a second polarisation adjusted pulse; and delaying the first polarisation adjusted pulse to cause an interference between the first polarisation adjusted pulse and the second polarisation adjusted pulse and generate an interference pulse using a phase interferometer, wherein a polarisation state of the interference pulse is dependent on a phase difference between the first phase of the first pulse and the second phase of the second pulse.

[0021] The methods described herein may be performed by software in machine readable form on a tangible storage medium e.g. in the form of a computer program comprising computer program code means adapted to perform all the steps of any of the methods described herein when the program is run on a computer and where the computer program may be embodied on a computer readable medium. Examples of tangible (or non-transitory) storage media include disks, thumb drives, memory cards etc. and do not include propagatedsignals. The software can be suitable for execution on a parallel processor or a serial processor such that the method steps may be carried out in any suitable order, or simultaneously.

[0022] This application acknowledges that firmware and software can be valuable, separately tradable commodities. It is intended to encompass software, which runs on or controls “dumb” or standard hardware, to carry out the desired functions. It is also intended to encompass software which “describes” or defines the configuration of hardware, such as HDL (hardware description language) software, as is used for designing silicon chips, or for configuring universal programmable chips, to carry out desired functions.

[0023] The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.Brief Description of the Drawings

[0024] Embodiments of the invention will be described, by way of example, with reference to the following drawings, in which:

[0025] Figure 1 shows a polarisation encoding system according to an embodiment of the present invention;

[0026] Figure 2 shows a photon source configured to sequentially output a first pulse with a first phase and a second pulse with a second phase;

[0027] Figure 3 shows a polarisation adjuster of the encoding system;

[0028] Figure 4 shows a phase interferometer of the polarisation encoding system; and

[0029] Figure 5 shows a single laser diode polarisation encoding system according to an embodiment of the invention.

[0030] Common reference numerals are used throughout the figures to indicate similar features.Detailed Description

[0031] Embodiments of the present invention are described below by way of example only. These examples represent the best mode of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved. The description sets forth the functions of the example and the sequence of stepsfor constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0032] Quantum key distribution (QKD) is a secure communication method which implements a cryptographic protocol involving components of quantum mechanics for distributing cryptographic keys. It enables two parties to produce a shared random secret key or cryptographic key known only to them, which can then be used to encrypt and decrypt messages. The BB84 QKD protocol is a well-known QKD protocol using photon polarisation states to encode the information. The BB84 QKD protocol uses a set of polarisation bases including at least two pairs of orthogonal photon polarisation bases (e.g. a set of bases including, without limitation, for example a rectilinear photon basis (e.g. vertical (0°) and horizontal (90°) polarisations) and diagonal photon basis (e.g. 45° and 135° polarisations) or the circular basis of left- and right-handedness etc.) In the BB84 protocol, QKD is performed between a sender device or intermediary device (e.g. referred to as Alice) and a receiver or first device (e.g. referred to as Bob or Carol). The sender device and receiver device are connected by a quantum communication channel which allows quantum information (e.g. quantum states) to be transmitted.

[0033] The quantum channel may be, without limitation, for example, an optical fibre or optical free space. Furthermore, the sender device and receiver device also communicate over a non-quantum channel or public classical channel, without limitation, for example a fibre optic channel, telecommunications channel, radio channel, broadcast radio or the internet and / or any other wireless or wired communications channel and the like. The present invention is applicable for use in both the quantum channel and the non-quantum or public classical channel.

[0034] Figure 1 shows a polarisation encoding system 100 according an embodiment of the present invention. The polarisation encoding system 100 comprises a photon source 120; a polarisation adjuster 140 for adjusting polarisation states of pulses, and a phase interferometer 160 for interfering adjacent optical pulses produced by the photon source 120, The polarisation encoding system 100 is capable of producing high-speed polarisation modulated pulses at low modulation voltages.

[0035] The connections between the photon source 120, the polarisation adjuster 140, and the phase interferometer 160 can be optical fibre; however, this is not essential. For example, it is understood that some connections between the photon source 120, the polarisation adjuster 140, and the phase interferometer 160 can be free space. It is also understood that some connections inside the photon source 120, the polarisation adjuster 140, and the phaseinterferometer 160 could be free space. For example, the phase interferometer 160 could be implemented with lenses and mirrors.

[0036] The polarisation encoding system 100 can be a QKD transmitter of a satellite or ground station that is part of a QKD communication system. The polarisation encoded pulse 166 of the polarisation encoding system 100 can be received by a QKD receiver connected by a quantum communication channel to the QKD transmitter.

