It is safe to say that, when the concept of the photon was first introduced, the generation of single photons was not being considered. Until the emergence of the ideas and methods of quantum optics, the `single-photon regime' was attained by strongly attenuating a laser beam to ensure that the probability of having more than one photon became negligible. However, such attenuated beams differ from `true' single photons in at least two respects: first, the vacuum probability is much higher than the probability of detecting a photon, so one gets predominantly a `no-photon' regime with occasional detection of a photon; second, the probability of getting two photons is never zero, but essentially follows a Poisson law.

Although the weak beam has been useful in quantum optics, the advent of quantum information science has placed stringent demands on optical sources, namely that sources produce single photons either on demand or heralded. In particular secure quantum cryptography and linear optical quantum computing depend on the availability of such single-photon sources. The combination of strict requirements for single photons, plus new technologies, is driving an exciting research effort into single-photon generation.

This Focus Issue recognizes that single-photon sources are rapidly developing and presents research articles covering the spectrum of activity in the field. Significant advances are reported for single-photon and photon-pair sources constructed from quantum dots in pillar microcavities, parametric down converters, falling neutral atoms and trapped ions in cavities, defects in diamond nanocrystals and a single molecule in a solid. In addition to producing single-photon pulses, one article reports on the latest developments regarding the production of photon number states in a microwave cavity. Another article discusses the application of a nitrogen-vacancy-based single-photon source to an experimental demonstration of quantum key distribution in open air.

Photodetection performs a critical role in assessing single-photon sources as well as heralding the arrival of a single photon based on detecting a correlated partner, and this issue presents the latest results on single-photon counting with a superconducting niobium nitride hot-electron bolometer. Another article provides a detailed study of how to characterize single-photon sources via intensity noise analyses. Photodetection is also important for post-selection-based processing for non-deterministic quantum information tasks, and this issue presents an analysis of photodetection and post-selection used in another way: enhancing the efficiency of single-photon sources by interferometry, photodetection and post-selection.

New Journal of Physics appreciates the rapid pace of development in the area of single photons on demand. This Focus Issue presents the latest developments and also points to a bright future with better photon sources.

Focus on Single Photons on Demand Contents

Photon statistics characterization of a single-photon source
R Alléaume, F Treussart, J-M Courty and J-F Roch

Photon statistics of a non-stationary periodically driven single-photon source
M Hennrich, T Legero, A Kuhn and G Rempe

On the measurement of two-photon single-mode coupling efficiency in parametric down-conversion photon sources
S Castelletto, I P Degiovanni, A Migdall and M Ware

Single-photon sources based on single molecules in solids
W E Moerner

Single-photon generation with InAs quantum dots
Charles Santori, David Fattal, Jelena Vuckovic, Glenn S Solomon and Yoshihisa Yamamoto

Visible single-photon generation from semiconductor quantum dots
Thomas Aichele, Valéry Zwiller and Oliver Benson

Enhanced correlated photon pair emission from a pillar microcavity
M Benyoucef, S M Ulrich, P Michler, J Wiersig, F Jahnke and A Forchel

Experimental open-air quantum key distribution with a single-photon source
R Alléaume, F Treussart, G Messin, Y Dumeige, J-F Roch, A Beveratos, R Brouri-Tualle, J-P Poizat and P Grangier

Post-processing with linear optics for improving the quality of single-photon sources
Dominic W Berry, Stefan Scheel, Casey R Myers, Barry C Sanders, Peter L Knight and Raymond Laflamme

A single-photon source based on a single Ca⁺ ion
Christian Maurer, Christoph Becher, Carlos Russo, Jürgen Eschner and Rainer Blatt

A calcium ion in a cavity as a controlled single-photon source
M Keller, B Lange, K Hayasaka, W Lange and H Walther

Quantum optics with single quantum dot devices
Valéry Zwiller, Thomas Aichele and Oliver Benson

The creation and detection of arbitrary photon number states using cavity QED
Benjamin T H Varcoe, Simon Brattke and Herbert Walther

Stable single-photon source in the near infrared
T Gaebel, I Popa, A Gruber, M Domhan, F Jelezko and J Wrachtrup

Colloidal CdSe/ZnS quantum dots as single-photon sources
X Brokmann, G Messin, P Desbiolles, E Giacobino, M Dahan and J P Hermier

Towards a periodic deterministic source of arbitrary single-photon states
Evan Jeffrey, Nicholas A Peters and Paul G Kwiat

Fabrication of a superconducting niobium nitride hot electron bolometer for single-photon counting
R Romestain, B Delaet, P Renaud-Goud, I Wang, C Jorel, J-C Villegier and J-Ph Poizat

High-quality asynchronous heralded single-photon source at telecom wavelength
Sylvain Fasel, Olivier Alibart, Sébastien Tanzilli, Pascal Baldi, Alexios Beveratos, Nicolas Gisin and Hugo Zbinden

Philippe Grangier, Institut d'Optique Théorique et Appliquée, Orsay, France
Barry Sanders, University of Calgary, Canada
Jelena Vuckovic, Stanford University, CA, USA