# International Conference on Quantum Cooperativity of Light and Matter 2023

**Date:** *October 10 – 13, 2023*

**Venue:** Lecture Hall H, Physics Department, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen

**Organizer:** TRR 306 QuCoLiMa

**Invited Speakers: **(in alphabetical order)

- Monika Aidelsburger (LMU Munich)
- Jacqueline Bloch (C2N – Université Paris-Saclay, CNRS)
- Thomas Ebbesen (Univ. of Strasbourg)
- Francesca Ferlaino (Univ. Innsbruck and IQOQI)
- Gerhard Rempe (MPQ Munich)
- Roderich Moessner (MPI for the Physics of Complex Systems)
- Tilman Pfau (Univ. of Stuttgart)
- Arno Rauschenbeutel (HU Berlin)
- Helmut Ritsch (Univ. of Innsbruck)

**Program:**

## Abstracts Invited Talks

### Monika Aidelsburger (MPQ & LMU München)

**Quantum simulation – Engineering & understanding quantum systems atom by atom**

*The computational resources required to describe the full state of a quantum many-body system scale exponentially with the number of constituents. This severely limits our ability to explore and understand the fascinating phenomena of quantum systems using classical algorithms. Quantum simulation offers a potential route to overcome these limitations. The idea is to build a well-controlled quantum system in the lab, which represents the problem of interest and whose properties can be studied by performing controlled measurements. In this talk I will introduce quantum simulators based on neutral atoms that are confined in optical arrays using laser beams. State-of-the-art experiments now generate arrays of several thousand particles, while maintaining control on the level of single atoms. I will show how these systems can be used to study the properties of topological phases of matter and to address fundamental questions regarding the thermalization of isolated quantum systems. In the end I will provide a brief outlook on new directions in the field based on the unique properties of alkaline-earth(-like) atoms.*

### Jacqueline Bloch (Université Paris-Saclay)

**Topological physics in non-linear polariton lattices**

*Semiconductor microcavities arranged into 1D or 2D lattices provide a versatile photonic platform to emulate Hamiltonians and probe their physical properties. The specificity of this system comes from its openess (the system presents loss and can be optically driven in a fully controlled way) and its huge Kerr nonlinarity.*

*In the present talk, I will explain how by engineering site by site the way we drive a polarito lattice, we can produce well controlled non-linear steady states. Some steady states are particularly interesting because they can modify the topological properties of the system, or induce topological properties in an otherwise trivial lattice.*

### Thomas Ebbesen (Université de Strasbourg)

**Manipulating Matter by Strong Coupling to the Vacuum Field**

*Over the past decade, the possibility of manipulating material and chemical properties by using hybrid light-matter states has stimulated considerable interest [1-3]. Such hybrid light-matter states can be generated by strongly coupling the material to the spatially confined electromagnetic field of an optical resonator. Most importantly, this occurs even in the dark because the coupling involves the electromagnetic fluctuations of the resonator, the vacuum field. After introducing the fundamental concepts, examples of modified properties of strongly coupled systems, such as chemical reactivity, charge and energy transport, superconductivity and magnetism, will be given to illustrate the broad potential of light-matter states.*

*[1] F.J. Garcia Vidal, C. Ciuti, T.W. Ebbesen, Science 373, eabd336 (2021)*

*[2] C. Genet, J. Faist, T.W. Ebbesen, Physics Today 74, 42 (2021)*

*[3] K. Nagarajan, A. Thomas, T.W. Ebbesen, J. Am. Chem. Soc. 143, 16877 (2021)*

### Francesca Ferlaino (Universität Innsbruck)

**Rotating dipolar quantum gases**

*The talk will focus on the latest results of our research on ultracold dipolar quantum gases in Innsbruck. In particular, we will focus on the creation of quantized vortices in both the BEC [1] and in two-dimensional circular supersolid phases [2-3]. While in condensates, the density is nearly homogeneous and the vortices are almost free to move, in supersolids, a state in which local density maxima and minima alternate periodically with a wavelength comparable with the very radius of the vortex core, the vortices find intersize equilibrium positions and experience a pinning force that limits their motion. Our experimental protocol uses an ultracold quantum gas of dysprosium atoms as the main resource, which is put into rotation by exploiting the new magnetostirring technique in which the atoms follow the rotational motion of an external magnetic field.*

*[1] L. Klaus, T. Bland et al., Nature Physics 18, 1453–1458 (2022).*

*[2] M. A. Norcia, C. Politi et al., Nature 596, 357-361 (2021).*

*[3] T. Bland et al., Phys. Rev. Lett. 128, 195302 (2022).*

### Roderich Moessner (MPI Dresden)

**Structured Random Driving**

*tba*

### Tilman Pfau (Universität Stuttgart)

**Quantum Optics based on dipolar interactions between hot atoms**

*Electrical dipolar interactions between Rydberg atoms are so strong that even for thermal atomic vapor the Rydberg blockade can be observed via the single photon emission of a blockaded ensemble. Additionally, the light induced dipolar interaction between two level atoms, the so-called Lorentz-Lorenz shift, can be observed in very thin cells as a 2D geometry of interacting dipoles as well as in 1D geometries, which are realized in integrated nano-photonic slot waveguides. The latter leads to a substantial and observable Purcell enhancement of the blue shift at telecom wavelengths due to the dipolar interaction. As an outlook we present the concept of an optical single thermal atom detector based on a freestanding photonic crystal cavity that enhances the atom light coupling to the strong coupling regime.*

