Registration Room: AlbaNova main building foyer

Opening address

• Axel Brandenburg (Nordita)

Room: Svedbergsalen (FD5)

Quantum computing and the limits of silicon miniaturisation

• Michelle Simmons (University of New South Wales, Australia)

Room: Svedbergsalen (FD5) Abstract: Down-scaling has been the leading paradigm of the semiconductor industry since the invention of the first transistor in 1947. However miniaturization will soon reach the ultimate limit, set by the discreteness of matter, leading to intensified research in alternative approaches for creating logic devices. One of the most exciting of these is quantum computation. We will present devices that address the ultimate limit of device miniaturization in silicon where we have patterned dopants in a crystalline environment with atomic precision to act as one-dimensional leads, single- electron transistors and control gates. In particular we demonstrate precision-single-atom transistors, spin-read- out in a scalable silicon quantum computing architecture and a direct measurement of exchange coupling in donor based systems. We will discuss the benefits of donors as qubits and address some of the challenges to achieving truly atomically precise devices in all three spatial dimensions.

The power of measurement in quantum control

• Daniel Burgarth (Aberystwyth University, UK)

Room: Svedbergsalen (FD5) Abstract: We show that adding something as simple as a single qubit measurement to a quantum system can drastically boost the complexity of its dynamics. In particular, we give an example where the ability to perform such a measurement can decide between quantum computation and a classically simulable system. We discuss the importance this result has on foundational aspects as well as practical matters.

Coffee break Room: Svedbergsalen (FD5)

Dipole-dipole bound Rydberg molecules

• Dieter Jaksch (University of Oxford, UK)

Room: Svedbergsalen (FD5) Abstract: In the first part of my talk I will discuss the physics of two and three ultracold Rydberg atoms interacting via the dipole-dipole interaction. These systems can form micrometer sized dimer molecules whose relative dynamics is governed by artificial gauge fields. In particular I will show that these fields exhibit magnetic monopoles and give rise to synthetic spin-orbit coupling. Furthermore, I will discuss three atom bound states that do not have a two atom equivalent.  The binding mechanism leading to these states is substantially different from Efimov physics. I will also show how these molecular states can be engineered in the laboratory and how the exaggerated properties of Rydberg atoms make their features directly observable using current experimental technology. In the second part of my talk I will discuss the prospect of forming strongly correlated electron gases starting from ultracold Rydberg atoms in optical lattices. I will describe our progress in electronic structure calculations for Rydberg atoms with electrons that are delocalized over the optical lattice. I will explain how this system might form a Rydberg crystal with strongly correlated electrons, a spatial periodicity of several hundred nanometers, and coherent dynamics on experimentally resolvable picosecond time scales. I will present the exciting properties that such an electronic system might possess and discuss some of the major challenges in realizing them.

Hybrid sensors based on color center spin in diamond and piezo-active layers

• Jianming Cai (Universität Ulm, Germany)

Room: Svedbergsalen (FD5) Abstract: The ability to measure weak signals such as pressure, force, electric field and temperature with nanoscale devices and high spatial resolution offers a wide range of applications in fundamental and applied sciences. Here we present a proposal for a hybrid device composed of thin film layers of diamond with colour centres and piezoactive elements for the transduction and measurement of physical signals. The magnetic response of a piezomagnetic layer to an external stress or a stress induced by a signal is shown to affect significantly the spin properties of nitrogen-vacancy centres in diamond. Under ambient conditions, realistic environmental noise and material imperfections, we show that this hybrid device can achieve significant improvements in sensitivity over the pure diamond-based approach in combination with nanometre-scale spatial resolution. Furthermore the proposed hybrid architecture offers novel possibilities for engineering strong coherent couplings between nanomechanical oscillator and solid state spin qubits.

Quantum valley Hall effect and other byproducts of the electromagnetic interaction
 among the electrons in graphene

• Eduardo Marino (Universidade Federal do Rio de Janeiro, Brazil)

Room: Svedbergsalen (FD5) Abstract: We use Pseudo Quantum Electrodynamics (PQED), a strictly 2D theory, in order to describe the full electromagnetic interaction of the p-electrons of graphene in a consistent formulation. By including the effects of the interaction on the vacuum polarization tensor and on the electron self-energy, we achieve the following physical results: 1) QVHE - We predict the onset of a spontaneous (interaction-driven) Quantum Valley Hall effect (QVHE) below a critical temperature of the order of $0.05$ K. The transverse (Hall) valley conductivity is evaluated exactly and shown to coincide with the one in the usual Quantum Hall effect. 2) DC-conductivity - By considering the corrections induced by PQED in the vacuum polarization tensor or, equivalently, in the current correlator, up to two-loops, we are able to obtain  a smooth zero-frequency limit in Kubo's formula. Thereby, we obtain in zeroth order, the usual expression for the minimal DC-conductivity plus higher- order corrections due to the interaction. These make our result, to the best of our knowledge, the closest to the experimental value. 3) Gap and Midgap States - We study the effects of the interaction on the electron self-energy and show that this produces a shift in the electron propagator poles. The energy spectrum is such that a set of P- and T- symmetric gapped electron energy eigenstates are dynamically generated, with an infinite number of midgap states. This discrete set of states are related to the QVHE in similar way the Landau levels are related to the ordinary Quantum Hall Effect.

