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Horizons for engineering strongly correlated fermions and bosons in 2D semiconductors

Bilayer electron systems made from 2D semiconductors are a leading platform to study correlated phases of bosons and fermions based on charges due to inherently strong intra- and interlayer interactions. Correlation effects can be further enhanced by introducing a lateral superlattice potential, which quenches the kinetic energy of resident charges. In this talk, I will discuss two distinct superlattice bilayer electron systems based on 2D semi-conductors and the correlated phases they host. The first system consists of moiré transition metal dichalcogenide (TMD) heterobilayer which is electrostatically coupled to a nearby monolayer TMD. The structure hosts a highly tunable Bose-Fermi mixture consisting of electrically injected dipolar excitons and charges, stabilized by strong correlations. The second system is a TMD heterobilayer where the TMD layers are separated by a thin dielectric tunnelling barrier, and a superlattice potential is electro-statically imprinted. Both layers in the system host correlated insulating states at integer and fractional fillings, and we observe signatures of an emergent dipolar exciton phase. Because the electrostatic potential for both layers originates from a common source, it realizes a perfectly aligned double moiré system, paving the way to explore the Bosonic correlations with tunable lattice geometry and the possibility of topological excitonic phases.

Direct observation of Forward-Volume-Spin-Waves foldover in a plain 200 nm YIG film by TR-MOKE

Spin-wave foldover, the hysteretic response that emerges when the spin-wave resonance frequency hardens with amplitude, is a hallmark of forward-volume spin waves (FVSWs) in thin garnets with out-of-plane magnetization [1]. It originates from a positive nonlinear (Kerr-type) frequency shift of both the uniform (FMR) and propagating modes, producing bistable windows under frequency or power sweeps and enabling robust thresholding functions. Recent experiments in microscale YIG waveguides established giant up-shifts (> GHz), broad hysteresis, and self-normalized emission that is nearly independent of drive once above threshold; features already leveraged to realize an all-magnonic repeater with amplitude gain and phase re-normalization for cascading logic [2]. Building on this physics, we investigate foldover in a 200 nm YIG film by time-resolved MOKE imaging of propagating spin waves versus frequency and magnetic field, complemented by power-dependent studies. We outline how k-resolved measurements enable extraction of the nonlinear shift coefficient and the mapping of stability. Beyond establishing the operating phase space, foldover-enabled limiters, switches, and repeaters suggest a compact route to robust magnonic interconnects and neuromorphic primitives [3]. 

[1] Wang et al., Sci. Adv. 9, eadg4609 (2023) 

[2] Wang et al., Nat Commun 15, 7577 (2024) 

[3] Wang et al., Phys. Rev. Applied 21, 040503 (2024)

Controlled magnon-phonon coupling in magnetic / nonmagnetic bilayers

Electrical excitation of self-hybridized exciton-polaritons in a van der Waals antiferromagnet

Van der Waals materials are at the forefront of solid-state research, with newly discovered layered systems extending the impact of two-dimensional physics. CrSBr is especially promising, combining strongly bound excitons, quasi-1D bands, high Néel temperature, air stability, and strong light-matter coupling. A key challenge has been electrical excitation of optically active quasiparticles in 2 dimensions. Here, we employ a novel electrical excitation mechanism, creating excitons by the near field coupling of tunneling electrons. We explore the different pathways of electrons in a van der Waals heterostructure, showing evidence for tunneling beyond the electrodes. In CrSBr, we reveal strong, linearly polarized electroluminescence confirming the excitonic origin, independent of the layer thickness. Efficiency changes across the Néel temperature, consistent with reduced oscillator strength, while the open electrode design further enables injection of hybrid exciton-polaritons into CrSBr governed by strong light-matter coupling.

