Written by Christopher Crockett
The goal of building a quantum computer is to harness the quirks of quantum physics to solve certain problems far faster than a traditional computer can. And at the heart of a quantum computer is the quantum bit, or qubit—the quantum equivalent of the 1s and 0s that underlie our digital lives.
“A qubit is the fundamental building block of quantum information science technology,” says Joseph Heremans, an electrical engineer at the US Department of Energy’s Argonne National Laboratory.
Traditional bits can be any sort of switch, anything that can flip from 0 to 1. But building a qubit takes something more.
“A qubit is essentially a quantum state of matter,” Heremans says. “And it has weird properties that allow you to store more information and process more information” than a traditional bit.
Fortunately, nature has provided lots of options, and engineers have cooked up a couple more.
Those weird properties include superposition (the ability to be in a mixed state, a weighted combination of 1 and 0) and entanglement (in which multiple qubits share a common quantum state). Both might seem like they’d be hard to come by. Fortunately, nature has provided lots of options, and engineers have cooked up a couple more.
Researchers are exploring more than half a dozen ways to implement qubits, with two promising approaches currently in focus: superconducting circuits and trapped ions.
Out in front
Ions—atoms that have lost one or more of their electrons—emerged as a promising qubit platform at the dawn of experimental quantum computing in the mid-1990s. In fact, the first qubit ever built was fashioned out of a single beryllium ion.
Ions are natural quantum objects: Two of the discrete energy levels of their remaining electrons can represent a 0 or 1; those energy levels are readily manipulated by lasers; and because ions are electrically charged, they are easily held in place by electromagnetic fields. Not much new needed to be invented to produce trapped-ion qubits. Existing technology could handle it.
Another upside of trapped ions is that they are stalwart defenders against a qubit’s greatest nemesis: loss of information. Quantum states are fragile, and superpositions stick around only if the qubits don’t interact with anything. A stray atom or an unexpected photon can collapse the quantum state. In physics speak, the qubit “decoheres.” And decoherence is the death knell to any quantum information technology.
“We want a system where we can manipulate it, because we want to do calculations, but the environment doesn’t talk to it too much,” says Kenneth Brown, an electrical engineer at Duke University.
Trapped ions check both boxes. Held safely in a darkened vacuum, they have a low interaction with the environment, he says.
Because of that robustness, trapped ions exhibit....