Lunch Room: 3rd floor, AlbaNova main building

Unexpected pairing of electrons in the IQHE regime

• Moty Heiblum (Weizmann Institute, Israel)

Room: Svedbergsalen (FD5) Abstract: Electron pairing is a rare phenomenon appearing only in a few unique physical systems, such as superconductors or Kondo-correlated quantum-dots. Here, we report on an unexpected “pairing” of electrons in the integer-quantum Hall effect (IQHE) regime. The pairing takes place in the interfering most outer edge channel within an electronic Fabry-Perot interferometer at a bulk filling factor ν_B > 2. We note three clear observations: (a) Aharonov-Bohm oscillations with magnetic-flux periodicity φ_0* = h/2e, with e the electron charge and h Planck’s constant; (b) An interference charge e* = 2e – revealed by shot noise measurements; and (c) Entanglement between the two most outer edge channels. While the exact mechanism of the pairing is not understood, we show that this unique phenomenon results from inter-edge channels interactions.

Holographic quantum Hall ferromagnetism

• Gordon Semenoff (University of British Columbia, Canada)

Room: Svedbergsalen (FD5) Abstract: A mechanism by which a probe D brane that is embedded in AdS geometry, carries electric charge and is subject to a magnetic field can have incompressible integer quantum Hall states will be discussed.  In all cases, these states are associated with dynamical symmetry breaking, similar to ``quantum Hall ferromagnetism'' which is observed in the integer quantum Hall regime in graphene and can be viewed as the realization of that phenomenon in a strongly coupled gauge field theory.

Coffee break Room: Svedbergsalen (FD5)

Topological states in higher orbitals

• Cristiane Morais Smith (Utrecht University, The Netherlands)

Room: Svedbergsalen (FD5) Abstract: I will discuss some properties of quantum Hall fluids, more specifically the so-called topological insulators, which exhibit a dissipation-less quantized spin-current. The current is generated by a coupling between the spin and the momentum of the electrons (spin-orbit interaction) and is protected by a topological invariant, i.e., it depends only on the topology of the material and not on its microscopic details. This quantized Hall  spin-current is analogous to the quantized charge current that occurs in semiconductors, in the quantum Hall regime [1].   The recent realization of "synthetic graphene" by the self-assembling of semiconducting nano-crystals into a honeycomb lattice has opened new  perspectives into the realization of topological materials in condensed matter [2]. By choosing the chemical elements in the nanocrystal, the spin-orbit coupling can be tuned to a great extent, thus allowing us to engineer new materials that could be useful for technological applications.   Topological states of matter are being studied not only in condensed matter, but also in quantum optics. By loading ultracold fermions or bosons  into optical lattices, it is possible to simulate cond-mat systems, thus custom tailoring model Hamiltonians which are supposed to describe complex  quantum systems. The recent experimental realization of a $p_x + i p_y$ Bose- Einstein condensate of Rb in a 2D optical lattice, for which  time-reversal symmetry is spontaneously broken, is a fascinating example of the numerous possibilities to be explored with those systems [3].       [1] N. Goldman, W. Beugeling, and C. Morais Smith, EPL 97, 23003 (2012).  [2] E. Kalesaki, C. Delerue, C. Morais Smith, W. Beugeling, G. Allen, and D. Vanmaekelbergh,  PRX 4, 011010 (2014).  [3] M. Ölschläger, T. Kock, G. Wirth, A. Ewerbeck, C. Morais Smith and A Hemmerich, New Journal Phys.15, 083041 (2013).

Stability of two-dimensional topological insulators

• Andrea Cappelli (INFN, Italy)

Room: Svedbergsalen (FD5) Abstract: Topological insulators possess non-chiral edge excitations that can interact and decay. The criteria of stability in presence of time-reversal symmetry are reviewed and put in relation with the existence of a discrete Z_2 anomaly. A general analysis is presented that include models with non- Abelian edge excitations.

Departure of buses to Stockholm City Hall Room: AlbaNova main entrance

Reception hosted by the President of the City Council / guided tour of Stockholm City Hall Room: Stockholm City Hall

Atom chips: quantum gases on the (sub)micron scale

• Peter Krüger (University of Nottingham, UK)

Room: Svedbergsalen (FD5) Abstract: Microtraps for cold atoms based on patterned surface-mounted structures (atom chips) form the basis of quantum devices utilizing ultracold atomic gases for many- body quantum state engineering and measurement. In this talk we will showcase some experiments that illustrate the capability of such devices for applications ranging from portable sensors to be used outside the laboratory to studies of dynamics in a one-dimensional gas of low-temperature bosons. New types of experiments become possible through structuring and manipulating quantum gases on (sub)micron scales, which requires the distance between trap and chip surface to be reduced to similar scales. We will present some of these new opportunities arising from the ability to shape the environment of the gas with a tailoring resolution on the order of and beyond its characteristic length scales (healing length). Conversely, a quantum gas trapped at a micron or less distance from a surface is a highly sensitive probe for microscopic magnetic fields that features high spatial resolution at the same time. We will discuss the obstacles on the way to further miniaturization of atom chips and how they can be overcome using novel materials and geometries.

