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Quantum Comms

Wednesday 26 June 2019

Quantum security refers to communications that use QKD and QRBG (Quantum Random Bit Generator) to securely transmit data. Networks up to 100km employing QKD are being developed and repeaters aim to support longer-distance distribution of cryptographic keys through such networks.

In the near-term, quantum random number generators and point-to-point quantum key distribution are at, or close to being, market-ready. QKD networks and the necessary quantum repeaters are mid-term to long-term technologies in terms of readiness.

The inherently random nature of quantum mechanics, means QRNG, unlike other technologies, generates truly random numbers resulting in stronger cryptographic keys. Commercially available QRNG systems are able to provide up to 1 Gbps outputs, demonstrating technology compatible with commercial servers and certified to national standards in several countries.

Quantum key distribution exploits quantum mechanics. It allows two parties to share a cryptographic key with the advantage of being able to detect if a third-party eavesdropper is intercepting communications. QKD in its most common form uses individual photons to convey the separate bits of a cryptographic key. QKD systems using fibre-optics are currently able to operate over standard telecoms fibre links, either as dedicated fibres or as fibres carrying other traffic. However, transmission through optical fibres is limited by losses that increase rapidly with distance. For example, single photons sent natively over 1,000km of optical fibre at rates of 10GHz, would require a receiver to wait over a hundred years to detect just one. This has limited the range of QKD to distances just over a hundred kilometres meaning that repeaters are needed to extend the range.

Existing telecommunication solves the problem of optical fibre losses by using amplifiers along a fibre to boost the signal. Unfortunately, these can’t be used in QKD networks because they would destroy the quantum characteristics of the photons. This isn’t helped by the fact that quantum physics precludes the copying of quantum information – so simply reproducing an amplified copy of the quantum data is impossible.

A solution to this is to use the weird properties of ‘entanglement’. A photon in an entangled pair is able to mutually share information (its state) with another, effectively replicating itself, even when separated over great distances. A quantum repeater, as with existing nonquantum repeaters, breaks-up the transmission length into lower-loss segments. A photon reaching the end of a segment influences a photon at the beginning of the next segment to create an entangled twin instantaneously. This allows quantum information to be preserved and, effectively, teleported into the next segment of the fibre-optical path for onward transmission. To achieve this a quantum memory is needed and a photon received by the quantum memory of one segment of the transmission line is not measured and destroyed but stored while the next transmission segment is made ready to receive and replicate the quantum information.

A critical element for this long distance QKD scheme is to create quantum memories for storing photon qubits. These memories need to be able to store qubits reliably for durations long enough to satisfy network transmission times – this means about 100ms for a global communication network. While they are not yet commercially available, they are being developed in a wide range of technology platforms such as ensembles of trapped atoms or ions or using solid-state crystals. Research at Australia’s Centre for Quantum Computation and Communication Technology has dramatically improved the storage time of a quantum memory. It can also operate in the same optical wavelength as current telecommunications infrastructure, making it compatible with existing the fibre-optic cables. Using erbium ions in a crystal, the researchers have been able to store quantum information for more than a second – a huge 10,000 times longer than other attempts and brings a global quantum network a step closer.

Nevertheless, there is also a work around called free-space QKD at hand. Links to satellites have been developed because optical losses for freespace transmissions increase much less rapidly with distance than fibre-optic cables. The Chinese government launched the Quantum Experiments at Space Scale (QUESS) satellite in 2016 and demonstrated QKD data transmission rates of several tens of kilohertz from low-Earth orbit to ground stations, requiring a relatively modest one metre aperture receiver telescope.

Dark Chatter

The QUESS satellite has created a QKD channel between Vienna and China, establishing the first secure intercontinental quantum video call. While the QUESS system has its limitations – it cannot operate in sunlight and needs line-of-sight to communicate with a ground station – this is, without doubt, an impressive achievement. Canada, Singapore, Japan and several European states each now have their own, or joint, satellite QKD project for low-Earth, medium-Earth, and even geostationary orbits.

Civil and military operations already base many strategic services on satellite communications and QKD may, in future, offer a much greater level of security for ground-to-satellite communications. This opens the possibility of a QKD satellite constellation offering high availability, secure communications between remote ground stations, with a high level of availability.

Core to QKD systems is the ability to provide single, identical photons for encoding reliably and on-demand, and to subsequently detect them accurately for decoding. One way to provide single photons is to attenuate regular laser pulses, containing many photons, to a single photon in each pulse. However, a growing opinion is that laser-based single-photon sources area bottle neck to ultraviolet to the mid-infrared spectral range, with dead-times as low as a few nanoseconds and ‘dark-counts’ (false detection noise) below one hertz.

In spite of these impressive developments, the widely held view is that data rates, cost-effective integration, and certification need to improve before QKD technologies will be widely deployed. This motivates many of those aiming to disrupt the telecommunications field with new quantum technologies. Robert Young of Quantum Base explains that the potential of QKD should not be limited to customers with deep pockets, so a focus should include developing scalable, practical products: “The promise of quantum communications is incredibly exciting, as it represents a new means to share information that is fundamentally secure.”

(Fearnside, A., 2019, Quantum Computing, Cambridge Wireless Journal, Vol. 2, Issue 4, p. 26-29).

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