The main seminar of the theory division usually takes place on Wednesdays at 11:30am in the Herbert-Walther Lecture Hall G0.25 next to the foyer. Attendance of all group members present at MPQ is expected. Talks should be about 45 minutes in length plus up to 15 minutes of question time; 60 minutes total should not be exceeded. A projector or a blackboard and a small flip chart are available in the lecture hall.

This page contains a list of past talks. To receive the announcements of upcoming talks please subscribe to the mpqtheory-announce mailing list, which is available to the public.

Currently, the Theory Seminar is taking place as an online webinar, available only for group members. Past talks and recordings are available on an internal page. (To access it, you will have to login to the website which is only possible over VPN.)

Organisers: Johannes Knörzer and Christoph Sünderhauf

11.3.2020

* Quantum East-like models *

Nicola Pancotti (MPQ)

In this talk, I will explain in detail the dynamical properties of the quantum East model, an interacting quantum spin chain inspired by simple kinetically constrained models of classical glasses. I will show the existence of a fast-to-slow transition throughout the spectrum that follows from a localization transition in the ground state. On the slow side, I will explicitly construct a large (exponential in size) number of non-thermal states which become exact finite-energy-density eigenstates in the large size limit, as expected for a true phase transition. Many of these excited states have large overlap with product states and can be approximated well by matrix product states at arbitrary energy densities. I will introduce the super-spin picture which allows to find a further large class of area-law states proved to display very slow relaxation. I will further support these results with extensive numerical analyses and show that the non-thermal features of the model may go far beyond the analytical constructions. Finally, I will show that similar dynamical properties can be found for a much larger class of models and can be extended also to two dimensional systems.

4.3.2020

* Markovian regimes in Quantum Many Body Systems *

Julian Roos (MPQ)

Long time evolution of the full state of quantum many body systems is generally out of reach due to build-up of entanglement. However, the computation of local observables only requires knowledge of the state of (small) subsystems. Is it possible to obtain a description of the reduced dynamics similarly to what is done in the fields of Quantum Optics and Open Quantum Systems (OQS)? We expect that such an endeavour is most promising in the simplest case, i.e. when the dynamics are Markovian (memoryless), and we thus study if such regimes do also exist in a many body setup. Here, the conditions that allow for the derivation of a Markovian master equation in the theory of OQS (Born-Markov) are not satisfied. In this talk I thus adopt a Quantum Information Theory perspective to identify interesting Markovian regimes (of a spin coupled to a XY spin chain) and explain the underlying physics responsible for Markovian/non-Markovian dynamics. arXiv:1912.09151

24.2.2020

* Causal Graphs: An Introduction *

Johannes Knörzer & Patrick Emonts (MPQ)

Modeling causality through directed acyclic graphs (DAGs) offers an appropriate language to think about and describe causal relationships between events. In this talk, we will talk about graphical models and how they are used in causal reasoning. We will explain useful concepts such as d-separation, the back-door criterion and do calculus. To demonstrate the usefulness of this framework, we are going to provide specific examples, with data taken from real studies.

12.2.2020

* Probing many-body quantum states with randomized measurements *

Andreas Elben (Universität Innsbruck)

Randomized measurements are a technique to probe many-body quantum states beyond familiar, low-order observables. In this talk, I introduce the technique using the example of measuring the second-order Rényi entropy in a trapped ion quantum simulator. Subsequently, I focus on two applications: First, I will present a scheme to measure (mixed-state) fidelities of two quantum states prepared on two, potentially very different, quantum devices. Importantly, the protocol requires only local measurements in randomized product bases and classical communication between the two devices. Second, I present protocols to access topological invariants for symmetry protected topological (SPT) phases, being quantized, nonlocal correlators of the many-body wavefunction. For one-dimensional spin systems, I show explicitly how to measure SPT invariants arising from inversion, time-reversal and unitary onsite symmetries. I illustrate the technique and its application in the context of the extended bosonic SSH model, as realized with Rydberg tweezer arrays.

