Institute for Quantum Materials and Technologies
IQMT Seminar
Time Speaker & Topic of the Talk
Wed 11:00
Zoom Seminar
Michael Marthaler
HQS Quantum Simulations
Lattice Models and Quantum Computing without Quantum Error Correction

Current quantum computers have relatively high error probabilities per operation. Therefore, any application has to use an extremely small number of operations and ideally be somewhat insensitive to the errors (e.g. decoherence). We discuss the number of operations needed for simulation of the electronic structure problem. We show that the number of operations and the mode of error accumulation seems somewhat favorable for lattice models. Given current limitations of quantum computers it is also important to compare to classical solvers like DMRG to understand at which point a quantum computer can really deliver a speed up of interest. We will also discuss our current work using DMRG and how it connects to our quantum computing goals.

Wed 11:00
Zoom Seminar
Titus Neupert
Condensed Matter Theory, Department of Physics University of Zurich
Solving a Small Hubbard Model on an IBM Quantum Computer

Fully programmable quantum computing and simulation devices have hit several milestones over the past few years, with a massively growing involvement of industrial research from companies such as IBM, Google, and Microsoft. The emerging quantum computing technology promises to be useful for applications such as quantum machine learning, but notably also for intrinsically quantum-mechanical calculations that appear in material science, quantum chemistry, or quantum many-body physics. In this talk, I will show how state-of-the-art quantum hardware performs in this domain of quantum problems. Concretely, I focus on solving the ground state structure of a fermionic Hubbard model on a small cluster of sites and demonstrate that qualitatively and quantitatively accurate results can be obtained. This is enabled by exploiting the symmetries of the problem, employing a hybrid quantum-classical variational algorithm, and a Lanczos-based error mitigation scheme. I will introduce each of these steps in detail and discuss the potential of the general workflow for upscaling.

Wed 11:00
Zoom Seminar
Jacob Biamonte
Laboratory for Quantum Information Processing
Skolkovo Institute of Science and Technology, Moscow, Russia
Introduction to Modern Quantum Algorithms: Optimisation, Simulation and Machine learning

This lecture begins with the promise of idealised, error corrected, quantum algorithms. With this goal defined, we then outline the contemporary capacity of quantum enhanced processors. Increasing the control and capacity of quantum simulators has resulted in a new class of quantum devices, utilising an iterative classical-to-quantum feedback process. This so-called “variational” approach to quantum computation was formally proven (in the noise-free setting) to represent a universal model of quantum computation. An exciting global research effort to understand the variational model is redefining the field of quantum computation. While the ultimate capacity of variational algorithms remains unknown, these methods represent an attractive approach due to the ease at which these algorithms can be realized experimentally. Contrary to analog quantum simulation, the variational approach forgoes the requirement of realising the targeted Hamiltonian directly in the laboratory, thus allowing the study of a wide variety of previously intractable target models experimentally. This comes at the cost of increased measurements and classical pre- and postprocessing.
The talk should be accessible to graduate students in physics.
- J. Biamonte et al. , Quantum Machine Learning, Nature 549, 195 (2017)
- J. Biamonte, Universal Variational Quantum Computation, arXiv:1903.04500 (2019)
- J. V. Akshay et al. , Reachability Deficits in Quantum Approximate Optimization, Phys. Rev. Lett. 104, 090504 (2020)

Wed 11:00
Zoom Seminar
Luca de' Medici
ESPCI, Paris
Electronic Compressibility and High-Tc Superconductivity: New Links

In multi-orbital Hubbard models including strong intra-atomic exchange a so-called "Hund's metal" phase is realized that shows, among other hallmarks, typically a strong differentiation in the degree of correlation of the conduction electrons based on their orbital character. This selective physics is also a key player in iron-based superconductors (FeSC), where a wealth of experimental evidences validates this theoretical picture. We will show that at the frontier between this Hund's metal phase and a conventional metal, the electronic compressibility is strongly enhanced or even divergent, and that the FeSC that have a high Tc are placed in our simulations on this frontier. This theoretical evidence of enhanced compressibility at the frontier between a phase with selective correlations and a more conventional metal is found in cuprates. We will outline the main indications for a common scenario of high-Tc superconductivity.

