Improving qubit storage for ultra-secure quantum telecommunications networks

A team of researchers at the University of Geneva have set the world record for qubit storage, moving one step closer to developing a quantum telecommunications network.

Quantum physics has already facilitated groundbreaking technological advancements in many fields, such as computing, smartphones, and GPS.

Developing quantum telecommunications networks

Now, it is leading to developments in cryptography (the study of secure communications methods that only enable the sender and recipient of a message to view its contents), with the goal of creating ultra-secure quantum telecommunications networks.

Quantum theories enable perfect authenticity and confidentiality for information (a qubit) when it is transmitted between two interlocutors by a particle of light (a photon), within an optical fibre. The phenomenon of superposition let the sender know immediately whether the photon conveying the message has been intercepted.

However, there are difficulties associated with developing ultra-secure telecommunications networks, namely, after a few hundred kilometres within an optical fibre, the photons that hold the qubits or ‘quantum bits’ (the information) vanish.

Improving qubit storage

Thus, scientists have needed to develop ‘repeaters,’ which are a type of ‘relay’ partly based on a quantum memory.

In 2015, the group led by Mikael Afzelius, a Senior Lecturer in the Department of Applied Physics at the Faculty of Science of the University of Geneva (UNIGE) were successful in storing a qubit carried by a photon for 0.5 milliseconds in a crystal (a ‘memory’).

This method enabled the photon to transfer its quantum state to the atoms of the crystal before it vanished. However, the event did not last long enough to facilitate the building of a larger network of memories, which is a necessary precondition for the advancement of long-distance quantum telecommunications.

The team of UNIGE researchers have now been able to to store a qubit in a crystal for 20 milliseconds, and in doing so, have set the world record for qubit storage. This has resulted in a massive step forward the development of long-distance quantum telecommunications networks.

The team’s methods and results have been published the journal Quantum Information.

Breaking storage records

On 22 March 2022, within the framework of the European Quantum Flagship programme, Mikael Afzelius’ group were able to massively increase the storage period considerably by storing a qubit for 20 milliseconds.

“This is a world record for a quantum memory based on a solid-state system, in this case a crystal. We have even managed to reach the 100 millisecond mark with a small loss of fidelity,” explained Afzelius.

Similarly to their past project, the team utilised crystals doped with europium, a rare earth metal, which is able to absorb and then re-emit light. These crystals were kept at -273,15°C (absolute zero), as 10°C above this temperature would result in the thermal agitation of the crystal obliterating the entanglement of the atoms.

“We applied a small magnetic field of one thousandth of a Tesla to the crystal and used dynamic decoupling methods, which consist in sending intense radio frequencies to the crystal. The effect of these techniques is to decouple the rare-earth ions from perturbations of the environment and increase the storage performance we have known until now by almost a factor of 40,” said Antonio Ortu, a post-doctoral fellow in the Department of Applied Physics at UNIGE.

These results represent a massive step forward in the development of long-distance quantum telecommunications networks. They also bring the storage of a quantum state carried by a photon to a time scale that can be approximated by humans.

Towards further development

However, there are still difficulties to overcome. “The challenge now is to extend the storage time further. In theory, it would be enough to increase the duration of exposure of the crystal to radio frequencies, but for the time being, technical obstacles to their implementation over a longer period of time prevent us from going beyond 100 milliseconds. However, it is certain that these technical difficulties can be resolved,” added Afzelius.

On top of this, the researchers must find methods to design memories that are able to store more than a single photon at one time, thereby having ‘entangled’ photons which will ensure confidentiality.

“The aim is to develop a system that performs well on all these points and that can be marketed within ten years,” concluded the researcher.

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