Scientific Series

Quantum superpositions of matter are unusually sensitive to decoherence by tiny momentum transfers, in a way that can be made precise with a new diffusion standard quantum limit.  Upcoming matter interferometers will produce unprecedented spatial superpositions of over a million nucleons. What sorts of dark matter scattering events could be seen in these experiments as anomalous decoherence?  We show that it is extremely weak but medium range interaction between matter and dark matter that would be most visible, such as scattering through a Yukawa potential. We construct toy models for these interactions, discuss existing constraints, and delineate the expected sensitivity of forthcoming experiments.  In particular, the OTIMA interferometer developing at the University of Vienna will directly probe many orders of magnitude of parameter space, and the proposed MAQRO satellite experiment would be vastly more sensitive yet.  This is a multidisciplinary talk that will be accessible to a non-specialized audience.

Magnetars are exceptional neutron stars with the highest magnetic
fields ( 10^15 gauss) in the universe, an unusual quasi steady X
radiation (10^35 ergs/sec) and also produce flares which are some of
the brightest events (10^46 ergs in one fifth of a second) to be
recorded. There is no satisfactory model of magnetars.
The talk will cover neutron stars and a new model for the origin of
the magnetic fields in which magnetars arise from a high baryon
density ( phase transition) magnetized core which forms when they are
born. The core magnetic field is initially shielded by the ambient
high conductivity plasma. With time the shielding currents dissipate
transporting the core field out, first to the crust and then breaking
through the crust to the surface of the star. Recent observations
provide support for this model which accounts for several properties
of magnetars and also enables us to identify new magnetars.

Topological phases of matter are phases of matter which are not characterized
by classical local order parameters of some sort. Instead, it is the global properties
of quantum many-body ground states which distinguish one topological phase from
another. One way to detect such global properties is to put the system on a topologically
non-trivial space (spacetime). For example, topologically ordered phases in (2+1)
dimensions exhibit ground state degeneracy which depends on the topology of the spatial manifold.
In this talk, I will discuss how one can use a {\it unoriented} space (spacetime)
to detect non-trivial properties of topological phases of matter in the presence
of discrete spacetime symmetry, such as time-reversal or reflection symmetry.
In particular, I will show how interaction effects on topological insulators and
superconductors can be understood using quantum anomalies on unoriented spacetime.

The evolution of many kinetic processes in 1+1 dimensions results in 2D directed percolative landscapes. The active phases of those models possess numerous hidden geometric orders characterized by distinct percolative patterns. From Monte-Carlo simulations of the directed percolation (DP) and the contact process (CP) we demonstrate the emergence of those patterns at specific critical points as a result of continuous phase transitions.These geometric transitions belong to the DP universality class and their nonlocal order parameters are the capacities of corresponding backbones. The multitude of conceivable percolative ordering patterns implies the existence of infinite cascades of such transitions in the models considered. We conjecture that such cascades of transitions is a generic feature of percolation as well as many other transitions with non-local order parameters.

The Reeh-Schlieder theorem says, roughly, that, in any reasonable quantum field theory,  for any bounded region of spacetime R, any state can be approximated arbitrarily closely by operating on the vacuum state (or any state of bounded energy) with operators formed by smearing polynomials in the field operators with functions having support in R.  This strikes many as counterintuitive, and Reinhard Werner has glossed the theorem as saying that “By acting on the vacuum with suitable operations in a terrestrial laboratory, an experimenter can create the Taj Mahal on (or even behind) the Moon!” This talk has two parts.  First, I hope to convince listeners that the theorem is not counterintuitive,  and that it follows immediately from facts that are already familiar fare to anyone who has digested the opening chapters of any standard introductory textbook of QFT.  In the second, I will discuss what we can learn from the theorem about how relativistic causality is implemented in quantum field theories.

Quantum field theory on curved space has long been studied for its interesting phenomenology, and more recently also as a means to obtain non-perturbative results in supersymmetric theories. In this talk I will describe the holographic dual for N=4 SYM coupled to massive N=2 flavors on spaces of constant curvature. With that in hand, I will discuss a topology-changing phase transition on S^4 and confront holographic computations with exact field theory results obtained using supersymmetric localization.

