Introduction

Quantum friction is an external potential added to the Hamiltonian that breaks time-reversal invariance so as to cool the system (decrease its total energy). It may be used to cool fermionic many-body systems with thousands of wavefunctions that must remain orthogonal. It is described in details in:

• A. Bulgac, M. M. Forbes, K. J. Roche, G. Wlazłowski, Quantum Friction: Cooling Quantum Systems with Unitary Time Evolution, arXiv:1305.6891

The quantum friction potential is given by:

  V_{\sigma}^{(qf)} = -\alpha \frac{\hbar\,\vec{\nabla}\cdot\vec{j}_\sigma}{\rho_0}

where \rho_0=\frac{k_F^3}{6\pi^2} is reference density. By construction, this potential removes any irrotational currents. Thus it provides a convenient method of removing phonon excitations from the system.