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Physical Review A

Detection of decay processes by means of quantum-jump statistics

Jan Grochmalicki, Maciej Lewenstein

Published Sep 1, 1989• 8 citations
Publisher

Abstract

Dehmelt's original idea of using the mechanism of quantum jumps for measurements is applied to the detection scheme of the dynamics of decay processes. The systems under consideration are the single atom inside a resonant optical cavity (waveguide) or the single trapped atom (ion) illuminated by the ionizing radiation. In each of the above cases the dark-period statistics of the side-level fluorescence is calculated and its specific features are pointed out.

DOI
10.1103/physreva.40.2517

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•Jan 1, 2006•0 citations
Dehmelt's original idea of using the mechanism of quantum jumps for measurements is applied to the detection scheme of the dynamics of decay processes. The systems under consideration are the single atom inside a resonant optical cavity (waveguide) or the single trapped atom (ion) illuminated by the ionizing radiation. In each of the above cases the dark-period statistics of the side-level fluorescence is calculated and its specific features are pointed out.
We consider a single three-level atom placed inside a resonant cavity and simultaneously probed in a double-resonance ladder configuration by a relatively strong laser beam. It is shown that photocounting statistics of a fluorescence signal induced by the probe, in certain circumstances, exhibits dark periods (i.e., quantum jumps are present), reflecting the statistical properties of the field inside the cavity. Comparisons are made between the number, thermal, coherent, and squeezed states. Specific features of the latter are pointed out.
A theory of the discrete photodetection of quantum jumps on the V configuration of atomic levels has been developed. A three-level source atom is placed in a cavity excited by a resonance fluorescence field. The cavity is tuned to exact resonance with an atomic transition. The cavity mode state is tested by a flux of unexcited (at the entrance) probe atoms passing through the cavity. The energy states of the outgoing probe atoms are detected by ionization chambers, which are assumed ideal. This a posteriori statistical information is indirectly related to the numerical characteristics of a measured quantum system consisting of the source atom and cavity mode. The “tuning” conditions for a discrete photodetector, i.e., the rules for choosing the parameters and durations of the interactions of the cavity mode with the probe and source atoms, intensities of the pump and probe fields that are necessary for observing quantum jumps from the “bright” state to the “dark” one and vice versa, have been determined. A two-state model that describes the dynamics of a quantum jump has been analyzed. The formulas have been obtained for the observable characteristics of quantum jumps: the mean residence time of the quantum system in quasistationary states (durations of the bright and dark periods), probabilities of quantum jumps, mean excitation levels of the quantized cavity mode, etc.
Complete density-matrix equations and a one-photon counting probability are used to examine the photon statistics of fluorescence light of a single three-level atom, weakly coupled to an external cavity of low QQQ value. The internal structure of the atom is supposed to be characterized by a weak and a strong transition. The strong transition is coherently driven by an external laser field, whereas no external field is acting on the weak transition. In free space the system exhibits quantum jumps. By placing the atom within the mirrors of an external cavity, tuned close to the transition frequency of the two upper levels, modifications in the quantum jump behavior of the atom are shown due to a modified spontaneous decay rate between the two states. In particular, it is demonstrated that the effect of the cavity is to modify the repetition rate of quantum jumps into the metastable dark state and the average duration of bright periods of fluorescence, whereas the average duration of dark periods is not changed in first order of the spontaneous decay rates involved. In view of current experiments in our laboratory a quantitative analysis is presented for the special case of a single indium ion located between the mirrors of a high-order symmetric confocal cavity.
Recent experiments with single atomic ions isolated in electromagnetic traps have demonstrated abrupt changes in observed fluorescence intensities, which have been interpreted as evidence for quantum jumps as the atom changes energy levels. This presents the seeming paradox of discontinuously changing observables from a system whose dynamical evolution is governed by the continuous optical Bloch equations. In this paper I present an analysis of the dynamics of a model three-level atom interacting with two laser fields in terms of the dressed states of the atom-laser system. These dressed states are eigenstates of a Hamiltonian which includes laser-atom interactions, and thus they simplify the discussion of the time evolution of the combined laser-atom system. This dressed-state description is particularly useful in the regime in which both laser intensities are strong enough to saturate their respective transitions. I calculate the radiative lifetimes of the different species of dressed states and the branching ratios for decay to other dressed states, and demonstrate the consequences of these quantities for the observed fluorescence pattern. The calculated duration of bright and dark periods shows a pronounced dependence on the detunings of the lasers.
We first use the quantum method to replicate the well-known results of a single atom relaxing, whilst demonstrating the intuitive picture it provides for dissipative dynamics. By use of individual "quantum trajectories", the method allows for simulation of systems inaccessible to ensemble treatments. This is shown by replicating resonance fluorescence, allowing us to concurrently demonstrate the method's facilitation of calculating photon statistics by the creation of discrete photon streams. To analyse these, we solidify the theoretical basis for, and implement, a computational method of calculating second-order coherence functions. A process by which to model interacting two-atom systems to allow for computation with the quantum jump method is then developed. Using this, we demonstrate cooperative effects leading to greatly modified emission spectra, before investigating the decoupling of states from dissipative and coherent interactions. Here, we find the novel insight provided by the quantum jump method both births and provides the tools with which to begin an investigation into the occurrence of macroscopic jumps and the formation of macroscopic dark periods in a system of two two-level dipole-dipole coupled atoms.
The control and manipulation of the atomic translational and internal degrees of freedom via atom-laser interaction is an essential tool in modern quantum optical experiments which allows for a large variety of applications. The present work is concerned with theoretical investigations which may shed some light on certain experiments in this field.In the first part of this work, cooperative effects in the fluorescence of dipole-dipole interacting atoms in a trap are studied. The famous macroscopic quantum jumps can occur for a single atom with a metastable state when illuminated by light. Then periods of fluorescence of fixed intensity alternate stochastically with dark periods. If one has two or three such atoms in a trap, there are dark periods and periods of single and multiple intensity, from overlap. The statistics of these periods is influenced by the dipole-dipole interaction between the atoms in a sensitive manner. This work is concerned with a theoretical description of these effects, including a discussion of the seemingly contradictory experimental results which have been obtained in this context. The main result indicates that the huge rate of double and triple jumps reported for three trapped Ba+ ions can in fact not be attributed to the dipole-dipole interaction.In the second part, the possible use of an evanescent wave atomic mirror as a measuring device for the properties of an atomic wave packet is investigated. The idea is taken from an experiment, in which such a mirror was used for the temporal diffraction of a cloud of laser cooled atoms dropping out of a magneto-optical trap. A model for the investigation of the atomic wave function with such a mirror is given, and the outcomes of different set-ups are investigated. The main result shows that with such a mirror one can sample the mod-squared of a spread-out wave function, and this opens the way for a realistic possibility for measuring this otherwise not easily accessible quantity.