Quantum light scattering at an electromagnetic time interface

Light-matter interactions in time-varying materials have attracted significant interest recently, uncovering novel electromagnetic phenomena and offering enhanced functionalities of photonic devices. However, compared to the research efforts and progresses that have taken place in the classical context, the quantum aspects of this emerging subject have been less explored. Here, we study quantum light scattering in an isotropic and nondispersive material with a suddenly changing refractive index, creating a time interface. By considering the case in which a forward and a backward propagating mode exist before the temporal discontinuity, we first show that the time interface transforms the bosonic mode operators and corresponding quantum states in terms of the two-mode squeeze operator. Our analysis then focuses on quantum state engineering and photon statistics of the scattered light, which reveals and connects various quantum optical phenomena: photon-pair production and destruction, photon bunching and antibunching, vacuum generation, quantum state discrimination, and quantum state freezing. In general, our work provides new fundamental insights about quantized light in time-varying media and supports further investigations on more sophisticated time-interface systems, including dispersive materials and photonic time crystals, with potential applications in future quantum photonic technologies.

Nonlinear polaritons in van der Waals heterostructure metasurfaces

Integrating metasurfaces with two-dimensional materials hold significant promise for creating ultrathin photonic devices. Here, we introduce van der Waals heterostructure metasurfaces, made from hBN-encapsulated WS2 monolayers. Room temperature strong coupling is confirmed from both absorption and emission. Moreover, ultralow excitation fluences (< 1 nJ/cm2) induce significant exciton-polariton nonlinearities, outperforming conventional approaches of about 3 orders of magnitude.

Non-Markovian open quantum system dynamics and applications to strong light matter interaction

In this talk i will present one possible approach to open quantum system dynamics which is based on solving the dynamics of both the system and the environment. I will present applications of this method to organic microcavity polaritons, quantum information and quantum measurement theory.

Quantum interference in heat

I will present our experimental work on bolometric detection of quantum interference in superconducting circuits. In particular we consider a driven system composed of a strongly coupled qubit and a cavity. In the second part of the talk I will discuss our theoretical results on thermalization and Poincare revivals of isolated systems of qubits.

An overview of the ultrafast, high-intensity systems and advancements

In this talk an overview of the high intensity systems made by Light Conversion will be covered. Yb:KGW lasers of high repetition rates and high energies, accompanied by the OPA and OPCPA systems to reach <10fs and the post compression of the pulse which is a compact alternative for OPCPA system.

Voltage-voltage correlations in gate-controlled superconducting nanowires

Since the discovery of the gate-controlled supercurrent effect in superconducting nanostructures, there has been an increasing push to develop a superconducting analog of the semiconductor field-effect transistor. However, this effect has lately been associated to alternate scenarios based on defect-induced gate leakage current. We have used voltage-voltage correlations to study the dynamics of voltage and phase in gate-controlled superconducting hybrid nanowire systems. Both the formation of single phase slips and the dynamics of phase slip centers exhibit Poissonian statistics. The dynamics of the phase slip centers is strongly correlated to the gate leakage current, most likely due to inelastic tunneling with phonon emission. Using the duality between the electronic shot noise and the phase slip noise, bunching of the phase slips is determined.

Ultra-High-Q Van der Waals Microresonators

Van der Waals (vdW) materials provide a promising platform for integrated photonics and optoelectronics, offering key advantages such as low optical losses, atomically flat surfaces, broad spectral tunability, strong light–matter interactions, seamless integration with diverse photonic architectures, and the ability to encompass a wide range of physical properties, including those of dielectrics, semiconductors, and superconductors. Here, we present a fabrication method that achieves ultra-high-quality-factor vdW microresonators, with values reaching up to one million. This universal nanofabrication strategy enables high-resolution patterning across diverse vdW materials, including insulators, semiconductors, ferroelectrics, and their heterostructures. Furthermore, we demonstrate the potential of these ultra-high-Q resonators to advance nonlinear optical applications.

