QUANTUM-Seminar

I discuss the concept of boundary time crystals, where the continuous time-translation symmetry breaking occurs only in a macroscopic fraction of a many-body quantum system. After introducing its definition and properties, we discuss in detail a solvable model where an accurate scaling analysis can be performed. The existence of the boundary time crystals is intimately connected to the emergence of a time-periodic steady state in the thermodynamic limit of a many-body open quantum system. I will also discuss the spreading of genuine multipartite correlations (GMC's) on such phases, showing results both for the (i) the structure (orders) of GMC's among its subsystem constituents, as well as (ii) their build-up dynamics for an initially uncorrelated state.

The toolkit of quantum technologies developed in atomic, molecular and optical physics are ideally suited to enhance the search for dark matter axions with masses above ~40 µeV. I will present an overview of a new experimental effort under construction at Imperial College, developing technologies to detect DFSZ axions with masses 120-250 µeV. We plan to use a large mode area Fabry-Perot cavity to efficiently convert axions into microwave photons. Compared to other geometries, the Fabry-Perot cavity can present a large mode volume and high Q, and can be easily tuned. To detect the microwaves, we will use an electron in a Penning trap as a single photon counter. Individual microwave absorption events will change the cyclotron state of the electron, causing measurable shifts in the trapped particle’s oscillation frequencies. This versatile device will also open other possible detection routes for alternative dark matter candidates and cosmological phenomena.

Excellent experimental control over quantum systems is increasingly bringing applications in quan-tum technologies within reach. In my talk, I will present our work on quantum technology applica-tions in various physical systems. Ultracold atoms have proven to be excellent platforms for studying quantum effects. In recent years, we have succeeded in introducing single Cs atoms as controlled impurities in an ultracold gas of Rb atoms. Spin-exchange collisions allow a very controlled transfer of energy quanta, and we use this transfer to operate the single atom as a machine in a magnetic field gradient. In another exper-imental setup, we address whether the significant energy differences resulting from the Pauli prin-ciple between ensembles of different fermionic and bosonic quantum statistics can be used as a novel form of energy to drive a quantum machine. Finally, I will present two new projects in the field of quantum technology. First, we are studying nanocrystals of diamond with a large number of NV centers in terms of collective effects and track-ing how the typical signatures in fluorescence lifetime and photon statistics change when larger agglomerates of nanocrystals show the transition to a bulk-like material. Second, I report on a new BMBF collaborative project, the quantum computing demonstrator pro-ject Rymax, which will simulate optimization problems from logistics and industry expressed as gra-phene problems on an array of single Yb atoms with Rydberg excitations.

Light pulses are an excellent tool to manipulate atoms, so that they move in superposition of different trajectories through space and time. In analogy to the concept of an optical interferometer, these branches can be brought to interference and used as sensors for gravity and other inertial forces. As such, atom interferometry has become a versatile tool technique high-precision quantum metrology, which ranges from gyroscopes to gravitational wave detection, as well as a testbed for the interface of relativity and quantum mechanics. At the same time, atoms not only possess a center-of-mass motion, but also internal degrees of freedom that are the very basis for atomic clocks. Making use of this additional property, one can in principle generate atomic clocks that move in superposition of different branches, interfere with each other, and therefore constitute a different probe of relativistic effects linked to time. This colloquium gives an introduction into the main concepts of atom interferometry and the toolbox necessary to manipulate atoms. While we explain basic examples of atom interferometers and state-of-the-art experiments, we also discuss current and ambitious proposals for high-precision tests of fundamental physics.

