Quantum cryptography is an emerging technology in which two parties may simultaneously generate shared, secret cryptographic key material using the transmission of quantum states of light. A unique aspect of quantum cryptography is that Heisenberg’s uncertainty principle ensures that if Eve attempts to intercept and measure Alice’s quantum transmissions, her activities must produce an irreversible change in the quantum states that are retransmitted to Bob. These changes will introduce an anomalously high error rate in the transmissions between Alice and Bob, allowing them to detect the attempted eavesdropping.
QKD is suitable for use in any key distribution application that has high security requirements including financial transactions, electoral communications, law enforcement, government, and military applications. Military is also transitioning to Quantum cryptography to takes advantage of the properties of matter in addition to the principles of mathematics to create a cryptosystem that cannot be broken with unlimited computing power (even with a quantum computer). QKD has already been successfully demonstrated in different contexts such as fibre-to- fibre, and free-space ground-toground as well as ground-to-air communications.
Current limitations of QKD are high cost of dedicated hardware, transmission distance limited to few tens of kilometres, and low key transmission rate. However, for many applications for which strong security conditions must be met, QKD will likely become an increasingly attractive option in the upcoming years. Now the technology is sufficiently mature to move to practical deployments.
Current best-in-class QKD protocols and implementations are limited to point-to-point key exchange. A future quantum-secure network will require a set of reconfigurable local nodes that can exchange keys individually with other nodes and mediate connections between two or more actors as a trusted cryptographic third party. Free-space quantum cryptographic platforms naturally lend themselves to reconfiguration, as nodes may be moved and reoriented to target new nodes with relative ease.
QKD is now about going over longer distances and cutting the costs. As well as being used on Earth it is also being tested in space and with drones. The goal is to have a fully functional constellation of satellites, not just one.
Drone to Ground Quantum Key Distribution Systems
Free-space quantum communication over any significant distance, will inevitably encounter signal degradation due to weather events (such as fog) and turbulence. These effects are significantly mitigated by launching the quantum signal from a higher altitude — in the case of turbulence, even an increase of tens of meters in launch height may reduce the effects of scintillation by orders of magnitude. Having access to an agile, reconfigurable QKD networking system will enable quantum cryptography to reach applications prohibited by current approaches, such as temporary networks in seaborne, urban, or battlefield situations.
Recent advances in Lithium battery technology and control theory have made multicopter drones (or Unmanned Aircraft System(s) – UAS) stable and reliable enough to be a viable candidate for an agile QKD node. Because even enterprise-level, non-military UAS have maximum payloads on the order of 10 kg, developing lightweight yet robust hardware for free-space QKD is a prerequisite for a UAS-based approach.
Project Q-DOS aims to deliver a QKD module using compact, cutting-edge photonic waveguide technology, which will allow low-SWaP aerospace requirements to be met. This module uses 1550 nm single photons to implement a BB84 protocol, and will enable the demonstration of a secure, high-speed optical communication data link (~0.5 Gbps) between a drone and a ground station. The targeted link range is 1 km.
In 2017, University of Bristol took Q-DOS : QKD for Drones with Optimal Size weight and power programme. Lightweight Unmanned Aerial Vehicles (UAVs) have seen a huge increase in commercial uptake in recent years, but their applications have been limited, in part by the inability to securely communicate highly sensitive data back to the ground. The aim of Project Q-DOS (QKD for Drones with Optimal Size weight and power) was to solve this problem by delivering a unique quantum encrypted communication system with an eavesdropping detection feature between an airborne platform and a ground-based station.
This hyper-secure system is based on Quantum Key Distribution (QKD), which provides future proof communications security combined with the novel ability to detect eavesdroppers. A key challenge is meeting the demanding Size, Weight and Power (SWaP) requirements as the system will have to be deployed on a lightweight (under 7kg) drone. This will be achieved by using the novel integrated quantum optical QKD chips combined with flight-proven optical communications system developed by project members. “We expect the outcome of this project not only to be a step change in capability in the secured drone market (both military and commercial), but to open up a significant number of areas, moving QKD away from niche applications and towards mainstream adoption.”
The airborne communications module, including the QKD terminal, tracking modules, traditional communications systems, optics and control electronics, must not exceed a mass of 5 kg and a power consumption of 20 W.
The Centre for Quantum Photonics at the University of Bristol is one of the world leaders in the development of integrated quantum photonics technology. It has a wealth of practical knowledge (also in free-space QKD experiments) which it hopes to contribute to the project. In particular, UoB will collaborate in the development of detector technologies and provide detector characterisation facilities, provide a combination fast and slow driving electronics, all combined with significant person-power of expertise of QKD systems and field experiments.
Quantum Communication between drones
Researchers from, The University of Illinois at Urbana-Champaign and The Ohio State University focused on developing a functioning optical payload for QKD capable of maintaining pointing between a transmitter node and receiver node while both are in flight. Research in this area is separated into several sub areas: fast, high-resolution optical stabilization; compact, independent, and identically distributed sources; and compact, lightweight singlephoton detection.
One of the critical challenge is signal acquisition and stabilization. Stabilization system that can compensate for the
normal movements of the UAS during flight. Initial acquisition is performed by rotating each UAS autonomously until each side “sees”’ their alignment laser returned from the other side’s corner-cube array. This ensures that the UAS themselves are approximately properly situated. Fine pointing is achieved by using steering mirrors to keep each drone’s quadrant detector locked on the partner drone’s alignment laser. After initial acquisition, each side centers the opposing side’s alignment laser on a local quadrantcell photodiode using its own steering mirrors. This orients each drone toward the other and establishes a robust link.
Spectrally filtered resonant-cavity light-emitting diodes operating at 650 nm will be used to generate the quantum photonic states as well as decoy states. For the transmitter, we are developing a polarization-based BB84 protocol that uses R/L polarization for the information channel and only the H polarization for security checking. These emitters can be modulated on the few nanosecond time scale and can be directly driven by a field-programmable gate array (FPGA). The receiver will use silicon avalanche photodiodes with custom-built, fast quench read-out and drive electronics. A FPGA will be used to time-tag the single-photon events and time synchronization across the platforms will be based on modulating the classical alignment and tracking beams. The microprocessor also interprets sensor data from the optical payloads (such as quadrant-cell photodiode signals) and instructs the payload steering mirrors how to respond
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