arXiv subjects

Kai Bongs

Publications and source records attributed to Kai Bongs.

At least 37 records · Page 2Linked to original sources

arXiv2017

Endoscopic imaging of quantum gases through a fiber bundle

We use a coherent fiber bundle to demonstrate the endoscopic absorption imaging of quantum gases. We show that the fiber bundle introduces spurious noise in the picture mainly due to the strong core-to-core coupling. By direct comparison with free-space pictures, we observe that there is a maximum column density that can be reliably measured using our fiber bundle, and we derive a simple criterion to estimate it. We demonstrate that taking care of not exceeding such maximum, we can retrieve exact quantitative information about the atomic system, making this technique appealing for systems requiring isolation form the environment.

cond-mat.quant-gas↗

arXiv2017

Observation of Stable Jones-Roberts Solitons in Bose-Einstein Condensates

We experimentally generate two-dimensional Jones-Roberts solitons in a three-dimensional atomic Bose-Einstein condensate by imprinting a triangular phase pattern. By monitoring their dynamics we observe that this kind of solitary waves are resistant to both dynamic (snaking) and thermodynamic instabilities, that usually are known to strongly limit the lifetime of dark plane solitons in dimensions higher than one. We additionally find signatures of a possible dipole-like interaction between them. Our results confirm that Jones-Roberts solitons are stable solutions of the non-linear Schrödinger equation in higher dimensions and promote these excitations for applications beyond matter wave physics, like energy and information transport in noisy and inhomogeneous environments.

cond-mat.quant-gas↗

arXiv2015

Macroscopic quantum resonators (MAQRO): 2015 Update

Do the laws of quantum physics still hold for macroscopic objects - this is at the heart of Schrödinger's cat paradox - or do gravitation or yet unknown effects set a limit for massive particles? What is the fundamental relation between quantum physics and gravity? Ground-based experiments addressing these questions may soon face limitations due to limited free-fall times and the quality of vacuum and microgravity. The proposed mission MAQRO may overcome these limitations and allow addressing those fundamental questions. MAQRO harnesses recent developments in quantum optomechanics, high-mass matter-wave interferometry as well as state-of-the-art space technology to push macroscopic quantum experiments towards their ultimate performance limits and to open new horizons for applying quantum technology in space. The main scientific goal of MAQRO is to probe the vastly unexplored "quantum-classical" transition for increasingly massive objects, testing the predictions of quantum theory for truly macroscopic objects in a size and mass regime unachievable in ground-based experiments. The hardware for the mission will largely be based on available space technology. Here, we present the MAQRO proposal submitted in response to the (M4) Cosmic Vision call of the European Space Agency for a medium-size mission opportunity with a possible launch in 2025.

quant-ph↗

arXiv2014

Design of a dual species atom interferometer for space

Atom interferometers have a multitude of proposed applications in space including precise measurements of the Earth's gravitational field, in navigation & ranging, and in fundamental physics such as tests of the weak equivalence principle (WEP) and gravitational wave detection. While atom interferometers are realized routinely in ground-based laboratories, current efforts aim at the development of a space compatible design optimized with respect to dimensions, weight, power consumption, mechanical robustness and radiation hardness. In this paper, we present a design of a high-sensitivity differential dual species $^{85}$Rb/$^{87}$Rb atom interferometer for space, including physics package, laser system, electronics and software. The physics package comprises the atom source consisting of dispensers and a 2D magneto-optical trap (MOT), the science chamber with a 3D-MOT, a magnetic trap based on an atom chip and an optical dipole trap (ODT) used for Bose-Einstein condensate (BEC) creation and interferometry, the detection unit, the vacuum system for $10^{-11}$ mbar ultra-high vacuum generation, and the high-suppression factor magnetic shielding as well as the thermal control system. The laser system is based on a hybrid approach using fiber-based telecom components and high-power laser diode technology and includes all laser sources for 2D-MOT, 3D-MOT, ODT, interferometry and detection. Manipulation and switching of the laser beams is carried out on an optical bench using Zerodur bonding technology. The instrument consists of 9 units with an overall mass of 221 kg, an average power consumption of 608 W (819 W peak), and a volume of 470 liters which would well fit on a satellite to be launched with a Soyuz rocket, as system studies have shown.

