Decoherence produced by a spin bath

Erik Torrontegui (Instituto de Física Fundamental, CSIC)

Any quantum system is never isolated and represents an open system. As result, the environment that surrounds the system introduces decoherence spoiling its quantum properties. Employing an alternative formalism to study open quantum systems, the Stochastic Surrogate Hamiltonian, we analyze dissipation and dephasing processes produced by a spin bath. For dissipation, we show that the Stochastic Surrogate Hamiltonian enables the simulation of thermalization beyond the weak and Markovian regimes. For dephasing, the relation with temperature is studied, showing that the effect depends on the dephasing mechanism itself. Refs: [1] E. Torrontegui and R. Kosloff, New J. Phys. 18, 093001 (2016). [2] R. Baer and R. Kosloff, J. Chem. Phys. 106, 21 (1997). [3] G. Katz, M. A. Ratner, and R. Kosloff, J. Phys. Chem. C 118, 21798 (2014).

Seminar Room, Serrano 121 (CFMAC)

Classical and semiclassical energy conditions

Prado Martín Moruno (Departamento de Física Teórica I, Universidad Complutense de Madrid)

The standard energy conditions usually associated to general relativity are (mostly) linear in the stress-energy tensor, and have clear physical interpretations in terms of geodesic focussing, but suffer the significant drawback that they are often violated by semi-classical quantum effects. In contrast, it is possible to develop non-standard energy conditions that are intrinsically non-linear in the stress-energy tensor, and which exhibit much better well-controlled behaviour when semi-classical quantum effects are introduced, at the cost of a less direct applicability to geodesic focussing. In this talk I will review the standard energy conditions and their various limitations. Then, I will briefly introduce the averaged energy conditions, and present in detail the nonlinear and the semi-classical energy conditions.

Seminar Room, Serrano 121 (CFMAC)

Edge transport over sub-millimeter distance in the 2D Topological Insulator InAs/GaSb

Enrique Díaz (Departamento de Física Fundamental, Universidad de Salamanca)

The InAs/GaSb double quantum well (DQW), a tunable two dimensional (2D) electron-hole system, was recently proposed as a 2D topological insulator (TI) [1]. While convincing evidence for transport by edge states in micrometer-sized devices has been reported in several experiments [2, 3, 4], the topological origin of these transport properties is still under debate [5]. In the present work we test the transport by edge states in several DQW’s with diferrent sizes InAs/GaSb to cover from normal insulator (NI) and TI samples up to unprecedented length scales and high magnetic fields. First of all, we address the low-temperature electrical transport in a local measurement configuration (Fig.1 (a)), both in zero-field and in the integer quantum Hall regime (Fig.1 (c)), with the latter indicating the realization of a non-trivial system [6]. Successively, making use of non-local transport measurements (Fig.1 (b), (d)), we consistently find evidence for transport by edge states over sub-millimeter distances, as tested using the methodology of Refs.[2, 3] and applying a resistor network model analogous to the one of Ref.[4]. Under high magnetic fields (up to B = 30 T), we detect persistent transport by edge states, with a clear suppression of backscattering in the case of perpendicular orientation. We analyse the response of the edge states to systematic inversions of the current contacts and/or of the polarity of the magnetic field. Many-body interactions can produce exotic novel ground states in these systems. An interacting electron and hole can spontaneously form excitons, i.e. a neutral bound state, provided that the exciton binding energy exceeds the energy separation between the single particle states. References [1] C.X. Liu, T.L Hughes, X.L. Qi, K. Wang and S.C Zhang, Phys. Rev. Lett. 100, 236601 (2008). [2] K. Suzuki, Y. Harada, K. Onomitsu, and K. Muraki, Phys. Rev. B 87, 235311 (2013). [3] K. Suzuki, Y. Harada, K. Onomitsu, and K. Muraki, Phys. Rev. B 91, 245309 (2015). [4] S. Mueller et al:, Phys. Rev. B 92, 081303(R) (2015). [5] F. Nichele et al:, arXiv:1511.01728. [6] B. Büttner et al:, Nature Phys. 7, 418 (2011).

