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Science

Quantum Control of the Inevitable

Marlan O. Scully, Shi-Yao Zhu

Published Sep 25, 1998• 48 citations
Publisher

Abstract

Long after its formulation, quantum mechanics still offers new surprises. In their Perspective, Scully and Zhu discuss recent efforts to control quantum states coherently with laser radiation. In particular, some new ways have been found to turn off spontaneous emission of radiation--a process that once seemed an inevitable process of atomic energy decay.

DOI
10.1126/science.281.5385.1973

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Long after its formulation, quantum mechanics still offers new surprises. In their Perspective, Scully and Zhu discuss recent efforts to control quantum states coherently with laser radiation. In particular, some new ways have been found to turn off spontaneous emission of radiation--a process that once seemed an inevitable process of atomic energy decay.
Introduction Ever since 1916/17 when Einstein argued [45] [46] that spontaneous emission must occur if matter and radiation are to achieve thermal equilibrium, physicists have believed that spontaneous emission is an inherent quantum mechanical property of atoms and that excited atoms inevitably radiate. This view, however, overlooks the fact that spontaneous emission is a consequence of the coupling of quantized energy states of atoms with the quantized radiation field, and is a manifestation of quantum noise or of emission ‘stimulated’ by ‘vacuum fluctuations’. An infinity of vacuum states is available to the photon radiated by an excited atom placed in free space, leading to the effective irreversibility of such emissions. If these vacuum states are modified, as for example by placing an excited atom between closely spaced mirrors or in a small cavity (essentially the Casimir effect), spontaneous emission can be greatly inhibited or enhanced or even made reversible. Recent advances in atomic and optical techniques have made it possible to control and manipulate spontaneous emission. A whole new branch of quantum optics called ‘cavity QED’ has developed since 1987 utilizing these dramatic changes in spontaneous emission rates in cavities to construct new kinds of microscopic masers or micromasers that operate with a single atom and a few photons or with photons emitted in pairs in a two-photon transition.
Ever since Einstein demonstrated that spontaneous emission must occur if matter and radiation are to achieve thermal equilibrium, physicists have generally believed that excited atoms inevitably radiate. Spontaneous emission is so fundamental that it is usually regarded as an inherent property of matter. This view, however, overlooks the fact that spontaneous emission is not a property of an isolated atom but of an atom-vacuum system. The most distinctive feature of such emission, irreversibility, comes about because an infinity of vacuum states is available to the radiated photon. If these states are modified—for instance, by placing the excited atom between mirrors or in a cavity—spontaneous emission can be greatly inhibited or enhanced.
It has been shown recently [see, for example, S.-Y. Zhu and M. O. Scully, Phys. Rev. Lett. 76, 388 (1996)] that a dynamical suppression of spontaneous emission can occur in a three-level system when an external field drives transitions between a metastable state and two decaying states. What is unusual in the decay scheme is that the decaying states are coupled directly by the vacuum radiation field. It is shown that the decay dynamics required for total suppression of spontaneous emission necessarily implies that the level scheme is isomorphic to a three-level \ensuremath{\Lambda} system, in which the lower two levels are both metastable and each is coupled to the decaying state. As such, the total suppression of spontaneous emission can be explained in terms of conventional dark states and coherent population trapping.
A three-level atom in a ΛΛΛ configuration trapped in an optical cavity forms a basic unit in a number of proposed protocols for quantum information processing. Through control with an appropriate laser, this system allows for efficient storage of cavity photons into long-lived atomic excitations, and their retrieval with high fidelity. This process presumes an adiabatic transfer through the `dark state', a coherent superposition of the two lower levels of the ΛΛΛ system, by a slow variation of the intensity of the control laser. We study the full quantum mechanics of this transfer process with a view to examine the nonadiabatic effects, as the control laser is varied in time. The nonadiabatic effects arise due to inevitable excitations of the system to states involving the highest level of the ΛΛΛ configuration, which is radiative. We formulate the problem in terms of the instantaneous eigenstates and solve it numerically, allowing for spontaneous decays from the excited level. We present detailed results for both storage and retrieval for a suitable range of variation rates and intensity of the control laser, and examine optimum conditions for the transfer process.
Abstract When quantum theories were first formulated a century ago, who would have guessed how much technological developments originating from quantum physics shape the way we live and interact today. Whether we work on a computer, use our mobile phones, or get a diagnosis based on magnetic resonance imaging: The understanding of quantum mechanics is the basis for all these technologies. In photonics, the laser and light‐emitting diodes are prime examples, with a current market of more than 12 billion US dollars for lasers alone. Now there are new quantum technologies on the horizon, promising a second quantum revolution.