Strongly correlated Fermions strongly coupled to light

Strong quantum correlations in matter are responsible for some of the most extraordinary properties of material, from magnetism to high-temperature superconductivity, but their integration in quantum devices requires a strong, coherent coupling with photons, which still represents a formidable technical challenge in solid state systems. In cavity quantum electrodynamics, quantum gases such as Bose-Einstein condensates or lattice gases have been strongly coupled with light. However, neither Fermionic quantum matter, comparable to electrons in solids, nor atomic systems with controlled interactions, have thus far been strongly coupled with photons. Here we report on the strong coupling of a quantum-degenerate unitary Fermi gas with light in a high finesse cavity. We map out the spectrum of the coupled system and observe well resolved dressed states, resulting from the strong coupling of cavity photons with each spin component of the gas. We investigate spin-balanced and spin-polarized gases and find quantitative agreement with ab-initio calculation describing light-matter interaction. Our system offers complete and simultaneous control of atom-atom and atom-photon interactions in the quantum degenerate regime, opening a wide range of perspectives for quantum simulation.

Jean-Philippe Brantut, Victor Youri Helson, Hideki Konishi, Kevin Etienne Robert Roux

Strong quantum correlations in matter are responsible for some of the most extraordinary properties of materials, from magnetism to high-temperature superconductivity, but their integration in quantum devices requires a strong, coherent coupling with photons, which still represents a formidable technical challenge in solid state systems. In cavity quantum electrodynamics, quantum gases such as Bose-Einstein condensates or lattice gases have been strongly coupled with light. However, neither Fermionic quantum matter, comparable to electrons in solids, nor atomic systems with controlled interactions, have thus far been strongly coupled with photons. Here we report on the strong coupling of a quantum-degenerate unitary Fermi gas with light in a high finesse cavity. We map out the spectrum of the coupled system and observe well resolved dressed states, resulting from the strong coupling of cavity photons with each spin component of the gas. We investigate spin-balanced and spin-polarized gases and find quantitative agreement with ab initio calculation describing light-matter interaction. Our system offers complete and simultaneous control of atom-atom and atom-photon interactions in the quantum degenerate regime, opening a wide range of perspectives for quantum simulation.

2020

Tailored-Potential Pyramidal Quantum Dot Heterostructures

Valentina Troncale

Semiconductor quantum dots are usually compared to artificial atoms, because their electronic structure consists of discrete energy levels as for natural atoms. These artificial systems are integrated in solid materials and can be localized with a spatial precision of the order of nanometers. Besides, they conserve their quantum properties even at quite high temperatures (∼ 10 K). These properties make quantum dots one of the most suitable systems for the realization of quantum devices and computers. However, the energy states and the optical properties of quantum dots are much more complicated than for atoms, because a quantum dot is never an isolated potential well. Instead, its electronic structure depends on the crystallin structure of the semiconductor material and on the Coulomb ion-electron and electron-electron correlations. In particular, the presence of several valence bands and their mixing, induced by quantum confinement, gives rise to novel properties which are still not completely understood and exploited in applications. To get a major advance in this field, a full deterministic control of the spatial shape of the quantum confinement is needed, combined with a deeper understanding of the connections between electronic and optical properties. This thesis work has these two main objectives. We realized and experimentally studied different quantum dot systems, in pyramidal hetero-structures grown with MOCVD techniques. These systems allowed the realization of several different geometries for the carrier confining potential, with a precision in the order of nanometers. The optical characterization has been obtained in particular by means of polarization-resolved microphotoluminescence, magneto-photoluminescence, excitation photoluminescence (PLE), and interferometry techniques. For single quantum dots, we have observed and characterized for the first time new excitonic complexes, arising from excited hole states. This allowed a full caracterizatioon of the valence band hole states in our peculiar system. By means of photon correlation measurements, we have also experimentally demonstrated that, even in presence of a large family of exciton states, these quantum dot systems can emit single photons. We have then realized much more complex quantum dot structures, double dot systems (quantum dot molecules) and a completely new system called Dot-in-Dot (DiD). This latter is composed by a small inner dot surrounded by an electrostatic potential well (which can be considered as an outer elongated dot). Such a composite system is characterized by a strong valence band mixing. This state superposition is however very sensitive to small variations of the confining potential. Therefore the degree of valence band mixing can be easily switched by the introduction of a week external field. Since the valence band mixing determines the polarization properties of the emitted light, the DiD changes the polarization properties of its emission spectrum under the action of an external field. In particular, we have experimentally demonstrated this effect for an external static magnetic field, while we have numerically predicted a very similar effect for a static electric field. In the latter case, the polarization switching is a direct consequence of the quantum confined Stark effect induced in the DiD. Hence the DiD appears to be an ideal candidate for realizing emitters of single photons with tunable and controllable polarization.

EPFL2010

Cavity Quantum Electrodynamics with strongly correlated fermions

Kevin Etienne Robert Roux

This thesis presents the first cavity quantum electrodynamics experiments performed with a degenerate gas of

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Li with strong atom-atom interactions. The first part of this manuscript describes the design and the building of the apparatus that has been especially developed to bring together a high-finesse optical cavity and a strongly interacting Fermi gas. I described how the cavity and all the laser-cooling procedure can be realized in the same vacuum chamber, thus speeding up the production cycle of the degenerate Fermi gas.This new experimental apparatus is the first of its kind combining these two field of quantum physics. Placing a quantum gas of Fermions within an optical resonator gives a important technical advantages, allowing for the fast, all-optical production of a degenerate gas of

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Li. We apply an technique that make it possible to modify the longitudinal structure of the cavity trap to cancel its lattice structure. It increases the phase space density after the evaporative cooling leading to a ultracold gas at temperature lower than ten percent of the Fermi temperature. We describe how magnetic field allows us to tune the interatomic interactions, making use of the broad Feshbach resonance of

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Li at

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G and how we characterize the thermodynamic properties of the ultracold Fermi gas. The direct observation of phase separation for a spin-imbalanced Fermi gas between a fully paired region at the cloud center, surrounded by a spin-polarized shell experimentally proves the apparition of superfluidity at low enough temperature.The first experiment showing the strong coupling between the cavity photons and the strongly interacting Fermi gas is shown in this manuscript. The observation of large avoided-crossings when performing cavity transmission spectroscopy experiment are the experimental smoking gun of the strong light-matter coupling regime. We observe the expected scaling of the light-matter coupling strength with the number of atoms in the gas, proving the coherent coupling of the atoms with the cavity field.The thirs part of this manuscript presents the first cavity quantum electrodynamics experiment where a pairs of atoms couple to the cavity photons, forming a new dressed state: the pair-polariton. This dressed state inherits from its atomic part the characteristics of the many-body physics of the strongly interacting Fermi gas. We confirm experimentally that the properties of the short-range two-body correlation function, know as Tan's contact, can directly be measured optically, on the pair-polariton transmission spectrum. We observe the coherent coupling of the ground state Fermion pairs with the cavity photons and use the pair-polariton to perform single shot, real-time, weakly destructive measurement of the short range two body correlation function. This new measurement of Tan's contact allows to follow in-time the evolution of a single system, contrasting with existing techniques.The last part of this thesis will show experiment carried far in the dispersive regime, where both the cavity resonance and the probe laser frequency are far detuned from the atomic resonance. We will discuss how we can, in this regime, measure the atom number evolution in-time, with a weak destructivity. We show that optical non-linearity emerges and depends on the atom-atom interaction strength. Last we implement a pump not aligned with the cavity axis that allows to create long-range interactions between atoms.

EPFL2022