Seminar Room, Serrano 121 (CFMAC)

Quantum Walks Gravity Simulation

Giuseppe di Molfetta (Laboratoire d'Informatique Fondamentale, CNRS/Université Aix-Marseille)

As we know, spacetime is not flat at the cosmological scale. In order to describe spacetime, in General Relativity theory (GR), we need a continuous and differentiable manifold and a formal way to account for the continuous distortion of the metrics. The main point is that changing coordinate systems should not affect physics laws (General Covariance). However at the Planck length, matter is not continuous and obeys Quantum Theory (QT). Although one century has passed, finding an intrinsically discrete counterpart of GR is still an open question. In fact, discretized GR does not turn out in just a mere finite difference scheme of the old formula. I recently showed that one way to describe a discrete curved spacetime is by using Quantum Walks. From a physical perspective a QW describes situations where a quantum particle is taking steps on a discrete grid conditioned on its internal state (say, spin states). The particle dynamically explores a large Hilbert space associated with the positions of the lattice and allows thus to simulate a wide range of transport phenomena. It is surprising that this unitary and local dynamics, defined on a rigid space-time lattice coincides in the continuous limit with the dynamical behavior of a quantum spinning-particle spreading on a curved spacetime. This could really turn out to be a powerful quantum numerical method to discretize GR.

Seminar Room, Serrano 121 (CFMAC)

Quantum optics in low dimensions: from fundamentals to applications

Alejandro González Tudela (Max-Planck-Institut für Quantenoptik)

Recent experimental developments in nanophotonics [1], circuit QED [2] and cold atoms [3] allow to engineer systems where quantum emitters couple to low dimensional photon-like reservoirs with non-trivial energy dispersion. Compared to three-dimensional and structureless baths, the interactions induced by such structured environments can be strongly enhanced and have long-range character. In this talk, I will show several phenomena that can emerge in these scenarios such as the existence of multi-photon bound states around single quantum emitters [4], the generation of tuneable long-range coherent interactions [5], or how one can boost the fidelities and efficiencies of non-classical states of light [6]. [1] Nature 508, 241–244 (2014), Nature Communications 5, 3808 (2014), Rev. Mod. Phys. 87, 347 (2015) [2] Nature Physics 13 (1), 48-52 (2017) [3] Nature Physics 8, 267–276 (2012),Physical Rev Lett. 101 (26), 260404 (2010) [4] Physical Review X 6 (2), 021027 (2016) [5] Nature Photonics 9 (5), 320-325 (2015), PNAS, 201603777 (2016) [6] Physical Review Letters 115 (16), 163603 (2015), New Journal of Physics 18 (4), 043041 (2016) arXiv:1603.01243

Seminar Room, Serrano 121 (CFMAC)

Anomalies (in) matter

Karl Landsteiner (IFT, (UAM-CSIC))

The concept of symmetry is one cornerstone of modern theoretical physics, quantum mechanics is another. Sometimes they are incompatible with each other. These incompatibilities are called anomalies. They constrain possible fermion spectra of gauge theories and explain otherwise forbidden processes such as the decay of the neutral pion into two photons. In the recent years however anomalies play an ever bigger role in a totally different realm of physics: condensed matter. In particular anomalies induce exotic new transport phenomena such as the chiral magnetic and the chiral vortical effects. I will review of chiral anomalies, anomaly induced transport phenomena and discuss some of its applications in a new exciting class of materials: the Weyl semimetals.

Seminar Room, Serrano 121 (CFMAC)

Machine Intelligence and quantum information processing

Peter Wittek (Institute of Photonic Sciences (ICFO))

Statistical learning theory is rich in results that changed the way we process and think about data. Furthermore, the theory offered a fresh perspective on artificial intelligence. Applications are countless and much progress has been made in applying learning schemes in quantum control problems. Quantum information processing and quantum computing are the next frontier for machine learning, but benefits work both ways: quantum-enhanced learning is also a promising research direction, but we must generalize known classical results to the quantum case to fully understand the limits and possibilities. This talk gives an introduction to the core concepts in learning theory, and then looks at the major challenges in the intersection of machine learning, artificial intelligence, and quantum information processing.

Seminar Room, Serrano 121 (CFMAC)