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Unsupervised learning of phase transitions: from principal component analysis to variational autoencoders

Published 7 Mar 2017 in cond-mat.stat-mech, cs.LG, and stat.ML | (1703.02435v2)

Abstract: We employ unsupervised machine learning techniques to learn latent parameters which best describe states of the two-dimensional Ising model and the three-dimensional XY model. These methods range from principal component analysis to artificial neural network based variational autoencoders. The states are sampled using a Monte-Carlo simulation above and below the critical temperature. We find that the predicted latent parameters correspond to the known order parameters. The latent representation of the states of the models in question are clustered, which makes it possible to identify phases without prior knowledge of their existence or the underlying Hamiltonian. Furthermore, we find that the reconstruction loss function can be used as a universal identifier for phase transitions.

Citations (300)

Summary

  • The paper demonstrates that unsupervised learning methods, PCA and VAEs, can extract latent parameters that mirror physical order parameters in phase transitions.
  • The study shows that non-linear VAEs are more effective than linear PCA in capturing the complex continuous symmetries of models like the 3D XY system.
  • The findings suggest that reconstruction loss in VAEs may serve as a universal indicator for phase transitions, paving the way for broader applications in physics.

Unsupervised Learning of Phase Transitions through PCA and Variational Autoencoders

The paper presents an exploration into the intersection of machine learning and statistical physics, particularly focusing on unsupervised learning techniques to identify phase transitions within physical models. The author investigates the use of principal component analysis (PCA) and variational autoencoders (VAEs) to determine latent parameters that effectively describe the states of the two-dimensional Ising and the three-dimensional XY models.

Summary and Analysis of Techniques

The research demonstrates how unsupervised learning can infer the macroscopic properties of physical systems from their microscopic configurations. By applying PCA and VAEs, the study reveals latent parameters closely aligned with known order parameters. For the Ising model, the extracted latent parameters correlate with the magnetization, capturing the order across different temperature regimes. In contrast, the XY model, which involves continuous symmetries, is adeptly handled by the variational autoencoders, underscoring the capability of VAEs in modeling complex phase transitions.

The transition from linear methods such as PCA to nonlinear techniques like VAEs is essential for adequately capturing the underlying features of these models. While PCA provides valuable insights, it is inherently limited by its linear nature and scaling inefficiency with larger datasets. By comparison, VAEs facilitate learning a probabilistic model for data comprising multiple continuous latent variables, thereby offering a more intricate understanding of the phase transitions. The network architecture utilized in VAEs effectively captures latent representations corresponding to order parameters, illustrating their applicability in identifying phases absent explicit prior definitions of order parameters or Hamiltonians.

Implications and Practical Considerations

This research holds several theoretical and practical implications. For physicists, these methods propose a novel way to determine order parameters in complex systems where standard approaches may falter, such as in topological states. The potential to discover previously unidentified phases or comprehend the nature of phase transitions in less understood models is profound. From a machine learning perspective, the role of VAEs not only as encoders but as instruments for physical interpretation of latent parameters could stimulate further advancements in hybrid approaches combining physics and ML.

The study additionally suggests using reconstruction loss in VAEs as a universal indicator for phase transitions. This indicates a promising direction for future research, potentially expanding the efficacy of unsupervised learning models to broader classes of physical systems. Adopting deep convolutional autoencoders may also reduce parameterization efforts while fostering better locality in feature discovery, serving computational efficiency in large-scale simulations.

Conclusion and Prospective Developments

In conclusion, this paper successfully showcases the harmonious application of unsupervised learning in elucidating phase transitions within physics. The integration of machine learning techniques into physical sciences not only provides a pathway for uncovering intricate physical phenomena but also opens doors to innovative computational frameworks in various domains. Future research could leverage the insights from this study to tackle more complex materials and quantum systems, thus further bridging the synergy between computational science and theoretical physics.

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