Optical lattice holds onto huge atoms for quantum computing

Ann Arbor, MI--Fast quantum computers could fulfill certain needs in science and cryptology, although their potential usefulness in run-of-the-mill computing is being debated. And, because it is astonishingly difficult to make even a basic version of a quantum computer, the debate is still largely based on theory.

So physicists, quantum-photonics researchers, cryptologists, and certain computer geeks would like to find out more about steps that have been taken toward making quantum computers real. In one of these steps, University of Michigan (U of M) physicists have built a better Rydberg atom trap. Rydberg atoms are highly excited, nearly-ionized atoms that can be thousands of times larger than their ground-state counterparts.

Big atoms, big interactions

As a result of their size, interactions between Rydberg atoms can be roughly a million times stronger than between regular atoms. This is why they could serve as faster quantum circuits, said Georg Raithel, a professor at the U of M. Raithel's team trapped the atoms in an optical lattice created by interfering laser beams.¹

"The optical lattice is better than any other Rydberg atom trap for quantum information processing or high-precision spectroscopy," Raithel said. "Compared with other traps, optical lattices minimize energy level shifts in the atoms, which is important for these applications."

Raithel and doctoral students Kelly Younge and Sarah Anderson started with ground-state atoms of the soft metal rubidium. At room temperature, the atoms whiz around at the speed of sound, or about 300 m/s. The researchers hit them with lasers to cool and slow them to 0.1 m/s.

"That's about the speed of a mosquito," Younge said. "Cooling lasers combined with a magnetic field allows us to trap the ground-state atoms. Then we excite the atoms into Rydberg states."

In a rubidium atom, just one electron occupies the outer valence shell. With precisely tuned lasers, the researchers excited this electron so that it moved 100 times farther away from the nucleus of the atom, which classified it as a Rydberg atom. That valence electron in this case is so far away from the nucleus that it behaves almost as if it's a free electron.

Optical lattice provides the grip

To trap the Rydberg atoms, the researchers took advantage of what's called the "ponderomotive force" that allows them to secure a whole atom by holding fast to one electron—the sole valence shell particle in the rubidium Rydberg atoms. The optical lattice is what provides the ponderomotive force.

The physicists used microwave spectroscopy to determine how the lattice affected the Rydberg atoms, and in general how the atoms behaved in the trap.

"Essentially, we could track the motion of the atoms during the experiment. We could tell if the atoms were sitting in the bottom of a well in the electromagnetic field, or if they were roaming over many wells. In this way, we could optimize the performance of the trap," Younge said.

The research is funded by the National Science Foundation and the National Defense Science and Engineering Graduate Fellowship Program.

REFERENCE:

1. K. C. Younge et al., Phys. Rev. Lett. 104, 173001 (2010).