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The 2002 experiment described in Manipulating the Quantum State of an Electrical Circuit demonstrates Ramsey interferometry in a circuit using superconductivity. An aluminum island between two small Josephson junctions and a larger readout Josephson junction formed the loop; a dilution refrigerator cooled it. Manipulation used the charge/flux bias point least sensitive to first-order fluctuations. Microwave pulse amplitude and duration were calibrated using Rabi oscillations. Two phase-coherent $\pi /2$ pulses then enclosed a variable free interval. A current pulse switched the large Josephson junction to finite voltage with state-dependent probability; a preamplifier and threshold discriminator counted repeated trials.^[ref]

As a readout control, switching was tested against pulse height at thermal equilibrium. The Ramsey interferometry oscillation period agreed with inverse frequency detuning, supporting relative quantum phase accumulation against the external oscillator. The decaying envelope indicated finite quantum coherence, while reduced contrast was attributed provisionally to population relaxation during readout. This supports controlled coherent evolution over a finite interval, without establishing perfect isolation or identifying one exclusive noise mechanism.^[ref]

An experimental physicist who coauthored the 1964 report of quantum interference in Josephson junctions. The work belongs to the experimental development of superconducting quantum interference devices: a superconducting circuit converts sensitivity to phase and magnetic flux into an electrical signal. It connects the Josephson effect with precision magnetic measurements.^[ref]

A superconducting loop containing one Josephson junction and coupled inductively to a radio-frequency resonator. Its flux-dependent nonlinear inductive response changes the resonator signal. A DC SQUID instead uses two junctions and can turn their critical-current interference into a voltage modulation under bias. Inductance, junction dynamics, noise and feedback determine the practical transfer function.

A SQUID uses Josephson junctions in a loop made from a superconductor to detect changes in magnetic flux.

A superconducting device uses superconducting behavior to perform a function such as generating a strong magnetic field, detecting flux, switching or manipulating quantum states. Examples include superconducting magnets, SQUIDs and Josephson junction circuits.

Its performance depends on the full operating environment: temperature, magnetic field, applied current, losses and thermal coupling. A superconducting component does not make every part of a device lossless. Junction interfaces, quasiparticles and vortices can produce measurable dissipation or noise even when the intended circuit element has a superconducting state.