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Physica Scripta

Absorption by the Numbers: Recent Experiments with Single Trapped and Cooled Ions

P. E. Toschek

Published Jan 1, 1988• 8 citations
Publisher

Abstract

Ion traps and cw dye lasers allow one to prepare small clouds of ions, or single ions. They can be detected even by absorptive techniques, such that the information is contained in one field mode only, the mode of the irradiating light. The noise can approach the limit. For excitation on weak and narrow lines, random breaks of the scattering on a competing resonance line can reveal the quantized signal absorption. These interruptions are the signature of quantum jumps which characterize the internal dynamics of a single atomic particle under quasi-continuous observation.

DOI
10.1088/0031-8949/1988/t23/032

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Ion traps and cw dye lasers allow one to prepare small clouds of ions, or single ions. They can be detected even by absorptive techniques, such that the information is contained in one field mode only, the mode of the irradiating light. The noise can approach the limit. For excitation on weak and narrow lines, random breaks of the scattering on a competing resonance line can reveal the quantized signal absorption. These interruptions are the signature of quantum jumps which characterize the internal dynamics of a single atomic particle under quasi-continuous observation.
An ion, or atom, confined in a ‘trap’ and laser-irradiated on its resonance line generates a quasi-continuous flow of scattered light, and makes the line split or shifted. When the ion is probe-laser excited, on a neighbouring dipole-forbidden line to a metastable state, the flow subsides. Also, the shift or dynamic Stark splitting of the resonance and of the dipole-forbidden line vanishes. De-excitation of the ion requires the probe laser to be re-tuned to the unshifted resonance. Cyclic scanning of this laser shows a mean frequency offset of the on-off jumps from the off-on jumps. This random hysteresis may cool or heat the ion, and lock an ensemble in the metastable state.
We briefly discuss the preparation of a single cold ion in an rf trap, and the concept of "quantum jumps" which requires a single atomic particle for verification. Recent corresponding experiments have demonstrated interruptions of the laser-excited resonance fluorescence, which are the signature of the particles performing quantum jumps to and from a metastable level, under weak excitation of the corresponding transition. The time density of the jumps obeys a Poissonian distribution in agreement with theoretical models. With three trapped ions, the high rate of multiple jumps suggests the presence of collective interaction.
A single trapped and cooled ion interacting with light is the realization of a fundamental system of quantum optics. Relevant viewpoints include the light scattering and its relation to the preparation of the ion, the dynamics of the ion's internal motion, and the dynamics of its vibrational motion. As cases in point, recent results are reviewed that illustrate those viewpoints: (i) The preparation of the ion, by spontaneous emission, in a superposition of energy eigenstates of quasi-permanent lifetime. (ii) The dispersive effect of quantum jumps of the ion, to and from a metastable state, that gives rise to internally self-locking the ion in either the light-scattering bright or the non-scattering dark state. (iii) The stochastic cooling of the ion by repeated null detection of its resonance scattering. These counter-intuitive phenomena characterize a physical system, where quantum mechanics models ensembles of observations.
We have used stored ion methods to improve resolution and sensitivity in optical spectroscopy. Single atomic ions have been confined by electric and magnetic fields, cooled by laser radiation pressure to temperatures on the order of 1 mK, and probed spectroscopically with narrowband lasers. The absorption resonance of a single Hg + ion has been observed by a decrease in the transmitted light intensity. An ultraviolet transition in Hg + has been observed with a linewidth of only 30 kHz. Quantum jumps to and from metastable levels of Hg + have been observed and used to determine radiative decay rates and to infer the existence of photon antibunching. Quantum jumps have also been observed in single Mg + ions.
When a single trapped ion is laser excited to a metastable state, its ``quantum jump'' inhibits the laser-excited light scattering on a resonance line. This event, and its inverse, modifies the distribution over the oscillator levels (n)(n)(n) of the ion's vibration in the trap. In particular, unsuccessful attempts to deexcite on vibrational sidebands decrease 〈n〉〈n〉〈n〉 to sub-Doppler values and may leave the ion in a ``trapped'' Fock state decoupled from the light.