[0037] The polarisation encoding system 100 can generate all pairs of orthogonal photon polarisation bases, i.e. Vertical (V) and Horizontal (H), Diagonal (D) and Anti-Diagonal (A), Right-Circular / Clockwise (R) and Left-Circular / Anti-Clockwise (L). In the illustrated embodiment, the polarisation encoding system 100 can generate any four polarisation states at any given time, for example V, H, D and A; or D, A, R and L. Accordingly, the polarisation encoded pulse 166 can be any of the above-mentioned polarisation states.

[0038] The polarisation encoding system 100 modulates the phase between consecutive pulses received from laser diode(s). Specifically, the photon source 120 modulates the phase of the consecutive pulses emitted from the photon source 120. Thereafter, the polarisation controller 140 adjusts the polarisation states of consecutive pulses emitted from the photon source 120. Finally, the polarisation adjuster 140 and the phase interferometer 160 of the polarisation encoding system 100 interferes the consecutive pulses received from the laser diodes of the photon source 120. In an embodiment, the polarisation encoding system 100 modulates phase changes between a first pulse 126 with a first phase and a second pulse 127 with a second phase emitted from the photon source 120. The first laser diode 121 and the second laser diode 123 of the polarisation encoding system 100 are in an optical injectionlocking configuration. Alternatively, the polarisation encoding system 100 modulates phase changes between a first pulse and a subsequent second pulse received from the same laser diode.

[0039] The polarisation encoding system 100 comprises a phase interferometer 160 to convert the phase difference between adjacent pulses into corresponding polarisation states. The two adjacent pulses are received by the phase interferometer 160 and caused to interfere with each other to generate an interference pulse by the phase interferometer 160. The interference pulse is the polarisation encoded pulse 166 of the polarisation encoding system 100. The polarisation state of the interference pulse is dependent on the phase difference between the first and second pulses received by the phase interferometer 160.

[0040] Figure 2 shows a photon source 120 configured to sequentially output a first pulse 126 with a first phase and a second pulse 127 with a second phase. The photon source 120 comprises an optical injection locking configuration. The optical injection locking configurationcomprises a first laser diode 121 , a second laser diode 123 and a circulator 125. The circulator 125 comprises three ports: a first port, a second port and a third port. The circulator 125 allows pulses to enter a port and only exit the next port. For example, a pulse entering the first port is output via the second port and a pulse entering the second port is output via the third port.

[0041] The first laser diode 121 and the second laser diode 123 can be laser diodes suitable for emitting classical pulses in the optical injection locking configuration. The first laser diode 121 is a phase preparation laser and the second laser diode 123 is a pulse preparation laser. The first laser diode 121 outputs a phase pulse 122 with a first phase to the first port of the circulator 125 and the second laser diode 123 outputs an injection locked pulse 124 to the second port of the circulator 125.

[0042] The first laser diode 121 is driven with a first electrical signal (not shown) to generate the phase pulse 122. The first electrical signal can be a direct current (DC) signal. According to the illustrated embodiment, the first electrical signal is substantially above a laser threshold of the first laser diode 121 . The first electrical signal comprises perturbations to control the phase of the phase pulse 122.

[0043] The second laser diode 123 is driven with a second electrical signal (not shown). The second electrical signal is a modulating signal. Specifically, the second electrical signal is a gain-switched signal, wherein the electrical voltage of the second electrical signal is modulated to bias the second laser diode 123 to be above or below the laser threshold of the second laser diode 123. Advantageously, the second electrical signal produces narrow optical pulses. The second electrical signal controls the intensity of the first pulse 126 and the second pulse 127 and consequently, when the photon source 120 emits the first pulse 126 and the second pulse 127. However, the phase of the pulse outputted by the second laser diode has a random phase when driven by the second electrical signal. Accordingly, to control the phase of the pulses emitted by the second laser diode 123, the first laser diode 121 and the second laser diode 123 are arranged in the optical injection locking configuration. The optical injection locking configuration enables the photon source 120 to emit controlled and timed injection locking pulses 124 that also exhibit stable wavelength, narrow linewidth and phase encoding properties of the phase pulse 122.