*These examples of thermal atoms acting as a reconfigurable strongly nonlinear medium in integrated nano-photonic circuits show that quantum optical applications on the single photon level are within reach.*

### Arno Rauschenbeutel (Humbold-Universität zu Berlin)

**Observation of superradiant bursts in a cascaded quantum system**

*Dicke superradiance describes the collective radiative decay of a fully inverted ensemble of two-level atoms. We experimentally investigate this effect for a chiral, i.e., direction-dependent light–matter coupling. Despite a fundamentally different interaction Hamiltonian which has a reduced symmetry compared to the standard Dicke case, we do observe a superradiant burst emission. This is enabled by coupling the atoms to a nanophotonic waveguide, which mediates unidirectional long-range dipole-dipole interactions between the emitters. We excite the atoms by a resonant, fiber-guided probe pulse that is much shorter than the excited state lifetime. We realize strong inversion, with about 80% of the atoms being excited, and study their subsequent radiative decay into the guided modes [1]. The burst occurs above a threshold number of atoms, and its peak power scales faster with the number of atoms than in the case of free-space Dicke superradiance. We measure the first-order coherence of the burst emission and experimentally distinguish two regimes, one dominated by the coherence induced during the excitation process and the other governed by vacuum fluctuations. Our results shed light on the collective radiative dynamics of cascaded quantum many-body systems, i.e., a system in which each quantum emitter is only driven by light radiated by emitters that are further upstream in the cascade. Our findings may turn out useful for generating multi-photon Fock states as a resource for quantum technologies.*

*References*

*[1] C. Liedl, Phys. Rev. Lett. 130, 163602 (2023)*

*[2] C. Liedl, arXiv:2211.08940 (2022)*

### Gerhard Rempe (MPQ München)

**Entanglement engineering in Quantum Networks**

### Helmut Ritsch (Universität Innsbruck)

**Minimalistic efficient quantum devices build of dipole coupled nano arrays of quantum emitters**

*An array of closely spaced, dipole coupled quantum emitters exhibits collective energy shifts as well as super- and sub-radiance with characteristic tailorable spatial radiation patterns. As a striking example we identify a sub-wavelength sized ring of exactly 9 identical dipoles with an extra identical emitter with a extra loss channel at the center as the most efficient configuration to deposit incoming photon energy to center without reemission. For very tiny structures below a tenth of a wavelength a full quantum description exhibits an even larger enhancement than predicted from a classical dipole approximation. Adding gain to such systems allows to design minimalistic classical as well as non-classical light sources.*

*On the one hand this could be the basis of a new generation of highly efficient and selective nano antennas for single photon detectors for microwaves, infrared and optical frequencies, while on the other hand it could be an important piece towards understanding the surprising efficiency of natural light harvesting molecules.*

*References: *

*Holzinger, Raphael, Mariona Moreno-Cardoner, and Helmut Ritsch. “”Nanoscale continuous quantum light sources based on driven dipole emitter arrays”, Appl. Phys. Lett. 2021 *

*Holzinger, Raphael, et al. “”Nanoscale coherent light source.”” Physical Review Letters 124.25 (2020): 253603*

## Abstract Laureate Talk

**Exploring spin at unconventional hybrid interfaces
** Angela Wittmann (JGU Mainz)

*Controlled manipulation of a system allows for systematic investigation of the underlying interactions and phenomena. Simultaneously, tunability also enables the development of novel materials systems and devices customized for specific applications. Here, we focus on materials systems that conventionally have not been used as active components in spintronic devices, specifically, hybrid molecule-magnetic interfaces. Molecules offer a unique way of controlling and varying the structure at the interface making it possible to precisely tune the underlying interactions and hybridization at the interface by molecular design. Particularly single-molecule magnets (SMMs) have recently gained significant attention as their multi-level quantum systems promise potential applications in data storage, quantum computing, and spintronics. However, so far, reading and controlling the SMMs has been highly challenging as the functionalization of SMMs on conventional thin-film devices often quenches the quantum character and, thus, the magnetic properties of the molecule due to hybridization effects. Here, we explore the hybridization effects between the SMMs and metal surfaces with the ultimate goal of functionalizing the quantum character in a collective response.*

## Abstracts Contributed Talks

### Petar Andrejic: Waveguide QED with Mössbauer Nuclei

We show that driving the waveguides at forward incidence instead allows for direct excitation of multiple guided modes, with centimetre scale attenuation lengths.

