= Quantronium Ramsey interference experiment
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The 2002 experiment described in https://arxiv.org/abs/cond-mat/0205343[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.<Manipulating the Quantum State of an Electrical Circuit by Denis Vion and others>[Manipulating the Quantum State of an Electrical Circuit, PDF pp. 1–4, Fig. 1 and Fig. 3; pulse calibration immediately following Fig. 3]{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.<Manipulating the Quantum State of an Electrical Circuit by Denis Vion and others>[Manipulating the Quantum State of an Electrical Circuit, PDF p. 2 readout test; Fig. 3B and PDF p. 4 Ramsey fringes and contrast discussion]{ref}
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