Bayesian estimation for collisional thermometry and time-optimal holonomic quantum computation

TL;DR

This thesis introduces a Bayesian framework for collisional quantum thermometry, enhancing temperature estimation precision, and explores non-adiabatic holonomic quantum gates, balancing speed and decoherence effects.

Contribution

It develops a practical Bayesian approach for collisional thermometry that saturates the Cramér-Rao bound and analyzes the optimal conditions for holonomic quantum gates in three-level systems.

Findings

Bayesian thermometry framework saturates the Cramér-Rao bound in long-time limit.

Analysis of prior information impacts on temperature estimation accuracy.

Identification of optimal regimes balancing RWA validity and decoherence for quantum gates.

Abstract

In this thesis we deal with two different topics. In the first half we investigate how the Bayesian formalism can be introduced into the problem of quantum thermometry -- a field which exploits the high level of control in coherent devices to offer enhanced precision for temperature estimation. In particular, we investigate concrete estimation strategies, with focus on collisional thermometry, a protocol where a series of ancillae are sent sequentially to probe the system's temperature. We put forth a complete framework for analyzing collisional thermometry using Bayesian inference. The approach is easily implementable and experimentally friendly. Moreover, it is guaranteed to always saturate the Cram\'er-Rao bound in the long-time limit. Subtleties concerning the prior information about the system's temperature are also discussed and analyzed in terms of a modified Cram\'er-Rao bound associated with Van Trees and Sch\"utzenberger. Meanwhile, in the last part of the thesis we approach the problem of non-adiabatic holonomic computation. Namely, we investigate the implementation based on $\Lambda$-systems. It is known that a three-level system can be used in a $\Lambda$-type configuration in order to construct a universal set of quantum gates through the use of non-Abelian nonadiabatic geometrical phases. Such construction allows for high-speed operation times which diminish the effects of decoherence. This might be, however, accompanied by a breakdown of the validity of the rotating-wave approximation (RWA) due to the comparable timescale between counter-rotating terms and the pulse length, which greatly affects the dynamics. Here, we investigate the trade-off between dissipative effects and the RWA validity, obtaining the optimal regime for the operation of the holonomic quantum gates.