In this regime, the embedded Mössbauer nuclei absorb and emit collectively into a super-position of these modes, with the resultant radiation field displaying pronounced interference beats on a micrometre scale. We show that this interference pattern leads to sub-radiance of the nuclear ensemble, with suppression of the dynamical beat at certain critical waveguide lengths.

We also consider structuring the nuclear ensemble into micrometre scaled patches, and show that it is feasible to engineer the resultant inter-nuclear coupling to create mesoscopic hopping models, with potential for applications in quantum simulation and experimental exploration of mesoscopic quantum dynamics.

### Thomas Basché: Molecular dimers: Electronic coupling and beyond

### Nico Baßler: Metasurface-based hybrid optical cavities for chiral sensing

### Michel Bockstedte: Theory of color centers in SiC towards the coupling of spin, light and mechanics

device and the achievement of cooperative effects such as the spin-spin interaction or superradiance is still

challenging. Not only technological complexity has to be controlled, but also competing physical mechanisms, even at the level of the color center itself,

have to be unraveled. Prototypical color centers such as the silicon vacancy couple light to spin via a fundamental quartet excitation and subsequent competing fluorescence or non-radiative, spin-selective relaxation recombination via low-spin intermediate states back into the ground state with spin sublevels and fine structure. Fine structure and spin relaxation are governed by spin-orbit and spin-spin interaction between the highly correlated defect states. Here we investigate such color centers by ab initio theory using correlated embedding methods (CI cRPA). Enabled by going beyond parameter-based analytic group theory, we provide important insight into the fundamental mechanims towards the coupling of light spin and mechanics.

### Soumya Chakraborty: Levitating the Future: Light, Magnet, and Fiber Unite!

Optical trapping of microparticles [1] in vacuum has emerged as a novel platform to study light-matter interaction free from environmental decoherence and mechanical loss. Magnetic microparticle adds another degree of freedom to the levitated system where the particle’s magnetization dynamics and the particle’s magnetically induced mechanical motion can be efficiently probed with light [2]. Lately, using a dual beam optical trapping scheme, we have successfully trapped 1 µm-diameter spheroidal YIG-like magnetic particle inside a twisted single-ring hollow-core photonic crystal fiber [3]. By applying an external static magnetic field on the trapped particle, we were able to observe rotational anisotropy of magnetic linear birefringence [4], originating from the rotation of the magnetic microparticle in the optical trap. Our experiment paves the way for subsequent studies related to quantum cooperativity at the single photon, phonon and magnon level.

Authors: Soumya Chakraborty, Gordon Wong, Monica Distaso, Ferdi Oda, Vanessa Wachter, Silvia Viola Kusminskiy, Philip Russell, and Nicolas Joly

References:

[1] D. S. Bykov, S. Xie, R. Zeltner, A. Machnev, G. K. L. Wong, T. G. Euser, and P. St. J. Russell, Long-range optical trapping and binding of microparticles in hollow-core photonic crystal fibre,” Light Sci. Appl. 7, 22 (2018).

[2] V. Wachter, V. A. S. V. Bittencourt, S. Xie, S. Sharma, N. Joly, P. St. J. Russell, F. Marquardt, and S. V. Kusminskiy, “Optical signatures of the coupled spin-mechanics of a levitated magnetic microparticle,” J. Opt. Soc. Am. B 38, 3858 (2021).

[3] F. Benabid, J.C. Knight and P. St. J. Russell, (2002). Particle levitation and guidance in hollow-core photonic crystal fiber. Opt. Expr. 10, 1195 (2022)

[4] W. Wettling, “Magnetooptical properties of YIG measured on a continuously working spectrometer,” Appl. Phys. 6, 367 (1975).”

### Günter Ellrott: Collectively Enhanced Giant Circular Dichroism of Germanium Nanohelix Square Lattice Arrays

[1] G. Ellrott, P. Beck, V. Sultanov, S. Rothau, N. Lindlein, M. Chekhova, V. Krstić, Adv. Photonics Res. 2023, DOI: 10.1002/adpr.202300159.

### Paul Fadler: Controlling long-range interactions by off-resonant driving

### Claudiu Genes: Cooperative quantum optics with molecules

While subwavelength separations are not easily achieved in standard quantum optics setups, molecular dimers and molecular aggregates (i.e.~arrays of identical molecules, such as J- and H-Aggregates) can feature deeply subwavelength separations on the nanometer scale. The downside of such systems is the much more complex structure, which introduces coupling of electronic degrees of freedom with intra- and inter-molecular vibrations. We have introduced a quantum Langevin equations approach to electron-vibron interactions for single molecules subject to either classical or cavity quantum light fields [2]. The extension of this method to many particles allowed us to benchmark the scaling of cooperative effects such as super- and subrradiance to molecular rings or chains, to quantify the effect of vibrations onto the operation of such systems as nanoscale coherent light sources [3] and to quantitatively describe couplings among collective electronic states via vibrations, in a process known as Kasha’s rule [4].

