CQED-22 Meeting

Forces are all around us, I will think of something to tell you related to our forceful life.

We at Swarovski Optik try to create the world's best binocular. The major ingredient for the best binocular is the customers needs. So what are the requirements concerning the optical system to fulfill these needs? How can the optical device provide the ultimate experience to the user's visual system? In this talk I would like to discuss some interesting facts that occur on this exciting quest.

The notions of symmetry and symmetry breaking are ubiquitous across physics. They play essential roles in a wide range of physical phenomena and have numerous applications, ranging from labelling the energy eigenstates of atoms and molecules to effectively describing quantum phase transitions and the topological classification of quantum matter. Here, we consider the matter-field interaction Hamiltonian in its full quantum form---cavity quantum electrodynamics (QED)---and show that by explicitly enumerating all continuous symmetries of the lowest-order, number conserving interaction term, we can classify all possible cavity-QED Hamiltonians. Our approach is general and provides a recipe to construct matter-field interaction Hamiltonians with desired continuous symmetries, paving the way to explore a plethora of exotic phenomena in cavity-QED settings.

Many cavity QED and quantum information processing protocols require high atom-photon interaction strength. This can be achieved by operating a cavity in the near-concentric regime minimising the beam waist and enhancing the local electric field, but at the cost of higher clipping losses at the finite-sized mirrors. We demonstrate how this limit can be overcome by moving away from the paradigm of spherical mirrors. Using a range of approaches, e.g. analytical, evolutionary algorithms, or machine learning, we design novel mirror shapes that give rise to improved cavity cooperativity.

Theoretical studies of superradiant lasing on optical clock transitions predict a superb frequency accuracy and precision closely tied to the bare atomic linewidth. Such a superradiant laser is also robust against cavity fluctuations when the spectral width of the lasing mode is much larger than that of the atomic medium. Despite recent major advances in this field, the experimental realization of a continuous wave superradiant laser has not yet been achieved. In my talk I will suggest a model of an optical atomic conveyor and present my latest results regarding the optimal parameters of its operation.

The usual supersolid implemented inside an optical cavity turns out to be impervious to the phononic excitations that determine the low-temperature properties of real materials. In my talk, I'm going to describe how to overcome this shortcoming and implement a nonrigid supersolid by making use of a multimode ring cavity.

I could provide a talk, summarising my research during my PhD studies. This can be grouped in 3 main topics: Ultracold atoms, quantum dots and superconducting qubits. Here are the links to the three papers: https://doi.org/10.22331/q-2019-06-03-149, https://doi.org/10.48550/arXiv.2102.00283, https://doi.org/10.48550/arXiv.2106.05342 The first two projects have been in collaboration with the Ritsch group. If it's prefered by the organizers, I could also just talk about one of those projects.

The technical Simulations at Liebherr will be introduced regarding different applications of theoretical physics in earth-moving machinery.

I will present an overview of the physics I have been studying over the last years: dipole-dipole interactions, collective spontaneous emission and some related applications like atomic clocks or superradiant lasers.

Quantum error correction is a crucial step to building useful quantum computers, needed to maintain precision in deep computations in spite of having faulty components with a nonzero error rate - as long as this rate is below a threshold value. The most promising quantum error correction scheme to date, the Surface Code, uses entanglement to distribute/hide logical qubits in several physical qubits. It is based on the so-called topological order in two-dimensional quantum spin models. Errors arising from entanglement with the environment can be investigated efficiently for the surface code, however, so-called coherent errors (resulting from 1/f noise) are harder to model. We investigated the combined effects of coherent errors and readout errors on the surface code, using a recently introduced mapping of the surface code to Majorana fermions. We have found numerically an error threshold that is slighlty lower for this combination of error sources than if there is no coherence in the errors.

QuantumCumulants.jl is a Julia framework for generalized mean field equations in open quantum systems. It automates the analytical derivation and numerical implementation for higher order equations, using the cumulant expansion method. I will live-program a CQED system of my current research with the toolbox.

I present our studies over the last several years related to the photon-blockade breakdown (PBB) effect, occurring most simply in a coupled system of a bosonic mode and a two-level system. Having been identified as a first-order dissipative phase transition in a thermodynamic limit where the coupling between the subsystems goes to infinity without affecting the system size [1] (hence the designation zero-dimensional), PBB was studied in a finite-size scaling approach [2], with finite-size scaling exponents determined numerically. I describe the experimental studies where we participated from the theoretical/computational side. PBB was first observed in a circuit QED platform [3], and the thermodynamic limit can also be modeled with artificial superconducting atoms [4]. [1] H. J. Carmichael, Breakdown of Photon Blockade: A Dissipative Quantum Phase Transition in Zero Dimensions, Phys. Rev. X 5, 031028 (2015). [2] A. Vukics, A. Dombi, J. M. Fink, P. Domokos, Finite-size scaling of the photon-blockade breakdown dissipative quantum phase transition, Quantum 3, 150 (2019). [3] J. M. Fink, A. Dombi, A. Vukics, A. Wallraff, and P. Domokos, Observation of the Photon-Blockade Breakdown Phase Transition, Phys. Rev. X 7, 011012 (2017). [4] R. Sett, F. Hassani, D. Phan, Sh. Barzanjeh, A. Vukics, and J. M. Fink: Approaching the thermodynamic limit of a first-order dissipative quantum phase transition in zero dimension, In preparation.

