Quantum Physics seminars

Sisi Zhou examines how quantum information methods can improve the precision of physical measurements. Qubit examples distinguish noise that quantum error correction can suppress from noise indistinguishable from the signal. The discussion extends to several qubits, phase estimation, many-body measurements and distributed sensors. A second part applies quantum-learning tools, including tomography, certification and purification, to estimation of several parameters. Because incompatible observables prevent all parameters from being measured optimally together, designing measurements is difficult. The talk presents practical protocols for several such sensing tasks that combine efficient implementation with sensitivity close to the optimum.

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.

Michel Devoret retraces the experimental discovery of macroscopic quantum tunnelling and quantised energy levels in superconducting electrical circuits. The lecture explains how Josephson circuits became controllable quantum systems and how these experiments opened a route from observing quantum behaviour in a macroscopic device to engineering artificial atoms. He connects this foundational work, recognised by the 2025 Nobel Prize in Physics shared with John Clarke and John Martinis, to the development of superconducting qubits and present approaches to quantum computing. The account links the original physical phenomena to the subsequent effort to build useful quantum processors from electrical circuits.

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.

Alain Aspect, John Clauser and Anton Zeilinger describe how questions about the foundations of quantum mechanics became experimentally testable and technologically productive. The lectures connect Bell’s inequalities with measurements of correlations between entangled photons, explaining how experiments discriminate between quantum predictions and local hidden-variable descriptions. Clauser and Aspect discuss the development of tests and the importance of their experimental assumptions and loopholes. Zeilinger develops the use of entanglement as a resource, including experiments on quantum teleportation and information processing. The programme follows the path from foundational doubts to controlled manipulation of quantum states, bringing together conceptual questions, experimental design and the origins of quantum information science.

Michel Devoret explains why a quantum measurement amplifier must transmit a signal without returning its own noise to the system being measured. He relates directional amplification to the breakdown of reciprocity, introducing Faraday rotation and microwave circulators used between superconducting circuits and measurement amplifiers. The lecture then examines the limitations of magnetic circulators: stray magnetic fields are undesirable near superconducting quantum circuits, and bulky components constrain the number of measurement channels inside a dilution refrigerator. Devoret develops a theoretical route to non-reciprocal microwave components using non-degenerate three-wave mixing in a frequency-conversion regime. The pump phase supplies an active analogue of the magnetic bias in the Faraday effect, allowing directionality to be engineered through phase differences.