Recently, scientists have successfully stored a qubit in a crystal for up to 20 milliseconds, setting a new record and laying an important foundation for the development of long-distance quantum communication networks.
In a cryostat, a laser is used to irradiate crystals used to store photon qubits, image from Antonio Ortu
Quantum physics has enabled advances in many technologies, such as computers, smartphones, GPS, and more. Multinational scientists are working on the field of quantum cryptography to develop more secure quantum communication networks. However, photons are easily lost after being transmitted hundreds of kilometers in the optical fiber, and the information is also lost. Therefore, in order to better store the information in photons, long-distance quantum communication is inseparable from the "repeater", which also means that the storage time of quantum information must be extended.
In 2015, The team of Mikael Afzelius in the Department of Applied Physics at the University of Geneva (UNIGE) In Switzerland successfully stored a qubit carried by photons in a crystal for a storage time of 0.5 milliseconds. In the process, photons transfer their quantum state to atoms in the crystal before disappearing. However, the duration is still not enough to build a larger storage network, and expanding the storage network is a prerequisite for the development of remote quantum communication.
This time, under the framework of the European Quantum Technology Flagship Program, the above-mentioned team broke through previous achievements and successfully achieved the world record of 20 milliseconds of qubit storage in crystals, taking an important step towards the development of long-distance quantum communication networks. The study was published in NATURE's partner journal NPJ Quantum Information.
Experimental system and apparatus, picture from the paper.
In this study, the team used crystals doped with rare earth europium, which absorbs light and then emits light (i.e., photoluminescence). These crystals are preserved at absolute zero, because once it exceeds absolute zero 10 °C, it destroys the entanglement between atoms within the crystal.
"We applied a small magnetic field of one-thousandth of a Tesla to the crystal and used dynamic decoupling methods, including sending a strong radio frequency to the crystal." Antonio Ortu, a postdoctoral researcher in the Department of Applied Physics at the University of Geneva, said, "These technologies are designed to decouple rare earth ions from environmental disturbances and increase current storage performance by nearly 40 times. ”
This research result is an important progress in the realization of long-distance quantum communication networks. "This is a world record based on quantum memory in solid-state systems (crystals), and with a slight loss of fidelity, we can even store for up to 100 milliseconds," the researchers said. ”
But there are still some challenges to face. "The challenge now is to further extend the storage time," Mikael Afzelius argues, arguing that it is theoretically possible to extend the time a crystal is exposed to RF, but a breakthrough in existing technology is required. In addition, the team needed to find a way to design a memory capable of storing multiple photons at the same time, entangled through photons, and ultimately achieving confidentiality.