[0044] The phase pulse 122 of the first laser diode 121 enters the first port of the circulator 125 and exits the second port of the circulator 125. The phase pulse 122 is thereafter injected into the second laser diode 123. The second laser diode 123 outputs an injection locked pulse 124. The injection locked pulse 124 has the same phase as the phase pulse 122. The wavelength, linewidth and jitter of the injection locked pulse 124 are also dependent on thephase pulse 122. This is because the injection locked pulse 124 inherits the properties of the phase pulse 122 when the phase pulse 122 is injected into the second laser diode 123. To produce the injection locked pulse 124, the second electrical signal is applied to the second laser diode 123. Thereafter, the injection locked pulse 124 enters the second port of the circulator 125 and then exits out of the third port of the circulator 125 as the first pulse 126..

[0045] The phase pulse 122 can be outputted by the first laser diode 121 according to a predetermined time interval. The second electrical signal and consequently the injection locked pulse 124 can be outputted by the second laser diode 123 according to another predetermined time interval, or the same set interval as the first laser diode 121.

[0046] The phase pulse 122 can be a continuous wave. The phase pulse 122 can be a continuous wave comprising perturbations to adjust the phase of the phase pulse 122. It is noted that this feature is not essential, and that the skilled person would understand that alternative method could be used to modify the phase of the phase pulse 122. The perturbations can be an intensity modulation of the phase pulse 122. The perturbations can be synchronised with the predetermined time interval of the second electrical signal of the second laser diode.

[0047] After the first pulse 126 is generated, repeats the above process with the first laser diode 121 outputting a phase pulse 122 with a second phase to the first port of the circulator 125 and the second laser diode 123 generates a second electrical signal and injects the phase pulse 122 with the second phase with the second laser diode 123 to generate a second injection locked pulse 124. The injection locked pulse 124 has the same phase as the second phase of the phase pulse 122. The second injection locked pulse 124 is sent to the second port of the circulator 125 and outputted at the third port of the circulator 125 as a second pulse 127.

[0048] Optionally, the first pulse 126 and the second pulse 127 can have the same amplitude or intensity. In the illustrated embodiment, <t>i and <t>2 indicate the phase differences 128 between the first pulse 126 and the second pulse 127, which dictates the polarisation state of the polarisation encoded pulse 166 at the output of the phase interferometer 160. At the output of the circulator 125, before entering the polarisation adjuster 140, both the first and second pulses 126, 127 have the same polarisation state, for example, Vertical (V).

[0049] As mentioned above, in the illustrated embodiment, the first laser diode 121 is a phase preparation laser at the first port of the circulator 125 and the second laser diode 123 is a pulse preparation laser at the second port of the circulator 125. The invention is not limited to this particular arrangement.

[0050] The optical injection locking configuration improves the resultant source linewidth, chirp, output power stability and timing jitter of the polarisation encoding system 100. The advantage of the optical injection locking configuration is its compactness, its reduction in modulation voltages and suitability for on-chip integration.

[0051] Advantageously, the second laser diode 123 inherits a phase noise of the first laser diode 121 , wherein the phase noise is related to the laser linewidth. Accordingly, this allows the first laser diode 121 to have a narrow linewidth because it can be driven far above threshold where spontaneous emission (instead of stimulated emission) is negligible.

[0052] Advantageously, the second laser diode 123 inherits the linewidth of the first laser diode 121 . The first laser diode 121 is driven above threshold and therefore the chirp of the second laser diode 123 is improved because chirp is a time varying change in the wavelength which often occurs in direct modulation when a laser is driven above and below threshold. This direct modulation results in a change in carrier density inside the cavity which modifies the refractive index of the gain medium.

[0053] Advantageously, the cavity of the second laser diode 123 acts as a resonant amplifier to first laser diode 121 ; however, without adding any spontaneous emission, so the output power of the entire optical locking configuration is more stable than using the second laser diode 123 alone . This is because the variation in output power of a laser about its average power is caused mostly by spontaneous emission.

[0054] Gain-switched lasers have issues due to the dominance of spontaneous emission when lasing below threshold. This means the carrier and photon density in the cavity of the laser diodes fluctuate randomly, so the laser diodes will start at a random level when each electrical pulse starts. This then means the time it takes to build up a positive gain for an optical pulse will also vary randomly. Advantageously, the optical injection locking configuration balances out these fluctuations which reduces the jitter.

[0055] Figure 3 shows a polarisation adjuster 140 of the polarisation encoding system 100. In the illustrated embodiment, the polarisation adjuster 140 comprises a polarisation controller 142. This is not essential, any other device capable of adjusting the polarisation state of the pulses 126, 127 be used.

