Conditional measurements, quantum feedback, and cold atoms in cavity QED

Joseph E. Reiner, Wade P. Smith, Luis A. Orozco

Andrei N. Soklakov

Wade P. Smith, Luis A. Orozco

Photon correlation measurements reveal the response of the conditional evolution of the cavity QED system to a novel quantum feedback protocol. A photodetection collapses the state of the system and triggers a feedback pulse with an adjustable delay and amplitude that alters the intensity driving the system. The conditional evolution of the system freezes into a new steady state where it resides until, after an amount of time determined by the experimenter, it re-equilibrates into the original steady state. We carry out a sensitivity analysis using a theoretical model with atomic detuning and make quantitative comparisons with measured results.

Thomas Wellens, Andreas Buchleitner

T. Valenzuela, Matteo Cristiani, Hannes Gothe +1 more

The quantum behavior of cold matter interacting with photons confined in high-finesse cavities is a subject of high interest for quantum communication, for studying properties of ultra-cold quantum gases, as well as for precision measurements. In particular promising schemes for enhanced laser cooling based on cavity feedback have been extensively studied both theoretically and experimentally. The paper presents the first results from a cavity QED experiment with ytterbium atoms which is suitable for cavity-QED-based atom-photon interfaces as well as for studying collective excitation of a cold atomic cloud interacting with the standing wave of a resonator, with the perspective of using this system for investigating new cooling mechanisms based on atom-cavity interaction.

Shay Hacohen-Gourgy, L. S. Martin

Developments over the last two decades have opened the path towards quantum technologies in many quantum systems, such as cold atoms, trapped ions, cavity-quantum electrodynamics (QED), and circuit-QED. However, the fragility of quantum states to the effects of measurement and decoherence still poses one of the greatest challenges in quantum technology. An imperative capability in this path is quantum feedback, as it enhances the control possibilities and allows for prolonging coherence times through quantum error correction. While changing parameters from shot to shot of an experiment or procedure can be considered feedback, quantum mechanics also allows for the intriguing possibility of performing feedback operations during the measurement process itself. This broader approach to measurements leads to the concepts of weak measurement, quantum trajectories, and numerous types of feedback with no classical analogs. These types of processes are the primary focus of this review. We introduce the concept of quantum feedback in the context of circuit-QED, an experimental platform with significant potential in quantum feedback and technology. We then discuss several experiments and see how they elucidate the concepts of continuous measurements and feedback. We conclude with an overview of coherent feedback, with application to fault-tolerant error correction.