Quantum Probes for Ohmic Environments at Thermal Equilibrium

It is often the case that the environment of a quantum system may be described as a bath of oscillators with an ohmic density of states. In turn, the precise characterization of these classes of environments is a crucial tool to engineer decoherence or to tailor quantum information protocols. Recently, the use of quantum probes in characterizing ohmic environments at zero-temperature has been discussed, showing that a single qubit provides precise estimation of the cutoff frequency. On the other hand, thermal noise often spoil quantum probing schemes, and for this reason we here extend the analysis to a complex system at thermal equilibrium. In particular, we discuss the interplay between thermal fluctuations and time evolution in determining the precision attainable by quantum probes. Our results show that the presence of thermal fluctuations degrades the precision for low values of the cutoff frequency, i.e., values of the order <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>&#969;</mi> <mi>c</mi> </msub> <mo>≲</mo> <mi>T</mi> </mrow> </semantics> </math> </inline-formula> (in natural units). For larger values of <inline-formula> <math display="inline"> <semantics> <msub> <mi>&#969;</mi> <mi>c</mi> </msub> </semantics> </math> </inline-formula>, decoherence is mostly due to the structure of environment, rather than thermal fluctuations, such that quantum probing by a single qubit is still an effective estimation procedure.