Dynamical analogue quantum simulators

• Jens Eisert (FU Berlin, Germany)

Room: Svedbergsalen (FD5) Abstract: Complex quantum systems out of equilibrium are at the basis of a number of long-standing questions in physics. This talk will be concerned on the one hand with recent progress on understanding how quantum many-body systems out of equilibrium eventually come to rest, thermalise and cross phase transitions, on the other hand with dynamical analogue quantum simulations using cold atoms [1-4]. In an outlook, we will discuss the question of certification of quantum simulators, and will how this problem also arises in other related settings, such as in Boson samplers [5,6]. [1] S. Braun, M. Friesdorf, S. S. Hodgman, M. Schreiber, J. P. Ronzheimer, A. Riera, M. del Rey, I. Bloch, J. Eisert, U. Schneider, arXiv:1403.7199. [2] M. Kliesch, M. Kastoryano, C. Gogolin, A. Riera, J. Eisert, arXiv:1309:0816. [3] S. Trotzky, Y.-A. Chen, A. Flesch, I. P. McCulloch, U. Schollwoeck, J. Eisert, I. Bloch, Nature Physics 8, 325 (2012). [4] A. Riera, C. Gogolin, M. Kliesch, J. Eisert, in preparation (2014). [5] C. Gogolin, M. Kliesch, L. Aolita, J. Eisert, in preparation (2014) and arXiv:1306.3995. [6] S. Aaronson, A. Arkhipov, arXiv:1309.7460.

Coffee break Room: Svedbergsalen (FD5)

Quantum simulations & devices with ultracold atoms

• Andrea Trombettoni (CNR-IOM DEMOCRITOS, Italy)

Room: Svedbergsalen (FD5) Abstract: In this talk I review the field of quantum simulations with ultracold atoms, and then discuss their use for realizing quantum devices, focusing in particular on the engineering of ultracold Josephson devices and Sagnac interferometers.

Global phase space study of coherence and entanglement in a double-well Bose-Einstein condensate

• David Campbell (Boston University, USA)

Room: Svedbergsalen (FD5) Abstract: Recently [1] we have shown that a "global phase space" (GPS) approach provides valuable understanding of the long-time coherence and Einstein-Podolsky-Rosen entanglement of a Bose-Einstein Condensate (BEC) trapped in a double-well optical lattice ("BEC dimer"). In particular, the GPS approach allows one to distinguish purely quantum effects from those which are captured by semi-classical methods. The GPS approach in Ref. (1) was applied in the limit of zero dissipation. After reviewing the key results in this limit, we extend the approach to allow for dissipation and again compare the results with relevant experiments. Surprisingly, although consistent with some prior exploratory studies, we find that dissipation can actually enhance coherence in certain instances, particularly around self-trapped modes, corresponding to fixed points in the classical phase space. We explain a number of interesting features of this enhancement and argue that, in spatially extended systems (corresponding to multi-well optical lattices), these localized, self-trapped modes may also play a role in enhancing coherence. [1] Holger Hennig, Dirk Witthaut, and David K. Campbell, Phys. Rev. A 86, 051640 [R] 2012.

Engineering phases of matter with light

• Jacob Taylor (Joint Quantum Institute/NIST, USA)

Room: Svedbergsalen (FD5) Abstract: By controlling the propagation of photons and generating strong interactions between them, novel quantum states of matter can be created and manipulated, enabling new approaches for quantum-limited measurement and computation.

Lunch Room: 3rd floor, AlbaNova main building

sl(2,R)- connections on circle bundles over space-time

• Michael Freedman (Microsoft Station Q, USA)

Room: Svedbergsalen (FD5) Abstract:   For the usual U(1) - principle bundle of electromagnetism with a U(1) - connection, A, magnetic charge can be calculated by integrating a local quantity (magnetic flux). This is no longer true if we allow the connection to boost the fibers (i.e. where SL(2,R) acts by linear fractional transformations on the fibers.) This strange phenomena of magnetic charge without magnet flux allows the construction of monopoles with exotic topologies. In condensed matter one can similarly imagine a circle valued order parameter (say of a superfluid) but with an sl(2,R)- connection created artificially in the cold-atom settings. In this case it is possible to produce a 2npi rotational flux from a connection that is a pure boost. The quantization condition on the boost is now expressed via hyperbolic geometry. Finally, a superconductor with sl(2,R)-connection on a circular order parameter is considered within Ginzburg- Landau theory. Each of these three examples calls for a different explanation of how the sl(2,R) behavior could arise.  This is joint work with Roman Lutchyn.

Meissner effect, artificial gauge fields, and topological Mott insulators

• Karyn Le Hur (Ecole Polytechnique, France)

Room: Svedbergsalen (FD5) Abstract: In this talk, we explore non-trivial phases of matter with topological properties as well as strong interactions. First, we introduce simple models exemplifying that (spin) Meissner currents can persist in insulating phases of matter such as Mott insulators. The topological aspect in this system emerges through the flux quantization phenomenon as in a superconductor, despite the presence of a Mott gap. Then, we discuss a possibility to engineer a two-dimensional topological Mott insulator, i.e., a topological band insulator driven by interaction effects (only). Finally, we address a bosonic analogue of the Haldane model on the honeycomb lattice and study the interplay bewteen Josephson physics, artificial gauge fields and Mott physics. The models presented here can be realized in cold atom experiments, in circuit Quantum Electrodynamics and Josephson junction systems.

Coffee break Room: Svedbergsalen (FD5)

Many-body cavity QED

• Jonas Larson (Stockholm University, Sweden)

Room: Svedbergsalen (FD5) Abstract: I discuss some new directions in the field of ultracold gases confined in optical resonators. For example, I demonstrate how some anomalous evolution in these systems can be understood in terms of synthetic non-Abelian gauge fields. Furthermore, I also analyze extended Dicke models, like an SU(3) version of the regular Dicke model, and characterize their phase diagrams.

