Letter

Nature 455, 510-514 (25 September 2008) | doi:10.1038/nature07288; Received 25 June 2008; Accepted 23 July 2008

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

Samuel Deléglise1, Igor Dotsenko1,2, Clément Sayrin1, Julien Bernu1, Michel Brune1, Jean-Michel Raimond1 & Serge Haroche1,2

  1. Laboratoire Kastler Brossel, Ecole Normale Supérieure, CNRS, Université Pierre et Marie Curie, 24 rue Lhomond, 75231 Paris Cedex 05, France
  2. Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France

Correspondence to: Serge Haroche1,2 Correspondence and requests for materials should be addressed to S.H. (Email: haroche@lkb.ens.fr).

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 reconstructed1 from an ensemble of copies through measurements on different realizations2, 3, 4. Reconstructing the state of a set of trapped particles shielded from their environment is an important step in the investigation of the quantum–classical boundary5. Although trapped-atom state reconstructions6, 7, 8 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 states9, Fock states with a definite photon number and 'Schrödinger cat' states (superpositions of coherent states with different phases10). These states are equivalently represented by their density matrices or Wigner functions11. 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 oscillations5. 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.

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