[1] M. Reitz, C. Sommer, and C. Genes, Cooperative Quantum Phenomena in Light-Matter Platforms, PRX Quantum 3, 010201 (2022).

[2] M. Reitz, C. Sommer and C. Genes, Langevin approach to quantum optics with molecules, Phys. Rev. Lett. 122, 203602 (2019).

[3] R. Holzinger, S. Oh, M. Reitz, H. Ritsch and C. Genes, Cooperative subwavelength molecular quantum emitter arrays, Phys. Rev. Research 4, 033116 (2022).

[4] R. Holzinger, N. S. Bassler, H. Ritsch and C. Genes, Scaling law for Kasha’s rule in photoexcited subwavelength molecular aggregates, arxiv: 2304.10236 (2023).

### Jonas Heimerl: Correlation and number statistics of free electrons triggered from metal needle tips

[1] S. Meier, J. Heimerl, P. Hommelhoff, Nat. Phys. 2023, https://doi.org/10.1038/s41567-023-02059-7

[2] R. Haindl, A. Feist, T. Domröse, et al., Nat. Phys. 2023, https://doi.org/10.1038/s41567-023-02059-7

[3] J. Heimerl, A. Mikhaylov, S. Meier, H. Höllerer, I. Kaminer, M. Chekhova, P. Hommelhoff, arXiv:2307.14153

### Jan Alexander Koziol: Quantum Phases of Dipolar Bosons

We study the particles on the bipartite square and honeycomb lattice, as well as on the non-bipartite triangular lattice.

We determine the quantum phase diagrams using a mean-field approach based on classical spins.

We describe a general approach to analyse diagonal ordering patterns in bosonic lattice models with algebraically decaying density-density interactions on arbitrary lattices.

The key idea is a systematic search for the energetically best order on all unit cells of the lattice up to a given extent.

Using resummed couplings we evaluate the energy of the ordering patterns in the thermodynamic limit using finite unit cells.

Our method provides a general framework to treat cristalline structures resulting from long-range interactions.

### Marjan Macek: Variational Monte Carlo in the Heisenberg picture

A considerate development in recent years has been the application of neural networks as the wave-function ansatz. Nevertheless, the time evolution of parametrized wave functions remains challenging. In this talk, we present our work on VMC in the Heisenberg picture, where, instead of the wave function, operators are parametrized and evolved in time.

### Raphaël Menu: Fractal ground state of one-dimensional Wigner crystals in periodic potentials

### Ralf Röhlsberger: New regimes in nuclear cooperative emission

The propagation of resonant radiation along the optical axis of an X-ray waveguide constitutes a special realization of the ‘super of superradiance’, conceptually introduced by Scully and coworkers to illustrate the physics of cooperative emission upon propagation in widely extended samples. A recent experimental realization of this regime involves front-coupling of X-rays into a single-mode planar waveguide containing ultrathin 57Fe layers.

Cooperative emission in forward scattering geometry through a set of N resonant scatterers, de-tuned at N equally spaced Doppler velocities, constitutes a Doppler frequency comb. This allows us to realize a nuclear quantum memory capable of storage and delayed release of X-ray pulses with a controlled temporal pulse shape, realized recently using high-brilliance synchrotron radiation at PETRA III (DESY, Hamburg) and ESRF (Grenoble).

A new regime of cooperative emission is encountered at X-ray free electron laser sources, where each pulse may contain hundreds of nuclear resonant photons per mode. Scattering of such pulses from an ensemble of Mössbauer atoms constitutes a new regime of cooperativity of light and mat-ter formed as a hybrid state of indistinguishable field quanta (photons) and matter quanta (atomic emitters).

Finally, if multiphoton cooperative emission is analyzed for higher-order photon correlations, fasci-nating perspectives emerge for X-ray imaging with electronic and nuclear fluorescence photons. Recent experiments at the European XFEL set a new benchmark in this field.

### Kai Phillip Schmidt: Competing light-matter assemblies

### Ferdinand Schmidt-Kaler: Collective light scattering of ion crystals to reveal spin-order

Crystals of trapped ions are excited with laser light. In a proper setting of experimental conditions, they act as near-to-prefect single photon emitters [1]. We record the photon correlation functions in space and time. We have demonstrated how the detection of first single photon can profoundly modify the collective spontaneous emission dynamics [2]. Moreover, we implement spin-dependent scattering and demonstrate signatures of spin-order.

[1] F. Schmidt-Kaler, J. von Zanthier, Collective Light emission of ion crystals in correlated Dicke states, in Photonic Quantum Technologies – Science and Applications, ISBN: 978-3-527-41412-3, Wiley-VCH, Berlin (2023)

[2] S. Richter, S. Wolf, J. von Zanthier, F. Schmidt-Kaler, Phys. Rev. Research 5, 013163 (2023)

### Tom Schmit: Quantum Kinetic Theory of Self-organization in Many-body Cavity QED

### Stella Stopkowicz: Ab initio modelling for molecular polaritons with and without an external magnetic field

Strong coupling can occur by either cooling the system to minimize the energy loss to the continuum, or by reducing the cavity volume such that the light-matter interaction is maximized leading to large Rabi splittings.

