New world record for qubit storage

New world record for qubit storage 

This article explains the New world record for qubit storage.

Quantum physics has permitted several technological developments. It is now expanding cryptography research to construct ultra-secure telecommunications networks. However, after a few hundred kilometers along an optical fiber, the photons carrying the qubits (information) vanish. They need ‘repeaters’, a form of ‘relay’ based on quantum memory. A team from UNIGE has established a world record by storing a qubit in a crystal for 20 milliseconds, paving the way for long-distance quantum telecommunications networks. This study is published in npj Quantum Information.

Quantum physics, developed in the 20th century, allows scientists to characterize atomic and particle behavior as well as electromagnetic radiation properties. Entanglement is the ability of two particles to impact each other instantaneously even at a distance (‘spooky action at a distance’) while superposition is the ability of a particle to be in several places at once. 

Quantum theories are now driving much of the research in cryptography, a field that combines message encoding techniques. Quantum theories allow flawless authentication and confidentiality of information (a qubit) when it is sent between two parties via an optical fiber. Superposition tells the sender if the photon carrying the message has been intercepted. 

Learning the code 

However, beyond a few hundred kilometers, the photons and the signal vanish. Since the signal cannot be reproduced or amplified without losing its quantum state, the challenge is to create ‘repeaters’ based on quantum memory. 

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Mikael Afzelius, a senior lecturer in the Department of Applied Physics at the University of Geneva (UNIGE), led a team that successfully stored a qubit (or ‘memory’) in a crystal for 0.5 milliseconds in 2015. Before dissipating, the photon could transfer its quantum state to the crystal’s atoms. The phenomena did not endure long enough to build a bigger network of memories, which is required for long-distance quantum telecommunications. 

DB record 

Mikael Afzelius’ team has now succeeded to store a qubit for 20 milliseconds, as part of the European Quantum Flagship program. “We even got to 100 milliseconds with a minor loss of fidelity”, says the researcher. The UNIGE researchers employed crystals doped with rare earth metals (in this case, euroopium) that can absorb light and re-emit it. The crystals were held at -273,15°C (absolute zero) since thermal agitation disrupts atomic entanglement beyond 10°C. 

“We employed a thousandth of a Tesla magnetic field on the crystal and sent it powerful radio frequencies to decouple it. “These strategies divorce rare-earth ions from environmental perturbations, increasing storage performance by almost a factor of 40,” explains Antonio Ortu, a postdoctoral fellow in UNIGE’s Department of Applied Physics. This finding represents a huge step forward in the creation of long-distance quantum networks. They also bring the storage of a quantum state carried by a photon to a human-scale. 

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In ten years, a good system 

But there are still many obstacles to overcome. “Now the problem is to extend storage time. However, for now, technical hurdles prevent us from going beyond 100 milliseconds of exposure to radio frequencies. 

The scientists will also have to create memories that can store many photons at once, ensuring confidentiality. In ten years, the researcher hopes to have a system that meets all of these criteria.

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