Observation of Superfluid Flow in a Bose-Einstein Condensed Gas

Abstract

We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred by a macroscopic object, a blue detuned laser beam, using nondestructive in situ phase contrast imaging. A critical velocity for the onset of a pressure gradient has been observed, and shown to be density dependent. The technique has been compared to a calorimetric method used previously to measure the heating induced by the motion of the laser beam.PACS 03.75.Fi, 67.40.Vs, 67.57.De Beginning with the London conjecture [1], BoseEinstein condensation has been considered crucial for the understanding of superfluidity. Since then, the weakly interacting Bose gas has served as an idealized model for a superfluid [2]. It has a phonon-like energy-momentum dispersion relation that does not allow for the generation of elementary excitations below a critical velocity, thus implying dissipationless flow at lower velocities. The onset of dissipation has been treated in the framework of the nonlinear Schrödinger equation [3][4][5][6][7] and shows an intriguing richness. Experiments on liquid helium could not test these theories, because superfluidity and dissipation are even more complex in this system due to the presence of strong interactions and surface effects [8].The creation of Bose-Einstein condensates in dilute gases has dramatically changed this situation, allowing for quantitative tests of microscopic theories using the tools and precision of atomic physics experiments [9]. A number of recent experiments have examined phenomenological features of superfluidity in gaseous BoseEinstein condensates. These include the observation of vortices [10,11], a non-classical moment of inertia [12], and suppression of collisions from microscopic impurities [13]. In previous work we found evidence for a critical velocity in a stirred condensate [14]. In this Letter we study the onset of dissipation with higher sensitivity using repeated in situ non-destructive imaging of the condensate. These images show the distortion of the density distribution around the moving object, thus directly probing the dynamics of the flow field that has been recently treated with different models [15][16][17][18][19][20]. The experimental setup was similar to the one used in our previous work [14]. Improvements in the evaporation strategy and decompression techniques allowed us to produce pure condensates with up to 5×10 7 sodium atoms, with densities ranging from 8.4×1013 to 3.5×10 14 cm −3 , corresponding to chemical potentials from 60 to 250 nK. We determined the Thomas-Fermi radius R z along the axial direction through in situ phase contrast imaging. The sound velocity at the center of the condensate was then evaluated through the relationship c s = 2πν z R z / √ 2, with ν z =20.1Hz being the axial trapping frequency. The macroscopic moving object was a 514 nm laser beam blue-detuned with respect to the sodium transitions, thereby creating an effective repulsive potential for the atoms. The beam is focused on the center of the condensate to a Gaussian 1/e 2 diameter o...

Room-temperature superfluidity in a polariton condensate

“…The density distribution shows a higher fluid density immediately ahead of the defect, similar to the case of atomic condensates when perturbed by a laser field scanned above the critical velocity 6 . Indeed, when the density is increased (10 7 pol/µm 2 ), the superfluid regime is recovered: the intensity peak before the defect disappears and the spatial distribution of the population becomes homogeneous around the defect (Fig.…”

“…1b. In contrast to experiments on superfluidity in atomic condensates 6,7 , here the position of the defect is kept fixed in space, while the speed of the quasi-particle flow is controlled by using the excitation angle, which is directly related to the polariton group velocity v g =hk p /m LP , where k p is the polariton wavevector and m LP the effective mass. intensities for a polariton flow moving upwards across an obstacle with an exciting angle chosen such that the flow speed is 19 µm/ps.…”

Vortices in a trapped dilute Bose-Einstein condensate

“…The gradual rise of this line reflects coupling from the ground state to the vortex mode caused by the driveĤ 1 in Eq. (150). Once during each Rabi cycle, the angular momentum approaches unity (bottom graph), and, at that time, the |2 state wave function approaches a pure vortex mode.…”

“…Recently, dissipation in a Bose-Einstein condensed gas was studied by moving a blue-detuned laser beam through the condensate [149,150]. The laser beam repels atoms from its focus and creates a moving macroscopic "hole" in the condensate.…”

Persistent Currents in a Bose-Einstein Condensate in the Presence of Disorder

“…Superfluidity includes a whole class of phenomena that appear in various quantum systems [1], such as liquid He, superconductors, and nuclei. More recently, a lot of effort has been put on the study of superfluid properties of dilute vapors of trapped atoms, including, for example, vortex states [2,3,4,5], transport properties [6], etc. Although these gases are very dilute, they are superfluid because of their extremely low temperature.…”