Charge insensitive qubit design derived from the Cooper pair box

TL;DR

The paper introduces the transmon, a superconducting qubit with increased charge noise immunity and strong qubit-photon coupling, achieved by operating at a high ratio of Josephson to charging energy, improving coherence times.

Contribution

It proposes the transmon qubit design, which significantly reduces charge noise sensitivity while maintaining control and coupling capabilities, advancing superconducting qubit technology.

Findings

Charge dispersion decreases exponentially with E_J/E_C.

Transmon exhibits reduced charge noise sensitivity.

Maintains sufficient anharmonicity for control.

Abstract

Short dephasing times pose one of the main challenges in realizing a quantum computer. Different approaches have been devised to cure this problem for superconducting qubits, a prime example being the operation of such devices at optimal working points, so-called "sweet spots." This latter approach led to significant improvement of $T_2$ times in Cooper pair box qubits [D. Vion et al., Science 296, 886 (2002)]. Here, we introduce a new type of superconducting qubit called the "transmon." Unlike the charge qubit, the transmon is designed to operate in a regime of significantly increased ratio of Josephson energy and charging energy $E_J/E_C$. The transmon benefits from the fact that its charge dispersion decreases exponentially with $E_J/E_C$, while its loss in anharmonicity is described by a weak power law. As a result, we predict a drastic reduction in sensitivity to charge noise relative to the Cooper pair box and an increase in the qubit-photon coupling, while maintaining sufficient anharmonicity for selective qubit control. Our detailed analysis of the full system shows that this gain is not compromised by increased noise in other known channels.