Integrability vs exact solvability in the quantum Rabi model

• Murray Batchelor (Chongqing University, China)

Room: Svedbergsalen (FD5) The quantum Rabi model, which describes the simplest interaction between light and matter, is a fundamental model with widespread applications in quantum physics. These include the interaction between light and trapped ions or quantum dots, and between microwaves and superconducting qubits. The model is also applicable to both cavity and circuit QED. Despite it’s simplicity, the eigenspectrum of the fully quantized version of the Rabi model has only recently been obtained exactly. The quantum Rabi model has been claimed to be integrable based on a criterion of quantum integrability without presupposing the existence of a set of commuting oper- ators. This talk will discuss the Yang-Baxter integrability of the quantum Rabi model. In particular, we contend that the model is in general not Yang-Baxter integrable, but rather is only integrable in the Yang-Baxter sense at two special parameter values. These considerations are extended to various physical generalizations of the Rabi model. This talk is based on joint work with Huan-Qiang Zhou.

Short break Room: Svedbergsalen (FD5)

Entanglement negativity in quantum field theories

• Pasquale Calabrese (Universita' di Pisa, Italy)

Room: Svedbergsalen (FD5) Abstract: The study of the entanglement content of many body quantum systems has prompted an intense research activity at the crossroad of different disciplines such as statistical mechanics, quantum information, condensed matter, and quantum field theory.  In this talk I will present systematic methods to calculate the entanglement negativity in the ground state of 1+1 dimensional quantum field theories, with particular emphasis on conformal invariant ones.

Entanglement crossovers in quantum impurity problems

• Hubert Saleur (CEA Saclay, France)

Room: Svedbergsalen (FD5) Abstract: We will discuss entanglement in the presence of an (impurity) RG flow such as the flow between weak and strong coupling fixed  points in the Kondo problem. We will explain how this entanglement is intrinsically non perturbative, and discuss ways to obtain it non perturbatively in some examples using form factors and or the Bethe ansatz.

Electron interactions in one dimension

• Michael Pepper (University College London, UK)

Room: Oskar Klein-auditoriet (FR4) Abstract: It will be shown that as the 1D confinement potential is weakened the electron wavefunctions relax in the second dimension and, in order to minimise the electron-electron repulsion, an array is formed in which a two row configuration is the ground state. This behaviour, which is the prelude to formation of a Wigner Lattice, will be discussed along with the spin incoherent regime. This arises in the low carrier concentration limit when the exchange energy between neighbouring electrons becomes small and the spin direction can no longer be defined. The role of a magnetic field on the formation of a two row ground state will be discussed and results presented showing how it can provide information on the magnitude of the interaction. When the confinement is sufficiently weak filling the levels with electrons can alter the normal sequence of the levels so that the, (one-electron), first excited state now becomes lower in energy than the normal ground state. Results will be presented on these effects and how the electrostatically defined order of the levels is altered by the electron-electron interaction.

Thermoelectric transport at a junction of multiple quantum wires

• Masaki Oshikawa (University of Tokyo, Japan)

Room: Oskar Klein-auditoriet (FR4) Transport in one-dimensional electron systems has been a fertile field of study. In particular, electron-electron interactions can be taken into account in the framework of Tomonaga-Luttinger liquid (TLL) theory. It leads to many interesting predictions, including a switching of the direction of the current by a magnetic field at a junction of 3 TLLs [1]. Recently, for non-interacting electrons, it was found that the thermoelectric efficiency can exceed Curzon-Ahlborn limit, if a magnetic field is applied to a junction of 3 one-dimensional channels [2]. Motivated by this finding, we will discuss thermal transport at junctions of 3 TLLs and report some preliminary results. References: [1] C. Chamon, M. O., I. Affleck, Phys. Rev. Lett. 91, 206403 (2003); JSTAT 2006, P02008. [2] K. Brandner, K. Saito, U. Seifert, Phys. Rev. Lett. 110, 070603 (2013).

Coffee break Room: Oskar Klein-auditoriet (FR4)

Conductance of flat bands with long-range Coulomb interactions

• Wolfgang Häusler (University of Augsburg, Germany)

Room: Oskar Klein-auditoriet (FR4) Abstract: Dispersionless (“flat”) electronic bands can arise throughout the Brillouin zone in certain multipartite lattices, besides ordinary dispersing bands. In such a flat band, hoppings between atomic orbitals interfere destructively which then leads to localization, a phenomenon denoted as “caging” of carriers. As a consequence, the system is insulating at zero temperature even when this band is partly filled, provided all other bands are either empty or completely filled. One may ask whether long range Coulomb interactions can alter this situation and cause finite conductivity. In the absence of kinetic energy, flat band carriers tend to Wigner crystallize. Here, this general observation is analyzed for the two-dimensional case specifically for the Sutherland or T3 –lattice where a conductivity is found, depending non-trivially on the carrier density at small flat band fillings.

Localization and interactions in topological bands

• Emil Bergholtz (FU Berlin, Germany)

Room: Oskar Klein-auditoriet (FR4) Abstract: I will discuss two related topics, namely the localization properties of Wannier functions in topological bands structures, and effects of interactions projected to such bands. In particular, I will discuss a novel approach towards obtaining maximally localized Wannier states based on compressed sensing, and new types of fractional topological insulators emerging due to interactions in partially filled bands with variable Chern number.

