Programm für das Sommersemester 2024
Thursdays, 14 Uhr c.t.
Institut für Physik
IPH Lorentzraum 05127
18.04.24  Dr. Felix Tennie, Imperial College, London, UK  
Nonlinear differential equations are ubiquitous in Physics, Engineering, Chemistry, Materials Science, and various other subjects. Numerical integration often requires resources exceeding current classical supercomputers. Quantum computing presents a fundamentally different computing paradigm. Quantum algorithms have a proven scaling advantage in many linear tasks such as Fourier transformation, matrix inversion, SVD, to name but a few. Yet, due to the linear evolution of quantum systems, integrating nonlinear dynamics on quantum computers is hard. In this talk I will present different approaches for integrating nonlinear differential equations on quantum computers, and will discuss their suitability for different types of quantum hardware.  
14 Uhr c.t., IPH Lorentzraum 05127  

25.04.24  Prof. Dr. Ralf Röhlsberger, DESY, Hamburg  
Using the highintensity radiation of the European Xray FreeElectron Laser, we recently succeeded to excite the sharpest atomic transition in the hard Xray range, the 12.4 keV nuclear resonance of the stable isotope Scandium45 [1].
With its extremely narrow natural linewidth of 1.4 femtoeV, it opens not only new possibilities for the development of a nuclear clock, but also for research linked to the foundations of physics, such as time variations of the fundamental constants, the search for dark matter as well as probing the foundations of relativity theory.
Furthermore, our experiment demonstrates the great potential of selfseeding Xray lasers with high pulse rates as a promising platform for the spectroscopy of extremely narrowband nuclear resonances.
The next steps towards a nuclear clock based on Scandium45 require a further increase of the spectral photon flux using improved Xray laser sources at 12.4 keV and the development of frequency combs reaching up to this energy.
[1] Yuri Shvyd’ko et al., Nature 622, 471 (2023)  
14 Uhr c.t., IPH Lorentzraum 05127  

02.05.24  Prof. Dr. Matthias Christandl, University of Copenhagen, Denmark  
In these days, we are witnessing amazing progress in both the variety and quality of platforms for quantum computation and quantum communication. Since algorithms and communication protocols designed for traditional 'classical' hardware do not employ the superposition principle and thus provide no gain even when used on quantum hardware, we are in need of developing specific quantum algorithms and quantum communication protocols that make clever use of the superposition principle and extract a quantum advantage. "Quantum hardware needs quantum software", so to say. Furthermore, due to noise in the qubits, known as decoherence, an additional quantumspecific software layer is required that emulates a perfect quantum machine on top of a noise one. I will demonstrate our recent work on this subject with theorems as well data from university and commercial quantum devices.  
14:00 Uhr s.t., IPH Lorentzraum 05127  

16.05.24  Prof. Kenneth R. Brown, Duke University, USA  
Conical intersections often control the reaction products of photochemical processes and occur when two electronic potential energy surfaces intersect. Theory predicts that the conical intersection will result in a geometric phase for a wavepacket on the ground potential energy surface, and although conical intersections have been observed experimentally, the geometric phase has not been directly observed in a molecular system. Here we use a trapped atomic ion system to perform a quantum simulation of a conical intersection. The ion’s internal state serves as the electronic state, and the motion of the atomic nuclei is encoded into the motion of the ions. The simulated electronic potential is constructed by applying statedependent optical forces to the ion. We experimentally observe a clear manifestation of the geometric phase using adiabatic state preparation followed by motional state measurement. Our experiment shows the advantage of combining spin and motion degrees for quantum simulation of chemical reactions. We conclude with a discussion of future simulation directions.  
14:00 Uhr s.t., IPH Lorentzraum 05127  