While a lot of research has focused on model Hamiltonians, some progress has also been made in terms of rigorous ab-initio formulations of the respective theory combining quantum electrodynamics with the highly accurate coupled cluster methods. [2]

Electronic states are also tunable by the application of a strong magnetic field. This leads to exciting chemistry and new bonding mechanisms [3] for which finite magnetic field coupled-cluster methods have been developed. [4-6]

Using both the cavity as well as the magnetic field as regulators, the aim is to explore the control over reactivity via investigating ground and excited states as well as molecular properties.

Within the dipole approximation, a scheme to calculate the energy of a molecule subject to a cavity has been developed and implemented at the QED-coupled-cluster level of theory including the option to switch on a finite magnetic field.

Here, preliminary results on the dispersion interaction of two H2 molecules subject to a cavity as well as a finite magnetic field are presented and the proposed further directions are discussed.

[1] T. W. Ebbesen, Acc. Chem. Res., 49, 2403 (2016)

[2] T. S. Haugland, E. Ronca, E. F. Kjønstad, A. Rubio and H. Koch, Phys. Rev. X, 10, 041043 (2020)

[3] K. K. Lange, E. I. Tellgren, M. R. Hoffmann, T. Helgaker, Science, 337, 6092 (2012)

[4] F. Hampe, S. Stopkowicz, J. Chem. Phys. 146, 154105 (2017)

[5] F. Hampe, S. Stopkowicz, J. Chem. Theory Comput. 15, 4036 (2019)

[6] F. Hampe, N. Gross, S. Stopkowicz, Phys. Chem. Chem. Phys. 22, 23522 (2020)

### Riccardo J. Valencia Tortora: A Rydberg platform for non-ergodic chiral quantum dynamics

### Silvia Viola Kusminskiy: Cavity Magnomechanics: Harnessing the Magnomechanical Coupling for Applications in the Microwave and Optical Regimes

### Joachim von Zanthier: Collective photon emission of correlated sources in the optical and the x-ray domain

For suitable detector positions and if the detection is unable to identify the individual photon sources, the ensemble cascades down the ladder of symmetric Dicke states each time a photon is recorded [1-5].

We apply this scheme to demonstrate collective super- and subradiance in the optical and the x-ray domain using trapped ions and incoherent light sources at 13.2 nm, respectively [6,7].

[1] C. Skornia et al., Phys. Rev. A 64, 063801 (2001).

[2] C. Thiel et al., Phys. Rev. Lett. 99, 193602 (2007).

[3] S. Oppel et al., Phys. Rev. Lett. 113, 263606 (2014).

[4] R. Wiegner et al., Phys. Rev. A 92, 033832 (2015).

[5] F. Schmidt-Kaler, J. von Zanthier, Collective Light emission of ion crystals in correlated Dicke states, in Photonic Quantum Technologies – Science and Applications, ISBN: 978-3-527-41412-3, Wiley-VCH, Berlin (2023)

[6] S. Richter, S. Wolf, J. von Zanthier, F. Schmidt-Kaler, Phys. Rev. Res. 5, 013163 (2023).

[7] T. Mährlein et al., to be published

### Abstracts Posters

### 1 - Yanis Abdedou: Generation of localized silicon vacancies in 4H-SiC

### 2 - Patrick Adelhardt: Quantum criticality of Heisenberg systems with long-range interactions

### 3 - Subrahmanya Siddardha Chelluri: Bosonic Quantum Error Correction (BQEC) for Quantum Repeaters and Networks with Binomial Codes

However, real memories have limited coherence times and therefore we apply BQEC to increase the effective lifetimes of the memories.

### 4 - Antonia Duft: High-order series expansions and crystalline structures for Rydberg atom arrays

expansions. We extend our investigations to the high- and low-field limit of the paradigmatic transverse-field Ising model, which is contained within the model of Rydberg atoms for a specific line in parameter space.

### 5 - Fabian Engelhardt: Magnon-phonon interaction for quantum information processing

[1] F. Engelhardt, V. A. S. V. Bittencourt, H. Huebl, O. Klein and S. Viola Kusminskiy, Phys. Rev. Appl. 18, 04059 (2022)

[2] M. Mueller, J. Weber, F. Engelhardt, V.A.S.V. Bittencourt, T. Luschmann, M. Cherkasskii, S. T. B. Goennenwein, S. Viola Kusminskiy, S. Gepraegs, R. Gross, M. Althammer and Hans Huebl, arXiv:2303.08429 (2023)

### 6 - Gage Harmon: Out-of-Equilibrium Dynamics and Phases of an Atomic BEC Coupled to an Optical Cavity

### 7 - André Hochreiter: Towards spin-phonon coupling in 4H-SiC: Fabrication and eigenfrequency analysis

### 8 - Sebastian Karl: Synchrotron tests of TEMPUS - a Timepix4 readout system for photon science experiments

### 9 - Emma King: Reservoir engineering quantum adiabatic dynamics

### 10 - Jan Alexander Koziol: Systematic analysis of crystalline phases in bosonic lattice models with algebraically decaying density-density interactions

We investigate the ground-state properties of the antiferromagnetic long-range Ising model on the triangular lattice and determine a six-fold degenerate plain-stripe phase to be the ground state for finite decay exponents. We also probe the classical limit of the Fendley-Sengupta-Sachdev model describing Rydberg atom arrays. We focus on arrangements where the atoms are placed on the sites or links of the Kagome lattice. Our method provides a general framework to treat cristalline structures resulting from long-range interactions.

