The realization of quantum computers, which in theory should be capable of far faster computations than conventional computers, remains a long-sought goal for researchers. New work at MIT takes a step in that direction, with a material that could serve as qubits — the basic units of information in quantum computing.
As described in the journal Science, the material is a collection of simple two-atom molecules made of sodium and potassium. By cooling them to an ultracold temperature — just a few hundred nanokelvins above absolute zero — researchers have demonstrated the molecules’ ability to retain information stored in them for hundreds of times longer than previously achieved.
According to MIT physics professor Martin Zwierlein, the choice to work with molecules rather than individual atoms came from the fact that molecules are more prone to interaction. “They can vibrate, they can rotate, and in fact they can strongly interact with each other, which atoms have a hard time doing. Typically, atoms have to really meet each other, be on top of each other almost, before they see that there's another atom there to interact with,” he said. "In order to make these qubits talk to each other and perform calculations, using molecules is a much better idea than using atoms.”
The use of sodium-potassium molecules provides a number of advantages, as well. “The molecule is chemically stable; if one of these molecules meets another one they don't break apart," Zwierlein added. Because it’s made from just two atoms, it’s also easier to cool. And, in a significant departure from most qubit materials that previously have been considered, they represent a system that may realize both storage and processing of quantum information.
Although it remains to be proven, such a system would theoretically be capable of carrying out thousands of quantum computations, or “gates,” within the time that information is retained, also known as coherence. Results could then be read optically through a microscope, revealing the molecules’ final state. In the case of the sodium-potassium molecules, coherence time is a single second — a veritable eternity in the context of quantum computing.
"We have strong hopes that we can do one so-called gate…in a fraction of a millisecond," Zwierlein said. "If you look at the ratio, you could hope to do 10,000 to 100,000 gate operations in the time that we have the coherence in the sample. That has been stated as one of the requirements for a quantum computer, to have that sort of ratio of gate operations to coherence times.”
Using an array of perhaps 1,000 molecules would make it possible to carry out calculations so complex that no existing computer could even begin to check the possibilities, Zwierlein added.
In practice, the advent of quantum computers is probably still a decade or more away. But in principle such devices could quickly solve problems such as factoring very large numbers — a process whose difficulty forms the basis of today's best financial transactions encryption systems.