Solid State Sources for Single Photons

We have combined cryogenic scanning probe techniques with laser spectroscopy of single molecules. By scanning a nanocrystal doped with fluorescent molecules through a local inhomogeneous electric field, we have been able to identify and locate two individual molecules that were separated by 12±2 nm. In addition, we have shown that these two molecules were coupled via the coherent dipole-dipole coupling, leading to two sub- and superradiant states. By increasing the intensity of the incident laser beam, we have been able to put into evidence a two-photon transition that takes place at the frequency midway between the original transition frequencies of the system. Furthermore, we have measured the photon statistics of the fluorescence light and shown that when exciting the system at the new frequency we observe photon bunching whereas excitation of the system at any of the original resonances yields photon antibunching. All our observations are verified by comparison with the outcome of calculations based on a master equation approach.

We have obtained pairs of correlated single photons from the emission cascade of an isolated InAs quantum dot. The cross correlation function of the two photons in a pair exhibits the co-existence of asymmetric bunching and antibunching features, which is the signature for their sequential emission with a definite order. This observation opens the way to the use of semiconductor quantum dots as triggered sources of photon pairs with strong quantum correlations for quantum information applications.

We have carried out the first full implementation of a quantum cryptography protocol using a stream of single photon pulses generated by a stable and efficient source operating at room temperature. The single photon pulses are emitted on demand by a single nitrogen-vacancy (NV) colour centre in a diamond nanocrystal (deliverable 1.2). The quantum bit error rate is less that 4.6 % and the secure bit rate is 5800bits/s. The secret key has been distributed over 50m in free space. Using the published criteria that warrant absolute secrecy of the key against any type of individual attacks, we have shown that the overall performances of our system reaches a domain where single photons have a measurable advantage over an equivalent system based on attenuated light pulses.

We report on the realisation of a stable solid-state room temperature source for single photons. It is based on the fluorescence of a single nitrogen-vacancy (NV) colour canter in a diamond nanocrystal. Antibunching has been observed in the fluorescence light under pulsed excitation. Our source delivers 10^5 s-1 single-photon pulses at an excitation repetition rate of 5MHz. The number of two-photon pulses is reduced by a factor of 14 compared to the strongly attenuated coherent sources.

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