WO2022084678A1

WO2022084678A1 - Aerial platforms based quantum communication

Info

Publication number: WO2022084678A1
Application number: PCT/GB2021/052723
Authority: WO
Inventors: David Grace; Yi Chu; Rupesh Kumar Parapatil SUBRAMANIAN; Ross DONALDSON
Original assignee: University Of York; Heriot-Watt University
Priority date: 2020-10-23
Filing date: 2021-10-21
Publication date: 2022-04-28
Also published as: GB202016889D0

Abstract

Methods, aerial platforms, a node of a communication network, and a communication network are disclosed. A method comprises the steps of sending first location data, indicative of a location of an aerial platform, from the aerial platform to at least one further node of a communication network via a first, non-optical, wireless communication channel, responsive to the first location data and to further location data indicative of a location of a further node, directing electromagnetic radiation from the aerial platform at the further node and/or receiving electromagnetic radiation at the aerial platform originating from the further node, and determining cryptographic key information responsive to at least one measurable quantum state of at least one photon of the electromagnetic radiation directed from and/or received by the aerial platform.

Description

SECURE COMMUNICATION The present invention relates to methods and systems for determining cryptographic key information at aerial platforms. In particular, but not exclusively, the present invention relates to quantum key distribution between an aerial platform and a further node of a communication network whereby a non-optical wireless communication channel is used to send location data from the aerial platform to the further node. There are multiple well-known mechanisms for communicating information from a first location to a second location that is remote from the first location. One of these mechanisms may be referred to as electronic communication whereby the exchange of information takes place at least partly via an electronic medium and/or an optical medium. For example, electronic communication may involve the transfer of signs, signals, writing, images, sounds, data, or intelligence of any nature transmitted in whole or in part by a wire, radio, electromagnetic, photo-electronic, or photo-optical system. Electronic communication is now ubiquitous in day to day life and the security of these electronic communications is thus often considered essential. This is especially the case for establishments such as financial institutions, online retailers, government bodies and the like where there is a strict requirement that the information being exchanged remains confidential. Typically, communications are secured by encrypting the information to be exchanged via a mathematical algorithm. For example, RSA is a well-known algorithm used by modern computers to encrypt and decrypt messages. Ideally, only the sender and receiver should be able to decrypt the information via such an algorithm. However, these algorithms are not provably secure, and the underlying security provided by them is based on mathematical problems which are believed to be difficult to solve in a timely manner. For example, the RSA algorithm assumes that finding the factors of a large composite number is difficult: when the factors are prime numbers, this problem is called prime factorisation. The concern is that as computers become more and more powerful, and especially if quantum computers become practical to implement, the time required to solve these mathematical problems may be vastly reduced leaving these algorithms essentially redundant for securing communications. Other methods for encrypting information to be exchanged have therefore been developed. One of these methods may be referred to generally as quantum communication. Quantum communication relies on the exchange of quantum states which exhibits phenomenon such as super-position; vacuum noise, quantum entanglement, etc. For example, one example of quantum communication is known as Quantum Key Distribution (QKD).

QKD is a cryptographic technique which offers theoretically secured cryptographic key delivery between two parties, typically named Alice (the transmitter) and Bob (the receiver). The security of QKD is based on the laws of quantum physics. The key shared via QKD is generated by quantum randomness, rather than an algorithm, meaning the shared key is robust to future advances in decryption algorithms and attacks from quantum computers. QKD relies on quantum superposition, quantum uncertainties, and/or quantum entanglement for secure key distribution/generation. Utilising QKD also enables identification of eavesdropper activity in a communication channel. These benefits make QKD an attractive cryptographic technique.

QKD over optical fibres has been an area of active research for decades. However, in order to reach longer transmission distances, amplification of the quantum state is not a viable solution. This is because deterministic amplification of quantum states induces noise, which swamps the encoding. There is a known loss per kilometre for currently deployed optical fibre (approximately 0.2 dB/km), which limits the achievable distance of QKD in optical-fibre. There are no efficient mechanisms for amplifying or repeating the quantum signal. Thus, due to the exponential loss of optical fibres with distance, long-distance secure key distribution over optical fibre becomes inefficient. Multi-hop links based on relay nodes can help overcome this limitation, however, additional security assumptions are required, such as that the relay nodes are trusted.

QKD via satellite has been considered as an alternative to deliver keys over large distances by utilising the free space quantum channel. Satellites located at less than 2000 km low earth orbit (LEO) provide much less attenuation than fibre at the same distance, thereby achieving higher key rate. However, when applying QKD from satellites, the primary source of attenuation is the beam divergence. Diffraction limited narrow beams are required to overcome the attenuation due to beam spreading over distance. In this case, accurate pointing, acquisition and tracking (PAT), at grad precision, is required to ensure that the ground beam footprint can cover the receiver telescope. In this case equipment such as InGaAs cameras, quadrant detectors and high precision gimbals are used at the transmitter to ensure the accuracy of the PAT. This conventional closed-loop PAT method requires beacons equipped by both transmitter and receiver to illuminate each other's camera sensors to adjust the pointing directions according to the position of the light spots detected by the quadrant position detectors. To help ensure the attenuation of the signal is not too great, a satellite will require has to be larger to capture more of the divergent beam, leading to high costs for the ground segment. The high costs of satellite operations and difficulties of equipment maintenance in space will also always make such implementations difficult. Another method of exploiting the free space quantum channel is QKD via aerial platforms. Aerial platforms include low altitude drones, medium altitude aircraft and also high altitude platforms (HAPs). HAPs are located in the stratosphere, typically 17-22km altitude and can be delivered using balloons, airships or fixed wing aircraft. HAPs are able to continuously cruise at the stratosphere of about 20 km altitude for several months. The renewable energy source equipped by HAPs can harvest energy to power the aircraft and the payload. They can be deployed rapidly and relocate globally according to their applications and tasks. There are two major types of HAPs, heavier-than-air (mainly fixed-wing HAPs) and lighter-than-air (free- floating balloons and airships) aircraft. Exploiting QKD from aerial platforms has not been widely considered. Compared with the predictable trajectory of satellites, the movements of aerial platforms are more random (because of the wind) and generate high angular speed and acceleration to the beam which brings more challenges to PAT of the optical system. Also, when considering QKD from aerial platforms, the conventional beacon-based PAT system adds complexity and weight to the payload carried by the aerial platform. At the moment many aerial platforms have limited payload capacity, for example most drones can only carry less than 500g payload, which makes weight management extremely important when applying QKD applications from aerial platforms. There are also associated costs and health and safety risks associated with using intense beacon light on the aerial platforms. It is an aim of the present invention to at least partly mitigate at least one of the above- mentioned problems. It is an aim of certain embodiments of the present invention to help provide a beaconless open-loop PAT system to facilitate quantum communications from and/or to aerial platforms. It is an aim of certain embodiments of the present invention to help provide free space quantum communications using beams of variable or fixed divergence, which do not require closed- loop tracking. It is an aim of certain embodiments of the present invention to help provide free space quantum communications using diverged beams from aerial platforms, e.g. high altitude platforms or the like. It is an aim of certain embodiments of the present invention to heip provide free space quantum communications using diverged beams using a cellular network to assist pointing, acquisition and tracking.

It is an aim of certain embodiments of the present invention to help implement a quantum communications link from aerial platforms without the need for fine PAT using a beacon, thereby significantly simplifying and speeding up the PAT process, as well as reducing costs and health and safety risks associated with intense beacon light

It is an aim of certain embodiments of the present invention to heip reduce the complexity of using quantum communications from aerial platforms, enable faster PAT for quantum communications from aerial platforms, reduce the need for mechanical agility of the PAT mechanism to deal with altitude changes in the aerial platform, reduce general payload size for quantum communications free-space delivery and reduce health and safety risks from intense laser hazard.

It is an aim of certain embodiments of the present invention to help provide quantum communication from aerial platforms with much lower cost than that is achievable with conventional techniques.

According to a first aspect of the present invention there is provided a method for providing cryptographic key information at an aerial platform via communication of at least one photon of electromagnetic radiation having at least one measurable quantum state, the method comprising the steps of sending first location data, indicative of a location of an aerial platform, from the aerial platform to at least one further node of a communication network via a first, non-optical, wireless communication channel, responsive to the first location data and to further location data indicative of a location of a further node, directing electromagnetic radiation from the aerial platform at the further node and/or receiving electromagnetic radiation at the aerial platform originating from the further node and determining cryptographic key information responsive to at least one measurable quantum state of at least one photon of the electromagnetic radiation directed from and/or received by the aerial platform.

Aptly, the method further comprises orienting a pointing axis of at least one optical element supported by the aerial platform based on the first and further location data without reference to one or more beacon signals. Aptly the method further comprises orienting the pointing axis with a pointing precision of up to 10 milliradians, optionally orienting the pointing axis with a pointing precision between 0.001 milliradians and 5 miliiradians, and optionally orienting the pointing axis with a pointing precision between 0.05 miliiradians and 1 milliradian.

Aptly the method further comprises directing electromagnetic radiation from the aerial platform in a direction towards the further node via orienting a pointing axis of at least one optical element supported by the aerial platform in a direction towards the further node, and directing electromagnetic radiation emitted from at least one source of electromagnetic radiation supported by the aerial platform towards the further node along the pointing axis.

Aptly the method further comprises receiving electromagnetic radiation at the aerial platform originating from the further node via orienting a pointing axis of at least one optical element supported by the aerial platform in a direction towards the further node, and receiving electromagnetic radiation emitted from at least one source of electromagnetic radiation supported by the further node along the pointing axis.

Aptly the method further comprises supporting the optical element on a two-axis or three-axis gimbal, and orienting the pointing axis of the optical element via controlling respective axes of the gimbal responsive to the first and further location data.

Aptly the optical element is a transmitting and/or receiving telescope and optionally includes one or more mirrors and/or one or more lenses.

