Ultra-sensitive magnetometry based on free precession of nuclear spins

Abstract

We discuss the design and performance of a very sensitive low-field magnetometer based on the detection of free spin precession of gaseous, nuclear polarized 3 He or 129 Xe samples with a SQUID as magnetic flux detector. The device will be employed to control fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron as well as in a new type of 3 He/ 129 Xe clock comparison experiment which should be sensitive to a sidereal variation of the relative spin precession frequency. Characteristic spin precession times after one day. Even in that sensitivity range, the magnetometer performance is statistically limited, and noise sources inherent to the magnetometer are not limiting. The reason is that free precessing 3 He ( 129 Xe) nuclear spins are almost completely decoupled from the environment. That makes this type of magnetometer in particular attractive for precision field measurements where a long-term stability is required.

In situ calibration of magnetic field coils using free-induction decay of atomic alignment

“…This precision is compatible with the expectation for a Cramér-Rao (CR) limited frequency determination [9] of a damped sine wave with a signal/ noise ratio that is 20 dB above shot noise. The fact that the achieved precision is two orders of magnitude worse than the precision of the individual frequency determinations can be traced back to the % precision of the individual measurements of the control voltages V i performed by the oscilloscope.…”

Design and performance of an absolute 3He/Cs magnetometer

“…SQUIDs (superconducting quantum interference devices) are highly sensitive magnetometers that have been deployed for detecting the 3 He FSP [18]. However, the additional technical complexity associated with the cryogenic cooling needed for the SQUID operation is an obstacle for operating these magnetometers under the experimental conditions of a room temperature nEDM experiment.…”

“…It has first been studied by Cramér and Rao [26], [27] who derived a lower bound for the frequency estimation variance of a signal of constant amplitude (CRLB) [28]. The corresponding bound for a damped oscillation was derived in [18] and reads…”

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“…Hence both the longitudinal and transverse relaxation times are extremely long (they range from minutes in this experiment to several hours or days in optimised situations [17,18,19]). These relaxation processes will thus be neglected.…”