Wed 11:00
Zoom Seminar
Leonardo Degiorgi
ETH Zürich, Department Physik
Nematicity in the Charge Dynamics of Iron-Based Superconductors

The divergent nematic susceptibility, obeying a simple Curie-Weiss power law over a large temperature interval, is empirically found to be a ubiquitous signature in several iron-based superconductors across their doping-temperature phase diagram. I will discuss the impact of nematicity in the optical response of selected iron pnictide and chalcogenide materials, over a broad spectral range, as a function of temperature and of tunable applied stress, which acts as an external symmetry breaking field. First, I will focus my attention on FeSe where we reveal an astonishing anisotropy of the optical response in the mid-infrared-to-visible spectral range. This bears testimony to an important polarization of the underlying electronic structure, due to an orbital-ordering mechanism, supplemented by orbital selective band renormalization. The far-infrared response of the charge dynamics in FeSe moreover allows establishing the link to the dc resistivity, emphasizing scenarios based on scattering by anisotropic spin-fluctuations. Second, I will offer a comprehensive optical investigation of the optimally hole-doped Ba0.6K0.4Fe2As2, for which we show that the stress-induced optical anisotropy in the infrared spectral range is reversible upon sweeping the applied stress and occurs only below the superconducting transition temperature. These findings demonstrate that there is a large electronic nematicity at optimal doping which extends right under the superconducting dome.

Wed 11:00
Zoom Seminar
Hermann Suderow
Department Física de la Materia Condensada, Instituto Nicolás Cabrera, IFIMAC, Universidad Autónoma de Madrid - Spain
Real Space Imaging of Electronic Correlations

I will briefly explain the basics of Scanning Tunneling Spectroscopy (STS), focusing on techniques to study strongly correlated electron systems at dilution refrigeration temperatures. I will first report on the discovery of a one-dimensional charge density wave (1D-CDW) which is a Moiré pattern between the atomic lattice and a hot spot for electronic scattering in the bandstructure of the hidden order (HO) state of URu2Si2. The Moiré is produced by fracturing the crystal in presence of a dynamical spin mode at low temperatures and its presence suggests that charge interactions are among the most relevant features competing with HO in URu2Si2. Then, I will discuss the consequences of band hybridization in the local density of states which lead to new insight into the local density of states on small-sized atomically flat areas in URu2Si2. Finally, I will briefly present suprising results in feedback driven atomic size Josephson junctions, which lead to an AC Josephson signal that will likely improve Josephson Scanning Spectroscopy.

Wed 14:00
Zoom Seminar
Xavier Waintal
Quantum Photonics, Electronics and Engineering Laboratory (PHELIQS)
Commissariat à l'énergie atomique et aux énergies alternatives (CEA)
Grenoble, France
Introduction to Quantum Computing by a Skeptic

I will expose the concepts of quantum computing for physicists and from a physicist's point of view. I will go through the construction of quantum error corrections and point out the difficulties and physical limitations for implementing this framework. I will, in particular, address the surface code and the concept of correctable versus non-correctable errors. This presentation is intended to an audience of physicists that mostly know about what the qubits are but not how we are supposed to make a computer out of them.
After the main talk, there will be a more specialized discussion session "Is quantum supremacy real?" for those who are interested in details of the recent Google experiment on quantum supremacy.

