Quantum Chip Sends Information by Bus

Linking superconducting circuits may lead to bigger quantum computers
qubit bus

Researchers are finally beginning to cobble together the basic elements of a quantum computer on a chip. Two teams report they have forged the first chip-based version of a quantum bus, a tool for mixing and swapping information between quantum bits (qubits).

The groups say their results are a key step along the way to making a full-scale quantum computer by piggybacking on modern chipmaking technology.

In normal computers, a bit is either 0 or 1, but a qubit can enter a so-called superposition of 0 and 1 simultaneously. Hundreds or thousands of qubits linked together would theoretically allow a quantum computer to break secret codes and search databases that would bog down today's fastest computers for decades.

The trick is fusing the quantum realm with modern electronics to scale up the number of qubits. One up-and-coming approach generates qubits from current flowing in superconducting material, which, when chilled to near absolute zero, carries electricity without resistance. A loop of superconductor broken by one or two gaps called Josephson junctions causes two different currents to flow through it at once.

In both new designs, researchers created an information conduit—a bus—from a squiggly wire between two such loops of aluminum.

When activated, a qubit loop transferred its superposition to the wire in the form of a microwave photon. The effect is "sort of like having a guitar string and plucking it," says physicist Raymond Simmonds of the National Institute of Standards and Technology in Boulder, Colo., who led one of the teams.

When plucked, his group's 7-millimeter- (0.3-inch-) long wire stored a photon for more than a microsecond before the second qubit absorbed it. The result, Simmonds says, is a basic memory circuit that can transfer a quantum state from one qubit to another.

Rob Schoelkopf of the Department of Applied Physics at Yale University and his group performed a similar trick with a longer wire that mixed a single quantum state between the two qubits, such that the 0 and 1 flip-flopped between them. Such mixing is essential to creating the link of entanglement between many qubits—the "fuel" on which a quantum computer runs.

Researchers should now be able to connect perhaps six qubits along such a bus, says Johannes Majer, a physicist in the Yale group. Others have linked up pairs of superconducting qubits, he says, but a bus allows pairs of qubits to bypass their closest neighbors and communicate directly. "If you want to build a quantum computer, you need something that is long-range and not [the] nearest-neighbor," Majer says.

There's still a snag, though: superpositions in superconducting qubits do not stay stable or coherent for very long. "The tricky part is, of course, getting the coherence of our qubits up," Simmonds says. The good news, he adds, is "now we've shown the elements for scaling work."

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