Trying to count people in a dark room by the sound of their breathing hits a wall of background noise — individual breaths are inaudible. Scientists working with quantum spins in solids face a similar challenge. Each spin (the electron's magnetic moment) has its own "voice," but it is drowned out by the technical noise of the instruments.

In the new study, physicists examined NV centers — defects in the diamond crystal lattice where a carbon atom is replaced by nitrogen and the neighboring site is vacant. These centers behave like tiny magnets, and their state can be read out with light. The key trick of the experiment is the method of optically detected magnetic resonance (ODMR) with a contrast exceeding 20% for one crystallographic orientation of spins. This was achieved by combining polarization-selective optical excitation and spin-to-charge conversion. In other words, the laser was tuned to "interrogate" only spins with a specific orientation and ignore the rest.

As a result, projection noise was directly measured in ensembles containing up to 43 spins, and the number of spins was literally counted from signal fluctuations. This is akin to determining the number of people in a room by the barely perceptible variations in the overall noise rather than by the volume of the chorus. This approach promises a significant gain in the sensitivity of magnetometers based on NV centers — all at room temperature, without bulky cryostats or strong magnetic fields.