Sources of single and entangled photons are of use for photon-based quantum information processing and quantum key distribution.

The defining characteristic of a single-photon source is that it emits one and only one photon at any given time. A single quantum emitter, such as an individual atomic particle, molecule, crystal defect, or semiconductor quantum dot, can be used to generate individual photons. The challenge in creating a useful single-photon source is to capture the emitted photons efficiently, and know that their characteristic parameters are predictable. A review article on this topic^[1] gives an overview the various physical systems under study, together with the measurement techniques used to characterise sources.

Several quantities define the performance and subsequent utility of any given single-photon source. To determine how well a source emits one and only one photon, the photon flux is subject to a coincidence measurement using a Hanbury-Brown & Twiss interferometer^[2]. The parameter measured is known as g⁽²⁾(τ=0); for a Poissonian source, g⁽²⁾(τ=0) = 1, and for an ideal single-photon source g⁽²⁾(τ=0) = 0. In practice g⁽²⁾(τ=0) can get quite close to zero, but is usually limited to a value slightly greater than this. This measurement is based on the particle nature of the photons.

The spectral purity of the emitted photons may be quantified by measuring their coherence length (and hence coherence time τ_c) with a Michelson interferometer; this measurement relies on the wave nature of the photons. The emitter's excited-state lifetime τ_s may be measured by a couple of different techniques, and together with the coherence time, determines the value of 2τ_s/τ_c. This is an important figure of merit, since it quantifies the degree of indistinguishability of the emitted photons. When the photons are Fourier-transform limited and completely indistinguishable, 2τ_s/τ_c = 1.

Indistinguishable photons are desirable for linear optics schemes to generate entangled photon states. A further measure of indistinguishability is given by the Hong-Ou-Mandel experiment^[3], where two photons are overlapped at a beamsplitter. If the photons are perfectly indistinguishable, then they stick together and do not exit the beamsplitter on separate ports. The measurement yields what is known as the overlap integral V, which assists in determining the viability of using a source to generate entangled photon states.

It should therefore be evident that a suite of metrics is necessary to determine the utility of a specific single-photon source for quantum information applications. NPL now has the facilities to perform all of these measurements. This is now being extended to include techniques for quantifying the fidelity of entangled state generation and quantum optical processes.

References

1. M Oxborrow and A G Sinclair, Contemporary Physics, vol. 46, p 173 (2005).
2. R Hanbury Brown and R Q Twiss, Nature, vol. 177, p 27 (1957).
3. C K Hong, Z Y Ou and L Mandel, Phys. Rev. Lett. vol. 59, p 2044 (1987).