Charge-insensitive qubit design derived from the Cooper pair box

TLDR

Short dephasing times hinder quantum computing, and operating superconducting qubits at optimal sweet spots has markedly improved T2 times, as demonstrated in Cooper pair box qubits. The authors introduce the transmon, a new superconducting qubit. The transmon operates at a high EJ/EC ratio, exponentially reducing charge dispersion while only modestly decreasing anharmonicity. The design predicts a drastic reduction in charge‑noise sensitivity and enhanced qubit‑photon coupling, with anharmonicity sufficient for selective control and no loss of performance from other noise sources.

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.