The operating principle of quantum sensors is conceptually straightforward. We use quantum systems prepared in a state that is maximally sensitive to the environmental parameter we wish to measure. As the system interacts with its surroundings, its quantum state changes. Our task is to detect this change. Since the evolution of the system depends on the value of the environmental parameter, we can infer its value from the observed change.

The more precisely we can measure the quantum system and the better we understand its interaction with the environment, the faster and more accurately we can determine the parameter of interest. An individual quantum measurement, however, provides little information because its outcome is inherently random. Measurements therefore have to be repeated, and the result of a quantum measurement is ultimately expressed as a probability. The uncertainty of the estimated parameter is determined by how accurately this probability can be established. Absolute precision would require an infinite number of measurements and is therefore fundamentally unattainable.

In classical measurements, statistical uncertainty typically decreases proportionally to the square root of the number of repetitions. Quantum measurements, however, can exploit entangled states, making it theoretically possible to achieve a precision that scales linearly with the number of measurements. In principle, one hundred measurements performed with an ideal quantum sensor could provide the same precision as ten thousand measurements with a classical device. Unfortunately, preparing the required entangled states is far from simple.