arXiv subjects

Georg M. Bruun

Publications and source records attributed to Georg M. Bruun.

At least 37 records · Page 2Linked to original sources

arXiv2022

Strong photon interactions from weakly interacting particles

The hybridization of light and matter excitations in the form of polaritons has enabled major advances in understanding and controlling optical nonlinearities. Entering the quantum regime of strong interactions between individual photons has however remained challenging since the strength of achievable polariton interactions is typically limited by the available interactions in the material. Here, we investigate collisions between dark-state polaritons in three-level systems and discover a resonant process that yields effective interactions, which are much larger than the underlying interaction between their matter constituents. We systematically investigate the underlying mechanism and demonstrate a substantial enhancement of polariton interactions by several orders of magnitude. This suggests a promising approach to quantum nonlinear optics in a range of physical settings, from atomic gases to excitons in semiconductors and two-dimensional bilayer materials.

cond-mat.quant-gas↗

arXiv2022

Equilibrium and non-equilibrium dynamics of a hole in a bilayer antiferromagnet

The dynamics of charge carriers in lattices of quantum spins is a long standing and fundamental problem. Recently, a new generation of quantum simulation experiments based on atoms in optical lattices has emerged that gives unprecedented insights into the detailed spatial and temporal dynamics of this problem, which compliments earlier results from condensed matter experiments. Focusing on observables accessible in these new experiments, we explore here the equilibrium as well as non-equilibrium dynamics of a mobile hole in two coupled antiferromagnetic spin lattices. Using a self-consistent Born approximation, we calculate the spectral properties of the hole in the bilayer and extract the energy bands of the quasiparticles, corresponding to magnetic polarons that are either symmetric or anti-symmetric under layer exchange. These two kinds of polarons are degenerate at certain momenta due to the antiferromagnetic symmetry, and we, furthermore, examine how the momentum of the ground state polaron depends on the interlayer coupling strength. The long time dynamics of a hole initially created in one layer is shown to be characterised by oscillations between the two layers with a frequency given by the energy difference between the symmetric and the anti-symmetric polaron. We finally demonstrate that the expansion velocity of a hole initially created at a given lattice site is governed by the ballistic motion of polarons. It moreover depends non-monotonically on the interlayer coupling, eventually increasing as a quantum phase transition to a disordered state is approached.

cond-mat.str-el↗

arXiv2022

Mobile impurity probing a two-dimensional superfluid phase transition

The use of atomically sized quantum systems as highly sensitive measuring devices represents an exciting and quickly growing research field. Here, we explore the properties of a quasiparticle formed by a mobile impurity interacting with a two-dimensional fermionic superfluid. The energy of the quasiparticle is shown to be lowered by superfluid pairing as this increases the compressibility of the Fermi gas, thereby making it easier for the impurity to perturb its surroundings. We demonstrate that the fundamentally discontinuous nature of the superfluid to normal phase transition of a two-dimensional system, leads to a rapid increase in the quasiparticle energy around the critical temperature. The magnitude of this increase exhibits a nonmonotonic behavior as a function of the pairing strength with a sizable maximum in the cross-over region, where the spatial extend of the Cooper pairs is comparable to the interparticle spacing. Since the quasiparticle energy is measurable with present experimental techniques, our results illustrate how impurities entangled with their environment can serve as useful probes for non-trivial thermal and quantum correlations.

cond-mat.quant-gas↗

arXiv2021

Polaritons in an electron gas -- quasiparticles and Landau effective interactions

Two-dimensional semiconductors inside optical microcavities have emerged as a versatile platform to explore new hybrid light-matter quantum states. The strong light-matter coupling leads to the formation of exciton-polaritons, which in turn interact with the surrounding electron gas to form quasiparticles called polaron-polaritons. Here, we develop a general microscopic framework to calculate the properties of these quasiparticles such as their energy and the interactions between them. From this, we give microscopic expressions for the parameters entering a Landau theory for the polaron-polaritons, which offers a simple yet powerful way to describe such interacting light-matter many-body systems. As an example of the application of our framework, we then use the ladder approximation to explore the properties of the polaron-polaritons. We furthermore show that they can be measured in a non-demolition way via the light transmission/reflection spectrum of the system. Finally, we demonstrate that the Landau effective interaction mediated by electron-hole excitation is attractive leading to red shifts of the polaron-polaritons. Our work provides a systematic framework to study exciton-polaritons in electronically doped two-dimensional materials such as novel van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2021