On the interacting Majorana chain

• Dirk Schuricht (Utrecht University, The Netherlands)

Room: Oskar Klein-auditoriet (FR4) Abstract: We study the effect of interactions on Kitaev's toy model for Majorana wires. We demonstrate that even though strong repulsive interaction eventually drive the system into a Mott insulating state the competition between the (trivial) band-insulator and the (trivial) Mott insulator leads to an interjacent topological insulating state for arbitrary strong interactions. We show that the exact ground states can be obtained analytically even in the presence of interactions when the chemical potential is tuned to a particular function of the other parameters. The ground states obtained are two-fold degenerate and differ in fermion parity, as is the case with the Kitaev/Majorana chain in a topological phase. We prove that the ground state is unique in each fermion parity sector and that there exists an energy gap. We propose a realisation in an array of superconducting islands with semiconducting nanowires, where a capacitive coupling between adjacent islands leads to an effective interaction between the Majorana modes.

Lunch Room: 3rd floor, AlbaNova Building

Afternoon free (guided tour of the Vasa museum 15:30,...) Room: .

Conference dinner / Stockholm archipelago boat tour Room: Kajplats 8

Transport in superconductor-semiconductor hybrid nanowires

• Charles Marcus (University of Copenhagen, Denmark)

Room: Oskar Klein-auditoriet (FR4) Abstract: This talk will review experiments in superconductor- semiconductor hybrid devices, including discussion of majorana end states, even-odd filling of N-S-N quantum dots, and a quantum phase transition in the so-called destructive phase of a cylindrical structures. Research supported by Danish National Research Foundation and by Microsoft.

Quantum critical point in topological SN junctions

• Ian Affleck (University of British Columbia, Canada)

Room: Oskar Klein-auditoriet (FR4) Abstract: A quantum wire with spin-orbit coupling, proximate to an s-wave superconductor, has a topological phase with a Majorana mode localized at each end of the superconducting part of the wire. If one of the Majorana modes couples to 2 or more channels of interacting electrons in the normal region an unusual type of frustration occurs, leading to a novel quantum critical point. I will discuss the properties of this critical point and prospects for observing it.

Coffee break Room: Oskar Klein-auditoriet (FR4)

Quantum transport in topological insulator nanowires

• Jens Bardarson (Max Planck Institute for the Physics of Complex Systems, Germany)

Room: Oskar Klein-auditoriet (FR4) Abstract: Topological insulators are a state of matter that is protected by time reversal symmetry. In 3D, it has an insulating bulk but a conducting surface which low energy electronic properties are well described by Dirac fermions. In this talk I will discuss what are the characteristic properties of this material when the surface is curved, such as in a cylindrical or rectangular nanowire. In particular, I focus on how one can observe these features in a transport experiment. I will then discuss what changes ones these wires are interfaced with superconductors.  In the talk, I will focus on then fundamental quantum aspects of these materials, namely, the role of time reversal symmetry in quantum mechanics and Kramers degeneracy; geometric phases such as the Berry phase and its interplay with Aharonov-Bohm phases; and finally, possibilities of creating and observing Majorana modes in these systems.

Phase junctions in one-dimensional topological superconductors

• Eddy Ardonne (Stockholm University, Sweden)

Room: Oskar Klein-auditoriet (FR4) Abstract: In this talk, we consider junctions in superconducting wires. In particular, we will study the difference `real' junctions, and so-called `phase-winding' junctions, and investigate the role topology plays. We briefly comment on some potential experimental setups, which could be used to test our results.

Exact results on the out-of-equilibrium Kondo model

• Edouard Boulat (University Paris Diderot)

Room: Oskar Klein-auditoriet (FR4) Abstract: Transport in nanoscale quantum devices can be described in some situations by quantum impurity models in which the low energy regime is often a strong coupling (SC) regime, the archetypical example maybe being the Kondo model. We have recently developed a framework for integrable models[1], in which we can exactly tackle various out-of-equilibrium situations for quantum impurities in their SC regime, using their equilibrium integrability properties.  It allows to compute directly the expansion of the universal scaling functions for physical quantities (like the electrical current), in principle to arbitrarily high order in the driving out-of-equilibrium, be it voltage, frequency,… In particular, we show how to apply this to the Kondo model : our approach successfully goes beyond known results for the electrical current and noise.   [1]: L.Freton and E.Boulat, arxiv/1303.7441, in review.

Lunch Room: 3rd floor, AlbaNova main building

Multiplicities of Majorana vortecies

• Roman Jackiw (MIT, USA)

Room: Oskar Klein-auditoriet (FR4) Abstract: Clifford-like algebra for mult-vortecies is established.

Topological Kondo effect

• Alexander Altland (Cologne University, Germany)

Room: Oskar Klein-auditoriet (FR4) Abstract: In this talk, we will briefly review recent progress in realizing Majorana fermion bound states in condensed matter devices. We will motivate a structure comprising semiconductor quantum wires coupled to a superconductor, and to external leads as the most generic device architecture realizing Majorana fermion transport by current date technology. In the main part of the talk we will discuss how a conspiracy of topological correlations and electrostatic interaction drive such 'Majorana quantum dots' to a fixed point fundamentally different from an ordinary Fermi liquid fixed point. At strong coupling, the system exhibits unconventional transport behavior and non-Fermi liquid correlations which make it distinct from any conventional quantum electronic device. We will close by discussing possible extensions of the elementary Kondo cell to more complex architectures.