23.05.24  Dr. Hans Keßler, Universität Hamburg  
In driven nonlinear systems, various kinds of bifurcations can be observed on their route to chaos. From the evolution of Floquet multipliers one can extract information which serves as a precursor for phase transitions and dynamical instabilities. This method is applied in classical nonlinear physics, for example, to obtain early warning signals.
Utilising our impressive control over an atomcavity platform, we are able to prepare our system in various dynamical regimes and study the bifurcation experimentally in a quantum gas to obtain insights that could potentially be applied to more complex systems.
We prepare a BoseEinstein condensate inside the centre of a cavity and pumping it perpendicular to the cavity axis with a standing wave light field. Upon crossing a critical pump strength, we observe a pitchfork phase transition from a normal to a steady state selforganized phase [1]. Employing an open three level Dicke model, this transition can be understood as a transition between two fixpoints, indicating a pitchfork bifurcation. If the pump strength is increased further, the system undergoes a Hopf bifurcation. This causes limit cycles, which have time crystalline properties, to emerge [2]. In this regime, our model no longer shows fixpoints but stable attractive periodic orbits [3]. For strong pumping, we observe a second bifurcation, in our case a NeimarkSacker bifurcation. Its main characteristics is an oscillation with two incommensurate frequencies, this may indicate the formation of a continuous time quasicrystal [4]. Finally, in the regime of very strong pumping, we observe chaotic dynamics with many contributing frequencies.
References:
[1] J. Klinder, et al., PNAS 112, 11 (2015)
[2] P. Kongkhambut, et al., Science 307 (2022)
[3] J. Skulte, et al., arXiv:2401.05332 (2023)
[4] P. Kongkhambut, et al., manuscript in preparation  
14 Uhr c.t., IPH Lorentzraum 05127  

06.06.24  Prof. Nicolò Defenu, ETH Zürich  
The concept of universality has shaped our understanding of manybody physics, but is mostly limited to homogenous systems. The seminar introduces a definition of universal scaling on a nonhomogeneous graph. The corresponding scaling theory is expected to depend only on a single parameter, the spectral dimension ds, which plays the role of the relevant parameter on complex geometries. We will then focus on a concrete example, the longrange diluted graph (LRDG), which allows to tune the value of the spectral dimension continuously. By means of extensive numerical simulations, we probe the scaling exponents of a simple instance of O(N) symmetric models on the LRDG showing quantitative agreement with the theoretical prediction of universal scaling in fractional dimensions.  
14:00 Uhr s.t., IPH Lorentzraum 05127  

13.06.24  Prof. Eugene Polzik, University of Copenhagen, Denmark  
Studies of extreme cases within quantum mechanics have always been particularly attractive. How macroscopic can objects be and still demonstrate unique quantum features, such as entanglement? What are the real limits of measurement precision in quantum mechanics? I will review our experiments where macroscopic objects are driven deep into the quantum regime. Observation of a quantum trajectory of motion in a quantum reference frame with, in principle, unlimited accuracy will be presented. A concept of a reference frame with an effective negative mass required for such observation will be introduced. Generation of an entangled EinsteinPodolskyRosen state between distant mechanical and atomic oscillators and progress towards application of those ideas to gravitational wave detection will be reported. Finally, a recent demonstration of entanglement enhanced magnetic induction tomography for medical applications will be presented.  
14 Uhr c.t., IPH Lorentzraum 05127  

20.06.24  Dr. Janis Nötzel, TUM, München  
In this talk, we address the question of how some theoretically predicted quantum advantages could be utilized in future system design. We start with an overview of various theoretical descriptions of quantum communication systems, focusing mainly on data transmission tasks involving topics such as Holevo capacity and entanglementassisted capacity of a quantum channel as well as the use of entanglement for coordination in multiple access scenarios. We give a brief overview of the state of the art of implementations before moving on to an applied perspective, where we start from the state of the art in today's network design and explore the potential role of the abovementioned quantum system descriptions for future networks.
We conclude the talk by formulating system design questions.  
14 Uhr c.t., IPH Lorentzraum 05127  