Aptly the method further comprises receiving the further location data at the aerial platform via a further, non-optical, wireless communication channel or storing the further location data in at least one memory associated 'with the aerial platform prior to the aerial platform attaining an altitude of greater than 500 metres.

Aptly the method further comprises prior to sending the first location data, establishing at least one non-optical wireless communication link between the aerial platform and the further node.

Aptly the non-optical communication link is provided by a common bi-directional, non-optical communication link supporting both the first and further non-optical communication channels, and the method further comprises sending the first location data in a first link direction and receiving the further location data in a remaining link direction. Aptly the method further comprises sending the first location data repeatedly or continuously and optionally in real time.

Aptly the method further comprises sending the first location data via a direct RF or mm-Wave or 3G or 4G or 5G communication channel.

Aptly the method further comprises receiving the further location data repeatedly or continuously and optionally in real time.

Aptly the method further comprises receiving the further location data via a direct RF or mm- Wave or 3G or 4G or 5G communication channel.

Aptly the method further comprises dynamically selecting a degree of beam divergence associated with electromagnetic radiation directed from the aerial platform towards the further node via at least one further optical element on the aerial platform, optionally wherein the further optical element is an adjustable beam expander.

Aptly the method further comprises selectively increasing beam divergence to assist in establishing an optical communication link or decreasing beam divergence to increase a rate for determining cryptographic key information.

Aptly the method further comprises dynamically selecting a degree of beam divergence between 0.1 and 10 milliradians.

Aptly the method further comprises determining the first location data via a GPS chip and/or DGPS chip and optionally an altimeter associated with the aerial platform.

Aptly the method further comprises responsive to electromagnetic directed and/or received by the aerial platform, establishing at least one optical wireless communication link between the aerial platform and the further node, the optical communication link supporting a first optical channel for sending at least one photon of electromagnetic radiation having at least one measurable quantum state and optionally a further optical channel for sending classical optical signals.

Aptly the method further comprises utilising a discrete-variable QKD protocol to implement discretised encoding of cryptographic key information in a quantum state of at least one photon of electromagnetic radiation directed and/or received by the aerial platform. Aptly the method further comprises utilising a continuous-variable QKD protocol to implement discretised encoding of cryptographic key information in a quantum state of at least one photon of electromagnetic radiation directed and/or received by the aerial platform.

Aptly the method further comprises responsive to the first location data and to further location data indicative of a plurality of respective locations of a plurality of further nodes, directing a respective beam of electromagnetic radiation from a plurality of beams of electromagnetic radiation from the aerial platform in a direction towards a respective further node.

Aptly each beam of electromagnetic radiation directed towards each respective further node provides a respective cell coverage area, the method further comprising directing the plurality of beams of electromagnetic radiation to provide a substantially contiguous cell coverage area.

Aptly the method further comprises directing a plurality of beams of electromagnetic radiation in a direction towards a further node; whereby each beam of electromagnetic radiation has an angular beam direction out of alignment with the remaining beams of electromagnetic radiation to thereby provide a substantially contiguous cell coverage area.

Aptly the aerial platform is a low altitude drone or a medium altitude aircraft or a high altitude platform.

According to a second aspect of the present invention there is provided a method for providing cryptographic key information at a further node of a communication network via communication of at least one photon of electromagnetic radiation having at least one measurable quantum state, the method comprising the steps of receiving first location data, indicative of a location of an aerial platform, at a further node of a communication network from the aerial platform via a first, non-optical, wireless communication channel, responsive to the first location data and to further location data indicative of a location of the further node, directing electromagnetic radiation from the further node at the aerial platform and/or receiving electromagnetic radiation at the further node originating from the aerial platform, and determining cryptographic key information responsive to at least one measurable quantum state of at least one photon of the electromagnetic radiation directed from and/or received by the further node.

Aptly the further node is remote from the aerial platform and is a stationary or mobile optical ground station. Aptly, the method further comprises orienting a pointing axis of at least one optical element supported by the further node based on the first and further location data without reference to one or more beacon signals.

Aptly the method further comprises orienting the pointing axis with a pointing precision of up to 10 milliradians, optionally orienting the pointing axis with a pointing precision between 0.001 milliradians and 5 milliradians, and optionally orienting the pointing axis with a pointing precision between 0.05 milliradians and 1 milliradian.

Aptly the method further comprises receiving the first location data repeatedly or continuously and optionally in real time.

Aptly the method further comprises receiving the first location data via a direct RF or mm- Wave or 3G or 4G or 5G communication channel.

Aptly the method further comprises dynamically selecting a degree of beam divergence associated with electromagnetic radiation directed from the further node towards the aerial platform via at least one further optical element on the further node, optionally wherein the further optical element is an adjustable beam expander.

According to a third aspect of the present invention there is provided an aerial platform configured to perform the method according to the first aspect of the present invention. Aptly the aerial platform Is a low altitude drone or a medium altitude aircraft or a high altitude platform.

Aptly the high altitude platform is a balloon or airship or fixed wing aircraft located in the stratosphere at an altitude between about around 17 and 22km.

According to a fourth aspect of the present invention there is provided a further node of a communication network configured to perform the method according to the second aspect of the present invention.

Aptly the further node is a stationary or mobile optical ground station.

According to a fifth aspect of the present invention there is provided an aerial platform, comprising at least one non-optical transmitter and/or transceiver element for transmitting non- optical wireless communication signals to a further node of a communication network, at least one processing element for determining location data, and wherein the processing element optionally comprises a GPS chip and/or DGPS chip, and wherein the processing element optionally further comprises an altimeter, at least one optical element for directing electromagnetic radiation at the further node and/or receiving electromagnetic radiation from the further node, at least one two-axis or three-axis gimbal for supporting the optical element, at least one source and/or at least one detector of electromagnetic radiation, and at least one further optical element for encoding and/or decoding at least one measurable quantum state on at least one photon of electromagnetic radiation.

Aptly the aerial platform further comprises at least one element for generating random numbers, wherein optionally the element for generating random numbers is a quantum random number generator.

According to a sixth aspect of the present invention there is provided a communication network comprising an aerial platform, and at least one further node of a communication network remote from the aerial platform; wherein the aerial platform is configured to send first location data, indicative of a location of the aerial platform, to the further node via a first, non-optical, wireless communication channel, direct electromagnetic radiation at the further node and/or receive electromagnetic radiation originating from the further node responsive to the first location data and to further location data indicative of a location of a further node, and determine cryptographic key information responsive to at least one measurable quantum state ot at least one photon of the electromagnetic radiation directed from and/or received by the aerial platform.

According to a seventh aspect of the present invention there is provided a method of quantum key distribution, the method comprising the step of exchanging information between an aerial platform and at least one further node of a communication network via at least one non-optical communication channel and at least one optical communication channel to thereby provide cryptographic key information at the aerial platform.

Certain embodiments of the present invention help to facilitate quantum communications from and/or to aerial platforms without the need for using beacons in the PAT system.

Certain embodiments of the present invention help enable accurate pointing of receiving/transmitting telescopes supported on aerial platforms based only on location data (e.g. DGPS co-ordinates).

Certain embodiments of the present invention help provide the use of beams of variable or fixed divergence during quantum communications from and/or to aerial platforms.

Certain embodiments of the present invention help provide free space quantum communications using diverged beams from aerial platforms, e.g. high altitude platforms or the like.

Certain embodiments of the present invention help provide quantum communications from and/or to aerial platforms 'which use a non-optical 'wireless communication channel to transmit location data concerning the location of the aerial platform.

Certain embodiments of the present invention help provide free space quantum communications from and/or to an aerial platform and use a cellular network to assist in pointing, acquisition and tracking.

Certain embodiments of the present invention help reduce the complexity of using quantum communications techniques with aerial platforms.

Certain embodiments of the present invention help enable PAT for quantum communications from and/or to aerial platforms more rapidly than conventional techniques. Certain embodiments of the present invention help reduce the need for mechanical agility of the PAT mechanism to deal with longitude, latitude and/or altitude changes in the aerial platform.

Certain embodiments of the present invention help provide a method of quantum key distribution between an aerial platform and another node of a communication network.

Certain embodiments of the present invention help reduce the weight required to be carried by an aerial platform which can establish a quantum communication link.

Certain embodiments of the present invention help to obviate the need to utilise intense laser beams on aerial platforms, thus lowering the health and safety risk.

Certain embodiments of the present invention help reduce the costs associated with quantum communication from aerial platforms compared to conventional techniques.

Certain embodiments of the present invention will now be described hereinafter, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates a communication network;

Figure 2a illustrates a flowchart of certain steps carried out by a high altitude platform;

Figure 2b illustrates a flowchart of certain steps carried out by an optical ground station;

Figure 3 illustrates a flowchart of certain steps performed to provide cryptographic key information at a high altitude aerial platform and an optical ground station;

Figure 4 illustrates a discrete variable QKD (DV-QKD) system for a high altitude platform (Alice) and an optical ground station (Bob);

Figure 5 illustrates a graph of quantum bit error rate against operational frequency for a DV- QKD system;

Figure 6 illustrates a graph of estimated sifted key rate for a DV-QKD system; Figure 7 illustrates a continuous variable QKD (CV-QKD) system for a high altitude platform (Alice) and an optical ground station (Bob);

Figure 8 illustrates a graph of secure key rate against channel loss for a CV-QKD system;

Figure 9 illustrates a high altitude platform communicating with multiple optical ground based stations;

Figure 10 illustrates a high altitude platform communicating with multiple optical ground based stations;

Figure 11 illustrates a high altitude platform communicating with an optical ground station with extended coverage;

Figure 12 illustrates a comparison of beam footprint for high altitude platforms and satellites;

Figure 13 illustrates a comparison between the use of wide and narrow beams from a high altitude aerial platform;

Figure 14 illustrates a graph of signal attenuation against visibility range under certain weather conditions;

Figure 15 illustrates a graph of signal attenuation against visibility range under certain weather conditions;

Figure 16 illustrates a graph of channel loss against line of sight distance;

Figure 17 illustrates a graph of Quantum Bit Error Rate (QBER) against channel loss at different times of the day;

Figure 18 illustrates a graph of channel loss against line of sight distance in foggy conditions;

Figure 19 illustrates a graph of channel loss against line of sight distance in rainy conditions;

Figure 20 illustrates a graph of channel loss against aperture size of a transmitter telescope; Figure 21 illustrates a graph of channel loss against line of sight distance with varying beam divergence angles;

Figure 22 illustrates a graph of QBER against line of sight distance with varying beam divergence angles; and

Figure 23 illustrates a graph of ground beam radius against beam divergence.