Coffee break Room: Oskar Klein-auditoriet (FR4)

Topological Kondo island Josephson-coupled to a superconductor

• Christophe Mora (Ecole Normale Supérieure, France)

Room: Oskar Klein-auditoriet (FR4) Abstract: We propose a device architecture with Majorana fermions predicted to host a manifold of non-Fermi quantum impurity states. The device consists of a floating superconducting island carrying one-dimensional nanowires with Majorana end states. The Majorana are tunnel-coupled to normal leads while the island is Josephson-coupled to a bulk superconductor. In this system, the quantum impurity, nonlocally encoded by the Majorana fermions, experiences both Kondo screening and resonant Andreev reflection processes. Surprisingly, we found that these two effects can coexist, leading to a ground state manifold with non-Fermi liquid continuous exponents. Our results were obtained using a combination of conformal field theory arguments, Abelian bosonization and an intuitive quantum Brownian motion analogy which explains the manifold in simple terms. We also found an illuminating analogy between our system and the two-channel two-impurity Kondo model. The predicted manifold and its non-Fermi liquid nature can be identified in charge transport measurements, where we predict the appearance of nonlocal conductances with a power law temperature dependence. The power law exponent is continuously tunable within the manifold by changing gate voltages.

Engineering non-abelian defects between one and two dimensions

• Ady Stern (Weizmann Institute, Israel)

Room: Oskar Klein-auditoriet (FR4) Abstract: In my talk I will describe how non-abelian topological defects may be engineered to occur in various systems, and focus on defects that go beyond Majorana fermions.

Short break Room: Oskar Klein-auditoriet (FR4)

Anyonics: Designing exotic circuitry with non-Abelian anyons

• Kirill Shtengel (University of California, Riverside, USA)

Room: Oskar Klein-auditoriet (FR4) Abstract: Non-Abelian anyons are widely sought for the exotic fundamental physics they harbour as well as for their possible applications for quantum information processing. Currently, there are numerous blueprints for stabilizing the simplest type of non-Abelian anyon, a Majorana zero energy mode bound to a vortex or a domain wall. One such candidate system, a so-called "Majorana wire" can be made by judiciously interfacing readily available materials; the experimental evidence for the viability of this approach is presently emerging. Following this idea, we introduce a device fabricated from conventional fractional quantum Hall states, s-wave superconductors and insulators with strong spin-orbit coupling. Similarly to a Majorana wire, the ends of our “quantum wire” would bind "parafermions", exotic non- Abelian anyons which can be viewed as fractionalised Majorana zero modes. I will briefly discuss their properties and describe how such parafermions can be used to construct new and potentially useful circuit elements which include current and voltage mirrors, transistors for fractional charge currents and "flux capacitors".

Driven graphene

• So-Young Pi (Boston University, USA)

Room: Oskar Klein-auditoriet (FR4) Abstract: We consider a driven graphene with a rotating Kekule mass and study the system through the lense of Floquet theory.

Poster session with wine & cheese Room: 3rd floor, AlbaNova main building

Quantum acoustics: Propagating phonons coupled to a superconducting qubit

• Per Delsing (Chalmers University of Technology, Sweden)

Room: Oskar Klein-auditoriet (FR4) Abstract: Mechanical movement can be quantized in the same way as light, into particles of vibration known as phonons. Recent experiments have shown that quantum information generated in qubits can be converted to vibrations in mechanical resonators, which resemble miniature drum skins or piano strings. Such resonators restrict motion to their discrete and stationary modes, and thus serve as local storage units for phonons. Here, we demonstrate the use of phonons as propagating carriers of quantum information, by coupling them strongly to a superconducting qubit. The phonons thus serve the same role as itinerant photons have in the field of quantum optics. Three different experiments are presented: i) Exciting the qubit with an electromagnetic signal we can “listen” to the SAW phonons emitted by the qubit. The low speed of sound also allows us to observe the emission of the qubit in the time domain, giving clear proof that the dominant coupling is acoustic. ii) Reflecting a SAW wave off the qubit, we observe a nonlinear reflection with strong reflection at low power and low reflection at high power. iii) Exciting the qubit with both an electromagnetic signal and with a SAW signal, we can do two tone spectroscopy on the qubit. Due to the low speed of sound and a potential for very strong coupling, the use of propagating phonons as quantum carriers allows regimes to be explored, which are difficult or impossible to reach with photons. This work was done in collaboration with M.V. Gustafsson, T. Aref, A. Frisk- Kockum, M.K. Ekström, and G. Johansson.

Quantum dynamics of a fluxonium device

• Leonid Glazman (Yale University, USA)

Room: Oskar Klein-auditoriet (FR4) Abstract: Recent development of a new type of a qubit, fluxonium, facilitated the observation of two elusive quantum effects. The first of the two is the interference between separated in space quantum phase slips. The second one is the produced by quasiparticles dissipative component of the Josephson current. The interference of phase slips manifested itself via inhomogeneous broadening of the qubit oscillations frequency. The dissipative effect of quasiparticles was quantified by measuring the T1 time of the qubit. This latter measurement actually resolved the so- called "cosine-phi" problem which existed since the time the Josephson effect has been predicted. This talk covers the theory of the fluxonium qubit, and its use in designing the experiments and interpreting the data.

Coffee break Room: Oskar Klein-auditoriet (FR4)

Topology and magnetism

• Yoshinori Tokura (RIKEN, Japan)

Room: Oskar Klein-auditoriet (FR4) Topology of spin texture in a solid is a source of emergent electromagnetic properties and functions. Under strong spin-charge coupling as well as relativistic spin-orbit coupling, topological spin textures give birth to exotic phenomena, such as colossal magnetoresistance, multiferroicity (magnetic ferroelectricity), skyrmions, topological insulator/semimetal, quantized- anomalous/topological Hall effects, etc. In particular, non- collinear and/or non-coplanar spin textures may generate the fictitious (emergent) magnetic and electric fields acting on the electrons, thus hosting the enhanced magneto- electric effect and topological Hall effects. Emergent electromagnetism in topological magnets and its possible application are presented.

