Luca Magazzù

Heat measurement of quantum interference

  1. Christoforus Dimas Satrya,
  2. Aleksandr S. Strelnikov,
  3. Luca Magazzù,
  4. Yu-Cheng Chang,
  5. Rishabh Upadhyay,
  6. Joonas T. Peltonen,
  7. Bayan Karimi,
  8. and Jukka P. Pekola
Quantum coherence plays a key role in the operation and performance of quantum heat engines and refrigerators. Despite its importance for the fundamental understanding in quantum thermodynamics
and its technological implications, coherence effects in heat transport have not been observed previously. Here, we measure quantum features in the heat transfer between a qubit and a thermal bath in a system formed of a driven flux qubit galvanically coupled to a λ/4 coplanar-waveguide resonator that is coupled to a heat reservoir. This thermal bath is a normal-metal mesoscopic resistor, whose temperature can be measured and controlled. We detect interference patterns in the heat current due to driving-induced coherence. In particular, resonance peaks in the heat transferred to the bath are found at driving frequencies which are integer fractions of the resonator frequency. A selection rule on the even/odd parity of the peaks holds at the qubit symmetry point. We present a theoretical model based on Floquet theory that captures the experimental results. The studied system provides a platform for studying the role of coherence in quantum thermodynamics. Our work opens the possibility to demonstrate a true quantum thermal machine where heat is measured directly.
arxiv pdf

Thermal rectification in a qubit-resonator system

  1. Luca Magazzù,
  2. Elisabetta Paladino,
  3. Jukka P. Pekola,
  4. and Milena Grifoni
A qubit-oscillator junction connecting as a series two bosonic heat baths at different temperatures can display heat valve and diode effects. In particular, the rectification can change
in magnitude and even in sign, implying an inversion of the preferential direction for the heat current with respect to the temperature bias. We perform a systematic study of these effects in a circuit QED model of qubit-oscillator system and find that the features of current and rectification crucially depend on the qubit-oscillator coupling. While at small coupling, transport occurs via a resonant mechanism between the sub-systems, in the ultrastrong coupling regime the junction is a unique, highly hybridized system and the current becomes largely insensitive to the detuning. Correspondingly, the rectification undergoes a change of sign. In the nonlinear transport regime, the coupling strength determines whether the current scales sub- or super-linearly with the temperature bias and whether the rectification, which increases in magnitude with the bias, is positive or negative. We also find that steady-state coherence largely suppresses the current and enhances rectification. An insight on these behaviors with respect to changes in the system parameters is provided by analytical approximate formulas.
arxiv pdf