John Chalker

Abstract:

These notes accompany lectures given in June 2025 at the summer school Fundamental Problems in Statistical Physics XVI. They offer a short introduction to random quantum circuits as simple models for generic many-body quantum systems. They give an outline of the motivation for introducing these models, starting from ideas of random matrix theory. They also provide a sketch of calculations of some of the quantities of most physical interest, based on an average over an ensemble of systems. These quantities give insights into operator spreading, entanglement dynamics and spectral correlations.

Abstract:

We study the competition between Haar-random unitary dynamics and measurements for unstructured systems of qubits. For projective measurements, we derive various properties of the statistical ensemble of Kraus operators analytically, including the purification time and the distribution of Born probabilities. The latter generalizes the Porter–Thomas distribution for random unitary circuits to the monitored setting and is log-normal at long times. We also consider weak measurements that interpolate between identity quantum channels and projective measurements. In this setting, we derive an exactly solvable Fokker–Planck equation for the joint distribution of singular values of Kraus operators, analogous to the Dorokhov–Mello–Pereyra–Kumar (DMPK) equation modelling disordered quantum wires. We expect that the statistical properties of Kraus operators we have established for these simple systems will serve as a model for the entangling phase of monitored quantum systems more generally.

Abstract:

We review the network model for the integer quantum Hall effect. The model provides a simplified description of Anderson localization in this context. It represents non-interacting electrons moving in two dimensions under the combined influence of a strong magnetic field and a smooth disordered potential. In this setting, electron eigenstates form disorder-broadened Landau levels and their character varies with energy across the Landau level. States in both the low-energy and the high-energy tails of the Landau level are localized, with a spatial extent characterized by the localization length. At the center of the Landau level there is a transition between phases with different quantized values of the Hall conductance and the localization length is divergent. The network model captures universal features of this transition.