Department of Plasma Physics and Astrophysics

Active control of atomic spins by the application of resonant optical and radio frequency (rf) fields has been used since 1950 to measure the coupling of atomic magnetic moments to external magnetic fields, especially when resonance behavior yields information on the fields. For ...

We present an experimental study of the spectra produced by optical/radio-frequency double resonance in which resonant linearly polarized laser light is used in the optical pumping and detection processes. We show that the experimental spectra obtained for cesium are in excellent agreement with a very general theoretical model developed in our group and we investigate the limitations of this model. Finally, the results are discussed in view of their use in the study of relaxation processes in aligned alkali vapors.

We have performed a comparison of laser (LsOPM) and lamp (LpOPM) pumped cesium vapor magnetometers. Although the LsOPM operated 50% above its shot-noise limit we found an intrinsic sensitivity of 15 fT/√Hz and 25 fT/√Hz for the LsOPM and the LpOPM, respectively. Two modes of operation, viz. the phase-stabilized and the self-oscillating modes, were investigated and found to yield a similar performance. We have compared the performance of the LsOPM and the LpOPM directly by simultaneous measurements of field fluctuations of a 2-μT magnetic field inside a multilayer magnetic shield and have used one of the magnetometers for an active field stabilization. In the stabilized mode we found a gradient instability of 25 fT within an integration time of 100 s, which represents an upper limit of the long-term stability of the magnetometers. Our research is motivated by the need for an improved control of magnetic fields and gradients in a planned neutron electric dipole experiment.

We present a theoretical study of the spectra produced by optical-radio-frequency double resonance devices, in which resonant linearly polarized light is used in the optical pumping and detection processes. We extend previous work by presenting algebraic results which are valid for atomic states with arbitrary angular momenta, arbitrary rf intensities, and arbitrary geometries. The only restriction made is the assumption of low light intensity. The results are discussed in view of their use in optical magnetometers.

ABSTRACT The evolution of the radio-frequency magnetic resonance spectrum of optically oriented potassium atoms in terrestrial magnetic fields as a function of the amplitude of the alternating magnetic field H 1 is investigated theoretically and experimentally. It is shown that among the set of observed n-quantum resonances which satisfy the selection rules ΔF=0 and Δm F±n (n is an integer), there is one resonance with the highest multiplicity n=4 (m F=2⇔m F=−2), which has exceptional properties. These qualities make this resonance a candidate for use in a weak-field quantum magnetometer with record-high characteristics. A preliminary investigation of an experimental model of a four-quantum potassium magnetometer is performed.

The evolution of the radio-frequency magnetic resonance spectrum of optically oriented potassium atoms in terrestrial magnetic fields as a function of the amplitude of the alternating magnetic field H 1 is investigated theoretically and experimentally. It is shown that among the set of observed n-quantum resonances which satisfy the selection rules ΔF=0 and Δm F±n (n is an integer), there is one resonance with the highest multiplicity n=4 (m F=2⇔m F=−2), which has exceptional properties. These qualities make this resonance a candidate for use in a weak-field quantum magnetometer with record-high characteristics. A preliminary investigation of an experimental model of a four-quantum potassium magnetometer is performed.

The design of a new quantum magnetometer for geomagnetic fields, an outgrowth of the idea of the so-called HFS magnetometer that uses microwave transitions between hyperfine sublevels of alkaline atoms, is presented. In contrast to the ordinary HFS magnetometer, which measures the frequency difference of two independent transitions, the new Λ-HFS magnetometer is based on the excitation of two transitions with a common level. The design under discussion retains the principal advantages of the prototype (negligible systematic errors and the absence of dead zones for a single sensor), with the electronic circuits being simplified significantly and the requirements on the short-term frequency stability of the reference oscillator being relaxed substantially. An analysis of the new scheme is based on the solution of the equation for the density matrix of a multilevel system without constraints on the power of the microwave fields.

Experiments aimed to determine the sensitivity and the optical shift of the output frequency of a magnetometer based on optical pumping of cesium atoms and intended for a neutron magnetic resonance stabilization system are described. A gradiometric arrangement used in these experiments makes it possible to reduce the effect of magnetic field variations on measurements.

A new version of the fast optically pumped magnetometer, an optically pumped balanced quantum magnetometer, built around a pair of symmetric transitions in the hyperfine structure of the 87Rb ground state is implemented for the first time. The noise-limited sensitivity of the prototype in terms of the variance is 6 pT for a measurement time of 0.1 s. The basic advantages of the new magnetometer are the absence of dead zones when it changes orientation relative to the magnetic field and an extremely low sensitivity to the drift of pumping parameters.

The design of a quantum magnetometer on cesium vapor is described. The device is intended for stabilization of the neutron resonance in experiments devoted to the search for the electric dipole moment (EDM) of neutron. The choice of this magnetometer type is justified and its potential sensitivity is estimated.

We report the results of an experimental investigation of the longitudinal relaxation time T 1 of the electronic polarization of the ground state of potassium atoms in cells with an antirelaxation coating on the walls. Investigations were performed for a number of cells at temperature T=24°C. The maximum recorded relaxation times τ are 3.6, 4.3, and 5.2 s for cells with diameter D=50,75, and 100 nm, respectively. These are the longest longitudinal relaxation times ever recorded in coated cells. The transverse relaxation time was measured for these cells at the same temperature: T 2=0.72,1.1, and 1.4 s, respectively.

Optimization of parameters of the M x resonance, which excited between Zeeman sublevels of a single HFS level of the ground state of an alkali metal in a vacuum cell in a circuit with a single beam for pumping and detection (M x magnetometer circuit), has been carried out. A simple model taking into account all main factors controlling the resolving power of the M x resonance (including spin-exchange broadening and absorption in a thick layer of the cell has been constructed. It is shown that the spin-resonance broadening of the resonance line is mainly determined by the requirements imposed on the optical thickness of the cell, which considerably restricts the realization of advantages of ultranarrow (<1 Hz) lines in magnetometry. The experiment confirming the efficiency of the model has been carried out.

A new model of a quantum magnetometer based on the principle of Cs-K tandem was developed and experimentally tested. The specific features of the magnetometer are a single absorption cell containing Cs and K vapors, digital synthesis of the potassium resonance frequency by multiplying the cesium resonance frequency by a conversion factor (the ratio of the atomic constants), and the use of four-quantum resonance in potassium atoms. Device readings were found to be insensitive (within 10 pT) to variations of the main operating parameters even if the variations go far beyond the normal service variability.

The design of a multichannel system of stabilization of the magnetic resonance of ultracold neutrons in a multichamber spectrometer developed in the search for the electric dipole moment (EDM) of the neutron is briefly described. The results of tests on a prototype of this system are presented, which show the possibility of ensuring the stability of resonance lines that is equivalent to a magnetic field variation not exceeding ∼30 fT over a 100-s period of EDM measurements.