[0056] The polarisation controller 142 adjusts the polarisation state of the pulses 126, 127 outputted by the circulator 125 to generate a polarisation adjusted pulse 144. In the illustrated embodiment, the polarisation controller 142 adjusts the polarisation state of the first and second pulses 126, 127 by 45 degrees. Accordingly, when a vertically polarised pulse is received by the polarisation controller 142, the polarisation controller 142 outputs a 45degrees diagonally polarised pulse 144 comprising horizontal and vertical components of equal intensity. It is not essential that the polarisation controller 142 adjusts the polarisation state of the first and second pulses 126, 127 by 45 degrees. The polarisation controller 142 can adjust the polarisation state of the first and second pulses by an angle, such that the polarisation adjusted pulse 144 is a superposition of two polarisation components. Optionally, the polarisation adjusted pulse 144 can be a superposition of two polarisation components that are orthogonal pairs.

[0057] The polarisation controller 142 can be an electric polarisation controller, such as a microprocessor-based polarisation controller, or a non-electric polarisation controller, such as a birefringence induced fibre bending polarisation controller.

[0058] Figure 4 shows a phase interferometer 160 of the polarisation encoding system 100. The phase interferometer 160 comprises an asymmetric Mach-Zehnder interferometer (AMZI) 161 , wherein the AMZI 161 comprises an input terminal 162, a first leg 163, a second leg 164, and an output terminal 165.

[0059] It is noted that the illustrated embodiments shows an AMZI; however, the skilled person would understand that other phase interferometers 160 could be used to interfere the consecutive pulses received from the photon source 120. For example, the skilled person could use a Michelson interferometer.

[0060] The AMZI 161 comprises an input terminal 162 and an output terminal 165. The input terminal 162 and the output terminal 165 are connected by the first leg 163 and the second leg 164, wherein the first leg 163 and second leg 164 are connected in parallel between the input terminal 162 and the output terminal 165.

[0061] The input terminal 162 can be an input power beam splitter (PBS) configured to split a pulse into two pulses. The PBS at the input terminal 162 splits the vertical component and the horizontal components of the diagonally polarised polarisation adjusted pulse 144 into two signals: a first split pulse and a second split pulse. The first split pulse has a vertical polarisation state and the second split pulse has a horizontal polarisation states. Accordingly, the two split pulses have polarisation states that are orthogonal to each other.

[0062] It is not essential that the input signal of the AMZI 161 is split into horizontal and vertical pulses. Equally, for example, the AMZI 161 could split the pulses into diagonal / anti- diagonal or right-circular / left-circular pairs, depending on the required output pulse of the polarisation encoder 144 and depending on the polarisation states of the polarisation adjusted pulse 144.

[0063] The two output pulses split from the input power beam splitter are respectively transmitted to a first leg 163 and second leg 164 of the AMZI 161 . In other words, the PBS at the input terminal 162 thereafter transmits the vertically polarised first split signal to the first arm 163 of the AMZI 161 and the horizontally polarised second split signal to the second arm 164 of the AMZI 161.

[0064] The first arm 163 causes a delay in the first split signal. In the illustrated embodiment, the first arm 163 and the second arm 164 can be optical fibres, wherein the first arm 163 induces the delay in the first split signal due to its longer length relative to the second arm 164. The delayed vertical signal and horizontal signal is recombined at the PBS at the output terminal 165. The delay of the vertical signal causes an interference between the delayed vertical signal and the horizontal signal at the output terminal 165 of the AMZI 161 . Specifically, the delay induced by the first leg 163 causes an interference between the horizontal component of the first pulse 126 and the vertical component of the second pulse 127.

[0065] It is noted that it is not essential that an AMZI is used to create the interference between horizontal component of the first pulse 126 and the vertical component of the second pulse 127. The skilled person would understand that other phase interferometers 160 are capable of generating two split pulses that have polarisation states that are orthogonal to each other.

[0066] The output terminal 165 can be an output PBS configured to combine the first split pulse and the second split pulse transmitted through the first leg 163 and the second leg 164 of the AMZI 161 . The interfered pulse at the output terminal 165 can be output directly from the AMZI and act as the polarisation encoded pulse 166 of the output pulse of the polarisation encoding system 100. The interfered pulse of the AMZI is defined as follows | I|J> (| H) +ei( i)| V))

[0067] Where (|>i is the phase difference between consecutive pairs of pulses, i.e. the first pulse 126 and the second pulse 127. Choosing (|>i to be any of 0, T 2, -T 2 or IT allows the preparation of any of the 4 states given below: -

[0068] Advantageously, the phase interferometer 160 according to the present invention, in particular the AMZI, maintains speed of changes in polarisation states whilst utilising lower modulation voltages compared to separate discrete laser diodes and fast phase modulators.

