Link�ping University researchers (L-R) Weimin Chen, Irina Buyanova and �Yuttapoom Puttisong

Scientists create leap forward for quantum computing

Scientists have devised a method for both “initializing and reading nuclear spins at room temperature”, which could help clear the way for quantum computing’s arrival in the future and supercede current digital computing.

With the help of a spin filter, Professor Weimin Chen and colleagues at Linköping University, Sweden succeeded in producing “a flow of free electrons with a given spin in a material at room temperature”.

This is the first time strong nuclear spin polarisation is demonstrated at room temperature by spin-polarised conduction electrons, says the research team.

The implication of the method’s success is significant simply because quantum computers are theoretically so powerful - potentially millions of times more powerful than today’s most powerful supercomputers.

Quantum computers perform many complex calculations at the same time and search large amounts of data at speeds that far exceed those available today.

“You could say that a quantum computer can think several thoughts simultaneously, while a traditional computer thinks one thought at a time,” says Weimin Chen, professor in the Division of Functional Electronic Materials at the Department of Physics, Chemistry and Biology at LiU.

Quantum computers perform these feats by harnessing atomic power and thus are able to contain multiple states simultaneously.

Current silicon-based digital computers function by using bits that exist in one of two states: a 0 or a 1.  Quantum computers, however, are not limited to two states. Information is encoded as quantum bits, or qubits, which can have any value between 1 and 0. Qubits represent microscopic particles such as atoms, ions, photons or electrons and their respective control devices. Together these elements form a computer memory and a processor.

The particles that act as qubits are manipulated through the use of various control devices,  such as ion traps (which use optical or magnetic fields -or a combination of both- to trap ions), optical traps (which use light waves) or quantum dots (for electrons).

Chen and co opted for nuclear spin as the qubit of choice because the nucleus is well protected from unwanted electromagnetic disturbance, which is a condition for keeping sensitive information in the qubit intact.

Starting, or initiating, the spin-based qubits required all the atomic nuclei to spin in the same direction, either ‘up’ or ‘down’ (clockwise or counter-clockwise).

The most common method for polarising nuclear spin is called dynamic nuclear polarisation, meaning simply that the electrons’ spin influences the nucleus to spin in the same direction.

It had hitherto not been possible to carry out dynamic nuclear polarisation at room temperature - only at lower temperatures, because spin orientation in electrons was easily lost at room temp, due to electrons being sensitive to outside disturbance.

This is because by looking at subatomic particles, they could be bumped and thereby change their value. To make a practical quantum computer, scientists have to devise ways of making measurements indirectly to preserve the system's integrity.

“We prove experimentally that the measurable magnetic field from the nuclei, as well as the strong polarisation of the nuclear spins in the material at room temperature, comes from the dynamic polarisation of the nuclear spin in the extra added Ga atoms,” says Chen.

The research shows that nuclear spin polarisation happens very quickly – potentially in less than a nanosecond (one-billionth of a second).

The method proposed also has the advantage of making use of free electrons. This makes it possible to control the polarisation of the spin in the nucleus electrically; in this way the information lying in the spin can both be initiated and read.

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