5.2.2020

* Parity Measurements in Action: Detecting Errors in the Surface Code and Heralding Itinerant Cat States using Superconducting Circuits. *

Christopher Eichler (ETH Zurich)

Parity measurements distinguish between quantum states with an even and an odd number of excitations without revealing any additional information about the state. Nondestructive parity detection allows one to project onto highly entangled states and therefore plays a central role in quantum error correction. In my talk, I discuss two complementary implementations of parity detection both realized in superconducting circuits. First, we measure the parity of the photon number of propagating radiation fields and thereby herald itinerant Schrödinger cat states [1]. Second, we perform ancilla-based parity detection in multi-qubit systems to implement the smallest viable instance of the surface code capable of detecting any single error [2]. Both experiments crucially rely on recent progress in performing high-fidelity, fast, and qubit-selective readout [3], and highlight the great potential of superconducting circuits for quantum information science. [1] J. - C. Besse, S. Gasparinetti, M. C. Collodo, T. Walter, A. Remm, J. Krause, C. Eichler, and A. Wallraff, arXiv:1912.09896 (2019); J. - C. Besse, S. Gasparinetti, M. C. Collodo, T. Walter, P. Kurpiers, M. Pechal, C. Eichler, and A. Wallraff, Phys. Rev. X 8, 021003 (2018) [2] C. Kraglund Andersen, A. Remm, S. Lazar, S. Krinner, N. Lacroix, G. J. Norris, M. Gabureac, C. Eichler, and A. Wallraff, arXiv:1912.09410 (2019); C. Kraglund Andersen*, A. Remm*, S. Lazar, S. Krinner, J. Heinsoo, J-C. Besse, M. Gabureac, A. Wallraff, and C. Eichler, NPJ Quantum Information 5, 69 (2019) [3] J. Heinsoo, C. Kraglund Andersen, A. Remm, S. Krinner, T. Walter, Y. Salathe, S. Gasparinetti, J-C. Besse, A. Potocnik, A. Wallraff, and C. Eichler, Phys. Rev. Applied 10, 034040 (2018); T. Walter, P. Kurpiers, S. Gasparinetti, P. Magnard, A. Potoćnik, Y. Salathe, M. Pechal, M. Mondal, M. Oppliger, C. Eichler, and A. Wallraff, Phys. Rev. Applied 7, 054020 (2017)

22.1.2020

* Confined phases of one-dimensional spinless fermions coupled to Z2 gauge theory *

Umberto Borla (TUM)

We investigate a quantum many-body lattice system of one-dimensional spinless fermions interacting with a dynamical Z2 gauge field. The gauge field mediates long-range attraction between fermions resulting in their confinement into bosonic dimers. At strong coupling we develop an exactly solvable effective theory of such dimers with emergent constraints. Even at generic coupling and fermion density, the model can be rewritten as a local spin chain. Using the Density Matrix Renormalization Group the system is shown to form a Luttinger liquid, indicating the emergence of fractionalized excitations despite the confinement of lattice fermions. In a finite chain we observe the doubling of the period of Friedel oscillations which paves the way towards experimental detection of confinement in this system. We discuss the possibility of a Mott phase at the commensurate filling 2/3.

17.1.2020

* Exceptional Topology of Non-Hermitian Systems *

Emil Bergholtz (Stockholm University)

Non-Hermitian Hamiltonians have become an important asset for the effective description of various physical systems that are subject to dissipation. Using simple examples, I will discuss several new topological phenomena and relate them to uniquely non-Hermitian concept of exceptional degeneracies at which both eigenvalues and eigenvectors coalesce. In particular, this includes the occurrence of Fermi arcs in two dimensions, three-dimensional knotted metals featuring open Fermi-Seifert surfaces, new symmetry protected phases and a bulk-boundary correspondence strikingly distinct from that known from the Hermitian realm. Along the way I will give examples on how these phenomena can be realised in settings ranging from optical setups with gain and loss, electric circuits, and mechanical systems, to quantum many-body systems such as electronic transport settings at material junctions. Reference: arXiv:1912.10048

15.1.2020

* Realizing generalized quantum measurement based on linear optics *

Kang-Da Wu (University of Science and Technology of China)

Quantum measurements are the key for extracting information from quantum systems and for connecting the quantum world with the classical world. Understanding the power and limitation of measurements is of paramount importance not only to foundational studies, but also to many applications, such as quantum tomography, metrology, and communication. I will give a talk on our recent experimental results on realizing both generalized measurement on a single quantum system and collective measurement on multiple-partite system based on linear optics, and their applications in quantum metrology, quantum resource theories and quantum thermodynamics.

Earlier seminars: 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2009, 2008, 2007, 2006, 2005