A dark horse candidate for the super powerful quantum computer of the future has now passed an important milestone. Researchers have made the first direct measurement showing they can forge a crucial quantum link between currents flowing through ultracold, superconducting wires.

Quantum computers would take advantage of a particle or other quantum system's ability to exist simultaneously in two states--namely, a superposition of 0 and 1. Combining many such quantum bits, or qubits, into a working quantum computer would allow its operators to perform feats impossible even on today's supercomputers, such as breaking gold-standard encryption schemes or conducting complex searches quickly.

To get qubits working together, they must be entangled, meaning that their quantum states have to be linked so that when one qubit is 0 the other is 1, and vice versa. Researchers had made strides in entangling multiple qubits made from ions, which have long lasting superpositions but must be shuffled around to coordinate many qubits. Another kind of qubit consists of a current in a superconducting metal wire. Such qubits could easily be linked by fashioning wires between them, but their superpositions are relatively easy to disturb, so researchers had yet to definitively show they could entangle two superconducting qubits.

To demonstrate this link, a group constructed two qubits, each one a complicated coil of aluminum wire [diamond shapes between ends of U-shape electrode above], and joined them together on a sapphire substrate, which boosts the duration of the superposition. When the system is cooled so the aluminum becomes superconducting, a small current can flow through the coil despite an insulating gap interrupting it. A microwave pulse puts one qubit into a superposition of two slightly different currents flowing in the same direction. The superposition should pass to the second like two tuning forks touching one another, creating the entanglement. To confirm the link, the researchers interrupted many successive superpositions with microwave pulses. Collectively the measurements told them that each qubit was indeed the mirror image of the other. "This is a big step," says lead author John Martinis of the University of California, Santa Barbara. "With this experiment and some others that will be done in the next year," he says, "we're going to become very competitive with ions." The report is published in the September 8 Science.