Pieter W. Claeys



Shortcuts to Dynamic Polarization

13 November 2020

New work on the arXiv: check out our new paper on Shortcuts to Dynamic Polarization! We combine our recent work on the integrability of the central spin model with our approximate counterdiabatic protocols to speed up dynamical polarization in central spin models. Dynamical polarization protocols polarize a spin bath by repeatedly transfering polarization from the central spin to the surrounding spins. This polarization transfer tends to occur adiabatically, and we here show how this adiabatic transfer can be sped up by introducing additional control fields — which can be realized experimentally through Floquet protocols. We further use the integrable structure of the central spin to fully characterize the performance of all presented protocols, and show how we can reduce the necessary time scales by orders of magnitude. Great work from graduate student Tamiro Villazon in collaboration with Boston University's Anatoli Polkovnikov and Anushya Chandran.

Dynamic polarization protocols aim to hyperpolarize a spin bath by transferring spin polarization from a well-controlled qubit such as a quantum dot or a color defect. Building on techniques from shortcuts to adiabaticity, we design fast and efficient dynamic polarization protocols in central spin models that apply to dipolarly interacting systems. The protocols maximize the transfer of polarization via bright states at a nearby integrable point, exploit the integrability-breaking terms to reduce the statistical weight on dark states that do not transfer polarization, and realize experimentally accessible local counterdiabatic driving through Floquet-engineering. A master equation treatment suggests that the protocol duration scales linearly with the number of bath spins with a pre-factor that can be orders of magnitude smaller than that of unassisted protocols. This work opens new pathways to cool spin baths and extend qubit coherence times for applications in quantum information processing and metrology.

Journal Club for Condensed Matter Physics

01 November 2020

Daniel Arovas (UC San Diego) was kind enough to recommend a recent paper by Anatoli Polkovnikov and me for the Journal Club for Condensed Matter Physics. The journal club presents A Monthly Selection of Interesting Papers by Distinguished Correspondents, and each recommendation is accompanied by a commentary. Read Daniel Arovas's commentary, titled A New Approach to the Classical-Quantum Correspondence, here — including some unpublished results!

I've been following this Journal Club for a while now and am always interested in their selection of papers, so I'm pretty proud of this!

Adiabatic Eigenstate Deformations in Phys. Rev. X

23 October 2020

Our paper on Adiabatic Eigenstate Deformations as a Sensitive Probe for Quantum Chaos has just been published in Physical Review X! Including short blurb on the Physical Review X homepage. Find the article here.

Popular summary: Quantum chaos and ergodicity, which underlies all of statistical mechanics and thermodynamics, manifests itself in a vast range of phenomena. While researchers have developed powerful mathematical tools for understanding the emergence of chaos in quantum systems, the same cannot be said for probes that verify that the system is indeed chaotic. Although standard measures of quantum chaos exist, they are generally not very sensitive. We propose a sensitive probe for quantum chaos that is in line with both quantum and classical definitions of chaos.

To understand the emergence of chaos in quantum systems, researchers rely on the eigenstate thermalization hypothesis. Briefly, the hypothesis says that quantum chaos is encoded in the set of allowed energy states of a quantum system, as opposed to classical chaos which is expressed through exponential sensitivity of particle trajectories to initial conditions. Our proposed probe of quantum chaos blends both definitions by quantifying the sensitivity of allowed energy states to small perturbations. Using an appropriate measure, we can detect quantum chaos orders of magnitude before standard measures and extract information about nontrivial dynamics close to nonchaotic regimes. We use this probe to establish the existence of a regime separating nonchaotic and chaotic systems, characterized by a maximal sensitivity of allowed energy states and anomalously slow (exponentially long) relaxation times.

This work shows that there is a maximally susceptible regime separating chaotic and nonchaotic systems. Our measure can probe this regime, which is completely invisible to standard measures such as level-spacing statistics.