Decoherence and dissipation during a quantumXOR gate operation

The dynamics of a generic quantum XOR gate operation involving two interacting qubits being coupled to a bath of quantum harmonic oscillators is explored. By use of the formally exact quasiadiabatic-propagator path-integral methodology we study the time-resolved evolution of this interacting and decohering two-qubit system in presence of time-dependent external fields. The quality of the XOR gate operation is monitored by evaluating the four characteristic gate quantifiers: fidelity, purity, the quantum degree, and the entanglement capability of the gate. Two different types of errors for the XOR operation have been modeled, i.e., ~i! bit-flip errors and ~ii! phase errors. The various quantifiers are systematically investigated vs the strength of the interqubit coupling and vs both, the environmental temperature and the ~Ohmic-like! bath-interaction strength. Our main findings are that these four gate quantifiers depend only very weakly on temperature, but are extremely sensitive to the bath-interaction strength. Interestingly enough, however, we find that the XOR gate operation deteriorates only weakly upon decreasing the interqubit coupling strength. This generic case study yields lower bounds on the quality of realistic XOR gate operations.