Single photon sources

At a single-photon source is a fluorescent light source, wherein no two or more photons are emitted simultaneously. Required single photons in particular in quantum information processing and quantum cryptography.

In contrast to the laser, in which to build a plurality of photons by stimulated emission at a coherent beam, the single-photon source based on spontaneous emission light. It is therefore not a very weak laser, but a structure which just wants to prevent the same time a second photon is emitted in the same direction.

Construction

The probability of spontaneous emission can be increased due to the Purcell effect by a high Q resonator. It is crucial that the emitting atom is kept as quiet as possible, which can be ensured by different arrangements:

Individual atoms or ions in cold traps

In particular, the original work by L. Mandel and H. Walther based on that ions were recorded in magnetic cooling traps. The wavelength of the emitted light then corresponds just to a transition of the ion. In order to avoid Doppler effect and thermal collisions, the ions must be cooled deep. Due to the photon antibunching effect is ensured that only a second photon is emitted when the ion absorbs another photon. Thus the photons have another minimum time interval, which is in the described systems on the order of 10 ns.

Color centers in solids

At color centers are defects in a solid state, such as a diamond or a semiconductor having a low band gap. In the solid state the atomic light sources have already been fixed, so no minimum and no strong magnetic field are necessary. Since it may be, for the defects to different effects, such as gaps, impurities, holes, charge barrier, so it is not at the emitted wavelength usually an ordinary transition of an atom.

Quantum dots

Quantum dots are nanoscopic atomic clusters on a semiconductor substrate. For the single photon source usually a CdSe - ZnS structure to a matrix, or InGaAs - GaAs structure is applied to a matrix. Quantum dots are a very good and effective way to generate single photons; However, the degree of indistinguishability of the photons is not very high and they are not as easy to use in quantum cryptography.

Parametric fluorescence

A frequently used method is the parametric fluorescence (English parametric down conversion, PDC) dar. Here a high-energy photon into two photons of half the energy is converted into a nonlinear crystal. Both photons may be entangled with each other, i.e., they have a common state even though they are spatially separated. The great advantage of this method is that the second photon can be used to determine the time at which the single photon leaving the single-photon source. This is a feature that allows many experiments in quantum optics and quantum information only.

Trivia

There are now finished first commercial single photon sources to buy.