Nature 448, 784-786 (16 August 2007) | doi:10.1038/nature06054; Received 11 April 2007; Accepted 26 June 2007

Generation of optical 'Schrödinger cats' from photon number states

Alexei Ourjoumtsev1, Hyunseok Jeong2, Rosa Tualle-Brouri1 & Philippe Grangier1

  1. Laboratoire Charles Fabry de l'Institut d'Optique, Université Paris-Sud, CNRS UMR 8501, 91127 Palaiseau, France
  2. Centre for Quantum Computer Technology, Department of Physics, University of Queensland, Brisbane, Queensland 4072, Australia

Correspondence to: Alexei Ourjoumtsev1 Correspondence and requests for materials should be addressed to A.O. (Email: alexei.ourjoumtsev@institutoptique.fr).

Schrödinger's cat1 is a Gedankenexperiment in quantum physics, in which an atomic decay triggers the death of the cat. Because quantum physics allow atoms to remain in superpositions of states, the classical cat would then be simultaneously dead and alive. By analogy, a 'cat' state of freely propagating light can be defined as a quantum superposition of well separated quasi-classical states2, 3—it is a classical light wave that simultaneously possesses two opposite phases. Such states play an important role in fundamental tests of quantum theory4, 5, 6, 7 and in many quantum information processing tasks, including quantum computation8, quantum teleportation9, 10 and precision measurements11. Recently, optical Schrödinger 'kittens' were prepared12, 13, 14; however, they are too small for most of the aforementioned applications and increasing their size is experimentally challenging. Here we demonstrate, theoretically and experimentally, a protocol that allows the generation of arbitrarily large squeezed Schrödinger cat states, using homodyne detection and photon number states as resources. We implemented this protocol with light pulses containing two photons, producing a squeezed Schrödinger cat state with a negative Wigner function. This state clearly exhibits several quantum phase-space interference fringes between the 'dead' and 'alive' components, and is large enough to become useful for quantum information processing and experimental tests of quantum theory.