International Conference on Quantum Cooperativity of Light and Matter 2023

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.

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.

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)

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).

Structured Random Driving
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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.

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)

Entanglement engineering in Quantum Networks

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

Grazing incidence X-ray waveguides have become a well established platform for X-ray quantum optics. In these systems, X-rays are scattered resonantly by Mössbauer transitions in atomic nuclei. Due to the indistinguishability of the nuclei and the recoil-free Mössbauer transititions, the collective emission and absorption of radiation plays a large role. Recently a formalism has been developed to describe the collective nuclear response using the classical electromagnetic Green’s function for the waveguide. However, so far these works have considered only translationally symmetric systems, and plane wave driving fields. In this regime, the spatial structure of the nuclei in the direction of propagation is insignificant, and pure single mode Dicke super-radiance is observed.

 

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.

Quantum coherences between electronically excited molecules are a signature of entanglement and play an important role in energy transport in molecular assemblies. Coherent signatures appear as an enhancement of the purely electronic transitions which is reflected by changes of the fluorescence spectra and lifetimes. Here, we have investigated molecular dimers at the bulk and single molecule level. While dimers composed of two perylene-diimide (PDI) dye molecules show the expected signatures of electronic coupling, additional processes have to be considered for dimers composed of two terrylene-diimide (TDI) molecules. Single molecule data suggest the coherent formation of two triplet states localized at the two TDI molecules out of the delocalized singlet state (singlet fission).

Quantum metasurfaces, i.e., two-dimensional subwavelength arrays of quantum emitters, can be employed as mirrors towards the design of hybrid cavities, where the optical response is given by the interplay of a cavity-confined field and the surface modes supported by the arrays. We show that, under external magnetic field control, stacked layers of quantum metasurfaces can serve as helicity-preserving cavities. These structures exhibit ultranarrow resonances and can enhance the intensity of the incoming field by orders of magnitude, while simultaneously preserving the handedness of the field circulating inside the resonator, as opposed to conventional cavities. The rapid phase shift in the cavity transmission around the resonance can be exploited for the sensitive detection of chiral scatterers passing through the cavity. We discuss possible applications of these resonators as sensors for the discrimination of chiral molecules.

Color centers in semiconductors such as silicon carbide (SiC) enable the implementation of solid-state quantum bits and single photon light sources. Exploring coherent coupling among individual color centers in a single

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.

Abstract:
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).”

Circular dichroism measurements on square lattice arrays of germanium nanohelices (~100 nm feature size) with dissymmetry factors outperforming plasmonic metal-based ensembles are presented[1]. The measured circular dichroism strength depends on the use of either spatially incoherent white light or spatially coherent light from a supercontinuum source. Classical numerical electrodynamic calculations do not reproduce the observed behavior for either light source. The unexpectedly high dissymmetry is attributed to the rise of cooperative coupling between individual helices within the array. Additionally, the numerical calculations do not take quantum mechanical effects into account as are present in germanium at the present length scales. Overall, this leads to an extraordinarily high response of the lattice of dielectric helices. The array thus can be considered as a periodic assembly of chiral nanoresonators, each of which are experiencing a nanocavity effect, which leads to strong chirally selective light-matter interactions.

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

Coupling condensed matter systems to quantum electromagnetic fields allows for mediated long-range interactions. In this paper we show, that the Coulomb repulsion between electrons can be used as intermediary allowing resonant enhancement of these interactions through external classical driving. This is done for the Fermi-Hubbard model at half filling coupled to a quantum electromagnetic field and a time periodic external drive. In the absence of the external drive, long-range interactions in the form of four-spin terms scale subextensively, while in the driven case resonant enhancement and extensive scaling of the former is possible. A comparision between two types of series expansion is made: A second order approach based on a spectroscopic spin-photon Hamiltonian and a fourth order scheme based on the full Hamiltonian. We find that the former approach can only capture the interactions properly under resonant driving of the quantum mode. There exist platforms that are already capable of reaching coupling strengths, where the mediated long-range interactions have a strong collective influence on the magnetic properties of existing materials. Finally, we comment on the emergence of a spatial structure in the long-range interaction upon the inclusion of many quantum modes.

Cooperative effects in complex, coupled quantum systems, cannot be understood by sole consideration of the individual constituents, as they arise from the interplay among them. Light-matter platforms provide an optimal playground for the observation and exploitation of quantum cooperative effects [1]. For example, structured subwavelength arrays of quantum emitters trapped in optical lattices, are ideal showcases of such cooperative behavior, as their optical response can be efficiently enhanced by controlling the hopping of surface excitations via the quantum electromagnetic vacuum induced dipole-dipole interactions.