Addressing light emission statistics and vibrational pumping of organic molecules in nanocavities

Theoretical analyses on light emission from quantum emitters typically focus on the case of simple two-level systems, which only provide information of the photons emitted without the assistance of vibrations (Zero-Phonon Line). However, in fluorescence experiments, light radiated from solid-state quantum emitters is usually filtered with the aim of detecting only the Stokes-shifted photons and discarding in this way the laser photons. We present a model to describe and characterize the statistics of the Stokes-shifted photons, as well as that of the Zero-Phonon-Line photons, scattered from a large variety of quantum emitters. We reveal the differences between the correlation of Zero Phonon-Line photons and of Stokes-shifted photons scattered from two interacting emitters and identify a variety of regimes of light emission statistics depending on the coupling strength, laser intensity and detuning between the emitters. Furthermore, metallic nanocavities can be used to enhanced vibrational signals from molecules located therein. In this context, a NanoParticle-on-a-Mirror configuration enhances particularly well the electromagnetic field, boosting non-linear vibrational phenomena of organic molecules. Among others, a nonlinear dependency of the Stokes and anti-Stokes Raman signals from organic molecules as a function of illumination intensity serves to identify the vibrational pumping regime in molecular optomechanics. We introduce the basics of the molecular optomechanical interaction between molecular vibrations and nanocavity photons, and identify the vibrational pumping regime in Surface-Enhanced Raman Spectroscopy (SERS) of organic molecules.

Fully differential photon statistics in complex nanophotonic systems

I will discuss our approach to obtain a quantum-optics like description in terms of a few discrete modes for the quantum light-matter interaction in arbitrary nanophotonic structures, while still taking into account the full nanophotonic complexity of light propagation and emission [1,2]. I will then discuss how photon correlations of the emitted light resolved in space, frequency, time and polarization for arbitrary electromagnetic environments can be accessed by including weakly coupled emitters acting as detectors into the system [3]. Finally, I will discuss some fundamental properties of light-matter interactions and show that single-emitter ultrastrong coupling can in fact not be reached with photons (quanta of the transverse electromagnetic field) [5]. 

[1] I. Medina, F. J. García-Vidal, A. I. Fernández-Domínguez, and J. Feist, “Few-Mode Field Quantization of Arbitrary Electromagnetic Spectral Densities”, Phys. Rev. Lett. 126, 093601 (2021). 

[2] M. Sánchez-Barquilla, F. J. García-Vidal, A. I. Fernández-Domínguez, and J. Feist, “Few-mode field quantization for multiple emitters”, Nanophotonics 11, 4363 (2022). 

[4] M. Lednev, D. Fernández de la Pradilla, F. Lindel, E. Moreno, F. J. García-Vidal, and J. Feist, “Spatially resolved photon statistics of general nanophotonic systems”, PRX Quantum 6, 020361 (2025) 

[5] D. Fernández de la Pradilla, E. Moreno, and J. Feist, There is no ultrastrong coupling with photons, arXiv:2508.00702 (2025)
 

Strong light-matter coupling between a dielectric nanocavity and a monolayer transition metal dichalcogenide

We show strong light-matter coupling between light in a dielectric nanocavity with deep sub-wavelength confinement and excitons in a monolayer of molybdenum ditelluride. Avoided crossing is demonstrated by both photoluminescence and reflection measurements, from which we extract a light-matter interaction strength of approximately 5 meV. The associated Rabi splitting is twice as large as the system’s losses. These values are in good agreement with values obtained by a novel exciton reaction coordinate formalism. The strong light-matter interaction, combined with low losses and sub-wavelength confinement of light, demonstrates a new regime of light-matter interactions where strong nonlinearities at the single-photon level are expected.

