arXiv subjects

T. Pfau

Publications and source records attributed to T. Pfau.

52 records · Page 3Linked to original sources

arXiv2006

Collective oscillations of dipolar Bose-Einstein condensates and accurate comparison between contact and dipolar interaction

We propose a scheme for the measurement of the s-wave scattering length $a$ of an atom or molecule with significant dipole-dipole interaction with an accuracy at the percent level. The frequencies of the collective oscillations of a Bose-Einstein condensate are shifted by the magnetic dipole interaction. The shift is polarization dependent and proportional to the ratio $ε_{dd}$ of dipolar and s-wave coupling constants. Measuring the differences in the frequencies for different polarization we can extract the value of $ε_{dd}$ and thus measure $a$. We calculate the frequency shifts for a large variety of non-axisymmetric harmonic traps in the Thomas-Fermi limit and find optimal trapping geometries to maximize the shifts.

cond-mat.other↗

arXiv2006

Expansion dynamics of a dipolar Bose-Einstein condensate

Our recent measurements on the expansion of a chromium dipolar condensate after release from an optical trapping potential are in good agreement with an exact solution of the hydrodynamic equations for dipolar Bose gases. We report here the theoretical method used to interpret the measurement data as well as more details of the experiment and its analysis. The theory reported here is a tool for the investigation of different dynamical situations in time-dependent harmonic traps.

cond-mat.other↗

arXiv2006

Trapping atoms on a transparent permanent-magnet atom chip

We describe experiments on trapping of atoms in microscopic magneto-optical traps on an optically transparent permanent-magnet atom chip. The chip is made of magnetically hard ferrite-garnet material deposited on a dielectric substrate. The confining magnetic fields are produced by miniature magnetized patterns recorded in the film by magneto-optical techniques. We trap Rb atoms on these structures by applying three crossed pairs of counter-propagating laser beams in the conventional magneto-optical trapping (MOT) geometry. We demonstrate the flexibility of the concept in creation and in-situ modification of the trapping geometries through several experiments.

quant-ph↗

arXiv2005

Spin-3 Chromium Bose-Einstein Condensates

We analyze the physics of spin-3 Bose-Einstein condensates, and in particular the new physics expected in on-going experiments with condensates of Chromium atoms. We first discuss the ground-state properties, which, depending on still unknown Chromium parameters, and for low magnetic fields can present various types of phases. We also discuss the spinor-dynamics in Chromium spinor condensates, which present significant qualitative differences when compared to other spinor condensates. In particular, dipole-induced spin relaxation may lead for low magnetic fields to transfer of spin into angular momentum similar to the well-known Einstein-de Haas effect. Additionally, a rapid large transference of population between distant magnetic states becomes also possible.

cond-mat.other↗

arXiv2005

Observation of dipole-dipole interaction in a degenerate quantum gas

We have investigated the expansion of a Bose-Einstein condensate (BEC) of strongly magnetic chromium atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to an anisotropic deformation of the expanding Cr-BEC which depends on the orientation of the atomic dipole moments. Our measurements are consistent with the theory of dipolar quantum gases and show that a Cr-BEC is an excellent model system to study dipolar interactions in such gases.

cond-mat.other↗

arXiv2005

Depolarisation cooling of an atomic cloud

We propose a cooling scheme based on depolarisation of a polarised cloud of trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the coupling between the internal spin reservoir of the cloud and the external kinetic reservoir via dipolar relaxation to reduce the temperature of the cloud. By optical pumping one can cool the spin reservoir and force the cooling process. In case of a trapped gas of dipolar chromium atoms, we show that this cooling technique can be performed continuously and used to approach the critical phase space density for BEC

quant-ph↗

arXiv2005

Atom Nano-lithography with Multi-layer Light Masks: Particle Optics Analysis

We study the focusing of atoms by multiple layers of standing light waves in the context of atom lithography. In particular, atomic localization by a double-layer light mask is examined using the optimal squeezing approach. Operation of the focusing setup is analyzed both in the paraxial approximation and in the regime of nonlinear spatial squeezing for the thin-thin as well as thin-thick atom lens combinations. It is shown that the optimized double light mask may considerably reduce the imaging problems, improve the quality of focusing and enhance the contrast ratio of the deposited structures.

quant-ph↗

arXiv2005

Probing the light induced dipole-dipole interaction in momentum space

We theoretically investigate the mechanical effect of the light-induced dipole-dipole interaction potential on the atoms in a Bose-Einstein condensate. We present numerical calculations on the magnitude and shape of the induced potentials for different experimentally accessible geometries. It is shown that the mechanical effect can be distinguished from the effect of incoherent scattering for an experimentally feasible setting.

quant-ph↗

arXiv2004

Observation of Feshbach resonances in an ultracold gas of ${}^{52}$Cr

We have observed Feshbach resonances in elastic collisions between ultracold ${}^{52}$Cr atoms. This is the first observation of collisional Feshbach resonances in an atomic species with more than one valence electron. The zero nuclear spin of ${}^{52}$Cr and thus the absence of a Fermi-contact interaction leads to regularly-spaced resonance sequences. By comparing resonance positions with multi-channel scattering calculations we determine the s-wave scattering length of the lowest $^{2S+1}Σ_{g}^{+}$ potentials to be $\unit[112(14)]{a_0}$, $\unit[58(6)]{a_0}$ and $-\unit[7(20)]{a_0}$ for S=6, 4, and 2, respectively, where $a_{0}=\unit[0.0529]{nm}$.

