Protocol for temperature sensing using a three-level transmon circuit

TL;DR

This paper introduces a novel in situ thermometry method for superconducting transmon circuits using three energy levels and a sequence of π gates, enabling temperature measurement without single-shot readout.

Contribution

The proposed protocol utilizes the first three levels of a transmon and projective dispersive readout for accurate thermometry without requiring single-shot measurement capabilities.

Findings

Validated in the 50 mK - 200 mK range

Achieved residual populations of 1%-20% for the first excited state

Achieved residual populations of 0.02%-3% for the second excited state

Abstract

We present a method for in situ temperature measurement of superconducting quantum circuits, by using the first three levels of a transmon device to which we apply a sequence of $\pi$ gates. Our approach employs projective dispersive readout and utilizes the basic properties of the density matrix associated with thermal states. This method works with an averaging readout scheme and does not require a single-shot readout setup. We validate this protocol by performing thermometry in the range of 50 mK - 200 mK, corresponding to a range of residual populations $1\%-20 \%$ for the first excited state and $0.02\%-3 \%$ for the second excited state.