Quantum communication and quantum encryption technology have become an important cornerstone for improving communication security. Quantum theory ensures the authenticity and secrecy of qubits when information (qubits, the fundamental unit of quantum computing and quantum information) is transmitted between two interlocutors via a single photon in an optical fiber. However, there is a major obstacle to establishing a long-distance quantum communication system: after the transmission of hundreds of kilometers, the photons will be lost, and the signal will also disappear. For this reason, "repeaters" based on quantum memory are born. To function well, you need to store quantum information for a little longer.
In 2015, a team led by Mikael Afzelius of the University of Geneva succeeded in storing qubits carried by a single photon inside a crystal (memory) for 0.5 milliseconds. In the process, the photon can transfer its quantum state to the atoms of the crystal before disappearing, but this duration is not long enough to build the larger storage network that is a prerequisite for the development of long-range quantum communication.
In the latest study, within the framework of the European Quantum Flagship program, the team of Mikael Afzelius managed to store a qubit for 20 milliseconds. To do this, they used crystals doped with europium, which are capable of absorbing light before emitting light.
"We applied a small magnetic field of one-thousandth Tesla to the crystal and used a dynamic decoupling method, including sending intense radio frequencies to the crystal, aimed at isolating the rare earth ions from environmental perturbations and improving the storage performance," the team said. Nearly 40 times higher."
Afzelius said: "This is a world record for quantum memory based on solid-state systems (crystals), and with a slight loss of fidelity, we can even reach 100 milliseconds, which is a long-range quantum communication network. significant progress in the field.”
Nonetheless, they still have quite a few issues to solve, such as extending the storage time further, allowing storage to last longer than 100 milliseconds without distorting it. In addition, they must design new types of memory that can store more than one photon at a time (making the photons "entangled"), "Our goal is to develop a system that performs well in all of these areas and bring it to market within a decade."