cond-mat.other↗

arXiv2003

Dipolar Relaxation in an ultra-cold Gas of magnetically trapped chromium atoms

We have investigated both theoretically and experimentally dipolar relaxation in a gas of magnetically trapped chromium atoms. We have found that the large magnetic moment of 6 $μ_B$ results in an event rate coefficient for dipolar relaxation processes of up to $3.2\cdot10^{-11}$ cm$^{3}$s$^{-1}$ at a magnetic field of 44 G. We present a theoretical model based on pure dipolar coupling, which predicts dipolar relaxation rates in agreement with our experimental observations. This very general approach can be applied to a large variety of dipolar gases.

quant-ph↗

arXiv2003

Determination of the s-wave Scattering Length of Chromium

We have measured the deca-triplet s-wave scattering length of the bosonic chromium isotopes $^{52}$Cr and $^{50}$Cr. From the time constants for cross-dimensional thermalization in atomic samples we have determined the magnitudes $|a(^{52}Cr)|=(170 \pm 39)a_0$ and $|a(^{50}Cr)|=(40 \pm 15)a_0$, where $a_0=0.053nm$. By measuring the rethermalization rate of $^{52}$Cr over a wide temperature range and comparing the temperature dependence with the effective-range theory and single-channel calculations, we have obtained strong evidence that the sign of $a(^{52}Cr)$ is positive. Rescaling our $^{52}$Cr model potential to $^{50}$Cr strongly suggests that $a(^{50}Cr)$ is positive, too.

quant-ph↗

arXiv2002

An Intense Source of Cold Rb Atoms from a Pure 2D-MOT

We present a 2D-MOT setup for the production of a continuous collimated beam of cold 87-Rb atoms out of a vapor cell. The underlying physics is purely two-dimensional cooling and trapping, which allows for a high flux of up to 6*10^(10) atoms/s and a small divergence of the resulting beam. We analyze the velocity distribution of the 2D-MOT. The longitudinal velocity distribution of the atomic beam shows a broad feature(FWHM=75m/s), centered around 50m/s. The dependence of the flux on laser intensity, geometry of the trapping volume and on pressure in the vapor cell was investigated in detail. The influence of the geometry of the 2D-MOT on the mean velocity of the cold beam has been studied. We present a simple model for the velocity distribution of the flux based on rate equations describing the general dependencies.

physics.atom-ph↗

arXiv2001

Continuous loading of a magnetic trap

We have realized a scheme for continuous loading of a magnetic trap (MT). ^{52}Cr atoms are continuously captured and cooled in a magneto-optical trap (MOT). Optical pumping to a metastable state decouples atoms from the cooling light. Due to their high magnetic moment (6 Bohr magnetons), low-field seeking metastable atoms are trapped in the magnetic quadrupole field provided by the MOT. Limited by inelastic collisions between atoms in the MOT and in the MT, we load 10^8 metastable atoms at a rate of 10^8 atoms/s below 100 microkelvin into the MT. After loading we can perform optical repumping to realize a MT of ground state chromium atoms.

quant-ph↗

arXiv2000

Continuous optical loading of a Bose-Einstein Condensate

The continuous pumping of atoms into a Bose-Einstein condensate via spontaneous emission from a thermal reservoir is analyzed. We consider the case of atoms with a three-level $Λ$ scheme, in which one of the atomic transitions has a very much shorter life-time than the other one. We found that in such scenario the photon reabsorption in dense clouds can be considered negligible. If in addition inelastic processes can be neglected, we find that optical pumping can be used to continuously load and refill Bose-Einstein condensates, i.e. provides a possible way to achieve a continuous atom laser.

quant-ph↗

arXiv2000

Amplification of light and atoms in a Bose--Einstein condensate

A Bose-Einstein condensate illuminated by a single off-resonant laser beam (``dressed condensate'') shows a high gain for matter waves and light. We have characterized the optical and atom-optical properties of the dressed condensate by injecting light or atoms, illuminating the key role of long-lived matter wave gratings produced by the condensate at rest and recoiling atoms. The narrow bandwidth for optical gain gave rise to an extremely slow group velocity of an amplified light pulse (1 m/s).

cond-mat.soft↗

arXiv2000

Bose-Einstein condensation with magnetic dipole-dipole forces

Ground-state solutions in a dilute gas interacting via contact and magnetic dipole-dipole forces are investigated. To the best of our knowledge, it is the first example of studies of the Bose-Einstein condensation in a system with realistic long-range interactions. We find that for the magnetic moment of e.g. chromium and a typical value of the scattering length all solutions are stable and only differ in size from condensates without long-range interactions. By lowering the value of the scattering length we find a region of unstable solutions. In the neighborhood of this region the ground state wavefunctions show internal structures not seen before in condensates. Finally, we find an analytic estimate for the characteristic length appearing in these solutions.

cond-mat↗