  We identify optimal operating conditions of an entangling two-qubit gate
realized by a capacitive coupling of two superconducting charge qubits in a
transmission line resonator (the so called "transmons"). We demonstrate that
the sensitivity of the optimized gate to 1/f flux and critical current noise is
suppressed to leading order. The procedure only requires a preliminary estimate
of the 1/f noise amplitudes. No additional control or bias line beyond those
used for the manipulation of individual qubits are needed. The proposed
optimization is effective also in the presence of relaxation processes and of
spontaneous emission through the resonator (Purcell effect).
