We geometrically characterize one-qubit dissipators of a Lindblad type. An efficient parametrization in terms of 6 linear parameters opens the way to various optimizations with local dissipation. As an example, we study maximal steady-state singlet fraction that can be achieved with an arbitrary local dissipation and two-qubit Hamiltonian. We show that this singlet fraction has a discontinuity as one moves from unital to nonunital dissipators and demonstrate that the largest possible singlet fraction is $\approx 0.654$. This means that for systems with a sufficiently entangled ground state there is a fundamental quantum limit to the lowest attainable energy. With local dissipation one is unable to cool the system below some limiting nonzero temperature.

• Received 16 January 2015

DOI:https://doi.org/10.1103/PhysRevA.91.052107