physics.ins-det↗

arXiv2014

Quantum Tests of the Einstein Equivalence Principle with the STE-QUEST Space Mission

We present in detail the scientific objectives in fundamental physics of the Space-Time Explorer and QUantum Equivalence Space Test (STE-QUEST) space mission. STE-QUEST was pre-selected by the European Space Agency together with four other missions for the cosmic vision M3 launch opportunity planned around 2024. It carries out tests of different aspects of the Einstein Equivalence Principle using atomic clocks, matter wave interferometry and long distance time/frequency links, providing fascinating science at the interface between quantum mechanics and gravitation that cannot be achieved, at that level of precision, in ground experiments. We especially emphasize the specific strong interest of performing equivalence principle tests in the quantum regime, i.e. using quantum atomic wave interferometry. Although STE-QUEST was finally not selected in early 2014 because of budgetary and technological reasons, its science case was very highly rated. Our aim is to expose that science to a large audience in order to allow future projects and proposals to take advantage of the STE-QUEST experience.

gr-qc↗

arXiv2014

Fluorescence detection at the atom shot noise limit for atom interferometry

Atom interferometers are promising tools for precision measurement with applications ranging from geophysical exploration to tests of the equivalence principle of general relativity, or the detection of gravitational waves. Their optimal sensitivity is ultimately limited by their detection noise. We review resonant and near-resonant methods to detect the atom number of the interferometer outputs and we theoretically analyze the relative influence of various scheme dependent noise sources and the technical challenges affecting the detection. We show that for the typical conditions under which an atom interferometer operates, simultaneous fluorescence detection with a CCD sensor is the optimal imaging scheme. We extract the laser beam parameters such as detuning, intensity, and duration, required for reaching the atom shot noise limit.

physics.atom-ph↗

arXiv2010

How to observe dipolar effects in spinor Bose-Einstein condensates

We study a spinor condensate of alkali atoms in F = 1 hyperfine state under the presence of an oscillating magnetic field. We find resonances which, due to the dipolar interactions, magnify the transfer of atoms from mF = 1 to mF = 0 Zeeman sublevel. These resonances occur at magnetic fields of the order of milligaus and are broad enough to enable observation of the famous Einstein-de Haas effect, which is solely a dipolar effect, in systems of cold alkali gases.

cond-mat.quant-gas↗

arXiv2010

Gap and screening in Raman scattering of a Bose condensed gas

We propose different spectroscopic methods to explore the nature of the thermal excitations of a trapped Bose condensed gas: 1) a four photon process to probe the uniform region in the trap center: 2) a stimulated Raman process in order to analyze the influence of a momentum transfer in the resulting scattered atom momentum distribution. We apply these methods to address specifically the energy spectrum and the scattering amplitude of these excitations in a transition between two hyperfine levels of the gas atoms. In particular, we exemplify the potential offered by these proposed techniques by contrasting the spectrum expected, from the {\it non conserving} Bogoliubov approximation valid for weak depletion, to the spectrum of the finite temperature extensions like the {\it conserving} generalized random phase approximation (GRPA). Both predict the existence of the Bogoliubov collective excitations but the GRPA approximation distinguishes them from the single atom excitations with a gapped and parabolic dispersion relation and accounts for the dynamical screening of any external perturbation applied to the gas. We propose two feasible experiments, one concerns the observation of the gap associated to this second branch of excitations and the other deals with this screening effect.

