Cavity quantum electrodynamics (cavity QED) is the study of the interaction between light confined in a reflective cavity and atoms or other particles, under conditions where the quantum nature of photons is significant. It could in principle be used to construct a quantum computer.
The case of a single 2-level atom in the cavity is mathematically described by the Jaynes–Cummings model, and undergoes vacuum Rabi oscillations
|e\rangle|n-1\rangle\leftrightarrow|g\rangle|n\rangle
n-1
n
If the cavity is in resonance with the atomic transition, a half-cycle of oscillation starting with no photons coherently swaps the atom qubit's state onto the cavity field's,
(\alpha|g\rangle+\beta|e\rangle)|0\rangle\leftrightarrow|g\rangle(\alpha|0\rangle+\beta|1\rangle)
Other interaction durations create entanglement between the atom and cavity field; for example, a quarter-cycle on resonance starting from
|e\rangle|0\rangle
(|e\rangle|0\rangle+|g\rangle|1\rangle)/\sqrt{2}
The 2012 Nobel Prize for Physics was awarded to Serge Haroche and David Wineland for their work on controlling quantum systems.[1]
Haroche shares half of the prize for developing a new field called cavity quantum electrodynamics (CQED) – whereby the properties of an atom are controlled by placing it in an optical or microwave cavity. Haroche focused on microwave experiments and turned the technique on its head – using CQED to control the properties of individual photons.[1]
In a series of ground-breaking experiments, Haroche used CQED to realize Schrödinger's famous cat experiment in which a system is in a superposition of two very different quantum states until a measurement is made on the system. Such states are extremely fragile, and the techniques developed to create and measure CQED states are now being applied to the development of quantum computers.