arXiv subjects

G. Hechenblaikner

Publications and source records attributed to G. Hechenblaikner.

13 recordsLinked to original sources

arXiv2010

Coupling characterization and noise studies of the Optical Metrology System on-board the LISA Pathfinder Mission

In this article we describe the first investigations of the complete engineering model of the Optical Metrology System (OMS), a key subsystem of the LISA Pathfinder science mission to space. The latter itself is a technological precursor mission to LISA, a space-borne gravitational wave detector. At its core, the OMS consists of four heterodyne Mach Zehnder interferometers, a highly stable laser with external modulator and a phase-meter. It is designed to monitor and track the longitudinal motion and attitude of two floating test-masses in the optical reference frame with a (relative) precision in the picometer and nanorad range, respectively. We analyze sensor signal correlations and determine a physical sensor noise limit. The coupling parameters between motional degrees of freedom and interferometer signals are analytically derived and compared to measurements. We also measure adverse cross-coupling effects originating from system imperfections and limitations and describe algorithmic mitigation techniques to overcome some of them. Their impact on system performance is analyzed in the context of the Pathfinder mission.

physics.ins-det↗

arXiv2004

Properties of quasi two-dimensional condensates in highly anisotropic traps

We theoretically investigate some of the observable properties of quasi two-dimensional condensates. Using a variational model based on a Gaussian-parabolic trial wavefunction we calculate chemical potential, condensate size in time-of-flight, release energy and collective excitation spectrum for varying trap geometries and atom numbers and find good agreement with recent published experimental results.

cond-mat.other↗

arXiv2004

Quasi-2D Confinement of a BEC in a Combined Optical and Magnetic Potential

We have added an optical potential to a conventional Time-averaged Orbiting Potential (TOP) trap to create a highly anisotropic hybrid trap for ultracold atoms. Axial confinement is provided by the optical potential; the maximum frequency currently obtainable in this direction is 2.2 kHz for rubidium. The radial confinement is independently controlled by the magnetic trap and can be a factor of 700 times smaller than in the axial direction. This large anisotropy is more than sufficient to confine condensates with ~10^5 atoms in a Quasi-2D (Q2D) regime, and we have verified this by measuring a change in the free expansion of the condensate; our results agree with a variational model.

cond-mat.other↗

arXiv2002

The Experimental Observation of a Superfluid Gyroscope in a dilute Bose Condensed Gas

We have observed a superfluid gyroscope effect in a dilute gas Bose-Einstein condensate. A condensate with a vortex possesses a single quantum of angular momentum and this causes the plane of oscillation of the scissors mode to precess around the vortex line. We have measured the precession rate of the scissors oscillation. From this we deduced the angular momentum associated with the vortex line and found a value close to $\hbar$ per particle, as predicted for a superfluid.

cond-mat.stat-mech↗

arXiv2001

The Moment of Inertia and the Scissors Mode of a Bose-condensed Gas

We relate the frequency of the scissors mode to the moment of inertia of a trapped Bose gas at finite temperature in a semi-classical approximation. We apply these theoretical results to the data obtained in our previous study of the properties of the scissors mode of a trapped Bose-Einstein condensate of $^{87}$Rb atoms as a function of the temperature. The frequency shifts that we measured show quenching of the moment of inertia of the Bose gas at temperatures below the transition temperature - the system has a lower moment of inertia that of a rigid body with the same mass distribution, because of superfluidity.

cond-mat.stat-mech↗

arXiv2001

Vortex nucleation in Bose-Einstein condensates in an oblate, purely magnetic potential

We have investigated the formation of vortices by rotating the purely magnetic potential confining a Bose-Einstein condensate. We modified the bias field of an axially symmetric TOP trap to create an elliptical potential that rotates in the radial plane. This enabled us to study the conditions for vortex nucleation over a wide range of eccentricities and rotation rates.

cond-mat.stat-mech↗

arXiv2001

Direct observation of irrotational flow and evidence of superfluidity in a rotating Bose-Einstein condensate

We observed the expansion of vortex-free Bose-condensates after their sudden release from a slowly rotating anisotropic trap. Our results show clear experimental evidence of the irrotational flow expected for a superfluid. The expansion from a rotating trap has strong features associated with the superfluid nature of a Bose-condensate, namely that the condensate cannot at any point be cylindrically symmetric with respect to the axis of rotation since such a wavefunction cannot possess angular momentum. Consequently, an initially rotating condensate expands in a distinctively different way to one released from a static trap. We report measurements of this phenomenon in absorption images of the condensate taken along the direction of the rotation axis.