cond-mat.stat-mech↗

arXiv2010

SAI: a compact atom interferometer for future space missions

Atom interferometry represents a quantum leap in the technology for the ultra-precise monitoring of accelerations and rotations and, therefore, for all the science that relies on the latter quantities. These sensors evolved from a new kind of optics based on matter-waves rather than light-waves and might result in an advancement of the fundamental detection limits by several orders of magnitude. Matter-wave optics is still a young, but rapidly progressing science. The Space Atom Interferometer project (SAI), funded by the European Space Agency, in a multi-pronged approach aims to investigate both experimentally and theoretically the various aspects of placing atom interferometers in space: the equipment needs, the realistically expected performance limits and potential scientific applications in a micro-gravity environment considering all aspects of quantum, relativistic and metrological sciences. A drop-tower compatible prototype of a single-axis atom interferometry accelerometer is under construction. At the same time the team is studying new schemes, e.g. based on degenerate quantum gases as source for the interferometer. A drop-tower compatible atom interferometry acceleration sensor prototype has been designed, and the manufacturing of its subsystems has been started. A compact modular laser system for cooling and trapping rubidium atoms has been assembled. A compact Raman laser module, featuring outstandingly low phase noise, has been realized. Possible schemes to implement coherent atomic sources in the atom interferometer have been experimentally demonstrated.

physics.space-ph↗

arXiv2009

Excitation spectrum of Mott shells in optical lattices

We theoretically study the excitation spectrum of confined macroscopic optical lattices in the Mott-insulating limit. For large systems, a fast numerical method is proposed to calculate the ground state filling and excitation energies. We introduce many-particle on-site energies capturing multi-band effects and discuss tunnelling on a perturbative level using an effectively restricted Hilbert space. Results for small one-dimensional lattices obtained by this method are in good agreement with the exact multi-band diagonalization of the Hamiltonian. Spectral properties associated with the formation of regions with constant filling, so-called Mott shells, are investigated and interfaces between the shells with strong particle fluctuations are characterized by gapless local excitations.

cond-mat.quant-gas↗

arXiv2009

Spontaneous pattern formation in an anti-ferromagnetic quantum gas

Spontaneous pattern formation is a phenomenon ubiquitous in nature, examples ranging from Rayleigh-Benard convection to the emergence of complex organisms from a single cell. In physical systems, pattern formation is generally associated with the spontaneous breaking of translation symmetry and is closely related to other symmetry-breaking phenomena, of which (anti-)ferromagnetism is a prominent example. Indeed, magnetic pattern formation has been studied extensively in both solid-state materials and classical liquids. Here, we report on the spontaneous formation of wave-like magnetic patterns in a spinor Bose-Einstein condensate, extending those studies into the domain of quantum gases. We observe characteristic modes across a broad range of the magnetic field acting as a control parameter. Our measurements link pattern formation in these quantum systems to specific unstable modes obtainable from linear stability analysis. These investigations open new prospects for controlled studies of symmetry breaking and the appearance of structures in the quantum domain.

cond-mat.quant-gas↗

arXiv2008

Self-Trapping of Bosons and Fermions in Optical Lattices

We theoretically investigate the enhanced localization of bosonic atoms by fermionic atoms in three-dimensional optical lattices and find a self-trapping of the bosons for attractive boson-fermion interaction. Because of this mutual interaction, the fermion orbitals are substantially squeezed, which results in a strong deformation of the effective potential for bosons. This effect is enhanced by an increasing bosonic filling factor leading to a large shift of the transition between the superfluid and the Mott-insulator phase. We find a nonlinear dependency of the critical potential depth on the boson-fermion interaction strength. The results, in general, demonstrate the important role of higher Bloch bands for the physics of attractively interacting quantum gas mixtures in optical lattices and are of direct relevance to recent experiments with 87Rb - 40K mixtures, where a large shift of the critical point has been found.

cond-mat.other↗

arXiv2008

Exact Solution of Strongly Interacting Quasi-One-Dimensional Spinor Bose Gases

We present an exact analytical solution of the fundamental system of quasi-one-dimensional spin-1 bosons with infinite delta-repulsion. The eigenfunctions are constructed from the wave functions of non-interacting spinless fermions, based on Girardeau's Fermi-Bose mapping, and from the wave functions of distinguishable spins. We show that the spinor bosons behave like a compound of non-interacting spinless fermions and non-interacting distinguishable spins. This duality is especially reflected in the spin densities and the energy spectrum. We find that the momentum distribution of the eigenstates depends on the symmetry of the spin function. Furthermore, we discuss the splitting of the ground state multiplet in the regime of large but finite repulsion.

