Quantum information research spans many different topics and disciplines in physics. NPL research into trapped ions, single photons and superconducting devices is now being directed towards the precision measurement challenges presented by these quantum technologies.

The most desirable system for creating a quantum information processor is not yet clear. All prospective systems must be considered against the 'Di Vincenzo Criteria' to determine their viability. This has been done in the quantum computation roadmap.

Many significant demonstrations in experimental quantum information research have been made using trapped ions, by the groups of Wineland at NIST Boulder, Blatt at Innsbruck, and Monroe at Michigan. For example, quantum gates^[1], algorithms^[2], and deterministic teleportation^[3] have been demonstrated using ion strings.

Quantum entanglement of ions in the string is a crucial feature of these advances. Entanglement is also a resource that permits precision measurements beyond the noise limit of a classical system. For N entangled particles, the signal-to-noise scales as 1/N, reaching the 'Heisenberg Limit' and surpassing the classical limit for unentangled particles that scales as 1/√N.

NPL is developing microfabricated ion trap chips and associated techniques for creating strings of entangled ions. These ion strings will be used to investigate precision measurements with entangled states, and develop capabilities to quantify the performance of ion trap processor chips.

Quantum information processing using superconducting circuits is a rapidly developing field. NPL is developing techniques for entangling superconducting circuits with single photons; a technology that has potential applications such as on-demand single photon generation in the microwave regime, quantum computing and quantum communication.

Products for quantum key distribution are now available in the marketplace from companies such as ID Quantique (IDQ), Toshiba Europe Research Ltd (TREL), and MagiQ Technologies yet there are no agreed standards to describe device performance. Single-photon sources and detectors are likely to be used in future generations of key distribution systems, as well as in photon-based quantum information processing schemes. The quantum cryptography roadmap sets out the technology's achievements and likely future direction. NPL is researching sources of single and entangled photons, with a view to developing a suite of metrics to describe the performance of such photon sources.

References

1. F. Schmidt-Kaler et al, Nature vol 422, p 408 (2003); D. Leibfried, et al, Nature vol 422, p 412 (2003).
2. S. Gulde, et al, Nature vol 421, 48 (2003); K.-A. Brickman et al, Phys. Rev. A vol 72, 050306R (2005).
3. M. D. Barrett, Nature vol 429, p 737 (2004); M. Riebe et al, Nature vol 429, p734 (2004).