The role of coherences in quantum thermodynamics

The role of coherences in quantum thermodynamics

Seminar author:Giovanni Vacanti

Event date and time:07/23/2015 03:30:pm

Event location:IFAE seminar room

Event contact:

In the context of quantum thermodynamics (QT), it is important to determine which states of a given system have genuine quantum features. In this regard, the concept of asymmetry with respect to time translation can be used to distinguish classical states from quantum states. Loosely speaking, while it might be possible to interprete the thermodynamic properties of states that are symmetric with respect to time translation (i.e. states that  are block-diagonal in the eigenbasis of the system’s Hamiltonian) using  the framework of classical statistical mechanics, the thermodynamics of states with non-zero off-diagonal elements (i.e. states with coherences) can only be understood in terms of quantum theory. Here, We consider the problem of keeping an arbitrary  state $\rho_s$ out of thermal equilibrium. Using a partial swapping model for thermalisation process, we find that counter-acting thermalisation using only a resource system which is in a stationary state at the initial time and a system-resource interaction that preserves the global energy is possible if and only if the target state $\rho_s$ is block-diagonal in the eigenbasis of the system’s Hamiltonian $H_s.$ We compute the extra work that must be provided in order to overcome this constraint. This quantity, which is interpreted as the work needed to preserve the coherences in the state, can be expressed in terms of the target state $\rho_s$ and the thermal equilibrium state $\rho_{\beta},$ and it is proportional to the symmetrized relative entropy between $\rho_s$ and $\rho_{\beta}.$