Spatial Bloch oscillations of a quantum gas in a "beat-note" superlattice

Abstract

We report the experimental realization of a new kind of optical lattice for ultra-cold atoms where arbitrarily large separation between the sites can be achieved without renouncing to the stability of ordinary lattices. Two collinear lasers, with slightly different commensurate wavelengths and retroreflected on a mirror, generate a superlattice potential with a periodic "beat-note" profile where the regions with large amplitude modulation provide the effective potential minima for the atoms. To prove the analogy with a standard large spacing optical lattice we study Bloch oscillations of a Bose Einstein condensate with negligible interactions in the presence of a small force. The observed dynamics between sites separated by ten microns for times exceeding one second proves the high stability of the potential. This novel lattice is the ideal candidate for the coherent manipulation of atomic samples at large spatial separations and might find direct application in atom-based technologies like trapped atom interferometers and quantum simulators.

Multimode Trapped Interferometer with Ideal Bose-Einstein Condensates

“…The beat-note superlattice potential along x is generated by overlapping two standing waves V 1 cos 2 k 1 x + V 2 cos 2 k 2 x with wavevectors k 1 = 2π/1.013 µm −1 and k 2 = 2π/1.12 µm −1 , and V 1 , V 2 are the lattice depths. For V 1 , V 2 of the order of a recoil energy the BEC experiences an effective lattice potential V eff (1 + sin(kx + φ))/2, with 2 , where M is the atomic mass, the spatial period 2π/k = π/(k 1 − k 2 ) equals 5.3 µm, and the phase φ depends on the relative phase between the two combined lattices [22]. Both lasers are frequency locked to the same optical reference cavity with a relative stability of ∼ 10 kHz, via sideband-locking that allows to tune φ dynamically by adjusting the radiofrequency of one sideband.…”

“…Indeed, beyond the effective potential approximation, the KD pulses diffract atoms also at momentum components associated with the two fundamental optical lattices, i.e. at integer multiples of 2 k 1,2 [22] and the atoms of these components are effectively lost for the purpose of the interferometer: due to their large momenta, they are driven in the anharmonic region of the ODT (if not outside). In practice, increasing β reduces N , the total number of atoms contributing to the interferometer signal; for this reason we work with β < 2.…”

“…Here we use an optical trap and take advantage of a large-spacing (∼ 5 µm) optical lattice, that reduces the recoil velocity, hence the oscillation amplitude, by a factor of 10, with respect to the commonplace lattice spacing of 0.5µm. Specifically, we create the periodic potential exploiting a recently developed technique, named "beat-note superlattice" [22] capable of realizing lattices with a large effective period in a retro-reflected configuration, with laser wavelengths of the order of 1 µm. In order to investigate experimentally the operation of the KD interferometer in the presence of external perturbations, we first observe the evolution of the different diffracted orders in the ODT, confirming a harmonic and symmetric evolution.…”