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).

When two electrons are emitted within a tightly constrained space-time volume at the nanometer-femtosecond scale, a substantial Coulomb interaction can be expected . Based on our ultrafast laser emission experiments from sharp needle tips, we have access to this intriguing few-electron correlation physics. In this talk, we present results on the Coulomb-induced energy anti-correlation between two electrons released from nanometer-sized tungsten needle tips triggered by femtosecond laser pulses [1]. We extract two critical parameters: (1) a mean energy splitting of 3.3 eV and (2) a correlation decay time of 82 fs. Both of these parameters will be of great importance for ultrafast electron microscopes, as demonstrated in a companion study performed inside of a TEM [2]. We show that by energetically filtering the electrons, it is possible to achieve pulsed electron beams with sub-Poissonian fluctuations, which are highly relevant for overcoming the shot-noise limit in imaging applications. Furthermore, we illustrate that in the strong field regime, where ponderomotive effects of the laser field come into play, the anti-correlation gap experiences significant modulation. We will also briefly summarize our results obtained jointly with the Chekhova group on electron emission from needle tips when driven with bright squeezed vacuum light, i.e., anti-squeezed light and that its counting statistics are transferred to the electrons [3].

 

[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

We present quantum phase diagrams of the hardcore Bosons with repulsive dipolar interactions.

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.

In variational Monte Carlo (VMC), the energy of the system is lowered to the ground state energy by optimizing the parameters of the ansatz wave function.

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.

We theoretically analyse the ground state of a one-dimensional Wigner crystal in the presence of a periodic potential. We determine the action of the Coulomb problem and show that, in the continuum limit, it is mapped to the one of a (1+1) Thirring model with Coulomb-interactions. The corresponding mean-field model is an antiferromagnetic spin chain with long-range Coulomb interactions. For finite chains this model predicts ordered structures that are commensurate with the periodic substrate, which form a devil’s staircase as a function of the charge density. The step-size of the devil’s staircase, indicating the interval of densities for which the commensurate phase is stable, scales as $1/\ln N$ with the number $N$ of charges. It finally vanishes in the thermodynamic limit, showing that the Coulomb repulsion dominates and the charges order at the positions that minimize the Coulomb repulsion.

Advances in x-ray source development together with the development of novel sample preparation methods enable access to new parameter regimes of cooperative emission. In this presentation I will talk about recent experiments and theoretical concepts that illustrate the potential to enter new regimes of cooperative emission employing the nuclear resonances of Mössbauer atoms.

 

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.

The collective behaviour of matter as well as its interplay with light is one of the most important topics of modern science. Understanding it is crucial in basic research, as it holds the key to a variety of correlated quantum many-body phenomena like spin liquids or superconductvity. At the same time, this understanding forms the basis for many (quantum) technological applications which define the modern era. We discuss light-matter systems, where strong matter-matter and strong light-matter interactions are present simultaneously. From a condensed matter perspective, one might expect to tune the properties of quantum materials by quantum light and from a quantum optics perspective one might engineer interesting novel facets of quantum light originating from such entangled light-matter systems. Our main focus is to investigate the quantum cooperativity of these correlated light-matter systems and the physical consequences of the induced long-range interactions. Specifically, we will discuss the cooperative properties of the paradigmatic Dicke-Ising model being the sum of a matter-matter Ising interaction and a quantum Rabi (Dicke) Hamiltonian.

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)

Ensembles of atoms strongly coupled to an optical cavity offer a formidable laboratory for studying the equilibrium and the out-of-equilibrium dynamics of long-range interacting systems in the quantum regime. In this talk, we present a formalism that allows us to sytematically characterize the steady state and dynamics of atomic ensembles in optical cavities, both in the quantum as well as in the semiclassical limit, and to establish a natural mapping to long-range models. We then apply this theory for determining metastable states in the presence of two cavity fields, which compete in selfordering the atoms. We show that our predictions are in quantitative agreement with experimental measurements performed at ETH. We finally discuss the nature and the lifetime of metastability in these systems.

