Apr 29, 1999 NEC researchers control a superconducting qubit for the first time

NMR had shown what a quantum algorithm looked like in practice, but its qubits could not be multiplied at will. Ions and nuclei are supplied by nature; a large quantum computer would need components that could be manufactured, the way transistors are printed onto silicon.

One candidate was the superconducting circuit. Cooled to a fraction of a degree above absolute zero, certain metals carry current without resistance, and a small circuit made from them can behave as a single quantum object. The question was whether such a circuit could hold a superposition and be steered by electrical signals alone.

In April 1999 Yasunobu Nakamura and his colleagues at the Japanese electronics company NEC reported that it could. Their device, a "Cooper-pair box", was a tiny island of superconducting metal whose two states differed by a single pair of electrons. A short voltage pulse placed the island in a superposition of both, and a probe junction read out the result. It was the first qubit built not from an atom but from an electronic circuit.

The quantum state was extremely fragile, far too short-lived for useful computation. But the device was made with lithography, like an ordinary chip, and its descendants became the qubits of many of the quantum computers built since, including those of IBM and Google.

Here we report the observation of quantum oscillations in a single-Cooper-pair box. By applying a short voltage pulse via a gate electrode, we can control the coherent quantum state evolution: the pulse modifies the energies of the two charge states non-adiabatically, bringing them into resonance. The resulting state—a superposition of the two charge states—is detected by a tunnelling current through a probe junction. Our results demonstrate electrical coherent control of a qubit in a solid-state electronic device.