[0069] The illustrated embodiment demonstrates that separating individual pulses into the orthogonal components of |H> and |V> and then delaying one of the orthogonal components with respect to the other, the AMZI can prepare any one of the four states (D, A, R and L) given above. Optionally, the inclusion of a quarter wave plate at a 45 degrees angle with respect to the fast axis of the interfered pulse at the output terminal 165 of the AMZI 161 will then transform these four states into the standard H, V, D and A states used in BB84 protocol. The quarter wave plate can be realised by another polarisation controller or a free space lens wave plate.

[0070] Figure 5 shows a single laser diode polarisation encoding system 200 according to an embodiment of the invention. The single laser diode polarisation encoding system 200 comprises the same features as the two-laser diode polarisation encoding system 100 of Figure 1 , except the photon source. The single laser diode polarisation encoding system 200 comprises a single laser photon source 220. The single laser photon source 220 comprises a single laser diode 221 . The single laser diode 221 generates both the first pulse 226 and the second pulse 227 of the photon source 220. The single laser diode 221 sends a plurality of pulses with the same polarisation state and different phases to encode polarisation states at the interfered pulse of the AMZI 161 .

[0071] The embodiments described above are fully automatic. In some examples a user or operator of the system may manually instruct some steps of the method to be carried out.

[0072] In the described embodiments of the invention parts of the system may be implemented as a form of a computing and / or electronic device. Such a device may comprise one or more processors which may be microprocessors, controllers or any other suitable type of processors for processing computer executable instructions to control the operation of the device in order to gather and record routing information. In some examples, for example where a system on a chip architecture is used, the processors may include one or more fixed function blocks (also referred to as accelerators) which implement a part of the method in hardware (rather than software or firmware). Platform software comprising an operating system or any other suitable platform software may be provided at the computing-based device to enable application software to be executed on the device.

[0073] Various functions described herein can be implemented in hardware, software, or any combination thereof. If implemented in software, the functions can be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer- readable media may include, for example, computer-readable storage media. Computer- readable storage media may include volatile or non-volatile, removable or non-removable media implemented in any method or technology for storage of information such as computerreadable instructions, data structures, program modules or other data. A computer-readable storage media can be any available storage media that may be accessed by a computer. By way of example, and not limitation, such computer-readable storage media may comprise RAM, ROM, EEPROM, flash memory or other memory devices, CD-ROM or other optical disc storage, magnetic disc storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disc and disk, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu- ray disc (BD). Further, a propagated signal is not included within the scope of computer- readable storage media. Computer-readable media also includes communication media including any medium that facilitates transfer of a computer program from one place to another. A connection, for instance, can be a communication medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fibre optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of communication medium. Combinations of the above should also be included within the scope of computer-readable media.

[0074] Alternatively, or in addition, the functionality described herein can be performed, at least in part, by one or more hardware logic components. For example, and without limitation, hardware logic components that can be used may include Field-programmable Gate Arrays (FPGAs), Program-specific Integrated Circuits (ASICs), Program-specific Standard Products (ASSPs), System-on-a-chip systems (SOCs), Complex Programmable Logic Devices (CPLDs), etc.

[0075] Although illustrated as a single system, it is to be understood that a computing device may be a distributed system. Thus, for instance, several devices may be in communication by way of a network connection and may collectively perform tasks described as being performed by the computing device. Although illustrated as a local device it will be appreciated that the computing device may be located remotely and accessed via a network or other communication link (for example using a communication interface).

[0076] The term 'computer' is used herein to refer to any device with processing capability such that it can execute instructions. Those skilled in the art will realise that such processing capabilities are incorporated into many different devices and therefore the term 'computer' includes PCs, servers, mobile telephones, personal digital assistants and many other devices.