The treatment of molecules in cavities is currently heavily investigated in terms of both theory as well as experiment. In these systems, light and matter states are hybridized, a cooperative phenomenon in which photons and electronically excited states can no longer be distinguished. Such hybrid states can be viewed as quasi-particles and are referred to as polaritons. When choosing the frequency of the light carefully, molecular processes can be activated or inhibited. [1]

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)

We propose a mechanism for engineering chiral interactions in Rydberg atoms via a directional antiblockade condition, where an atom can change its state only if an atom to its right (or left) is excited. The scalability of our scheme enables us to explore the many-body dynamics of kinetically constrained models with unidirectional character. We observe non-ergodic behavior via either scars, confinement, or localization, upon simply tuning the strength of two driving fields acting on the atoms. We discuss how our mechanism persists in the presence of classical noise and how the degree of chirality in the interactions can be tuned, providing paths for investigating a wide range of models.

Cavity magnonic systems are ideally suited to explore the range of possibilities opened by tailoring the interactions between photons, phonons, and magnons. In this talk I will discuss the different coupling mechanisms and propose applications ranging from wavelength conversion to optical probing of the coupled spin mechanics.

We consider successive measurement of photons at particular positions emitted from an ensemble of incoherently emitting sources.

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

Silicon carbide (SiC) is a wide band gap semiconductor with extensive applications. It is a host for optically addressable defects such as the silicon vacancy (𝑉_𝑆𝑖), carbon vacancy (𝑉_𝐶) or di-vacancy (𝑉_𝑆𝑖 𝑉_𝐶 ). Notably, the silicon vacancy has a millisecond coherence time, high spectral stability and close to transform limited photon emission as well as a pronounced emission into the zero-phonon line (ZPL) making SiC a good candidate for quantum applications (quantum information, computing, sensing …). For quantum computing applications, photon losses must be strictly limited which can only be achieved with appropriate photonic structures. During the last decade, progress has been made toward the development of scalable quantum photonic technologies on SiC. Accurate generation of defects into waveguides or resonators being one of the key aspect to maximize their coupling and efficiency, for this several techniques were developed to generate silicon vacancies such as laser writing, ion implantation through mask with holes, implantation with a focused ion beam (FIB). With this in mind, we proceed to implant samples using a femtosecond laser, a Cs-FIB and a Ga-FIB. A comparison will be made between these processes to chose which one is more adapted to accurately generate single shallow silicon vacancies while minimizing surface damages.

The majority of numerical approaches investigating long-range quantum systems is restricted to one-dimensional systems and systems in two dimensions with quickly decaying long-range interactions. While models with discrete symmetries like the long-range transverse-field Ising model have been studied thoroughly, much less is known about long-range models with continuous symmetries where long-range interactions can cricumvent the Hohenberg-Mermin-Wagner theorem in one dimension allowing the spontaneous breaking of continuous symmetries or can give rise to massive excitations violating Goldstone’s theorem. We study the breakdown of the rung-singlet phase in the quasi one-dimensional Heisenberg ladders as well as two-dimensional Heisenberg bilayer systems with algebraically decaying long-range interactions. To this end we use the method of perturbative continuous unitary transformations (pCUT) as a white graph expansion with classical Monte Carlo simulations yielding high-order series in the thermodynamic limit about the limit of isolated dimers. This allows us to determine the critical point as well as critical exponents as a function of the decay exponent.

Memory-assisted quantum repeaters provide a way to overcome the exponential channel loss in long-distance secure communication.

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

We investigate a model of hardcore bosons on the links of a Kagome lattice subject to a long-range decaying van-der-Waals interaction. This model is known to be the relevant microscopic description of Rydberg atom arrays excited by a detuned laser field which has been realized in experiments recently. Particular interest lies on this system as it is an engineerable quantum platform which has been predicted to to host a topological phase. We investigate the quantum phase diagram for different limiting cases with a main focus on the low interaction-strength limit where we apply high-order linked cluster

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.

We study strong linear magnon-phonon interaction for applications in quantum information processing. In a first project we have shown optimal broad-band frequency conversion between optical and microwave fields [1]. Here we took advantage of the well established magnon-microwave and optomechanical coupling and combined it with the magnon-phonon interaction as mediator. Using estimates for linewidths and coupling strength in Yttrium Iron Garnet (YIG), the resulting two-stage conversion chain yields unity conversion efficiencies and large band widths on the order of linewidths. In a next step we will characterise this setup for application in state teleportation with the objective to combine conversion and teleportation in a single geometry. In addition we will briefly outline a recent experiment that studies magnon-phonon interactions in magnetic bilayers [2].

