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arXiv:2003.08459 · Physical Review A

Precise control of magnetic fields and optical polarization in a time-orbiting potential trap

Adam Fallon, Seth Berl, Edward Moan, Charles A. Sackett

Published Aug 10, 2020• 2 citations
arXivHTMLPDF

Abstract

A time-orbiting potential trap confines neutral atoms in a rotating magnetic field. The rotation of the field can be useful for precision measurements, since it can average out some systematic effects. However, the field is more difficult to characterize than a static field, and it makes light applied to the atoms have a time-varying optical polarization relative to the quantization axis. These problems can be overcome by using stroboscopic techniques, where either a radio-frequency field or a laser is applied in pulses that are synchronized to the rotating field. By using these methods, the magnetic field can be characterized with a precision of 10 mG and light can be applied with a polarization error of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$.

DOI
10.1103/physreva.102.023108

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A time-orbiting potential trap confines neutral atoms in a rotating magnetic field. The rotation of the field can be useful for precision measurements, since it can average out some systematic effects. However, the field is more difficult to characterize than a static field, and it makes light applied to the atoms have a time-varying optical polarization relative to the quantization axis. These problems can be overcome by using stroboscopic techniques, where either a radio-frequency field or a laser is applied in pulses that are synchronized to the rotating field. By using these methods, the magnetic field can be characterized with a precision of 10 mG and light can be applied with a polarization error of 5\ifmmode×\else\texttimes\fi10\ensuremath−55\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}5\ifmmode×\else\texttimes\fi10\ensuremath−5.
Methods of trapping neutral spin-polarized particles are proposed using highly nonuniform magnetic fields rotating at low frequencies, down to the resonance of the particle's orbiting motion. Such fields beget new trapping mechanisms of vast possibilities. The new resonance methods and the new traps of zero-size equilibrium orbit should give much tighter and more stable confinement than previously known. They also give the possibility of a high, resonance selectivity in magnetic separation and evaporative cooling. A general analysis of trapping with rotating fields is given.
Magneto-Optical Traps (MOTs) have long been used to produce samples of cold trapped neutral atoms, which can be used in the measurement of a variety of physical quantities and theories. Until recently, one limitation of this type of trap was the necessity for the presence of a relatively large magnetic field which would decay only slowly after the trapping mechanism was turned off. This residual magnetic field is expected to partially destroy any atomic polarization induced, for example, by optical pumping. As a result, the precision of any physical measurement which requires polarization is limited. We will discuss the construction of our version of a newer type of MOT, the AC-MOT [2], which is designed specifically so as to minimize residual magnetic fields. We have found that our AC-MOT has lifetimes and cloud sizes similar to those we measured in our DC-MOT. We intend to use a trap similar to this in upcoming nuclear beta decay parity-violation measurements. We also discuss the numerical evolution of the optical Bloch equations in the presence of transverse and longitudinal magnetic fields, so as to quantify the effect of a magnetic field on atomic polarization.
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Electromagnetic trapping of neutral atoms is a new branch of applied physics that has potential for use in very many areas, including metrology and high-resolution precision spectroscopy. It has the possibility of leading toward the spectroscopic ideal of an isolated atom at rest, available for interaction with electromagnetic field probes. We present an introduction to trapping in both dc magnetic fields and optical fields (laser beams). The basic ideas and fundamental limitations are discussed and the first successful experiments are reviewed.