The potential of quantum computing is immense, but the distances over which entangled particles can reliably carry information remains a massive hurdle. The tiniest of disturbances can make a scrambled mess of their relationship.
To circumvent the problem, quantum computing researchers have found ways to stabilize long lengths of optical fibers or used satellites to preserve signals through the near-vacuum of space.
Yet there’s more to a quantum-based network than a transmission. Scientists struggled to crack their long sought-after goal of developing a system of interconnected units or ‘repeaters’ that can also store and retrieve quantum information much like classical computers do, to extend the network’s reach.
Now, a team of researchers have created a system of atomic processing nodes that can contain the critical states created by a quantum dot at wavelengths compatible with existing telecommunications infrastructure.
It requires two devices: one to produce and potentially entangle photons, and another ‘memory’ component that can store and retrieve the all-important quantum states within those photons on demand without disturbing them.
“Interfacing two key devices together is a crucial step forward in allowing quantum networking, and we are really excited to be the first team to have been able to demonstrate this,” says quantum optics physicist and lead author Sarah Thomas, from the Imperial College London (ICL).
Made partly in Germany and assembled at ICL, the newly proposed system places a semiconductor quantum dot capable of emitting a single photon at a time in a cloud of hot rubidium atoms, serving as quantum memory. A laser turns the memory component ‘on’ and ‘off’, allowing the photons’ states to be stored and released from the rubidium cloud on demand.
The distances over which this particular system could transmit quantum memories haven’t been tested – it’s just a proof-of-concept prototype in a basement lab, one based on photons that aren’t even entangled. But the feat could lay a solid foundation for the quantum internet, better than relying on entangled photons alone.
“This first-of-its-kind demonstration of on-demand recall of quantum dot light from an atomic memory is the first crucial step toward hybrid quantum light-matter interfaces for scalable quantum networks,” the team writes in their published paper.
Researchers in quantum computing have been trying to link up photon light sources and processing nodes that store quantum data for some time, without much success.
“This includes us, having tried this experiment twice before with different memory and quantum dot devices, going back more than five years, which just shows how hard it is to do,” says study co-author Patrick Ledingham, an experimental quantum physicist from the University of Southampton in the UK.
Part of the problem was that the photon-emitting quantum dots and atomic ‘memory’ nodes used so far were tuned to different wavelengths; their bandwidths incompatible with each other.
In 2020, a team from China tried chilling rubidium atoms to lure them into the same entangled state as the photons, but those photons then had to be converted to a suitable frequency for transmitting them along optic fibers – which can create noise, destabilizing the system.
The memory system designed by Thomas and colleagues has a bandwidth wide enough to interface with the wavelengths emitted by the quantum dot and low enough noise so as not to disturb entangled photons.
While the feat is significant, the researchers are still working to improve their prototype. To create quantum network-ready devices, they want to try extending storage times, increasing the overlap between the quantum dots and atomic nodes, and shrinking the size of the system. They also need to test their system with entangled photons.
For now, it remains a tenuous thread, but one day we could see this technology or something like it covering the world in a web of delicate yet stable quantum networks.
The study has been published in Science Advances.
