Exotic physics can happen when quantum particles come together and talk to each other. Understanding such processes is challenging for scientists, because the particle interactions can be hard to glimpse and even harder to control. Moreover, modern computer simulations struggle to make sense of all the intricate dynamics going on in a large group of particles. Luckily, atoms cooled to near zero temperatures can provide insight into this problem.
Lasers can make cold atoms mimic the physics seen in other systems—an approach that is familiar terrain for atomic physicists. They regularly use intersecting laser beams to capture atoms in a landscape of rolling hills and valleys called an optical lattice. Atoms, when cooled, don't have enough energy to walk up the hills, and they get stuck in the valleys. In this environment, the atoms behave similarly to the electrons in the crystal structure of many solids, so this approach provides a straightforward way to learn about interactions inside real materials.
But the conventional way to make optical lattices has some limitations. The wavelength of the laser light determines the location of the hills and valleys, and so the distance between neighboring valleys—and with that the spacing between atoms—can only be shrunk to half of the light's wavelength. Bringing atoms closer than this limit could activate much stronger interactions between them and reveal effects that otherwise remain in the dark.
Now, a team of scientists from the Joint Quantum Institute (JQI), in collaboration with researchers from the Institute for Quantum Optics and Quantum Information in Innsbruck, Austria, has circumvented the wavelength limit by leveraging the atoms' inherent quantum features, which should allow atomic lattice neighbors to get closer than ever before. The new technique manages to squeeze the gentle lattice hills into steep walls separated by only one-fiftieth of the laser's wavelength—25 times narrower than possible with conventional methods. The work, which is based on two prior theoretical proposals, was recently published in Physical Review Letters .
In most optical lattices, atoms are arranged by repeating smooth dips in the intensity of laser light—a mechanism that also works with non-quantum objects like bacteria or even glass beads. But this ignores many inherent quantum characteristics of the atoms. Unlike glass beads, atoms, prompted by laser light of certain colors, can internally switch between different quantum versions of themselves, called states. The team exploits this property to build lattices that effectively replace the rolling hills with spiky features.
"The trick is that we don't rely on the light's intensity by itself," explains Yang Wang, a postdoctoral researcher at the JQI and the lead author of the paper. "Instead, we use light as a tool to facilitate a quantum mechanical effect. And that creates the new kind of landscape for the atoms."
To create this lattice, the researchers ensnare the atoms in a two-toned light pattern. Each color is chosen so that it can change an atom's internal state on its own, but when the two colors overlap, the more intense color at each spot takes charge and decides which internal state the atom lands in. But this pattern is not smooth—there are vast valleys where the atom prefers one state, interrupted by thin strips where it should switch. The rules of quantum mechanics dictate that every time an atom changes its state, the atom must pay a price in the form of energy, just like climbing a hill. While a smooth transition may appear as a Sunday stroll to the atom, large changes over shorter distances quickly evolve into an increasingly steep hike. In the experiment, the thin strips inside the light pattern are so narrow, that they look like insurmountable walls to the atom, so it avoids them and gets stuck in between.
These sharp walls are an important first step in the quest to bring atoms even closer. The new technique still provides plenty of room for atoms to travel within the wide, flat plains, but researchers plan to reduce this freedom by adding more barriers. "As we take steps to confine the atoms further and further, quantum effects between the atoms should become increasingly important," says Trey Porto, a JQI Fellow and an author of the paper. "This has a practical side effect, because it also increases the temperature that we need to be at to see weird quantum behavior. Cooling is quite difficult, so this would make the physics that we're after more easily attainable."
The research team says that this tool may also be useful for future quantum chemistry experiments, allowing scientists to bring atoms close enough to engage in a small-scale, highly-controlled reaction.
Explore further: Team invents new way to 'see' the quantum world
More information: M. Łącki et al. Nanoscale "Dark State" Optical Potentials for Cold Atoms, Physical Review Letters (2016). DOI: 10.1103/PhysRevLett.117.233001 Y.
Wang et al. Dark State Optical Lattice with a Subwavelength Spatial Structure, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.120.083601