[qmeso-seminar] два семинара 17 октября

[Quantum Mesoscopics Dept.]

СЕМИНАР СЕКТОРА КВАНТОВОЙ МЕЗОСКОПИКИ

В пятницу 17 октября будет заслушано два доклада:


1) В 15:00

Валентин Мыльников (ФТИ им. А.Ф. Иоффе)

Двухфотонный параметрический осциллятор: бистабильность и время
декогеренции


В докладе будет рассмотрен квантовый осциллятор с двухфотонной
накачкой и нелинейной диссипацией. В данной системе возникают
два метастабильных состояния, между которыми происходит
переключение за экспоненциально большое время. Основное
внимание будет уделено исследованию времени переключения между
метастабильными состояниями. Экспоненциальная зависимость
времени переключения будет оценена из инстантонного подхода в
формализме функционального интеграла Келдыша. Префактор будет
вычислен в рамках P-представления.


2) В 16:00

Дмитрий Львов


Thermometry based on a superconducting qubit

Abstract
In this thesis, we demonstrate operation of a thermometer based on a superconducting transmon qubit. We experimentally show that by measuring qubit’s population distribution, and its respective effective temperature, it is possible to measure temperature of an object the qubit is thermalized with. However, due to the qubit’s quantum nature, it is very sensitive to any other source of excitation, present in the system, which can be treated as a separate noise source or a heat bath. Consideration of a qubit, coupled to several uncorrelated noise sources constitutes an important part of the thesis. We investigate the applicability range of the qubit thermometer in terms of dynamic range, operation speed, accuracy and precision. While the upper bound on the dynamic temperature range is mainly defined by the material properties of the superconductor used in the qubit fabrication, namely, by the superconducting gap, the lower temperature is set by coupling to parasitic excitation sourc
 es. The population measurements of a transmon qubit are based on an algorithm which uses -pulses for swapping the populations of the three lowest energy levels of the transmon. The largest impact on the accuracy and precision of the measurement is defined by the signal-to-noise ratio and quality of the qubit control pulses. The precision limit is ultimately defined by the quantum Cramér-Rao bound, highlighting the statistical origin of this thermometry method.

Finally, we utilize the transmon qubit for thermometry of a mesoscopic heat bath located on the same chip. The heat bath is a normal metal resistor, whose temperature was controlled with normal metal/insulator/superconductor (NIS) junctions. By coupling the transmon to the resistor capacitively, and performing the population measurements, we could observe linear dependence between the qubit effective temperature and temperature of the resistor. Moreover, while the designed mechanism of the qubit-resistor coupling was photonic, we managed to observe another channel of interaction. At large bias voltages applied to the NIS-junction, which sets temperature of the resistor, the junction starts to emit nonequilibrium phonons that can break Cooper-pairs in the superconducting qubit. This is a nonlocal effect, in which nonequilibrium quasiparticles lead both to change of the qubit population distribution, and significant suppression of the relaxation time.

    1. Azat Gubaydullin, George Thomas, Dmitry S. Golubev, Dmitrii Lvov, Joonas T. Peltonen and Jukka P. Pekola. Photonic heat transport in three terminal superconducting circuit. Nature Communications, 13:1552, 10 pages, March 2022.
    2. S. A. Lemziakov, B. Karimi, S. Nakamura, D. S. Lvov, R. Upadhyay, C. D. Satrya, Z.-Y. Chen, D. Subero, Y.-C. Chang, L. B. Wang, J. P. Pekola. Photonic heat transport in three terminal superconducting circuit. Journal of Low Temperature Physics, 217:54-81, 28 pages, May 2024.
    3. Dmitrii S. Lvov, Sergei A. Lemziakov, Elias Ankerhold, Joonas T. Peltonen, and Jukka P. Pekola. Thermometry Based on a Superconducting Qubit. Physical Review Applied, 23(5), 054079, 20 pages, May 2025.



[Трансляция: https://zoom.us/j/96948399564?pwd=RXlKbW96Qm1tZEV6SHB0Q2JYeTdrdz09]

подробнее: http://qmeso.itp.ac.ru/seminars.php?abstract=true