Why Can Quantum Sensors Measure Magnetic Fields as Tiny as a Femtotesla?

Introduction

A femtotesla (fT) is 10⁻¹⁵ tesla, one quadrillionth of a tesla. Yet quantum magnetometers can detect fields at or below this scale because they do not simply measure a magnetic field directly. Instead, they use quantum states whose behaviour changes predictably when exposed to a magnetic field. That change becomes the measurement signal.

The key is the extraordinary sensitivity of spin.

Electrons and atoms possess quantum angular momentum, or spin, which interacts with magnetic fields. A field changes the energy of different spin states, shifting their resonance frequency.

By measuring this tiny frequency, phase or optical change very precisely, the magnetic field can be inferred.

The Concept: Turn an Invisible Field into a Measurable Quantum Change

In an atomic magnetometer, a laser first polarizes atoms such as rubidium or cesium inside a small vapour cell. An external magnetic field causes their collective spins to precess, much like a spinning top.

The precession rate, known as the Larmor frequency, is proportional to the magnetic field.

The sensor then reads this precession optically. Under the spin-exchange relaxation-free (SERF) regime, unwanted spin relaxation can be dramatically reduced, allowing extremely small fields to be detected. NIST has demonstrated a ⁸⁷Rb atomic magnetometer with sensitivity of 4.5 fT/√Hz.

Another approach uses nitrogen-vacancy (NV) centres in diamond. Here, a nitrogen atom replaces a carbon atom next to an empty lattice site. The resulting defect has quantum spin states whose energy changes with magnetic field. Lasers and microwaves interrogate these states, converting the magnetic field into an optical signal.

What Materials Are Used?

There is no single “quantum sensor material.” Different architectures use different quantum systems:

• Alkali atoms: rubidium and cesium are widely used in optical atomic magnetometers.
• Diamond: NV centres provide a solid-state alternative that can operate from cryogenic to above-room temperatures.
• Superconductors: SQUIDs use superconducting circuits and quantum interference to achieve exceptional sensitivity, but require cryogenic cooling.
• Silicon and glass: microfabricated vapour cells can package atomic sensors into millimetre-scale devices.

The Trade-Off: Sensitivity Is Not Everything

Femtotesla sensitivity comes with engineering compromises.

Atomic magnetometers can achieve extraordinary sensitivity but often require magnetic shielding, controlled fields, lasers and carefully managed temperature and atomic density. SERF sensors, in particular, work best in very low magnetic fields.

SQUIDs offer outstanding sensitivity but need cryogenic cooling, increasing system complexity, size and cost.

NV-diamond sensors trade some ultimate sensitivity for robustness, compactness, spatial resolution and operation under much broader environmental conditions. Their sensitivity is also limited by spin coherence, photon collection and material quality.

The important lesson is that quantum sensing does not eliminate noise, it gives engineers a remarkably sensitive quantum reference against which extremely small changes can be measured. The future therefore lies not simply in achieving the lowest number of femtotesla, but in balancing sensitivity, bandwidth, spatial resolution, power, environmental stability and cost for the application.

References for Further Reading

1. NIST — Sensors for a Magnetic World
2. NIST — Femtotesla Atomic Magnetometry in a Microfabricated Vapor Cell
3. Barry et al., Sensitivity Optimization for NV-Diamond Magnetometry, Reviews of Modern Physics
4. Budker, Shaffer & Kitching, Atom-Based Quantum Sensing of Electromagnetic Fields, Optica (2025)
5. NIST — NV-Center Magnetometry