Rabi oscillation

A Rabi oscillation is the rhythmic back-and-forth of a two-level quantum system when it is driven by a steady field tuned close to the energy gap between its two states. Instead of jumping once and staying put, the system cyclically climbs to the upper state, falls back to the lower one, climbs again, and so on, sloshing probability smoothly between the two levels like a pendulum.

This is what you see when the driving is strong and the timescale short — exactly the regime where Fermi's golden rule, with its single steady rate, does not apply. Rather than a one-way drift toward the final state, you get coherent oscillation. By choosing how long to apply the field, you can leave the system fully excited, fully relaxed, or in any superposition in between, which is the basis of controlling qubits and spins.

The pace of the oscillation is set by the Rabi frequency, which grows with the strength of the drive. The effect was first studied by Isidor Rabi in molecular-beam experiments and now underpins nuclear magnetic resonance, atomic clocks, and quantum computing, where a precisely timed pulse is used to flip a qubit from one state to another or to place it in a deliberate superposition.