Quantum memory sets new world record for storage time
Computers, smartphones, GPS: quantum physics has enabled many technological advances. It is now opening up new fields of cryptography research with the aim of developing ultra-secure telecommunications networks. There's a hurdle, however: After traveling for hundreds of kilometers in the fiber, the photons that carry the qubits (information) disappear. Therefore, they need "repeaters" - mostly based on quantum memory.
Now, a research team from the University of Geneva (UNIGE) has succeeded in storing a quantum bit in a crystal (a "memory") for 20 milliseconds, setting a new world record and taking an important step towards the development of long-range quantum telecommunications networks step. The related research results were published in npj quantum information [1], a sub-journal of the journal Nature, under the title of "Storing photon time-bin qubits in rare earth doped crystals for up to 20 milliseconds".
Crystal for storing photonic qubits, irradiated with laser light in cryostat
Long-range quantum communication requires repeaters
By breaking classical physics, quantum theory brought about a real revolution and introduced concepts not found in the macroscopic world, such as superposition, which describes the possibility of a particle being in multiple places at the same time, or entanglement, which describes two The ability of particles to momentarily influence each other even at a distance ("spooky action at a distance").
Quantum theory is now at the heart of much research in cryptography, a discipline that incorporates techniques for encoding information. Quantum theory guarantees the perfect authenticity and secrecy of information (qubits) as they travel between two interlocutors via photons in optical fibers. The superposition phenomenon lets the sender know immediately if the photon transmitting the information has been intercepted.
However, there is a major hurdle in developing long-range quantum communication systems: beyond a few hundred kilometers, photons are lost and signals are lost. Since the signal cannot be replicated or amplified, it will lose the quantum state that guarantees its confidentiality. So the challenge is to find a way to repeat the signal without changing it, especially by creating quantum memory-based "repeaters".
In 2015, a team led by Mikael Afzelius, Senior Lecturer in the Department of Applied Physics at the Faculty of Science of the University of Geneva (UNIGE), succeeded in storing a qubit carried by a photon in a crystal for 0.5 milliseconds. This process allows the photon to transfer its quantum state to the atoms of the crystal before disappearing. However, this phenomenon did not last long enough to build larger memory networks, a prerequisite for the development of long-range quantum telecommunications.
Qubit storage record
Within the framework of the European Quantum Flagship programme, the team of Mikael Afzelius managed to significantly increase this duration by storing one qubit for 20 milliseconds. The average memory output fidelity is F =(85 ± 2)%, and the average number of photons per qubit is μin =0.92 ± 0.04.
"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 100 milliseconds with a slight loss of fidelity," the researchers said.
Qubit storage for 20 ms (a), 50 ms (b) and 100 ms (a)
Like their previous work, the UNIGE scientists used crystals doped with a rare earth metal (in this case, europium), which absorb light and then re-emit it. These crystals are kept at about -273.15°C (absolute zero) because thermal perturbations of the crystals can disrupt the entanglement of atoms by 10°C above this temperature.
Antonio Ortu, a postdoctoral fellow at the Department of Applied Physics at the University of Geneva, explains: "We applied a small magnetic field of one thousandth of a Tesla to the crystal and used a dynamic decoupling method, which sends strong radio frequencies to the crystal. The effect of these techniques is Decoupling rare-earth ions from environmental perturbations and improving the memory performance we have known to date by a factor of nearly 40."
This research is a major advance in long-range quantum communication networks. They also brought the storage of quantum states carried by photons to timescales that humans can estimate.
Efficient systems in ten years
However, some challenges remain to be addressed. "The challenge now is to extend the storage time even further," Afzelius said. "In theory, increasing the duration of the crystal's exposure to RF would be sufficient, but for now, the technical barriers to implementing these techniques over longer periods of time prevent us from exceeding 100 ms. However, these technical difficulties can certainly be solved.
Scientists must also find ways to design memories that can store multiple photons at the same time, thus having "entangled" photons, which would guarantee secrecy. "Our goal is to develop a system that performs well in all of these areas and can be brought to market within a decade," the researchers concluded.
Link:
[1] https://www.nature.com/articles/s41534-022-00541-3
[2] https://www.nanowerk.com/nanotechnology-news2/newsid=60127.php
