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Physical Review Letters

Feedback on the Motion of a Single Atom in an Optical Cavity

Thomas H. Fischer, Peter Maunz, Pepijn W. H. Pinkse, Thomas Puppe, Gerhard Rempe

Published Apr 4, 2002• 97 citations
Publisher

Abstract

We demonstrate feedback on the motion of a single neutral atom trapped in the light field of a high-finesse cavity. Information on the atomic motion is obtained from the transmittance of the cavity. This is used to implement a feedback loop in analog electronics that influences the atom's motion by controlling the optical dipole force exerted by the same light that is used to observe the atom. In spite of intrinsic limitations, the time the atom stays within the cavity could be extended by almost 30% beyond that of a comparable constant-intensity dipole trap.

DOI
10.1103/physrevlett.88.163002

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We demonstrate feedback on the motion of a single neutral atom trapped in the light field of a high-finesse cavity. Information on the atomic motion is obtained from the transmittance of the cavity. This is used to implement a feedback loop in analog electronics that influences the atom's motion by controlling the optical dipole force exerted by the same light that is used to observe the atom. In spite of intrinsic limitations, the time the atom stays within the cavity could be extended by almost 30% beyond that of a comparable constant-intensity dipole trap.
An experiment is described where slow laser-cooled atoms are injected into a high-finesse optical cavity. Single atoms are trapped in the cavity light field and observed by measuring the light intensity transmitted through the cavity. Feedback-based methods are realized to extend the time the atom stays in the cavity. The influence of atoms on the cavity transmission and the light force on atoms in a cavity are also analyzed theoretically. This allows to study the motion of an atom in the cavity and to interpret the experimental transmission signals. The current limitations for the storage time are identified and a strategy to overcome them is suggested. Apart from that, a method is proposed which allows a two-dimensional position measurement of atoms in the cavity.
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.
Summary form only given. The strong coupling between an atom and the light field in an optical microresonator causes a single atom to alter the transmission of the cavity significantly, allowing its continuous observation. At the same time, the strong coupling leads to novel optical forces acting on the atom. It was predicted that it should be possible to trap an atom with single intracavity photons. Using an atomic fountain, we inject single ultracold neutral atoms in the standing-wave light field of a high-finesse optical cavity. When an atom is detected in the cavity, the laser light pumping the cavity is suddenly switched to a higher value. This increases the depth of the optical dipole potential and traps the atom in the cavity field.
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.
We report on trapping a single neutral atom in the standing-wave light field of a high-finesse optical cavity containing one photon on average, a single-photon optical trap, or SPOT for short. This trap has the novel feature that the light field is also used to observe the atom in real time. The oscillatory motion of the trapped atom induces well-resolved oscillations of the light intensity. Periodic structure is visible in the fourth-order intensity correlation function, attributed to long-distance flights of the atom along the standing wave. The finite duration of those flights provides evidence for cavity-mediated cooling of atoms. We discuss the various mechanisms determining the trapping time and compare the results with a quantum-jump Monte Carlo simulation to interpret the observed signals.
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.