We will summarize recent progress made within our group on self-assembled quantum dot device development for quantum repeater and quantum computer applications. A particular emphasis will be on semiconductor quantum dots embedded in circular Bragg grating cavities. For scalability, spatially deterministic placement of quantum dots in bullseye cavities is pursued and tuning by electric and strain fields are implemented. To apply electric fields, a new device design for electrically contactable circular Bragg grating cavities in labyrinth geometry is employed. In(Ga)As/GaAs quantum dots (QDs) are very attractive candidates to confine single excitons and single spins serving as solid state qubits in a mature semiconductor platform. While these qubits can be directly manipulated by optical means, both optical and electrical excitation of the QDs can be implemented to efficiently generate single photons or entangled photon pairs on demand. Light-matter interaction in coupled quantum dot-cavity systems can be widely controlled by embedding the QDs into microcavities. In this presentation, we will summarize recent progress made within our group and plans on device development with self-assembled quantum dots intended for quantum repeater and quantum computer applications [1]. A particular emphasis will be on semiconductor quantum dots embedded in circular Bragg grating cavities [2,3]. For scalability, spatially deterministic placement of quantum dots in bullseye cavities is pursued and techniques for tuning by electric and strain fields are implemented. To apply electric fields, a new device design for electrically contactable circular Bragg grating cavities in labyrinth geometry is employed [4]. We report on the challenges experienced in obtaining high performance devices based on circular Bragg grating cavities and figures of merits achieved, outlining the prospects for these devices in quantum technology applications. We are grateful for financial support of this work by the German Federal Ministry of Education and Research (BMBF) within the projects Q.Link.X, QR.X, MHLASQU, PhotonQ and QD-E-QKD. Expert technical assistance by Silke Kuhn, Adriana Wolf and Margit Wagenbrenner is gratefully acknowledged. 1. C.-Y. Lu and J.-W. Pan, Nature Nanotechnology 16, 1294-1296 (2021) 2. J. Scheuer and A. Yariv, IEEE J. Quantum Electron. 39, 1555-1562 (2003) 3. M. Davanco, M. T. Rakher, D. Schuh, A. Badolato, and K. Srinivasan, Appl. Phys. Lett. 99, 041102 (2011) 4. Q. Buchinger, S. Betzold, S. Höfling and T. Huber-Loyola, Appl. Phys. Lett. 122, 111110 (2023)

Chemical exchange processes play an important role for various soluble molecular systems with cavities. Hyperpolarized 129Xe gives insights into the underlying kinetics and structure parameters by providing a spin system that comes with several NMR spectroscopic advantages. It contributes to the understanding of synthetic molecules for binding greenhouse gases or characterizes hydrophobic pockets in naturally occurring proteins. This talk gives an overview how 129Xe is a powerful probe to explore such cavities. The nuclear spin of an inert gas is a useful “spy” that reveals, e.g., hidden states of different molecular symmetry. It also enables “spin counting” to quantify the attoliter volume in hollow protein structures. Such structures, normally used by bacteria to adjust their buoyancy in water, may one day also improve medical magnetic resonance imaging. Bacterial gas vesicles fill up with the harmless noble gas xenon according to the ideal gas law and have the potential to serve as powerful MRI contrast agents.

Penning traps are used to perform motional-frequency measurements to deliver the most precise and accurate cyclotron-frequency values of many atomic/molecular ions and charged (anti)particles. By applying laser cooling, it is possible to better control those effects arising from deviations of the Penning trap from the ideal configuration. Since laser cooling can be only performed on a few ion species, a single laser-cooled (auxiliary) ion can be used to cool the target ion, forming an ordered structure along the magnetic-field axis of the Penning trap. At the University of Granada, we have built a 7-T Penning-trap platform and generated the (unbalanced) Coulomb crystals 42Ca+-40Ca+, 232Th+ -40Ca+ and 232ThO+-40Ca+, establishing the basis to perform quantum Penning-trap mass spectrometry and with prospects to explore laser-spectroscopy. In this talk, I will present the first Penning-trap eigenfrequency measurements using two-ion Coulomb crystals, from the detection of fluorescence photons, and will show the work carried out towards the final goal in our lab, including the recent upgrade of the laboratory after the installation of a new cryogen-free magnet.

Entanglement-based quantum networks hold out the promise of new capabilities for secure communication, distributed quantum computing, and interconnected quantum sensors. However, only a handful of elementary quantum networks have been realized to date. I will present results from our prototype network, in which two calcium ions are entangled with one another over a distance of 230 m, via a 520 m optical fiber channel linking two buildings. The ion-ion entanglement is based on ion-photon entanglement mediated by coherent Raman processes in optical cavities. I will discuss the advantages of trapped ions for quantum networks and the role that cavities can play as quantum interfaces between light and matter at network nodes. After examining the key metrics for remote entanglement, we will consider the necessary steps to extend this work to long-distance networks of entangled quantum processors.

Because of atomic hydrogen’s simplicity, its energy levels can be precisely described by theory. This has made hydrogen an important atom in the development of quantum mechanics and quantum electrodynamics (QED). While one can use hydrogen spectroscopy to determine the Rydberg constant and the proton charge radius, a discrepancy of these constants determined through different transitions, or in different species, can indicate new physics. Such discrepancies currently persist between different measurements in hydrogen and muonic hydrogen. With this motivation in mind, I will discuss several precision spectroscopy measurements of hydrogen as Colorado State University including a relatively recent measurement of the hydrogen 2S-8D two-photon transition, a measurement of the hydrogen 2S hyperfine splitting, and our future plans to measure several relatively narrow 2S-nS transitions in hydrogen. If these latter measurements are successful, they could provide some of the most precise measurements of the Rydberg constant along with insight into the experimental discrepancies.