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Superconducting Qubits Tie the NOT Gate

Researchers close in on a small quantum computer built from loops of superconducting metal
qubit chip



TU DELFT
Researchers are creeping closer to a small quantum computer built from loops of superconducting metal. A team has demonstrated a key ingredient of such a computer by using one superconducting loop to control the information stored on a second. Combined with other recent advances, the result may pave the way for devices of double the size in the next year or two—closer to what other quantum computing candidates have achieved, says physicist Hans Mooij of the Delft University of Technology in the Netherlands.

Unlike today's computers, which process information in the form of 0s and 1s, a quantum computer would achieve new levels of power by turning bits into fuzzy quantum things called qubits (pronounced cue-bits) that are 0 and 1 simultaneously. In theory, quantum computers would allow hackers to crack today's toughest coded messages and researchers to better simulate molecules for designing new drugs and materials.

Mooij and co-workers created the quantum version of a basic building block of computer logic, called the controlled-not (CNOT) gate, which switches a bit from 0 to 1 or vice versa if a second bit is set to 1—or does nothing if that bit is set to 0. "You need something like a controlled-not gate to make every quantum algorithm you might need," Mooij says. Their gate, described in this week's Nature, consisted of two side-by-side loops of aluminum cooled to a few kelvins (nearly -459 degrees Fahrenheit). A current flowing around one loop created a magnetic field that influenced the current flowing around the second loop.

The researchers set up the loops so that a clockwise current in one loop, called the control qubit, switched the direction of the current in the second loop (the target qubit). If they applied a magnetic field, however, the control qubit current flowed clockwise and counterclockwise simultaneously—a "superposition" of 0 and 1—which in turn caused the current in the target qubit to both switch and not switch directions.

Mooij says the next step is to connect multiple qubits together with CNOT gates. A report published in May described a way to control the interaction between two very similar qubits by inserting a third loop between them, which Mooij says might allow his group to extend its technique to three or four qubits. "You have to fight for every next step," he says, "but it's moving."

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