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Understanding the Hubbard model is crucial for investigating various quantum many-body states and its fermionic and bosonic versions have been largely realized separately. Recently, transition metal dichalcogenides heterobilayers have emerged as a promising platform for simulating the rich physics of the Hubbard model. In this work, we explore the interplay between fermionic and bosonic populations, using a $\rm{WS}_2$/$\rm{WSe}_2$ heterobilayer device that hosts this hybrid particle density. We independently tune the fermionic and bosonic populations by electronic doping and optical injection of electron-hole pairs, respectively. This enables us to form strongly interacting excitons that are manifested in a large energy gap in the photoluminescence spectrum. The incompressibility of excitons is further corroborated by measuring exciton diffusion, which remains constant upon increasing pumping intensity, as opposed to the expected behavior of a weakly interacting gas of bosons, suggesting the formation of a bosonic Mott insulator. We explain our observations using a two-band model including phase space filling. Our system provides a controllable approach to the exploration of quantum many-body effects in the generalized Bose-Fermi-Hubbard model., Main text: 7 pages, 5 figures. Supplementary Material: 10 pages, 7 figures

Magnetic properties of materials ranging from conventional ferromagnetic metals to strongly correlated materials such as cuprates originate from Coulomb exchange interactions. The existence of alternate mechanisms for magnetism that could naturally facilitate electrical control have been discussed theoretically but an experimental demonstration in an extended system has been missing. Here, we investigate MoSe$_2$/WS$_2$ van der Waals heterostructures in the vicinity of Mott insulator states of electrons forming a frustrated triangular lattice and observe direct evidence for magnetic correlations originating from a kinetic mechanism. By directly measuring electronic magnetization through the strength of the polarization-selective attractive polaron resonance, we find that when the Mott state is electron doped the system exhibits ferromagnetic correlations in agreement with the Nagaoka mechanism., 10 pages, 8 figures

Laser induced shift of atomic states due to the AC-Stark effect has played a central role in cold-atom physics and facilitated their emergence as analog quantum simulators. Here, we explore this phenomena in an atomically thin layer of semiconductor MoSe$_2$, which we embedded in a heterostructure enabling charge tunability. Shining an intense pump laser with a small detuning from the material resonances, we generate a large population of virtual collective excitations, and achieve a regime where interactions with this background population is the leading contribution to the AC-Stark shift. Using this technique we study how itinerant charges modify -- and dramatically enhance -- the interactions between optical excitations. In particular, our experiments show that the interaction between attractive polarons could be more than an order of magnitude stronger than those between bare excitons.

We develop a theoretical model for the interaction strength between a pair of exciton-polaritons with finite and parallel static electric-dipole moments. In stark contrast to simple perturbative arguments predicting the interaction to be reduced due to the hybridisation with cavity photons, we find that the strong coupling to a cavity mode leads to a drastic enhancement of the polariton interactions as compared to bare excitons. This enhancement is caused by the small polariton mass due to its photonic component, which increases the kinetic cost associated with minimising the dipole-dipole repulsion. Microscopic numerical calculations for one-dimensional geometries show that the interaction strength for the lower polariton branch is maximal when the exciton and the cavity mode are close to resonance, and can easily exceed state-of-the-art cavity decay rates for realistic confinement lengths, thus entering the polariton blockade regime. Analytical extension of our results to two-dimensional polariton systems highlight dipolar polaritons as a most promising solid-state platform to realize strongly interacting photonic systems., Main manuscript: 8 pages, 5 figures. Appendix: 4 pages, 3 figures

We propose a novel platform to realize highly-tunable, frustrated Hubbard physics with topological order in multi-layer moire structures. Identifying a layer degree of freedom as a pseudo spin, allows us to retain SU(2) symmetry, while controlling ring exchange processes and concurrently quenching the kinetic energy by large external magnetic fields. This way, a broad class of interacting Hofstadter states and their transitions can be studied. Remarkably, in the limit of strong interactions the system becomes Mott insulating and we find exceptionally stable spin liquid phases which are induced by the magnetic field. As the magnetic flux can be easily tuned in moire systems, our approach provides a promising route towards the experimental realization and control of topologically ordered phases of matter. We also discuss how layer pseudo-spin can be probed in near-term experiments, Comment: 9+3 pages, 4+5 figures

