This Atom Makes Quantum Computing Light Work

Abstract concept of a photon interacting with a particle. agsandrew/Shutterstock.

The elusive dream of quantum computing might seem far off, but researchers everywhere are trying to figure out solutions to get us closer to these powerful computers. One team of scientists from the University of Toronto has succeeded in making an important tool in computing, a logic gate, from a single type of atom. It uses clever optical effects to transmit information. The research is published in Nature Physics.

Logic gates are powerful instruments in computing. They are a sort of doorway for information, but not just any information can go through. They have to abide by a particular set of predefined rules. The simplest example is binary logic gates where they receive either an input of one or zero. One signals "on" and zero signals "off."

In computers, they typically take the form of diodes or transistors, but engineers are keen to make them smaller. Quantum computers have components that consist of only a few atoms. Reducing computer components to this size is challenging, but the creation of a single-atom logic gate should help the challenge of making everything mini a tiny bit easier. 

In this research, the logic gate takes the form of a rubidium atom. A 'cloud' of atoms are cooled down to a millionth of a degree above absolute zero. It is problematic to take precise measurements at room temperature since atoms jiggle around, however when chilled to near absolute zero the rubidium atoms are much easier to manage. 

The atoms are then ready to receive information. In this instance, the information comes in the form of a single photon, which zooms towards the rubidium atoms. "It tickles it a little bit, changes how it affects the second photon, and nevertheless keeps propagating through after the interaction is complete," study author Aephraim Steinberg explained to IFLScience.

The rubidium atoms don't directly absorb the photon; this would introduce huge changes in the atom. Instead, the photon's electromagnetic field just alters the electron configuration slightly.

An artist's rendition of what occurs when one photon goes through a carefully prepared atomic medium at the same time as a pulse including many photons. The change in color represents phase shifts picked up by each pulse. Amir Feizpour.

This change in electron configuration alters the way the rubidium atoms react to light. When the second beam of light propagates through the rubidium atom, the beam undergoes a phase shift. In this way, the rubidium atom is acting a little bit like glass. Atoms that have been "tickled" by an initial photon "slow [the laser beam] down a little bit, bend it, and give it what we technically call a phase shift," said Steinberg.

But what information is this laser beam transmitting? Well, much like our simple logic gate that recorded a one or zero, on or off, this atomic logic gate has two settings: entangled or unchanged. The information is in the second laser beam; after being blasted through the rubidium, it either has a phase shift, or it is unchanged. This corresponds to either a one or a zero.

The scientists were delighted since it is intrinsically difficult to get photons to "talk" (interact) with each other. "This kind of work opens up a new regime in optics where we can see interactions between individual particles of light, something that wasn't really possible over the last 50 years," Steinberg added.

He continued: "Light has this funny property that it doesn't interact with itself, it's not like marbles that can bounce into each other; photons travel through each other without any interaction." 

This experiment shines new light on the fundamental physics that might lead to fabled quantum computers. But there is still a long way to go. Using one type of atom that acts as a logic gate is very far away from an entire computer with an array of logic gates that all work together. "The challenge is going to be to assemble these gates in some scalable fashion," said Steinberg. "Trying to figure out if you've got one, how do you make ten, how do you make 100, how do you get them to talk to one another? 

"I'd say there are plenty of possibilities for longer-term applications that we don't know about yet."

Correction: where article said there was only one atom has now been corrected to a cloud of atoms.

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