Superconducting quantum magnetometer with independent pumping

Konstantin K. Likharev, Oleg V. Snigirev, S.S. Tinchev

Cort Johnson, Peter D. D. Schwindt

In recent years, atomic magnetometers (AMs) have demonstrated sub-femtotesla sensitivities and have emerged as potential replacements for superconducting quantum interference devices (SQUIDs). In an AM, a circularly polarized pump laser aligns the electron spins in a cloud of alkali metal vapor. Changes occur in the optical properties of the vapor when its collective moment interacts with an external magnetic field, resulting in a measurable polarization rotation of a linearly polarized probe beam. We present a unique pump/probe scheme in which a pump beam tuned to the87Rb D1 line (795 nm) and a probe beam tuned to the D2 line (780 nm) share a single optical axis. A dichroic waveplate is used to circularly polarize the pump beam while leaving the probe beam linearly polarized. Based upon this scheme, we have developed a small profile, fiber-coupled AM. By independently optimizing pump/probe power and detuning, we are able to achieve intrinsic single-channel sensitivity of <; 5 fT/√Hz, which is suitable for demanding applications such as magnetoencephalography (MEG). Furthermore, through straightforward adaptations of the current design, it should be possible to create dense, reconfigurable arrays of AMs for whole-head MEG.

B. V. Vasiliev, V. G. Grebinnik, V. G. Simkin

A. A. Baranov, Ермак Сергей Викторович, E. A. Sagitov +2 more

The contributions of the vector and scalar components to the magnetically dependent microwave transition frequency light shift are analyzed and the compensation of these components is experimentally demonstrated for the 87Rb atoms optically oriented by a laser tuned to the D 2 line of the head doublet. The Allan variance is studied as a function of the averaging time for a tandem of optically pumped quantum magnetometers (OPQMs), one of which is based on a low-frequency spin oscillator while another is based on a quantum microwave discriminator with a resonance frequency that corresponds to magnetically dependent transitions between HFS sublevels with the extremal value of the magnetic quantum number. It is shown that the compensation of the scalar and vector components of the light shift in OPQMs reduces the Allan variance at averaging times that exceed hundreds of seconds compared to a quantum discriminator based on the magnetically independent 0–0 transition. In this case, the minimal Allan variance in OPQMs at the end resonance is achieved at considerably longer averaging times than in the case of the quantum discriminator that is tuned to the 0–0 transition frequency.

Manas Pandey

Stationary multi-magnetometer arrays are routinely deployed in geomagnetic observatories, laboratory shielded rooms, and ground-based monitoring stations. The standard analysis pipeline reduces each sensor to an independent power spectrum, discarding any inter-sensor structure that is itself diagnostic of measurement health and of localised magnetic activity. This paper develops a purely theoretical framework for a deliberately simple visualisation that maps the short-time Fourier (STFT) power spectra of three concurrent magnetometers into the red, green, and blue channels of a single image: the \emph{cross-sensor RGB spectrogram}. Inter-sensor coherence appears as neutral grey or white, while spectral energy that is unique to one or two sensors stands out as saturated colour. We formalise the construction of the image, derive its time-frequency resolution properties, give an explicit account of the per-channel normalisation choice, and present a colour-anomaly taxonomy that distinguishes coherent broadband activity, single-sensor faults, asymmetric pairwise sources, and slow temporal drift. A companion long-window variant is described for resolving features in the ultra-low frequency (ULF) band. The construction is presented without reference to any particular dataset or implementation; it is intended as a self-contained methodological building block that can be inserted into any monitoring pipeline whose front end is a synchronously sampled magnetometer triad. Because the construction operates on scalar magnitude time series alone, it applies equally to classical fluxgate sensors and to quantum magnetometers -- optically pumped magnetometers (OPMs), nitrogen-vacancy (NV) centre arrays, and superconducting quantum interference devices (SQUIDs) -- where distinguishing quantum-limited noise from technical artefacts is a central diagnostic challenge.

Jiahui Li, Irina V. Novikova

We experimentally demonstrate that a squeezed probe optical field can improve the sensitivity of magnetic field measurements based on nonlinear magneto-optical rotation (NMOR) with an amplitude-modulated pump when compared to a coherent probe field under identical conditions. To realize an all-atomic magnetometer prototype, we utilize a nonlinear atomic interaction, known as polarization self-rotation, to produce a squeezed probe field. An independent pump field, amplitude-modulated at the Larmor frequency of the bias magnetic field, allows us to extend the range of most sensitive NMOR measurements to sub-Gauss magnetic fields. While the overall sensitivity of the magnetometer is rather low ( > 250 p T / H z ), we clearly observe a 15 % sensitivity improvement when the squeezed probe is used. Our observations confirm the recently reported quantum enhancement in a modulated atomic magnetometer [ Phys. Rev. Lett. 127 , 193601 ( 2021 ) PRLTAO 0031-9007 10.1103/PhysRevLett.127.193601 ].