Seminar Program WS 2023/2024

Walther-Meißner-Institute, Bavarian Academy of Sciences and Humanities


Friday, 11.15 h



Seminar Room 143, Walther-Meißner-Institute,
Walther-Meißner-Str. 8, D-85748 Garching






Lukas Schamriss
Department of Physics, Chair of Theoretical Physics II
FAU Erlangen, Germany
Zoom Link: tum-conf.zoom.us/j/65538017969
(Passcode: wmi-rabl)

Quantum criticality of the frustrated transverse-field Ising model on a triangular bilayer using directly evaluated enhanced perturbative continuous unitary transformations

Ising models in a transverse field are paradigmatic models for quantum phase transitions of various universality classes which occur depending on the lattice geometry and the choice or antiferromagnetic of ferromagnetic coupling. We investigate the quantum phase diagram of the antiferromagnetic transverse-field Ising model (TFIM) on a triangular bilayer with an Ising interlayer coupling. Without a field, the model is known to host a classically disordered ground state, and in the limit of decoupled layers it exhibits the 3d-XY transition of the corresponding single layer model. Our starting point for the unknown parts of the phase diagram is a high-order perturbative calculation from the limit of isolated dimers. Enhanced perturbative continuous unitary transformations (epCUTs) are used to derive series expansions for the ground-state energy and the energy gap. These are refined by directly evaluated epCUTs (deepCUTs) which provide estimates which coincide with the perturbative series up to its respective order and add a non-perturbative correction. These allow to draw conclusions about the nature of occurring quantum phase transitions. 

Nikolaj Aagaard Larsen
Niels Bohr Institute, University of Copenhagen
Jagtvej 155 A, 2200 Copenhagen N., Denmark
Zoom Link: tum-conf.zoom.us/j/65538017969
(Passcode: wmi-rabl)

Effective Negative Mass and Frequency Down-Conversion in Two-Tone Optomechanics

Since the turn of the century, the somewhat mystical sounding 'negative mass' oscillators have been explored as a tool for quantum back action evasion and entanglement generation in bipartite quantum oscillator systems, both experimentally and theoretically. While the physical realisations range from spin-polarized atomic ensembles to mechanical oscillators of various kinds, optomechanical implementations provide several advantages, such as the high quality factors which can be achieved. However so far the mechanical systems have generally been limited by the need to embed them in narrow cavities, and the requirement of frequency matching them, which has limited their use for hybrid setups.

In this talk we describe a theoretical proposal on how to overcome these limitations, by employing measurement based feedback on a two-tone driven optomechanical system. The scheme we propose makes no assumptions on the degree of sideband resolution, and accounts from dynamical back-actions effects arising both from detuning of the lasers from the cavity and from asymmetric laser drives. Furthermore, the effective oscillator generated by our scheme has a tuneable mass sign and a resonance which is downconverted by several orders of magnitude.

The work presented in this talk was completed during my MSc and generalises the original proposal by Zeuthen et al. in [PRX QUANTUM 3, 020362 (2022)]

Alexander Poddubny
Weizmann Institute of Science

Zoom Link: tum-conf.zoom.us/j/65538017969
(Passcode: wmi-rabl)

Mesoscopic Non-Hermitian Skin Effect

We discuss a generalization of the non-Hermitian skin effect to finite-size photonic structures with neither gain nor loss in the bulk and purely real energy spectrum under periodic boundary conditions (PBC) [1]. We show that such systems can still have significant portions of eigenmodes concentrated at the edges and that this edge concentration can be linked to the non-trivial point-gap topology of the size-dependent regularized PBC spectrum, accounting for the radiative losses. As an example, we consider the chiral waveguide quantum electrodynamics platform with an array of atoms coupled to the waveguide [2]. The proposed mesoscopic analog of the non-Hermitian skin effect could be potentially applied to other seemingly lossless photonic structures, such as chiral resonant all-dielectric metamaterials.

[1] A. Poddubny, J. Zhong, Sh. Fan, arXiv:2310.04025 
[2] A.S. Sheremet, M.I. Petrov, I.V. Iorsh, A. V. Poshakinskiy, and A.N. Poddubny, Rev. Mod. Phys. 95, 015002 ( 2023)
Vera Bader
University of Augsburg

Zoom Link: tum-conf.zoom-x.de/j/63894710937

Meeting ID: 638 9471 0937
Passcode: 681324

2D spin-orbit coupled frustrated magnets

The magnetism in layered transition metal compounds is governed by the individual interplay of various actors like Hund’s coupling, spin-orbit coupling (SOC), crystal field effect, and frustration. Two compounds based on Ru, Na2RuO3 and Na3RuO4, and two compounds based on Co, Na2BaCo(PO4)2 and Na2SrCo(PO4)2, are presented. For the four compounds the synthesis, the structural characterization, and the thermodynamic properties are described.