Optical probing of Wigner crystallization in monolayer WSe2 via diffraction of longitudinal excitons

Monolayer transition metal dichalcogenides (TMDs) are characterized by relatively large carrier effective masses and suppressed screening of the Coulomb interaction, which substantially enhances the correlation effects in these structures. The direct band gap allows to effectively optically probe these correlations. Here, we present an experimental observation of Wigner crystallization in mono layer WSe2 probed by the measurement of the exciton diffraction on the Wigner crystal (WC) periodic potential. We observe the formation of the WC phase in the absence of external mag netic fields at temperature range T < 26 K and carrier concentrations n < 2 × 1011 cm−2. The direct observation of the exciton diffraction is enabled by the strong exciton longitudinal-transverse splitting induced by the long-range intervalley exchange interaction, leading to the large detuning between main exciton peak and first diffraction peak. Our findings highlight that the valley degree of freedom of charge carriers in TMDs facilitates optical probing of correlated electron phases in these structures.

Floquet control of gain and loss in quantum fluids of light by GHz acoustic strain

Time-periodic modulation of a multi-level system on a time-scale faster than the coherence time enables exquisite control of the population transfer between states, which has implications for a wide-range of quantum systems, as well as for the realization of Floquet engineering. Here, we apply this concept to non-equilibrium light-matter Bose-Einstein-like condensates (BEC) of exciton-polaritons in a semiconductor microcavity by energy-modulating the excitonic-component of the polariton with large and tunable amplitude using monochromatic GHz-frequency strain of bulk acoustic waves. We study a multimode polariton BEC under spatial confinement, where (without the modulation) the population is predominantly in excited states. The modulation leads to multiple anti-crossings, which results in efficient transfer of polaritons to the ground state. Remarkably, BEC coherence is maintained over the modulation period, which leads to the emission of coherent sub 50 ps pulses (corresponding to the frequency combs in the emission spectra). Our model shows that the modulation leads to a very fast and tunable control of the gain and loss governed by an interplay between the bosonic stimulation and the adiabatic Landau-Zener-like population transfer [1]. The demonstrated results are relevant for the Floquet engineering of a wide range of light-matter systems due to the universal character of the acoustic modulation, as well as tunable single- or multi-wavelength ultrafast pulsed laser-like emission for novel information technologies. 

[1] A. Kuznetsov et al., ArXiv arXiv:2506.05874v1 (2025)

Quantum geometry in condensed matter physics and photonics

We will briefly describe why quantum geometry is important both in condensed matter physics and photonics, and then discuss our theoretical results on the relation of quantum geometry to superconductivity, Bose-Einstein condensation, and light-matter interactions, as well as our experimental observation of the quantum geometric tensor in plasmonic lattices.

Dynamic Quantum Transport in Mesoscopic Conductors

Dynamic quantum transport explores how time-dependent fields govern the flow of charge and heat at the nanoscale, where quantum coherence plays a central role. In my talk, I will introduce the field of dynamic quantum transport and present two of our recent works on negative interference currents and high-frequency heat pulses. First, I will show how interfering scattering paths through an electronic Fabry–Pérot cavity can make the electric current turn negative, even though the applied voltage pulses are always positive [1]. I will then go on to discuss how heat pulses can be emitted into a mesoscopic conductor by performing work on an electrode to change its temperature. We then investigate the properties of the heat pulses, for instance, by using a Hong-Ou-Mandel interferometer to examine if the heat pulses bunch or antibunch as they collide at an electronic beam splitter. 

[1] "Negative currents in Fabry-Pérot cavities are caused by interfering paths," M. Saha , L. Horray, P. Portugal, and C. Flindt, Phys. Rev. B. 112, L081408 (2025), Editors' Suggestion 

[2] "Heat Pulses in Electron Quantum Optics," P. Portugal, F. Brange, and C. Flindt, Phys. Rev. Lett. 132, 256301 (2024)