A mobile ion in a Fermi sea

The remarkable single particle control of individual ions combined with the versatility of ultracold atomic gases makes hybrid ion-atom system an exciting new platform for quantum simulation of few- and many-body quantum physics. Here, we study theoretically the properties of a mobile ion immersed in a quantum degenerate gas of fermionic atoms. Using an effective low-energy atom-ion interaction together with a well established approach that includes exactly two-body correlations, we calculate the full spectral response of the ion and demonstrate the existence of several quasiparticle branches, which are charged analogues of the Fermi polaron observed in neutral atomic gases. Due to the long-range nature of the atom-ion interaction, these ionic Fermi polarons have several properties distinct from their neutral counterparts such as the simultaneous presence of several stable states and smooth transitions from repulsive to attractive polarons with increasing interaction strength. Surprisingly, the residue of the ionic polaron is shown to increase with the Fermi density for fixed interaction strength, which is in marked contrast to the neutral polaron. The properties of the ionic polaron approach that of the neutral polaron only in the low density limit where the average interparticle spacing is larger than the characteristic length of the atom-ion interaction. We finally analyse the effects of the Fermi gas on the molecular ions, which are bound atom-dimer states.

cond-mat.quant-gas↗

arXiv2021

Nonlinear optical response of resonantly driven polaron-polaritons

Exciton polaritons in two-dimensional semiconductors inside microcavities are powerful platforms to explore hybrid light-matter quantum systems. Here, we study a macroscopic coherent population of the lowest energy state of polaron-polaritons, which are quasiparticles formed by the dressing of exciton polaritons by particle-hole excitations in a surrounding electron gas. Using a non-perturbative many-body theory to describe exciton-electron correlations combined with a non-equilibrium theory for the macroscopically populated state, we show that the electrons strongly affect the collective properties of the polaron-polaritons. This stems from the dependence of the polaron-polariton energy and the interaction between them on the electron density, which leads to strong nonlinearities. We identify stable and unstable regimes of the polaron-polaritons by calculating its excitation spectrum, and show that they result in prominent hysteresis effects when the electron density is varied. Our results should be readily observable using present experimental technology.

cond-mat.mes-hall↗

arXiv2021

Quantum geometry and flat band Bose-Einstein condensation

We study the properties of a weakly interacting Bose-Einstein condensate (BEC) in a flat band lattice system by using multiband Bogoliubov theory, and discover fundamental connections to the underlying quantum geometry. In a flat band, the speed of sound and the quantum depletion of the condensate are dictated by the quantum geometry, and a finite quantum distance between the condensed and other states guarantees stability of the BEC. Our results reveal that a suitable quantum geometry allows one to reach the strong quantum correlation regime even with weak interactions.

cond-mat.quant-gas↗

arXiv2021

Excitation Spectrum and Superfluid Gap of an Ultracold Fermi Gas

Ultracold atomic gases are a powerful tool to experimentally study strongly correlated quantum many-body systems. In particular, ultracold Fermi gases with tunable interactions have allowed to realize the famous BEC-BCS crossover from a Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer (BCS) superfluid of weakly bound Cooper pairs. However, large parts of the excitation spectrum of fermionic superfluids in the BEC-BCS crossover are still unexplored. In this work, we use Bragg spectroscopy to measure the full momentum-resolved low-energy excitation spectrum of strongly interacting ultracold Fermi gases. This enables us to directly observe the smooth transformation from a bosonic to a fermionic superfluid that takes place in the BEC-BCS crossover. We also use our spectra to determine the evolution of the superfluid gap and find excellent agreement with previous experiments and self-consistent T-matrix calculations both in the BEC and crossover regime. However, towards the BCS regime a calculation that includes the effects of particle-hole correlations shows better agreement with our data.

cond-mat.quant-gas↗

arXiv2021

Stability and breakdown of Fermi polarons in a strongly interacting Fermi-Bose mixture

We investigate the properties of a strongly interacting imbalanced mixture of bosonic $^{41}$K impurities immersed in a Fermi sea of ultracold $^6$Li atoms. This enables us to explore the Fermi polaron scenario for large impurity concentrations including the case where they form a Bose-Einstein condensate. The system is characterized by means of radio-frequency injection spectroscopy and interspecies interactions are widely tunable by means of a well-characterized Feshbach resonance. We find that the energy of the Fermi polarons formed in the thermal fraction of the impurity cloud remains rather insensitive to the impurity concentration, even as we approach equal densities for both species. The apparent insensitivity to high concentration is consistent with a theoretical prediction, based on Landau's quasiparticle theory, of a weak effective interaction between the polarons. The condensed fraction of the bosonic $^{41}$K gas is much denser than its thermal component, which leads to a break-down of the Fermi polaron description. Instead, we observe a new branch in the radio-frequency spectrum with a small energy shift, which is consistent with the presence of Bose polarons formed by $^{6}$Li fermions inside the $^{41}$K condensate. A closer investigation of the behavior of the condensate by means of Rabi oscillation measurements support this observation, indicating that we have realized Fermi and Bose polarons, two fundamentally different quasiparticles, in one cloud.

