Madrid. A research team at the University of Chicago took the first critical steps toward creating a new type of quantum computer, based on phonon splitting.
When a song is played, what sounds like a continuous wave of music is actually transmitted as tiny packets of quantum particles called phonons.
The laws of quantum mechanics hold that these particles are fundamentally indivisible, but researchers at the Pritzker School of Molecular Engineering (PME) are exploring what happens when you try to split one.
In two first-of-its-kind experiments, a team led by Andrew Cleland used a device called an acoustic beam splitter to split phonons to demonstrate their quantum properties. By showing that such a divider can be used both to induce a special quantum superposition state for a phonon and to create further interference between two of them, the team took the first critical steps towards creating a new type of quantum computer. The results are published in the journal Science.
In the experiments, the researchers used phonons that are nearly a million times louder in pitch than the human ear can hear. Cleland and his team previously discovered how to create and detect individual phonons and were the first to entangle two of them.
To show the quantum capabilities of these phonons, the team, including Cleland graduate student Hong Qiao, created a beam splitter that can split one sound beam in half, transmitting one and reflecting the other back to its source (there are already for light and is used to demonstrate the quantum capabilities of photons). The entire system, including two qubits (basic units of information in quantum computing) to generate and detect phonons, works at extremely low temperatures and uses individual surface acoustic wave particles from those particles, which travel on a material, in this case niobate from lithium.
However, quantum physics says that a single phonon is indivisible. So when the team sent one to the beam splitter, instead of splitting up it went into quantum superposition, a state in which the phonon is reflected and transmitted at the same time. Observing (measuring) the phonon causes this quantum state to collapse into one of two outputs.
The quantum superposition is transferred from the phonon to the two qubits. The researchers measured it, which produced “gold standard proof that the beam splitter creates a quantum entangled state,” Cleland said in a statement.
second experiment
In the second experiment, the team wanted to show an additional fundamental quantum effect that had been first demonstrated with photons in the 1980s. Now known as the Hong-Ou-Mandel effect, when two identical photons are sent from opposite directions into a beam splitter at the same time, the overlapping outputs interfere so that both photons always travel together, in one output direction or the other.
Importantly, the same thing happened when the team did the experiment with phonons: the overlapping output means that only one of the two detector qubits captures phonons, going one way but not the other. Although qubits only have the ability to pick up a single phonon at a time, not two, the one placed in the opposite direction never “hears” such a particle, which shows that both phonons are going in the same direction. This phenomenon is called two-phonon interference.
Getting those particles into this entangled quantum state is a much bigger leap than doing it with photons. The phonons used here, while indivisible, still require trillions of atoms working together in a quantum mechanical way. And if it rules physics only in the smallest realm, it raises questions about where classical physics ends and begins; this further proves that transition.
“All of those atoms have to behave in a coherent way to support what quantum mechanics says they should do,” Cleland said. “It’s kind of amazing. The strange aspects of that part of physics are not limited by size.
The power of quantum computers lies in the “weirdness” of the quantum realm. By harnessing the strange quantum powers of superposition and entanglement, researchers hope to solve previously intractable problems. One approach to doing this is to use photons, in what is called a “linear optical quantum computer.”
A linear mechanical quantum computer, which would use phonons instead of photons, might have the ability to perform new types of calculations. “The result confirms that we have the technology we need to build one,” Cleland said.
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