Measuring effective temperatures of qubits using correlations

Initialization of a qubit in a pure state is a prerequisite for quantum computer operation. Qubits are commonly initialized by cooling to their ground states through passive thermalization or by using active reset protocols [1, 2]. To accurately quantify quality of the initialization and to shed light on origin of spurious excitation one requires an accurate tool to measure the excited state population [3]. We propose a new technique of finding the excited state population of a qubit using measurement correlations in time. We experimentally implement the proposed method using a circuit QED platform and compare its performance with previously developed techniques [4]. The method does not rely on having high-fidelity measurement or the higher energy levels. We measure the spurious qubit population with accuracy of up to ≈0.01% and discuss its possible sources.
[1] D. Egger et al., Phys. Rev. Applied 10, 044030 (2018).
[2] P. Magnard et al., Phys. Rev. Lett.121, 060502 (2018).
[3] X. Y. Jin et al., Phys. Rev. Lett.114, 240501 (2015).
[4] A. Kulikov et al., in preparation