September 07, 2009

Another New State of Matter

Journal Science: Realization of an Excited, Strongly Correlated Quantum Gas Phase

Ultracold atomic physics offers myriad possibilities to study strongly correlated many-body systems in lower dimensions. Typically, only ground-state phases are accessible. Using a tunable quantum gas of bosonic cesium atoms, we realized and controlled in one-dimensional geometry a highly excited quantum phase that is stabilized in the presence of attractive interactions by maintaining and strengthening quantum correlations across a confinement-induced resonance. We diagnosed the crossover from repulsive to attractive interactions in terms of the stiffness and energy of the system. Our results open up the experimental study of metastable, excited, many-body phases with strong correlations and their dynamical properties.

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The researchers produced a quantum gas made up of bosonic caesium atoms in a vacuum chamber. Then, they generated an optical lattice using two laser beams; the lattice confined the atoms to vertical, one-dimensional structures with up to 15 atoms aligned in each 'tube'. The laser beams prevented the atoms from shifting out of line or changing places. Once this was achieved, the scientists used a magnetic field to tune the interaction among the atoms.

'By increasing the interaction energy between the atoms (attraction interaction), the atoms start coming together and the structure quickly decays,' explained Dr Naegerl. This is called the 'Bosenova effect'. When the interaction energy is minimised, the atoms are able to repel instead of attract each other; this allows them to align vertically and regularly along a one-dimensional structure. The resulting system is stable.

The researchers observed a surprising effect when the interactions were switched from strongly repulsive to strongly attractive. They achieved 'an exotic, gas-like phase, where the atoms are excited and correlated but do not come together and the 'Bosenova effect' is absent', said Dr Naegerl.

According to co-investigator Elmar Haller of the University of Innsbruck, the phase was predicted four years ago. 'We have now been able to realise it experimentally for the first time,' he stated.

The experimental setup will be used in future studies to investigate the properties of quantum wires, which have until now been extremely difficult to observe. Further research on low-dimensional structures may also shed light on the functioning of high-temperature superconductors.

4 pages of supplemental material

Lattice loading.
We produce a BEC of Cs atoms in the lowest hyperfine sublevel with hyperfine quantum
numbers F = 3 and mF = 3 in a crossed beam dipole trap with trap frequencies !x;y;z =
2  (15; 20; 13) Hz, where z denotes the vertical direction. The BEC is adiabatically transferred from the dipole trap to the array of tubes by exponentially ramping up the power in the lattice laser beams with waists  350 m within 500 ms. The repulsive interaction causes the atoms to move radially outwards during the initial phase of the lattice loading in response to the strong local compression. We use this effect to vary the total number of tubes loaded and hence the atom number per tube by setting a3D for the loading process to values between 40 a0 and 350 a0. For the data set in the repulsive regime (Fig.3A, circles), we exponentially ramp down the crossed beam dipole trap during the loading process and reach longitudinal and transversal trap frequencies of !D = 2  15:4(1) Hz and !? = 2  13:1(1) kHz with a transversal confinement length a? = 1440(6) a0. Here, depending on the regime of interaction to be studied, the number of atoms in the central tube is set to values between 8 and 25. For the data set in the sTG regime (Fig.3A, squares) we increase !D to 2  115:6(3) Hz to reduce the vertical extent of the sample and hence the variation of the magnetic field across the atom cloud. For this, we keep the depth of the crossed beam dipole trap constant during the loading process and then ramp up the power in one of the beams within 100 ms. The number of atoms in the central tube is set to values between 8 and 11.

Array of 1D tubes....

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