Single photons on demand

One hundred years since the introduction of the photon to explain the photoelectric effect, researchers are working towards creating single photons on demand. Quantum information technology is an important driver of this research effort, with single photons operating as carriers of quantum information for optical quantum computing and for quantum cryptography. Ideally, the creation of single photon on demand means providing exactly one photon, in a transform-limited wavepacket, precisely when it is required. In reality, a variety of criteria are used to characterize a single photon source, depending on the envisioned application. Weak coherent pulses, obtained by strongly attenuating a laser beam, in order to get closer to the ‘single-photon regime’, provide a good comparison scale. Such pulses have a Poissonian photon number statistics, which means that they present a large vacuum component (i.e. many pulses are empty), as well as non-negligible multiphoton contributions (i.e. a fraction of the pulses contain two photons or more). By contrast, a good source of single photons should be efficient, i.e. the vacuum component (the likelihood of not getting a photon) should be small. For a given probability to get a photon, the probability to get two photons or more should be negligible compared to the Poisson value. In addition it is desirable for applications that the source can produce a train of single photons on demand at a high repetition rate. The capability to produce single photons at or near room temperature is also desirable from a practical viewpoint. For quantum information applications, a rapid production of single photons is required to produce a large number of qubits per unit of time. Shaping of the spatio-temporal mode of the single photon is also important to provide optimal coupling efficiency with components of the quantum network, such as communication channels, quantum memory and detectors. In addition, optical quantum computation requires that all photonic qubits are identical, i.e., indistinguishable. In recognition of the importance of single photons and the rapid advances in the field of research, the open-access journal New Journal of Physics has published a Focus Issue on "Single Photons on Demand". Representing the absolute state-of-the-art in the field, the issue features articles from some of the leading research groups working on single photons. This review of the contributions therefore provides an overview of the research area in 2004 and an inspiration of what is to come. The advances reported in the issue include new and improved sources of single photons, characterization and applications of single photons, improved efficiency of single photon sources by interferometry and accepting the output field based on only certain photon counting outcomes, and preparing intracavity photon number states. Physical realizations of sources are quite varied, including quantum dots in pillar microcavities, parametric down-conversion, falling neutral atoms, trapped atoms in optical cavities, single defects in diamond nanocrystals, and individual molecules in a solid. The New Journal of Physics presents the latest developments in all these areas. Moerner, from Stanford University, presents results on using single terrylene molecules in host crystals of p-terphenyl, which operate at low (liquid helium) and room temperature. Controllable emission was demonstrated for single molecules in using adiabatic rapid passage. Terrylene is a particularly interesting source as it is exhibits high photostability under continuous, intense irradiation and can operate effectively at room temperature by exploiting fast pumping into vibrational sidebands of the electronically excited state. The single-photon emission at a detected count rate of 300,000 photons per second exhibits strong antibunching, which indicates a negligible rate of multiphoton events. Alléaume and co-workers from CNRS, the École Normale Supérieure, Laboratoire Kastler Brossel, and the Université Pierre et Marie Curie, employ the molecule DiIC18 as a room temperature single-photon source with a good photostability. They achieve strong antibunching and realized a careful analysis of intensity noise, in order to demonstrate an explicit sub-Poissonian photon statistics (negative value of the Mandel parameter) for the emitted train of single photons. Heinrich et al, from the Max-Planck Institut für Quantenoptik, obtain single photons from neutral atoms in an optical cavity. The cooled atoms fall randomly through a high-finesse optical cavity and are driven by a periodic sequence of laser pulses. The output light is strongly antibunched, but the Poisson statistics of the number of atoms in the cavity prevents sub-Poissonian photon statistics from the source. A collaboration by Santori et al, involving Stanford University, University of Tokyo and NTT Basic Research Laboratories of Japan, demonstrate single-photon generation with InAs quantum dots. These quantum dots are in pillar microcavities (see Fig. 1 for the schematic, scanning-electron microscope image of an actual pillar structure, and the optical excitation scheme for the quantum dot), and the effects on performance of the excitation wavelength and polarization, the collection bandwidth and polarization, are studied in detail. Measured results for efficiency of generating single photons and the purity of the states are reported, and they discuss prospects for improving these devices. Aichele et al from Humboldt University and from ETH in Zürich also consider single photons from quantum dots. They report photoluminescence measurements from single InP and CdSe quantum dots, which produce visible photons in the visible spectral range 510–690 nm, and characterize this photoluminescence by the autocorrelation function and via Fourier spectroscopy on several transitions in the quantum dot. In particular they observe and interpret carrier trapping and recapture in InP quantum dots, which leads to anomalies in the measured correlation functions. In an overlapping collaboration with the work reported in the previous paragraph, Zwiller et al from ETH in Switzerland and Humboldt University discuss single quantum dots as singlequantum emitters with all the requirements to generate single photons at visible and near-infrared wavelength. They also show that single quantum dots can be used to generate non-classical states of light, from single photons to photon triplets. A collaboration between the Universities of Stuttgart, Bremen and Würzburg, reported by Benyoucef et al, study enhanced