The interaction between a mobile quantum impurity and a bosonic bath leads to the formation of quasiparticles, termed Bose polarons. The elementary properties of Bose polarons, such as their mutual interactions, can differ drastically from those of the bare impurities. Here, we explore Bose polaron physics in a two-dimensional nonequilibrium setting by injecting $\sigma^-$ polarised exciton-polariton impurities into a bath of coherent $\sigma^+$ polarised polaritons generated by resonant laser excitation of monolayer MoSe$_2$ embedded in an optical cavity. By exploiting a biexciton Feshbach resonance between the impurity and the bath polaritons, we tune the interacting system to the strong-coupling regime and demonstrate the coexistence of two new quasiparticle branches. Using time-resolved pump-probe measurements we observe how polaron dressing modifies the interaction between impurity polaritons. Remarkably, we find that the interactions between high-energy polaron quasiparticles, that are repulsive for small bath occupancy, can become attractive in the strong impurity-bath coupling regime. Our experiments provide the first direct measurement of Bose polaron-polaron interaction strength in any physical system and pave the way for exploration and control of many-body correlations in driven-dissipative settings., Comment: 7 pages, 5 figures

Quantum confinement is the discretization of energy when motion of particles is restricted to length scales smaller than their de Broglie wavelength. The experimental realization of this effect has had wide ranging impact in diverse fields of physics and facilitated the development of new technologies. In semiconductor physics, quantum confinement of optically excited quasiparticles, such as excitons or trions, is typically achieved by modulation of material properties - an approach crucially limited by the lack of insitu tunability and scalability of confining potentials. Achieving fully tunable quantum confinement of optical excitations has therefore been an outstanding goal in quantum photonics. Here, we demonstrate electrically controlled quantum confinement of neutral excitons in a gate-defined monolayer p-i-n diode. A combination of dc Stark shift induced by large in-plane fields and a previously unknown confining mechanism based on repulsive interaction between excitons and free charges ensures tight exciton confinement in the narrow neutral region. Quantization of exciton motion manifests in multiple discrete, spectrally narrow, voltage-dependent optical resonances that emerge below the free exciton resonance. Our measurements reveal several unique physical features of these quantum confined excitons, including an in-plane dipolar character, one-dimensional center-of-mass confinement, and strikingly enhanced exciton size in the presence of magnetic fields. Our method provides an experimental route towards creating scalable arrays of identical single photon sources, which will constitute building blocks of strongly correlated photonic systems.

The elementary optical excitations in two dimensional semiconductors hosting itinerant electrons are attractive and repulsive polarons -- excitons that are dynamically screened by electrons. Exciton-polarons have hitherto been studied in translationally invariant degenerate Fermi systems. Here, we show that electronic charge order breaks the excitonic translational invariance and leads to a direct optical signature in the exciton-polaron spectrum. Specifically, we demonstrate that new optical resonances appear due to spatially modulated interaction between excitons and electrons in an incompressible Mott state. Our observations demonstrate that resonant optical spectroscopy provides an invaluable tool for studying strongly correlated states, such as Wigner crystals and density waves, where exciton-electron interactions are modified by the emergence of new electronic charge or spin order., Main text: 9 pages, 5 figures; Supplemental Material: 9 pages, 5 figures

Van der Waals heterostructures combining two-dimensional magnetic and semiconducting layers constitute a promising platform for interfacing magnetism, electronics, and optics. Here, we use resonant optical reflection spectroscopy to the observe magnetic proximity effect in a gate-tunable MoSe$_2$/CrBr$_3$ heterostructure. High quality of the interface leads to a giant zero-field splitting of the K and K' valley excitons in MoSe$_2$, equivalent to an external magnetic field of 12 T, with a weak but distinct electric field dependence that hints at potential for electrical control of magnetization. The magnetic proximity effect allows us to use resonant optical spectroscopy to fully characterize the CrBr$_3$ magnet, determining the easy-axis coercive field, the magnetic anisotropy energy, and critical exponents associated with spin susceptibility and magnetization.

