Washington, Sept 27 : Scientists at Yale University have achieved two major steps towards putting quantum computers into real practice – sending a photon signal on demand from a qubit onto wires, and transmitting the signal to a second, distant qubit.

Over the past several years, the research team of 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.

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

In order to successfully perform the experiments, the researchers had to control electrical signals corresponding to one single photon. In comparison, a cell phone emits about 1023 photons per second.

Furthermore, the extremely low energy of microwave photons mandates the use of highly sensitive detectors and experiment temperatures just above absolute zero.

“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’. 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,” said Prof. Schoelkopf.

“The first breakthrough reported 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 cell phones and ovens, their sources do not produce just one photon. This new system creates a certainty of producing individual photons,” said postdoctoral associates Andrew Houck and David Schuster, lead co-authors on the first paper.

“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, you want there to be exactly one,” they said.

In the second study, postdoctoral associate Johannes Majer and graduate student Jerry Chow 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,” Majer and Chow said in their study.

“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,” added Prof. Schoelkopf.

The study appears in the sequential issues of Nature on September 20 and September 27. (With inputs from ANI)