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Concept
Phase qubit
Summary
In quantum computing, and more specifically in superconducting quantum computing, the phase qubit is a superconducting device based on the superconductor–insulator–superconductor (SIS) Josephson junction, designed to operate as a quantum bit, or qubit.
The phase qubit is closely related, yet distinct from, the flux qubit and the charge qubit, which are also quantum bits implemented by superconducting devices. The major distinction among the three is the ratio of Josephson energy vs charging energy (the necessary energy for one Cooper pair to charge the total capacitance in the circuit):
For phase qubit, this ratio is on the order of 106, which allows for macroscopic bias current through the junction;
For flux qubit it's on the order of 10, which allows for mesoscopic supercurrents (typically ~300 nA);
For charge qubit it's less than 1, and therefore only a few Cooper pairs can tunnel through and charge the Cooper-pair box. However, transmon can have a very low charging energy due to the huge shunt capacitance, and therefore have this ratio on the order of 10~100.
A phase qubit is a current-biased Josephson junction, operated in the zero voltage state with a non-zero current bias.
A Josephson junction is a tunnel junction, made of two pieces of superconducting metal separated by a very thin insulating barrier, about 1 nm in thickness. The barrier is thin enough that electrons, or in the superconducting state, Cooper-paired electrons, can tunnel through the barrier at an appreciable rate. Each of the superconductors that make up the Josephson junction is described by a macroscopic wavefunction, as described by the Ginzburg–Landau theory for superconductors. The difference in the complex phases of the two superconducting wavefunctions is the most important dynamic variable for the Josephson junction, and is called the phase difference , or simply "phase".
Official source
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