Breakthroughs pave way for quantum computers

Scientists at Yale University have published details of two breakthroughs that could help pave the way for next-generation super-fast quantum computers.

The researchers have managed to send a photon signal on demand from a qubit onto wires, and transmit the signal to a second distant qubit.

Professors Robert Schoelkopf and Steven Girvin have explored the use of solid-state devices resembling microchips as the basic building blocks in the design of a quantum computer.

Today, for the first time, they report that superconducting qubits, or artificial atoms, have been able to communicate information not only to their nearest neighbour, but to a distant qubit on the chip.

This research now moves quantum computing from "having information" to " communicating information", the scientists stated.

In the past information had only been transferred directly from qubit to qubit in a superconducting system.

Schoelkopf and Girvin's team has engineered a superconducting communication 'bus' to store and transfer information between distant qubits on a chip.

This work, according to Schoelkopf, is the first step in making the fundamentals of quantum computing useful.

The first breakthrough is the ability to produce on demand, and control, single discrete microwave photons as the carriers of encoded quantum information.

While microwave energy is used in cellphones and ovens, their sources do not produce just one photon. This new system creates a certainty of producing individual photons.

The scientists said that it is not very difficult to generate signals with one photon on average, but it is quite difficult to generate exactly one photon each time. To encode quantum information on photons, there has to be exactly one.

"We are reporting the first such source for producing discrete microwave photons, and the first source to generate and guide photons entirely within an electrical circuit," said Professor Schoelkopf.

In order to successfully perform these experiments, the researchers had to control electrical signals corresponding to one single photon.

"In this work we demonstrate only the first half of quantum communication on a chip - quantum information efficiently transferred from a stationary quantum bit to a photon or 'flying qubit'," said Professor Schoelkopf.

"However, for on-chip quantum communication to become a reality, we need to be able to transfer information from the photon back to a qubit."

Postdoctoral associate Johannes Majer and graduate student Jerry Chow, lead co-authors of the second paper, added a second qubit and used the photon to transfer a quantum state from one qubit to another.

This was possible because the microwave photon could be guided on wires, similarly to the way fibre optics can guide visible light, and carried directly to the target qubit.

"A novel feature of this experiment is that the photon used is only virtual, winking into existence for only the briefest instant before disappearing," said Majer and Chow.

Together the new Yale research constitutes the first demonstration of a " quantum bus" for a solid-state electronic system.