Foundational step towards a quantum internet

Quantum computers promise to revolutionise information technology this century. Based on the rules of quantum mechanics, the very nature of their hardware means they will be able to solve problems beyond the reach of classical computers. And scientists are also working on how best to build a network of these machines in order to create a ‘quantum internet’ of sorts.

Engineers at Caltech, the California Institute of Technology, have discovered that by embedding atoms of the rare-earth element ytterbium in an optical cavity they are able to control and measure a stable form of quantum information in a solid. The system they have developed has the potential to share that information over thousands of kilometres using photons.

"This ticks most of the boxes,” said Caltech’s Professor Andrei Faraon, who led the research team. “It's a rare-earth ion that absorbs and emits photons in exactly the way we'd need to create a quantum network.

“This could form the backbone technology for the quantum internet."

Dr John Bartholomew is a co-author of the Nature paper and worked on the project at Caltech. This year he joined the University of Sydney Nano Institute and School of Physics.

He said: “These rare-earth atoms have great appeal for quantum technologies but several challenges had to be overcome to get things working at the single atom level. I’ve worked on overcoming these challenges since starting my PhD at the Australian National University 12 years ago.

“I saw the nanophotonic cavities pioneered at Caltech as the best shot for making this breakthrough.”

Dr Bartholomew now leads the Quantum Integration Laboratory at the University of Sydney. Here he hopes to build on the University’s demonstrated strengths in photonics and quantum technologies.

“The next big steps are to increase the performance and scale of this hardware and I can't wait to tackle these challenges at the University of Sydney by designing new materials and building integrated devices,” he said.

How will it work?

As they can with classical computers, engineers would like to be able to connect multiple quantum computers to share data and work together – creating a ‘quantum internet’. This would open the door to several applications, including solving computations that are too large to be handled by a single machine and establishing provably secure communications using quantum cryptography.

In order to work, a quantum network needs to be able to transmit information between two points without altering the quantum properties of the information being transmitted. The idea is to use one of the fundamental quantum properties of matter, which is entanglement. This is where the information of quantum objects remains dependent on each other, even if separated by an arbitrary distance.

One current model works like this: a single atom or ion acts as a quantum bit (or qubit) storing information via one if its properties, such as the direction of its angular momentum, known as ‘spin’. To read that information and transmit it elsewhere, the atom is excited with a pulse of light, causing it to emit a photon whose spin is entangled with the spin of the atom. The photon can then transmit the information entangled with the atom over a long distance via fibre-optic cable.

Doing that is harder than it sounds, however. Finding atoms that you can control and measure that also aren't too sensitive to magnetic or electric field fluctuations that cause errors, or decoherence, is challenging.

"Solid-state emitters that interact well with light often fall victim to decoherence; that is, they stop storing information in a way that's useful from the prospective of quantum engineering," said Caltech’s Dr Jon Kindem, lead author of the Nature paper.

Meanwhile, atoms of rare-earth elements, which have properties that make the elements useful as qubits, tend to interact poorly with light.

To overcome this challenge, researchers led by Professor Faraon constructed a nanophotonic cavity about 10 microns (0.01 millimetres) in length, sculpted from a piece of crystal.

The crystal was made in such a way that light inside it would bounce around in predictable patterns.

They then identified a charged atom, or ion, of the rare-earth element ytterbium was then placed at the centre of the cavity where it could receive a beam of photons. The optical cavity allows for light to bounce back and forth down the beam multiple times until it is finally absorbed by the ion.

In the Nature paper, the team showed that the cavity modifies the environment of the ion such that whenever it emits a photon, more than 99 percent of the time that photon remains in the cavity, where scientists can efficiently collect and detect that photon to measure the state of the ion. This results in an increase in the rate at which the ion can emit photons, improving the overall effectiveness of the system.

In addition, the ytterbium atoms store information for 30 milliseconds. That doesn’t sound long, but it’s long enough for light to transport that information nearly 6000 kilometres – about the distance from Sydney to Jakarta and enough time to cross continental Europe, Asia, Australia or the US.

The team's current focus is on creating the building blocks of a quantum network. Next, they hope to scale up their experiments and connect two quantum bits, Professor Faraon said.

Declaration

This research was funded by the US National Science Foundation, the US Air Force Office of Scientific Research and the Institute for Quantum Information and Matter at Caltech and used the Kavli Nanoscience Institute Laboratory at Caltech.