Quantum Mechanics seminars

Can quantum uncertainty resolve cosmological singularities within the Wheeler–DeWitt description itself? Introducing an observer's proper time turns the problem into evolution resembling scattering in a central potential. The talk describes a smooth quantum bounce for a radiation-filled universe, with large scale-factor uncertainty, and explains why anisotropy introduces a critical attractive potential. That harder regime motivates extending the classical BKL account of singularities into quantum theory.

Representing quantum states through measurement probabilities can preserve subsystem structure or faithfully track nearby states, but not necessarily both. This creates robustness problems for axiomatic reconstructions based on generalized probabilistic theories. The talk introduces an operational alternative that treats a state as an observer's knowledge, aiming to compare possible physical theories while avoiding instability under small changes of the theory.

Operational definitions identify subsystems through the transformations an agent can perform. This talk classifies complementary, bicommutant-closed subsystems using unitary channels, whose disregard for global phase permits possibilities beyond conventional operator subalgebras. The classification involves graded subalgebras. Some resulting subsystems are nontrivial yet support no measurements, raising a foundational choice between excluding them and accepting agents that act without being able to observe.

This talk reexamines black-hole information through the completeness of observables at infinity in gravitational theories. That completeness challenges radial factorization of the bulk state space and assumptions underlying familiar information-loss arguments. Within this account, information is already represented asymptotically. Restricting an observer's available measurements can nevertheless produce Page curves, interpreted as redistribution among accessible observables rather than information newly escaping the hole.

Oren Renard studies security of quantum position verification based on f-routing. The work derives resource lower bounds for uniformly generated attackers by connecting successful attacks to the computational complexity of the routed function. Separate results cover arbitrary quantum channels, Pauli-sparse unitaries and Clifford+T circuits. Time and space hierarchies yield explicit functions secure against polynomial or quasipolynomial numbers of attacker qubits under these computational restrictions, extending earlier bounds while increasing the complexity of honest evaluation.

Euclidean wormholes can make a theory appear nonfactorizing when it has effectively averaged over hidden alternatives. This talk develops a categorical analogue for spatial wormholes. In that formulation, apparent failure of state-space factorization can instead arise from restricting an underlying holographic field theory to neutral states, offering a different interpretation of Einstein–Rosen bridges.