cond-mat.other↗

arXiv2008

Localization and delocalization of ultracold bosonic atoms in finite optical lattices

We study bosonic atoms in small optical lattices by exact diagonalization and observe a striking similarity to the superfluid to Mott insulator transition in macroscopic systems. The momentum distribution, the formation of an energy gap, and the pair correlation function show only a weak size dependence. For noncommensurate filling we reveal in deep lattices a mixture of localized and delocalized particles, which is sensitive to lattice imperfections. Breaking the lattice symmetry causes a Bose-glass-like behavior. We discuss the nature of excited states and orbital effects by using an exact diagonalization technique that includes higher bands.

cond-mat.other↗

arXiv2008

Heteronuclear molecules in an optical lattice: Theory and experiment

We study properties of two different atoms at a single optical lattice site at a heteronuclear atomic Feshbach resonance. We calculate the energy spectrum, the efficiency of rf association and the lifetime as a function of magnetic field and compare the results with the experimental data obtained for K-40 and Rb-87 [C. Ospelkaus et al., Phys. Rev. Lett. 97, 120402 (2006)]. We treat the interaction in terms of a regularized delta function pseudopotential and consider the general case of particles with different trap frequencies, where the usual approach of separating center-of-mass and relative motion fails. We develop an exact diagonalization approach to the coupling between center-of-mass and relative motion and numerically determine the spectrum of the system. At the same time, our approach allows us to treat the anharmonicity of the lattice potential exactly. Within the pseudopotential model, the center of the Feshbach resonance can be precisely determined from the experimental data.

cond-mat.other↗

arXiv2007

Resonant Einstein-de Haas effect in a rubidium condensate

We numerically investigate a condensate of $^{87}$Rb atoms in an F=1 hyperfine state confined in an optical dipole trap. Assuming the magnetic moments of all atoms are initially aligned along the magnetic field we observe, after the field's direction is reversed, a transfer of atoms to other Zeeman states. Such transfer is allowed by the dipolar interaction which couples the spin and the orbital degrees of freedom. Therefore, the atoms in $m_F=0,-1$ states acquire an orbital angular momentum and start to circulate around the center of the trap. This is a realization of the Einstein-de Haas effect in systems of cold gases. We find resonances which amplify this phenomenon making it observable even in very weak dipolar systems. The resonances occur when the Zeeman energy on transfer of atoms to $m_F=0$ state is fully converted to the rotational kinetic energy.

cond-mat.other↗

arXiv2007

Evolution from a Bose-Einstein condensate to a Tonks-Girardeau gas: An exact diagonalization study

We study ground state properties of spinless, quasi one-dimensional bosons which are confined in a harmonic trap and interact via repulsive delta-potentials. We use the exact diagonalization method to analyze the pair correlation function, as well as the density, the momentum distribution, different contributions to the energy and the population of single-particle orbitals in the whole interaction regime. In particular, we are able to trace the fascinating transition from bosonic to fermi-like behavior in characteristic features of the momentum distribution which is accessible to experiments. Our calculations yield quantitative measures for the interaction strength limiting the mean-field regime on one side and the Tonks-Girardeau regime on the other side of an intermediate regime.

cond-mat.other↗

arXiv2006

Dynamics of F=1 87Rb condensates at finite temperatures

We investigate the dynamics of a F=1 spinor Bose-Einstein condensate of 87Rb atoms confined in a quasi-one-dimensional trap both at zero and at finite temperature. At zero temperature, we observe coherent oscillations between populations of the various spin components and the formation of multiple domains in the condensate. We study also finite temperature effects in the spin dynamics taking into account the phase fluctuations in the Bogoliubov-de Gennes framework. At finite T, despite complex multidomain formation in the condensate, population equipartition occurs. The length scale of these spin domains seems to be determined intrinsically by nonlinear interactions.

cond-mat.mes-hall↗