Protocol for temperature sensing using a three-level transmon circuit

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 50-200 mK, corresponding to a range of residual populations 1%-20% for the first excited state and 0.02%-3% for the second excited state.