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 instantly affect each other even at a distance (“ranged spooky action”).
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. As 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.
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 solid-state system, in this case a crystal. We even managed to reach the 100 millisecond mark with a small loss of fidelity”, enthuses the researcher. 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 stored 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 a thousandth of a Tesla to the crystal and used dynamic decoupling methods, which involve sending intense radio frequencies to the crystal. These techniques have the effect of decoupling rare earth ions from environmental disturbances and increasing the storage performance that we have known so far by a factor of almost 40”, explains Antonio Ortu, post-doctoral fellow at the Department of Applied Physics Research 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 timescale 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 time. In theory, it would suffice to increase the duration of exposure of the crystal to radio frequencies, but for the moment, technical obstacles to their implementation over a longer duration prevent us from going beyond 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.
Reference: “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, March 15, 2022, npj Quantum Information.