A Caltech Library Service

Non-Equilibrium Quantum Dynamics in a Disordered Ising Magnet


Simon, Christopher (2024) Non-Equilibrium Quantum Dynamics in a Disordered Ising Magnet. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tm3k-yk55.


The quantum two-level system, or “qubit,” is a simple platform that nonetheless displays fundamentally non-trivial quantum behavior. The rare-earth magnet LiHoF₄ is a natural physical representation of a system of coupled qubits. With its uncommonly high crystal anisotropy, LiHoF₄ can be mapped to the problem of the Ising model in a transverse field. However, while this Ising approximation can quantitatively predict much of the equilibrium behavior, quantum corrections, originating from off-diagonal terms in the dipolar interaction that generate quantum fluctuations, are crucial in driving non-equilibrium dynamics when subject to an external drive. Furthermore, quenched disorder can be introduced through chemical substitution, which, through the dipolar interaction, generates spatially random pinning fields, as well as internal transverse fields, which drive quantum fluctuations. Noise measurements on the disordered ferromagnet LiHo0.65Y0.35F4 show critical behavior, whose statistics are driven from the underlying pinning distribution, while measurements on LiHo0.40Y0.60F4 display non-critical behavior that can only be attributed to quantum co-tunneling processes. This is the first demonstration of crackling noise in a ferromagnet in the purely quantum regime. Furthermore, pump-probe susceptibility measurements on the decoupled cluster glass show the system being driven out of equilibrium with astonishingly weak drives, due to resonant transitions arising from off-diagonal dipolar terms σiz σjx. Non-linear sample response is observable in inelastic Raman scattering measurements, and these spin clusters also exhibit asymmetric Fano resonances with high Q-factors of ~10⁵. Quantum interference effects can be tuned to fully decouple one of the dressed states from the others, rendering the sample transparent to the drive. This is analogous to optical systems that display electromagnetically-induced transparency, but at 100 Hz frequencies!

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:non-equilibrium quantum dynamics disorder anisotropy magnetism
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Applied Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Rosenbaum, Thomas F.
Thesis Committee:
  • Endres, Manuel A. (chair)
  • Refael, Gil
  • Minnich, Austin J.
  • Rosenbaum, Thomas F.
Defense Date:1 December 2023
Non-Caltech Author Email:csimontang (AT)
Funding AgencyGrant Number
Office of Basic Energy Sciences (BES)DE-SC0014866
Air Force Office of Scientific Research (AFOSR)FA9550-20-1-0263
Record Number:CaltechTHESIS:12182023-202554036
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for section 2.6 adapted for sections 3.4, 3.5 adapted for sections 1.5, 2.4, 2.5, 2.7
Simon, Christopher0009-0003-6129-2078
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16268
Deposited By: Christopher Simon
Deposited On:02 Jan 2024 20:24
Last Modified:16 Jan 2024 23:47

Thesis Files

[img] PDF - Final Version
See Usage Policy.


Repository Staff Only: item control page