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arXiv:quant-ph/0508052 · New Journal of Physics

Resurrection of Schrödinger's cat

Jae-Seung Lee, A K Khitrin

Published Aug 24, 2006• 2 citations

Abstract

It is experimentally demonstrated that a quantum state, destroyed by uncontrollable natural decoherence, can be purified by using results of projective measurement and converted into a desired target pure state. The physical system is a cluster of seven dipolar-coupled nuclear spins of single-labelled 13 C-benzene in liquid crystal. 13 C spin plays the role of a device for measuring the protons' 'cat' state, a superposition of states with six spins up ( alive ) and six spins down ( dead ). Information about the state, stored in the 13 C spin, is used to bring the protons' subsystem into the target alive state, while the excess entropy produced by decoherence is transferred to the 'measuring device', the 13 C spin.

DOI: 10.1088/1367-2630/8/8/144

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It is experimentally demonstrated that a quantum state, destroyed by uncontrollable natural decoherence, can be purified by using results of projective measurement and converted into a desired target pure state. The physical system is a cluster of seven dipolar-coupled nuclear spins of single-labelled 13 C-benzene in liquid crystal. 13 C spin plays the role of a device for measuring the protons' 'cat' state, a superposition of states with six spins up ( alive ) and six spins down ( dead ). Information about the state, stored in the 13 C spin, is used to bring the protons' subsystem into the target alive state, while the excess entropy produced by decoherence is transferred to the 'measuring device', the 13 C spin.

It is experimentally demonstrated that an arbitrary quantum state of a single spin 1/2, a| upward arrow+b| downward arrow, can be converted into a superposition of the two ferromagnetic states of a spin cluster: a| upward arrow upward arrow... upward arrow upward arrow+b| downward arrow downward arrow... downward arrow downward arrow. The physical system is a cluster of seven dipolar-coupled nuclear spins of single-labeled 13C-benzene molecules in a liquid-crystalline matrix. In this complex system, the pseudopure ground state and the required controlled unitary transformations have been implemented. The experimental scheme can be considered as an explicit model of quantum measurement.

Schrodinger's famous cat has long been misunderstood. According to quantum theory and experiments with entangled systems, an entangled state such as the Schrodinger's cat state is neither a superposition of states of either subsystem nor a superposition of compound states of the composite system, but rather a nonlocal superposition of correlations between pairs of states of the two subsystems. The entangled post-measurement state that results from an ideal measurement is not paradoxical, but is merely a coherent superposition of two statistical correlations at "zero phase angle," i.e. at 100% positive correlation. Thus the state of the radioactive nucleus and Schrodinger's cat is as follows: an undecayed nucleus is 100% positively correlated with an alive cat, and (i.e. superposed with) a decayed nucleus is 100% positively correlated with a dead cat. The superposition consists merely in the fact that both correlations are simultaneously true. Despite many published statements to the contrary, this superposition is not paradoxical. It is in fact what one expects intuitively.