In the drawings like reference numerals refer to like parts.

Figure 1 illustrates a communication network 100 including a high altitude platform (HAP) 110 and an optical ground station (OGS) 120 providing a beaconless open-loop PAT system. It will be appreciated that the HAP may be a balloon or airship or fixed wing aircraft or the like located in the stratosphere at an altitude between about around 17 and 22km. It will also be appreciated that other aerial platforms (e.g. a low altitude drone or a medium altitude aircraft or the like) may be used instead of a HAP. It will be appreciated that the OGS may be any ground-based structure or set of structures providing functionality for communicating with an aerial platform via optical (and non-optical) wireless communications. For example, the OGS may be a building (e.g. an observatory), a tent, a cell tower, a vehicle or the like. It will thus be appreciated that the OGS may be a stationary or non-stationary (i.e. mobile) ground station. It will also be appreciated that a non-ground-based station may be used instead of an OGS in accordance 'with certain other embodiments of the present invention. For example, the non- ground-based station may be an aerial vehicle that flies at an altitude lower than an altitude of an aerial platform such as a low altitude airplane or helicopter and may be an aerial vehicle that is less than 1000m above a surface at a local ground level (e.g. a hot air balloon, a drone or the like).

The HAP 110 includes an RF or mm-wave transceiver 130. The RF/mm-wave transceiver enables location data indicative of a location of the HAP 110 to be transmitted to the OGS 120. The RF/mm-wave transceiver 130 also enables location data indicative of a location of the OGS 120 to be received at the HAP 110 from the OGS 120. When the OGS is a stationary ground station, it will be appreciated that the location data indicative of the location of the OGS may be stored in a memory (not shown) associated with the HAP 110 prior to the HAP reaching its’ desired altitude. In other words, prior to the HAP 110 attaining an altitude of, for example, greater than 500 metres. It will also be appreciated that any other transceiver that enables signals to be sent via non-optical wireless communication channels may be used instead of using an RF/mm-wave transceiver. For example, a 3G, 4G or 5G transceiver may be implemented according to certain other embodiments of the present invention. A 3G/4G/5G transceiver enables the HAP 110 to communicate via a cellular network. In this sense it will thus be appreciated that a non-optical wireless communication channel refers to the type of electromagnetic medium used to convey information. For example, the non-optical wireless communication channel may be a radio communication channel. It will thus be appreciated that non-optical communications are non-optical in the sense that they use electromagnetic radiation within a predetermined frequency range outside the optical frequency range. For example, the non-optical communications may be within a frequency range from 10kHz - 1THz. Aptly, the non-optical communications may be within a frequency range from 100kHz - 100GHz.

The HAP 110 also includes a DGPS chip 140. The DGPS chip enables the location of the HAP 110 to be determined in real-time. The DGPS chip 140 generates location data indicative of the location of the HAP 110. The location data may indicate the latitude and longitude of the HAP 110. The location data may also indicate the HAP’s altitude. This location data can then be sent to the OGS 120 via the RF/mm-wave transceiver 130 as described above. Differential GPS (DGPS) is able to achieve centimetre level accuracy when the remote site is close to the base station and sub-meter level accuracy when the remote site is 100 km away from the base station. A non-optical wireless communication link established between the HAP 110 and the OGS 120 allows the HAP 110 to send the location data to the OGS 120 continuously. The HAP 110 may also receive location data indicative of the location of the OGS 120 continuously, it will be appreciated that any other means for determining location of the HAP 110 may be used instead of the DGPS chip 140. For example, a GPS chip may be employed. The HAP 110 may also include an altimeter (not shown) to determine the altitude of the HAP 110. An altimeter may be helpful when there are too few satellites in communication with the HAP 110 to accurately determine the altitude of the HAP 110 via the DGPS chip 140 (or via a GPS chip).

The HAP 110 also includes a three-axis gimbal 150. That is, a gimbal that is effectively a set of three gimbals each offering a degree of freedom (roil, pitch and yaw). It will be appreciated that two-axis gimbals with two degrees of freedom may also be utilised. The location data indicative of the location of the HAP 110 is provided to the gimbal 150 from the DGPS chip 140. The location data indicative of the location data of the OGS 120 is also provided to the gimbal 150. This location may either be provided via the RF/mm-wave transceiver 130 (once the location data has been received) or from a memory (not shown) located on the HAP 110. These options were discussed above. The HAP 110 also includes a telescope 160 supported on the gimbal 150. The telescope includes lenses and/or mirrors. It will be appreciated that other optical elements allowing electromagnetic radiation to be directed and/or received may also be utilised. The telescope enables optical signals to be directed by and/or received at the HAP 110. That is to say that the telescope 160 may be oriented such that electromagnetic radiation can be directed from the HAP 110 in a direction towards the OGS 120 and/or such that electromagnetic radiation can be received at the HAP 110 from the OGS 120. For example, once the gimbal 150 receives both sets of location data, the three degrees of freedom (i.e. the three axes) of the gimbal 150 are manipulated in order to control a pointing axis (not shown) of the telescope 160. In the illustration of Figure 1 , the pointing axis would generally need to be oriented parallel to the lines representing the QKD link and FSO link such that the HAP 110 and the OGS 120 can communicate efficiently. The pointing axis may be oriented with a pointing precision of up to 10 milliradians. For example, the pointing precision may be between 0.001 milliradians (1 microradian) and 10 milliradians. Alternatively the pointing precision may be between 0.001 milliradians and 5 milliradians. Alternatively the pointing precision may be between 0.01 milliradians (10 microradians) and 1 milliradian. Alternatively the pointing precision may be between 0.01 milliradians and 0.5 milliradians (500 microradians). Alternatively the pointing precision may be between 0.01 milliradians and 0.1 milliradians (100 microradians). For example, the pointing precision may be approximately 0.1 milliradians. The pointing axis of the telescope 160 is oriented without reference to any beacon signals.

In particular, conventional beacon-based aerial platform systems rely on a beacon and detector system whereby a high-power laser beam (i.e. a beacon) is emitted from the aerial platform in a direction towards the ground station to provide a beacon signal. In these conventional systems, the beacon is aimed from the aerial platform in a general direction towards the ground station and is then swept along a spiral pathway of continuously decreasing radius until the ground station detects the beacon’s signal. Thereafter, the aerial platform and ground station operate a closed-loop feedback system such that the direction in which the beacon is aimed is continuously modified 'when needed so that the beacon remains approximately on the centre of a quadrant detector located on the ground station. This is effectively the acquisition and tracking part of a so-called PAT process. This closed-loop feedback maintains the aim of the telescopes on the aerial platform and the ground station to ensure the aerial platform and ground station can communicate with one another via optical signals.

In contrast to these conventional systems, according to certain aspects of the present invention as communication of location data takes place between the HAP 110 and the OGS 120 via a non-optical wireless communication channel (e.g. a wireless communication such as a radio communications channel or the like), there is no need for any beacon signals. Certain aspects of the present invention thus do not require the acquisition and tracking elements of a conventional PAT process. This helps to reduce cost and complexity of the HAP 110 and OGS 120. This also reduces the payload (i.e. the weight the HAP carries) needed on the HAP 110 allowing for other ancillary equipment to be included on the HAP 110. The removal of the need for high power laser beams also helps to reduce the health and safety risk associated with using such beams.

Figure 1 also illustrates that the HAP 110 includes a source 170 of photons of electromagnetic radiation having some measurable quantum property or state. For example, the measurable quantum property of the photons may be their polarisation, phase, quadrature or the like. Aptly the electromagnetic radiation may have a wavelength of between 300 nm and 2000 nm. Aptly the electromagnetic radiation may have a wavelength of approximately 550nm or approximately 690nm or approximately 850nm or approximately 1550nm. In Figure 1 the source 170 is referred to as a DV-QKD or CV-QKD source. These two options are discussed in further detail below. It will also be appreciated that whilst a source 170 is shown on the HAP 110, the source may in fact be present on the OGS 120. In that case, instead of providing a source, the HAP would include a detector that is able to measure a quantum property or state of photons of electromagnetic radiation. The HAP may also include both the source 170 and a detector for measuring quantum states.

The HAP 110 also includes a free space optical (FSO) transceiver 180. The FSO transceiver 180 includes a source and a detector of ‘classical’ electromagnetic radiation. In other words, electromagnetic radiation that has not been specifically encoded to have any measurable quantum states. Aptly the electromagnetic radiation may have a wavelength of between 300 nm and 2000 nm. Aptly the electromagnetic radiation may have a wavelength of approximately 550nm or approximately 690nm or approximately 850nm or approximately 1550nm. The FSO transceiver 180 enables classical optical signals to be exchanged between the HAP 110 and the OGS 120. These classical optical signals can for example be used to transmit and receive messages for post-processing of data related to QKD between the HAP 110 and the OGS 120 once quantum signal communication has been completed at the HAP 110 and the OGS 120 using a DV-QKD or CV-QKD protocol (discussed in more detail below).

