Scientists at the University of California, Santa Barbara, and at Google reported on Wednesday in the journal Nature that they had made a significant advance that brings them a step closer to developing a quantum computer.
Researchers have been pursuing the development of computers that exploit quantum mechanical effects since the 1990s, because of their potential to vastly expand the performance of conventional computers. The goal has long-remained out of reach, however, because the computers are composed of basic elements known as qubits that have remained, despite decades of engineering research, highly unstable.
In contrast to a bit, which is the basic element of a conventional computer and can represent either a zero or a one, a qubit can exist in a state known as superposition, in which it can represent both a zero and a one simultaneously.
If the qubits are then placed in a so-called entangled state — physically separate but acting with many other qubits as if they are connected — they can represent a vast number of values simultaneously.
To date, matrices of qubits that are simultaneously in superposition and entangled have eluded scientists because they are ephemeral, with the encoded information dissipating within microseconds.
The University of California, Santa Barbara, and Google researchers reported, however, that they had succeeded in creating an error-correction system that stabilizes a fragile array of nine qubits. The researchers said they had accomplished this by creating circuits in which additional qubits are used to observe the state of the computing qubits without altering their state.
An important asterisk remains, however, according to scientists who read an early version of the paper. The Nature paper stated that the reserchers had succeeded in preserving only the limited “classical” states, rather than the more complex quantum information that would be needed to create a system that outperforms today’s computers.
The importance of the advance is that the scientists have developed evidence that the system becomes more stable as they interconnect more qubits in the error-checking array. This suggests that far larger arrays of qubits, composed of thousands or tens of thousands of qubits, might be able to control the errors that have until now bedeviled scientists.
“We have for the first time in the long history of quantum computing an actual device, where we can test all of our ideas about error detection,” said Rami Barends, a quantum electronics engineer at Google and one of the authors of the paper.
Julian Kelly, another Google quantum electronics engineer, said that there remained significant challenges in manufacturing materials for quantum computing.
In some cases, the scientists are able to rely on existing semiconductor technology, but there are many steps for which they will have to invent approaches.
The research was reported by scientists working in the laboratory of John M. Martinis, a physicist at the University of California, Santa Barbara. In September, Google announced that it would join efforts to build a quantum computer as part of the recently established Quantum Artificial Intelligence Laboratory. Under that agreement, Dr. Martinis joined Google while keeping his teaching role, and members of his laboratory became Google employees.
While the researchers described their new circuit as a significant advance, they acknowledged that they had not yet solved all of the problems that prevent the building of a working quantum computer.
“While the basic physical processes behind quantum error correction are feasible, many challenges remain, such as improving the logic operations behind error correction and testing protection from phase-flip errors,” the scientists noted in a statement posted on the company’s website.
In a discussion of the Nature paper on his website, which focuses on advances in quantum computing, the M.I.T. physicist Scott Aaronson suggested that the achievement represents about half the progress required to build a fully functional quantum computer.
Google is not the only computing company collaborating with academic researchers in advancing quantum computing. IBM is working with scientists at Yale, and Microsoft is working separately with researchers at the University of California, Santa Barbara, supporting the Station Q research laboratory it created there in 2006.
Microsoft has chosen a high-risk approach known as topological quantum computing, a different way of achieving the same qubit error correction that the Google group is pursuing.
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