Single atom-photon quantum interfaces

Atom-photon interfaces will be one of the core building blocks in future quantum information protocols. While photons are ideal carriers for transporting quantum information over long distances, atoms can be used to store and process information.

A very clean implementation of quantum information storage uses the internal degrees of freedom of single, isolated atoms. By localizing a single atom in free space and coupling it to external light fields, we investigate single atom-single photon interactions, which are important for a basic understanding of the underlying physical processes of quantum interfaces.

Experimental Setup

We prepare a cloud of cold 87Rubidium atoms using a combination of laser cooling and trapping in a magneto-optical trap (MOT). By overlapping a tightly focussed optical dipole trap with the MOT, we can trap a single atom only. Two or more atom trapping events are prevented by the collisonal blockage effect.


Single atom Trap Setup

B - beam block, AL - aspheric lens, L1 - lens for collecting single atom fluorescence, L2 - lens for dipole trap laser collimation, F - filters, DM - dichoric mirror.

MOT

The picture above shows a cloud of cold 87Rubidium atoms trapped confined in the MOT - the whitish dot in between the two lens. A dipole trap beam (depicted in the setup as a yellow trace) is focussed into the cloud to trap a single atom.


Signature of a single atom

The manifestation of a single atom in the trap is an anti-bunching effect in the atomic fluorescence. It is revealed in the dip in the histogram of time difference between two photodetection events observed when the atomic fluorescence is coupled to single photon detectors in a Hanbury-Brown-Twiss configuration.

Anti-bunching dip from single atom fluorescence

Our measurement reveals clearly anti-bunching for zero time delay, after taking into account for the noise caused by accidental pair events due to detector dark counts and ambient scattering. This clearly proves that we have a single atom in the observation region of our two detectors.

Reference:

[1] N. Schlosser, G. Reymond, P. Grangier, Phys. Rev. Lett. 89, 023005 (2002)
[2] N. Schlosser, G. Reymond, I.E. Protsenko, and P. Grangier, Nature 411, 1024 (2001)
[3] M. Weber, et al., Phys. Rev. A 73, 043406 (2006)