Magnon BEC in spin-polarized atomic hydrogen gas

Bose-Einstein condensation of cold atoms implies a macroscopic occupation of the ground state accompanied by the long range correlations between the particles of the ensemble. Similar phenomena can be observed also in the systems of wave like excitations or quasi-particles such as magnons. In cold gases spin waves occur due to the exchange interaction in atomic collisions. This leads to the identical spin rotation effect (ISRE) and wave-like propagation of the transversal component of spin or magnetization. Magnons in spin-polarized atomic hydrogen gas can be trapped and controlled by magnetic forces in a similar manner as ordinary atoms possessing magnetic moments. We show experimentally that at high enough density of magnons, they accumulate in a single state and exhibit long-term coherence. We observed a variety of spin wave modes caused by the ISR effect with strong dependence on the spatial profile of the polarizing magnetic field. The ISR magnons of atomic hydrogen are a high field seeking excitations, and are trapped in regions of strong magnetic field. At some critical value of density of hydrogen atoms a sharp and strong peak emerged in the ESR spectrum, and we argue that these effects of spontaneous coherence can be interpreted as Bose-Einstein condensation of magnons.

Multiferroic Order and Electromagnons at the Monolayer Limit

Conventional materials hosting exotic quantum phases typically have complex atomic structures, inhomogeneities from defects, impurities, and dopants making it difficult to rationally engineer their electronic properties. This can be overcome using van der Waals (vdW) materials and their heterostructures. Among these, vdW multiferroics are particularly promising for engineering novel magnetoelectric responses at the atomic scale. Recent work has established that monolayer NiI2 hosts robust type-II multiferroicity, raising the prospect of ultrafast, low-power spintronic devices controlled by electric fields. I will discuss our recent results on understanding multiferroicity in van der Waals materials in the monolayer limit using low temperature scanning tunneling microscopy and spectroscopy. Our findings provide atomic-scale evidence on the mechanism of multiferroicity in NiI2 and related compounds and we are also able to visualize the collective excitations of the multiferroic orders, electromagnons. More broadly, they demonstrate the versatility of vdW heterostructures in enabling multiferroic phenomena and in designing emergent quantum states beyond those found in nature

Engineering topological non-Hermitian quantum and photonic matter with modulated losses

Losses and coupling to an environment, usually considered detrimental effects, provide an exceptional new strategy to engineer new forms of quantum matter. Here we will show that engineered modulated losses enable creating topological many-body states that emerge purely from the existence of loss in the system. We first [1] show that modulated optical systems enable the realization of a whole family of quasiperiodic non-Hermitian states, featuring criticality and topological excitations. Second [2], by leveraging an integrated photonics platform, we demonstrate the experimental realization of topological excitations from modulated optical loss. Third [3], we demonstrate that effective topological non-Hermitian descriptions naturally arise in strongly correlated Kondo lattice materials, in particular giving rise to topological Kondo excitations. Finally [4], we demonstrate that generic exponentially complex non-Hermitian many-body models can be solved by leveraging tensor network methods, directly enabling imaging topological excitations. Our results put forward modulated losses as a versatile strategy to engineer topological non-Hermitian matter, enabling the realization of topological excitations solely from engineered environmental coupling. 

[1] Phys. Rev. Research 6, 023004 (2024)
[2] Nature Materials 24, 1393-1399 (2025)
[3] Phys. Rev. Lett. 134, 116605 (2025)
[4] Phys. Rev. Lett. 130, 100401 (2023)

Generation of Polarization Entangled Photons with Layered Semiconductors

The generation of photon pairs through nonlinear frequency conversion processes, such as spontaneous parametric down conversion (SPDC) is an essential element of future quantum technologies. For such sources to be technologically relevant, they must be compatible with integration and directly generate entangled photon pairs. In both respects, it has recently been realized that certain ultra-thin nonlinear media, such the transition-metal dichalcogenides (TMDs), have great potential. Carrying out nonlinear optics in ultrathin media has unique characteristics due to the broad phase-matching and the ability to use higher-loss materials and not suffer the effects of loss. In this talk I will show that we can exploit the crystal symmetries of ultrathin TMD to directly generate polarization entanglement on the nanoscale. I will then show that we microscopically engineer the TMDs, stacking several layers to introduce slight periodic poling, to enhance the generation rate by orders of magnitude, while preserving the nanoscopically generated entanglement. TMDs can also natively generate different classes of polarization entangled states, including the well-known Bell states, which have applications in quantum computing and quantum communication, and the so-called N00N states which provide advantages in metrology. Finally, I will discuss our plans to measure broadband frequency entanglement, measure two-photon Hong-Ou-Mandel interference, and enhance the entanglement purity by cooling the TMDs.

