title: Individual Quantum Probes for Optimal Thermometry.
abstract: The unknown temperature of a sample can be estimated with minimal disturbance by putting it in thermal contact with an individual quantum probe. If the interaction time is sufficiently long so that the probe thermalizes, the temperature can be read-out directly from its steady state. Here we prove that the optimal quantum probe, acting as a thermometer with maximal thermal sensitivity, is an effective two-level atom with a maximally degenerate excited state. When the total interaction time is insufficient to produce full thermalization, we optimize the estimation protocol by breaking it down into sequential stages of probe preparation, thermal contact, and measurement. We observe that frequently interrogated probes initialized in the ground state achieve the best performance. For both fully and partly thermalized thermometers, the sensitivity grows significantly with the number of levels, though optimization over their energy spectrum remains always crucial.
authors.affiliation: Departament de Física, Universitat Autònoma de Barcelona - E08193 Bellaterra, Spain.
authors.forename: Luis A
authors.initials: LA
authors.lastname: Correa
authors.affiliation: Departament de Física, Universitat Autònoma de Barcelona - E08193 Bellaterra, Spain.
authors.forename: Mohammad
authors.initials: M
authors.lastname: Mehboudi
authors.affiliation: School of Mathematical Sciences, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom.
authors.forename: Gerardo
authors.initials: G
authors.lastname: Adesso
authors.affiliation: Departament de Física, Universitat Autònoma de Barcelona - E08193 Bellaterra, Spain.|Institució Catalana de Recerca i Estudis Avançats - E08011 Barcelona, Spain.
authors.forename: Anna
authors.initials: A
authors.lastname: Sanpera