[0077] Those skilled in the art will realise that storage devices utilised to store program instructions can be distributed across a network. For example, a remote computer may store an example of the process described as software. A local or terminal computer may accessthe remote computer and download a part or all of the software to run the program. Alternatively, the local computer may download pieces of the software as needed, or execute some software instructions at the local terminal and some at the remote computer (or computer network). Those skilled in the art will also realise that by utilising conventional techniques known to those skilled in the art that all, or a portion of the software instructions may be carried out by a dedicated circuit, such as a DSP, programmable logic array, or the like.

[0078] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. Variants should be considered to be included into the scope of the invention.

[0079] Any reference to 'an' item refers to one or more of those items. The term 'comprising' is used herein to mean including the method steps or elements identified, but that such steps or elements do not comprise an exclusive list and a method or apparatus may contain additional steps or elements.

[0080] As used herein, the terms "component" and "system" are intended to encompass computer-readable data storage that is configured with computer-executable instructions that cause certain functionality to be performed when executed by a processor. The computerexecutable instructions may include a routine, a function, or the like. It is also to be understood that a component or system may be localized on a single device or distributed across several devices.

[0081] Further, as used herein, the term "exemplary" is intended to mean "serving as an illustration or example of something".

[0082] Further, to the extent that the term "includes" is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term "comprising" as "comprising" is interpreted when employed as a transitional word in a claim.

[0083] The figures illustrate exemplary methods. While the methods are shown and described as being a series of acts that are performed in a particular sequence, it is to be understood and appreciated that the methods are not limited by the order of the sequence. For example, some acts can occur in a different order than what is described herein. In addition, an act can occur concurrently with another act. Further, in some instances, not all acts may be required to implement a method described herein.

[0084] Moreover, the acts described herein may comprise computer-executable instructions that can be implemented by one or more processors and / or stored on a computer-readable medium or media. The computer-executable instructions can include routines, sub-routines, programs, threads of execution, and / or the like. Still further, results of acts of the methods can be stored in a computer-readable medium, displayed on a display device, and / or the like.

[0085] The order of the steps of the methods described herein is exemplary, but the steps may be carried out in any suitable order, or simultaneously where appropriate. Additionally, steps may be added or substituted in, or individual steps may be deleted from any of the methods without departing from the scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

[0086] It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable modification and alteration of the above devices or methods for purposes of describing the aforementioned aspects, but one of ordinary skill in the art can recognize that many further modifications and permutations of various aspects are possible. Accordingly, the described aspects are intended to embrace all such alterations, modifications, and variations that fall within the scope of the appended claims.

Claims

Claims1. A polarisation encoding system comprising: a photon source configured to output a first pulse with a first phase and a second pulse with a second phase, wherein: the first pulse is outputted before the second pulse; and a polarisation state of the first pulse and a polarisation state of the second pulse are the same; a polarisation adjuster configured to adjust the polarisation state of the first pulse and the polarisation state the second pulse to respectively generate a first polarisation adjusted pulse and a second polarisation adjusted pulse; and a phase interferometer configured to delay the first polarisation adjusted pulse to cause an interference between the first polarisation adjusted pulse and the second polarisation adjusted pulse and generate an interference pulse, wherein a polarisation state of the interference pulse is dependent on a phase difference between the first phase of the first pulse and the second phase of the second pulse.

2. A system according to claim 1 , wherein the photon source comprises: a first laser diode configured to be driven by a first electrical signal above a laser threshold of the first laser diode to output a phase pulse, wherein the phase pulse sets the phase of the first phase and the second phase; and a second laser diode configured to be driven by a second electrical signal modulated above or below a laser threshold of the second laser diode to output an injection locked pulse, wherein the second electrical signal sets an intensity of the first pulse and the second pulse.

3. A system according to claim 2, wherein the first laser diode and the second laser diode are in an optical locking configuration.

4. A system according to claim 3, wherein the optical locking configuration comprises a circulator, wherein: a first port of the circulator is connected to the first laser diode; a second port of the circulator is connected to the second laser diode; and the third port of the circulator is an output of the photon source.

5. A system according to claims 2 - 4, wherein the phase pulse is configured to be outputted at a first repetition rate and the second electrical signal is configured to be outputted at asecond repetition rate, wherein the first repetition rate is a continuous wave with small perturbations in between the second repetition rate.

6. A system according to claim 1 , wherein the photon source comprises a single laser diode configured to output the first pulse and the second pulse.

7. A system according to any preceding claim, wherein the polarisation adjuster is a polarisation controller.

8. A system according to any preceding claim, wherein the polarisation adjuster configured to adjust the first polarisation adjusted pulse and the second polarisation adjusted pulse such that both the first polarisation adjusted pulse and the second polarisation adjusted pulse are superposition of a first polarisation component and a second polarisation component, wherein the two polarisation components are orthogonal pairs.