According to certain aspects of the present invention, the OGS 120 includes several elements providing similar functionality to the corresponding elements present on the HAP 110 as discussed hereinabove. For example, the OGS 120 includes an RF or mm-wave transceiver 135, a DGPS chip 145, a three-axis gimbal 155, a telescope 165, and an FSO transceiver 185. As discussed above, instead of having a source of photons of electromagnetic radiation having some measurable quantum property or state, in Figure 1 it is illustrated that the OGS 120 has a detector 175 that is able to measure a quantum property or state of photons of electromagnetic radiation.

The RF/mm-wave transceiver 135 enables location data indicative of a location of the HAP 110 to be received at the OGS 120. The RF/mm-wave transceiver 135 also enables location data indicative of a location of the OGS 120 to be sent to the HAP 110 from the OGS 120. It will also be appreciated that any other transceiver that enables signals to be sent via non- optical wireless communication channels may be used instead of using an RF/mm-wave transceiver 135. For example, a 3G, 4G or 5G transceiver may be implemented according to certain other embodiments of the present invention. A 3G/4G/5G transceiver enables the HAP 110 to communicate via a cellular network.

The DGPS chip 145 enables the location of the OGS 120 to be determined in real-time. The DGPS chip 145 generates location data indicative of the location of the OGS 120. The location data may indicate the latitude and longitude of the OGS 120. The location data may also indicate altitude. This location data can then be sent to the HAP 110 via the RF/mm-wave transceiver 135 as described above. Alternatively, the location data may have already been stored in memory on the HAP 110. Differential GPS (DGPS) is able to achieve centimetre level accuracy when the remote site is close to the base station and sub-meter level accuracy when the remote site is 100 km away from the base station. A non-opticai wireless communication link established between the HAP 110 and the OGS 120 allows the OGS 120 to send the location data to the HAP 110 continuously. The OGS 120 also receives location data indicative of the location of the HAP 110 continuously. It will be appreciated that any other means for determining location of the OGS 120 may be used instead of the DGPS chip 145. For example, a GPS chip may be employed. The OGS 120 may also include an altimeter (not shown) to determine the altitude of the OGS 120. When the OGS is a stationary ground station, it will be appreciated that the location data indicative of the location of the OGS 120 may be stored in a memory (not shown) associated 'with the OGS 120. Thus, the OGS 120 may not need a DGPS or GPS chip.

The three-axis gimbal 155 is a gimbal that is effectively a set of three gimbals each offering a degree of freedom (roll, pitch and yaw). It will be appreciated that two-axis gimbals with two degrees of freedom may also be utilised. The location data indicative of the location of the HAP 110 is provided to the gimbal 155 via the RF/mm-wave transceiver 135 (once it is received from the HAP 110). The location data indicative of the location data of the OGS 120 is also provided to the gimbal 150. This location data may either be provided via DGPS chip 145 or from a memory (not shown) located on the OGS 120. These options were discussed above.

The telescope 165 is supported on the gimbal 155. The telescope includes lenses and/or mirrors. It will be appreciated that other optical elements allowing electromagnetic radiation to be directed and/or received may also be utilised. The telescope enables optical signals to be directed by and/or received at the OGS 120. That is to say that the telescope 165 may be oriented such that electromagnetic radiation can be directed from the OGS 120 in a direction towards the HAP 110 and/or such that electromagnetic radiation can be received at the OGS 120 from the HAP 110. For example, once the gimbal 155 receives both sets of location data, the three degrees of freedom (i.e. the three axes) of the gimbal 155 are manipulated in order to control a pointing axis (not shown) of the telescope 165. In the illustration of Figure 1 , the pointing axis would generally need to be oriented parallel to the lines representing the QKD link and FSO link such that the HAP 110 and the OGS 120 can communicate efficiently. The pointing axis of the telescope 165 is also oriented without reference to any beacon signals as discussed above with respect to the HAP 110.

The detector 175 is able to measure a quantum property or state of photons of electromagnetic radiation. For example, the measurable quantum property of the photons may be their polarisation, phase, quadrature or the like. In Figure 1 the detector 175 is referred to as a DV- QKD or CV-QKD detector. These two options are discussed in further detail below. It will also be appreciated that whilst a detector 175 is shown on the OGS 120, the detector 175 may in fact be present on the HAP 110 (as previously discussed). In that case, a source of photons of electromagnetic radiation having some measurable quantum property or state would be provided on the OGS 120. The OGS 120 may also include both the detector 175 and a source of photons of electromagnetic radiation having some measurable quantum property or state.

The FSO transceiver 185 includes a source and a detector of ‘classical’ electromagnetic radiation. In other words, electromagnetic radiation that has not been specifically encoded to have any measurable quantum states. The FSO transceiver 185 enables classical optical signals to be exchanged between the HAP 110 and the OGS 120. These classical optical signals can for example be used to transmit and receive encrypted messages between the HAP 110 and the OGS 120 once cryptographic key information has been provided at the HAP 110 and the OGS 120 using a DV-QKD or CV-QKD protocol (discussed in more detail below). Three communication iinks are illustrated in Figure 1 . A QKD link 190, an FSO link 192 and a non-optical link 194. Although the QKD link 190 and the FSO link 192 are shown as separate links, it will be appreciated that these links may be provided as a unitary link.

The QKD link 190 is the link that has been established between the HAP 110 and the OGS 120 in order to send photons of radiation having some measurable quantum state. The QKD link represents an optical wireless communication link. As is illustrated in Figure 1 , the QKD link 190 is a uni-directional wireless communication link. That is to say that photons are only sent from the HAP 110 towards the OGS 120. However, it will be appreciated that the QKD link may also be a bi-directional wireless communication link according to certain other embodiments of the present invention. The QKD link 190 supports optical wireless communication channels for sending photons that have been encoded with a quantum state. The link is the logical connection that is established between the HAP 110 and the OGS 120 in order to convey information used for determining cryptographic key information.

The FSO link 192 is the link that has been established between the HAP 110 and the OGS 120 in order to send classical optical signals. In other words, in order to send optical signals that have not been specifically encoded 'with any quantum states. The FSO link 192 also represents an optical wireless communication link. As shown in Figure 1 , the FSO link 192 is a bi-directional communication link. However, it will be appreciated that this link may also be a uni-directional wireless communication link according to certain other embodiments of the present invention. The FSO link 192 is not necessary in order to determine cryptographic key information. As such, certain embodiments of the present invention may not require an FSO link (and therefore may not require FSO transceivers 180, 185). It will be appreciated that the FSO link 192 also supports optical wireless communication channels.

The non-optical wireless communication link 194 is the link that has been established between the HAP 110 and the OGS 120 in order to exchange location data relating to the HAP 110 (and the OGS 120 when required). As shown in Figure 1 , this link is a common bi-directional communication link. The link supports non-optical wireless communication channels. The location data indicative of the location of the HAP 110 is sent in one link direction and the location data indicative of the location of the OGS 120 is sent in a remaining link direction. As discussed above, the location data can be sent repeatedly and continuously in either direction and can be sent in real-time.

To establish a uni-directional communication link, link setup information is transmitted from the HAP 110 (or the OGS 120) and received at the OGS 120 (or the HAP 110). No acknowledgement of receipt is transmitted. After this takes place, the uni-directional link is ready to be used. When establishing a bi-directional communication link, link setup information is transmitted from the HAP 110 (or the OGS 120) and received at the OGS 120 (or the HAP 110) and an acknowledgement of successful reception is then sent back to the HAP 110 (or the OGS 120). After this takes place, the bi-directional link is ready to be used. Thus, from the above it will be appreciated that whilst a communication channel refers to the type of electromagnetic medium used to convey information, a link is the logical connection between nodes that has been established using a protocol.

An adjustable beam expander (not shown) can also be included on the HAP 110 to dynamically control or select beam divergence of optical signals (either from the source 170 or the transceiver 180) sent to the OGS 120 to allow the system to adapt to different conditions. For example, a degree of beam divergence between 0.1 and 10 milliradians may be dynamically selected. For example, an adjustable beam expander can increase beam divergence to lower the difficulty of acquiring the QKD link 190 between the HAP 110 and the OGS 120. The adjustable beam expander can also decrease beam divergence to increase the quantum key rate. The FSO link 192 can be used to feedback for example the real-time key rate received based on a QKD application at the OGS 120 to the HAP 110 to adjust the beam divergence, or the OGS 120 can remotely control the beam divergence based on its requirements. The FSO link 192 also can ensure that location information is updated which allows the QKD link 190 to be maintained even when one or both of the HAP 110 and the OGS 120 move. Atmospheric compensation methods based on a look-up table can also be used to deal with refraction or other atmospheric effects. It will be appreciated that an adjustable beam expander may also be included on the OGS 120.

The process of determining cryptographic key information via a QKD protocol is discussed with respect to Figures 2 and 3. Figure 2a shows a flowchart 200 of certain steps carried out by the HAP 110 illustrated in Figure 1 . Figure 2b shows a flowchart 205 of certain steps carried out by the OGS 120 illustrated in Figure 1 .

Figure 2a includes a first establishing step 210 involving establishing an RF/mm-wave communication link with the OGS 120. The link may be established as discussed above 'with respect to Figure 1 . Once a non-optical wireless communication link has been established, a first sending step 220 involves sending location data indicative of the location of the HAP 110 to the OGS 120. The location data is referred to in Figure 2a as ‘HAP GPS’. As illustrated in Figure 2a, a checking step 230 involves checking for reception of location data indicative of the location of the OGS 120 at the RF/mm-wave transceiver 130. The checking step 230 may be carried out before, after or simultaneously with the first sending step 220. If it is determined at a confirming step 240 that no OGS location data has been received, then checking step 230 is repeated until OGS location data is received. In Figure 2a, the location data indicative of the location of the OGS 120 is referred to as ‘OGS GPS’. Alternatively, as discussed above with respect to Figure 1 , the OGS location data may be stored in memory on the HAP 110. In this case, the flowchart shown in Figure 2a may proceed directly from the first sending step 220 to a further sending step 250 (indicated by arrow 270) as the OGS location data does not need to be received by the HAP 110. This further sending step 250 involves sending the HAP and OGS location data to the three-axis gimbal 150 located on the HAP 110. This results in the axes of the gimbal 150 being manipulated such that the pointing axis of the telescope 160 is oriented in a direction towards the OGS 120. The telescope 160 is therefore put in a suitable orientation to be able to direct electromagnetic radiation from the source 170 or the FSO transceiver 180 on the HAP 110 in a direction towards the OGS 120. The telescope 160 may also receive electromagnetic radiation from the OGS 120. Once the pointing axis of the telescope 160 is correctly oriented (and also when the telescope of the OGS 165 is correctly oriented; see Figure 2b), a further establishing step 260 involves establishing a QKD link and an FSO link. These links may be established as discussed above with respect to Figure 1 . As also discussed above with respect to Figure 1 , only a QKD link may need to be established. The process of determining cryptographic key information then proceeds as shown in Figure 3.