 

[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)

We study the out-of-equilibrium dynamics of a laser-driven Bose-Einstein condensate coupled to a single mode of an optical cavity. In the regime of large red cavity detuning and below a threshold for the laser intensity, the atoms remain spatially homogeneous, and the cavity photon number is close to zero. Above threshold, the system enters a superradiant phase where the atoms form a coherent Bragg grating that supports constructive interference of scattered laser photons. This results in a macroscopic cavity field that can reach a stationary state, which stabilizes the atoms into a self-organized structure. In our work, we focus on the regime where the effective cavity detuning depends strongly on the dispersive AC Stark shift, and where the cavity relaxation rate is fast compared to the typical atomic relaxation rate. In this regime, the cavity field is slaved to the atomic state, and the effective cavity detuning strongly depends on the atomic pattern. This positive feedback between the atomic pattern and cavity field allows for a parameter regime where the cavity field is unable to stabilize the atomic configuration. Instead, the system enters a dynamical phase where the atomic pattern and cavity field exhibit oscillations. We analyze this behavior using a mean-field approach that describes the coupled dynamics of the atoms and cavity field. Our analysis demonstrates the emergence of limit cycle and chaotic phases. In addition, working in the bad-cavity regime allows us to derive equations of motion where the cavity degrees of freedom are eliminated, which massively improves the integration time. We benchmark and validate these equations of motion and showcase that the existence of limit cycle phases do not require a treatment of the cavity field and atoms to be on equal timescales.

SiC is an outstanding material platform with its unique properties. We show our latest result in B03: from monolithic fabrication of cantilever to their eigenfrequency and Q-factor analysis. Creating the foundation for spin-phonon coupling on the SiC platform.

The Timepix4 detector readout ASIC has been designed to overcome the shortcomings of currently available x-ray detectors in both timing resolution and active area. TEMPUS is a readout system for this detector ASIC currently under development, designed to support nanosecond timing and a sustained event rate of 1Mhits/mm2/s. We report first tests of a TEMPUS prototype at the P01 beamline of the PETRA III synchrotron. In these tests, detector timing resolutions of less than 30ns FWHM were achieved at photon energies ranging from 6keV to 14.4keV.

Achieving efficient adiabatic transfer in the presence of decoherence is important for the realization of fast, accurate adiabatic quantum processes in the NISQ era. In our work, we extend the concept of reservoir engineering to quantum trajectories, with the aim of tailoring a time-dependent non-unitary dynamics for which the master equation solution is the target trajectory. The efficiency of the protocol is determined by the fidelity of attaining the target, as well as the transfer time. In the simple setting of a two-level system (qubit), we solve the optimization problem for minimizing the time at a fixed infidelity. Results for the protocol based on reservoir engineering are benchmarked against the unitary case. While transitions out of the target trajectory are generally not favorably suppressed for the open system, the quadratic Grover scaling of the transfer time is retained.

We propose 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. We apply the proposed approach to the atomic limit of the extended Bose-Hubbard model on the triangular lattice at fillings $f=1/2$ and $f=1$.

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.

The transverse-field Ising model with quenched disorder is studied in one and two dimensions at zero temperature by stochastic series expansion quantum Monte Carlo simulations. Using a sample-replication method we are able to determine distributions of pseudo-critical points, from which critical shift and width exponents $\nu_{s/w}$ are extracted by finite-size scaling. The critical points extrapolated to infinite systems are confirmed and refined by an analysis of averaged binder ratios. Scaling of the averaged magnetisation at the critical point is used further to determine the order-parameter critical exponent $\beta$ and the critical exponent $\nu_{av}$ of the average correlation function. The dynamical scaling in the Griffiths phase is investigated by measuring the local susceptibility in the disordered phase and the critical exponent $z′$ is extracted.

Using a spin-carrying electron on a 1D quantum helix as a model for the chiral motion of charges within chiral matter, the co-existence of two spin-polarised charge transport anisotropies is disclosed. These depend on the helix handedness (or helicity), spin-orientation and electrical current direction. The presence of the anisotropies allows mapping quantum optical concepts onto charge transport approaches, for instance, providing an analog to atom-state/photon-field entanglement

We study the Ising model in a light-induced quantized transverse field (qTFIM) [1, 2] using quantum Monte Carlo to investigate the influence of light-matter interactions on correlated quantum matter. To avoid a direct sampling of the photons, we develop a quantum Monte Carlo algorithm based on the recently introduced wormhole algorithm for spin-boson systems [3], in which the bosonic degrees of freedom are integrated out analytically. By this means, the bosons induce a retarded spin-spin interaction in imaginary time [3]. In contrast to the Ising interaction inherent to the model, this induced interaction is also non-local in space.

