Even quantum computers need to keep their cool. Now, researchers have built a tiny nanoscale refrigerator to keep qubits cold enough to function.
Classical computers require built-in fans and other ways to dissipate heat, and quantum computers are no different. Instead of working with bits of information that can be either 0 or 1, as in a classical machine, a quantum computer relies on “qubits”, which can be in both states simultaneously – called a superposition – thanks to the quirks of quantum mechanics. Those qubits must be shielded from all external noise, since the slightest interference will destroy the superposition, resulting in calculation errors. Well-isolated qubits heat up easily, so keeping them cool is a challenge.
Also, unlike in a classical computer, qubits must start in their low-temperature ground states to run an algorithm. Qubits heat up during calculations, so if you want to run several quantum algorithms one after the other, any cooling mechanism must be able to do its job quickly. A standard fan just won’t cut it.
Now, Mikko Möttönen at Aalto University in Finland and his colleagues have built the first standalone cooling device for a quantum circuit. It could eventually be integrated into many kinds of quantum electronic devices – including a computer.
The team built a circuit with an energy gap dividing two channels: a superconducting fast lane, where electrons can zip along with zero resistance, and a slow resistive (non-superconducting) lane. Only electrons with sufficient energy to jump across that gap can get to the superconductor highway; the rest are stuck in the slow lane.
If some poor electron falls just short of having enough energy to make the jump, it can capture a photon from a nearby resonator to get a boost. As a result, the resonator gradually cools down.
Over time this has a selective chilling effect on the electrons as well: the hotter electrons jump the gap, while the cooler ones are left behind. The process removes heat from the system, much like how a refrigerator functions.
Spiros Michalakis at the California Institute of Technology draws a loose analogy with the famous thought experiment known as Maxwell’s Demon, in which an intelligent being presides over a box of gas atoms divided into two chambers. The demon allows only the hottest, or most energetic, atoms to pass through an opening in the wall dividing the two chambers, resulting in a sharp difference in temperature between the two.
There is no demon in the quantum fridge, but it works in a similar way, Michalakis says. “It’s kind of like a gate similar to Maxwell’s Demon, where you only allow electrons with energy above a certain threshold to cross,” he said.
The next step will be to build the device and cool actual qubits with it, being careful not to accidentally destroy the superposition when the fridge is shut down. Möttönen is confident enough in eventual success that he has applied for a patent for the device.
“Maybe in 10 to 15 years, this might be commercially useful,” he said. “It’s going to take some time, but I’m pretty sure we’ll get there.”