It is widely assumed that photons cannot be manipulated using electric or magnetic fields. Even though hybridization of photons with electronic polarization to form exciton-polaritons has paved the way to a number of groundbreaking experiments in semiconductor microcavities, the neutral bosonic nature of these quasiparticles has severely limited their response to external gauge fields. Here, we demonstrate polariton acceleration by external electric and magnetic fields in the presence of nonperturbative coupling between polaritons and itinerant electrons, leading to formation of new quasiparticles termed polaron-polaritons. We identify the generation of electron density gradients by the applied fields to be primarily responsible for inducing a gradient in polariton energy, which in turn leads to acceleration along a direction determined by the applied fields. Remarkably, we also observe that different polarization components of the polaritons can be accelerated in opposite directions when the electrons are in ν=1 integer quantum Hall state., Physical Review X, 10 (1), ISSN:2160-3308

Two dimensional semiconductors provide an ideal platform for exploration of linear exciton and polariton physics, primarily due to large exciton binding energy and strong light-matter coupling. These features, however, generically imply reduced exciton-exciton interactions, hindering the realisation of active optical devices such as lasers or parametric oscillators. Here, we show that electrical injection of itinerant electrons into monolayer molybdenum diselenide allows us to overcome this limitation: dynamical screening of exciton-polaritons by electrons leads to the formation of new quasi-particles termed polaron-polaritons that exhibit unexpectedly strong interactions as well as optical amplification by Bose-enhanced polaron-electron scattering. To measure the nonlinear optical response, we carry out time-resolved pump-probe measurements and observe polaron-polariton interaction enhancement by a factor of 50 ($0.5 ��$eV $��$m$^2$) as compared to exciton-polaritons. Concurrently, we measure a spectrally integrated transmission gain of the probe field of $\gtrsim 2$ stemming from stimulated scattering of polaron-polaritons. We show theoretically that the non-equilibrium nature of optically excited quasiparticles favours a previously unexplored interaction mechanism stemming from a phase-space filling in the screening cloud, which provides an accurate explanation of the strong repulsive interactions observed experimentally. Our findings show that itinerant electron-exciton interactions provide an invaluable tool for electronic manipulation of optical properties, demonstrate a new mechanism for dramatically enhancing polariton-polariton interactions, and pave the way for realisation of nonequilibrium polariton condensates.

Nonperturbative coupling between cavity photons and excitons leads to formation of hybrid light-matter excitations termed polaritons. In structures where photon absorption leads to creation of excitons with aligned permanent dipoles, the elementary excitations, termed dipolar polaritons, are expected to exhibit enhanced interactions. Here, we report a substantial increase in interaction strength between dipolar polaritons as the size of the dipole is increased by tuning the applied gate voltage. To this end, we use coupled quantum well structures embedded inside a microcavity where coherent electron tunneling between the wells controls the size of the excitonic dipole. Modifications of the interaction strength are characterized by measuring the changes in the reflected intensity of light when polaritons are driven with a resonant laser. Factor of 6.5 increase in the interaction strength to linewidth ratio that we obtain indicates that dipolar polaritons could be used to demonstrate a polariton blockade effect and thereby form the building blocks of many-body states of light., 12 pages, 4 figures. Supplementary note

Advent of new materials such as van der Waals heterostructures, propels new research directions in condensed matter physics and enables development of novel devices with unique functionalities. Here, we show experimentally that a monolayer of MoSe2 embedded in a charge controlled heterostructure can be used to realize an electrically tunable atomically-thin mirror, that effects 90% extinction of an incident field that is resonant with its exciton transition. The corresponding maximum reflection coefficient of 45% is only limited by the ratio of the radiative decay rate to the linewidth of exciton transition and is independent of incident light intensity up to 400 Watts/cm2. We demonstrate that the reflectivity of the mirror can be drastically modified by applying a gate voltage that modifies the monolayer charge density. Our findings could find applications ranging from fast programmable spatial light modulators to suspended ultra-light mirrors for optomechanical devices.

Resonant laser spectroscopy of a negatively charged self-assembled quantum dot is utilized to measure the temperature of a three dimensional fermionic reservoir down to 100mK. With a magnetic field applied to the quantum dot the single charged ground state is split by the Zeeman energy. As the quantum dot is in tunnel contact with a thermal electron reservoir, a thermal occupation of the quantum dot spin states is enforced by co-tunneling processes. Resonant laser induced fluorescence is used in order to measure the thermal quantum dot spin state population., Comment: 11 pages, 4 figures