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)

Intensity interferometry is a reemerging astronomical imaging technique, benefiting immensely from the recent improvements in (single) photon detection instrumentation. Our goal is to perform spatial correlations of A-type stars in the blue using ultra high-rate single photon detectors. We present a setup for the C2PU telescope at the Calern observatory, Nice, France, featuring hybrid single photon counting detectors (HPDs) with which we successfully measured temporal correlations of three different stars – Vega, Altair and Deneb. The setup showed remarkable stability and very efficient coupling of the starlight to the photodetectors, owed mainly to the large active area of the HPDs. However, the HPDs saturation count rate of 10 MHz can easily be reached for bright stars at C2PU. Under these conditions 50% of potential photon events are lost due to the dead time of the detector, diminishing the measurement’s SNR substantially. We are thus developing a custom single photon detection system, aiming to exceed 100MHz in sustained single photon count rate, using a 18 mm diameter (chevron) micro-channel plate photomultiplier tube. The single photon events are digitized using tailored discrimination electronics and a self-developed Tapped-Delay-Line Time-To-Digital-Converter (TDL TDC) to handle the high throughput rate of the detection system. First tests reveal that high resolution spatial intensity interferometry experiments are within reach at 1 m diameter class telescopes within one night of observation time for bright stars.

In this research, we investigate the strong atom-cavity interaction of cold 174Yb atoms within a high-finesse optical cavity (length 5 cm, finesse 45 000), where the 174Yb atoms are trapped in a MOT, by using the narrow intercombination 1S0 – 3P1 transition. We present the investigation of the cavity output spectra for variable atom number, cavity detuning, as well as trap/pump light detuning and also we compare the properties of the cavity output light with free-space scattered light (fluorescence) in order to identify the coupling mechanisms and dissipation channels of the coupled system.

A highly efficient single-photon source is an indispensable building block for emerging quantum technologies. In this project, we aim at realizing a quasi-deterministic source of single photons by embedding single organic molecules in a metallo-dielectric antenna which is manufactured by bonding a solid immersion lens to an etched glass substrate. So far, this concept has only been verified at room temperature [1]. We will showcase our latest results on measuring collection efficiencies at cryogenic temperatures where molecules have a lifetime-limited linewidth.

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).

We investigate scenarios of laser cooling of quantum emitters inside a driven optical cavity in the bad cavity regime, characterized by the Purcell enhancement of spontaneous emission. For a single

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.

Nanometer-sized metal tips provide an optimal system for studying nonlinear light-matter interactions on metals. In this contribution, we investigate the dynamic correlation of two electrons emitted by a single femtosecond laser pulse. Furthermore, we report the statistics of the emitted electrons, initially emitted from classical laser pulses. We then show how the statistics of bright-squeezed vacuum can be applied to the emission statistics of electrons.

In this work, we consider a topological magnon insulator and study spin induced polarisation due to the dynamical magnetoelectric effect. The system under consideration is a spin model ferromagnetic Heisenberg insulator on a honeycomb lattice with next nearest neighbour Dzyaloshinskii Moriya interaction. The low energy excitations of this model exhibits topological band structure with chiral edge modes in the bulk band gap. The goal of the project is to device an optical probe for the chiral edge modes through parametric amplification under external driving.

X-ray photons have very desirable properties: they carry a large momentum, are robust, have exceptional penetration depth, good detection efficiency and remarkable focusing potential, far from any practical diffraction-limit constraint. Once properly controlled, x-ray photons could enable unique quantum technology applications and could be used for sensing with unprecedented simultaneous energy and spatial resolution for material science, magnetism or biochemical samples.

 

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.

Modification of spontaneous decay in space and time is a central topic of quantum physics. It has been predominantly investigated in cavity quantum electrodynamic systems. Altered spontaneous decay may equally result from correlations among the emitters in free space, as observed in super- and subradiance. Yet, preparation of an entangled quantum state and the resulting modified emission pattern has not been observed so far due to the lack of ultra-fast multi-pixelated cameras. Using two trapped ions in free space, we prepare their state via projective measurements and observe their corresponding collective photon emission. Depending on the direction of detection of the first photon, we record fundamentally different emission patterns, including super- and subradiance. Our results demonstrate that the detection of a single photon may fundamentally determine the subsequent collective emission pattern of an atomic array, here represented by its most elementary building block of two atoms stored in an ion trap.

