Question 1

What is a Rabi oscillation?

Accepted Answer

A Rabi oscillation is the coherent, periodic transfer of population between the two levels of a quantum system when it is driven by a near-resonant oscillating field. The probability of finding the atom in the excited state swings smoothly between 0 and a maximum and back again.

Question 2

What is the Rabi frequency?

Accepted Answer

The Rabi frequency Omega is the rate at which population cycles between the two levels on resonance. It equals the dipole matrix element times the field amplitude divided by the reduced Planck constant: Omega = d·E/hbar. Stronger driving gives faster oscillations.

Question 3

What is detuning?

Accepted Answer

Detuning Delta is the difference between the drive frequency and the atomic transition frequency. When Delta is zero the drive is on resonance; non-zero detuning speeds up the oscillation but reduces the maximum excited-state population.

Question 4

What is the generalised Rabi frequency?

Accepted Answer

The generalised (effective) Rabi frequency is Omega_R = sqrt(Omega^2 + Delta^2). It sets the actual oscillation rate when there is detuning, and the oscillation amplitude is reduced by the factor Omega^2/Omega_R^2.

Question 5

What is a pi pulse?

Accepted Answer

A pi pulse is a drive applied for exactly the time needed to complete half a Rabi cycle, Omega_R·t = pi. On resonance it fully inverts the atom, moving all population from the ground state to the excited state — equivalent to a qubit NOT gate.

Question 6

What is a pi/2 pulse?

Accepted Answer

A pi/2 pulse drives the system for a quarter Rabi cycle, Omega_R·t = pi/2. On resonance it creates an equal superposition of ground and excited states — the Bloch vector ends up lying in the equatorial plane. It is the building block of Ramsey interferometry.

Question 7

What is the Bloch sphere?

Accepted Answer

The Bloch sphere is a geometric picture of a two-level quantum state. The north pole is the ground state, the south pole is the excited state, and every pure state is a point on the surface. Driving the atom rotates the Bloch vector around an axis set by the drive and detuning.

Question 8

How do Rabi oscillations relate to qubits?

Accepted Answer

A qubit is a controllable two-level system, so Rabi oscillations are exactly how single-qubit gates are performed. By choosing the pulse length, phase and detuning, an experimenter rotates the qubit to any desired state on the Bloch sphere.

Question 9

Why do real Rabi oscillations decay?

Accepted Answer

In real systems spontaneous emission and dephasing gradually destroy coherence, so the oscillations damp out and the population settles toward a steady value. This idealised simulation shows the coherent, undamped limit to make the physics clear.

Question 10

Who discovered Rabi oscillations?

Accepted Answer

They are named after Isidor Isaac Rabi, who developed the theory of magnetic resonance in molecular beams in the late 1930s. His work won the 1944 Nobel Prize in Physics and underpins NMR, atomic clocks and quantum computing.

🔬 Rabi Oscillations — Driven Two-Level Atom

What did you find?