Measuring two temperatures using a single thermometer

Abstract: We consider the question: Is it possible to measure two temperatures simultaneously using a single thermometer? Under common circumstances, where the thermometer can interact with only one bath at a time and the interaction leads to complete thermalization, this is clearly impossible because the final state of the thermometer would be independent of the temperature of the first bath. In this work, we show that this task can indeed be accomplished with the assistance of quantum control. In particular, we consider a composite particle with multiple quantum degrees of freedom (DoF) as a temperature sensor, where one of the DoF -- termed as internal DoF -- is susceptible to the local temperature, thereby functioning as a thermometer, whereas another DoF -- termed external DoF -- is quantum-controlled. We leverage the entanglement between the aforementioned DoF in a composite particle for two-temperature thermometry by preparing the external DoF in a quantum superposition, exposing the internal DoF to two local temperatures. We show that such a particle used in a Mach-Zehnder type interferometer, or a quantum switch -- which allows quantum control over the order of application of quantum channels -- can be used to estimate two temperatures simultaneously, thus affirming our main proposition. For each of these setups, we obtain the variance in the estimated temperatures through the multi-parameter Cram\'er-Rao bound, and compare their performances based on the range of total variance of the two temperatures estimated. On benchmarking all the setups based on the total variance of the estimated temperatures, we find that a quantum switch with a qudit probe outperforms other setups. On restricting our probe to be a qubit, we find that quantum switch performs equally well as a Mach-Zehnder type interferometer.