Wed 11:00
Zoom Seminar
Jens Müller
Goethe University Frankfurt
Learning from Noise - Fluctuation Spectroscopy of Correlated Electrons in Molecular Conductors

So-called 1/f-type fluctuations are ubiquitous in nature, and can be found in such diverse contexts as light curves of quasars, the human heartbeat, earthquakes, road traffic or classical music. In this seminar, we aim to give a broad and pedagogical overview of such fluctuation phenomena in condensed-matter systems and discuss how 'noise can be turned into a signal' by the method of fluctuation spectroscopy. We first give an example where understanding the microscopic origin of the fluctuations in semiconductor-based Hall sensors helps to improve the signal-to-noise ratio and hence the device performance. In the main part of the talk we then will discuss recent results on the low-frequency dynamics of strongly correlated electrons in low-dimensional molecular metals, which may be considered model systems for studying the Mott metal-insulator transition, a key phenomenon in many-body physics. Our findings range from glassy structural dynamics, critical slowing down of charge fluctuations at the Mott transition, and recent results on the formation of polar nanoregions in charge-driven ferroelectrics.

Thu 11:00
Rolf Lortz
Hong Kong University of Science and Technology
Topological Superconducting Phases in 2D and 3D Materials

In this talk I give an overview of our recent research on 2D and 3D topological superconducting materials. I will focus on the doped topological insulator Bi2Se3 and on heterostructures between a quantum anomalous Hall insulator and a superconductor. A nematic topological superconductor has an order parameter symmetry, which spontaneously breaks the crystalline symmetry in its superconducting state. This state can be observed, for example, by thermodynamic or upper critical field experiments in which a magnetic field is rotated with respect to the crystalline axes, but also directly from the anisotropic gap symmetry in scanning tunneling probe experiments. We present a study on the upper critical field of the Nb-doped Bi2Se3 for various magnetic field orientations parallel to the basal plane of the Bi2Se3 layers. The data clearly demonstrate a two-fold symmetry that breaks the three-fold crystal symmetry. This provides strong experimental evidence that Nb-doped Bi2Se3 is a nematic topological superconductor similar to the Cu- and Sr-doped Bi2Se3, and rules out earlier suggestions that the finite magnetic moment of the intercalated Nb ions could instead induce a chiral superconducting state. We then show that in doped Bi2Se3, the nematic order arises from a multicomponent order parameter where superconductivity is the primary order and the nematic order an intertwined secondary order. Such a state of matter with a multi-component order parameter can give rise to a vestigial order. In the vestigial phase, the primary order is only partially melted, leaving a remaining symmetry breaking behind, an effect driven by strong classical or quantum fluctuations. We present the observation of a partially melted superconductor in which pairing fluctuations condense at a separate phase transition and form a nematic state with broken Z3 symmetry High-resolution thermal expansion, specific heat and magnetization measurements reveal that this symmetry breaking occurs at Tnem≈3.8 K above Tc≈3.25 K, along with an onset of superconducting fluctuations. Thus, before Cooper pairs establish long-range coherence at Tc, they fluctuate in a way that breaks the rotational invariance at Tnem and induces a distortion of the crystalline lattice. With the recent discovery of the quantum anomalous Hall insulator, which exhibits the conductive quantum Hall edge states without external magnetic field, it becomes possible to create a novel topological superconductor by introducing superconductivity into these edge states. In this case, two distinct topological superconducting phases with one or two dispersive chiral Majorana edge modes were theoretically predicted, characterized by Chern numbers (N) of 1 and 2, respectively. We present spectroscopic evidence from Andreev reflection experiments for the presence of chiral Majorana modes in a Nb / (Cr0.12Bi0.26Sb0.62)2Te3 heterostructure with distinct signatures attributed to two different topological superconducting phases. The results are in qualitatively good agreement with the theoretical predictions.

