Hi all,

This afternoon's talk has been postponed to tomorrow morning at 10:30 in HIT K52. David Gross will speak on "The failure of highly entangled states to be useful for measurement-based quantum computation and the failure of infinite-dimensional models to respect Lieb-Robinson bounds."

Abstract: I will give a black board talk on two totally unrelated results, emphasizing (or dropping) the one or the other based on the audience's demands.

In the first part, I show that measurement-based computation on quantum states with a high value of the geometric measure of entanglement can be classically efficiently simulated. It is further shown that generic states are subject to this effect. A slightly over-simplified summary reads "Most quantum states are too entangled to be useful". The argument is mostly elementary and can be completely proved in a black board session without putting too much strain on the audience's attention.

The second topic concerns information propagation in many-body systems. In finite-dimensional spin chains, localized perturbations propagate no faster than a given "speed of sound" characteristic of the system's Hamiltonian. This fact, known by the name of Lieb-Robinson bound, plays a surprisingly fundamental role in many rigorous arguments ascertain classical simulatability of some many-body systems. One could expect similar results to hold even for infinite-dimensional systems in the presence of energy constraints. However, we construct a drastic counter-example to any such conjecture.

References: arXiv:0810.4331, arXiv:0808.3581.

Cheers, Roger