Quench dynamics in quantum integrable models

• Natan Andrei (Rutgers University, USA)

Room: Oskar Klein-auditoriet (FR4) Abstract: I will describe a formulation for studying the quench dynamics of integrable systems generalizing an approach by Yudson  and apply it to the evolution dynamics of the  Lieb-Liniger system, a gas of bosons moving on the continuous line and interacting via a short range potential. The formalism allows us to quench the system from any initial state. Considering first a finite number of bosons on the line. I will show that for any value of repulsive coupling  the system asymptotes towards a strongly repulsive gas for any initial state, while for an attractive coupling, the system forms a maximal bound state that dominates at longer times. In either case the system equilibrates but does not thermalize, an effect that is consistent with prethermalization. Then considering the system in the thermodynamic limit - with the number of bosons and the system size sent to infinity at a constant density with  the long time limit taken subsequently- I'll discuss the equilibration of the system for strong but finite positive coupling and show it equilibrates to a GGE (generalized Gibbs ensemble) for translationally invariant initial states with short  range correlations. For initial states with long range correlations a generalizedGGE emerges. If the initial state is strongly non-translationally invariant the system does not equilibrate.  I will give some examples of quenches: from a Mott insulator initial state or from a domain wall configuration. Then I will show that if the coupling constant is negative the GGE fails for most initial states. The latter result extends to all models with bound states such as the XXZ or the Hubbard model. 
If time permits I shall discuss also the quench dynamics of the XXZ Heisenberg chain and of a mobile impurity in an interacting Bose gas.

Glimmers of a quantum KAM theorem: Insights from quantum quenches in one-dimensional Bose gases

• Robert Konik (Brookhaven National Laboratory, USA)

Room: Oskar Klein-auditoriet (FR4) Abstract: We consider quantum quenches in one dimensional Bose gases where we prepare the gas in the ground state of a parabolic trap and then release it into a small cosine potential.  This cosine potential breaks the integrability of the 1D gas which absent the potential is described by the Lieb-Liniger model.  We explore the consequences of this cosine potential on the thermalization of the gas. We argue that the integrability breaking of the cosine does not immediately lead to ergodicity inasmuch as we demonstrate that there are residual quasi-conserved quantities post-quench.  We demonstrate that the quality of this quasi-conservation can be made arbitrarily good.

Lunch Room: 3rd floor, AlbaNova main building

Algebraic Bethe ansatz and tensor networks

• Vladimir Korepin (Stony Brook University, USA)

Room: Oskar Klein-auditoriet (FR4) Abstract: A relation between Bethe Ansatz states and Matrix Product State is established. This helps to improve numerics for solvable models and for generic as well.

Quantum environment for long-lasting coherence

• Paola Verrucchi (Instituto dei Sistemi Complessi - CNR, Italy)

Room: Oskar Klein-auditoriet (FR4) Decoherence is one of the main obstacles placed in the way of the correct functioning of quantum devices. It is an ubiquitous phenomenon, due to the unavoidable interaction between a quantum principal system and its environment, which becomes particularly disruptive when quantum properties are to be exploited and controlled. Despite being an ordinary effect, decoherence is not easily describable in a general framework, as it depends on several details of the physical setup. In this work, we make use of a recently proposed method (PNAS 110, 6748 (2013)) for studying the dynamical evolution of a generic quantum system subject to decoherence. From such treatment, an analytical expression for a consistent measure of the coherence time, emerges, and formally shows how, and why, decoherence depends on the number of dynamical variables of the environment. Based on this result we propose a strategy for effectively reduce decoherence, and finally implement it in two exemplifying situations where decoherence must be kept under control.

Coffee break Room: Oskar Klein-auditoriet (FR4)

Testing time reversal symmetry in artificial atoms

• Amir O. Caldeira (Universidade Estadual de Campinas, Brazil)

Room: Oskar Klein-auditoriet (FR4) Over the past several decades, a rich series of experiments has repeatedly verified the quantum nature of superconducting devices, leading some of these systems to be regarded as artificial atoms. In addition to their application in quantum information processing, these `atoms' provide a test bed for studying quantum mechanics in macroscopic limits. Regarding the last point, we present here a feasible protocol for directly testing time reversal symmetry in a superconducting artificial atom. Time reversal symmetry is a fundamental property of quantum mechanics and is expected to hold if the dynamics of the artificial atom strictly follow the Schroedinger equation. However, this property has yet to be tested in any macroscopic quantum systems. The test we propose is based on the verification of the microreversibility principle, which on top of its own importance, provides us with a viable approach to verifying quantum work fluctuation theorems - an outstanding challenge in quantum statistical mechanics. For this, we outline a procedure that utilizes the microreversibility test in conjunction with numerical emulations of Gibbs ensembles to verify these theorems over a large temperature range.

An order parameter for quantum impurity systems at criticality

• Abolfazl Bayat (University College London, UK)

Room: Oskar Klein-auditoriet (FR4) Abstract: We address the issue of quantum criticality in quantum impurity systems. In contrast to ordinary bulk quantum phase transitions, the notion of a conventional order parameter which exhibits scaling is notably missing at an impurity quantum critical point. We here explore the possibility to use the Schmidt gap, which is an observable obtained from the entanglement spectrum, as an order parameter. A case study of the two-impurity Kondo model confirms that the Schmidt gap faithfully captures the scaling behavior by correctly predicting the critical exponent of the dynamically generated length scale at the quantum critical point.