Figure 2b includes a first establishing step 215 involving establishing an RF/mm-wave communication link with the HAP 110. The link may be established as discussed above with respect to Figure 1 . Once a non-optical wireless communication link has been established, a first sending step 225 involves sending location data indicative of the location of the OGS 120 to the HAP 110. The location data is referred to in Figure 2b as ‘OGS GPS’. As discussed above with respect to Figure 2a, if the OGS location data is stored in memory on the HAP, the first sending step 225 may not be needed. Turning back to Figure 2b, a checking step 235 involves checking for reception of location data indicative of the location of the HAP 110 at the RF/mm-wave transceiver 135. This checking step 235 may be carried out before, after or simultaneously with the first sending step 225. If it is determined in a confirming step 245 that no HAP location data has been received, then the checking step 235 is repeated until HAP location data is received. In Figure 2b, the location data indicative of the location of the HAP 110 is referred to as ‘HAP GPS’. A further sending step 255 involves sending the HAP and OGS location data to the three-axis gimbal 155 located on the OGS 120. This results in the axes of the gimbal 155 being manipulated such that the pointing axis of the telescope 165 is oriented in a direction towards the HAP 110. The telescope 165 is therefore put in a suitable orientation to be able to direct electromagnetic radiation from the FSO transceiver 185 on the OGS 120 in a direction towards the HAP 110 (and to direct electromagnetic radiation from a DV/CV-QKD source when one is present on the OGS). The telescope 165 may also receive electromagnetic radiation from the HAP 110. Once the pointing axis of the telescope 165 is correctly oriented (and also when the telescope of the HAP 160 is correctly oriented; see Figure 2a), a further establishing step 265 involves establishing a QKD link and an FSO link. These links may be established as discussed above with respect to Figure 1 . As also discussed above with respect to Figure 1 , only a QKD link may need to be established. The process of determining cryptographic key information then proceeds as shown in Figure 3.

It is not necessary for the QKD link to be permanently established. In QKD, secure keys can be transferred when there is a link established between the source and receiver. For example, a QKD link established for only 1% of the time in which QKD signals are sent will still enable QKD to be successful. A higher key rate will however be achieved if the link is established for 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100% of the time. In order to ensure the secrecy of the quantum key, it is only required to perform a definite minimum for the quantum signal transmission. This further enables the source and receiver PAT system to function less accurately than with conventional systems operating with beacons.

Figure 3 illustrates a flowchart 300 for determining cryptographic key information using discrete variable quantum key distribution (DV-QKD). Discrete-variable QKD (DV-QKD) is the generic term used to describe QKD protocols 'which implement dlscretised encoding of quantum key information on weak-coherent pulses, single-photon sources, or quantum entangled photon pairs. The dlscretised encoding and single-photon level optical signals limits the number of measurements an eavesdropper can make, limiting the effectiveness of attacks. In an ideal case, active interference of the quantum signal will cause errors in the shared communication key, revealing the eavesdropper.

In more detail, in DV-QKD, the key information is encoded on discrete degrees-of-freedom of the quantum optical states. The quantum states can be generated using pseudo-deterministic or probabilistic entangled photon sources. Quantum state superposition or quantum entanglement are utilised for the secure transmission of the key and to reveal eavesdroppers in the communications channel. Both types of sources and quantum phenomena can be used to construct DV-QKD protocols based on a degree-of-freedom of the quantum state. In optical- fibre implementations, protocols based on phase and time-bin encoding are traditionally used, as those degrees-of freedom are more robust to transmission in optical fibre. Polarisation based protocols, such as the BB84 protocol (discussed below), are traditionally used in free- space communications, due to the robustness of polarisation to atmospheric transmission. When a receiver (called Bob) receives the quantum state from the optical channel, he has no apriori information about the quantum state encoded by the sender (called Alice), and uses his own quantum random number generator (QRNG) to select the decoding basis set. Bob then records the measurement outcome, using single-photon detectors, as well as his own basis set selection. That information is stored for further processing of the key. In postprocessing, Alice and Bob use an authenticated communication channel to perform basis set reconciliation, which sifts out the measurements where Alice and Bob randomly chose the same basis set for encoding and measurement. Alice and Bob then follow post-processing techniques to reduce the amount of errors in the shared encryption key and information potentially leaked to an eavesdropper.

The general protocol operation is outlined in Figure 3. in summary, the HAP 110 (referred to herein as ‘Alice’) first establishes an optical link with the OGS 120 (referred to herein as ‘Bob’). Alice then transmits her quantum DV-QKD states to Bob, where they are measured by Bob. Alice and Bob perform basis set reconciliation to ensure the same basis set was chosen for the transmitter and receiver. Alice and Bob then estimate the quantum bit error rate, by revealing the encoding measured, for a small number of states, allowing them to estimate the amount of information an eavesdropper may have. Alice and Bob apply error correction and privacy amplification codes to reduce the amount of information and eavesdropper could have gained about the shared quantum key. After this process, Alice and Bob now have a quantum secure one-time pad key to use. This process is described in more detail below.

The first step 310 shown in Figure 3 relates to establishing the optical QKD link between the HAP 110 and the OGS 120. This step is equivalent to the further establishing steps (260, 265) in Figures 2a and 2b discussed above.

Once the QKD link is established, a second step 320 involves generating and encoding measurable quantum states on photons of electromagnetic radiation. In particular, in a quantum communication session the HAP 110 generates a long sequence of photons and encodes the photons with random quantum states from a set of quantum states with predefined classical bit values. The HAP 110 then performs the step (not shown) of sending these encoded photons to the OGS 120. This involves providing the encoded photons at the telescope 160 of the HAP 110 in order to direct the photons in a direction towards the OGS 120. This process was discussed above with respect to Figure 2. It will be appreciated that according to certain other embodiments of the present invention the photons may also be encoded using a distribution with undefined classical bit values. In other words, continuous variable quantum key distribution (CV-QKD) may be used, in CV-QKD, direct measurement of the quantum signal does not reveal the key, but data-post processing causes a common key to be determined at the HAP 110 or ‘Alice’ and the OGS 120. CV-QKD is discussed in further detail below.

The third step 330 shown in Figure 3 involves the OGS 120 measuring the photons of electromagnetic radiation received from the HAP 110. This involves the OGS 120 receiving the photons via the telescope 165 and providing the photons to the detector 175 such that the quantum state of the photons can be measured. For the purposes of basis reconciliation and error correction, the HAP 110 and OGS 120 each keep a record of which quantum states were sent and 'which states were measured as well as the basis used to prepare and measure the quantum states.

The fourth step 340 shown in Figure 3 refers to performing basis set reconciliation by the HAP 110 and the OGS 120. To do this, the HAP 110 and the OGS 120 communicate via an authenticated communication channel to confirm which basis sets were chosen by the HAP 110 and the OGS 120 for each encoded photon sent to the OGS 120. The authenticated communication channel may be the non-optical 'wireless communication channel provided via the non-optical link 194 or alternatively an optical communication channel provided via the FSO link 192. At the end of this step 340, the HAP 110 and the OGS 120 disregard any measured photons where the same basis was not used for encoding and measuring. As such, the HAP 110 and the OGS 120 are left 'with a sifted subset of measurements 'where the same basis was used to prepare and measure the quantum state of the photons. This allows the HAP 110 and the OGS 120 to determine the quantum bit error rate (QBER) which provides a signature of eavesdropping.

At the fifth step 350 shown in Figure 3, the HAP 110 and the OGS 120 perform a process of estimating the quantum bit error rate (QBER). To do this, the HAP 110 and OGS 120 share with one another which quantum states were prepared and which quantum states 'were measured, respectively for a subset of the photons (after basis reconciliation). If there is a substantial QBER (the maximum is defined by the protocol), then it is likely that an eavesdropper was interfering with the communication channel and trying to steal the shared secret key. If the QBER goes beyond the maximum bound QBER, the shared keys are not secure and a new key sharing process must be initiated. In other words, the QBER provides the HAP 110 and the OGS 120 with an indication of how much information about the key that an eavesdropper may have. It will be appreciated that if CV-QKD is utilised, instead of QBER, comparing the variance of a part of the sifted quadrature values reveals the presence of eavesdropping as noise; this is called excess noise variance. Unlike DV-QKD, since CV-QKD quadrature values are analogue values, and an additional post-processing is employed to convert the analogue values to binary digits.

In order to reduce the amount of key information that an eavesdropper may have access to, the sixth step 360 shown in Figure 3 involves performing error correction. In particular, if the QBER or excess noise are below the permissible limit for secure key generation, the HAP 110 and OGS 120 apply classical error correction techniques such as: cascade or low-density parity check (LDPC) on the rest of the sifted key and convert it to an error corrected key, which is ideally a perfectly correlated string of bits.

