Controlling light-matter interactions with two-dimensional semiconductors at cryogenic temperatures

Efficient interactions between solid-state systems and photons are the basis for the emerging quantum technologies. An outstanding challenge in facilitating light-matter interactions can be solved by the introduction of an optical resonator. By trapping the photons in a small volume, the interaction time with the solid-state system is increased in such quantum optics experiments. This thesis focuses on light-matter interactions with two-dimensional semiconductor transition-metal dichalcogenides. Strongly bound electron-hole pairs (excitons) in transition-metal dichalcogenides exhibit large oscillator strength and offer a valley degree-of-freedom in the band structure that interacts with specific polarization of light, this makes them an excellent prospect for quantum optics experiments. In this work, we realize light-matter interactions with monolayer tungsten diselenide in two experiments using different optical resonators. In the first experiment, we investigated a hybrid system combining localized plasmons, on the surface of gold nanodisks, with a monolayer tungsten diselenide. The coherent interference of excitons with plasmons yield an asymmetric optical response, known as Fano line-shape, in the limit of weak light-matter coupling. This optical response of the hybrid system is corroborated with a three-level model. In addition, magnetic field-induced valley-dependent exciton energy splitting is harnessed to achieve chiral reflection. The second experiment develops a modular tunable cavity at cryogenic temperatures. A major technological challenge involving scalable cryogenic experiments is mechanical vibrations. After a thorough understanding of the functionality of a closed-cycle cryostat, a variety of vibration-reduction techniques were applied to develop an open cavity setup. In comparison to the vibrations on the standard closed-cycle cryostat, we attain 50-fold reduction to reach the root-mean-square stability of less than 100 pm over the entire period of a cooling cycle. This enables the operation of a high-finesse cavity at low temperatures. Subsequently, the versatility of the platform was demonstrated in a controlled experiment with monolayer tungsten diselenide. Exciton-polaritons were observed in the high cooperativity strong-coupling regime.Effiziente Wechselwirkungen zwischen Festkörpersystemen und Photonen bilden die Grundlage für die aufstrebenden Quantentechnologien. Die zentrale Herausforderung bei der Kontrolle von Wechselwirkungen zwischen Licht undMaterie kann durch die Verwendung eines optischen Resonators gelöst werden. Durch Einfangen der Photonen in einem kleinen Volumen wird die Wechselwirkungszeit mit dem Festkörpersystem in solchen quantenoptischen Experimenten erhöht. Diese Arbeit konzentriert sich auf Wechselwirkungen zwischen Licht und Materie in zweidimensionalen halbleitenden Übergangsmetalldichalkogeniden. Stark gebundene Elektron-Loch-Paare (Exzitonen) in Übergangsmetalldichalkogeniden weisen eine große Oszillatorstärke auf und bieten einen weiteren Pseudospin-Freiheitsgrad in der Bandstruktur, der mit spezifischer Polarisation des Lichts interagiert. In dieser Arbeit realisieren wir in zwei Experimenten Licht-Materie-Wechselwirkungenmit monolagigem Wolframdiselenid mit unterschiedlichen optischen Resonatoren. Im ersten Experiment untersuchten wir ein Hybridsystem, das lokalisierte Plasmonen auf der Oberfläche von Goldnanoscheiben mit monolagigem Wolframdiselenid kombiniert. Kohärente Interferenz von Exzitonen mit Plasmonen ergibt eine asymmetrische spektrale Antwort - als Fano-Linienformbekannt - im Grenzfall schwacher Licht-Materie-Kopplung. Diese optische Antwort des Hybridsystems wird durch ein Drei-Niveau-Modell bestätigt. Zusätzlich wird die magnetfeldinduzierte Energieaufspaltung der Exzitonen genutzt, um chirale Reflexion zu ermitteln. Das zweite Experiment entwickelt einen modular abstimmbaren optischen Resonator bei kryogenen Temperaturen. Eine große technologische Herausforderung bei skalierbaren kryogenen Experimenten sind mechanische Schwingungen. Basierend auf einem gründlichen Verständnis der Funktionalität eines Kryostaten mit geschlossenem Kreislauf wurden verschiedene Techniken zur Schwingungsreduzierung implementiert, um einen Aufbau mit offenen optischen Resonatoren zu entwickeln. Im Vergleich zu Vibrationen des Standardsystems wurde eine 50-fache Verminderung erreicht, und somit über den gesamten Zeitraum eines Kühlzyklus eine mittlere Stabilität von weniger als 100 pm gewährleistet. Dies ermöglicht den Betrieb eines optischen Resonators mit hoher Finesse bei niedrigen Temperaturen. Anschließend wurde die Vielseitigkeit des Systems in einemkontrollierten Experimentmit monolagigem Wolframdiselenid demonstriert. Exziton-Polaritonen wurden im Regime starker Kopplung mit hoher Kooperativität beobachtet

