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Generation of optical ‘Schrödinger cats’ from photon number states


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.

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Figure 1: Preparing squeezed ‘Schrödinger cat’ states from Fock states using a single homodyne detection.
Figure 2: Theoretical performance.
Figure 3: Experimental set-up.
Figure 4: Experimental results.
Figure 5: Influence of experimental imperfections.

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This work was supported (in France) by the EU IST/FET project COVAQIAL and the ANR/PNANO project IRCOQ, and (in Australia) by the US Army Research Office and the DTO, the Australian Research Council and Queensland State Government. H.J. thanks T. C. Ralph and M. S. Kim for discussions.

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Correspondence to Alexei Ourjoumtsev.

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Supplementary information

Supplementary Methods

This file contains Supplementary Methods which present a detailed theoretical analysis of the proposed protocol. After giving the general proof of principle, authors discuss the influence of its intrinsic limitations. Finally, an analytic model is presented which accounts for all the imperfections of the actual experiment. (PDF 240 kb)

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Ourjoumtsev, A., Jeong, H., Tualle-Brouri, R. et al. Generation of optical ‘Schrödinger cats’ from photon number states. Nature 448, 784–786 (2007).

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