Gating Ratchet for Cold Atoms

Abstract

We demonstrate experimentally a gating ratchet with cold rubidium atoms in a driven near-resonant optical lattice. A single-harmonic periodic modulation of the optical potential depth is applied, together with a single-harmonic rocking force. Directed motion is observed as a result of the breaking of the symmetries of the system. DOI: 10.1103/PhysRevLett.100.040603 PACS numbers: 05.60.Cd, 05.40.ÿa, 05.45.ÿa, 05.70.Ln The rectification of Brownian motion in the absence of net applied bias forces is an intriguing phenomenon, termed the ratchet effect, which has recently attracted much attention in different fields [1][2][3][4][5][6][7][8]. In fact, ratchets have been implemented in a variety of physical systems [9][10][11][12][13][14][15] ranging from solid state devices and optical trap setups to granular gases and nanopores in polymer films. All the different implementations of the ratchet effect can be traced back to two main elements. First, the system has to be driven out of equilibrium, so to overcome the restrictions imposed by the second principle of thermodynamics. Second, the relevant symmetries of the system, which would otherwise inhibit directed motion, have to be broken.In the rocking ratchet [2], particles in a periodic potential experience also an applied ac force. The applied force is zero average and time symmetric. However, there is an eventual net flux due to the symmetry-breaking anisotropy of the potential. The same effect can be obtained for a spatially symmetric potential and a biharmonic drive, with the time symmetry of the system controlled by the symmetry of the drive [4 -7,11].In the gating ratchet [16], particles experience an oscillating potential which is spatially symmetric. A zeroaverage and time-symmetric ac force is also applied. A current can be generated following a gating effect, with the lowering of the potential barriers synchronized with the motion produced by the additive force. This mechanism has to be contrasted with the previously demonstrated acdriven ratchets with additive biharmonic driving [11], in which the underlying mechanism is harmonic mixing [17].In this work we present the experimental demonstration of a gating ratchet with cold rubidium atoms in a dissipative optical lattice. We demonstrate that the gating mechanism can indeed lead to the generation of a current and show that such a current generation is controlled by the symmetries of the system.Before presenting the experimental results, it is important to carry out the symmetry analysis for the gating ratchet. This will serve as a guide for the experimental work. We consider a weakly damped particle in an amplitude modulated symmetric potential Vx1 mt. A rocking force Ft is also applied. The Langevin equation for the particle of mass M isHere x is the position of the particle at the time t, and and are the damping coefficient and a stationary Gaussian noise, respectively. Both the amplitude modulation mt and the rocking force Ft are single-harmonic fields:For the symmetry analysis, the noise term t ca...

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