Programm für das Sommersemester 2022

Thursdays, 14 Uhr c.t.

Institut für Physik
IPH Lorentzraum 05-127

21.04.22Prof. Dr. Angela Wittmann, Institut für Physik, Universität Mainz
According to Moore’s law, the development of charge-based devices is fast approaching its limits. In response to this, spintronics has become one of the most promising alternatives. Its paradigm-changing nature lies in making use of the spin degree of freedom in addition to the electron’s charge and mass for encoding information. Particularly regarding the development of novel and versatile devices for different applications, tunable spintronic properties at hybrid interfaces in unconventional materials systems are of paramount interest. In the first part of this talk, I would like to give a brief insight into spin currents in hybrid molecule-magnet systems. We will explore the unique tunability of spin injection and diffusion by molecular design. The second part will focus on unravelling the domain structure in antiferromagnetic thin films.
14 Uhr c.t., Lorentz-Raum 05-127

28.04.22Prof. Dr. Stephan Schlemmer, I. Physikalisches Institut, Universität zu Köln, Germany
Ions play a key role in the chemical evolution of our universe. The process of star and planet formation is tightly connected to the presence and abundance of these species as I will discuss in my presentation. Molecular spectra are diagnostic tools for various astrophysical environments and their temporal evolution. However, laboratory spectra of most ions relevant to astrophysics are not available. Moreover, predicted spectra from ab-initio theory are not nearly accurate enough to guide astrophysical searches. Therefore, laboratory spectra of molecular ions are needed. I will report on progress towards recording high-resolution spectra from the microwave to visible range using our unique and innovative methods of action spectroscopy in ion traps and how in some cases our traditional picture of molecular structure is challenged. References B.A. McGuire, O. Asvany, S. Brünken, and S. Schlemmer, Nature Review Physics (2020) 2, 402–410.
14 Uhr c.t., Lorentz-Raum 05-127

04.05.22Prof. Florian Schreck, University of Amsterdam
Ultracold quantum gases are excellent platforms for quantum simulation and sensing. So far these gases have been produced using time-sequential cooling stages and after creation they unfortunately decay through unavoidable loss processes. This limits what can be done with them. For example it becomes impossible to extract a continuous-wave atom laser, which has promising applications for precision measurement through atom interferometry [1]. I will present how we achieve continuous Bose-Einstein condensation and create condensates (BECs) that persist in a steady-state for as long as we desire. Atom loss is compensated by feeding fresh atoms from a continuously replenished thermal source into the BEC by Bose-stimulated gain [2]. Our experiment is the matter wave analog of a cw optical laser with fully reflective cavity mirrors. The only step missing to create a continuous-wave atom laser beam is the addition of a coherent atom outcoupling mechanism. In addition this BEC may give us access to interesting driven-dissipative quantum phenomena over unprecedented timescales. The techniques we developed to create the continuous source of thermal atoms are also nicely suited to tackle another challenge: the creation of a continuously operating superradiant clock [3,4,5,6]. These clocks promise to become more rugged and/or more short-term stable than traditional optical clocks, thereby opening new application areas. In the second part of my talk I will present how we are developing two types of superradiant clocks within the European Quantum Flagship consortium iqClock [4,5,6].
14:00 Uhr s.t., HS Institut für Kernphysik, at Zoom

Veranstaltungstag und -ort geändert

12.05.22Prof. Dr. Christoph Becher, Universität des Saarlandes
Quantum bits based on solid-state spins are promising and potentially scalable systems for the implementation of quantum technologies ranging from quantum information processing to quantum-enhanced sensing and metrology. Ideally, they combine individually addressable spins with very long coherence times, optical emission spectra with narrow homogeneous and inhomogeneous broadenings and bright single-photon emission. In this respect, impurity-vacancy color centers in diamond based on group-IV elements (SiV, GeV, SnV, PbV) have emerged as interesting systems promising to combine all desired favorable properties. Here, we report on recent experiments on SnV centers where we find promising optical and spin properties: The negatively charged SnV(-) center is a bright single photon emitter, showing a narrow inhomogeneous distribution of zero phonon lines in a high-temperature annealed sample and truly lifetime-limited transition linewidths down to 20 MHz. We explore the charge transition cycle upon resonant excitation which leads to shelving in the dark SnV(2-) state and find that illumination with a second light field in the blue spectral range leads to fast and efficient initialization into the desired negative charge state. The charge-stabilized SnV(-) center exhibits exceptional spectral stability with very small spectral diffusion (4 MHz on a homogenous linewidth of 25 MHz over 1 hour) and promising spin dephasing time (5 µs, measured via coherent population trapping). We discuss possible applications in quantum communication, based on the prospects of coherent spin manipulation and generation of indistinguishable single photons, as well as in quantum sensing by exploiting the option of all-optical sensing schemes. J. Görlitz et al., Coherence of a charge stabilised tin-vacancy spin in diamond, npj Quantum Inf. 8, 45 (2022).
14 Uhr c.t., Lorentz-Raum 05-127, at Zoom