27.06.24  Prof. Oded Zilberberg, Universität Konstanz  
Topological classification of matter has become crucial for understanding (meta)materials, with associated quantized bulk responses and robust topological boundary effects [1]. Topological phenomena have also recently garnered significant interest in nonlinear systems [2]. In particular, weak nonlinearities can result in parametric gain, leading to “nonHermitian” metamaterials and the associated topological classification of open systems [3]. Here, we venture into this expanding frontier using an approach that moves away from quasilinear approximations around the closed system classification. We harness instead the topology of structural stability of vector flows, and thus propose a new topological graph invariant to characterize nonlinear outofequilibrium dynamical systems via their equations of motion. We exemplify our approach on the ubiquitous model of a dissipative bosonic Kerr cavity, subject both to one and twophoton drives. Using our classification, we can identify the topological origin of phase transitions in the system, as well as explain the robustness of a multicritical point in the phase diagram. We, furthermore, identify that the invariant distinguishes population inversion transitions in the system in similitude to a Z2 index. Our approach is readily extendable to coupled nonlinear cavities by considering a tensorial graph index.
References
[1] T. Ozawa, H. M. Price, A. Amo, N. Goldman, M. Hafezi, L. Lu, M. Rechtsman, D. Schuster, J. Simon, O. Zilberberg, and I. Carusotto, Rev. Mod. Phys. 91, 015006 (2019).
[2] A. Szameit, and M. C. Rechtsman, Nat. Phys. (2024).
[3] K. Ding, C. Fang, and G. Ma, Nat. Rev. Phys. 4, 745 (2022).  
14 Uhr c.t., IPH Lorentzraum 05127  

11.07.24  Prof. Nir BarGill, Hebrew University, Jerusalem  
The study of open quantum systems, quantum thermodynamics and quantum manybody spin physics in realistic solidstate platforms, has been a longstanding goal in quantum and condensedmatter physics. In this talk I will address these topics through the platform of nitrogenvacancy (NV) spins in diamond, in the context of bath characterization, purification (or cooling) of a spin bath as a quantum resource and for enhanced metrology and sensing.
I will first describe our work on characterizing noise using robust techniques for quantum control ([1], in collaboration with Ra’am Uzdin). This approach suppresses sensitivity to coherent errors while enabling noise characterization, providing a useful tool for the study of complicated open quantum systems, with the potential for contributions to enhanced sensing. I will then present a general theoretical framework we developed for Hamiltonian engineering in an interacting spin system [2]. This framework is applied to the coupling of the spin ensemble to a spin bath, including both coherent and dissipative dynamics [3]. Using these tools I will present a scheme for efficient purification of the spin bath, surpassing the current stateoftheart and providing a path toward applications in quantum technologies, such as enhanced MRI sensing.
Finally, if time permits, I will describe our work in using NVbased magnetic microscopy to implement quantum sensing in various modalities. I will present advanced techniques for improving sensing bandwidth using compressed sensing and machine learning. Demonstrations of NV sensing capabilities will include measurements of 2D vdW magnetic materials, and specifically the phase transition of FGT through local imaging of magnetic domains in flakes of varying thicknesses [4], as well as a technique for sensing radical concentrations through the change in the charge state of shallow NVs ([5], in collaboration
with Uri Banin).
1. P. PENSHIN ET. AL., SUBMITTED.
2. K. I. O. BEN’ATTAR, D. FARFURNIK AND N. BARGILL, PHYS. REV. RESEARCH 2, 013061 (2020).
3. K. I. O. BEN’ATTAR ET. AL., IN PREPARATION.
4. G. HAIM ET. AL., IN PREPARATION.
5. Y. NINIO ET. AL., ACS PHOTONICS 8, 7, 19171921 (2021).  
14 Uhr c.t., IPH Lorentzraum 05127  

Koordination:  Kontakt: 
Prof. Dr. Peter van Loock Dr. Lars von der Wense  Andrea Graham 