cond-mat.stat-mech↗

arXiv2001

Calculation of mode coupling for quadrupole excitations in a Bose-Einstein condensate

In this paper we give a theoretical description of resonant coupling between two collective excitations of a Bose condensed gas (BEC) on, or close, to a second harmonic resonance. Using analytic expressions for the quasi-particle wavefunctions we show that the coupling between quadrupole modes is strong, leading to a coupling time of a few milliseconds (for a TOP trap with radial frequency 100 Hz and 10^4 atoms). Using the hydrodynamic approximation, we derive analytic expression for the coupling matrix element. These can be used with an effective Hamiltonian (that we also derive) to describe the dynamics of the coupling process and the associated squeezing effects.

cond-mat.soft↗

arXiv2000

The experimental observation of Beliaev damping in a Bose condensed gas

We report the first experimental observation of Beliaev damping of a collective excitation in a Bose-condensed gas. Beliaev damping is not predicted by the Gross-Pitaevskii equation and so this is one of the few experiments that tests BEC theory beyond the mean field approximation. Measurements of the amplitude of a high frequency scissors mode, show that the Beliaev process transfers energy to a lower lying mode and then back and forth between these modes. These characteristics are quite distinct from those of Landau damping, which leads to a monotonic decrease in amplitude. To enhance the Beliaev process we adjusted the geometry of the magnetic trapping potential to give a frequency ratio of 2 to 1 between two of the scissors modes of the condensate. The ratios of the trap oscillation frequencies $ω_y / ω_x$ and $ω_z / ω_x$ were changed independently, so that we could investigate the resonant coupling over a range of conditions.

cond-mat.stat-mech↗

arXiv2000

Bose-Einstein condensation in a stiff TOP trap with adjustable geometry

We report on the realisation of a stiff magnetic trap with independently adjustable trap frequencies, $ω_z$ and $ω_r$, in the axial and radial directions respectively. This has been achieved by applying an axial modulation to a Time-averaged Orbiting Potential (TOP) trap. The frequency ratio of the trap, $ω_z / ω_r$, can be decreased continuously from the original TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein condensate (BEC) into this trap and obtained very good agreement between its observed anisotropic expansion and the hydrodynamic predictions. Our method can be extended to obtain a spherical trapping potential, which has a geometry of particular theoretical interest.

cond-mat.stat-mech↗

arXiv2000

Observation of harmonic generation and nonlinear coupling in the collective dynamics of a Bose condensate

We report the observation of harmonic generation and strong nonlinear coupling of two collective modes of a condensed gas of rubidium atoms. Using a modified TOP trap we changed the trap anisotropy to a value where the frequency of the m=0 high-lying mode corresponds to twice the frequency of the m=0 low-lying mode, thus leading to strong nonlinear coupling between these modes. By changing the anisotropy of the trap and exciting the low-lying mode we observed significant frequency shifts of this fundamental mode and also the generation of its second harmonic.

cond-mat.stat-mech↗

arXiv1999

Bose-Einstein condensation in a rotating anisotropic TOP trap

We describe the construction and operation of a time-orbiting potential trap (TOP trap) that has different oscillation frequencies along its three principal axes. These axes can be rotated and we have observed Bose-Einstein condensates of 87^Rb with a rotating ellipsoidal shape. Under these conditions it has been predicted that quantized vortices form and are stable.

cond-mat.stat-mech↗

arXiv1999

Observation of the scissors mode and superfluidity of a trapped Bose-Einstein condensed gas

We report the observation of the scissors mode of a Bose-Einstein condensed gas of 87^Rb atoms in a magnetic trap, which gives direct evidence of superfluidity in this system. The scissors mode of oscillation is excited by a sudden rotation of the anisotropic trapping potential. For a gas above T_c (normal fluid) we detect the occurrence of oscillations at two frequencies, with the lower frequency corresponding to the rigid body value of the moment of inertia. Well below T_c the condensate oscillates at a single frequency, without damping, as expected for a superfluid.

cond-mat.stat-mech↗