Vector Magnetometry With NV Diamond Ensembles

Short answer: Ensembles of nitrogen-vacancy (NV) centres in diamond can measure magnetic fields, and vector magnetometers measure all three field components at once. A 2018 experiment reported a simultaneous vector magnetometer with about 50 pT sensitivity per component, and a 2020 paper described a microwave-free design. A 2020 review says present ensemble devices are orders of magnitude away from theoretical sensitivity limits.

Why are NV ensembles used for magnetometry?

The 2018 Physical Review Applied abstract notes that optical NV-diamond measurements provide high-sensitivity, broadband magnetometry under ambient or extreme physical conditions. The four fixed crystallographic axes in diamond enable vector sensing free from heading errors. A 2020 Reviews of Modern Physics article says solid-state spin systems including NV centres are an increasingly favoured quantum sensing platform.

What is the gap to theory?

The 2020 review states that present NV ensemble devices have sensitivities orders of magnitude away from theoretical limits, which both handicaps existing implementations and curtails the application space. It analyses present and proposed approaches to improve the sensitivity of broadband ensemble NV-diamond magnetometers, and names improvements to the spin dephasing time, the readout fidelity and the host diamond material properties as the most promising avenues. It aims to give a foundation for new techniques.

What did the 2018 vector magnetometer do?

The authors demonstrate a vector magnetometer that simultaneously measures all Cartesian components of a dynamic magnetic field using an ensemble of NV centres in single-crystal diamond. Multichannel lock-in detection extracts the field-dependent spin-resonance shifts of NVs along all four tetrahedral axes from the optical signal on a single detector. The sensor works from near dc, at 5 Hz, up to a 12.5 kHz measurement bandwidth, and achieves approximately 50 pT sensitivity for each component, in the units given in the abstract, which did not render fully in the page text we read. The authors describe this as, to date, the highest demonstrated sensitivity of a full vector magnetometer employing solid-state spins. They report a fourfold measurement speed-up compared with optimized devices that interrogate the four orientations in sequence, and say the technique could extend to pulsed sensing protocols and parallel wide-field magnetic imaging.

What about the microwave-free approach?

A 2020 Physical Review Applied paper notes that vector magnetometry often needs microwaves applied sequentially or simultaneously, which limits use at cryogenic temperature. The authors propose and demonstrate a microwave-free vector magnetometer that measures all Cartesian components using NV ensembles. It uses the level anticrossing in the triplet ground state at 102.4 mT and allows measurement of longitudinal and transverse fields with bandwidth from zero to the megahertz range. They report a root-mean-square noise floor in all directions, but the unit did not render in the abstract text, so the figure is not quoted here. They say the technique may extend to single NV sensors and to nanoscale measurements at ambient temperature, which is a prospect.

How to read this

The two experiments use different methods and operating fields, so their performance cannot be compared directly. All figures are the authors' own results from laboratory devices. This post is based on abstracts and visible summary text, and full texts were not checked. It makes no claim about commercial sensors.

FAQ

What does vector magnetometry measure?

All components of the magnetic field, not just its magnitude. The 2018 device reads three Cartesian components simultaneously.

What speed-up did simultaneous measurement give?

A fourfold measurement speed-up compared with devices that read the four NV orientations in sequence, according to the 2018 abstract.

Why microwave-free?

Microwaves are a limit for operation at cryogenic temperature, according to the 2020 paper, which uses a level anticrossing instead.

How far are ensemble sensors from their limits?

The 2020 review says orders of magnitude away from theoretical limits.