An introductory example to quantum sensing

Quantum sensing refers to the theory and methods that use a quantum system to measure real physical magnitudes such as magnetic field intensities, particle speeds, energy levels, etc.

In this video, the potential of a quantum system is altered by an unknown scalar variable ‘a’ that we want to measure. This variable could represent some physical magnitude like the electric field. Our system has only two levels, denoted as |0) and |1) (a mathematical notation called Bra-Ket used commonly in quantum mechanics to describe levels), both of which remain constant over time except for a cumulative phase, a unit complex number that can appear in the coefficients of quantum states, which is different for each level.

This phase will change faster or slower depending on our parameter ‘a’. In quantum physics, we can use a superposition of these two states, something that classically would be impossible. Can you imagine being on the ground floor and on the first floor of a building? Well, it is possible in the quantum world! We can represent this state as [|0) + |1)] / √2. By letting this state evolve over time we can find the relative phase difference between the |0) and the |1) part of the state. In a lab, we could do this by comparing a particle in this newly evolved state with a particle in the initial superposition form. By measuring how similar they are we can calculate the relative phase that the new state has acquired, and with this phase, we can finally retrieve the elusive parameter ‘a’ that we were looking for.