Cavity-polaritons in semiconductor photonic structures have emerged as a test bed for exploring non-equilibrium dynamics of quantum fluids in an integrated solid-state device setting. Several recent experiments demonstrated the potential of these systems for revealing quantum many-body physics in driven-dissipative systems. So far, all experiments have relied on fully integrated devices with little to no flexibility for modification of device properties. Here, we present a novel approach for realizing confined cavity-polaritons that enables in-situ tuning of the cavity length and thereby of the polariton energy and lifetime. Our setup is based on a versatile semi-integrated low-temperature fiber-cavity platform, which allows us to demonstrate the formation of confined polaritons (or polariton boxes) with unprecedented quality factors. At high pump powers, we observe clear signatures of polariton lasing. In the strong-confinement limit, the fiber-cavity system could enable the observation of the polariton-blockade effect.

The interaction between a single confined spin and the spins of a Fermionic reservoir leads to one of the most spectacular phenomena of many body physics -- the Kondo effect. Here we report the observation of Kondo correlations in optical absorption measurements on a single semiconductor quantum dot tunnel-coupled to a degenerate electron gas. In stark contrast to transport experiments, absorption of a single photon leads to an abrupt change in the system Hamiltonian and a quantum quench of Kondo correlations. By inferring the characteristic power law exponents from the experimental absorption line-shapes, we find a unique signature of the quench in the form of an Anderson orthogonality catastrophe, originating from a vanishing overlap between the initial and final many-body wave-functions. We also show that the power-law exponents that determine the degree of orthogonality can be tuned by applying an external magnetic field which gradually turns the Kondo correlations off. Our experiments demonstrate that optical measurements on single artificial atoms offer new perspectives on many-body phenomena previously studied exclusively using transport spectroscopy. Moreover, they initiate a new paradigm for quantum optics where many-body physics influences electric field and intensity correlations.

Cavity quantum-electrodynamics experiments using an atom coupled to a single radiation-field mode have played a central role in testing foundations of quantum mechanics, thus motivating solid-state implementations using single quantum dots coupled to monolithic nano-cavities. In stark contrast to their atom based counterparts, the latter experiments revealed strong cavity emission, even when the quantum dot is far off resonance. Here we present experimental and theoretical results demonstrating that this effect arises from the mesoscopic nature of quantum dot confinement, ensuring the presence of a quasi-continuum of transitions between excited quantum dot states that are enhanced by the cavity mode. Our model fully explains photon correlation measurements demonstrating that photons emitted at the cavity frequency are essentially uncorrelated with each other even though they are generated by a single quantum dot., 5 pages, 4 figures

Cavity quantum electrodynamics has attracted substantial interest, both due to its potential role in the field of quantum information processing and as a testbed for basic experiments in quantum mechanics. Here, we show how cavity quantum electrodynamics using a tunable photonic crystal nanocavity in the strong coupling regime can be used for single quantum dot spectroscopy. From the distinctive avoided crossings observed in the strongly coupled system we can identify the neutral and single positively charged exciton as well as the biexciton transitions. Moreover we are able to investigate the fine structure of those transitions and to identify a novel cavity mediated mixing of bright and dark exciton states, where the hyperfine interactions with lattice nuclei presumably play a key role. These results are enabled by a deterministic coupling scheme which allowed us to achieve unprecedented coupling strengths in excess of 0.15 meV., Comment: 5 pages, 3 figures

We demonstrate electromagnetically induced transparency (EIT) in a sample of rubidium atoms, trapped in an optical dipole trap. Mixing a small amount of $\sigma^-$-polarized light to the weak $\sigma^+$-polarized probe pulses, we are able to measure the absorptive and dispersive properties of the atomic medium at the same time. Features as small as 4 kHz have been detected on an absorption line with 20 MHz line width., Comment: 5 pages, 8 figures

We have performed all-optical measurements of spin relaxation in single self-assembled InAs/GaAs quantum dots (QD) as a function of static external electric and magnetic fields. To study QD spin dynamics we measure the degree of resonant absorption which results from a competition between optical spin pumping induced by the resonant laser field and spin relaxation induced by reservoirs. Fundamental interactions that determine spin dynamics in QDs are hyperfine coupling to QD nuclear spin ensembles, spin-phonon coupling and exchange-type interactions with a nearby Fermi sea of electrons. We show that the strength of spin relaxation generated by the three fundamental interactions can be changed by up to five orders of magnitude upon varying the applied electric and magnetic fields. We find that the strength of optical spin pumping that we use to study the spin relaxation is determined predominantly by hyperfine-induced mixing of single-electron spin states at low magnetic fields and heavy-light hole mixing at high magnetic fields. Our measurements allow us to determine the rms value of the hyperfine (Overhauser) field to be ~15 mTesla with an electron g-factor of g_e=0.6 and a hole mixing strength of |epsilon|^2 = 0.0005., Comment: submitted to Physical Review B, 19 Pages in RevTeX4 format

