Major step taken toward error-free computing

Stable quantum information could make possible solving problems in record time

ENTANGLED  Tiny electrical circuits have given physicists unprecedented control over the quantum states of units of information called qubits. The qubits appear as five crosses (center).

Erik Lucero/UCSB

Quantum computing has overcome an important barrier: Scientists have achieved nearly perfect control over a bit of quantum information in a way that could bring them a step closer to error-free calculations.

All digital information comes in tiny packets called bits. In consumer devices, bits are chunks of magnetic or electric material that flip between two distinct states. But thanks to quantum weirdness, certain minuscule objects called quantum bits, or qubits, can exist in two states at once. Physicists have connected multiple qubits with each other to share one overall “entangled” state. Using entanglement, rudimentary quantum computers can run multiple calculations at once and solve simple problems like factoring 15 into 3 and 5 (SN: 3/10/12, p. 26). Because each additional qubit doubles a device’s processing power, future quantum computers should complete tasks far more rapidly than conventional machines do.

But quantum computing has a downside: Quantum states are easily shattered, especially as the number of entangled qubits increases. John Martinis, a physicist at the University of California, Santa Barbara, compares a classical bit to a coin resting flat on a table: The coin won’t flip unless the table gets a really hard shake. A qubit, by contrast, is like a coin standing on edge — the slightest jiggle topples it. Theorists in the 1990s suggested that qubits arranged in a checkerboard could overcome this fragility by monitoring and correcting errors in their neighbors, creating communal stability. Even in this scheme, however, individual qubits’ states would need to come out correctly after at least 99 out of 100 state-changing computations; otherwise, errors would multiply throughout the grid. No device containing more than three qubits has yet achieved this 99 percent stability threshold for each qubit.

Seeking to make such an unflappable qubit, Martinis and colleagues report in the April 24 Nature that they built tiny electrical circuits, each roughly the size of a grain of sand, from superconducting aluminum wire and ultrathin barriers of aluminum oxide. When cooled to 30 thousandths of a degree Celsius above absolute zero, electrons slosh back and forth, or resonate, around the circuits without encountering resistance. Information can be encoded in this resonance to make a qubit.

Using the grid computing idea, Martinis and colleagues lined up five of their qubits and electrically linked each to its nearest neighbors. The researchers then etched larger circuits that allowed them to change individual qubits’ states with tiny pulses of electricity. Using these pulses, the scientists found they could control one qubit’s state more than 99.9 percent of the time. For two entangled neighboring qubits, the fidelity dropped to 99.4 percent, still above the 99 percent threshold. When they entangled all five at once, the researchers could control the qubits’ state 81.7 percent of the time.

Achieving such precise control in a system with so many qubits is “a great milestone for quantum information processing,” says physicist Raymond Laflamme of the University of Waterloo in Canada.

“It’s quite a spectacular achievement,” agrees Simon Devitt, a theoretical physicist at Ochanomizu University in Tokyo. He says the result provides a clear path to a quantum computer: “Once you satisfy the error correction requirements, then the rest is engineering.”

Yale University’s Robert Schoelkopf, who invented the sloshing-electron qubit that Martinis’ team used, says the team has made “a significant advance.” But he says a practical quantum computer would require even stabler qubits. 

Editor’s Note: This story was updated May 6, 2014, to correct the approximate size of the qubit circuits and to correc t the temperature those circuits were cooled to.

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