Extracting information on the environment by dynamically controlled qubit probes

ANALIA ZWICK

Stuttgart

Seminario; Seminar at Stuttgart University, invited by Dr Joerg Wratchrup; 2015

Controlled quantum spins are sensitive probes of the environment and a powerful tool for characterizing highly complex quantum systems at a molecular or atomic scale. Novel quantum technologies requiring high sensitivity at the nanoscale are based on quantum spin probes serving as magnetometers, thermometers, sensors for imaging or monitoring biological process. We explore the ability of a qubit probe to characterize unknown parameters of its environment. By resorting to quantum estimation theory, we analytically find the ultimate bound on the precision of estimating key parameters of a broad class of ubiquitous environmental noises which the qubit may probe [1]. These include the probe-bath coupling strength, the correlation time of generic types of bath spectra, the power laws governing these spectra, as well as their dephasing times. By optimizing the dynamical control on the probe under realistic constraints one may attain the maximal accuracy bound on the estimation of these parameters by the least number of measurements possible. Applications of this protocol that combines dynamical control and estimation theory tools to quantum sensing are illustrated for a nitrogen-vacancy center in diamond used as a probe. In addition, we reveal a critical behavior, akin to a phase transition, of the information obtainable by a qubit probe concerning the memory time of environmental fluctuations [2]. This criticality emerges only when the probe is subject to suitable dynamical control aimed at inferring the memory time. A sharp transition is then anticipated between two dynamical phases characterized by either a short or long memory time compared to the probing time. This phase-transition of the environmental information is a fundamental feature that characterizes open quantum-system dynamics and facilitates the attainment of the highest estimation precision of the environment memory-time under practical experimental limitations.[1] A. Zwick, G.A. Álvarez, and G. Kurizki. arXiv:1507.03281 (2015).[2] A. Zwick, G.A. Álvarez, and G. Kurizki. arXiv:1509.06516 (2015).