Condensed Matter Physics Seminar: Janet Anders

Abstract:
The interaction of nanoscale and quantum systems with their environment can be relatively strong, and alter the equilibrium state. For open quantum systems, explicit expressions of these so-called mean force (MF) equilibrium states have been missing. I will first report on useful analytic expressions of these states, valid for a general quantum system in contact with a bosonic bath [1]. These are illustrated for the well-known spin-boson model, for which we provide the first classification of coupling regimes, from weak to ultrastrong, and for both the quantum and classical setting [2]. The utility of these theoretical concepts is demonstrated by predicting the equilibrium state’s magnetisation of a magnetic material (e.g. nickel) as a function of temperature. We show that including the environment-induced effects gives a much improved match with experimental data, also in the low temperature regime which has resisted accurate atomistic modelling [3]. 
In the second part of my talk, I will briefly report on our recent results on optimal (global) quantum thermometry [4]. This new framework is most relevant for small data sets, such as those in cold atom experiments where some scans for a single data point can take half a day. This theoretical framework, based on Bayesian principles, has successfully been used to reduce estimation errors in release-recapture experiments [5].
 
[1] Weak and Ultrastrong Coupling Limits of the Quantum Mean Force Gibbs State, J. Cresser, J. Anders, PRL 127, 250601 (2021)
[2] Quantum-classical correspondence in spin-boson equilibrium states at arbitrary coupling, F. Cerisola, et al., NJP 26, 053032 (2024)
[3] Accounting for Quantum Effects in Atomistic Spin Dynamics, M. Berritta, et al., PRB 109, 174441 (2024)
[4] Global Quantum Thermometry, J. Rubio, J. Anders, L. A. Correa, PRL 127, 190402 (2021)
[5] Optimal cold atom thermometry using adaptive Bayesian strategies, J. Glatthard, et al., PRX Quantum 3, 040330 (2022)