Single photon source
A single photon source is a fluorescent light source that never emits two or more photons at the same time . Individual photons are needed in particular in quantum information processing and quantum cryptography .
construction
The probability of spontaneous emission can be increased due to the Purcell effect by using a resonator of high quality . It is crucial that the emitting atom is kept as still as possible, which can be ensured by various arrangements:
Individual atoms or ions in cold traps
In particular, the original work of L. Mandel and H. Walther was based on the fact that ions were trapped in magnetic cold traps . The wavelength of the emitted light then corresponds to a transition of the ion. In order to avoid the Doppler effect and thermal collisions, the ions have to be cooled down deeply. The " photon antibunching " effect ensures that a second photon is only emitted when the ion has absorbed another photon. Accordingly, the photons have a minimal time interval from one another, which in the systems described is of the order of 10 ns.
Color centers in the solid
In color centers is defects in a solid, eg. B. a diamond or a semiconductor with a low band gap. The atomic light sources are already firmly fixed in the solid body, which is why no cryogenic temperatures and no strong magnetic field are necessary. Since the defects can be a matter of various effects, such as spaces, foreign atoms, holes, charge barriers, the emitted wavelength is usually not a normal transition of an atom.
There are commercial single photon sources based on diamond voids.
Quantum dots
Quantum dots are nanoscopic clusters of atoms on a semiconductor substrate . For the single photon sources, a CdSe structure is usually applied to a ZnS matrix or InGaAs structure to a GaAs 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 so easy to use in quantum cryptography .
Parametric fluorescence
A frequently used method is parametric fluorescence ( parametric down conversion , PDC). Here, a high-energy photon is converted into two photons of half the energy in a non-linear crystal. Both photons can be entangled with one another , i.e. that is, 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 when the single photon leaves the single photon source. This is a property that makes many experiments in quantum optics and quantum information possible in the first place.
literature
- Philippe Grangier , Barry Sanders, Jelena Vuckovic: Focus on Single Photons on Demand . In: New Journal of Physics . tape 6 , 2004, doi : 10.1088 / 1367-2630 / 6/1 / E04 .
Web links
- parametric down conversion ( memento of September 26, 2008 in the Internet Archive )
Individual evidence
- ^ HJ Kimble, M. Dagenais, and L. Mandel: Photon antibunching in resonance fluorescence . In: Phys. Rev. Lett. No. 39 , 1977, pp. 691 .
- ^ F. Diedrich and H. Walther: Nonclassical radiation of a single stored ion . In: Phys. Rev. Lett. No. 58 , 1987, pp. 203 .
- ↑ Brahim Lounis, Michel Orrit: Single-photon sources . In: Rep. Prog. Phys. tape 68 , no. 5 , April 21, 2005, p. 1129 , doi : 10.1088 / 0034-4885 / 68/5 / R04 .
- ↑ C. Braig: Solid-state-based single photon source. (PDF; 2.7 MB) Diploma thesis at the Faculty of Physics. LMU Munich, December 14, 2001, accessed on March 5, 2012 (German, see introduction).
- ↑ Igor Aharonovich, Dirk Englund, Milos Toth: Solid-state single-photon emitters . In: Nature Photonics . tape 10 , 2016, p. 631-641 , doi : 10.1038 / nphoton.2016.186 .
- ↑ SPS1.01. (PDF; 416 kB) (No longer available online.) Quantum Communications Victoria, archived from the original on September 12, 2009 ; Retrieved May 1, 2010 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