### 11 - Calvin Krämer: Quantum-critical properties of random transverse-field Ising models extracted by quantum Monte Carlo methods

### 12 - Vojislav Krstic: Quantum helix: Two-fold spin-anisotropic transport and its quantum optics analogs

### 13 - Anja Langheld: Quantum Monte Carlo simulation of the Ising model in a light-induced quantized transverse field

The method is applied to the unfrustrated antiferromagnetic qTFIM in the presence of a longitudinal field, for which mean-field considerations [2] suggest a rich quantum phase diagram including continuous phase transitions and a nontrivial intermediate phase. Our numerical findings confirm the presence of this intermediate phase. However, the extent of this phase is much smaller than anticipated and certain phase transitions turn out to be of first order rather than of second order.

[1] J. Rohn et al., Phys. Rev. Research 2, 023131 (2020)

[2] Y. Zhang et al., Sci Rep 4, 4083 (2014)

[3] M. Weber et al., Phys. Rev. Lett. 119, 097401 (2017)

### 14 - Verena Leopold: Towards spatial correlations of A-type stars in the blue using ultra high-rate single photon detectors

### 15 - Ke Li: Coupling and dissipation in a cavity QED system of Yb atoms trapped on a narrow line

### 16 - Siwei Luo: Coupling a Defined Number of Molecules to a Dielectric Antenna

The second part of our project is concerned with molecular dimers. We have performed a detailed localization microscopy study on dibenzanthanthrene (DBATT) molecules, where two molecules are interconnected via a nm-length linker [2]. Our results are the first step towards the routine investigation of cooperative phenomena with molecular emitters.

[1] X.-L. Chu et al., Nature Photonics 11, 58 (2017).

[2] F. Mikhail et al., ChemistrySelect 6, 39 (2021).

### 17 - Julian Lyne: Purcell cavity cooling of quantum emitters

two-level emitter exhibiting a closed electronic transition, this translates into a modification of the cooling rate, dictated by a large single emitter cooperativity. This mechanism can be applied to quantum emitters without closed transitions, which is the case for molecular systems, where the Purcell effect can mitigate the loss of excitation from the cooling cycle. We extend our analytical

formulation to the many particle case governed by weak individual coupling but collective strong Purcell enhancement. We do not find evidence that the collective coupling leads to an improvement in the efficiency of kinetic energy removal at the individual particle level.

### 18 - Stefan Meier: Coulomb-based electron correlations and statistics of ultrashort electron pulses

### 19 - Ipsika Mohanty : Magneto-electric effect in 2D topological materials

### 20 - Adriana Palffy: Topological features and cooperative effects at x-ray frequencies

In this work, possible avenues to exploit topological effects for x-ray quantum control will be discussed. The starting point are x-ray quantum optics phenomena in thin-film cavities interacting resonanty with x-ray light. These are 2D nanostructures which exploit evanescent coupling of x-rays to form a standing wave over the cavity layers. One or more of these layers contain Mössbauer nuclei which are resonantly driven by the cavity x-ray field. In cavities with several such layers with resonant nuclei, the coupling between layers can be controlled via cooperative effects. We investigate theoretically structures with many layers coupled by the cavity field, ideally mimicking a Su-Schrieffer-Heeger model and exploiting topological effects to control x-ray photons.

### 21 - Stefan Richter: Simultaneous super- and subradiant light emission of two stored ion in free space

### 22 - Sayan Roy: Resetting in the quantum dynamics of a particle in a long-ranged tight-binding model and subject to repeated measurements

consider the case of measurements being done both at regular and at random time intervals. In this setup, the ballistic propagation of the particle is found to be constrained by the repeated measurement protocol, which yields a detection probability less than 1. The detection probability is obtained analytically by using a perturbation-theory approach. To get advantage of the ballistic propagation, we extend the model by resetting at a constant rate, and find the optimal resetting rate required to maximize the detection probability. We finally determine the dependence of the detection probability on the range of the interaction.

### 23 - Ansgar Schaefer: Far field observation of coherent and incoherent light scattering from $^{40}$Ca$^+$ ion crystals in the framework of a ladder-system

These phenomenons can be combined to measure interference patterns in the far field of a $^{40}$Ca$^+$ ion-chain, emitting the light at $393$ nm wavelength, while only driving the $S_{\frac{1}{2}} \rightarrow D_{\frac{5}{2}}$ transition with a 729 nm laser, and the $D_{\frac{5}{2}} \rightarrow P_{\frac{3}{2}}$ transition with an 854 nm laser.

