Physics

A central question in quantum computation is to identify which problems can be solved faster on a quantum computer. A Holy Grail of the field would be to have a theory of quantum speed-ups that delineates the physical mechanisms sustaining quantum speed-ups and helps in the design of new quantum algorithms. In this talk, we present such a toy theory for the study of a class of quantum algorithms for algebraic problems, including Shor’s celebrated factoring algorithm. Our theory is an extension of Gottesman’s stabilizer formalism based on elements of group and hypergroup theory. Using our methods, we develop classical simulation algorithms for Clifford-like circuits containing quantum Fourier transforms as well as new quantum algorithms for hidden symmetries and hyper-symmertry problems. During the talk, we will discuss the role of resources such as entanglement, interference and contextuality within our formalism and connect quantum speed-ups therein to the presence of precise algebraic structures.

Based on the following works:

[1] https://arxiv.org/abs/1210.3637
[2] https://arxiv.org/abs/1409.3208
[3] https://arxiv.org/abs/1409.4800
[4] https://arxiv.org/abs/1509.05806

Tensor networks offer an efficient representation of many-body wave-functions in an exponentially large Hilbert space by exploiting the area law of ground state quantum entanglement. I will start with a gentle introduction to the tensor network formalism. Then I will describe its application to realizing Wilson's renormalization group directly on quantum lattice models (e.g. quantum spin chains), with emphasis on the RG fixed points corresponding to conformal field theories. We will see how to define both global and local scale transformations on the lattice in such a way that, intriguingly, conformal invariance can be tested (and conformal data extracted) right at the UV cut-off scale.

When small, hard particles are suspended in a fluid, they make it more resistant to flow. The higher the particle concentration, the higher the viscosity. Add enough particles and fluid stops flowing entirely, becoming a jammed solid - this makes intuitive sense.

Less intuitive and more intriguing are suspensions that flow smoothly if pushed gently, but that suddenly solidify if you push too hard. This behaviour is called Discontinuous Shear Thickening (DST). You can try it yourself by mixing cornstarch with water - in the right proportions, the mixture will flow smoothly when stirred gently, but will refuse to flow at all if stirred too hard.

More than an interesting kitchen trick, DST has important real-world consequences. It can cause catastrophic failure of industrial pumping equipment, but can also have life-saving applications to bulletproof vests.

For many years, the mechanism behind DST was unclear, but we have very recently found a new and stunningly simple explanation based on the idea that the contacts between particles become less lubricated and more frictional as the force between them increases. Although this dependence is typically gradual, when a fluid gets close to the “jamming” point, global instabilities can result in the sudden switching from liquid to solid.

Michael Cates (Lucasian Professor of Mathematics, University of Cambridge) will explain this peculiar form of “bulletproof custard” with a few equations, plenty of diagrams, and even some hands-on demonstrations.

Scale invariant transfer matrices and Hamiltonians Abstract We investigate the possibility of strictly scale invariant transfer matrices in quantum spin chains based on a certain planar algebra, both as operators and as quadratic forms.

Recent explorations of the space of quantum field theories have provided novel topological and geometric information about this space. This voyage has resulted in the solution of some long-standing questions:  the computation of sought-after topological invariants (Gromov-Witten invariants) by a new physics-based approach, the first instance of exact correlation functions in a four-dimensional QFT and the unearthing of the action of dualities on the basic observables of three dimensional gauge theories.

Experimentalists are getting better and better at building qubits, but no matter how hard they try, their qubits will never be perfect.  In order to build a large quantum computer, we will almost certainly need to encode the qubits using quantum error-correcting codes and encode the quantum circuits using fault-tolerant protocols.  This will eventually allow reliable quantum computation even when the individual components are imperfect.  I will review the current state of the art of quantum fault tolerance and discuss progress towards answering the most important questions that will enable large fault-tolerant quantum computers.

I discuss, from a quantum information perspective, recent proposals of Maldacena, Ryu, Takayanagi, van Raamsdonk, Swingle, and Susskind that spacetime is an emergent property of the quantum entanglement of an associated boundary quantum system. I review the idea that the informational principle of minimal complexity determines a dual holographic bulk spacetime from a minimal quantum circuit U preparing a given boundary state from a trivial reference state. I describe how this idea may be extended to determine the relationship between the fluctuations of the bulk holographic geometry and the fluctuations of the boundary low-energy subspace. In this way we obtain, for every quantum system, an Einstein-like equation of motion for what might be interpreted as a bulk gravity theory dual to the boundary system. If time permits I will comment on the link to Brownian quantum circuits and tensor networks. 

Should we revisit the concept of space solely based on quantum mechanics?

Do we need a radically new physical principle to address the problem of quantum gravity?

In this talk I will adress these questions. I will review what are the central challenges one faces when trying to understand the theory of quantum gravity from first principles and focus on the main one which is non-locality.

I will present a collection of results and ideas that have been developed in the recent years that provides a radical new perspective on these issues.

One of the central concept I'll present is the idea that locality has to be made relative, and how this idea goes back to one of the founder of quantum mechanics: Max Born. I'll also explain how these new ideas remarkably force us to revisit the concept of space itself and propose a natural generalization that incorporate quantum mechanics in its fabric called modular space. I'll also sketch how these foundational ideas quite unexpectedly links with the most recent developments on the geometry of string theory, and generalized geometry.

Entanglement is fundamental to quantum mechanics. It is central to the EPR paradox and Bell’s inequality. Tensor network states constructed with explicit entanglement structures have provided powerful new insights into many body quantum mechanics. In contrast, the saddle points of conventional Feynman path integrals are not entangled, since they comprise a sequence of classical field configurations. The path integral gives a clear picture of the emergence of classical physics through the constructive interference between such sequences, and a compelling scheme for adding quantum corrections using diagrammatic expansions.  We combine these two powerful and complementary perspectives by constructing Feynman path integrals over sequences of matrix product states, such that the dominant paths support a degree of entanglement. We develop a general formalism for such path integrals and give a couple of simple examples to illustrate their utility [arXiv:1607.01778]. 

Dark energy, the driver of the accelerated expansion of the universe, remains a conundrum. New physics may well be needed to explain it; from Lorentz invariance, this new physics should contain scalar fields, which should be straightforward to detect — so where are they? Physicists have now realized that scalar fields can hide from detection by three distinct screening mechanisms, which respectively rely on nonlinear features in the scalar’s kinetic energy, potential energy, or coupling to normal matter. These fields go by names such as galileons, chameleons, f(R) gravity, or symmetrons and range from theoretically well-motivated and natural to speculative. We will present new techniques to detect these particles, relying on interferometry with cold atoms in vacuum. The low density of such atoms compared to bulk matter avoids triggering some screening mechanisms, while the high sensitivity of the interferometer overcomes other screening mechanisms by brute force. We will show limits ruling out substantial regions of parameter space for chameleons and symmetrons [Science 349, 849 (2015)] and argue that an interferometer with 10,000 fold improved sensitivity will be able to search the gamut of screened scalar fields. A positive identification would revolutionize particle physics and cosmology, but even a null result would have important implications for our understanding of the dark sector.