How can the lifetime of a qubit - the quantum memory unit - be extended?
Quantum computers excite the imagination and are a fertile ground for research and industrial development in the largest companies in the world. These computers, which are still taking their first steps, are expected to process information at a tremendous speed and on a huge scale that is not at all possible with the existing technology. They may allow us to solve mathematical problems that have no answer, or to deal with the quantum properties of the world that are currently accessible to us mainly at a theoretical level.
While a normal computer operates using a bit, a memory unit that can be in state 0 or 1, in quantum memory units, or qubit, the information is stored simultaneously in both 0 and 1 - a state known as superposition. One qubit is equivalent to many bits in its processing power, but it is extremely sensitive to the environment, and therefore loses the information contained in it within fractions of a second.
New research of Dr. Or Katz and Roy Shaham from the laboratory of Dr Ofer Furstenberg, from the Department of Physics of Complex Systems at the Weizmann Institute of Science, in collaboration with the "Raphael" company, is part of a theoretical and practical project that offers a new way to preserve information in qubits for particularly long periods of time.
For 20 years, a quantum memory system has been researched, which stores the information with the help of a property unique to the electron that surrounds the nucleus of the atom and is called spin. In a certain abstraction, we can say that spin is a rotation of the electron around its axis. In well-defined states, where the spin is relatively stable, it is possible to imagine that the electron rotates clockwise or counterclockwise. Another characteristic is that when two atoms collide, their spins keep their original directions or switch between them. Thus, the spin is considered a system capable of storing information. For example, you can decide that clockwise spin is 1 and counterclockwise spin is 0.
In order for the atomic memory to become quantum, the information we want to store is created using a laser beam of photons that is in a state of superposition. The laser beam is launched into a closed container that contains billions of gas atoms, and reverses the spin direction of one of the electrons in one of the gas atoms - without us being able to identify who it is. Later, the atoms of the gas can be excited, so that the same electron whose spin has been reversed will launch a photon in a precise direction that continues the original laser beam. Thus we got a quantum memory unit with a relatively long lifetime, although not more than a few tenths of a second.
The electrons of a rare noble gas have no spin, but their atomic nucleus has a spin - capable of maintaining its direction even for several months
The scientists of the Weizmann Institute hypothesized that using a similar system of photons and gas, which so far has not been used in the context of quantum computing, may significantly extend the life time of the qubit. This is a system where the closed cell contains two types of gas. First, an alkali gas, such as rubidium or potassium, absorbs the information from the photon in the laser beam, then transfers it through collisions to a rare noble gas, such as helium-3.
The electrons of a rare noble gas have no spin, but their atomic nucleus has a spin - capable of maintaining its direction even for several months. To read the memory, the alkaline gas is excited, which received the quantum information from the noble gas through collisions, and which emits it in the form of a photon. In an article they published in early 2021, researchers from Dr. Furstenberg's lab presented part of the complex process, and demonstrated the connection between photons and spin in the atomic nucleus - and back to photons.
The researchers estimate that while in classical systems it takes a long time to transfer the information from the electrons of the alkaline gas to the nuclei of the noble gas, the requirements of a quantum memory allow this to be done very quickly. This is because all that is required is communication between one photon and one electron of the alkali gas and one nucleus of the noble gas. This is because the goal is to create a state of superposition, no matter which of the atoms will collide and transfer the information from one to the second and to the third. In another article they recently published, the researchers demonstrated the construction of a system with optimal conditions, which allows the passage of information to take place at a very high efficiency and has the potential to create a stable system of quantum memory.
However, the road to the realization of such a system in the quantum field is still long. The researchers' theoretical studies and experiments deal, step by step, with the innovative idea - to lead to the breakthrough that is expected to serve technologies for communication between quantum computers using photons. Following them, several research groups were established in the world that are also examining the new direction and joining the challenge, designed to keep the qubit alive.
