Computers, smartphones, GPS: quantum physics has enabled many technological advances. It is now opening up new fields of analysis in cryptography (the artwork of coding messages) with the purpose of growing ultra-secure telecommunications networks. There is one impediment, nevertheless: after a number of hundred kilometers inside an optical fiber, the photons that carry the qubits or ‘quantum bits’ (the data) disappear. They due to this fact want ‘repeaters’, a sort of ‘relay’, that are partly based mostly on a quantum reminiscence. By managing to retailer a qubit in a crystal (a “memory”) for 20 milliseconds, a staff from the University of Geneva (UNIGE) has set a world file and brought a serious step in direction of the event of long-distance quantum telecommunications networks. This analysis will be discovered within the journal npj Quantum Information.
Developed throughout the 20th century, quantum physics has enabled scientists to explain the habits of atoms and particles in addition to sure properties of electromagnetic radiation. By breaking with classical physics, these theories generated an actual revolution and launched notions with out equal within the macroscopic world akin to superposition, which describes the chance for a particle to be in a number of locations directly, or entanglement, which describes the flexibility of two particles to have an effect on one another instantaneously even at a distance (‘spooky action at a distance’).
Quantum theories are actually on the coronary heart of a lot analysis in cryptography, a self-discipline that brings collectively methods for encoding a message. Quantum theories make it doable to ensure excellent authenticity and confidentiality for data (a qubit) when it’s transmitted between two interlocutors by a particle of sunshine (a photon) inside an optical fiber. The phenomenon of superposition let the sender know instantly whether or not the photon conveying the message has been intercepted.
Memorizing the sign
However, there’s a main impediment to the event of long-distance quantum telecommunication programs: past a number of hundred kilometers, the photons are misplaced and the sign disappears. Since the sign can’t be copied or amplified — it could lose the quantum state that ensures its confidentiality — the problem is to discover a manner of repeating it with out altering it by creating ‘repeaters’ based mostly, particularly, on a quantum reminiscence.
Crystal used for storing photonic qubits and illuminated by a laser in a cryostat, an instrument for acquiring cryogenic temperatures. Credit: (c) Antonio Ortu
In 2015, the staff led by Mikael Afzelius, a senior lecturer within the Department of Applied Physics on the Faculty of Science 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 course of allowed the photon to switch its quantum state to the atoms of the crystal earlier than disappearing. However, the phenomenon didn’t final lengthy sufficient to permit the development of a bigger community of recollections, a prerequisite for the event of long-distance quantum telecommunications.
Storage file
Today, inside the framework of the European Quantum Flagship program, Mikael Afzelius’ staff has managed to extend this 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”, enthuses the researcher. As of their earlier work, the UNIGE scientists used crystals doped with sure metals referred to as ‘rare earths’ (europium on this case), able to absorbing mild after which re-emitting it. These crystals have been saved at -273,15°C (absolute zero), because beyond 10°C above this temperature, the thermal agitation of the crystal destroys 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,” explains Antonio Ortu, a post-doctoral fellow 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 bring the storage of a quantum state carried by a photon to a time scale that can be estimated by humans.
An efficient system in ten years
However, there are still several challenges to be met. “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,” says Mikael Afzelius.
The scientists will also have to find ways of designing memories capable of storing more than a single photon at a time, and thus of having ‘entangled’ photons which will guarantee confidentiality. “The aim is to develop a system that performs well on all these points and that can be marketed within ten years,” concludes the researcher.
Referemce: “Storage of photonic time-bin qubits for up to 20 ms in a rare-earth doped crystal” by Antonio Ortu, Adrian Holzäpfel, Jean Etesse and Mikael Afzelius, 15 March 2022, npj Quantum Information.
DOI: 10.1038/s41534-022-00541-3