 

We analyze the dynamics of a quantum particle moving according to a long ranged tight-binding Hamiltonian and which is additionally subject to repeated projective measurements by a detector placed at the target site. We

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.

Ions chains, interacting with a monochromatic light field, behave like an optical grating. This has previously been demonstrated by recording optical interference patterns in their far field. Independently, optical three level systems, interacting with 2 monochromatic light fields, can emit fluorescent radiation with the combined frequency of the two driving fields.

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.

Strongly correlated matter systems intertwined with light-matter interactions of comparable strength offer a promising playground for understanding the influence of quantum light onto correlated matter and vice versa. As a paradigmatic exemplary system we investigate the Dicke-Ising model [1, 2], containing the described competition of interactions.

 

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

Color centers in 4H Silicon Carbide, such as the Silicon Vacancy (VSi) and Di-Vacancy (VCVSi), are promising qubits and single photon sources for the implementation of quantum applications. They feature strongly localized defect states, forming high and low spin states opening the path towards a wide range of spin manipulation protocols using optical or mechanical resonator modes.

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.

We propose a variational quantum eigensolver ansatz, which enables us to explore the phase structure of the multi-flavor Schwinger model in the presence of a chemical potential. Our ansatz is capable of incorporating relevant model symmetries via parameter constrains and can be implemented on circuit-based as well as measurement-based quantum devices.

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.

We observe optical gain and laser emission from a medium of a few thousand Ytterbium-174 atoms which are magneto-optically trapped (MOT), using their 1𝑆0 → 1𝑃1 transition at 399 nm, inside a 5-cm long high-finesse cavity. The cavity output is observed as continous wave lasing on the 1 𝑆0 → 3 𝑃1 intercombination line at 556 nm when the atoms are laser-pumped on the same transition. The physics behind the observation is understood as a multi-photon lasing mechanism involving the MOT transition [1]. By heterodyne analysis, we analyse the frequency characteristics of the system versus pump and cavity detuning. By cooling the atoms near the Doppler limit of the intercombination transition, we observe an increase in atomic density and a corresponding reduction of the laser threshold. In the future, we will explore corresponding phenomena on the spin-forbidden 1 𝑆0 → 3 𝑃0 transition with the help of the 3P to 3S repumping transitions at 680nm and 770nm.

The potential of quantum computers to solve the problems in quantum many-body physics have been growing with the development of classical-quantum hybrid algorithms. In this paper, we investigate the quantum simulation of transverse-field Ising model in 1-D and 2-D lattice, and the problems associated with the same. We propose a method to approximate the ground-state energy of transverse-field Ising model in the thermodynamic limit by introducing a method to combine numerical linked-cluster expansions (NLCE) with the vari-

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.

Dysprosium is a fascinating candidate for studying cooperative and

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.

We studied the light-matter interaction between vortex light carrying orbital angular momentum and a single trapped ion. We developed a theoretical framework to predict the effect of the different light field gradients on different quadrupole transitions and their respective sidebands. By placing the single ion in the vortex beam phase-singularity, we developed STED inspired techniques for imaging its center of mass wavefunction. Moreover, we demonstrated the highly surprising appearance of coherent motional excitation perpendicularly to the light propagation direction.

We investigate the coupled spin-mechanics of an electrically levitated ellipsoidal nanodiamond with a single embedded nitrogen-vacancy center in a Paul trap. We characterize the system and show the possibility to reach the ultrastrong coupling regime between the nitrogen-vacancy spin system and the rotational motion of the nanodiamond. Further, we study the effects of initial rotation to investigate the manifestation of the Barnett effect.

The success of deep learning encourages people to explore the border of artificial intelligence (AI). Although the advance in high-performance computing units plays a significant role in this process, its inefficiency in energy usage and other disadvantages arouse an engagement in the study of specific hardware for machine learning. In this project, we explore the possibility of doing neuromorphic computing with

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.

The simulation of physical systems is a vital part of contemporary research in many different areas, especially

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.

Ion traps are ideal candidate platforms for quantum simulators of interacting spin systems.
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)
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Hybridized light-matter states, called polaritons, can arise when molecules are placed in cavities. By precisely selecting the frequency of the light, molecular processes can be activated or inhibited, modified or controlled. On this poster, we present highly accurate ab initio quantum electrodynamics coupled cluster methods capable of describing molecules in cavities in the regime of strong coupling. Furthermore, we develop and investigate the influence of an external magnetic field in arbitrary orientation and strength which can be seen as a further regulator to manipulate the electron structure of the molecule.