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Teubner-Texte zur Physik

Quantum Optics of Accelerated 2-level Systems on Arbitrary Stationary Trajectories

Jürgen Audretsch

Published Jan 1, 1996• 0 citations
Publisher

Abstract

Recently, a new physical picture for the spontaneous excitation of a uniformly accelerated two-level atom has been put forward [1]. Following a quantum optical approach it is based on the distinction of the two competing mechanisms which operate to excite an atom in the quantum vacuum: vacuum fluctuations and radiation reaction. Vacuum fluctuations tend to excite an atom in its ground state and de-excite it in an excited state. On the other hand, radiation reaction leads always to a loss of internal excitation energy. For an inertial atom in the ground state, the two contributions cancel exactly, so that a very sublime balance between vacuum fluctuations and radiation reaction prevents the spontaneous excitation of the atom [2]. If the atom is in the excited state, both contribution add up to the well-known spontaneous emission rate. It has been shown in [1] that uniform acceleration will disturb this balance. Spontaneous transitions from the ground state to the excited state become possible: the Unruh effect [3].

DOI
10.1007/978-3-663-01204-7_47

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Recently, a new physical picture for the spontaneous excitation of a uniformly accelerated two-level atom has been put forward [1]. Following a quantum optical approach it is based on the distinction of the two competing mechanisms which operate to excite an atom in the quantum vacuum: vacuum fluctuations and radiation reaction. Vacuum fluctuations tend to excite an atom in its ground state and de-excite it in an excited state. On the other hand, radiation reaction leads always to a loss of internal excitation energy. For an inertial atom in the ground state, the two contributions cancel exactly, so that a very sublime balance between vacuum fluctuations and radiation reaction prevents the spontaneous excitation of the atom [2]. If the atom is in the excited state, both contribution add up to the well-known spontaneous emission rate. It has been shown in [1] that uniform acceleration will disturb this balance. Spontaneous transitions from the ground state to the excited state become possible: the Unruh effect [3].
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.
We consider a multilevel hydrogen atom in interaction with the quantum electromagnetic field and separately calculate the contributions of the vacuum fluctuation and radiation reaction to the rate of change of the mean atomic energy of the atom for uniform acceleration. It is found that the acceleration disturbs the vacuum fluctuations in such a way that the delicate balance between the contributions of vacuum fluctuation and radiation reaction that exists for inertial atoms is broken, so that the transitions to higher-lying states from ground state are possible even in vacuum. In contrast to the case of an atom interacting with a scalar field, the contributions of both electromagnetic vacuum fluctuations and radiation reaction to the spontaneous emission rate are affected by the acceleration, and furthermore the contribution of the vacuum fluctuations contains a nonthermal acceleration-dependent correction, which is possibly observable.
We discuss the radiative level shifts of a uniformly accelerated atom moving between two infinite reflecting plates and interacting with a massless scalar field in the vacuum state. The atom, supposed to be a two-level system, accelerates in a direction parallel to the conducting plates. We evaluate separately the contributions of vacuum fluctuations and radiation reaction field to the energy shift of the atomic levels, and discuss their dependence on acceleration, atomic position and cavity length.
Abstract We investigate a two-level atom interacting with a massless real scalar field inside an infinitely long cylindrical cavity, and calculate separately the contributions of vacuum fluctuations and radiation reaction to the mean atomic energy-change rate when the atom moves along the cavity axis. Using the formalism suggested by Dalibard, Dupont-Roc, and Cohen-Tannoudji, we analyze both inertial and uniformly accelerated motion, and discuss the behavior in the low-acceleration and high-acceleration regimes. For the inertial atom, we further introduce a finite cavity quality factor Q to phenomenologically account for cavity dissipation, which regularizes the resonant singularity of the ideal lossless cavity and yields finite resonant peaks. For a uniformly accelerated excited-state atom in the low-acceleration regime, the energy-change rate remains finite at the nominal resonance points, but exhibits pronounced oscillatory enhancement near resonance. In certain parameter regions, the cavity result can exceed both the corresponding inertial result in the same cavity and the uniformly accelerated result in free space, revealing the combined effect of acceleration-induced broadening and the discrete transverse mode structure of the cylindrical boundary. In the high-acceleration regime, a ground-state atom may undergo spontaneous excitation, while the cavity resonance structure is strongly suppressed and the energy-change rate gradually approaches the corresponding free-space value. These results show that the cylindrical geometry can significantly reshape the radiative response of accelerated atoms, enhancing acceleration-induced effects at low acceleration while washing out cavity-induced resonances at high acceleration.
The normalized two-time intensity-fluctuation functions are calculated for the spontaneous-emission radiation from a system of NNN two-level atoms. The radiated field, when the system decays from the excited state to the ground state, does not look like a Gaussian or a coherent field.