Scientific Series

I will discuss fine tuning in modified gravity models that can account for today’s dark energy. I will introduce some models where the underlying cosmological constant may be Planck scale but starts as a redundant coupling which can be eliminated by a field redefinition. The observed vacuum energy arises when the redundancy is explicitly broken. I’ll give a recipe for constructing models that realize this mechanism and satisfy all solar system constraints on gravity, including one based on Gauss-Bonnet gravity which provides a technically natural explanation for dark energy.

In frustrated systems, competing interactions lead to complex phase diagrams and sometimes entirely new states of matter. Frustration often arises from the lattice geometry, leaving the system delicately balanced between a variety of possible orders. A number of normally weak effects can lead to a lifting of this degeneracy. For example, I will discuss how quantum fluctuations can stabilize a supersolid phase, where the system is at once both a crystal and a superfluid. Frustrated magnets are promising candidates for realizing spin liquid phases with exotic \'topological order\', and new kinds of quantum phase transitions that have no classical analog. Bio: Ashvin Vishwanath received his MSc from IIT Kanpur in 1996, and PhD from Princeton in 2001. After holding the Papparlardo fellowship at MIT, he joined UC Berkeley in 2004, where he is currently associate professor of physics. He specializes in theoretical condensed matter physics, especially magnetism, superconductivity and other correlated quantum phenomena in solids and cold atomic gases. Ashvin is the recipient of several awards, most recently a Sloan fellowship (2004), a Hellman foundation fellowship (2006), and an NSF CAREER Award (2007).

A quantum channel models a physical process in which noise is added to a quantum system via interaction with its environment. Protecting quantum systems from such noise can be viewed as an extension of the classical communication problem introduced by Shannon sixty years ago. A fundamental quantity of interest is the quantum capacity of a given channel, which measures the amount of quantum information which can be protected, in the limit of many transmissions over the channel. In this talk, I will show that certain pairs of channels, each with a capacity of zero, can have a strictly positive capacity when used together, implying that the quantum capacity does not completely characterize a channel\'s ability to transmit quantum information. As a corollary, I will show that a commonly used lower bound on the quantum capacity - the coherent information, or hashing bound - is an overly pessimistic benchmark against which to measure the performance of quantum error correction because the gap between this bound and the capacity can be arbitrarily large.

According to the second law of thermodynamics the entropy of a system cannot decrease by adiabatic state transformations. In quantum mechanics, the \'degree of entanglement\' of a state cannot increase under state transformations of a certain kind (local operations assisted by classical communication) In this talk I will explore the significance of the analogy between these two statements.

After using the complex Hilbert space formalism for quantum theory for so long, it is very easy to begin to take for granted features like projection operators and the projection postulate, the algebra of observables, symmetric transition probabilities, linear evolution, etc.... Over the past 50 years there have been many attempts to gain a better understanding of this formalism by reconstructing it from different kinds of (sometimes) physically motivated assumptions. By looking at how the above features are motivated and used in different reconstructions, it becomes clear just how special and restrictive many of them are. The question is then what a theory which does not have some of these features looks like. Another interesting question is whether there are any reasons to be suspicious of postulating them in reconstructions or when trying to generalize or apply the quantum formalism to untested situations.

The QUaD experiment has recently released CMB polarization results at el>200 which are the most sensitive to date. The predicted series of peaks in the EE spectrum are shown to be present for the first time while BB remains undetectable. After briefly reviewing the motivation for polarization measurements I will move on to the experiment, observations, analysis technique and the final results. Finally I will mention on-going efforts to detect gravitational wave B modes.

There are many results showing that the probability of entanglement is high for large dimensions. Recently, Arveson showed that the probability of entanglement is zero when the rank of a bipartite state is no larger than half the dimension of the smaller space. We show that that the probability of entanglement is zero when the rank of a bipartite state is no larger than half the maximum of the rank of its two reduced density matrices. Our approach is quite different from that of Arveson and uses a different measure. But both approaches show that the separable states lie in a lower dimensional manifold given a reasonable parameterization of the separable states. This is joint work with Elisabeth Werner, using on characterizations of the extreme points of qubit channels given by Ruskai, Szarek and Werner and the extreme points of entanglement breaking channels given by M. Horodecki, Shor and Ruskai.

During the last two decades Alain Connes developed Noncommutative Geometry, which allows to unify two of the basic theories of modern physics: General Relativity and the Standard Model of Particle Physics. In the noncommutative framework the Higgs boson, which had previously to be put in by hand, and many of the ad hoc features of the standard model, appear in a natural way. The aim of my talk is to motivate this unification from basic physical principles and to give a flavour of its derivation. I will give an overview of the basic tools such as almost-commutative spectral triples and the spectral action principle. The latter allows to derive the Einstein-Hilbert Lagrangian and the Standard Model Lagrangian together with a set of relations among the Standard Model parameters.

I will report on efforts to implement a new method for simulating concatenated quantum error correction, where many levels of concatenation are simulated together explicitly. That is, the approach involves a Monte Carlo simulation of a noisy circuit involving many thousands of qubits, rather than tens of qubits previously. The new approach allows the threshold and resource usage of concatenated quantum error correction to be determined more accurately than before. Also, the approach makes it possible to better study the effects of circuit optimizations and message-passing algorithms [Poulin, PRA, 2006] on the performance of fault-tolerant concatenated quantum error correction. Such studies are necessary in order make a proper comparison with competing families of error-correction protocols, such as those involving surface codes. In the talk, a range of new numerical results will be presented.

We will discuss the missing pieces in the understanding of the effective field theory description of string creation, the T-dual of the Hanany-Witten effect, both in the open and closed string picture. We explain the origin of the \'bare\' Chern-Simons term, so far added in by hand. There however remain unsettled issues concerning the need to modify the DBI action and the interpretation of this term in M-theory.