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Frontiers in Optics

Towards Measurement of Tunneling Times of a Bose-Einstein Condensate

Shreyas Potnis, R. Chang, R. Ramos, Alex Hayat, Aephraim M. Steinberg

Published Jan 1, 2012• 0 citations
Publisher

Abstract

We present progress towards measuring tunneling times of massive particles. A variety of 1D scattering potentials with 1.2 μm resolution are created, which we use to engineer our momentum distribution, and study tunneling times.
DOI
10.1364/ls.2012.ltu4i.5

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Tunnelling is one of the phenomena at the heart of quantum mechanics. Despite its central role in physics, the duration of the tunnelling process has been debated since the development of quantum theory without a definite answer. In this thesis, the question of how long a particle takes to tunnel is investigated experimentally using ultracold atoms. These systems provide an ideal platform to address this question due to their isolation from the external environment and the ability to create almost arbitrary potentials for the atoms. As the tunnelling time experiment requires low temperatures, the realization and characterization of wave packets below the nanokelvin regime are studied first. A 16x reduction of the effective temperature of the atoms is obtained through delta-kick cooling. However, the achieved temperatures are so low that a new method, dubbed the “knife-edge technique”, is developed to characterize the final atomic state accurately. Through this new method, a final temperature of 900 ± 200 pK is found. Finally, the tunnelling time is investigated using a Bose-Einstein condensate in a one dimensional waveguide intersected by a 1.3 μm barrier. As the atoms tunnel through the thin potential, their spins time the tunnelling event through the Larmor precession induced by a pseudo-magnetic field localized inside the barrier. The precession angle is found by performing tomography on the spin state of the tunnelling atoms. With the knowledge of the Larmor frequency and the rotation angles, we find a tunnelling time of 0.65(7) ms. This experiment achieves a long-sought tunnelling time measurement with a clear interpretation, and its results show good quantitative agreement with theory based on the weak measurement formalism.
In this thesis I will describe my experiments to study spin transport of spin Bose Einstein Condensates (BECs) with synthetic spin-orbit coupling. First I will describe procedures used to reproduce previous experiments with synthetic gauge fields in Bose-Einstein Condensates, from constant vector potentials to synthetic electric fields and synthetic spin-orbit coupling as well as all of the necessary calibration experiments. Next I will describe new experiments measuring spin transport induced by synthetic spin-dependent electric fields in spin-orbit coupled Bose-Einstein Condensates (BECs). The one dimensional (1D) spin-orbit coupling (SOC) is created with counter propagating Raman lasers which couple hyperfine sub-levels and momentum states of 87Rb, allowing us to engineer spin dependent vector potentials. Quickly lowering the Raman laser intensity, decreases the spin-orbit Raman coupling strength (Ω), separates the spin vector potentials and applies opposite synthetic electric fields to the two dressed spin BECs. We allow them to oscillate in opposite directions within the optical trap (exhibiting a spin dipole mode) and measure the time evolution of their momentum and density after time-of-flight (TOF). The damping of the oscillations as the spin BECs collide is found to drastically increase for larger final Raman coupling. Over longer time scales, the bare spins' oscillations damp and thermalize. However, with Raman coupling, dressed spins' oscillations are overdamped and accompanied by rich excitations in the BEC but less thermalization.