A new study shows how 'splitting' sound takes us one step closer to a new type of quantum computer

The research led by Prof. Andrew Cleland from the Pritzker School of Molecular Engineering (PME) at the University of Chicago demonstrated two experiments using an acoustic beamsplitter to split the phonons. 

This helped them observe two fundamental quantum phenomena previously only seen in photons.

Beam splitter with single phonons

Splitting, in the context of quantum computing, refers to the ability to manipulate and control quantum states. 

In their experiments, the scientists employed phonons with frequencies far beyond the range of human hearing. For the first of the two experiments, the team used the acoustic beam splitter to split a phonon.

Upon entering the beam splitter, the phonon entered a superposition state, simultaneously being transmitted and reflected. This happens due to a phenomenon known as interference. 

To preserve the superposition state, the phonon was captured in two qubits. Qubits are the basic units of quantum information analogous to bits in classical computers. 

The information about which qubit captured the phonon is unavailable until a measurement (or observation) is performed on the system. This causes the superposition state to collapse, revealing the information.

Before measurement, the phonon's presence was spread across both qubits, demonstrating the characteristic of quantum superposition. But, measuring the two-qubit superposition revealed a phenomenon central to quantum computing called entanglement.

Entanglement refers to a correlation between two or more qubits, such that the state of one qubit depends on the state of the others, even when physically separated, and cannot be described independently.

Two phonon interference 

For the next experiment, the team wanted to demonstrate the Hong-Ou-Mandel effect, which was first seen with photons in the 1980s. When two identical photons are transmitted in opposite directions into a beam splitter simultaneously, they interfere. The superposed state is such that the two photons are found traveling together in the same direction.

The team showed that when two identical phonons were sent into a beamsplitter simultaneously, the superposed outputs exhibited interference, resulting in the two phonons always being detected together in one of the output directions

This effect, known as two-phonon interference, provided evidence that both phonons were traveling in the same direction despite the qubits' inability to capture two phonons simultaneously.

Both experiments were conducted at exceedingly low temperatures and employed surface acoustic wave phonons on lithium niobate, a material conducive to quantum phenomena.

The demonstration of two weird or spooky quantum phenomena with phonons was exceptional. Harnessing the quantum properties of entanglement and superposition with phonons is essential for building linear mechanical quantum computers. 

These computers could open avenues for all kinds of new computations. In a press release, the lead author of the study, Cleland said, "The success of the two-phonon interference experiment is the final piece showing that phonons are equivalent to photons. The outcome confirms we have the technology we need to build a linear mechanical quantum computer." 

The findings of the study are published in the journal Science.

Study abstract:

Linear optical quantum computing provides a desirable approach to quantum computing, with only a short list of required computational elements. The similarity between photons and phonons points to the interesting potential for linear mechanical quantum computing using phonons in place of photons. Although single-phonon sources and detectors have been demonstrated, a phononic beam splitter element remains an outstanding requirement. Here we demonstrate such an element, using two superconducting qubits to fully characterize a beam splitter with single phonons. We further use the beam splitter to demonstrate two-phonon interference, a requirement for two-qubit gates in linear computing. This advances a new solid-state system for implementing linear quantum computing, further providing straightforward conversion between itinerant phonons and superconducting qubits.