To compare to theory, a model for the steady state in the outlined system has been calculated using the Liouville-von-Neumann equation, correctly predicting the frequency dependence of light’s intensity.

The findings suggests that there is a tradeoff between the selectivity of the 729 transition, and a high visibility of the interference patterns.

Furthermore, a question of particular interest is the spectrum of the scattered light, in the coherent and the incoherent cases, as the broader spectrum of the incoherently scattered light should prevents reabsorption effects.

For the poster session, the theoretical foundations of the process will be illustrated with particular attention to the dependence on the various parameters involved, i.e. the frequency, power and polarization of both driving lasers.

As the selectivity of the 729 transition can be used to have the scattering depending on the energy level of the ions, this is a significant finding towards the question whether the spin structure of an ion chain can be determined from their interference pattern.

### 24 - Andreas Schellenberger: (Almost) Everything is a Dicke model

Coming from the limit of low light-matter couplings and large system sizes, we show how to map different variants of the Dicke-Ising model in the low-energy regime onto the Dicke model, which is exactly solvable. The found gap-closing points for the full Dicke-Ising model match with the proposed phase transitions obtained by mean-field theory [1]. We accompany and verify our findings with calculations on finite systems, using exact diagonalization and our newly developed perturbation-theory method pcst++ [3].

[1] J. Rohn et al., Phys. Rev. Research, 2, 2020

[2] Y. Zhang et al., Sci. Rep., 4, 2014

[3] L. Lenke et al., Phys. Rev. A, 108, 2023

### 25 - Maximilian Schober: Effective Spin-Hamiltonians for Color Centers from an Ab Initio Framework

To engineer interfaces for these applications, a comprehensive understanding of the underlying spin physics is essential. The spin-spin and spin-orbit interactions, for instance, contribute to the zero-field splitting and facilitate non-radiative transitions normally prohibited by optical selection rules.

In recent years, progress has been made by constructing effective model Hamiltonians based on fundamental group theoretical analysis [1] and first principles techniques such as Density Functional Theory (DFT). Despite some successes [2], these approaches often fall short due to the inaccessibility of fundamental coupling parameters and the correlated multi-determinantal nature of the low and high-spin multiplets involved.

Here we report progress on an ab initio route based on spin-restricted DFT and CI-cRPA [3], which enables the fully-fledged treatment of the active multiplet structure of color centers beyond the limitation of the former methods. On this basis we describe the spin-spin and spin-orbit coupling of the silicon vacancy crucial for understanding the spin-relaxation path. Even for the well-investigated NV center in diamond, new aspects of the coupling became clear. Our results will pave way for the development of novel coupling scenarios of color centers to resonators.

[1] Soykal, Ö. O., Dev, P., & Economou, S. E. (2016). Silicon vacancy center in 4 H-SiC: Electronic structure and spin-photon interfaces. Physical Review B, 93(8), 081207.

[2] Biktagirov, T., Schmidt, W. G., & Gerstmann, U. (2020). Spin decontamination for magnetic dipolar coupling calculations: Application to high-spin molecules and solid-state spin qubits. Physical Review Research, 2(2), 022024.

[3] Bockstedte, M., Schütz, F., Garratt, T., Ivády, V., & Gali, A. (2018). Ab initio description of highly correlated states in defects for realizing quantum bits. npj Quantum Materials, 3(1), 31.

### 26 - Stephan Schuster: Exploring the phase structure of the multi-flavor Schwinger model in the presence of a chemical potential with quantum computing

We show via numerical simulations that our ansatz is able to resolve the phase structure of the model and can approximate the ground state with high accuracy.

Moreover, we perform proof-of-principle simulation on superconducting, gate-based quantum hardware. Our results show that our approach is suitable for current intermediate-scale quantum hardware and can be readily implemented on existing quantum devices.

### 27 - Saran Shaju: Lasing on narrow lines in trapped cold Yb atoms

### 28 - Sumeet Sumeet: Quantum simulation of the transverse field Ising model in the thermodynamic limit

ational quantum eigensolver (VQE) algorithm. To this end, we use the Hamiltonian variational ansatz for the VQE algorithm to compute the ground-state energy for finite graphs for a lattice model. These graphs are then embedded on the infinite lattice using numerical linked-cluster expansions to approximate the ground state of a lattice model in the thermodynamic limit.

### 29 - Ishan Varma: Towards light scattering experiments from ultracold Dysprosium atoms

collective effects in dense ultra-cold media. With the largest ground-state

magnetic moment of all elements in the periodic table (10 Bohr magnetons),

it offers a platform to study the effect on scattering of

light due to competition between magnetic dipole-dipole interactions

(DDI) and light induced correlations. In a sufficiently dense regime,

the strong magnetic DDI significantly influence the propagation of light

within the atomic sample. In particular, we want to look at signatures

of collective light scattering phenomena like Super-radiance and Subradiance.

