# Quantum circuit refrigerator for superconducting circuits : qubit reset and microwave gain

- Quantenschaltungskühlschrank für supraleitende Schaltungen: Qubit-Reset und Mikrowellenverstärkung

Hsu, Hao; DiVincenzo, David (Thesis advisor); Hassler, Fabian (Thesis advisor)

*Aachen : RWTH Aachen University (2021, 2022)*

Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

Quantum computing has become one of the pillars in the research of quantum technology. According to one of the DiVincenzo’s criteria, quickly and accurately resetting a qubit is necessary to realize quantum computers. Recently, a "quantum circuit refrigerator" (QCR) consisting of a voltage-biased superconductor-insulator-normal-metal-insulator-superconductor (SINIS) tunnel junction has been experimentally and theoretically demonstrated to cool superconducting resonators. In the first part of this thesis work, we extend the theory developed previously to study a QCR coupled to a generic superconducting circuit, where the Fermi’s golden rule is used to calculate the QCR-induced transition rates. We discuss specific examples, including QCR-transmon and QCR-capacitively shunted flux qubit systems and predict 99.99% reset fidelity in a reset time of few to few tens of nanoseconds, depending on the scenario. In addition, the QCR-induced decay rate of the coupled qubit can be turned on and off by changing the bias voltage of the QCR, with an on/off ratio larger than 10^5 for typical experimental parameters. Therefore, the QCR seems to be a promising tool for qubit reset and for detailed studies of open quantum systems. To study in more details the dynamics of the excess charge on the normal-metal island, we derive a master equation for a QCR-two level system. We find that starting with a steady state excess charge distribution on the normal-metal island, thanks to slower charge relaxation rate than the bare qubit decoherence rate in the off state and the QCR-induced qubit decay rate, the distribution always remains in its steady state, thus validating the theory presented above. Based on the master equation approach, we also consider applying an ac voltage tocontrol the QCR, which enables a 10^4 on/off ratio tuned by the drive amplitude. We achieve an order-of-magnitude drop in the qubit excitation within a duration of 40 ns and a residual reset infidelity below 10^−4, which is comparable to the dc-control QCR.Finally, we study a so-called dot QCR system where the normal-metal island of the regular QCR is replaced with a quantum dot, such that the corresponding charging energy becomes the largest energy scale. Also using the master equation approach, we find out that though the dot QCR-induced decay rate is low for qubit reset, there exists a voltage regime which cannot be found in the regular QCR, where the photon-assisted tunnelings serve as a pumping mechanism. We investigate a possible application of the resulting microwave gain by coupling the dot QCR to a resonator and find that the delivered maximum power to be 38 femtowatts. Interestingly, we find that the Fano factor of such a microwave source can be lower than unity, implying a non-classical state of light, which may be useful in quantum metrology.

### Identifier

- DOI: 10.18154/RWTH-2021-11569
- RWTH PUBLICATIONS: RWTH-2021-11569