One of the biggest issues facing the development of quantum computers—tomorrow's supercomputers based on the strange principles of quantum physics—is keeping everything cool. Electronics make heat, and while your laptop and smartphone can use fans or heat-absorbing water tanks, those just won't cut it for quantum computing, which will take advantage of the quirks of quantum mechanics to create computers that calculate at insane speeds.

"When you start to make electronics smaller and denser, not only are you making much more heat in the same amount of volume, but it's much harder for the heat to flow outward," says Peter Nalbach, a theoretical physicist at the University of Hamburg, Germany.

At this stage, our early attempts at quantum computers have to be kept at a temperature barely hovering above the insanely cold, dead-standstill of absolute zero. If you're trying to develop a large-scale quantum computer, Nalbach, says, "at a certain point, you'll have to actively transport heat out of the spot where it's created," Until now, engineers had no idea exactly how to do this.

But Nalbach and his colleagues have just published the schematics of a method to individually target and cool the physical building blocks of tomorrow's quantum computers. In their outline, recently published in the physics journal Physical Review Letters, the physicists show how they can halve the temperature of individual quantum dots—nano-sized pieces of crystal that are currently being investigated as qubits for quantum computers.

Spin Cooling

Here's how the nano-cooler design works: Jochen Brüggemann, a physicist with the team, says you can think of the quantum dot crystal "sort of like a small island, which can temporarily trap an electron." The physicists sandwich their quantum dot—between two tiny electromagnetic prongs. To cool that quantum dot, they first produce an electrical current between the two prongs, creating a short pathway for electrons to jump from one prong to the other. And as electrons travel with the current, some of the electrons will stop and chill that quantum dot island en route.

Normally, Brüggemann says, this would actually heat up the quantum dot (by jamming up that island with energetic electrons). But the physicists do something tricky, by taking advantage of a quantum mechanical property called spin.

Think of each electron as a small rotating ball; in simple terms, the spin of each electron is just the direction it's rotating. The physicists polarize their prongs in such a way that one of them—the prong generating the electrons—mostly produces electrons that spin upward, while the other—the prong grabbing the electrons on the other side—for the most part will accept only electrons spinning upwards.

Some of the up-spinning electrons will get stuck on that quantum dot island, and because of the magnetic current, they really want to leave. But to do so, the electrons have to physically change their spin to match the destination prong. Changing spin is possible, but it's an energy-intensive chore.

And this is how the cooling happens: These castaway electrons will borrow energy from ambient vibrating movement of the quantum dot (the quantum dot's temperature, essentially) and use it to flip their spin so they can and pass on through to the other side. According to Brüggemann, this constant stream of spin-flipping electrons continually cools the quantum dot by to half its ambient temperature.

Applying the Design

While Brüggemann and Nalbach are excited about this new method of cooling, they're quick to temper the possibility that we might see their design helping to produce quantum computers in the near future. Because many designs for quantum computers are still being developed, it's hard to say how well this schematic would mesh with the future technology. "Although we're now seeing a lot of other [experimental] work with spin-polarized electrons, and that makes it more likely we could adapt this type of cooling."

In addition, Brüggemann says, because he and his colleagues have yet to bulid or test their design in the laboratory, it's very difficult to figure out how efficient their cooling method would be in practice. "And everything comes down to efficiency," Nalbach says.

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