Bose condensation of interacting bosons in a two-dimensional random potential is studied. The Gross-Pitaevskii equation is solved to determine the spatially-varying order parameter and the localization length as a function of the disorder, the interaction strength, and the condensate density. A finite temperature of condensation is obtained thereby. The results are applied to determination of the superradiant decay of excitons in a GaAs quantum well., Comment: 10 pages, 5 figures, LaTeX 2e

The light-matter interaction can be utilized to qualitatively alter physical properties of materials. Recent theoretical and experimental studies have explored this possibility of controlling matter by light based on driving many-body systems via strong classical electromagnetic radiation, leading to a time-dependent Hamiltonian for electronic or lattice degrees of freedom. To avoid inevitable heating, pump-probe setups with ultrashort laser pulses have so far been used to study transient light-induced modifications in materials. Here, we pursue yet another direction of controlling quantum matter by modifying quantum fluctuations of its electromagnetic environment. In contrast to earlier proposals on light-enhanced electron-electron interactions, we consider a dipolar quantum many-body system embedded in a cavity composed of metal mirrors and formulate a theoretical framework to manipulate its equilibrium properties on the basis of quantum light-matter interaction. We analyze hybridization of different types of the fundamental excitations, including dipolar phonons, cavity photons, and plasmons in metal mirrors, arising from the cavity confinement in the regime of strong light-matter interaction. This hybridization qualitatively alters the nature of the collective excitations and can be used to selectively control energy-level structures in a wide range of platforms. Most notably, in quantum paraelectrics, we show that the cavity-induced softening of infrared optical phonons enhances the ferroelectric phase in comparison with the bulk materials. Our findings suggest an intriguing possibility of inducing a superradiant-type transition via the light-matter coupling without external pumping. We also discuss possible applications of the cavity-induced modifications in collective excitations to molecular materials and excitonic devices., Physical Review X, 10 (4), ISSN:2160-3308

In this dissertation, we explore the interplay between strongly correlated electrons and optical polaritons confined to two dimensions. This is implemented experimentally with gallium arsenide quantum wells hosting a two-dimensional electron gas (2DEG). The structure is further embedded in an optical microcavity to reach the strong light-matter coupling regime. Optically created excitons in the quantum well act as impurities in the surrounding electron gas, forming collective excitations termed Fermi polarons. The resulting polaron-polariton modes are used to study the quantum Hall effect and allow tailoring photonic properties via control over the electrons. The electronic ground states in the integer and fractional quantum Hall regimes are investigated using polariton spectroscopy. This tool allows probing the spin polarization of the 2DEG by optical means. Many-body spin textures are studied around integer filling, in particular for the case of vanishing g-factor which is expected to favor large skyrmions. Using an optimized device structure, we advance polariton spectroscopy by drastically reducing unwanted modifications of the electron density upon optical illumination. We observe coupling of the polaron-polaritons to different fractional quantum Hall states as the filling factor is varied. In a second part, a polariton Hall bar device is fabricated to investigate the connection between electronic and polariton transport in two dimensions. We demonstrate acceleration of polaritons by shaping the electron density with external electric and magnetic fields. For a spin polarized electron gas, we demonstrate the creation of spin density gradients. They are used to route polaritons on the Hall bar device in a spin selective manner, reminiscent of an optical spin Hall effect. In the last part, we present four-wave mixing experiments performed with polaron-polaritons. The nonlinear optical response of polaritons is measured while they are coupled to different quantum Hall states. A surprising increase in nonlinear signal is found for the specific filling fractions 2/3 and 2/5. These results demonstrate enhanced polariton-polariton interactions both compared to other fillings and to the case of exciton-polaritons without 2DEG. This constitutes a step towards polariton blockade and suggests that nonlinear optics may allow us to extract properties of the correlated electronic states beyond linear spectroscopy.