Collective quantum beats without an initial superposition

The phenomenon of quantum beats is a well-established process, completely understood, and long used as a spectroscopic technique in various systems. Here we demonstrate two new aspects in understanding and using quantum beats - (i) coupling to the electromagnetic vacuum allows for beating without an initial superposition between the excited levels [1], and (ii) by detecting the transmission in the forward direction in a superradiant burst, quantum beats can be collectively enhanced, increasing the signal strength useful in systems with low signal-to-noise. We derive an analytical expression for the vacuum-induced collective quantum beats from a superradiant ensemble of three-level V-system atoms. We experimentally observe such dynamics from the forward emission of magneto-optically trapped 85Rb atoms illuminated by a weak drive field resonant on only one transition. After a sudden turn-off of the drive, the subsequent radiative dynamics exhibit amplification of the quantum beats proportional to the overall decay rate, in excellent agreement with the theory. These new aspects of quantum beats may be utilized to enhance spectroscopic precision, generate entangled photons, and study the delayed feedback dynamics in waveguide systems.

[1] G. C. Hegerfeldt and M. B. Plenio, Quantum Opt.6, 15 (1994).