Two-photon gateway and real-time feedback control of a single atom in a cavity

Hyosub E. Kim, Woojun Lee, Jaewook Ahn

Alexander Kubanek

This thesis reports on experiments with single atoms strongly coupled to a single mode of a high-finesse optical cavity. In the first part, it is shown that the motion of the atom in an intracavity dipole trap can be observed with high spatial and temporal resolution. Depending on the atom's current position, the dipole trap is switched to control the motion of the atom. The feedback acts in real time, faster than one oscillation period of the atom in the trap. In the second part the anharmonic energy level structure of the system is investigated. This anharmonicity allows one to selectively populate states containing two energy quanta in a two-photon process, resulting in a nonlinear response of the system with respect to the drive laser intensity. The photon statistics of the light transmitted through the cavity shows photon bunching and super-Poissonian fluctuations, indicating the transmission of photon pairs typical for a two-photon gateway.

Alexander Kubanek, M. Koch, Christian Sames +4 more

Alexander Kubanek, Alexei Ourjoumtsev, I. Schuster +4 more

Single atoms absorb and emit light from a resonant laser beam photon by photon. We show that a single atom strongly coupled to an optical cavity can absorb and emit resonant photons in pairs. The effect is observed in a photon correlation experiment on the light transmitted through the cavity. We find that the atom-cavity system transforms a random stream of input photons into a correlated stream of output photons, thereby acting as a two-photon gateway. The phenomenon has its origin in the quantum anharmonicity of the energy structure of the atom-cavity system. Future applications could include the controlled interaction of two photons by means of one atom.

Alexander Kubanek, M. Koch, Christian Sames +4 more

T W Lynn, K Birnbaum, H J Kimble

Recent realizations of single-atom trapping and tracking in cavity QED open the door for feedback schemes which actively stabilize the motion of a single atom in real time. We present feedback algorithms for cooling the radial component of motion for a single atom trapped by strong coupling to single-photon fields in an optical cavity. Performance of various algorithms is studied through simulations of single-atom trajectories, with full dynamical and measurement noise included. Closed loop feedback algorithms compare favorably to open-loop "switching" analogs, demonstrating the importance of applying actual position information in real time. The high optical information rate in current experiments enables real-time tracking that approaches the standard quantum limit for broadband position measurements, suggesting that realistic active feedback schemes may reach a regime where measurement backaction appreciably alters the motional dynamics.