= Josephson junction
{c}
A <Josephson junction> couples two regions with <superconductivity> through a weak link. A tunnel barrier is one implementation.
The remarkable possibility is a steady <electric current> across an insulating barrier at zero <electric potential difference>. This is the <DC Josephson effect>, carried by coherent transfer of <Cooper pairs>. It differs from <single-electron superconducting tunneling>, which probes the <quasiparticle> excitation spectrum and produces a dissipative <current-voltage characteristic>. Both contributions can occur in the same superconducting tunnel device. <Brian D. Josephson> predicted the pair contribution in 1962; his paper treats transfer that leaves the <quasiparticle> distribution unchanged. <Possible new effects in superconductive tunnelling by Brian D. Josephson>[Josephson (1962), pp. 251–252, opening paragraph and equation (3)]{ref}
The <Josephson effect> turns a difference of <quantum phases> into a measurable <electric current>, and an applied <electric potential difference> into a rate of phase advance. The <Giaever gap experiment> instead turns an <electric potential difference> into an excitation <energy> scale. These complementary measurements expose two aspects of <superconductivity>: its coherent <order parameter> and its gapped <quasiparticle> spectrum. The microscopic pairing calculation belongs to <BCS theory> and the <BCS gap equation>. A <tunnel diode> uses a different device, a semiconductor <p-n junction>.
Adding <capacitance> and an external bias makes the <Gauge-invariant Josephson phase> dynamical. <Current-biased Josephson phase dynamics> explains how its trapped <energy levels> and escape can be measured electrically.
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