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Itinerant Magnets & Quantum Information
Correlated spin fluctuations · Quantum Fisher Information · Quantum sensing
At SBQMI, I am investigating itinerant magnets that do not develop conventional long-range magnetic order. These materials are interesting because their spin fluctuations extend broadly in energy and wavevector, carrying quantum correlations that are distributed across many degrees of freedom rather than localized in a simple ordered state. Characterizing those correlations is difficult with standard probes — the inelastic neutron spectra are diffuse and broad, and are often attributed simply to disorder or short-range fluctuations without further analysis.
A central part of this program is developing Quantum Fisher Information (QFI) as a practical tool for extracting entanglement information directly from neutron scattering structure factors. QFI provides a rigorous lower bound on multipartite entanglement and is computable from experimental data without any model assumptions about the ground state. This connects condensed-matter neutron measurements to the language of quantum information science and gives a concrete, experiment-based metric for assessing whether a material's quantum correlations are strong enough to be useful for sensing or computing applications.
The Quantum Fisher Information density FQ is related to the dynamic spin structure factor S(q, ω) by an integral over all frequencies at a given wavevector. This sum-rule relation means that QFI is directly accessible from inelastic neutron scattering data without any model assumptions. When FQ/N exceeds k, it certifies that at least (k+1)-partite entanglement is present in the system — meaning at least k+1 spins are genuinely quantum-mechanically entangled.
In practice, extracting QFI from neutron data requires wide energy-transfer coverage and careful absolute normalization, both of which are achievable on modern time-of-flight spectrometers at SNS. The goal of this part of the program is to establish a reliable experimental protocol for QFI extraction across different classes of quantum magnets, and to use the resulting entanglement benchmarks to identify which materials are worth pursuing for quantum information applications.
Not every itinerant magnet will host strong quantum correlations. The challenge is identifying systems where the interplay between kinetic energy, exchange interactions, and geometric frustration produces a strongly correlated paramagnetic state rather than a conventional ordered phase or a trivial paramagnet. I am growing single crystals of candidate materials at SBQMI and characterizing their bulk magnetic and thermodynamic properties using PPMS and MPMS.
Measurements of susceptibility, specific heat, and resistivity as functions of temperature and field provide a first map of the magnetic phase diagram and flag systems with anomalous correlations that are worth investigating with neutron scattering. This synthesis-driven approach allows the neutron beam time to be focused on the most promising candidates rather than spent on routine survey measurements.
The connection to quantum information science here is direct. QFI is the same quantity that determines the sensitivity of a quantum sensor operating at the Heisenberg limit — the fundamental quantum bound on measurement precision. A material with high multipartite entanglement in its spin fluctuations could in principle be used as a quantum-enhanced sensing medium for weak magnetic fields, operating in a regime where classical sensors are limited by shot noise. Identifying which magnetic materials achieve high QFI at accessible temperatures and fields is therefore a concrete step toward quantum magnetic materials with real technological value.