Thermometry based on superconductive qubit

Thermometry of superconductive qubit attracts special attention due to prospects of monitoring temperature with minimal invasion, properties of quantum systems with high accuracy at extremely low temperatures [1]. Studies of qubit thermalization process shed light onto interaction between the qubits and their effective environment, contributing to the field of open quantum systems and quantum thermodynamics. We employ a superconducting transmon qubit for quantum thermometry at sub-kelvin temperatures [2]. The thermometry principle is based on the steady-state qubit population measurements, done by applying different sequences of π-pulses [3]. We present a comprehensive experimental study of a qubit thermometer and consider different application cases, such as measurements of the cryostat temperature, or thermometry of a weakly coupled heat bath having a variable temperature. The heat bath is comprised of a mesoscopic resistor with a controllable temperature, realized by NIS-junction thermometry [4]. We provide a thermal model for the qubit coupled to several heat baths representing the cryostat and an effective environment. We discuss various limitations of the qubit thermometer in terms of its dynamic range, sensitivity, accuracy, operation speed etc., and compare our results with previous theoretical and experimental works. We consider influence of the qubit relaxation and coherence times and, respectively, and finite fidelity of the control pulses on the accuracy of the presented method and compare it with a numerical model. **References** [1] M. Mehboudi, A. Sanpera, and L. A. Correa, Thermometry in the quantum regime: recent theoretical progress, Journal of Physics A: Mathematical and Theoretical 52, 303001 (2019). [2] Dmitrii S. Lvov, Sergei A. Lemziakov, Elias Ankerhold, Joonas T. Peltonen, and Jukka P. Pekola. Thermometry based on a superconducting qubit. arXiv:2409.02784, 2024. [3] A. Sultanov, M. Kuzmanovi´c, A. V. Lebedev, and G. S. Paraoanu, Protocol for temperature sensing using a three-level transmon circuit, Applied Physics Letters 119, 144002 (2021). [4] A. V. Feshchenko, L. Casparis, I. M. Khaymovich, D. Maradan, O.-P. Saira, M. Palma, M. Meschke, J. P. Pekola, and D. M. Zumb¨uhl, Tunnel-junction thermometry down to millikelvin temperatures, Phys. Rev. Appl. 4, 034001 (2015).