Photonic quantum devices based on atomic vapors at room temperature combine the advantages of atomic vapors being intrinsically reproducable as well as semiconductor-based concepts being scalable and integrable. One key device in the field of quantum information are on-demand single-photon sources. Comparable to similar realizations using cold atoms , it has been supposed to realize room-temperature single-photon sources by combining the two effects of four-wave mixing and Rydberg blockade . Driving the four-wave mixing cycles in a pulsed manner, single photons are generated on demand.
The essential conditions of coherence [3,4] and sufficient Rydberg-Rydberg interaction strengths  have already been demonstrated. Up to now, the third condition has been missing: That is performing the excitation cycles in a low-dimensional geometry in order to obtain an excitation blockade of the whole volume by only one Rydberg excitation.
We realize this spatial confinement in transversal direction by focusing one of the excitation beams by means of a high-NA lens, and in longitudinal direction by using vapor cells of about one micrometer inner thickness (“µ-cells”).
In order to deal with reasonable numbers of atoms in such a small volume, we exploit the fact that in thermal equilibrium there are large numbers of adsorbed atoms on the glass surface. We can easily photo-desorb them shortly before the four-wave mixing cycles and increase by this the optical thickness in the excitation volume by one order of magnitude.
By means of these techniques we are now able to generate single photons at 780nm. With a Rydberg blockade radius of 1.0µm and a cell thickness of 1.2µm, we observe pure anti-bunching in the light statistics of the coherent emission field, deviating with a significance of three standard deviations from Poisson-type statistics.
We obtain generation efficiencies of currently up to 5% per four-wave mixing cycle, depending on experimental parameters.
We systematically investigate the disappearance of the anti-bunching by increasing the cell thickness. The results are also compared to measurements at smaller blockade radii.
This technique can be further improved by investigating different Rydberg states or different multi-level schemes which allow exploiting the latest developments in laser research.
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 M. M. Müller et al., PRA 87, 053412 (2013)
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Fabian Ripka, Harald Kübler, and Robert Löw, "On-demand single-photon source based on thermal rubidium (Conference Presentation)," Proc. SPIE 10674, Quantum Technologies 2018, 106740N (Presented at SPIE Photonics Europe: April 24, 2018; Published: 29 May 2018); https://doi.org/10.1117/12.2309776.5788750515001.
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