Condensed Matter Physics seminars

Carolina Paiva develops exact matrix product states for strongly interacting electrons in lattice bands. Conformal-field-theory correlation functions already provide exact representations of fractional quantum Hall trial wavefunctions, including Laughlin states. Extending the construction to fractional Chern insulators is obstructed by the lattice length scale. The talk shows how ideal Chern bands overcome that obstruction and derives an exact representation of Laughlin model states in a hybrid Wannier basis on a torus.

Luca Delacretaz investigates a nonperturbative description of Fermi surfaces, whose many low-energy excitations, collective modes, entanglement and possible non-Fermi-liquid behavior are difficult to handle with conventional field theory. Bosonization describes their dynamics using a collective field in phase space, but quantizing that field has been a longstanding obstacle beyond one dimension. The talk presents an exact description through a particular large-N limit of a level-one U(N) Wess–Zumino–Witten model, with a hierarchy of irrelevant corrections. Matrix degrees of freedom capture noncommutative phase space, and solvable strong-coupling dynamics removes the apparent excess of collective-field modes without dividing the Fermi surface into patches.

Samuel Garratt presents rigorous upper limits on entanglement entropy for locally interacting quantum many-body states at fixed energy. Ground states usually exhibit an area law, unlike generic states whose entanglement scales with volume, and gapless systems can introduce corrections. The framework constrains ground-state entanglement for gapped and gapless systems in arbitrary spatial dimension, follows the transition toward volume-law behavior as energy increases, and bounds the computational resources needed to calculate response functions at zero temperature. These results connect spectral information with the cost of tensor-network calculations.

John Clarke, Michel H. Devoret and John M. Martinis trace the discovery that a macroscopic electrical circuit can display quantum tunnelling and discrete energy levels. Clarke connects early superconducting measuring devices with experiments on current-biased Josephson junctions. Microwave-induced resonances and the crossover from thermal activation to temperature-independent escape provide tests of quantum behaviour. Devoret explains how superconducting circuits became controllable artificial atoms, with Josephson elements enabling engineered energy spectra, qubits and quantum-limited amplifiers. He considers what these designed systems make possible beyond experiments with natural atoms. Martinis follows the development from early junction experiments to superconducting quantum processors. He discusses energy-level quantization, tunnelling, photon generation, quantum computational experiments, and the fabrication and error-control challenges involved in building useful machines.

Neural networks can learn and recognize subtle correlations in high dimensional inputs. However, neural networks are simply many-body systems with strong non-linearities and disordered interactions. Hence, many-body physical systems with similar interactions should be able to show neural network-like behavior. Here we show neural network-like behavior in the nucleation dynamics of promiscuously interacting molecules with multiple stable crystalline phases. Using a combination of theory and experiments, we show how the physics of the system dictates relationships between the difficulty of the pattern recognition task solved, time taken and accuracy. This work shows that high dimensional pattern recognition and learning are not special to software algorithms but can be achieved by the collective dynamics of sufficiently disordered molecular systems.