There is, of course, a catch. The mere act of measuring or observing a qubit can strip it of its computing potential. So researchers have used quantum entanglement  in which particles are linked so that measuring a property of one instantly reveals information about the other, no matter how far apart the two particles are  to extract information. But creating and maintaining qubits in entangled states has been tremendously challenging.

“We’re at the stage of trying to develop these qubits in a way that would be like the integrated circuit that would allow you to make many of them at once,” said Rob Schoelkopf, a physicist who is leader of the Yale group. “In the next few years you’ll see operations on more qubits, but only a handful.”

The good news, he said, is that while the number of qubits is increasing only slowly, the precision with which the researchers are able to control quantum interactions has increased a thousandfold.

The Santa Barbara researchers said they believe they will essentially double the computational power of their quantum computers next year.

John Martinis, a physicist who is a member of the team, said, “We are currently designing a device with four qubits, and five resonators,” the standard microelectronic components that are used to force quantum entanglement. “If all goes well, we hope to increase this to eight qubits and nine resonators in a year or so.”

Two competing technological approaches are also being pursued. One approach involves building qubits from ions, or charged atomic particles, trapped in electromagnetic fields. Lasers are used to entangle the ions. To date, systems as large as eight qubits have been created using this method, and researchers believe that they have design ideas that will make much larger systems possible. Currently more than 20 university and corporate research laboratories are pursuing this design.

In June, researchers at Toshiba Research Europe and Cambridge University reported in Nature that they had fabricated light-emitting diodes coupled with a custom-formed quantum dot, which functioned as a light source for entangled photons. The researchers are now building more complex systems and say they can see a path to useful quantum computers.

A fourth technology has been developed by D-Wave Systems, a Canadian computer maker. D-Wave has built a system with more than 50 quantum bits, but it has been greeted skeptically by many researchers who believe that it has not proved true entanglement. Nevertheless, Hartmut Neven, an artificial-intelligence researcher at Google, said the company had received a proposal from D-Wave and NASA’s Jet Propulsion Laboratory to develop a quantum computing facility for Google next year based on the D-Wave technology.