Research: Quantum Computers Can Be Better Than the Sum of Their Parts

Pobody’s nerfect—not even the indifferent, calculating bits that are the foundation of computers. But for the first time, a team that includes University of Maryland researchers has demonstrated that an assembly of quantum computing pieces can be better than the shakiest individual parts used to make it.

In a paper published today in the journal Nature, the team that includes Christopher Monroe, a fellow of the Joint Quantum Institute and a College Park Professor of Physics, along with other UMD scientists and colleagues from Duke University, shared how they took this landmark step toward reliable, practical quantum computers.

In their experiment, the researchers combined several qubits—the quantum version of bits that encode information in typical computers as zeros and ones—to function together as a single unit. This “logical qubit” is based on a quantum error correction code that can detect and correct an error that occurs in one of the 13 individual qubits that compose the logical qubit “team.” Additionally, the logical qubit design is fault-tolerant—that is, capable of containing errors to minimize their negative effects.

The demonstration bolsters the great promise of quantum computers, which are theoretically capable of operations beyond the scope of standard, or “classical” computers, in part because qubits are far more flexible than regular bits, and not constrained to just being zero or one. But quantum errors have long dogged the effort to scale up these futuristic machines to greater levels of power; unlike transistors that encode information in normal computer chips, a qubit is susceptible to errors from tiny environmental disruptions like a vibration or temperature change that knock it out of its quantum state.

But a group of qubits that work as a team can help get around such limitations, said Monroe, who’s also co-founder and chief scientist at IonQ, a quantum company in College Park that’s based in part on technology he developed as a UMD researcher.

“Qubits composed of identical atomic ions are natively very clean by themselves,” he said. “However, at some point, when many qubits and operations are required, errors must be reduced further, and it is simpler to add more qubits and encode information differently. The beauty of error correction codes for atomic ions is they can be very efficient and can be flexibly switched on through software controls.”

This is the first time that a logical qubit has been shown to be more reliable than the most error-prone step required to make it. The experiment demonstrated that the team could confirm that it correctly created the logical qubit in a desired quantum state 99.4% of the time, compared to the approximate 98.9% success rate of the six quantum processes (called quantum operations) that they used to make it.

That might not sound like a big difference, but it’s a crucial step in the quest to build much larger quantum computers. If the six quantum operations were assembly line workers, each focused on one task, the combined error rate of the workers would result in the line only producing useful products 93.6% of the time, far lower than the 99.4% effectiveness rate when the “workers” collaborate to minimize the chance of quantum errors compounding and ruining the result.

While it might seem uneconomical to use so many individual qubits and steps just to make something to function as a single qubit, the unique computational capabilities of quantum computers could make logical qubits a small price to pay. If quantum computers can be made trustworthy, they will be powerful devices capable of computations expected to revolutionize sectors including healthcare, security and finance.

The results were achieved using Monroe’s ion-trap system at UMD, which uses up to 32 individual charged atoms—ions—that are cooled with lasers and suspended over electrodes on a chip. The ions can then be used as qubits through further laser manipulations.

“We have 32 laser beams,” said Monroe. “And the atoms are like ducks in a row; each with its own fully controllable laser beam. I think of it like the atoms form a linear string and we're plucking it like a guitar string. We're plucking it with lasers that we turn on and off in a programmable way. And that's the computer; that's our central processing unit.”

By successfully creating a fault-tolerant logical qubit with this system, the researchers have shown that careful, creative designs have the potential to unshackle quantum computing from the constraint of the inevitable errors of the current state of the art.

“What's amazing about fault tolerance, is it's a recipe for how to take small unreliable parts and turn them into a very reliable device,” said Kenneth Brown, a professor of electrical and computer engineering at Duke and a co-author on the paper. “And fault-tolerant quantum error correction will enable us to make very reliable quantum computers from faulty quantum parts.”

In addition to Monroe and Brown, co-authors of the paper are JQI graduate student Laird Egan; JQI research scientist Marko Cetina; JQI graduate students Andrew Risinger, Daiwei Zhu and Debopriyo Biswas; Duke University physics graduate student Dripto M. Debroy; Duke postdoctoral researchers Crystal Noel and Michael Newman; and Georgia Institute of Technology graduate student Muyuan Li.