At the seventh step 370 of Figure 3, the HAP 110 and the OGS 120 perform privacy amplification to reduce the information leaked to an eavesdropper during the quantum communication session as well as during classical post processing. The HAP 110 and the OGS 120 for example apply universal hashing on the error corrected key in order to amplify the privacy of the final key. The amount of privacy amplification, reduction in the size of the error correcting key, is decided on the estimation of eavesdropped information from QBER or excess noise. In generic form, the final key rate equation can be written as:

K = I (A: B) - min {I (A: E), I(B: E)}

Here, l(A:B) is the mutual information between Alice and Bob, l(A:E) is the information between Alice and Eve, which has to be taken into account for direct reconciliation where Bob corrects his noisy measurement outcomes with respect to Alice. I(B:E) is the information between Bob and Eve, in the case of reverse reconciliation where Alice corrects her data in order to match 'with the noisy version of Bob’s measurement outcomes.

Figure 4 illustrates a schematic 400 of one type of DV-QKD protocol based on polarisation encoding called BB84. According to certain embodiments of the present invention which utilise this particular protocol, the HAP transmitter (Alice) 405 has four laser sources 420I-4 to encode four different polarisations. These polarisations make up two basis sets. One basis set contains horizontal (H) and vertical (V) polarisations. The other, orthogonal, basis set contains polarisations of 45° (D1 ) and 135° (D2). To produce these polarisations, the laser sources 420<-4 may for example be passed through linear polarisers (not shown). Alice uses a quantum random number generator (QRNG) 430 to randomly select the basis set to use for each photon and also the encoding of each photon to use when sending to Bob 410. The lasers are attenuated down to the single-photon level to give a mean photon number of less than 1 per pulse. For example, 0.2 photons per pulse. For example, this may require use of an optical attenuation element (not shown). Alice’s 405 optical system allows the four laser beam paths to be coupled together into the free-space channel 440 via polarisation beamsplitters 450i 2 and 50:50 beamsplitter 460. Bob’s 410 receiver passively selects the basis set for the measurement with a 50:50 beamsplitter 470. Bob 410 may also include a QRNG (not shown) for selecting which basis set to use 'when measuring. Each output of the receiver 50:50 beamsplitter 470 has a measurement which can fully distinguish one of the basis sets. To detect photons in the H, V basis set, Bob has a polarisation beamsplitter 480i which directs V photons towards detector 1 , 490i and H photons to detector 2, 4902. To detect photons in the DI , D2 basis set, Bob has a half-wave plate 495 and a polarisation beamsplitter 4802 'which directs D1 photons towards detector 3, 490s and D2 photons to detector 4, 490₄. As an example of a protocol in operation, in the BB84 protocol, Alice 410 has two sets of paired attenuated laser sources 420M, which are used to encode four quantum states. Individual lasers within a pair are used to directly encode the binary key bits. Having two sets of paired lasers enables two basis sets for encoding and decoding, creating the quantum superposition states. Alice uses a quantum random number generator (QRNG) 430 to select one basis set for encoding and uses the QRNG 430 again to select the random key bit. Alice records her basis set choice and the key bit and transmits the encoded quantum state to Bob through an optical channel. Bob then measures the transmitted photons and thereafter post-processing is performed as described with respect to Figure 3 above.

Figure 5 illustrates a graph 500 of quantum bit error rate against operational frequency for a DV-QKD system. In particular, Figure 5 shows an estimation of the quantum bit error rate (QBER) for the BB84 protocol for various operational clocking frequencies, and the dependency of the QBER on the optical channel losses. As the signal to noise ratio reduces due to loss of photons, there will be an increase in the number of misinterpreted quantum measurements, leading to errors in the shared key. To help overcome errors, error correction codes are implemented, however these are most efficient below a certain threshold. An increase in QBER at a given channel loss can be interpreted as interference by an eavesdropper. To overcome channel loss and reduce QBER, operation at higher frequencies is required, which is also highlighted in Figure 6.

Figure 6 illustrates a graph 600 of estimated sifted key rate for a DV-QKD system. In particular, Figure 6 gives an estimation of the sifted key rate - the photon events that remain after the basis set reconciliation process, with channel loss for various operational frequencies. As can be seen, as the operational frequency increases, so does the achieved sifted key rate. The combination of QBER and sifted key rate give the final secure key rate, if there is a high QBER, more sifted key needs to be sacrificed to ensure the final shared key is secure.

As discussed hereinabove, instead of using DV-QKD protocols for sharing secret keys, CV- QKD may also be implemented according to certain embodiments of the present invention. CV-QKD uses amplitude and phase of light to carry secure key information. At lower intensity, near to vacuum, light shows uncertainty in the exact value of amplitude and phase. CV-QKD relies on this fundamental uncertainty, referred as vacuum noise, in order to distribute keys securely. Since the vacuum noise prevents precise measurements on the amplitude and phase of the transmitted optical signal, any action of the eavesdropping induces additional noise which can be detected at the receiver. In a typical CV-QKD signal transmission system, a coherent state of light is first amplitude and phase modulated and then this is sent to the receiver through a communication channel. At the receiver, a vacuum noise sensitive detector jointly measures the amplitude and phase - called quadrature measurement. Under key distribution conditions, the quadrature (amplitude and phase) of the transmitted and received signals are correlated. From this correlation, channel loss and excess noise by the eavesdropper can be estimated, if the excess noise is under the permitted level of the channel loss, a secure key can be extracted by performing error correction and privacy amplification.

For example, CV-QKD may utilise a Gaussian modulated coherent state protocol, GMCS. Here, Alice generates a sequence of random amplitude and phase modulated coherent states |α > = re^-iθ such that the distribution of the quadrature, X_A = r cos θ, and P_A = r sin θ, follows a normal distribution with variance, V_A, and mean zero. Here, |r | ² is the intensity of the coherent signal which corresponds to a few photons per pulse, on average. 0 is the relative phase of the coherent signal with respect to an intense reference signal, referred as the local oscillator (LO). The LO is either generated at Alice and sent to Bob along with the coherent state, which is referred to as a transmitted local oscillator (TLO) scheme. Alternatively, the LO is generated locally at Bob, which is referred to as a local local oscillator (LLO) scheme. In an LLO scheme, two different lasers are used to generate the coherent states and the LO (i.e. one at Alice and another at Bob), a common phase reference is also established between the Alice and Bob which is achieved by sending a phase reference pulse, R_ref from Alice to Bob.

Bob then randomly measures one of the quadrature using a shot noise limited homodyne receiver. This is performed by mixing the input quantum signals with an intense LO on a symmetric beam splitter. The outputs of the beam splitter are individually detected using reverse biased PIN photodiodes, the photocurrents are then subtracted from each other and amplified. The amplified output represents a noisy version of Alice’s quadrature values X_A or P_A, depending on 0 or 90° relative phase with respect to LO. This would create a correlated data set of quadrature values between Alice and Bob for the raw key. It will be appreciated that heterodyne detection may also be used.

CV-QKD operation from HAPs is similar to the DV-QKD operations discussed above, with an additional data processing layer in figure 3. That is, instead of estimating the QBER, in CV- QKD data post processing estimates channel transmission and excess noise. Figure 7 shows a schematic 700 of a generalized CV-QKD transmitter 710 located on the HAP and a receiver 720 located on the OGS. The transmitter 710 illustrates how electromagnetic radiation from a laser source 730 is passed through an amplitude modulator 740 and a phase modulator 750 before being sent to the receiver 720 over the free space channel 760. The receiver combines the optical signal sent from the transmitter 710 with a local oscillator 770 at a beamsplitter 780 before measuring the quadrature of the transmitted optical signal via homodyne or heterodyne detection with detectors 790_1-2 (e.g. PIN diodes). Figure 8 illustrates a graph 800 of the estimated secure key rate of a CV-QKD system, over a free-space quantum link at 1 MHz clock rate.

By diverging an optical beam sent from the HAP to the OGS, it is also possible that multiple receivers within the beam’s footprint can be served with quantum keys concurrently within a time period. Multiple diverged beams can also enable simultaneous transmission of quantum keys over a wider area, with a contiguous beam pattern formed on the ground. This can be used to provide coverage and capacity for multiple receivers. Such receivers could for example be mobile.

Figure 9 shows a communication network 900 and illustrates the case when there are multiple DV/CV-QKD sources on the HAP that can simultaneously perform QKD with multiple users. These users can be geographically spread over a wide area and may be mobile.

Figure 10 shows a communication network 1000 and illustrates the case when there are multiple DV/CV-QKD sources on the HAP that can simultaneously perform QKD to multiple users. However, in this case the beams are designed to ensure that there is contiguous coverage capability for QKD delivery over a wide area. In other words, the individual beams form a respective cell coverage area and the multiple cell coverage areas of all the beams form a contiguous area of coverage. Figure 11 shows a communication network 1100 and illustrates the case where a single DV/CV-QKD source on the HAP is used with multiple beams, which help provide contiguous coverage. In this case it is assumed that there may be pitch/roll/yaw or lateral movements which cannot be effectively corrected by the PAT system. However, in this situation the key can be received from anyone of the beams when illuminated by a source of photons encoded with quantum states. Additionally, if a user is happy to have intermittent or sporadic key delivery then the source need not illuminate the user continually. This could further be used to reduce the requirements of the PAT system.

Figure 12 illustrates a schematic diagram 1200 of a comparison of the beam footprint when using a HAP compared to 'when using a satellite. It will be appreciated that when using HAPs, the attenuation due to beam spreading over distance is further reduced compared with beam spreading from satellites. The difference in general brings 25 dB to 30 dB less attenuation when applying QKD from HAPs.

