Physics MS Thesis Defense by Cahit Kargı

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

PHYSICS

MS THESIS DEFENSE BY CAHİT KARGI

Title: Non-reciprocal Heat Transfer in Quantum Systems

Speaker: Cahit Kargı

Time: August 3rd, 2018, 13:00

Place: ENG 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Prof. Dr. Özgür E. Müstecaplıoğlu (Advisor, Koç University)

Asst. Prof. Dr. Menderes Işkın (Koç University)

Prof. Dr. Mauro Paternostro (Queen’s University Belfast)

Abstract:

Abundance of heat makes it a good candidate as an energy resource, but our ability in manipulating heat is so limited. As a new way of heat control, thermal analogs of non-reciprocal electronic devices, such as a diode, transistor, etc., caught attention in many works. Recently, these works started to propose devices in the quantum domain, and the proposed quantum thermal devices, especially diodes as our focus in this thesis, still have optimization problems. In the cases of two interacting qubits diodes, heat rectification is absent for resonant qubits. In this thesis, we consider a quantum thermal diode composed of two interacting qubits, coupled with an optomechanical-like coupling. We derive the global master equation and calculate the heat current for both flat and Ohmic spectral densities to show the diode behavior. Quality of the diode is quantified by a measure, called rectification factor. We numerically calculate the rectification factor for a wide range of system parameters, including weak and strong coupling regimes. We show that the unit rectification factor is obtained for various parameters both in high and low temperature ranges. Most importantly, almost unit rectification is possible even when the qubits have resonant transition frequencies. We explain the physical mechanism leading to all these results, and we show that the mechanism relies on allowed transition and/or bath couplings being asymmetric. We also demonstrate that the asymmetry in transitions is achieved by an asymmetry in free Hamiltonians of subsystems and/or interaction among them. Demonstrations for the sources of asymmetry are demonstrated with two toy models. One of them is a single qubit, which is the smallest possible diode, and the other is a three-level atom. Even though these two systems show diode behaviors, there is very little control of the rectification direction, and two interacting qubits provide versatile control of the diode.