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Depth imaging

In recent years there has been increasing interest in the development of single‑photon counting lidar for long‑range three‑dimensional imaging for a number of remote sensing applications.

One reason for this is the recent availability of Geiger-mode (Gm) arrays which provide full frame data acquisition with single-photon sensitivity and picosecond resolution.

The technology has also found applications in airborne surveillance where long-range target identification through turbulence presents an engineering challenge.

The use of lower power laser sources means that single-photon detection will have greater level of covertness and is less likely to exceed eye‑safety thresholds. Applications such as airborne surveillance using active imaging impose limits on system weight, size and volume and necessitate low-power laser sources and highly sensitive optical detection.

Single‑photon lidar is a candidate technology that has the potential to meet these challenging requirements.

Quantum information

Quantum information is an active research field where quantum properties of single qubits are exploited to enhance speed, versatility, and efficiency of measuring and manipulating quantum information.

Quantum systems can provide information theoretical security to many modern digital communications protocols enhancing their security against malicious parties.

Quantum key distribution (QKD) and quantum digital signatures (QDS) are some of these alternative communications protocols which aim to remove the potential vulnerabilities of public-key encryption systems.

Such systems, however, require very stringent conditional operation which limit the communication distances to only a few hundreds of kilometres. Therefore, new sophisticated probabilistic amplification techniques need to be adopted in order to extend such distances whilst limiting the amount of noise introduced by the overall mechanism.

Another fundamental branch of quantum information is randmoness generation and statistical testing.

Quantum phenomena are described by the intristic probabilistic nature of physical systems thus defining the perfect candidate as quantum random generators (QRNG).

Single‑photon detection

Single‑photon detection at visible and low‑end near infrared wavelengths (in the range 400 to 1060 nm) based on silicon (Si) is a relatively mature technology with several efficient devices commercially available but single‑photon detectors for longer wavelengths are still largely experimental.

This relative lack of maturity is of increased relevance when undertaking quantum information experiments in optical fibre as the majority of installed optical fibre is single‑mode at wavelengths around 1300 nm and 1550 nm.

The most common semiconductor‑based devices are single‑photon avalanche diode (SPAD) detectors, which are reverse biased avalanche photodiodes(APDs) biased above the avalanche breakdown voltage in the Geiger mode.

Combination of Si devices with new structured germanium (Ge) substrates would enhance both low‑noise capabilities and infra‑red optical sensing ranges.

Research is concentrating on improving detection efficiency and reducing dark count rates, as well as improving packaging for practical systems.