In Na2RuO3 SOC leads to the formation of a J = 0 ground state singlet and the system is non-magnetic. In frustrated systems incommensurate magnetic structures are commonly observed. In this context the anisotropic triangular lattice compound Na3RuO4 is introduced. The study of frustrated magnets is largely motivated by the search for a quantum spin-liquid (QSL). The concept of QSLs will be given with focus on triangular lattice compounds and in this context the compound Na2BaCo(PO4)2 previously reported as QSL candidate is presented. The compound is further compared to its chemical sibling Na2SrCo(PO4)2 and the structural differences can directly be related to the differences of the thermodynamic properties.

Benjamin Lienhard
Princeton University

Zoom Link: tum-conf.zoom-x.de/j/61239564020

Meeting-ID: 612 3956 4020
Kenncode: 857419

Interfacing with Quantum Information Processors—From Readout to Control

To realize the vision of useful quantum computing, balancing the effort required for quantum system control, particularly in measurements during system characterization and calibration, is essential. This effort should be low enough to compensate for system parameter changes and fast enough for periodic recalibration. While theoretical models offer insights into the general structures of quantum control landscapes, they may fall short in fully representing real quantum systems. On the contrary, complete system characterization can provide accurate numerical models, but this process is often cumbersome. In contrast, model-free learning control, although resource-intensive, presents a data-driven calibration technique. As quantum systems increase in size, both avenues become more measurement-intensive.

During this presentation, I will delve into the protocols we have devised to enhance superconducting qubit readout. Additionally, I will introduce a novel technique that combines residual modeling with neural networks to extract quantum processor dynamics. This approach provides a comprehensive representation of experimentally observed dynamics with minimal effort. The significance of this low-resource methodology lies in its ability to facilitate the calibration of closed-system models, allowing for the initiation of reinforcement-learning gate-calibration agents. This approach holds the potential to scale up and address the challenges associated with quantum control, offering a promising trajectory for the future of quantum computation.


Masashi Shiraishi
Kyoto University, Japan

Zoom link: tum-conf.zoom-x.de/j/67362682375

Meeting-ID: 673 6268 2375
Kenncode: 111600

Tolopogical Spintronics/Orbitronics

Topological quantum materials (TQMs), such as topological insulators, topological superconductors, Weyl semimetals and Weyl ferromagnets, have been collecting tremendous attention in modern condensed matter physics. In spintronics/spin-orbitronics perspectives, TQMs have been playing significant role because of its intriguing spin states that allows spin transport, spin conversion and manifestation of novel spin physics in them.

In this talk, I introduce recent studies on spin physics using TQMs by focusing on WTe2 [1](Weyl semimetal), Co2MnGa [2](Weyl ferromagnet), PbSnTe [3](topological crystalline insulator) and one of the mother materials to host topological states, Bi [4], where spin detection, spin conversion, observation of ferroelectric Berry curvature dipole and g-factor control for efficient spin conversion are realized. If time permits, I also briefly introduce our recent challenge to detect p-wave superconductivity in topological superconductor [5].

[1] K. Ohnishi, M. Shiraishi et al., Adv. Electron. Mater. 9, 2200647 (2023).
[2] M. Aoki, M. Shiraishi et al., Nano Lett. (2023).
[3] T. Nishijima, M. Shiraishi et al., Nano Lett. 23, 2247 (2023).
[4] N. Fukumoto, R. Ohshima, M. Shiraishi et al., PNAS e2215030120 (2023).
[5] K. Ohnishi, R. Ohshima, M. Shiraishi et al., submitted.

Anirban Bhattacharjee
Tata Institute of Fundamental Research, Mumbai, India.

Zoom link: tum-conf.zoom-x.de/j/63989616967 
Meeting-ID: 639 8961 6967
Kenncode: 987859

Towards on-demand, all-to-all connectivity in a superconducting qubit network using a ring-resonator based coupler

Our work addresses one of the key challenges in development of large-scale quantum computing systems, namely the challenge of long-range and on-demand connectivity between qubits. Achieving increased connectivity in a multi-qubit network can reduce gate count during algorithm execution. We have recently demonstrated the use of a ring resonator in 3D architecture to provide beyond nearest-neighbour connectivity in a planar architecture with fixed coupling. However, fixed coupling architectures can lead to coherent errors due to the cross-Kerr effect between all coupled qubits. To address this, we are introducing tunable couplers between each qubit and the ring resonator designed on a 2D planar architecture. This enables on-demand activation of couplingbetween any connected qubits while avoiding coherent errors. The coupler design uses a flux-biased
Josephson junction. I will present our work on this coupler design using finite-element simulations and experimental progress.

