Frequency Modulation of Ultracold Atoms by an Oscillating Barrier

Andrew J. Pyle, Charles Fancher, Megan Ivory +5 more

Andrew J. Pyle

A quantum pump is a device that transports particles through a circuit with localized time-varying potentials, and without the need for an external applied voltage or chemical potential. Quantum pumping was originally proposed in the context of electron transport in nanowires, but has proven difficult to implement. The ultracold atom approach represents a possible route around the current experimental bottleneck. We present an experiment to study 1D quantum mechanical scattering by an amplitude-modulated barrier. This experiment represents a first step toward implementing a quantum pump for ultracold atoms based on two such barriers modulated out of phase with one another. A single oscillating barrier imparts or subtracts kinetic energy in discrete amounts from the scattered atoms. In this manner, the energy spectrum of the scattered atoms resembles a comb with a tooth spacing proportional to the oscillation frequency of the barrier. This effect is analogous to the frequency modulation of a radio wave to add sidebands to a carrier frequency, where the carrier is the kinetic energy of ultracold atoms and the oscillating barrier provides the modulating signal. Numerical simulations of the scattering process confirm this basic scattering picture. We present dual chamber, dual-species a atom chip apparatus that produces quasi-pure Rubidium-87 Bose-Einstein condensates (BEC) of 10,000 atoms used to study the quantum scattering dynamics. The proposed experiment operates by releasing a BEC from a relaxed magnetic chip trap and directing it horizontally towards a tightly focused laser beam that serves as an oscillating barrier. A magnetic field gradient provided by is used to control the vertical motion of the BEC. Detection is carried out with a time of flight technique to resolve discrete atomic packet sidebands. This method can be used to study momentum sideband generation with a BEC in the presence of no or weak interactions. Dark-ground imaging can be used to detect small atom number BECs, which will have the benefit of weaker interactions.

Klaus Stefan Drese, Martin Holthaus

C. Yüce

Berry's phase is investigated for ultracold atoms in a frequency modulated optical lattice. It is shown that Berry's phase appears due to Bloch oscillation and the periodic motion of the optical lattice. Particularly, Berry's phase for ultracold atoms under the gravitational force in an oscillating tight-binding optical lattice is calculated analytically. It is found that the Berry's phase depends linearly on the amplitude of the oscillation of the optical lattice.

W. K. Hensinger, Hartmut Häffner, Antoine Browaeys +8 more

Sheng-Chang Li, Li-Chen Zhao

We investigate the properties of phase transitions in a weakly coupled ultracold atom-molecule boson system based on the phase space analysis approach. We identify the pure molecular and mixed atom-molecule phases of the system by different values of the time average of molecular population. When applying a periodic modulation on the energy detuning, we find that the atom-molecule mixing dynamics can be well controlled. For both high- and low-frequency modulations, it is shown that the transition points can be shifted effectively by choosing parameters of the periodic modulation appropriately. The analytical expressions for the dependence of the transition parameters on the modulation parameters are also obtained for the two cases. In particular, when the modulation with a frequency near the atom-molecule transition frequency is applied, the resonance between the periodic modulation and atom-molecule Rabi oscillation will emerge, which can affect the atom-molecule mixing dynamics dramatically and lead to the complex chaos phenomenon.