transmon

Transmon is a type of superconducting qubit used in quantum computing. It is a variant of the Cooper-pair box in which a Josephson junction is shunted by a large capacitor, forming a circuit whose energy spectrum is deliberately made weakly sensitive to the background charge. By increasing the ratio EJ/EC, with EJ the Josephson energy and EC = e^2/(2C) the charging energy, the device enters the transmon regime where charge noise produces only small dephasing. The charge dispersion of the qubit energy levels decreases exponentially with the square root of EC/EJ.

Design and operation: The transmon consists of a Josephson junction (often arranged as a SQUID for tunability) in parallel with a large shunt capacitance. It is typically coupled to a microwave resonator in circuit quantum electrodynamics. The two lowest energy states form the qubit; higher levels form an anharmonic ladder, which allows selective addressing of transitions. Readout is achieved via dispersive shift of the resonator or via other readout schemes.

History: The transmon was introduced in 2007 by Koch, Yu, Gambetta and colleagues as a charge-insensitive qubit for superconducting quantum information. The name reflects reduced sensitivity to charge noise compared to the Cooper-pair box.

Impact and use: Transmons have become widely used in superconducting quantum processors due to their relatively simple fabrication, strong anharmonicity, and robust coherence properties. They are a standard building block in circuit quantum electrodynamics, used in many research efforts and in demonstrations of scalable quantum processors, including Google's Sycamore. See also superconducting qubit, circuit quantum electrodynamics, Josephson junction.