On Magnetic Models in Wavefunction Ensembles
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  • Release Date: 2024-09-11
Playlist
  • quantum magnetism
  • wavefunction ensembles
  • large deviations
Chapter
00:11
The Infamous Boundary
00:31
A Schrödingerist Thermodynamics
01:06
Definition of Wavefunction Statistical Ensembles
01:33
What We Proved
01:57
The Non-Linear Term: Wavefunction Energy (WFE)
02:24
Schrödingerist Curie–Weiss Model
02:44
Meaning of Wavefunction Energy
03:21
Testing Wavefunction Energy
Video Introduction

This video is adapted from 10.3390/e25040564

In a wavefunction-only philosophy, thermodynamics must be recast in terms of an ensemble of wavefunctions. In this perspective we study how to construct Gibbs ensembles for magnetic quantum spin models. We show that with free boundary conditions and distinguishable “spins” there are no finite-temperature phase transitions because of high dimensionality of the phase space. Then we focus on the simplest case, namely the mean-field (Curie–Weiss) model, in order to discover whether phase transitions are even possible in this model class. This strategy at least diminishes the dimensionality of the problem. We found that, even assuming exchange symmetry in the wavefunctions, no finite-temperature phase transitions appear when the Hamiltonian is given by the usual energy expression of quantum mechanics (in this case the analytical argument is not totally satisfactory and we relied partly on a computer analysis). However, a variant model with additional “wavefunction energy” does have a phase transition to a magnetized state. (With respect to dynamics, which we do not consider here, wavefunction energy induces a non-linearity which nevertheless preserves norm and energy. This non-linearity becomes significant only at the macroscopic level.) The three results together suggest that magnetization in large wavefunction spin chains appears if and only if we consider indistinguishable particles and block macroscopic dispersion (i.e., macroscopic superpositions) by energy conservation. Our principle technique involves transforming the problem to one in probability theory, then applying results from large deviations, particularly the Gärtner–Ellis Theorem. Finally, we discuss Gibbs vs. Boltzmann/Einstein entropy in the choice of the quantum thermodynamic ensemble, as well as open problems.

 

Follow-up pre-print: https://doi.org/10.48550/arXiv.2403.13699

GitHub repository for the computational part: https://github.com/leodecarlo/Computing-Large-Deviation-Functionals-of-not-identically-distributed-independent-random-variables

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De Carlo, L.; Wick, W.D. On Magnetic Models in Wavefunction Ensembles. Encyclopedia. Available online: https://encyclopedia.pub/video/video_detail/1356 (accessed on 13 October 2024).
De Carlo L, Wick WD. On Magnetic Models in Wavefunction Ensembles. Encyclopedia. Available at: https://encyclopedia.pub/video/video_detail/1356. Accessed October 13, 2024.
De Carlo, Leonardo, William D. Wick. "On Magnetic Models in Wavefunction Ensembles" Encyclopedia, https://encyclopedia.pub/video/video_detail/1356 (accessed October 13, 2024).
De Carlo, L., & Wick, W.D. (2024, September 11). On Magnetic Models in Wavefunction Ensembles. In Encyclopedia. https://encyclopedia.pub/video/video_detail/1356
De Carlo, Leonardo and William D. Wick. "On Magnetic Models in Wavefunction Ensembles." Encyclopedia. Web. 11 September, 2024.
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