19.05.22Dr. Ronald Ulbricht, Max-Planck-Institut für Polymerforschung
The negatively-charged nitrogen-vacancy defect (NV–) possesses an interesting combination of spin and optical properties that can potentially be exploited in applications such as solid-state qubits, highly sensitive electric and magnetic field probes and single-photon emitters. Within the diamond bandgap, the NV– centre forms an optically accessible two-level quantum system which consists of a spin-triplet ground state of 3A2 symmetry and a spin-triplet excited state of 3E symmetry. Two more electronic levels, both being spin-singlet states (1E and 1A1), are situated within the bandgap. NV centres can also exist in the neutral charge state (NV0). The predominantly utilized feature of the NV– centre is the spin-triplet 3A2 state that can be manipulated with microwave radiation and its spin state read out via the PL yield of the triplet transition as optically detected magnetic resonance (ODMR). Recently, photoelectric detection of magnetic resonance (PDMR) has been demonstrated as an alternative that utilizes state-selective ionization of NV– to NV0 and photocurrent detection. Despite being one of the best-studied solid-state defects, the non-equilibrium dynamics of NV centres are not yet fully understood, particularly with respect to charge conversion. We present results using time-resolved spectroscopic techniques such as transient absorption spectroscopy, photocurrent spectroscopy and THz time-domain spectroscopy to investigate the dynamics of ensembles of NV centers in bulk diamond after photoexcitation by probing the transient response of its optical signatures. Two separately wavelength-tunable femtosecond pulses (450-1040nm) for excitation, combined with broadband spectral probing (400-1650nm) over timescales reaching from fs to ms enable us to probe all relevant optical transitions in a time-resolved fashion, providing a direct measure of complex processes such as photoionization and charge conversion. Variation of the concentration of single substitutional nitrogen (Ns) in different samples permits us to characterize their influence on NV dynamics. We probe the electronic dynamics of both NV0 and NV– centres. For the latter one, we characterize the whole spin polarization cycle and find two additional localized electronic states. We find that recombination of electrons from the conduction band after photoionization of NV– via 3E proceeds through two distinct relaxation channels. Using photocurrent spectroscopy, we also experimentally determine the photon energy threshold for photoionization from 3E.
14 Uhr c.t., IPH Lorentzraum (05-127)

02.06.22Prof. Dr. Peter Krüger, PTB Berlin
Magnetic fields are ubiquitous in nature and since a long time also in technology. Yet, there are many open questions, needs for research and emerging new applications. Standards need to be set or refined, and more accurate calibrations are required by industrial adopters of new technologies. A particular challenge and opportunity arise at the lowest end of the spectrum of magnetic fields. With demonstrated measurement sensitivities beyond the femtotesla (per root Hertz) scale, the neuronal activities of the brain following a peripheral nerve stimulus become detectable in a single trial, for example. While even the foundations of physics can be tested at the frontier of lowest metrological noise floors, a current trend is to make magnetic field measurement and imaging viable in application contexts beyond quantum physics laboratories. Here, we will discuss such developments in terms of sensor developments, measurement environments and key use cases. We will focus on atomic gas-based probes of DC and slowly varying magnetic fields. With trapped ultracold gases, high resolution field mapping can be achieved with relevance to material developments such as indium tin oxide replacements for next-generation touch screens and solar panels. On the other hand, cells containing thermal atomic vapours can provide highest field sensitivities as part of optically pumped magnetometers with use in clinical neurology or current-density imaging in electric vehicle batteries.
14 Uhr c.t., Lorentz-Raum 05-127

09.06.22Dr. Danila Barskiy, Johannes Gutenberg Universität
Nuclear magnetic resonance (NMR) is undergoing renaissance: the advent of hyperpolarization techniques allows enhancing weak nuclear signals by orders of magnitude, making possible measurements which were previously considered impractical, e.g., monitoring metabolism in vivo. Since large magnetic fields are no longer necessary to generate large nuclear magnetization, non-conventional signal detection approaches are now being considered as an alternative to conventional, inductive schemes. In my talk, I will focus on our recent work demonstrating that parahydrogen-based spin chemistry can be used for generating hyperpolarized analytes for low (<1 tesla) and ultralow-field (<100 nT) NMR. The presented method is applicable to a wide range of small molecules possessing exchangeable protons and have potential applications for chemical analysis and measuring NMR spectra from natural extracts and biological fluids using portable spectroscopic tools.
14 Uhr c.t., Lorentzraum 05-127

