New form of silicon targets quantum computers

Its developers say a new form of silicon called Q-silicon could be just the ticket. Researchers at North Carolina State University reported the material in the journal Materials research letter says it has properties suitable not only for quantum computing but also for lithium-ion batteries.

“To fool Mother Nature, you have to overcome thermodynamic limitations, so you have to do this very, very quickly.”
—Jay Narayan, North Carolina State University

Silicon typically comes in three forms: crystalline, in which the atoms have an ordered structure; amorphous, where atoms are randomly located; and polycrystalline, in which smaller crystalline units are randomly connected. In crystalline form, silicon atoms are stacked like carbon atoms in a diamond, with four atoms forming the corners of a pyramid.

Q-silicon has a random arrangement of diamond-like pyramids that results in more densely packed atoms and less empty space. Jay Narayan, professor of materials science and engineering at NCSU, and his colleagues created Q-silicon by blasting amorphous silicon with nanosecond-long high-power laser pulses, then cool it in 1/5 microsecond.

That’s fast enough that conventional thermodynamics can’t rearrange the atoms back to one of silicon’s three naturally occurring forms. “To fool Mother Nature, you have to overcome thermodynamic limitations, so you have to do this very, very quickly,” says Narayan.

How can Q-silicon be used for qubits or batteries?

The researchers show that Q-silicon reveals properties never before seen in conventional silicon. First, it is ferromagnetic at room temperature. Ferromagnetism, a property that causes a material to become magnetized when placed in an external magnetic field and then retain that magnetized state. Ferromagnetism is commonly found in metals such as iron and nickel and arises from the bulk nature of atoms in solids. Their magnetic dipoles can be aligned by external fields and then retain their positions when those fields disappear. But if individual electrons in those materials can be isolated, then the spins of those electrons – which themselves can be increased or decreased or an intermediate quantum combination of both – can also be used as qubits, as a means of encoding quantum information.

The even number of electrons in carbon and silicon usually means that their charges both exist in pairs with opposite spins, canceling each other’s magnetic fields. So trapping and manipulating individual electron spins in silicon is often not an option for engineers and materials scientists. Ferromagnetism requires single electrons or unpaired spins, Narayan said. However, “with rapid melting and cooling, we can create unpaired spins that are ferromagnetic,” he says. “The idea is that if silicon can have an unpaired spin then you can store information in that spin.”

Harnessing spin is a challenge, and attempts have been made to read the spin state of phosphorus atoms implanted in silicon as a path to quantum computers. Q-silicon could make it simpler to take advantage of the spin of silicon atoms, Narayan said. “Now you can make quantum computers and all kinds of other interesting applications,” he said, “because Q-silicon is ferromagnetic at room temperature.”

Furthermore, when doped with boron atoms, the researchers report that Q-silicon becomes a superconductor. Known superconductors typically only demonstrate their superconducting capacity at very low temperatures, so there is skepticism about any reports of room-temperature superconductors.

The superconductors with the highest temperatures at ambient pressure known to date become superconductors below 130 kelvin. Narayan and his colleagues show that boron-doped Q-silicon transitions to a superconducting state at 174 K.

Narayan said the researchers plan to demonstrate a quantum computer based on Q-silicon in the near future. But they are also looking to develop the material’s potential for battery applications. “We will create high-energy and high-performance lithium and sodium-ion batteries,” he said.

To use Q-silicon for batteries, Narayan said they will combine Q-silicon with another related material, called Q-carbon, which they discovered in 2015. These two materials both absorb more lithium ions than materials made from graphite. Anodes are used in batteries today. Graphite has the capacity to store an electric current of 200 milliamperes per gram, he said. In contrast, he says, Q-carbon has a capacity of 500 mA/g. Meanwhile, he said, Q-silicon boasts a current of 1,000 mA/g. “Put them together and they create the best anode for a lithium-ion battery,” he claims.

The researchers collaborated with German company Koening Systems to form a startup called Q-Power Battery.