Transmon

thumb|upright=0.8|Eigenenergies E_m(first three levels, m = 0, 1, 2) of the qubit Hamiltonian as a function of the effective offset charge n_g for different ratios E_J/E_c. Energies are given in units of the transition energy E_{01}, evaluated at the degeneracy point n_g = 0.5. The zero point of energy is chosen as the bottom of the m = 0 level. The charge qubit (small E_J / E_c, top) is normally operated at the n_g = 0.5 "sweet spot" where fluctuations cause less energy shift and the anharmonicity is maximal. Transmon (large E_J / E_c, bottom) energy levels are insensitive to fluctuations but the anharmonicity is reduced. In quantum computing, and more specifically in superconducting quantum computing, a transmon is a type of superconducting charge qubit designed to have reduced sensitivity to charge noise. The transmon was developed by Jens Koch, Terri M. Yu, Jay Gambetta, Andrew Houck, David Schuster, Johannes Majer, Alexandre Blais, Michel Devoret, Steven M. Girvin, and Robert J. Schoelkopf at Yale University and Université de Sherbrooke in 2007. Its name is an abbreviation of the term transmission line shunted plasma oscillation qubit; one which consists of a Cooper-pair box "where the two superconductors are also [capacitively] shunted in order to decrease the sensitivity to charge noise, while maintaining a sufficient anharmonicity for selective qubit control". thumb|upright=0.8|left|A device consisting of four transmon qubits, four quantum buses, and four readout resonators fabricated by [[IBM and published in npj Quantum Information in January 2017.]]

The transmon achieves its reduced sensitivity to charge noise by significantly increasing the ratio of the Josephson energy to the charging energy. This is accomplished through the use of a large shunting capacitor. The result is energy level spacings that are approximately independent of offset charge. Planar on-chip transmon qubits have T1 coherence times approximately 30 μs to 40 μs. Recent work has shown significantly improved T1 times as long as 95 μs by replacing the superconducting transmission line cavity with a three-dimensional superconducting cavity, and by replacing niobium with tantalum in the transmon device, T1 is further improved up to 0.3 ms. These results demonstrate that previous T1 times were not limited by Josephson junction losses. Understanding the fundamental limits on the coherence time in superconducting qubits such as the transmon is an active area of research.