When we listen to our favorite song, 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 quantum particles are fundamentally indivisible and
therefore cannot be split, but researchers at the Pritzker School of Molecular Engineering
(PME) at the University of Chicago are exploring what happens when you try to split a phonon.
In two experiments – the first of their kinds – a team led by Prof. Andrew Cleland
used a device called an acoustic beamsplitter to “split” phonons and thereby demonstrate
their quantum properties.
By showing that the beamsplitter can be used to both induce a special quantum superposition
state for one phonon, and further create interference between two phonons, the research team took
the first critical steps toward creating a new kind of quantum computer.
The results are newly published in the journal Science and built on years of breakthrough
work on phonons by the team at Pritzker Molecular Engineering.
In the experiments, researchers used phonons that have roughly a million times higher pitch
than can be heard with the human ear.
Previously, Cleland and his team figured out how to create and detect single phonons and
were the first to entangle two phonons.
To demonstrate these phonons’ quantum capabilities, the team created a beamsplitter that can split
a beam of sound in half, transmitting half and reflecting the other half back to its
source.
Beamsplitters already exist for light and have been used to demonstrate the quantum
capabilities of photons.
The whole system, including two qubits to generate and detect phonons, operates at extremely
low temperatures and uses individual surface acoustic wave phonons, which travel on the
surface of a material, in this case lithium niobate.
However, quantum physics says a single phonon is indivisible.
So when the team sent a single phonon to the beamsplitter, instead of splitting, it went
into a quantum superposition, a state where the phonon is both reflected and transmitted
at the same time.
Observing (measuring) the phonon causes this quantum state to collapse into one of the
two outputs.
The team found a way to maintain that superposition state by capturing the phonon in two qubits.
A qubit is the basic unit of information in quantum computing.
Only one qubit actually captures the phonon, but researchers cannot tell which qubit until
post-measurement: In other words, the quantum superposition is transferred from the phonon
to the two qubits.
The researchers measured this two-qubit superposition, yielding gold standard proof that the beamsplitter
is creating a quantum entangled state.
In the second experiment, the team wanted to show an additional fundamental quantum
effect that had first been 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 beamsplitter at the same time, the superposed outputs interfere so
that both photons are always found traveling together, in one or the other output directions.
Importantly, the same happened when the team did the experiment with phonons – the superposed
output means that only one of the two detector qubits captures phonons, going one way but
not the other.
Though the qubits only have the ability to capture a single phonon at a time, not two,
the qubit placed in the opposite direction never “hears” a phonon, giving proof that
both phonons are going in the same direction.
This phenomenon is called two-phonon interference.
Getting phonons into these quantum-entangled state is a much bigger leap than doing so
with photons.
The phonons used here, though indivisible, still require quadrillions of atoms working
together in a quantum mechanical fashion.
And if quantum mechanics rules physics at only the tiniest realm, it raises questions
of where that realm ends and classical physics begins; this experiment further probes that
transition.
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 – itself
could have the ability to compute new kinds of calculations.
The success of the two-phonon interference experiment is the final piece showing that
phonons are equivalent to photons.
Unlike photon-based linear optical quantum computing, the UChicago platform directly
integrates phonons with qubits.
That means phonons could further be part of a hybrid quantum computer that combines the
best of linear quantum computers with the power of qubit-based quantum computers.
The next step is to create a logic gate – an essential part of computing – using phonons,
on which Cleland and his team are currently conducting research.