Classical ratchet potentials, which alternate a driving potential with periodic random dissipative motion, can account for the operation of biological motors. We demonstrate the operation of a quantum ratchet, which differs from classical ratchets in that dissipative processes are absent within the observation time of the system (Hamiltonian regime). An atomic rubidium Bose-Einstein condensate is exposed to a sawtooth-like optical lattice potential, whose amplitude is periodically modulated in time. The ratchet transport arises from broken spatiotemporal symmetries of the driven potential, resulting in a desymmetrization of transporting eigenstates (Floquet states). The full quantum character of the ratchet transport was demonstrated by the measured atomic current oscillating around a nonzero stationary value at longer observation times, resonances occurring at positions determined by the photon recoil, and dependence of the transport current on the initial phase of the driving potential.
We demonstrate a scheme for the Fourier synthesis of periodic optical
potentials with asymmetric unit cells for atoms. In a proof of principle
experiment, an atomic Bose-Einstein condensate is exposed to either symmetric
or sawtooth-like asymmetric potentials by superimposing a conventional standing
wave potential of $\lambda/2$ spatial periodicity with a fourth-order lattice
potential of $\lambda/4$ periodicity. The high periodicity lattice is realized
using dispersive properties of multiphoton Raman transitions. Future
applications of the demonstrated scheme could range from the search for novel
quantum phases in unconventionally shaped lattice potentials up to
dissipationless atomic quantum ratchets.Comment: 4 figure
In our Letter, the expression given at the bottom of the right half of page 1 for the splitting between the first two excited Bloch bands has an error. With the given definitions of the potential depths, the correct value for the splitting is jV 2 1 =16E r þ e i' V 2 j=2. Furthermore, the given formula for the critical acceleration in the Landau-Zener tunneling probability between the first and second excited Bloch bands was given incorrectly. The correct value here is a c ¼ Á 2 =ð8@ 2 kÞ (actually, the previously quoted value refers to the result for the lowest band gap [1]). However, these mistakes do not influence the other results and the conclusion of the Letter.[1] C. Zener, Proc. R. Soc. A 137, 696 (1932).
We demonstrate an atom laser using all-optical techniques. A Bose-Einstein condensate of rubidium atoms is created by direct evaporative cooling in a quasistatic dipole trap realized with a single, tightly focused CO2-laser beam. An applied magnetic field gradient allows the formation of the condensate in a field-insensitive m(F)=0 spin projection only, which suppresses fluctuations of the chemical potential from stray magnetic fields. A collimated and monoenergetic beam of atoms is extracted from the Bose-Einstein condensate by continuously lowering the dipole trapping potential in a controlled way to form a novel type of atom laser.
We report on an experimental study of quantum transport of atoms in variable periodic optical potentials. The band structure of both ratchet-type asymmetric and symmetric lattice potentials is explored. The variable atom potential is realized by superimposing a conventional standing wave potential of lambda/2 spatial periodicity with a fourth-order multiphoton potential of lambda/4 periodicity. We find that the Landau-Zener tunneling rate between the first and the second excited Bloch band depends critically on the relative phase between the two spatial lattice harmonics.
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