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arXiv:quant-ph/0501123 · Physical Review Letters

Experimental Demonstration of Quantum State Expansion in a Cluster of Dipolar-Coupled Nuclear Spins

Jae‐Seung Lee, Anatoly K. Khitrin

Published Apr 22, 2005• 27 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevlett.94.150504

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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.
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.