9. A system according to any preceding claim, wherein the phase interferometer comprises: a first leg and a second leg; an input terminal and an output terminal, wherein: the first leg and the second leg are connected in parallel between by the input terminal and the output terminal, wherein the first leg is longer than the second leg and the phase interferometer is configured to transmit the first polarisation adjusted pulse through the first leg to delay the first polarisation adjusted pulse; and the interference of the first polarisation adjusted pulse and the second polarisation adjusted pulse is at the output terminal.

10. A system according to claim 9, when dependent on claim 8, wherein the phase interferometer is configured to transmit, for both polarisation adjusted pulses, the first polarisation component to the first leg and transmit the second polarisation component to the second leg, as a result cause an interference between the first polarisation component of the first polarisation adjusted pulse and the second polarisation component of the second polarisation adjusted pulse.11 . A system according to claim 9 or 10, wherein the phase interferometer is an asymmetric Mach Zehnder interferometer, AMZI.

12. A method of using a polarisation encoding system, the method comprising:outputting a first pulse with a first phase and a second pulse with a second phase from a photon source, wherein: the first pulse is outputted before the second pulse; and a polarisation state of the first pulse and a polarisation state of the second pulse are the same; adjusting the polarisation state of the first pulse and the polarisation state the second pulse using a polarisation adjuster to respectively generate a first polarisation adjusted pulse and a second polarisation adjusted pulse; and delaying the first polarisation adjusted pulse to cause an interference between the first polarisation adjusted pulse and the second polarisation adjusted pulse and generate an interference pulse using a phase interferometer, wherein a polarisation state of the interference pulse is dependent on a phase difference between the first phase of the first pulse and the second phase of the second pulse.

13. A method according to claim 12, wherein outputting a first pulse with a first phase and a second pulse comprises: outputting, by a first laser diode driven by a first electrical signal above a laser threshold of the first laser diode to output a phase pulse, the phase pulse to set the first phase and the second phase; and outputting, by a second laser diode driven by a second electrical signal modulated above or below a laser threshold of the second laser diode to output an injection locked pulse, the injection locked pulse, wherein the second electrical signal sets an intensity of the first pulse and the second pulse.

14. A method according to claim 13, wherein the first laser diode and the second laser diode are in an optical locking configuration.

15. A method according to claim 14, wherein the optical locking configuration comprises a circulator, wherein: a first port of the circulator is connected to the first laser diode; a second port of the circulator is connected to the second laser diode; and the third port of the circulator is an output of the photon source.

16. A method according to claims 13 - 15, wherein the phase pulse is outputted at a first repetition rate and the second electrical signal is outputted at a second repetition rate,wherein the first repetition rate is a continuous wave with small perturbations in between the second repetition rate.

17. A method according to claim 12, wherein outputting a first pulse with a first phase and a second pulse comprises a single laser diode outputting the first pulse and the second pulse.

18. A method according to any of claims 12 - 17, wherein the polarisation adjuster is a polarisation controller.

19. A method according to any of claims 12 - 18, wherein the polarisation adjuster adjusts the first polarisation adjusted pulse and the second polarisation adjusted pulse such that both the first polarisation adjusted pulse and the second polarisation adjusted pulse are superposition of a first polarisation component and a second polarisation component, wherein the two polarisation components are orthogonal pairs.

20. A method according to any preceding claim, wherein a phase interferometer comprises: a first leg and a second leg; an input terminal and an output terminal, wherein: the first leg and the second leg are connected in parallel between by the input terminal and the output terminal, wherein the first leg is longer than the second leg and the phase interferometer is configured to transmit the first polarisation adjusted pulse through the first leg to delay the first polarisation adjusted pulse; and the interference of the first polarisation adjusted pulse and the second polarisation adjusted pulse is at the output terminal.21 . A method according to claim 20, when dependent on claim 19, wherein the phase interferometer is configured to transmit, for both polarisation adjusted pulses, the first polarisation component to the first leg and transmit the second polarisation component to the second leg, as a result cause an interference between the first polarisation component of the first polarisation adjusted pulse and the second polarisation component of the second polarisation adjusted pulse.

22. A system according to claims 20 or 21 , wherein the phase interferometer is an asymmetric Mach Zehnder interferometer, AMZI.

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