Steady state entanglement beyond thermal limits

Classical engines turn thermal resources into work, which is maximized for reversible operations. The quantum realm has expanded the range of useful operations beyond energy conversion, and incoherent resources beyond thermal... [ view full abstract ]

Classical engines turn thermal resources into work, which is maximized for reversible operations. The quantum realm has expanded the range of useful operations beyond energy conversion, and incoherent resources beyond thermal reservoirs. This is for example the case of entanglement generation in a driven-dissipative protocol, which we analyze here as a continuous quantum machine.

Entanglement is a valuable resource in many quantum information and quantum computation protocols. In this work, we propose and describe from a theoretical perspective a device which is able to generate a significant amount of steady state entanglement without the need for a coherent driving field, thus proving that entanglement creation could be practically implemented relying only on disordered energy sources (i.e. incoherent pumps). Making use of the theory of open quantum systems and statistical mechanics we provide a description of the structure and behaviour of the proposed set-up in terms of effective temperatures and effective thermal baths, thus revealing explicitly its possible interpretation as a nanoscale thermal machine.

We show that for such machine, the more irreversible the process the larger the concurrence (i.e. bipartite entanglement) in the steady state. Maximal concurrence and entropy production are reached for the hot reservoir being at negative effective temperature, beating the limits set by classic thermal operations on an equivalent system. Irreversibility is quantified by the steady state entropy production rate, a concept derived from the field of stochastic thermodynamics and extended to a quantum case in which effective temperatures are involved.

 A practical implementation is discussed in a quantum optical model of a pair of incoherently driven non-interacting qubits resonantly coupled to a quantized and strongly dissipative cavity mode. Special attention is paid to realistic parameters in view of realizations and experimental tests in solid state and cavity QED systems, such as semiconductor quantum dots in photonic crystal cavities and circuit QED with superconducting resonators.

References:

[1] F. Tacchino, A. Auffèves, M.F. Santos and D. Gerace, Physical Review Letters 120, 063604 (2018).