Tue 11:00
Gediminas Simutis
Laboratoire de Physique des Solides, Université Paris - Saclay & CNRS
Tuning Quantum Magnets

When cooled down, most magnetic materials form long-ranged structures of the magnetic moments. The situation is markedly different when strong quantum fluctuations are present in low dimensional materials or lattices with highly frustrated networks of spins. Such systems collectively called quantum magnets may exhibit unconventional magnetism and even maintain a fluctuating quantum state down to the lowest temperatures.
Sometimes one of the best ways to reveal these new ground states and their excitations is by perturbing such systems. In this talk, I will present several ways to modify the quantum magnets using chemical substitution as well as application of high pressure. First, I will show how spectral modifications can be achieved by diluting low dimensional magnets and what effects the impurities have on magnetic ordering as well as the universal scaling properties [1]. I will then talk about a different way to perturb such systems: high pressure. In particular, I will discuss the search and discovery of new phases in bond-frustrated iridate and ruthenate magnets. The links between the exact modification of the lattice and the resulting magnetic phases will be demonstrated in some of Kitaev materials [2].
Finally, I will discuss arguably the most interesting perturbation - carrier doping. After an overview of previous efforts to dope a spin liquid, I will present our ongoing study of the interplay between the charge and magnetic degrees of freedom in an iridium-based geometrically frustrated hyperkagome lattice [3].
[1] Phys. Rev. Lett. 111, 067204 (2013); Phys. Rev. 95, 054409 (2017)
[2] Phys. Rev. B 98, 104421 (2018); Phys. Rev. Lett. 120, 237202 (2018)
[3] Phys. Rev. Lett. 99, 137207 (2007); Scientific Reports 4, 6818 (2014)

Tue 14:30
Max Hirschberger
RIKEN CEMS, Center for Emergent Matter Science, Japan
Scalar spin chirality on the nanoscale: Material exploration and emergent electrodynamics

Scalar spin chirality is a pervasive concept in contemporary research on magnetism in condensed matter, especially as relates to the coupling between magnetic order and the conduction electrons. In metallic magnets, non-coplanar spin structures on the nanoscale can exert a giant emergent magnetic field on charge carriers, with potential applications in spintronics. The emergent magnetic field may also be harnessed in the future design of new types of correlated systems with protected surface modes. In this talk, I will outline our work on magnets with non-coplanar order on different length scales: (1) The canted ferromagnetic state in the correlated pyrochlore oxide Nd2Mo2O7 is a representative example of non-coplanarity within a single crystallographic unit cell [1,2]. We discuss our experimental efforts of tuning the chemical potential in this material by chemical substitution, and new theoretical insight into the qualitatively different effects of scalar spin chirality and spin-orbit coupling on the electronic band structure [3]. (2) The metallic antiferromagnet Dy3Ru4Al12 with breathing Kagome network of rare-earth sites serves as an example of an intermediate class (λmag ~ 1.5 nm): Its non-coplanar magnetic order (in zero field) can be described as antiferromagnetic stacking of strongly coupled spin-trimers with tilted all-in or all-out configuration [4]. We reveal high sensitivity of the trimer arrangement to external magnetic field, which leads to a large anomalous Hall effect when net scalar spin-chirality emerges above 1 Tesla. (3) Finally, I will discuss our recent work on the formation of nanometer-sized skyrmion spin-vortices in the centrosymmetric rare earth intermetallics Gd2PdSi3 [5] and Gd3Ru4Al12 [6] with Heisenberg Gd3+ moments and modulation length λmag ~ 2.5-3 nm. In contrast to the more widely studied case of skyrmions in non-centrosymmetric material platforms such as bulk B20 compounds or magnetic interfaces (λmag ~ 5-200 nm), skyrmions emerge here in absence of the Dzyaloshinskii-Moriya interaction. We report the phase diagrams of these compounds, reveal their magnetic order using scattering and real space imaging techniques, and discuss their large topological Hall and Nernst responses [7].
[1] Y. Taguchi et al., Science 291, 2573
[2] Y. Taguchi et al., Physical Review Letters 90, 257202
[3] M. Hirschberger et al., manuscript in preparation
[4] S. Gao, M. Hirschberger et al., arXiv:1908.07728
[5] T. Kurumaji, T. Nakajima, M. Hirschberger, et al., Science 365, 914
[6] M. Hirschberger, T. Nakajima, et al., arXiv:1812.02553, accepted to Nat. Comms.
[7] M. Hirschberger, L. Spitz, et al., arXiv:1910.06027