Short break Room: Oskar Klein-auditoriet (FR4)

Hybrid atom-optomechanics

• Philipp Treutlein (University of Basel, Switzerland)

Room: Oskar Klein-auditoriet (FR4) Abstract: In optomechanics, laser light is used for cooling and control of the vibrations of micromechanical oscillators, with many similarities to the cooling and trapping of atoms. Laser light can also be used to couple the motion of ultracold atoms in a trap to the vibrations of a mechanical oscillator. In the resulting hybrid system the atoms can be used for sympathetic cooling of the oscillator, creating atom-oscillator entanglement, and controlling the oscillator on the single-phonon level. We have realized a hybrid mechanical system in which ultracold atoms and a micromechanical membrane are coupled by radiation pressure forces. The atoms are trapped in an optical lattice, formed by retro-reflection of a laser beam from an optical cavity that contains the membrane as mechanical element. When we laser cool the atoms, we observe that the membrane is sympathetically cooled from ambient to millikelvin temperatures through its interaction with the atoms. Sympathetic cooling with ultracold atoms or ions has previously been used to cool other microscopic systems such as atoms of a different species or molecular ions up to the size of proteins. Here we use it to efficiently cool the fundamental vibrational mode of a macroscopic solid-state system, whose mass exceeds that of the atomic ensemble by ten orders of magnitude. Our hybrid system operates in a regime of large atom-membrane cooperativity. With realistic improvements it enables ground-state cooling and quantum control of low-frequency oscillators such as membranes or levitated nanoparticles, in a regime where purely optomechanical techniques cannot reach the ground state. References: [1] A. Jöckel, A. Faber, T. Kampschulte, M. Korppi, M. T. Rakher, and P. Treutlein, "Sympathetic cooling of a membrane oscillator in a hybrid mechanical-atomic system", submitted (2014). [2] B. Vogell, K. Stannigel, P. Zoller, K. Hammerer, M. T. Rakher, M. Korppi, A. Jöckel, and P. Treutlein, "Cavity-enhanced long-distance coupling of an atomic ensemble to a micromechanical membrane", Phys. Rev. A 87, 023816 (2013). [3] P. Treutlein, C. Genes, K. Hammerer, M. Poggio, and P. Rabl, "Hybrid Mechanical Systems", in: "Cavity Optomechanics", ed. by M. Aspelmeyer, T. Kippenberg, F. Marquardt (Springer). preprint arXiv:1210.4151 (2012). [4] S. Camerer, M. Korppi, A. Jöckel, D. Hunger, T. W. Hänsch, and P. Treutlein, "Realization of an optomechanical interface between ultracold atoms and a membrane", Phys. Rev. Lett. 107, 223001 (2011).

Quantum engineering of states in graphene

• Eva Andrei (Rutgers University, USA)

Room: Svedbergsalen (FD5) Abstract: Scanning tunneling microscopy and spectroscopy (STM/STS) provides direct access to the Dirac fermion quasiparticles in graphene and their interactions - with the environment and with each other- through the local density of states. I will describe STM/STS experiments on graphene with emphasis on three results: a) tuning the band structure in twisted graphene layers; b) tuning screening by populating Landau levels; c) Kondo screening of single vacancies.

Electric-dipole-induced universality for Dirac fermions in graphene

• Alessandro De Martino (City University London, UK)

Room: Svedbergsalen (FD5) Abstract: I will present a study of electric dipole effects for massive Dirac fermions in graphene. The dipole potential accommodates towers of infinitely many bound states exhibiting a universal Efimov-like scaling hierarchy. The number of towers is determined by the strength of the dipole moment, but at least one tower is always present. The corresponding eigenstates show a characteristic angular asymmetry, observable in tunnel spectroscopy. I will then discuss the low-energy dipole scattering problem and show that the charge transport properties inferred from scattering states are highly isotropic.

Coffee break Room: Svedbergsalen (FD5)

Photonic multipartite entanglement

• Mohamed Bourennane (Stockholm University, Sweden)

Room: Svedbergsalen (FD5) Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographic protocols, and fundamental tests of quantum theory. Spontaneous parametric down-conversion is the most effective source for polarization entangled photon pairs. I show that a class of entangled 6-photon states can be directly created by parametric down-conversion. These states exhibit perfect quantum correlations and a high robustness of entanglement against photon loss. Therefore these states are well suited for new types of quantum communication. Bound entanglement is one of the most puzzling forms of entanglement. Being a peculiar form of entanglement, bound entanglement emerges in certain mixed quantum states. This form of entanglement is not distillable by local operators and classical communication. Bound entangled states are different from both the free entangled (distillable) and separable states. I report on the experimental evidence of the existence of bound entangled state, the so-called Smolin state.

Quantum correlations: when, how and why?

• Adan Cabello (University of Seville, Spain)

Room: Svedbergsalen (FD5) Abstract: In classical physics, any set of observables is jointly measurable. In contrast, any graph in which nodes represent observables and edges represent joint measurability is realizable in quantum theory. Moreover, correlations between quantum measurements violate some Bell and noncontextuality inequalities satisfied by theories in which observables are all jointly measurable. Remarkably, quantum violations occur only for some inequalities and only up to specific limits. Here we use a recently introduced approach to answer three questions that are important to understand quantum theory: (i) Which inequalities are violated? (ii) Which violations are possible? (iii) Why?

Closing remarks

• Pasquale Sodano (International Institute of Physics, Brazil)

Room: Svedbergsalen (FD5)