This poster reports on the progress made in generating dense samples

of ultracold dysprosium atoms. We plan to optically transport

atoms into a home-built science cell with high optical access. A high

NA custom objective, designed and assembled in-house, will then be

used to create dense atomic samples inside this cell. We evaluate the

performance and discuss the installation of the custom objective in

our experimental system. Further, an outlook is given on future measurements

exploring collective and cooperative effects in the generated

sample.

### 30 - Maurizio Verde: Single ion manipulation with vortex light

### 31 - Vanessa Wachter: Coupled spin mechanics of levitated nanorotors

### 32 - Qingshan Wang: Neuromorphic Computing with Synchronizing systems

synchronizing systems. Here we demonstrate that an artificial neural network consisting of coupled oscillators can be used for neuromorphic computing by testing its ability to learn and do some benchmark tasks. We will continue the research by testing the model with more complex tasks and exploring the effect of architecture, the possibility of physical implementation, and new learning methods.

### 33 - Yannik Weber: Digital Quantum Simulation of the Ising-Dicke-Model

in the investigation of manybody systems. Therefore the advancement of quantum computation presents a

great potential to accelerate any research in this field, since for many calculation tasks in this field quantum

hardware potentially offers a speedup over classical algorithms. In the light of this potential we investigate the

quantum simulation of a paradigmatic model of light-matter interaction, the Dicke-Ising model. The focus in

this work is the simultaneous simulation of spins and bosons and their interaction in a purely digital manner via

variational quantum algorithms. We ultimately want to calculate a phase diagram of the system highlighting

collective effects like superradiance, but we will start with mean field considerations and the calculation of

fluctuation.

### 34 - Benjamin Zenz: Detection of Spin-order from collective photon scattering and new experimental development

The effective spins are encoded within the internal energy levels of the ions. By applying optical

fields, long-range and tunable spin-spin interactions can be generated, and the final quantum state

of the ion chain should be read out [1].

We present a way to efficiently read out the spin order of an ion crystal by detecting collective,

coherent photon scattering in the far-field. The which-way information associated to the position

of the scattering ion is erased and the different photon paths interfere among each other [2].

We use a two-photon process for 40Ca+ ions within a segmented Paul trap. A first laser excite

the valence’s electron of the ion from its ground state to a long-lived metastable state through the

narrow quadrupole transition 4S1/2 ↔ 3D5/2 near 729 nm. A second laser, driving the dipole

transition 3D5/2 ↔ 4P3/2 near 854 nm is used to excite the electron to a short-lived state from

which it decays back to the ground state. By exploiting this optical scheme, it is possible to achieve

spin-dependent and background-free photon scattering. With these additions, it will be possible

to efficiently determine the spin order of the ion crystal based on the interference pattern.

Experimental data demonstrate the appearance two different regimes, depending on the interplay

between the dipole and the quadrupole transition. In one regime the visibility of the interference

pattern is high, in the other the transition is highly spin-selective. Finaly, we show how to find an

optimum out of these two regimes.

[1] C. Monroe, W.C. Campbell, L.-M. Duan, Z.-X. Gong, A.V. Gorshkov, P.W. Hess, R. Islam, K.

Kim, N.M. Linke, G. Pagano, P. Richerme, C. Senko, and N.Y. Yao Rev. Mod. Phys. 93, 025001

(2021)

[2] S. Wolf, J. Wechs, J. von Zanthier, and F. Schmidt-Kaler Phys. Rev. Lett. 116, 183002 (2016)

References

1

### 35 - Laurenz Monzel: Molecules in Cavities: Ab initio QED for Polaritonic wave functions with and without a magnetic field

### 36 - Zyad Shehata: Photon correlations of trapped calcium ion crystal

### Bus shuttle

There will be a bus shuttle connecting the main locations (hotel, venue, evening venues) during the conference.

**Tuesday**

18:00: Lecture Hall -> Get-Together at Kitzmann Bräuschänke (bus for non-locals only)

**Wednesday**

8:30: Novotel -> Lecture Hall (for hotel guests)

21:00: Lecture Hall -> Novotel (for hotel guests)

**Thursday**

8:30: Novotel -> Lecture Hall (for hotel guests)

15:00: Lecture Hall -> Erlangen inner city for City Tour (for city tour participants)

17:30: Erlangen inner city (Busbahnhof Erlangen) -> Conference Dinner at Fischerei Oberle (for all participants)

22:30: Fischerei Oberle -> Novotel/Busbahnhof Erlangen (for all participants)

**Friday**

8:30: Novotel -> Lecture Hall (for hotel guests)

13:30 Lecture Hall -> Erlangen Busbahnhof (for non-locals)

### Locations

Hotel: Novotel, Hofmannstrasse 34, 91052 Erlangen

Get-Together: Kitzmann Bräuschänke, Südliche Stadtmauerstraße 25, 91054 Erlangen

Conference Dinner: Fischerei Oberle, Am Deckersweiher 24, 91056 Erlangen