I will talk about Helmut's wish to have a MOT and the consequences of his influence on my way of thinking, and, even more importantly on my way of acting.

A very short overview of Patent Law.

Dipole-coupled subwavelength quantum emitter arrays respond cooperatively to external light fields as they may host collective delocalized excitations (a form of excitons) with super- or subradiant character. Deeply subwavelength separations typically occur in molecular ensembles, where in addition to photon-electron interactions, electron-vibron couplings and vibrational relaxation processes play an important role. While vibrations are typically considered detrimental to coherent dynamics, we show that molecular dimers or rings can be operated as platforms for the preparation of long-lived dark superposition states aided by vibrational relaxation.

Quantum dot superluminiscence diodes (QDSLD) provide incoherent broad-band radiation in the infrared regime. Remarkably, QDSLDs can be tuned by temperature to suppress intensity fluctuation to approximatly g_2=1.33 over a terahertz-wide band. We will present a brief description of this effect.

The optical properties of a quantum emitter such as its excitation lifetime or transition frequency can be strongly modified when placed in close proximity of a second emitter, as electromagnetic vacuum fluctuations can mediate strong dipole-dipole interactions between them. Such effects are maximized if the dipoles are arranged in a regular array, for which interference of the radiated waves can be maximal, leading to striking phenomena such as the emergence of subradiant guided modes that can propagate almost lossless through the array. In this talk we will show that a regular ring-shaped structure of emitters possess very special optical properties, which could be used for instance as a nanoscale coherent light source, an absorptive frequency-selective antenna or even are already at work in natural existing light harvesting complexes.

Time crystals spontaneously break the continuous time translation symmetry of an interacting system. In other words, interacting atoms can spontaneously rearrange into periodic motion after an infinitesimal perturbation of a many-body state. In this talk I will present some preliminary results on continuous time crystals in optical cavities.

We will discuss an attractive force caused by light induced dipole-dipole interactions in freely expanding ultracold Rb atoms. This collective, light-triggered effect results in a self-confining potential with interesting features: it exhibits nonlocal properties, is attractive for both red and blue-detuned light fields and induces a remarkably strong force that depends on the gradient of the atomic density.

If time permits, I could provide experience of our ongoing project "Global Heat" funded by the Erasmus+ program of the European Union. The focus is aside from getting to know other cultures that our students think of sustainable and realistic solutions. So far the exchange weeks served to make measurements on Gaisbergferner (Austria), the coast of Tjärno (Sweden) and Mount Etna (Italy). The final ideas and reports will be written in autumn in Belgium, where the coordinating partner invites to the final exchange week. I consider this talk as optional, but if interest exists, I could provide the final presentations of the students such to get an idea what level of education is realistic at the age of 17-18 and also some photos. As I arrive only on Saturday, this has to be taken in consideration.

Coherent light, either classical or quantum, as in the case of optical cavities, has the power to strongly modify and eventually enhance material properties. Different competing theoretical approaches are currently emerging to describe photon-electron interactions in the presence of vibronic coupling. I discuss progress we have recently made in developing a quantum Langevin equations approach to quantum optics with molecules. At the level of a single or a few molecules, this method can analytically describe effects such as polariton cross-talk, Purcell modification of branching ratio and incoherent FRET (Förster resonance energy transfer) migration of energy [1,3]. In the mesoscopic limit, where many molecules are coupled to a single cavity mode, I discuss strategies for incorporating frequency and orientational disorder, near field couplings and vibrational relaxation in an analytical model showing the degradation of the VRS (Vacuum Rabi splitting) at high densities [2]. References [1] M. Reitz, C. Sommer and C. Genes, Langevin approach to quantum optics with molecules, Phys. Rev. Lett. 122, 203602 (2019). [2] M. Reitz, C. Sommer, F, Mineo and C. Genes, Molecular polaritonics in dense mesoscopic disordered ensembles, Phys. Rev. Research 3, 033141 (2021) . [3] M. Reitz, C. Sommer and C. Genes, Cooperative quantum phenomena in light-matter platforms, PRX Quantum 3, 010201 (2022).