The Motion of a Qubit.
Prepare a qubit in either of two states. Measure each directly a thousand times, and both can produce roughly half zeros and half ones. Now apply a microwave pulse before measurement: one state can become zero every time, the other one. The pulse reveals a difference that the first measurement could not see—their relative phase.
A qubit stores information in the amplitudes of two basis states, zero and one. Their sizes determine the probabilities in a direct measurement; their relative angle affects how they combine when the qubit is rotated. On a superconducting chip, a Josephson junction makes the circuit’s energy levels unevenly spaced, helping a microwave pulse address the qubit without exciting higher levels.
We follow the state on a Bloch sphere, where microwave pulses rotate its representation. During a wait, the relative phase between the zero and one components changes. A second pulse turns that phase difference into a measurable change in the fraction of ones. This is a Ramsey experiment.
The pattern of Ramsey fringes reveals how quickly the qubit’s phase slips relative to the microwave reference. Frequency fluctuations blur the pattern across repeated runs. Energy relaxation reduces the excited-state population. Measuring both effects helps researchers understand how a real device differs from an ideal qubit.
