Frontiers Physics World  January 2018

Qubits swapped from cold atom to crystal

Light link A hybrid information network with two quantum nodes – rubidium atoms (red dots) and a doped crystal with praseodymium ions (white cube). (ICFO / Scixel)

Quantum information has been transferred between a cold atomic gas and a solid crystal using photons. Carried out by Nicolas Maring and colleagues at the Institute of Photonic Sciences (ICFO) at the Barcelona Institute of Science and Technology in Spain, the work could lead to significant advances in quantum computing and even to the creation of a “quantum internet”.

A big challenge in building a quantum computer is transferring quantum bits (qubits) of information between the “quantum nodes” of a system. These nodes can consist of different types of matter, including cold atomic gases and solid crystals doped with impurities. If two nodes are the same, it is relatively straightforward to transfer qubits – in the form of single photons, for example. In this process, one node emits a qubit-encoded photon that is then absorbed by another node.

In practical quantum communication systems, however, it is often better to use different types of quantum nodes to perform different functions. This is because some nodes are better than others at doing certain tasks. Cold atomic gases can easily produce qubit-encoded photons, for example, while doped solids can store quantum information for long periods.

The snag is that different types of nodes usually emit and process photons at different wavelengths and bandwidths, making qubit transfer between nodes tricky. “It’s like having nodes speaking in two different languages,” says Maring. “For them to communicate, it is necessary to convert the single photon’s properties so it can efficiently transfer all the information between these different nodes.”

In the ICFO study, a “hybrid” quantum network link between two different quantum nodes in separate labs was established for the first time. The first node, a gas of laser-cooled rubidium atoms, produced a qubit-encoded 780 nm photon. The photon was then converted to 1552 nm and sent down an optical telecoms fibre into a next-door lab, where it was converted again to a wavelength of 606 nm. Its quantum information was then processed by a second quantum node – a crystal doped with praseodymium ions. It could store qubits for 2.5 μs while retaining most of the original quantum information.

The research could lead to the creation of quantum networks that take advantage of the different processing and storage capabilities of different quantum nodes. The ICFO scientists hope that larger scale, more complex hybrid networks will be built, made from many different nodes and links between them (Nature 551 485).

Sam Jarman