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How To Build A Phononic Computer

Physicists have designed the building blocks of quantum computer that works using sound

The science of how light interacts with matter is called quantum electrodynamics or QED and the theory on which it is based is one of the crowning achievements of 20th century physics.

Today, it lies at the heart of an emerging technology called circuit-QED, in which photons trapped on a silicon chip are made to interact with superconducting devices called artificial atoms, which have various energy levels just like real atoms.   

This is a promising tool for quantum computation. Circuit-QED devices manipulate quantum information as it is transferred from light to matter and vice versa. And the fact that this all takes place on a single chip allows unprecedented control.

But there is another way of doing this kind of  quantum information processing that could be just as promising. Instead of relying on light, this uses quantum packets of sound called phonons. 

Phonons are quantum vibrations that travel through the lattice that makes up materials, much in the same way that sound passes through the air. Usually they are messy and incoherent things that we see as heat and noise, things that we usually want to minimise or remove entirely.

But in recent years, physicists have begun to examine ways to create phonons intentionally, to trap them and make them coherent. 

That’s given them a number of interesting ideas. Since vibration is essentially heat, this kind of control has led to various innovative cooling mechanisms on the nanoscopic scale. And since any quantum object can also carry quantum information, various physicists are thinking about ways of using phonons for quantum computation.

But before they can do that they need a way to manipulate the quantum information that phonons carry. And nobody has worked out a good way to do that.

Today, however, Rusko Ruskov and Charles Tahan at the University of Maryland say they know how to do it. Their idea is the phononic equivalent of circuit-QED. So instead of trapping a photon in a cavity and making it interact with an artificial atom, these guys want to do it with phonons: quantum phonodynamics.

Their scheme is relatively straight forward. They begin by creating a silicon membrane–a one dimensional silicon crystal about 200 nanometres thick. They engineer properties of this membrane such to create a kind of waveguide that steers phonons.

They create a phonon by zapping this membrane with a laser, which sends a quantum packet of vibration through the lattice.   

The equivalent of an atom trapped in an optical cavity is single atom of boron or aluminium which distorts the lattice. It is this distortion that interacts with the photon.

However, physicists can use an external magnetic field to make the distortion take several different energy levels. This changes the way the phonon interacts in a way that processes the information it carries.

Finally, the phonon passes into another region of the silicon lattice with a band gap that has been engineered to convert the phonon into a photon, which can then be measured.

That sounds like a good plan. Physicists already know that circuit-QED is a powerful way of manipulating quantum information. So the only question is over the feasibility of the design for quantum phonodynamics. 

The difficulty, of course, will be building this thing, which as Ruskov and Tahan say will be “a seminal achievement.”

Quantum information processing is crowded area with many ideas, plans and devices competing for interest and funding. Phononics has the benefit of being an emerging field with broad application in various exotic forms of computing. We’ll be watching with interest to see how it pans out.

Ref: On-Chip Quantum phonodynamics

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