07/21/2025
By Brenden Bowen

The Kennedy College of Science, Department of Physics & Applied Physics, invites you to attend a Ph.D. Dissertation defense by Brenden Bowen entitled "Quantum information probes of renormalization and many-body correlations'"

Degree: Doctoral
Date: Thursday, July 31, 2025
Time: 10 a.m.
Location: Hybrid, Olsen 109; Contact: brenden_bowen@student.uml.edu for Zoom link.

Committee:

  • Chair Nishant Agarwal, Department of Physics & Applied Physics, University of Massachusetts Lowell
  • Archana Kamal, Department of Physics and Astronomy, Northwestern University
  • Hugo Ribeiro, Department of Physics & Applied Physics, University of Massachusetts Lowell

Abstract

For decades, renormalization has been an essential technique in field-theoretic descriptions of natural phenomena, where the absence of a UV-complete description yields an abundance of divergent quantities. While the renormalization prescription has been thoroughly refined for equilibrium systems, consistently extending this prescription to systems that necessitate the inclusion of finite-time effects, such as cosmological or condensed matter systems, is an important area of research. In this thesis, we apply quantum information techniques to study renormalization in out-of-equilibrium systems. We first perform a direct perturbative analysis of the renormalizability of interacting quantum field theories, using quantum mutual information to characterize correlations between infinitesimal shells in momentum space. Initializing the system in the free theory vacuum and quenching to the interacting theory, we find that the late-time mutual information relaxes to that of the interacting vacuum. We then show that, as a function of shell separation, the mutual information decays in super-renormalizable theories, becomes constant in renormalizable (marginal) theories, and grows monotonically for non-renormalizable interactions. Next, we probe the renormalization group (RG) flow of an environment indirectly through the effective dynamics of a quantum sensor. We consider a central spin coupled through a one-to-all dephasing interaction to a transverse-field Ising model (TFIM) and employ a Jordan-Wigner transformation to map the TFIM to a massive Majorana field theory, in which the quantum critical point of the TFIM corresponds to the massless case. Treating the central spin as an open system, we use the (non-Markovian) Redfield equation to obtain the decoherence rate as a function of Majorana mass and temperature. We find that the decoherence rate is time-independent at RG fixed points, signaling Markovian dynamics, and otherwise flows towards its value at one of the stable fixed points.