Talks

A recent breakthrough in quantum computing has been the realization that quantum computation can proceed solely through single-qubit measurements on an appropriate quantum state. One exciting prospect is that the ground or low-temperature thermal state of an interacting quantum many-body system can serve as such a resource state for quantum computation. The system would simply need to be cooled sufficiently and then subjected to local measurements. It would be unfortunate, however, if the usefulness of a ground or low-temperature thermal state for quantum computation was critically dependent on the details of the system's Hamiltonian; if so, engineering such systems would be difficult or even impossible. A much more powerful result would be the existence of a robust ordered phase which is characterized by the ability to perform measurement-based quantum computation. I’ll discuss some recent results on the existence of such a computational phase of matter. I’ll first outline some positive results on a phase of a toy model that contains the cluster state. Then, in a realistic model of coupled spin-1 particles, I’ll demonstrate the existence of a computational phase. This result is obtained by using a local measurement sequence to “renormalize” the state to a computationally-universal fixed point. Together, these results reveal that the characterization of computational phases of matter has a rich, complex structure – one which is still poorly understood. Joint work with Gavin Brennen, Akimasa Miyake, and Joseph Renes.

Gravitational waves provide a unique way to study the Universe. From 2005 to 2007, the Laser Interferometer Gravitational-wave Observatory (LIGO) took data at design sensitivity. After describing gravitational waves and how LIGO works, I will discuss the status of searches for those waves and current astronomical constraints imposed by those searches. Data taking resumed in summer 2009 with enhanced LIGO detectors and the European Virgo detectors. I will discuss plans for combined electromagnetic and gravitational observing campaigns. Finally, I will highlight the prospects for gravitational-wave astronomy with Advanced LIGO over the next decade.

Underlying the standard cosmological model is the assumption that it is possible to coarse-grain the energy density of the Universe, and that the dynamical and optical properies of space-time should be well modelled by the result. However, even if the average coarse-grained geometry does have the same dynamical properties as the fine-grained system it is intended to imitate, there are good reasons to suspect that the optical properties may be different. To investigate this we consider a simple model of the Universe in which the matter content is in the form of uniformly distributed discrete islands, rather than a continuous fluid. It is found that in the appropriate limits the resulting large-scale dynamics of the model approach those of an FRW universe, while the optical properties do not. We find the angular diameter distance, luminosity distance and redshifts that would be measured by observers inside such a space-time, and use preliminary results to show that the effect on estimates of the cosmological constant can be of the order of 10%.

Ultrarelativistic heavy-ion collisions are one of the most difficult problems for theoretical physicists: they probe non-abelian dynamics deep in the non-perturbative (strong coupling) regime in a many-body system, are highly dynamical (strong gradients), exhibit collective behavior, and involve phase transitions. Fluid dynamics with input from holography is surprisingly good at describing some aspects of experimental data in heavy-ion collisions. I will review some of this successful description, its limitations, and point out open problems that one may be able to understand using gauge/gravity duality.