Is the graviton a truly massless spin-2 particle, or can the graviton have a small mass? If the mass of the graviton is of order the Hubble scale today, it can potentially help to explain the observed cosmic acceleration. Previous attempts to study massive gravity have been spoiled by the fact that a generic potential for the graviton leads to an instability called the Boulware-Deser ghost. Recently, a special potential has been constructed which avoids this problem while maintaining Lorentz invariance. In this talk I will present recent arguments that suggest that the requirement of avoiding the Boulware-Deser ghost (or other degrees of freedom) is so powerful that the kinetic term for a massive graviton is fixed as well. In fact it must be exactly the same as in General Relativity. This is remarkable as we derive the structure of General Relativity on the basis of stability requirements, not on a symmetry principle. 

The gauge color code is a quantum error-correcting code with local syndrome measurements that, remarkably, admits a universal transversal gate set without the need for resource-intensive magic state distillation. A result of recent interest, proposed by Bombin, shows that the subsystem structure of the gauge color code admits an error-correction protocol that achieves tolerance to noisy measurements without the need for repeated measurements, so called single-shot error correction. Here, we demonstrate the promise of single-shot error correction by designing a two-part decoder and investigate its performance. We simulate fault-tolerant error correction with the gauge color code by repeatedly applying our proposed error-correction protocol to deal with errors that occur continuously to the underlying physical qubits of the code over the duration that quantum information is stored. We estimate a sustainable error rate, i.e. the threshold for the long time limit, of ~0.31% for a phenomenological noise model using a simple decoding algorithm.

We analyse different classical formulations of General Relativity in the Batalin (Frad-
kin) Vilkovisky framework with boundary, as a first step in the program of CMR [1] quantisation. Success and failure in satisfying the axioms will allow us to discriminate among the different descriptions, suggesting that some are more suitable than others in view of perturbative quantisation. Based on a joint work with A. Cattaneo [2, 3] we will present the details of the application of the BV-BFV formalism to the Einstein-Hilbert and Palatini-Holst formulations of General Relativity. We show that the two descriptions are no longer equivalent from this point of view, and we discuss possible interpretations of this result.

[1] A. S. Cattaneo, P. Mnëv, N. Reshetikhin, Classical BV theories on manifolds with boundary, Comm. Math. Phys. 332, 2, 535-603 (2014); A.S. Cattaneo, P. Mnëv, N. Reshetikhin, Perturbative quantum gauge theories on manifolds with boundary, arXiv:1507.01221
[2] A. S. Cattaneo, M. Schiavina, BV-BFV analysis of General Relativity. Part I: Einstein Hilbert action, arXiv:1509.05762 (2015).
[3] A. S. Cattaneo, M. Schiavina, BV-BFV analysis of General Relativity. Part II: Palatini Holst action, in preparation; A. S. Cattaneo, M. Schiavina, On time, in preparation.

We study the representation theory of truncated shifted Yangians. These algebras arise as quantizations of slices to Schubert varieties in the affine Grassmannian. We will describe the combinatorics of their highest weights, which is encoded in Nakajima's monomial crystal. We also prove Hikita's conjecture in this context.

Plasma-filled magnetospheres can extract energy from a spinning black hole and provide the power source for a variety of observed astrophysical phenomena. These magnetospheres are described by the highly nonlinear equations of force-free electrodynamics. Typically these equations can only be solved numerically, but they become amenable to analytic solution in the extremal limit when the black hole achieves maximal angular momentum and an infinite-dimensional conformal symmetry emerges in the high-redshift region near its horizon. This constitutes an example of critical behavior in astronomy. We use the near-horizon scaling symmetry of the extreme Kerr metric to determine universal properties of physics near rapidly spinning black holes.

In this talk I address the problem of simultaneously inferring unknown quantum states and unknown quantum measurements from empirical data. This task goes beyond state tomography because we are not assuming anything about the measurement devices. I am going to talk about the time and sample complexity of the inference of states and measurements, and I am going to talk about the robustness of the minimal Hilbert space dimension. Moreover, I will describe a simple heuristic algorithm (alternating optimization) to fit states and measurements to empirical data. For this algorithm the dataset does not need to be quantum. Hence, the proposed algorithm enables us to interpret general datasets from a quantum perspective. By analyzing movie ratings, we demonstrate the power of quantum models in the context of item recommendation which is a key discipline in machine learning. We observe that quantum models can compete with state-of-the-art algorithms for item recommendation. Based on joint work with Aram Harrow. Relevant preprints: arXiv:1412.7437 and arXiv:1510.02800.