Developing long-distance quantum telecommunications networks — ScienceDaily

Computers, smartphones, GPS: quantum physics has enabled many technological advances. It is now opening up new fields of research in cryptography (the art of encrypting messages) with the aim of developing ultra-secure telecommunications networks. However, there is an obstacle: after a few hundred kilometers in an optical fibre, the photons which transport the qubits or “quantum bits” (information) disappear. They therefore need “repeaters”, a sort of “relay”, which are partly based on a quantum memory. By managing to store a qubit in a crystal (a “memory”) for 20 milliseconds, a team from the University of Geneva (UNIGE) has set a world record and taken a major step towards the development of long-distance quantum telecommunications networks. . This research can be found in the journal npj Quantum Information.

Developed during the 20th century, quantum physics has allowed scientists to describe the behavior of atoms and particles as well as certain properties of electromagnetic radiation. By breaking with classical physics, these theories have generated a real revolution and introduced notions without equivalent in the macroscopic world such as superposition, which describes the possibility for a particle to be in several places at the same time, or entanglement, which describes the ability of two particles to affect each other instantaneously even at a distance (“spooky action at a distance”).

Quantum theories are today at the heart of much research in cryptography, a discipline that brings together techniques for encoding a message. Quantum theories make it possible to guarantee perfect authenticity and confidentiality of information (a qubit) when it is transmitted between two interlocutors by a particle of light (a photon) within an optical fiber. The superposition phenomenon allows the transmitter to know immediately if the photon carrying the message has been intercepted.

Memorize the signal

However, there is a major obstacle to the development of long-distance quantum telecommunications systems: beyond a few hundred kilometers, the photons are lost and the signal disappears. Since the signal cannot be copied or amplified — it would lose the quantum state which guarantees its confidentiality — the challenge is to find a way to repeat it without altering it by creating “repeaters” based in particular on a quantum memory .

In 2015, the team of Mikael Afzelius, lecturer in the Department of Applied Physics of the Faculty of Sciences of the University of Geneva (UNIGE), succeeded in storing a qubit carried by a photon for 0.5 milliseconds in a crystal (a ‘Memory’). This process allowed the photon to transfer its quantum state to the atoms of the crystal before disappearing. However, the phenomenon did not last long enough to allow the construction of a larger network of memories, a prerequisite for the development of long-distance quantum telecommunications.

Storage record

Today, as part of the European Quantum Flagship program, Mikael Afzelius’ team has succeeded in significantly increasing this duration by storing a qubit for 20 milliseconds. “This is a world record for a quantum memory based on a semiconductor system, in this case a crystal. We even managed to reach the 100 millisecond mark with a small loss of fidelity”, enthuses the searcher. As in their previous work, the UNIGE scientists used crystals doped with certain metals called “rare earths” (europium in this case), capable of absorbing light and then re-emitting it. These crystals were kept at -273.15°C (absolute zero), because beyond 10°C above this temperature, the thermal agitation of the crystal destroys the entanglement of atoms.

“We applied a small magnetic field of a thousandth of a Tesla to the crystal and used dynamic decoupling methods, which involve sending intense radio frequencies to the crystal. The effect of these techniques is to decouple the rare earth ions from the disturbances and multiply by almost 40 the storage performance that we knew until now”, explains Antonio Ortu, post-doctoral researcher in the Department of Applied Physics at UNIGE. The results of this research constitute a major advance for the development of long-distance quantum telecommunications networks.They also reduce the storage of a quantum state carried by a photon to a time scale that can be estimated by humans.

A successful system in ten years

However, there are still several challenges to overcome. “The challenge now is to further extend the storage period. In theory, it would be enough to increase the duration of exposure of the crystal to radio frequencies, but for the moment, technical obstacles to their implementation on a longer duration prevent us from exceeding 100 milliseconds. However, it is certain that these technical difficulties can be solved,” says Mikael Afzelius.

Scientists will also need to find ways to design memories that can store more than one photon at a time, and therefore have “entangled” photons that will ensure privacy. “The objective is to develop a system which is efficient on all these points and which can be marketed within ten years”, concludes the researcher.

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Material provided by University of Geneva. Note: Content may be edited for style and length.