Figure 13 illustrates a schematic diagram 1300 of a comparison between the use of wide and narrow beams from a HAP. It will be appreciated that potential issues which may cause the beam footprint to miss the receiver telescope are the accuracy of the DGPS, the pointing accuracies of both sites and the random movements of the aerial platforms. These issues are lessened 'when using wide beams, and wide field-of-view receivers. Figure 13 demonstrates the effects of using narrow and wide beams. The aerial platforms usually experience random movements when airborne because of the 'wind. These movements can easily cause the beam to misalign with the target if the light source has narrow beam divergence (ground beam footprint is on the similar magnitude as the receiver telescope aperture). On the other hand a beam with wide divergence creates a larger beam footprint which makes it difficult to miss the receiver telescope (more tolerable pointing accuracy requirements) when the optical system on the aerial platform suffers from the similar level of random movements. Additionally using wide beams brings robustness of the PAT system against the effect of beam wandering. Diverging the beam however increases the attenuation due to beam spreading, link budget analysis is thus necessary to determine the amount of divergence the system is able to afford (link budget analysis is discussed in detail below). For utilising QKD between a HAP and an OGS, the link budget should be closed such that enough photons arrive at the receiver telescope so that sufficient cryptographic key information can be determined. There are many factors affecting the link budget including the transmission distance, wavelength, optical design, time, optical components, weather, and background noise. In order to achieve continuity of service, it is helpful to have the link budget closed with these varying factors. A lower link distance provides more tolerance to attenuation and operating potential during daylight which can compensate any of the above disadvantages. The station-keeping and long endurance capabilities of HAPs allow the QKD services to be delivered to certain regions continuously, unlike the unavoidable service window of QKD from LEO satellites. The ease of HAP launching and maintenance can close the gaps of QKD services by using multiple HAPs simultaneously.

If the parameters of the optical components at the receiver are known, the power of the background noise P_b varies according to the brightness of the day as follows:

where H_b is the brightness of the day, Ω_fov is the receiver field of view (FoV), A_rec is the area of the telescope aperture and B is the bandwidth of the optical filter. H_b varies at different times of the day, and the typical values are 150 (daytime with illuminating cloud), 15 (hazy daytime), 1.5 (clear daytime), 1.5x 10^-3 (full moon night), 1.5x 10^-4 (new moon night) and 1.5x 10^-5 (moonless night) (all values are given in Wm^-2Srμm). Across the range, there is maximum 70 dB difference in the background noise power, 'which highlights a challenge in operating QKD during daytime. Some optical filters can achieve 0.1 nm or better bandwidth, and the typical value of 0.1 nm is used in the link budget computation. Quantum optical states can be generated with a narrow bandwidth, justifying the filter choice. With temperature stability on the HAP, the wavelength of the quantum optical states can be kept within the window of the optical filter. The velocity of the HAP platforms is also not large enough to cause a significant Doppler shift in wavelength.

FoV of the receiver determines the amount of light (noise and the desired signal) collected by the telescope which reaches the detector. The optical receiver is normally a multi-lens system so obtaining an accurate FoV could be difficult without the detailed design of the system. A common receiver design uses a Schmidt-Cassegrain telescope followed by a collimation lens to produce a collimated beam for the downstream optical components. The assumption is often made that changing the receiver telescope aperture (with the same focal ratio) does not affect the rest of the system. So the receiver can be considered as a two-lens system where the first lens is the telescope and the second lens represents the rest of the optical components (which remain the same). The FoV of a lens can be expressed as:

where D is the lens diameter and F is the focal length. For example, D may be the detector diameter and F may be the effective focal length of the optical system. The effective focal length of a two-lens system can be obtained by:

where A is the focal length of the telescope, f₂ is the focal length of the other lenses and d is the distance between two lenses. Based on the previous assumptions, when varying the telescope aperture size, f₁ changes linearly with the aperture size, whereas the terms f₂ and f₁ - d remain the same. So the effective focal length of the system changes linearly with the aperture size. It can then be concluded that the FoV n_fov decreases linearly with an increasing aperture size, which captures the benefit of using larger telescopes (receiving less background noise directly on the quantum detector(s)).

There is also link attenuation (geometric loss) resulting from the natural spreading of the beam. The geometric losses are typically the dominant losses in a free-space QKD implementation.

Link attenuation can be expressed as:

where D_rx and D_tx are the receiver/transmitter telescope aperture size, R_LoS is the line-of-sight (LoS) distance between the two optical terminals, and 9 is the beam divergence. In the HAP scenario R_LoS can be computed by:

where H_HAP is the altitude of the HAP and a is the elevation angle which varies between 0° and 90°. 6 can be computed by:

where 2 is the operating wavelength.

The optical signals between the ground and the HAP propagates in the atmosphere, which will experience molecular absorption L_ma caused by the molecules of water and carbon dioxide. The amount of attenuation depends on the link distance and wavelength, some typical values of L_ma are 0.13 dB/km at 550 nm, 0.01 dB/km at 690 nm, 0.41 dB/km at 850 nm and 0.01 dB/km at 1550 nm.

Different weather conditions also cause attenuation when the optical signal propagates through the atmosphere. Fog causes attenuation because its’ particle size is comparable to the wavelength of the optical source. Large snowflakes can potentially block the optical path completely. Attenuation caused by fog may be given by:

where the units of L_fog are in dB/km, V is the visibility range in km, and p is the size distribution coefficient of scattering given by:

The attenuation of snow is given by:

The attenuation of rain is given by:

Both L_snow and L_rain are also expressed in units of dB/km.

Figure 14 and Figure 15 show graphs 1400 and 1500 respectively of the resulting attenuation under different weather conditions against the visibility range. It can be observed that once the visibility range falls below 2 km, the rain and snow attenuation increases significantly. Snow always causes large attenuation due to the size of the snowflakes.

In the HAP scenario the distance that the optical signal propagates in weather R_w can be computed by:

where H_w is the altitude of the weather, which varies at a few km with rain and snow, or subkm with fog.

There are also different types of attenuation coming from inside the optical system itself. The performance of the PAT system may affect the link budget when the beams are narrow. The random movements and vibrations of HAPs could potentially cause difficulties to the PAT system to achieve accurate alignment of narrow beams. The attenuation due to misalignment is given as:

where θj is the divergence angle of the pointing jitter. The other effect which could cause similar misalignment error is the beam wander. When propagating through a turbulent atmosphere, the beam experiences random deflection caused by the turbulent eddies and the centroid of the beam is randomly displaced. The displacement variance (in m²) can be computed as:

where r₀ is the atmosphere field parameter.

Optical components at the receiver can also bring additional attenuation L_rx, for example some attenuations are time filtering loss (4.2 dB), APD detector efficiency (4.2 dB), interference filter (3 dB), non-ideal optics (3 dB), diode coupling loss (2 dB). These losses are used in the link budget analysis as a benchmark. Summing the different attenuations described above the total loss can be given by (method 1 ):

L_T- — L_p + L_geo + L_maR_LoS + L_WR_W + L_rx where L_w is the conditional attenuation caused by different weather. Another method to estimate the loss is as follows (method 2):

where method 2 may be referred to as NanoBob and where the beam divergence θ is estimated as follows:

and θ_atm is the atmosphere turbulence included divergence angle computed as:

The term L_atm is the atmospheric attenuation due to Rayleigh scattering and absorption (3 dB is given as a typical value), the three terms T_t, T_r and T_p are the efficiency of the transmitter telescope, receiver telescope and pointing (all are given 0.8 as typical values).

The HAP altitude and elevation angles together result in the Line of Sight (LoS) link distance varying from 20 km to 230 km. Figure 16 shows the channel loss of both link budget methods at different LoS link distance. The NanoBob method has slightly higher loss across most link distance range, partially resulted from the overestimated beam divergence. Attenuation due to weather conditions is not considered in Figure 16.

Figure 17 shows a graph 1700 of the QBER at different times of the day with varying background noise levels (varying brightness of the sky). The protocol used is BB84 operating at 500 MHz. It is a symmetric basis state protocol, with a quantum signal and one decoy signal, the mean photon numbers were 0.5 and 1 , with probabilities 0.8 and 0.2 respectively. The receiver’s detectors are high performance InGaAs single-photon avalanche diodes (SPADs) with a single-photon detection efficiency of 25%, a detector dead time of 18 ps, a detector size of 64.5 μm (fibre core diameter coupled to detector), and a dark count rate of 500 counts per second.

Figure 18 shows a graph 1800 of the channel loss with different levels of fog (500 m above the ground) existing near the ground receiver. Together considering the results of Figure 17, the system is able to operate with levels of fog within the regular HAP operating elevation angles (20° or higher, equivalent to 60 km or less LoS distance). For the day-time with illuminating cloud scenario, the system can operate with the presence of moderate or light fog, but the range is reduced with the presence of heavy fog.

Figure 19 shows a graph 1900 of the channel loss with different levels of rain (5 km above the ground). The overall trend of the channel loss is similar to the situation with fog. Similarly the system is able to operate with levels of rain within the regular HAP operating elevation angles.

From the link budget analysis discussed above, it will be appreciated that generous link margin can be observed in most operating conditions. This indicates that additional attenuation can be offset with higher tolerance of the PAT system by diverging the optical beam. When the beam is able to create a large footprint at the OGS, beacons are no longer be needed because the coarse PAT using Differential Global Positioning System (DGPS) provides enough precision. The high-precision DGPS is able to provide centimetre-level accuracy on the positions of the HAP and the OGS which are sufficient for diverged beams. Moreover removing the beacons will reduce the HAP payload weight significantly.

To diverge the beam either a TX telescope with small aperture size can be used or a diverging lens can be applied to the beam. Figure 20 shows a graph 2000 of the channel loss with varying TX/RX telescope aperture size when the HAP operates at 20° elevation. Increasing the TX telescope aperture size reduces the beam divergence. Reduced channel loss is expected when using larger TX telescope however the results of method 1 show the opposite when using TX telescopes larger than 0.12 m while the NanoBob method shows the expected performance. The difference is caused by the ways that both methods capture the pointing errors. Method 1 uses an absolute divergence angle of the pointing jitter to estimate the attenuation due to misalignment and the NanoBob uses the fixed pointing efficiency T_p = 0.8. The divergence of the pointing jitter θj is 5 μrad and this is related to the precision of the PAT system (e.g. the gimbal) which is not dependent on the size of the telescopes. When decreasing the beam divergence 6 the attenuation L_p increases and makes L_P a dominant factor thereby increasing the channel loss. These results also indicate the trade-off between high-cost high-precision PAT system with narrow beams and low-cost high-tolerance PAT system with wide beams. The channel loss of NanoBob method is less affected by the different RX aperture size because, the dominant factor of the denominator inside the logarithm is T_tT_pT_r rather than D_rx in.

