= Martinis-Devoret-Clarke circuit experiments
{c}
The 1985 spectroscopy experiment used <niobium>-based tunnel <Josephson junctions> on <silicon>, thermally anchored with final bias <low-pass filters> to a <dilution refrigerator>. Calibrated <germanium> and <cobalt> thermometers measured <temperature>. A filtered <coaxial cable> injected <microwaves>; repeated switches to nonzero <electric potential difference> measured escape with irradiation on and off. Bias scans at fixed <microwave> <frequency> resolved separate enhancements matching calculated unequal <energy level> spacings. Raising <temperature> merged the structure toward classical response. Junction parameters were calibrated in the classical regime; reducing the <superconducting critical current> with a <magnetic field> checked thermal readout. Parameter uncertainties limited absolute resonance positions.<Energy-Level Quantization in the Zero-Voltage State of a Current-Biased Josephson Junction by John M. Martinis and others>[Original paper, pp.1543–1545, apparatus and Figs.2–4; p.1546, numerical boundary condition]{ref}
The companion escape experiment mounted a tunnel <Josephson junction> on an attenuating <coaxial cable>, cold-anchored its bias filters, ramped the <electric current>, and counted switches to nonzero <electric potential difference>. Rapid current shutoff limited heating. Classical escape and <microwave> resonance calibrated the <superconducting critical current>, <capacitance> and environmental loading. Low-temperature escape agreed with the quantum prediction using these calibrated parameters. A <magnetic field> reduced the <superconducting critical current> to keep the same apparatus thermally activated at lower <temperature>, testing excess noise. Frequency-dependent loading contributed uncertainty; the predicted <damping> correction was smaller than combined errors, so the experiment did not resolve that correction.<Measurements of Macroscopic Quantum Tunneling out of the Zero-Voltage State of a Current-Biased Josephson Junction by Michel H. Devoret and others>[Original paper, pp.1909–1910, calibration, Fig.2 noise control and dissipation limit]{ref}
Together these tests connect spectral structure and escape of the same kind of collective coordinate. They do not constitute a demonstration of coherent gate operations on a <qubit>, or prove that every macroscopic coordinate retains quantum behavior under arbitrary environmental coupling. The attraction is more concrete: a fabricated <electrical circuit> admits both an independently measurable classical model and quantitative quantum tests of its phase dynamics.
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