robotics//sensor//magnetometer//optically pumped magnetometer

An optically pumped magnetometer measures magnetic fields by watching how a vapour of alkali atoms responds to them, and it is the sensor that made wearable brain recording possible: it approaches the sensitivity of a SQUID without the liquid helium, so it fits in a small module that sits on the scalp. It is also used in geophysics and to record the heart's magnetic field.

An optically pumped magnetometer measures magnetic fields by watching how a vapour of alkali atoms responds to them, and it is the sensor that made wearable brain recording possible: it approaches the sensitivity of a SQUID without the liquid helium, so it fits in a small module that sits on the scalp. It is also used in geophysics and to record the heart's magnetic field.

A small glass cell holds the vapour, potassium, rubidium or caesium, heated to raise its density. A circularly polarized laser, the pump, lines up the spins of the atoms; a magnetic field makes those spins precess, and a second beam, the probe, crosses the cell and is rotated in proportion to how far they have turned. The rotation of the light is the reading.

The most sensitive mode works only near zero field.

In the spin-exchange relaxation-free regime the atoms stay coherent because the field is tiny; a 1 cm³ cell then reaches about 1 fT per root hertz, enough for the brain, but only inside a room that has cancelled the Earth's field around it.

The gain over a SQUID helmet is distance. A SQUID array sits inside a rigid flask a few centimetres from the head; an OPM rests on the scalp and moves with it, so it receives a stronger signal and lets the person move.

The cost is the environment. Shielded rooms of mu-metal and aluminium, often with active coils, bring the background down to a few femtotesla per root hertz; outside them the vapour sees the city's magnetic noise, about 10810^{8}108 fT.

Not every OPM runs in that regime: caesium magnetometers that work in the Earth's field, used in surveying, are far less sensitive (around 300 fT per root hertz) but need no shielding.

The cell runs hot (potassium cells are operated around 150 to 200 °C), so a wearable sensor needs insulation between the cell and the skin.