Coherence Times of Bose-Einstein Condensates beyond the Shot-Noise Limit via Superfluid Shielding

We demonstrate a new way to extend the coherence time of separated Bose-Einstein condensates that involves immersion into a superfluid bath. When both the system and the bath have similar scattering lengths, immersion in a superfluid bath cancels out inhomogeneous potentials either imposed by external fields or inherent in density fluctuations due to atomic shot noise. This effect, which we call superfluid shielding, allows for coherence lifetimes beyond the projection noise limit. We probe the coherence between separated condensates in different sites of an optical lattice by monitoring the contrast and decay of Bloch oscillations. Our technique demonstrates a new way that interactions can improve the performance of quantum devices.Samsung Scholarship FoundationNational Science Foundation (U.S.) (MIT-Harvard Center for Ultracold Atoms. Grant 1506369)United States. Air Force Office of Scientific Research. Multidisciplinary University Research Initiative (Grants FA9550-14-1-0035 and W911NF-14-1-0003

Open-cavity in closed-cycle cryostat as a quantum optics platform

The introduction of an optical resonator can enable efficient and precise interaction between a photon and a solid-state emitter. It facilitates the study of strong light-matter interaction, polaritonic physics and presents a powerful interface for quantum communication and computing. A pivotal aspect in the progress of light-matter interaction with solid-state systems is the challenge of combining the requirements of cryogenic temperature and high mechanical stability against vibrations while maintaining sufficient degrees of freedom for in-situ tunability. Here, we present a fiber-based open Fabry-P\'{e}rot cavity in a closed-cycle cryostat exhibiting ultra-high mechanical stability while providing wide-range tunability in all three spatial directions. We characterize the setup and demonstrate the operation with the root-mean-square cavity length fluctuation of less than $90$ pm at temperature of $6.5$ K and integration bandwidth of $100$ kHz. Finally, we benchmark the cavity performance by demonstrating the strong-coupling formation of exciton-polaritons in monolayer WSe$_2$ with a cooperativity of $1.6$. This set of results manifests the open-cavity in a closed-cycle cryostat as a versatile and powerful platform for low-temperature cavity QED experiments.Comment: 10 pages, 8 figure

Open-Cavity in Closed-Cycle Cryostat as a Quantum Optics Platform

The introduction of an optical resonator can enable efficient and precise interaction between a photon and a solid-state emitter. It facilitates the study of strong light-matter interaction, polaritonic physics and presents a powerful interface for quantum communication and computing. A pivotal aspect in the progress of light-matter interaction with solid-state systems is the challenge of combining the requirements of cryogenic temperature and high mechanical stability against vibrations while maintaining sufficient degrees of freedom for in situ tunability. Here, we present a fiber-based open Fabry-Pérot cavity in a closed-cycle cryostat exhibiting ultrahigh mechanical stability while providing wide-range tunability in all three spatial directions. We characterize the setup and demonstrate the operation with the root-mean-square cavitylength fluctuation of less than 90 pm at temperature of 6.5 K and integration bandwidth of 100 kHz. Finally, we benchmark the cavity performance by demonstrating the strong-coupling formation of exciton polaritons in monolayer WSe2 with a cooperativity of 1.6. This set of results manifests the open cavity in a closed-cycle cryostat as a versatile and powerful platform for low-temperature cavity QED experiments