Qubit systems as quantum probes for the temperature of an Ohmic environment

This thesis work fits into the framework of quantum decoherence and open quantum systems, prominent topics in various aspects of quantum technology. In particular, the thesis addresses the use of quantum probes to precisely characterise the emergent properties of a complex system. In our approach, the quantum probe is an open quantum system interacting with its environment, which is the complex system under investigation; the inherent fragility of the quantum probes against decoherence is the key feature making the overall scheme very sensitive. The specific topic addressed in the thesis is quantum thermometry, which aims to apply quantum effects in order to estimate the temperature of a sample, taking advantage of the high precision that quantum measurements might be able to reach. One of the main methods of quantum thermometry consists in letting a quantum system, e.g. a qubit in a particular state, interact with the sample of interest (remarkably without energy exchange!) and then being able from the resulting quantum correlation to deduce the temperature of the above-mentioned sample performing a measure on the qubit system only. It will be addressed the problem of finding the optimal procedure to infer the temperature of a thermal bath of boson harmonic oscillators in Ohmic regime (ranging from sub-Ohmic to super-Ohmic regime), letting set of two qubits in different initial states interact with the environment. To fulfil this purpose quantum probing and the methods dictated by quantum estimation theory will be exploited. In the light of the above, it is evident the reason why a decoherence study is needed: almost all the information we can gather depends on how the quantum probe behaves after the interaction with the thermal bath.