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Quantum computing: Speed tested in cloud of cold atoms

GEORGIA TECH (US) — Physicists are using clouds of ultra-cold atoms to establish the top speed at which a network of quantum computers could communicate.

The system includes multiple small quantum computers that would work together much as today’s multi-core supercomputers team up to tackle big digital operations. The individual computers in such a system could communicate information using the clouds, known as Bose-Einstein condensates (BECs), where the cold atoms all exist in exactly the same quantum state.

The method is a proposed solution for the loss of order in the systems—a problem known as quantum decoherence—which worsens as the number of bits in a quantum computer increases.

A team of physicists at the Georgia Institute of Technology examined how this Bose-Einstein communication might work. The researchers determined the amount of time needed for quantum information to propagate across their BEC.

“What we did in this study was look at how this kind of quantum information would propagate,” says Chandra Raman, an associate professor in Georgia Tech’s School of Physics. “We are interested in the dynamics of this quantum information flow not just for quantum information systems, but also more generally for fundamental problems in physics.”

Spreads like a drop of dye

The researchers first assembled a gaseous Bose-Einstein condensate that consisted of as many as three million sodium atoms cooled to nearly absolute zero.

To begin the experiment, they switched on a magnetic field applied to the BEC that instantly placed the system out of equilibrium. That triggered spin-exchange collisions as the atoms attempted to transition from one ground state to a new one. Atoms near one another became entangled, pairing up with one atom’s spin pointing up, and the other’s pointing down.

These images show that quantum correlations in the Georgia Tech Bose-Einstein condensate are highly concentrated at first (top graph), then slowly diffuse outward (lower two graphs). The peaks show the localization of the correlations. (Credit: Chandra Raman/Georgia Tech)

This pairing of opposite spins created a correlation between pairs of atoms that moved through the entire BEC as it established a new equilibrium.

The researchers, who included graduate student Anshuman Vinit and former postdoctoral fellow Eva Bookjans, measured the correlations as they spread through the cloud of cold atoms.

At first, the quantum entanglement was concentrated in space, but over time, it spread outward like drop of dye diffuses through water.

“You can imagine having a drop of dye that is concentrated at one point in space,” Raman says. “Through diffusion, the dye molecules move throughout the water, slowly spreading throughout the entire system.”

The research could help scientists anticipate the operating speed for a quantum computing system composed of many cores communicating through a BEC.

“This propagation takes place on the time scale of ten to a hundred milliseconds,” Raman explains. “This is the speed at which quantum information naturally flows through this kind of system. If you were to use this medium for quantum communication, that would be its natural time scale, and that would set the timing for other processes.”

‘A unifying principle’

Though relevant to communication of quantum information, the process also showed how a large system undergoing a phase transition does so in localized patches that expand to attempt to incorporate the entire system.

“An extended system doesn’t move from one phase to another in a uniform way,” says Raman. “It does this locally. Things happen locally that are not connected to one another initially, so you see this inhomogeneity.”

Beyond quantum computing, the results may also have implications for quantum sensing—and for the study of other physical systems that undergo phase transitions.

“Phase transitions have universal properties,” Raman notes. “You can take the phase transitions that happen in a variety of systems and find that they are described by the same physics. It is a unifying principle.”

Raman hopes the work will lead to new ways of thinking about quantum computing, regardless of its immediate practical use.

“One paradigm of quantum computing is to build a linear chain of as many trapped ions as possible and to simultaneously engineer away as many challenges as possible,” he says. “But perhaps what may be successful is to build these smaller quantum systems that can communicate with one another.

“It’s important to try as many things as possible and to keep an open mind. We need to try to understand these systems as well as we can.”

The research is scheduled to be published in the April 19 online version of the journal Physical Review Letters. The US Department of Energy and the National Science Foundation funded the work.

Source: Georgia Tech

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