Researchers store a quantum bit for a record-breaking 20 milliseconds

 

Crystal used for storing photonic qubits and illuminated by a laser in a cryostat, an instrument for obtaining cryogenic temperatures. Credit: Antonio Ortu

Quantum physics has enabled various technological advancements, including computers, smartphones, and GPS. With the goal of establishing ultra-secure telecommunications networks, it is now opening up new fields of research in cryptography (the art of coding signals). However, there is one stumbling block: after a few hundred kilometres within an optical cable, the photons that carry the qubits (information) vanish. As a result, they require "repeaters," a type of "relay" that is partly based on quantum memory. A team from the University of Geneva (UNIGE) has established a world record by storing a qubit in a crystal (a "memory") for 20 milliseconds, paving the way for the construction of long-distance quantum telecommunications networks. This research can be found in the journal npj Quantum Information.

Quantum physics, which was developed in the twentieth century, allows scientists to describe the behaviour of atoms and particles, as well as some features of electromagnetic radiation. By defying classical physics, these theories ushered in a real revolution, introducing concepts like superposition, which describes the ability of a particle to be in multiple places at the same time, and entanglement, which describes the ability of two particles to affect each other instantly even at a distance ("spooky action at a distance").

Many studies in cryptography, a topic that brings together strategies for encoding a message, are increasingly based on quantum theories. When information (a qubit) is communicated between two interlocutors by a particle of light (a photon) within an optical cable, quantum theories allow for absolute authenticity and confidentiality. The phenomenon of superposition informs the sender whether the message-carrying photon has been intercepted.

Memorizing the signal

However, there is a significant stumbling block to the creation of long-distance quantum telecommunication systems: beyond a few hundred kilometres, the photons vanish and the signal vanishes. Because the signal cannot be reproduced or amplified because it would lose its quantum state, which ensures its anonymity, the problem is to discover a means to repeat it without changing it by developing "repeaters" based on quantum memory in particular.

Successfully 2015, a team led by Mikael Afzelius, a senior lecturer at the University of Geneva's Faculty of Science's Department of Applied Physics, succeeded in storing a qubit delivered by a photon in a crystal for 0.5 milliseconds (a "memory"). Before fading, the photon was able to convey its quantum state to the crystal's atoms. The event, however, did not endure long enough to allow the formation of a broader network of memory, which is required for the development of long-distance quantum telecommunications.

Storage record

Mikael Afzelius' team has now succeeded to dramatically improve this length by storing a qubit for 20 milliseconds as part of the European Quantum Flagship programme. "This is the first time a quantum memory based on a solid-state system, in this case a crystal, has broken the world record. We've even gotten to 100 milliseconds with only a minor loss of quality "The researcher is enthused. The UNIGE researchers employed crystals doped with particular metals known as "rare earths" (europium in this case), which are capable of absorbing light and then re-emitting it, as they did in prior research. These crystals were held at -273,15°C (absolute zero) because thermal agitation of the crystal disrupts atom entanglement beyond 10°C above this temperature.

"We employed dynamic decoupling methods to transfer powerful radio frequencies to the crystal and applied a modest magnetic field of one thousandth of a Tesla to it. These strategies have the effect of decoupling rare-earth ions from environmental perturbations and increasing storage performance by nearly a factor of 40 over what we've seen so far "Antonio Ortu, a postdoctoral fellow at UNIGE's Department of Applied Physics, explains why. The findings of this study are a significant step forward in the construction of long-distance quantum telecommunications networks. They also reduce the storage of a quantum state carried by a photon to a human-estimable time scale.

An efficient system in 10 years

However, there are still a few obstacles to overcome. "Now the task is to prolong the storage time even further. In principle, increasing the duration of the crystal's exposure to radio frequencies would suffice, but for the time being, technical challenges in implementing them over a longer period of time preclude us from going beyond 100 milliseconds. These technical issues, on the other hand, are certain to be resolved "Mikael Afzelius agrees.

Scientists will also have to figure out how to make memory that can store more than one photon at a time, resulting in "entangled" photons that guarantee confidentiality. "The goal is to design a system that excels in all of these areas and can be commercialised within 10 years," the researcher concludes.

Reference:

• Antonio Ortu et al, Storage of photonic time-bin qubits for up to 20 ms in a rare-earth doped crystal, npj Quantum Information (2022). DOI: 10.1038/s41534-022-00541-3