cond-mat.quant-gas↗

arXiv2021

Non-equilibrium quantum dynamics and formation of the Bose polaron

Advancing our understanding of non-equilibrium phenomena in quantum many-body systems remains among the greatest challenges in physics. Here, we report on the experimental observation of a paradigmatic many-body problem, namely the non-equilibrium dynamics of a quantum impurity immersed in a bosonic environment. We use an interferometric technique to prepare coherent superposition states of atoms in a Bose-Einstein condensate with a small impurity-state component, and monitor the evolution of such quantum superpositions into polaronic quasiparticles. These results offer a systematic picture of polaron formation from weak to strong impurity interactions. They reveal three distinct regimes of evolution with dynamical transitions that provide a link between few-body processes and many-body dynamics. Our measurements reveal universal dynamical behavior in interacting many-body systems and demonstrate new pathways to study non-equilibrium quantum phenomena.

cond-mat.quant-gas↗

arXiv2020

Charged polarons and molecules in a Bose-Einstein Condensate

We investigate the properties a mobile ion immersed in a Bose-Einstein condensate (BEC) using different theoretical approaches. A coherent state variational ansatz predicts that the ion spectral function exhibits several branches in addition to polaronic quasiparticle states, and we employ a diagrammatic analysis of the ion-atom scattering in the BEC to identify them as arising from the binding of an increasing number of bosons to the ion. We develop a simplified model describing the formation of these molecular ions showing that their spectral weight scales with the number of bound atoms. The number of atoms in the dressing cloud around the ion are calculated from thermodynamic arguments, and we finally show that the dynamics ensuing the injection of an ion into the BEC exhibits various regimes governed by coherent quasiparticle propagation and decay.

cond-mat.quant-gas↗

arXiv2020

Observing the emergence of a quantum phase transition -- shell by shell

Many-body physics describes phenomena which cannot be understood looking at a systems' constituents alone. Striking manifestations are broken symmetry, phase transitions, and collective excitations. Understanding how such collective behaviour emerges when assembling a system from individual particles has been a vision in atomic, nuclear, and solid-state physics for decades. Here, we observe the few-body precursor of a quantum phase transition from a normal to a superfluid phase. The transition is signalled by the softening of the mode associated with amplitude vibrations of the order parameter, commonly referred to as a Higgs mode. We achieve exquisite control over ultracold fermions confined to two-dimensional harmonic potentials and prepare closed-shell configurations of 2, 6 and 12 fermionic atoms in the ground state with high fidelity. Spectroscopy is then performed on our mesoscopic system while tuning the pair energy from zero to being larger than the shell spacing. Using full atom counting statistics, we find the lowest resonance to consist of coherently excited pairs only. The distinct non-monotonic interaction dependence of this many-body excitation as well as comparison with numerical calculations allows us to identify it as the precursor of the Higgs mode. Our atomic simulator opens new pathways to systematically unravel the emergence of collective phenomena and the thermodynamic limit particle by particle.

cond-mat.quant-gas↗

arXiv2020

Mobile impurity in a Bose-Einstein condensate and the orthogonality catastrophe

We analyze the properties of an impurity in a dilute Bose-Einstein condensate (BEC). First the quasiparticle residue of a static impurity in an ideal BEC is shown to vanish with increasing particle number as a stretched exponential, leading to a bosonic orthogonality catastrophe. Then we introduce a variational ansatz, which recovers this exact result and describes the macroscopic dressing of the impurity including its back-action onto the BEC as well as boson-boson repulsion beyond the Bogoliubov approximation. This ansatz predicts that the orthogonality catastrophe also occurs for mobile impurities, whenever the BEC becomes ideal. Finally, we show that our ansatz agrees well with experimental results.

cond-mat.quant-gas↗

arXiv2019

Critical slowdown of non-equilibrium polaron dynamics

We study the quantum dynamics of a single impurity following its sudden immersion into a Bose-Einstein condensate. The ensuing formation of the Bose polaron in this general setting can be seen as impurity decoherence driven by the condensate, which we describe within a master equation approach. We derive rigorous analytical results for this decoherence dynamics, and thereby reveal distinct stages of its evolution from a universal stretched exponential initial relaxation to the final approach to equilibrium. The associated polaron formation time exhibits a strong dependence on the impurity speed and is found to undergo a critical slowdown around the speed of sound of the condensate. This rich non-equilibrium behaviour of quantum impurities is of direct relevance to recent cold atom experiments, in which Bose polarons are created by a sudden quench of the impurity-bath interaction.

