Quantum simulations are being tricked into becoming more realistic

May 18, 2021

Quantum simulations are getting closer to putting the cat in (or out of) the box. (Pixabay/Oleg Gamulinskiy)

Classical computers cannot unlock the quantum domain, so scientists rely on makeshift simulations to study the elusive field — and these programs are soon to reach another level of realism with the discovery of a new way to observe long-distance interactions between lithium atoms, stand-ins for quantum particles.

Growing interest in quantum technology has brought a demand for detailed understanding of quantum mechanisms. Among the slew of groups trying to help enumerate the domain's properties is a team that published a paper Monday in APS Physics describing a platform to place lithium atoms in a state that lets them interact with each other from far away.

"In my field, everybody always quotes Richard Feynman, who said, at some point, that the only way to study quantum many-body systems is to build a different one that behaves the same way," lead author Elmer Guardado-Sanchez, a Ph.D. candidate at Princeton University, told The Academic Times.

The quantum domain features tons of counterintuitive properties, the most talked-about one being superposition — illustrated by the famous Schrödinger's cat thought experiment, where the animal exists and doesn't exist at the same time. 

It also deals with entanglement, which means two separate systems — even on other sides of the universe — rely on each other to the point where if one is interrupted, the other is, too. However, these concepts and several others in the domain are already technically known; the hurdle resides in studying them with existing technology.

"Writing down the equations is not really that hard," Guardado-Sanchez said. "We've known quantum mechanics for a while now, but actually solving them using classical computers — it's not really feasible."

The rise of quantum computing is meant to solve this problem, but until these computers become widely available, makeshift quantum simulations as per Feynman's recommendation are necessary. In fact, they're vital for building a quantum computer in the first place.

"I like to think about them kind of like analog quantum computers," Guardado-Sanchez explained. "The very old IBM machines that you have to physically change cables and you get one input, one output — it's essentially what we do, but with lasers and mirrors and magnetic fields."

The new simulations use lithium atoms to behave as quantum particles — namely, electrons. These atoms have one valence electron, located on the outermost ring surrounding the nucleus. The team uses light to create a "fake solid," or an interference pattern generated by a laser, referred to as an optical lattice.

"In actual materials, electrons interact with each other via just simple charged forces," Guardado-Sanchez said. "All simulations that we've been able to do, with atoms on optical lattices, have not been able to have that character in them."

Because the lithium atoms have the valence electron, when they're very close together, they're able to interact with one another. Observing such short-distance interactions helps scientists understand short-distance quantum dynamics. However, these atoms are also neutral, meaning they don't have a strong enough charge to continue interacting with another atom when farther away.

"That essentially limits you to systems that have what [are] called contact interactions," Guardado-Sanchez said. "So we need to play tricks — we couple them to a Rydberg state."

The Rydberg state can excite lithium's valence electron and push it outward even farther from the nucleus. Because an atom's nucleus is positive — it carries protons — and the electron is negative, a magnetic force called a dipole moment is generated. If several atoms were placed into this state, their charges would interact — even at long distances. That's called a Van der Waals interaction.

"The more interesting part was that, well, it worked," Guardado-Sanchez said. "It's kind of like playing games with these quantum states of matter."

"We still went for it, even knowing that it might not pan out," he added. "We were happy to see that it did actually manage to reach kind of like a parameter space, where we could observe something interesting."

This is the first time that such a state has been created with lithium, in a system where particles can move around and in a mechanism that has observable dynamics.

Further, because typical Rydberg states are extremely strong and short-lived, the team adjusted the traditional Rydberg-dressing technique such that it led to a quantum superposition — again, with roots tracing back to Schrödinger's beloved pet — of Rydberg state and ground state. 

"By playing this trick," he said, "you can reduce the strength of the interaction — which is fine, we don't need that strong interaction — at the benefit of increasing the lifetime of this Rydberg dressing."

The study, "Quench Dynamics of a Fermi Gas with Strong Nonlocal Interactions," published May 17 in APS Physics, was authored by Elmer Guardado-Sanchez, Benjamin M. Spar and Waseem S. Bakr, Princeton University; Peter Schauss, University of Virginia; Ron Belyansky, Przemyslaw Bienias and Alexey V. Gorshkov, NIST/University of Maryland; Jeremy T. Young, NIST/University of Maryland and University of Colorado, Boulder; and Thomas Iadecola, Iowa State University. 

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