A team from the Institute for Laser Physics at the University of Hamburg, Germany has pioneered a new way of imaging quantum gases – collections of atoms only a fraction above absolute zero. Using a series of trapping and expansion techniques, the researchers magnify the spatial distribution of the atoms by up to a factor of 90, making it possible to measure the number of atoms, the correlations between them and the patterns they form with far greater precision than before. The technique could pave the way for exploring complex physics phenomena that were not previously accessible.
Using highly controlled laboratory experiments to simulate analogous quantum phenomena is one of the most prominent and promising paths to understanding complex quantum behaviour. These so-called quantum simulation experiments are particularly useful because they can exactly simulate the dynamics of, for example, condensed matter systems where numerical or theoretical methods fail.
Quantum gases in optical lattices make excellent quantum simulator candidates because they can be controlled and manipulated with high precision. Optical lattices are created by overlapping a series of laser beams to produce a periodic, egg-carton-like pattern of dips in potential energy, enabling atoms in the gas to be trapped at each dip or “site” (see figure for a visualization of the atoms in an optical lattice).
To exploit the favourable properties of these quantum simulators, one needs to accurately image each site at small length scales. In 2D quantum gas simulators, where each site is typically occupied by a single atom, well-established microscopy techniques already exist for imaging the system at a smaller scale by direct imaging with high optical resolution. However, current microscopy techniques for 2D systems fail for 3D systems due to, for example, many atoms occupying a single site. “Microscopy techniques are very important for insight into quantum many-body systems,” explains Christof Weitenberg, a junior group leader at the University of Hamburg, who led the study together with Klaus Sengstock. “We wanted to push current techniques further to both new regimes and to less technical complexity.”
Magnifying the matter-wave
To image an everyday object, one would use a glass lens to magnify its optical image, exploiting the wave properties of light. To image their 3D quantum gas, the Hamburg group instead harness the wave-like nature of the gas itself and use a so-called matter-wave lens as a magnifier. This lens is not a physical lens made from glass; instead, it is a series of techniques designed to manipulate the gas’s shape.
Initially, the 3D quantum gas is tightly trapped in an optical lattice with sites spaced less than a micrometre apart, making the structures too small to be imaged directly. By changing the shape of the trap for a fixed time and then turning off the trap completely to let the gas expand freely, the researchers cause the gas to evolve through a series of geometries such that it returns to its original shape, but with a significant magnification. They can then make precise measurements, even resolving the small atomic motion inside the lattice sites. “This is particularly exciting because we demonstrate that the technique works for 3D systems with several atoms per lattice site; conditions where, up to this moment, high-resolution imaging was not possible,” says Luca Asteria, a PhD student at the University of Hamburg and the study’s first author.
To benchmark their imaging technique, Asteria and colleagues measured the thermodynamic properties of the quantum gas to demonstrate its transition to a Bose-Einstein condensate, a state of mater in which all atoms behave as one “super-atom”. This transition can be characterized by measuring atom numbers across the lattice and thus made an ideal test.
Exploring novel and unexplored regimes
Now that the imaging technique has been established and shown to precisely image 3D quantum gases, the researchers are keen to investigate new regimes of quantum many-body dynamics in complex lattice geometries. “We are currently exploring the physics in 3D systems on the single-site scale,” Weitenberg says. “We have already found intriguing phenomena, such as an emerging density wave upon applying a strong tilt to the lattice.”
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In future experiments, the team plans to study topological states, in which atoms at the edge of the 3D system behave differently from atoms within its bulk. These states have been proposed theoretically but have not yet been observed experimentally in these systems. The team believe its methods could be extended to single-atom sensitivity, which would allow for deep insights into strongly interacting quantum systems.
The research is published in Nature.