cond-mat.quant-gas↗

arXiv2018

Dilute Fluid Governed by Quantum Fluctuations

Understanding the effects of interactions in complex quantum systems beyond the mean-field paradigm constitutes a fundamental problem in physics. Here, we show how the atom numbers and interactions in a Bose-Bose mixture can be tuned to cancel mean-field interactions completely. The resulting system is entirely governed by quantum fluctuations -- specifically the Lee-Huang-Yang correlations. We derive an effective one-component Gross-Pitaevskii equation for this system, which is shown to be very accurate by comparison with a full two-component description. This allows us to show how the Lee-Huang-Yang correlation energy can be accurately measured using two powerful probes of atomic gases: collective excitations and radio-frequency spectroscopy. Importantly, the behavior of the system is robust against deviations from the atom number and interaction criteria for canceling the mean-field interactions. This shows that it is feasible to realize a setting where quantum fluctuations are not masked by mean-field forces, allowing investigations of the Lee-Huang-Yang correction at unprecedented precision.

cond-mat.quant-gas↗

arXiv2018

Induced $p$-wave pairing in Bose-Fermi mixtures

Cooper pairing caused by an induced interaction represents a paradigm in our description of fermionic superfluidity. Here, we present a strong coupling theory for the critical temperature of $p$-wave pairing between spin polarised fermions immersed in a Bose-Einstein condensate. The fermions interact via the exchange of phonons in the condensate, and our self-consistent theory takes into account the full frequency/momentum dependence of the resulting induced interaction. We demonstrate that both retardation and self-energy effects are important for obtaining a reliable value of the critical temperature. Focusing on experimentally relevant systems, we perform a systematic analysis varying the boson-boson and boson-fermion interaction strength as well as their masses, and identify the most suitable system for realising a $p$-wave superfluid. Our results show that such a superfluid indeed is experimentally within reach using light bosons mixed with heavy fermions.

cond-mat.quant-gas↗

arXiv2018

Landau Effective Interaction between Quasiparticles in a Bose-Einstein Condensate

Landau's description of the excitations in a macroscopic system in terms of quasiparticles stands out as one of the highlights in quantum physics. It provides an accurate description of otherwise prohibitively complex many-body systems, and has led to the development of several key technologies. In this paper, we investigate theoretically the Landau effective interaction between quasiparticles, so-called Bose polarons, formed by impurity particles immersed in a Bose-Einstein condensate (BEC). In the limit of weak interactions between the impurities and the BEC, we derive rigorous results for the effective interaction. They show that it can be strong even for weak impurity-boson interaction, if the transferred momentum/energy between the quasiparticles is resonant with a sound mode in the BEC. We then develop a diagrammatic scheme to calculate the effective interaction for arbitrary coupling strengths, which recovers the correct weak coupling results. Using this, we show that the Landau effective interaction in general is significantly stronger than that between quasiparticles in a Fermi gas, mainly because a BEC is more compressible than a Fermi gas. The interaction is particularly large near the unitarity limit of the impurity-boson scattering, or when the quasiparticle momentum is close to the threshold for momentum relaxation in the BEC. Finally, we show how the Landau effective interaction leads to a sizeable shift of the quasiparticle energy with increasing impurity concentration, which should be detectable with present day experimental techniques.

cond-mat.quant-gas↗

arXiv2018

Stabilising Fulde-Ferrel-Larkin-Ovchinnikov Superfluidity with long-range Interactions in a mixed dimensional Bose-Fermi System

We analyse the stability of inhomogeneous superfluid phases in a system consisting of identical fermions confined in two layers that are immersed in a Bose-Einstein condensate (BEC). The fermions in the two layers interact via an induced interaction mediated by the BEC, which gives rise to pairing. We present zero temperature phase diagrams varying the chemical potential difference between the two layers and the range of the induced interaction, and show that there is a large region where an inhomogeneous superfluid phase is the ground state. This region grows with increasing range of the induced interaction and it can be much larger than for a corresponding system with a short range interaction. The range of the interaction is controlled by the healing length of the BEC, which makes the present system a powerful tunable platform to stabilise inhomogeneous superfluid phases. We furthermore analyse the melting of the superfluid phases in the layers via phase fluctuations as described by the Berezinskii-Kosterlitz-Thouless mechanism and show that the normal, homogeneous and inhomogeneous superfluid phases meet in a tricritical point. The superfluid density of the inhomogeneous superfluid phase is reduced by inherent gapless excitations, and we demonstrate that this leads to a significant suppression of the critical temperature as compared to the homogeneous superfluid phase.

cond-mat.quant-gas↗