Decoherence and gate performance of coupled solid-state qubits

Solid-state quantum bits are promising candidates for the realization of a scalable quantum computer. However, they are usually strongly limited by decoherence due to the many extra degrees of freedom of a solid-state system. We investigate a system of two solid-state qubits that are coupled via σ_z⁽ⁱ⁾⊗σ_z^(j) type of coupling. This kind of setup is typical for pseudospin solid-state quantum bits such as charge or flux systems. We evaluate decoherence properties and gate quality factors in the presence of a common and two uncorrelated baths coupling to σ_z, respectively. We show that at low temperatures, uncorrelated baths do degrade the gate quality more severely. In particular, we show that in the case of a common bath, optimum gate performance of a controlled-PHASE gate can be reached at very low temperatures, because our type of coupling commutes with the coupling to the decoherence, which makes this type of coupling interesting as compared to previously studied proposals with σ_y⁽ⁱ⁾⊗σ_y^(j) coupling. Although less pronounced, this advantage also applies to the controlled-NOT gate.