Reconstruction of non-classical cavity field states with snapshots of their decoherence

• Published in: Nature (London) Vol. 455; no. 7212; pp. 510 - 514
• Main Authors: Bernu, Julien, Dotsenko, Igor, Brune, Michel, Haroche, Serge, Sayrin, Clément, Deléglise, Samuel, Raimond, Jean-Michel
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25-09-2008; Nature Publishing; Nature Publishing Group
• Subjects: Atoms; Atoms & subatomic particles; Classical and quantum physics: mechanics and fields; Exact sciences and technology; Foundations, theory of measurement, miscellaneous theories (including aharonov-bohm effect, bell inequalities, berry's phase); Fundamental areas of phenomenology (including applications); Humanities and Social Sciences; letter; Microscopes; multidisciplinary; Nonclassical field states; squeezed, antibunched and sub-poissonian states; operational definitions of the phase of the field; phase measurements; Optics; Physics; Properties; Quantum mechanics; Quantum optics; Quantum Physics; Quantum theory; Rubidium; Science; Science (multidisciplinary); Standard deviation; United States; State reconstruction; Wigner function; Fock state; Schrödinger's cat state; Quantum optics; Quantum decoherence; Radiation matter interactions; Coherent states; CQED; QND; quantum; Schrodinger cat; Fock states; measurement
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature07288

The state of a microscopic system encodes its complete quantum description, from which the probabilities of all measurement outcomes are inferred. Being a statistical concept, the state cannot be obtained from a single system realization, but can instead be reconstructed from an ensemble of copies through measurements on different realizations. Reconstructing the state of a set of trapped particles shielded from their environment is an important step in the investigation of the quantum-classical boundary. Although trapped-atom state reconstructions have been achieved, it is challenging to perform similar experiments with trapped photons because cavities that can store light for very long times are required. Here we report the complete reconstruction and pictorial representation of a variety of radiation states trapped in a cavity in which several photons survive long enough to be repeatedly measured. Atoms crossing the cavity one by one are used to extract information about the field. We obtain images of coherent states, Fock states with a definite photon number and 'Schrödinger cat' states (superpositions of coherent states with different phases). These states are equivalently represented by their density matrices or Wigner functions. Quasi-classical coherent states have a Gaussian-shaped Wigner function, whereas the Wigner functions of Fock and Schrödinger cat states show oscillations and negativities revealing quantum interferences. Cavity damping induces decoherence that quickly washes out such oscillations. We observe this process and follow the evolution of decoherence by reconstructing snapshots of Schrödinger cat states at successive times. Our reconstruction procedure is a useful tool for further decoherence and quantum feedback studies of fields trapped in one or two cavities.