Talks

As we get closer to build a quantum computer, the main remaining challenge is handling the noise that aflicts quantum systems.

Topological methods, in their various forms, have become the main contestants in the quest for succesfully overcoming noise. A good deal of their strength and versatility is due to their rather unique physical flavour, which keeps giving rise to surprising developments.

I will discuss the main driving forces behind my research on topological quantum error correction, as well as exciting recent directions, including unexpected connections with high energy physics.

In 2015 the LIGO detectors observed gravitational waves from two distinct stellar-mass binary black hole mergers. This long awaited feat now opens avenues to explore astrophysical questions which cannot, or are difficult to, be answered purely by electromagnetic means. Massive stars which end their lives in a pair-instability supernova are not thought to leave a remnant behind, meaning there should exist a gap in the black hole mass spectrum. In this talk I will discuss whether LIGO observations can tell us something about this apparent mass gap. 

LIGO is currently operating in its second science run, and like its first, alerts are sent to electromagnetic partners whenever a candidate event is identified. I will also discuss the efforts being made to capture the signature of an electromagnetic counterpart, some of the facilities involved and the hurdles which need to be overcome to make a confident association between a gravitational-wave signal and an electromagnetic transient.

When utilized appropriately, the path-integral offers an alternative to the ordinary quantum formalism of state-vectors, selfadjoint operators, and external observers -- an alternative that seems closer to the underlying reality and more in tune with quantum gravity. The basic dynamical relationships are then expressed, not by a propagator, but by the quantum measure, a set-function $\mu$ that assigns to every (suitably regular) set $E$ of histories its generalized measure $\mu(E)$. (The idea is that $\mu$ is to quantum mechanics what Wiener-measure is to Brownian motion.) Except in special cases, $\mu(E)$ cannot be interpreted as a probability, as it is neither additive nor bounded above by unity. Nor, in general, can it be interpreted as the expectation value of a projection operator (or POVM). Nevertheless, I will describe how one can ascertain $\mu(E)$ experimentally for any specified $E$, by means of an arrangement which, in a well-defined sense, acts as an $E$-pass filter. This raises the question whether in certain circumstances we can claim to know that the event $E$ actually did occur.

REFERENCE:

Alvaro Mozota Frauca and Rafael Dolnick Sorkin, How to Measure the Quantum Measure, Int J Theor Phys 56: 232-258 (2017), arxiv:1610.02087

We study the eigenstate thermalization hypothesis in chaotic conformal field theories (CFTs) of arbitrary dimensions by computing the reduced density matrices of small size in energy eigenstates. We show that in the infinite volume limit this operator is well-approximated by a “universal” density matrix which is its projection to the primary operators that have nonzero thermal one-point functions. These operators in all two-dimensional CFTs and holographic higher-dimensional CFTs are the polynomials of stress tensor. We compute the time-dependent two-point correlators and Renyi entropies of the universal density martix, and compare the results to the thermal Gibbs state and black holes in various dimensions. We demonstrate that the two-dimensional universal density matrix is close in trace distance to the reduced Gibbs state. We put forward the truncation of density matrix to the stress tensor sector as a “hydrodynamic” method to study the out-of-equilibrium dynamics of strong-coupled field theories

Recently a new solution to the hierarchy problem was proposed which makes use of the cosmological evolution of a light scalar field, a scanner, instead of symmetry or anthropic arguments to select a small Higgs mass. In the original proposal this scanner field could be the QCD axion and thus such class of solution became known as ``relaxion’’. Two central features required of the relaxion are a reason for why a small Higgs mass is special from the scanner’s viewpoint and a mechanism fo the scanner to dissipate its energy in order to stop in a value associated with small Higgs mass. In this talk we propose a novel mechanism that achieves both goals and opens new possibilities for the relaxion. We show that particle production can create an effective friction force for the relaxion and show how it can be used to ``select’’ the TeV scale as a special energy scale from reasonable initial conditions. This allows for the scanning to happen much faster than in previous relaxion models, which usually require very large amounts of inflation, and also allows for the scanning to take place after inflation has ended.

Pulsars are some of physics and astrophysics’ most exotic objects, and they have already earned two Nobel Prizes. We currently know of about 2500 of them in our Galaxy, but a small subset, the millisecond pulsars (MSPs), are truly remarkable. These systems are notoriously hard to detect, yet their numbers have more than doubled in the past 5 years via surveys using the world’s most sensitive
telescopes, new instrumentation, and huge amounts of computing. Specialized “timing” observations of these systems, accounting for each and every one of the billions of rotations of the stars, are providing fantastic results in basic physics. In this talk I’ll focus on the efforts to directly detect gravitational waves from super-massive black hole binaries, make strong-field tests of general relativity, and determine the nature of the densest form of matter known in the universe.