Steady-state entanglement of two coupled qubits

The maximum entanglement allowed between two coupled qubits in the steady state established by independent incoherent sources of excitation is reported. Asymmetric configurations where one qubit is excited while the other dissipates the excitation are optimal for entanglement, reaching values three times larger than with thermal sources. The reason is the purification of the steady-state mixture (that includes a Bell state) thanks to the saturation of the pumped qubit. Photon antibunching between the cross emission of the qubits is proposed to experimentally evidence such large degrees of entanglement.