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Physical Review A

One-atom detection using resonance ionization spectroscopy

G. S. Hurst, Munir Hasan Nayfeh, Jack Phillip Young

Published Jun 1, 1977• 126 citations
Publisher

Abstract

A new measuring technique which can detect a single atom of a given kind, even in the presence of ${10}^{19}$ or more atoms of another kind, with good space and time resolution, was utilized to study the density fluctuation of less than 100 cesium atoms in a small volume of space filled with inert gases. Repeated measurements of the absolute number of atoms in a defined volume at an arbitrary time were recorded for the first time in order to obtain a statistical distribution giving the fluctuation of the number of atoms around the mean value. Numerous other physics applications of the one-atom detector are briefly described.

DOI
10.1103/physreva.15.2283

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A new measuring technique which can detect a single atom of a given kind, even in the presence of 1019{10}^{19}1019 or more atoms of another kind, with good space and time resolution, was utilized to study the density fluctuation of less than 100 cesium atoms in a small volume of space filled with inert gases. Repeated measurements of the absolute number of atoms in a defined volume at an arbitrary time were recorded for the first time in order to obtain a statistical distribution giving the fluctuation of the number of atoms around the mean value. Numerous other physics applications of the one-atom detector are briefly described.
A group at Oak Ridge National Laboratory reports that by means of the new technique of resonance ionization spectroscopy they have observed single atoms of cesium. They say that one atom in an environment of 1019 atoms or molecules of another kind can be measured with their method, and they do not believe this to be a limit. In subsequent experiments, the group has observed density fluctuations of atoms, absolute photodissociation cross sections, time-resolved diffusion and chemical reaction of atoms, and the detection of cesium atoms from the spontaneous fission of californium nuclei.
Resonance ionization spectroscopy, RIS, is a multistep photon absorption process in which the final state is the ionization continuum of an atom. The RIS process can be saturated with available pulsed lasers, so that one electron can be removed from each atom of the selected type. This method was first applied to the determination of the absolute number of He(21S2^{1}S21S) excited states produced when pulsed beams of protons interacted with helium gas. Laser schemes for RIS are classified into five basic types; with these, nearly all of the elements can be detected with commercially available lasers. A periodic table is included showing schemes applicable to all of the elements except He, Ne, F, and Ar. A compact theory of the RIS process is presented which delineates the conditions under which rate equations apply. Questions on the effects of collisional line broadening, laser coherence time, and nonresonant multiphoton ionization processes are discussed. The initial demonstration of one-atom detection of Cs is described. By using laser beams to saturate the RIS process and by using proportional counters as single-electron detectors, one-atom detection is made possible. With RIS, one-atom detection is highly selective, has the ultimate in sensitivity, and has excellent space and time resolution. Furthermore, a modification of the technique in which single electrons (or single ions) are detected with a channel electron multiplier permits single-atom detection in a vacuum. Resonance ionization spectroscopy has application to classical phenomena such as the diffusion of free atoms, the chemical reaction of free atoms with a gas, the measurement of concentration fluctuations in a dilute vapor, and a variety of other atomic fluctuation phenomena. The authors describe how RIS can be used for photophysics measurements such as far wing collisional line broadening, measurements of photoionization cross sections for excited states, and collisional redistribution among excited states. Modern applications include the detection of single atoms in individual ionization tracks, such as those created by the binary fission of a parent atom, and the extension of this technique to the detection of stable or unstable daughter atoms in time coincidence with the decay of a parent atom. These methods are beig developed for use in low-level counting to determine, for example, the flux of solar neutrinos on the earth.
Single magnetically trapped 87Rb^{87}\text{R}\text{b}87Rb atoms on an atom chip are detected with $67\ifmmode\pm\else\textpm\fi{}12\text{ }%$ efficiency by three photon ionization and subsequent ion detection in a channel electron multiplier. State selective detection of single atoms and high-resolution optical spectroscopy on trapped atom clouds are demonstrated. The temperature and particle density of a trapped atomic gas is monitored in situ and in real time with negligible atom loss due to ionization below 5%.
We have developed a simple theoretical model describing multi-atom signals that could be measured in experiments on resonant energy transfers in an ensemble of a few Rydberg atoms. We have shown that an efficiency of the selective field-ionization detector, which is less than 1, leads to the mixing up of the spectra of the resonant energy transfer registered for various numbers of detected Rydberg atoms. This may impede the possible observations of dipole blockade or coherent two-atom interaction required to perform basic quantum gates. The formulae are presented, which help to estimate an actual mean Rydberg atom number in an excitation volume per one exciting laser pulse at a given detection efficiency. We have also found that a measurement of relationship of the amplitudes of resonances observed in the one- and two-atom signals provides a straightforward determination of the absolute detection efficiency and actual mean Rydberg atom number. This novel method is advantageous as it is independent of the specific experimental conditions. We also performed a testing experiment on the resonant energy transfers in a small excitation volume of the velocity selected Na atomic beam. The observed one- and two-atom resonances were analyzed and compared with the theoretical predictions. A good agreement between experiment and theory in the width and amplitudes of the resonances has confirmed the validity of simple approximations used in the developed theoretical model.
Abstract This chapter discusses how, from the abstract statevector, one can extract reliable statements about what is observed, following the author’s favorite approach. It begins by defining “stuff” the absolute magnitude squared of the scalar product of the state vector with a basis. Singled out is the eigenbasis of the particle number operator in an arbitrary volume V. A criterion for “observability” is given: one can say that the particle number in V is observed when the ratio of standard deviation of this operator to its mean value is “sufficiently small.” If the criterion is satisfied, one can say that the mean value is the number of particles in V. The chapter goes on to provide examples of application of the criterion. Among these, one shows that, during a measurement, it is possible for the state of an apparatus to “briefly” not be observable, but the time interval of unobservability is less than human perception time. Lastly, the chapter discusses “tail” issue, that collapse never goes to completion in a finite time. It argues that the tiny amplitude state (superposed with the observed state) can be neglected since it gives rise to no observable particles.