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Physical Review Letters

Excitation of an Atomic Electron to a Coherent Superposition of Macroscopically Distinct States

Michael W. Noel, Carlos R. Stroud

Published Sep 2, 1996• 147 citations
Publisher

Abstract

An atomic electron is prepared in a state closely analogous to Schr\"odinger's coherent superposition of ``live cat'' and ``dead cat.'' The electronic state is a coherent superposition of two spatially localized wave packets separated by approximately $0.4\ensuremath{\mu}\mathrm{m}$ at the opposite extremes of a Kepler orbit. State-selective ionization is used to verify that only every other atomic level is populated in the ``cat state,'' and a Ramsey fringe measurement is used to verify the coherence of the superposition.

DOI
10.1103/physrevlett.77.1913

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An atomic electron is prepared in a state closely analogous to Schr"odinger's coherent superposition of live cat'' and dead cat.'' The electronic state is a coherent superposition of two spatially localized wave packets separated by approximately 0.4\ensuremathμm0.4\ensuremath{\mu}\mathrm{m}0.4\ensuremathμm at the opposite extremes of a Kepler orbit. State-selective ionization is used to verify that only every other atomic level is populated in the ``cat state,'' and a Ramsey fringe measurement is used to verify the coherence of the superposition.
A "Schrodinger cat"-like state of matter was generated at the single atom level. A trapped 9Be+ ion was laser-cooled to the zero-point energy and then prepared in a superposition of spatially separated coherent harmonic oscillator states. This state was created by application of a sequence of laser pulses, which entangles internal (electronic) and external (motional) states of the ion. The Schrodinger cat superposition was verified by detection of the quantum mechanical interference between the localized wave packets. This mesoscopic system may provide insight into the fuzzy boundary between the classical and quantum worlds by allowing controlled studies of quantum measurement and quantum decoherence.
The Schrödinger's Cat paradox was proposed in 1935 by Edwin Schrodinger, one of the founders of quantum mechanics, as an attempt to visualize the macroscopic realization of a quantum superposition state. A cat is placed in a sealed box together with a vial of poison. A two-state particle (e.g. an electron) is sent into a detector in the box resulting either in a broken or an intact vial and a dead or live cat, respectively. The main problem consists in whether the superposition state of a microscopic particle can be transferred upon the macroscopic cat, that is, whether the cat can exist in a superposition state, being simultaneously dead and alive. Since the standard Copenhagen interpretation is unable to assign any reality to the quantum superposition state, the paradox finds no resolution within the regime of this interpretation. Von Neumann's insistence on the uniform treatment of both microscopic (quantum) and macroscopic (classical) objects according to the laws of quantum mechanics provides a more consistent framework for the resolution of the paradox. In particular, the discovery of the phenomenon of decoherence, whereby the disappearance of the quantum interferences at the macro level is accounted for, suggests the onset of an extremely efficient interference relaxation process (10-23 s) upon the interaction of the two state particle with the detector. As a result, Schrodinger's cat can exist macroscopically either as dead or alive and never as a combination of both. Decoherence not only aids the resolution of the Schrodinger's Cat paradox but also sheds light upon the mechanisms by which the macro-world emerges from the microscopic quantum realm.