  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.
