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Physics World

Order and chaos with frozen ions

Wolfgang Qunit, Wolfgang P. Schleich, HERBERT WALTHER

Published Aug 1, 1989• 4 citations
Publisher

Abstract

A single ion at rest, unperturbed by its environment and forced into such a state for hours – once only a physicist's dream – has now been achieved by the combination of electromagnetic traps and laser technology. The Penning trap and the dynamical Paul trap developed in the 1930s and the late 1950s respectively, provide the experimenter with a unique tool to isolate a single ion from its surroundings. Tunable lasers can then be used to force the ion to fluoresce; simultaneously, as will be described, it is cooled to milli- or even micro-Kelvin temperatures. An ion driven into saturation by a sufficiently high laser intensity so that it spends half of the time in the excited state and half in the ground state, scatters roughly 108 photons per second. This leads to a high detection probability and at the same time to a reduction of the ion's kinetic energy via photon recoil.

DOI
10.1088/2058-7058/2/8/22

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A single ion at rest, unperturbed by its environment and forced into such a state for hours – once only a physicist's dream – has now been achieved by the combination of electromagnetic traps and laser technology. The Penning trap and the dynamical Paul trap developed in the 1930s and the late 1950s respectively, provide the experimenter with a unique tool to isolate a single ion from its surroundings. Tunable lasers can then be used to force the ion to fluoresce; simultaneously, as will be described, it is cooled to milli- or even micro-Kelvin temperatures. An ion driven into saturation by a sufficiently high laser intensity so that it spends half of the time in the excited state and half in the ground state, scatters roughly 108 photons per second. This leads to a high detection probability and at the same time to a reduction of the ion's kinetic energy via photon recoil.
The limitations to the achievement of low kinetic energies for laser cooling of single ions confined in electromagnetic traps are discussed. Sideband cooling of an ion in an rf (Paul) trap is reexamined including the effects of finite laser bandwidth and the energy of the rf micromotion. The micromotion is the oscillatory motion of the ion at the same frequency as the rf voltage applied to the trap electrodes. Sideband cooling of ions in a Penning trap is examined for the first time. In both cases, cooling to the zero-point energy of the ion in the trap should be possible and a method for verifying this condition is suggested. The implications for high-resolution, high-accuracy spectroscopy are investigated. Under certain conditions, the uncertainty in the second-order Doppler shift may be significantly less than 1 part in 1018{10}^{18}1018. .AE
Abstract The ions trapped, whether in the Paul trap or in the Penning trap, usually have energies of the order of 1 eV, that is, a temperature of about 10 000 K. High temperature means high kinetic energy, and this is un favourable to most trap-based experiments for the following reasons. First, with high kinetic energy, amplitudes of ion oscillations are high and consequently the chances of ion losses are high; this results in loss of sensitivity. Second, high kinetic energy enhances both the first-order and the second-order Doppler effects; so, unless the ions are cooled, all spectroscopic experiments are adversely affected in a major way. While isolation of the ions in the trapped condition does not permit ion energy to be easily dissipated to the surroundings, for the same reason, the trapped ions can be effectively cooled and, once cooled, they can be retained cold in the trap. In recent years, most of the trap-based experiments have used ion cooling of one kind or the other. In this chapter we present an overview of various cooling methods and their applications.
Single trapped ions represent elementary quantum systems that are well isolated from the environment. They can be brought nearly to rest by laser cooling, and both their internal electronic states and external motion can be coupled to and manipulated by light fields. This makes them ideally suited for quantum-optical and quantum-dynamical studies under well-controlled conditions. Theoretical and experimental work on these topics is reviewed in the paper, with a focus on ions trapped in radio-frequency (Paul) traps.
Single trapped ions trapped in Paul traps correspond to ideal candidates for realization of extremely accurate optical atomic clocks and practical studies of the light–atom interactions and nonlinear mechanical dynamics. These systems benefit from both, the superb isolation of the ion from surrounding environment and excellent control of its external and internal\ndegrees of freedom, at the same time, which makes them exquisite platforms for experimental studies and applications of light matter interaction at its most fundamental level. The exceptional degree of control of single or few ion's state enabled in past decade number of major advancements in the applications from the fields of experimental quantum information\nprocessing and frequency metrology, including recent realization of scalable Shor's\nalgorithm, fractional uncertainties of the frequency measurements close to 10-18 level, or simulations of complex quantum many-body effects. These results, together with the rapid advancements in the production of low-noise segmented micro-traps, promise prompt access to long-desired regimes of quantum optomechanics and further development and applications\nof optical atomic clocks.
We recently used a compact Penning trap to capture and isolate highly charged ions extracted from an electron beam ion trap (EBIT) at the National Institute of Standards and Technology. Isolated charge states of highly stripped argon and neon ions with total charge $Q\ensuremath{\ge}10$, extracted at energies of up to $4\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.16em}{0ex}}Q$ eV, are captured in a trap with well depths of $\phantom{\rule{0.16em}{0ex}}\ensuremath{\approx}(4--12)Q$ eV. Here we discuss in detail the process to optimize velocity tuning, capture, and storage of highly charged ions in a unitary Penning trap designed to provide easy radial access for atomic or laser beams in charge exchange or spectroscopic experiments, such as those of interest for proposed studies of one-electron ions in Rydberg states or optical transitions of metastable states in multiply charged ions. Under near-optimal conditions, ions captured and isolated in such rare-earth Penning traps can be characterized by an initial energy distribution that is $\ensuremath{\approx}60$ times narrower than typically found in an EBIT. This reduction in thermal energy is obtained passively, without the application of any active cooling scheme in the ion-capture trap.
An individual atomic ion localized in the center of a small Paul rf quadrupole trap is a promising object for ultimate resolution laser spectroscopy because, broadly speaking, the ion may be brought to "A State of Complete Rest in Free Space" by side band cooling. As a consequence, all Doppler shifts vanish. "Free Space" is approximated insofar as the electric trapping field vanishes in the center of the trap and there is no Stark effect. Neither need there be a Zeeman Effect as magnetic fields may be controlled down to the micro-Gauss range. Naturally, there is no transit time broadening. Minute laser powers provided by harmonic generators suffice for saturation of optical transitions as well focused beams may be used. Millionfold atomic amplification of the single-ion fluorescence from a metastable level may bring resolutions of 1 part in 1018 within reach. Furthermore, the close localization of an atomic particle in space is one of the most fundamental problems in physics and worthy of study on its own merit.