Figure 21 illustrates a graph 2100 presenting the channel loss with different beam divergence during moonless night 'with 1 mW transmitted signal power using a 0.1 m transmitter telescope and a 0.4 m receiver telescope. Together with the QBER model presented in Figure 17, the QBER with different beam divergence can be obtained (shown in the graph 2200 of Figure 22). It can be observed that within the regular operating elevation angle of the HAP, QKD can remain operational even at almost 3 mrad beam divergence. The beam with larger beam divergence also provides the opportunity of using low-cost gimbals in the PAT system. Many low-cost off-the-shelf gimbals have pointing precisions on the level of 0.1 mrad so using mrad level beam divergence can minimise the beam misalignment caused by the low pointing precisions. The difference between the precision of the DGPS signal and the size of the ground beam footprint also contributes to the tolerance of the overall pointing accuracy.

Figure 23 shows a graph 2300 of the ground beam radius with different beam divergence values when a light source on the HAP (20 km altitude) points at target locations at various elevation angles. The link budget analysis uses the geometric attenuation, pointing loss models, background noise estimation model, and losses caused by the receiver optical components discussed above. The estimated received signal power takes into account the transmitted signal power and various types of attenuations mentioned in the link budget analysis model. In Figure 23, the 1 mrad beam and 3 mrad beam result in 10 m and 30 m radius ground beams respectively, which are all magnitudes larger than the centimetre precision of DGPS. This indicates that the pointing precisions of the low-cost gimbals and the precision of the DGPS signals can all be tolerated.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to” and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Features, integers, characteristics or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or ail of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of the features and/or steps are mutually exclusive. The invention is not restricted to any details of any foregoing embodiments. The invention extends to any novel one, or novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader’s attention is directed to all papers and documents 'which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS:

1 . A method for providing cryptographic key information at an aerial platform via communication of at least one photon of electromagnetic radiation having at least one measurable quantum state, the method comprising the steps of: sending first location data, indicative of a location of an aerial platform, from the aerial platform to at least one further node of a communication network via a first, non-optical, wireless communication channel; responsive to the first location data and to further location data indicative of a location of a further node, directing electromagnetic radiation from the aerial platform at the further node and/or receiving electromagnetic radiation at the aerial platform originating from the further node; and determining cryptographic key information responsive to at least one measurable quantum state of at least one photon of the electromagnetic radiation directed from and/or received by the aerial platform.

2. The method as claimed in claim 1 , further comprising: orienting a pointing axis of at least one optical element supported by the aerial platform based on the first and further location data without reference to one or more beacon signals.

3. The method as claimed in claim 2, further comprising: orienting the pointing axis with a pointing precision of up to 10 milliradians, optionally orienting the pointing axis with a pointing precision between 0.001 milliradians and 5 milliradians, and optionally orienting the pointing axis with a pointing precision between 0.05 milliradians and 1 milliradian.

4. The method as claimed in any preceding claim, further comprising: directing electromagnetic radiation from the aerial platform in a direction towards the further node via: orienting a pointing axis of at least one optical element supported by the aerial platform in a direction towards the further node; and directing electromagnetic radiation emitted from at least one source of electromagnetic radiation supported by the aerial platform towards the further node along the pointing axis. The method as claimed in any preceding claim, further comprising: receiving electromagnetic radiation at the aerial platform originating from the further node via: orienting a pointing axis of at least one optical element supported by the aerial platform in a direction towards the further node; and receiving electromagnetic radiation emitted from at least one source of electromagnetic radiation supported by the further node along the pointing axis. The method as claimed in any one of claims 2 to 5, further comprising: supporting the optical element on a two-axis or three-axis gimbal; and orienting the pointing axis of the optical element via controlling respective axes of the gimbal responsive to the first and further location data. The method as claimed in any one of claims 2 to 6, wherein the optical element is a transmitting and/or receiving telescope and optionally includes one or more mirrors and/or one or more lenses. The method as claimed in any preceding claim, further comprising: receiving the further location data at the aerial platform via a further, non- opticai, wireless communication channel or storing the further location data in at least one memory associated 'with the aerial platform prior to the aerial platform attaining an altitude of greater than 500 metres. The method as claimed in any preceding claim, further comprising: prior to sending the first location data, establishing at least one non-optical wireless communication link between the aerial platform and the further node, and optionally wherein the non-optical communication link is provided by a common bidirectional, non-optical communication link supporting both the first and further non- optical communication channels, and the method further comprises: sending the first location data in a first link direction and receiving the further location data in a remaining link direction. The method as claimed in any preceding claim, further comprising: sending the first location data repeatedly or continuously and optionally in real time. The method as claimed in any preceding claim, further comprising: sending the first location data via a direct RF or mm-Wave or 3G or 4G or 5G communication channel. The method as claimed in any one of claims 8 to 11 , further comprising: receiving the further location data repeatedly or continuously and optionally in real time. The method as claimed in any one of claims 8 to 12, further comprising: receiving the further location data via a direct RF or mm-Wave or 3G or 4G or 5G communication channel. The method as claimed in any preceding claim, further comprising: dynamically selecting a degree of beam divergence associated with electromagnetic radiation directed from the aerial platform towards the further node via at least one further optical element on the aerial platform, optionally wherein the further optical element is an adjustable beam expander, the method optionally further comprising: selectively increasing beam divergence to assist in establishing an optical communication link or decreasing beam divergence to increase a rate for determining cryptographic key information, and the method optionally further comprising: dynamically selecting a degree of beam divergence between 0.1 and 10 milliradians. The method as claimed in any preceding claim, further comprising: determining the first location data via a GPS chip and/or DGPS chip and optionally an altimeter associated with the aerial platform. The method as claimed in any preceding claim, further comprising: responsive to electromagnetic directed and/or received by the aerial platform, establishing at least one optical wireless communication link between the aerial platform and the further node, the optical communication link supporting a first optical channel for sending at least one photon of electromagnetic radiation having at least one measurable quantum state and optionally a further optical channel for sending classical optical signals. The method as claimed in any preceding claim, further comprising: utilising a discrete-variable QKD protocol to implement discretised encoding of cryptographic key information in a quantum state of at least one photon of electromagnetic radiation directed and/or received by the aerial platform. The method as claimed in any one of claims 1 to 16, further comprising: utilising a continuous-variable QKD protocol to implement discretised encoding of cryptographic key information in a quantum state of at least one photon of electromagnetic radiation directed and/or received by the aerial platform. The method as claimed in any preceding claim, further comprising: responsive to the first location data and to further location data indicative of a plurality of respective locations of a plurality of further nodes, directing a respective beam of electromagnetic radiation from a plurality of beams of electromagnetic radiation from the aerial platform in a direction towards a respective further node, and optionally wherein each beam of electromagnetic radiation directed towards each respective further node provides a respective cell coverage area, optionally the method further comprising: directing the plurality of beams of electromagnetic radiation to provide a substantially contiguous cell coverage area. The method as claimed in any one of claims 1 to 18, further comprising: directing a plurality of beams of electromagnetic radiation in a direction towards a further node; whereby each beam of electromagnetic radiation has an angular beam direction out of alignment with the remaining beams of electromagnetic radiation to thereby provide a substantially contiguous cell coverage area. A method for providing cryptographic key information at a further node of a communication network via communication of at least one photon of electromagnetic radiation having at least one measurable quantum state, the method comprising the steps of: receiving first location data, indicative of a location of an aerial platform, at a further node of a communication network from the aerial platform via a first, non- optical, wireless communication channel; responsive to the first location data and to further location data indicative of a location of the further node, directing electromagnetic radiation from the further node at the aerial platform and/or receiving electromagnetic radiation at the further node originating from the aerial platform; and determining cryptographic key information responsive to at least one measurable quantum state of at least one photon of the electromagnetic radiation directed from and/or received by the further node. An aerial platform configured to perform the method as claimed in any of claims 1 to 20, wherein optionally the aerial platform is a low altitude drone or a medium altitude aircraft or a high altitude platform, further optionally 'wherein the high altitude platform is a balloon or airship or fixed wing aircraft located in the stratosphere at an altitude between about around 17 and 22km. A further node of a communication network configured to perform the method as claimed in claim 21 , wherein optionally the further node is a stationary or mobile optical ground station. An aerial platform, comprising: at least one non-optical transmitter and/or transceiver element for transmitting non-optical wireless communication signals to a further node of a communication network; at least one processing element for determining location data, and wherein the processing element optionally comprises a GPS chip and/or DGPS chip, and wherein the processing element optionally further comprises an altimeter; at least one optical element for directing electromagnetic radiation at the further node and/or receiving electromagnetic radiation from the further node; at least one two-axis or three-axis gimbal for supporting the optical element; at least one source and/or at least one detector of electromagnetic radiation; and at least one further optical element for encoding and/or decoding at least one measurable quantum state on at least one photon of electromagnetic radiation. A communication network comprising: an aerial platform; and at least one further node of a communication network remote from the aerial platform; wherein the aerial platform is configured to: send first location data, indicative of a location of the aerial platform, to the further node via a first, non-optical, wireless communication channel; direct electromagnetic radiation at the further node and/or receive electromagnetic radiation originating from the further node responsive to the first location data and to further location data indicative of a location of a further node; and determine cryptographic key information responsive to at least one measurable quantum state of at least one photon of the electromagnetic radiation directed from and/or received by the aerial platform. A method of quantum key distribution, the method comprising the step of: exchanging information between an aerial platform and at least one further node of a communication network via at least one non-optical communication channel and at least one optical communication channel to thereby provide cryptographic key information at the aerial platform.