Magneto-optical properties of Van-der-Waals Exciton-Polaritons

Two dimensional materials have emerged as a new and interesting platform for studies of tightly bound exciton in ultimately thin materials. Meanwhile, various types of 2D- or quasi 2D materials have become available that feature giant light-matter interactions, charge tunability, and intriguing magnetic and topological properties. These features can be exploited for implementing novel photonic devices, and for fundamental, as well as quantum photonic investigations in the framework of cavity quantum electrodynamics. I will discuss the implementation of our open optical cavity in a liquid-helium-free magneto-optical cryostat. It is ideally suited for the study of exciton-polaritons based on van-der-Waals materials, even in most complex geometries. I will address examples of such experiments, with a focus on magnetic-optical properties of charge-correlated exciton-polaritons in the regime of strong light-matter interaction in moiré lattices as well as in the van-der-Waals magnet CrSBr.

Molecular photodynamics under strong light-matter coupling: polaritons or an optical illusion?

When molecules interact collectively with the confined electromagnetic fields inside an optical cavity, hybrid light-matter excitations known as polaritons, can emerge. These states, which embody quantum correlations between electronic and photonic degrees of freedom, have been proposed to reshape molecular potential energy landscapes and control photochemical dynamics. Yet, whether such cavity-induced modifications truly arise from polariton formation remains an open question. In this talk, I will describe our recent efforts combining molecular dynamics simulations with experiments, to probe the mechanisms behind cavity-modified molecular dynamics. Our results suggest that polariton formation alone does not strongly alter molecular photodynamics, challenging prevailing interpretations and motivating a re-examination of the role of coherence and correlation in polaritonic chemistry.

Polaritonic BECs vs Extended Lasing States in Organic Microcavities for Room Temperature All-Optical Simulations

Non-equilibrium Bose-Einstein condensates (BEC) in photonic systems are promising for exploring and engineering phases of matter under extreme conditions and can be used to perform analogue simulations at both cryogenic and room temperatures. In order to establish a robust and scalable platform for computing based on non-equilibrium photonic condensates problems of tuneability and reconfigurability should be addressed. Here we demonstrate the realisation of coherent coupling between individually pumped lasing states and formation of spatially extended macroscopically occupied states in a weakly coupled microcavity filled with liquid crystals (LC) and P580 dye (dye-doped LCMC) at room temperature. The saturation of the optical transition in P580 laser dye leads to the blueshift of the photoluminescence above the lasing threshold similar to the mechanism of the emission blueshift reported for strongly coupled organic microcavities. It results in a ring-shaped emission profile in the momentum space and sufficient in-plane momentum of the coherent emission leading to coherent energy exchange between spatially separated individually pumped states, similar to the well-known behaviour of polaritonic BEC in GaAs systems at cryogenic temperatures. We bring optical reconfigurability by controllable driving of each lasing state with a focused non-resonant optical excitation and utilise it to form a coupled state in a dyad, 1D chain, and a 2D lattice of condensates with the coupling strength defined by the distance between the pumping spots, pump power, and polarisation of the pump. We demonstrate the possibility of going beyond the nearest-neighbour limit of coupling in a chain of coherently coupled emitters by controlling the polarisation of the pump. Furthermore, we show electrical control over interaction between coupled lasing states by applying external voltage. High birefringence of the LC provides wide range tuneability of the dispersions for the microcavity optical modes with different polarisation and allows for control over coupling efficiency by changing the in-plane component of the emission.