[1] S.Hazra, A.Bhattacharjee, et.al. Phys. Rev. Applied 16, 02 (2021) 024018.
[2] Y.Chen et.al., Phys. Rev. Lett. 113, 22(2014) 220502

Prof. Dr. Bart van Wess
Zernike Institute for Advanced Materials, University of Groningen,
Groningen, The Netherlands

Zoom link: tum-conf.zoom-x.de/j/61851161461

Meeting-ID: 618 5116 1461
Kenncode: 763513
Spin waves and magnons in two-dimensions: Emergent physics and new applications

Spintronics addresses the research on, and applications of, the transport, control and manipulation of the angular momentum of electrons, their “spin”. Spin currents are usually carried by mobile electrons, in both magnetic and nonmagnetic metals and semiconductors. In contrast, spin waves (magnons) are propagating wave like excitations of the magnetic order in ferro and antiferromagnetic materials. In recent years these have been shown as very effective carriers of spin current and spin information, even in electrically insulating materials [1][2].

In this talk I will give an introduction of spintronics in two-dimensional Van der Waals materials (including graphene), their heterostructures and devices. I will then show that spin currents can also be carried by spin waves in two-dimensional ferromagnetic and antiferromagnetic materials, and explain how their different (anti) ferromagnetic properties are reflected in the magnon spectrum and magnon transport [3]. I will conclude with an outlook of new devices and applications which will become possible by the combination of two-dimensional materials and their heterostructures with (spin wave) spintronics.


  1. Long-distance transport of magnon spin information in a magnetic insulator at room temperature, L.J. Cornelissen, et al., Nature Physics 11 (12) (2015).
  2. Giant magnon spin conductivity in ultrathin yttrium iron garnet films, X.Y. Wei, et al.,  Nature Materials 21 (12) (2022).
  3. Long-distance magnon transport in the van der Waals antiferromagnet CrPS4, D.K. de Wal, et al., Phys Rev. B107 (18) (2023).


Christian Schade
Humboldt-Universität zu Berlin

Zoom link: tum-conf.zoom-x.de/j/69748214073

Meeting-ID: 697 4821 4073
Kenncode: 541431
Experimentally testing a multiverse interpretation of quantum mechanics via circuit QED experiments with concurrent, incentivized human observers

A novel type of psychophysical experiments is proposed that is supposed to test a multiverse interpretation of quantum mechanics against the standard interpretation, based on the clustered-minds multiverse (Schade 2015; 2018; 2020; 2022). These experiments involve human observers and manipulate the number, preferences, and state of information of them (the paradigm is experimental-economics based). The development of the final set-up will be shown to be a three-stage process: (1) Standard quantum optical set-up that might pose, in different versions, different ‘practical difficulties. (2) Set-up with Rydberg atoms (Rauschenbeutel et al. 1999) that would work but cannot currently be carried out in any lab of the world (according to a discussion with Arno Rauschenbeutel). Experiments employing cavity QED (ibid), which do the same and can be implemented. The experiments will be testing an inequality to be presented and explained.


Piotr Grochowski

University of Innsbruck & IQOQI Innsbruck

Zoom Link: tum-conf.zoom.us/j/65538017969
(Passcode: wmi-rabl)

Quantum control of continuous systems via nonharmonic potential modulation

The preparation of a continuous-variable system in a non-Gaussian quantum state is of paramount importance in various aspects of quantum science. The generation of non-Gaussian states requires a nonlinear resource, often introduced through coupling to an auxiliary degree of freedom, such as a two-level system. On the other hand, some continuous-variable systems already possess intrinsic nonlinearity in the potential of a canonical variable. These nonharmonicites in the potential are typically used to define a qubit within continuous-variable systems. In contrast, we explore methods for utilizing this intrinsic nonlinearity to generate and control states beyond the two-dimensional subspace. Specifically, we present a theoretical proposal for preparing and manipulating a state of a single continuous-variable degree of freedom confined to a nonharmonic potential. By utilizing optimally controlled modulation of the potential’s position and depth, we demonstrate the generation of non-Gaussian states, including Fock, Gottesman-KitaevPreskill, multi-legged-cat, and cubic-phase states, as well as the implementation of arbitrary unitaries within a selected two-level subspace. Additionally, we propose protocols for single-shot orthogonal state discrimination and algorithmic cooling and analyze the robustness of this control scheme against noise. Since all the presented protocols rely solely on the precise modulation of the effective nonharmonic potential landscape, they are relevant to several experiments with continuous-variable systems, including the motion of a single particle in an optical tweezer or lattice, or current in circuit quantum electrodynamics. Moreover, the proposed protocols can be utilized in systems with very weak nonharmonicities, e.g., levitated nanoparticles.


Helena Reichlová
FZU - Institute of Physics
Czech Academy of Sciences
Prague, Czechia

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Previous Semesters: <WS 2021/22> <SS 2022> <WS 2022/23><SS 2023>

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