Our lab and experiments
Asymmetric LandauZener Tunneling in a Periodic Potential
Abstract:
Using a simple model for nonlinear LandauZener tunneling between two energy bands of a BoseEinstein condensate in a periodic potential, we find that the tunneling rates for the two directions of tunneling are not the same. Tunneling from the ground state to the excited state is enhanced by the nonlinearity, whereas in the opposite direction it is suppressed. These findings are confirmed by numerical simulations of the condensate dynamics. Measuring the tunneling rates for a condensate of rubidium atoms in an optical lattice, we have found experimental evidence for this asymmetry.
The phenomenon of LandauZener (LZ) tunneling [1] is a basic quantum mechanical process. It is based on the solution of the Schrödinger equation for a twolevel dynamics when a parameter of the Hamiltonian system is time dependent.
Recently, LZ tunneling within a periodic potential was studied for a nonlinear twolevel system in which the level energies depend on the occupation of the levels [2,3]. It was discovered that a nonlinearity with a positive sign enhances the tunneling probability between the ground band and the first excited band.
We explored, theoretically and experimentally, the LandauZener tunneling between Bloch bands of a BoseEinstein condensate in an accelerated optical lattice. The optical lattice depth controls the tunneling barrier, while the optical lattice acceleration controls the time dependence of the Hamiltonian. We showed that the meanfield nonlinearity produces an asymmetry for the tunneling probability. More precisely, the tunneling probability from the lower energy adiabatic state to the upper one is enhanced, while the inverse tunneling probability is suppressed.
The asymmetry in the tunneling transition probabilities can be explained qualitatively as follows: The nonlinear term of the Schrödinger equation acts as a perturbation whose strength is proportional to the energy level occupation. If the initial state of the condensate in the lattice corresponds to a filled lower level of the state model, then the lower level is shifted upward in energy while the upper level is left unaffected. This reduces the energy gap between the lower and upper levels and enhances the tunneling. On the contrary, if all atoms fill the upper level, then the energy of the upper level is increased while the lower level remains unaffected. This enhances the energy gap and reduces the tunneling.
Band structure of a BEC in an optical lattice (V0 = 2 Erec) and LZ tunneling [ground to excited state (a) and excited to ground state (b)]. When the BEC is accelerated across the edge of the Brillouin zone (BZ) at quasimomentum 1, LZ tunneling can occur. Further acceleration will result in the condensate part in the upper level undergoing LZ tunneling to higher bands with a large probability (due to the smaller gaps between higher bands), leaving them essentially unaffected by the lattice. After the first crossing of the edge of the BZ, increasing the lattice depth and decreasing the acceleration leads to a much reduced tunneling rate from the groundstate band at successive BZedge crossings.
LandauZener tunneling between the two lowest energy bands of a condensate inside an optical lattice is
investigated in the following way (see Fig. 1). Initially, the condensate is loaded adiabatically into one of
the two bands. Subsequently, the lattice is accelerated in such a way that the condensate crosses the edge of
the Brillouin zone once, resulting in a finite probability for tunneling into the other band (higherlying
bands can be safely neglected as their energy separation at the edge of the Brillouin zone is much larger
than the band gap). After the tunneling event, the two bands have populations reflecting the LandauZener
tunneling rate (assuming that, initially, the condensate populated one band exclusively). In order to
experimentally determine the number of atoms in the two bands, we then increase the lattice depth (from 2
E
_{rec} to 4 E
_{rec}) and decrease the acceleration (from 3 m/s
^{2} to 2 m/s
^{2}).
In this way, successive crossings of the band edge will result in a much reduced LandauZener tunneling probability
(of order a few percent). The fraction of the condensate that after the first tunneling event populated the
groundstate band will, therefore, remain in that band, whereas the population of the first excited band will
undergo tunneling to the second excited band with a large probability (around 90%) as the relevant gap is smaller
by a factor of 5 for our parameters. Once the atoms have tunneled into the second excited band, they
essentially behave as free particles since higherlying band gaps are smaller still, so they will no longer
be accelerated by the lattice. In summary, using this experimental sequence we selectively accelerate that
part of the condensate further that populates the groundstate band. In practice, in order to get a good
separation between the two condensate parts after a time of flight, we accelerate the lattice to a final
velocity of 4  6 v
_{rec} [14] and absorptively image the condensate after 22 ms.
LZ tunneling rate r within the twolevel model as a function of the nonlinear parameter C for different accelerations. Results are for V
_{0} = 2.2 E
_{rec}, in (a) for a
_{L} = 0.8; 3.2; 5.6 m/s
^{2} from bottom to top, and in (b) for a
_{L} = 2.9 m/s
^{2}.
In (a), the dashed (solid) lines correspond to tunneling from the excited (groundstate) band. In (b) experimental results denoted by open (filled) symbols correspond to tunneling from the excited (ground state) band.
In summary, we have numerically simulated LandauZener tunneling between two energy bands in a periodic potential and found that, in the presence of a nonlinear interaction term, an asymmetry in the tunneling rates arises. Experimentally, we have measured these tunneling rates for different values of the interaction parameter and found qualitative agreement with the simulations. Future experiments could probe the complicated and timedependent tunneling behavior due to the changing tunneling rate for multiple crossings of the zone edge. Furthermore, when the two lowest bands are initially equally populated, the tunneling behavior should again be linear. We also note here that the phenomenon of asymmetric tunneling should be a rather general feature of quantum systems exhibiting a nonlinearity. For instance, calculating the energy shift due to a nonlinearity for two adjacent levels of a harmonic oscillator, one finds that both levels are shifted upwards in energy, the shift being proportional to the population of the respective level. The energy difference between the levels, therefore, decreases if only the lower state is populated and increases if all the population is in the upper level.

L. D. Landau, Phys. Z. Sowjetunion 2, 46 (1932);
G. Zener, Proc. R. Soc. London, Ser. A 137, 696 (1932).

O. Zobay and B.M. Garraway
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B.Wu and Q. Niu
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M. JonaLasinio, O. Morsch, M. Cristiani, N. Malossi, J. H. Mueller, E. Courtade, M. Anderlini, and E. Arimondo
Asymmetric LandauZener Tunneling in a Periodic Potential
Phys. Rev. Lett. 91, 23, 230406