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Quantum Sensors Are Already Here: A Review of Quantum Sensing of Magnetic Fields

By Bharath Selvaraj posted 29 Jun 2020

(Medium.com) In this, the first of a promised series of articles, the authors discuss “Quantum Sensing of Magnetic Fields.”
Magnetic field is often the easiest thing for a quantum system to measure.
The most common one for quantum sensors is the familiar Zeeman effect, where magnetic field causes one energy level of the quantum system to split into multiple ones, with the energy gap proportional to the applied field. A quantum sensor will typically directly measure this splitting and extract the B-field from the result.
But just because a quantum sensor responds to a magnetic field, does not mean it can make a useful magnetometer! This is because you need to be able to miniaturise it. Except for a few extremely specific applications, a sensor is no good if it requires a big vacuum chamber and a room full of lasers. This is what makes atoms less ideal than solid-state sensors of comparable specs.
Secondly, the market is magnetometers is very busy.
On the “budget” end, integrated circuits which act as your phone compass are extremely small, reliable and cost about 1$.
On the “high performance” end, you find the SQUID (which stands for superconducting quantum interference device). These (quantum) sensors have been around since the 1960s, and nowadays reach mind-boggling sensitivities of ~1 fT \sqrt(s). The downside? They need to sit in a cryogenic environment!
The challenge for the new wave of quantum magnetometers is the need to find niches which are not satisfied with the performance of cheap classical sensors, but for which SQUIDs seem to bulky or expensive.
Physicists are actively studying a plethora of quantum magnetometers, each with its own pros and cons. The article discusses the two leading quantum sensing technologies:
1) Nitrogen-vacancy (NV) centers in diamond — probably the smallest magnetometer in the world. An NV center is an artificially synthesised defect that can be thought of as a single spin frozen inside diamond. This is essentially the product already available from Swiss start-ups Qnami and QZabre.
2) Atomic vapor cells — high performance quantum sensor. A vapor cell operates on the same principle of Zeeman effect we already discussed. However, instead of probing a spin (defect) embedded in diamond, we measure spins of atoms flying around a glass cell. The system is prepared and detected optically with a laser beam.
A recent high-performance unit, produced by Twinleaf, weighs 100g and achieves sensitivity <200 fT \sqrt(s). Perhaps this is the beginning of a wave of next-generation magnetometers?

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