23.06.22Assoc. Prof. Dr. Hartmut Häffner, University of California, Berkeley, USA
I will discuss two experiments recently completed at UC Berkeley. In the first, we couple two ions via image charged induced in a 0.6 mm long wire to each other. While the efforts were aimed at establishing the much needed interface between individual quantum processors, the coupling can be used to cool, control and detect particles not accessible laser control and are thus relevant for precision metrology as recently shown by the BASE collaboration for cooling (anti)protons. In the second set of experiments, I will discuss a test of a nonlinear extension to quantum mechanics. Already in the early 80's, S. Weinberg and others were wondering whether the laws of nature at the quantum scale are nonlinear (Ann.Phys. (N.Y.), 194, 336-386 (1989)). Interest dropped rapidly when it was shown that those extensions violate causality. However, recently Kaplan and Rajendran (arXiv:2106.10576 [hep-th]) managed to add nonlinear and state-dependent terms without violating causality. Interestingly this extension rendered the existing experimental tests ineffective. I will discuss why a quantum mechanical object tied to a macroscopic object (such as an ion trap) provides a more stringent test to Rajendran and Kaplan's hypothesis and present experimental bounds ruling it out at the 1E-11 level.
14 Uhr c.t., Lorentz-Raum (05-127)

30.06.22Prof. Dr. Reinhold Walser, TU Darmstadt, Institut für Angewandte Physik
We will present the efforts to model realistic cold quantum gas experiments in (3+1)D for long times and large distances for the use in precision interferometry on ground, in the ZARM drop tower and in orbit.
14 Uhr c.t., Lorentzraum 05-127

07.07.22Prof. Dr. Svetlana Malinovskaya, Stevens Institute of Technology, New Jersey, USA
Atoms in their highly excited electronic states, referred to as Rydberg atoms, have extraordinary nonlinear optical properties. Such atoms are highly polarizable and interact with each other via the dipole-dipole or the van-der-Waals interactions. Owing to these interactions, Rydberg atoms in optical traps possess the condensed matter-like collective behavior. They serve as a viable platform to study quantum many-body physics. Spin degrees of freedom of trapped Rydberg atoms bring rich new physics including quantum magnetism, quantum phases, and entanglement - a crucial resource in many quantum information and quantum communication tasks. In this talk, I will present a study of alkali rubidium atoms trapped in an optical lattice and controllably excited to the Rydberg states by linearly chirped laser pulses [1]. I will introduce a quantum control methodology to create entangled states of two typical classes, the W and the Greenberger-Horne-Zeilinger (GHZ) [2]. I will show that the entangled states of Rydberg atoms can be used to create the multiphoton entangled radiation states in a cavity and in free space [3]. The methodology exploits chirped-pulse adiabatic passage and provides a key step toward the resolution of a general problem of creating entanglement in high-dimensional quantum entities.
14 Uhr c.t., Lorentz-Raum 05-127

14.07.22Dr. Robert Löw, GEPRIS Universität Stuttgart
The research and the spectroscopy of hot vapors carries great potential, ranging from fundamental research to robust applications in the context of quantum technologies. In the past decades the spectroscopy of atomic and molecular gases at room temperature has lost some attention due to the focus on cold atomic systems. Still, due to their experimental simplicity, their robustness, and their fundamental nature, they hold the promise to realize real-world quantum devices. Their narrow-band transitions and high optical depths enable such vapor cell science to implement excellent sensors, reference and metrologic devices or building blocks in quantum optics. In this talk I will focus on optical non-linearities induced by atom-atom interactions, either by highly excited Rydberg states or for low lying states via the resonant dipole-dipole interactions. These non-linearities are manifest at the single photon level and can be exploited to generate and process non-linear light fields. As a platform we us a variety of cell types, where the most advanced ones involve integrated photonic waveguides and microresonators.
14 Uhr c.t., IPH Lorentzraum 05-127

21.07.22Prof. Dr. Giovanna Morigi, Universität des Saarlandes
In this talk I will discuss the role of noise in determining the topology of a neural network, whose dynamics is governed by biologically inspired model. I will then give an outlook on how these concepts could be implemented for engineering quantum target dynamics via a noisy environment.
14 Uhr c.t., Lorentz-Raum 05-127

Koordination: Kontakt:

Dr. Arne Wickenbrock
Institut für Physik und HIM Mainz

Andrea Graham
Institut für Physik