By listening to quantum atoms, we talk together thanks to the acoustics.

What happens when quantum physicists are unhappy with the limitations of quantum mechanics when they try to study densely packed atoms? EPFL obtains metamaterials, an engineering material that exhibits exotic properties.

The frustrated physicist is a Ph.D. student Matthew Paddleski. In collaboration with Hervé Lissek and Romain Fleury at the EPFL’s Institute of Wave Engineering, Padlewski has built a new acoustic system to explore condensed matter and its macroscopic properties, while avoiding the highly sensitive properties inherent in quantum phenomena. Additionally, acoustic systems can be tuned to study properties beyond solid-state physics. The results can be found in Physical Review B.

“We essentially built a playground inspired by quantum mechanics that can be tuned to study a variety of systems. Our metamaterials are made up of highly tunable active elements that allow us to synthesize phenomena that extend beyond the realm of nature,” says Paddleskey. “Potential applications include wave manipulation and induced energy for telecommunications. Setups may provide clues to harvest energy from, for example, waves.”

Schrodinger’s Cat, Quantum Difficult

In quantum mechanics, cats live dead in a box until they interfere with the system by measuring it. In this case, it is done by opening the box. From a purely quantum perspective, cats lie in the superposition of two possible states. It is a state that may be dead and may be alive.

Cats cannot die and survive at the same time. It is a thought experiment devised by Irwin Schrodinger in 1935, demonstrating the complexity of quantum concepts imagined beyond quantum scales such as cat scales.

The delicate nature of quantum physics, which makes the observation of solid states extremely difficult, arises from the act of measuring the system that measures the system. That said, physicists know how to indirectly probe electronic states and infer corresponding properties.

Modeling quantum phenomena using sound waves

But there is another phenomenon in which Schrodinger’s cat makes perfect sense in the macroscopic world, and that’s what we can interact with: sound.

For example, when you take the sound of your own voice, you know that the reason someone’s voice is unique and rich is because you hear the entire frequency. The frequency spectrum is distinctive in certain sounds, but also explains why the piano has its own tone, or why the trumpet sounds different from the trombone.

As a rule, in addition to the fundamental frequency, or fundamental state, you can hear all the high frequencies known as harmonics at the same time. Borrowing the language from quantum physics, we actually hear the superposition of many states at once. Or, by the similarity of Schrödinger’s cat, the cat is dead and alive, and we can hear it.

“Quantum stochastic waves are waves after all. Why aren’t they modeled with sound?” Padlewski says. “To directly verify the electronic state of a solid state without perturbation is like a blind person stepping on a busy street without using a cane. However, acoustics allow us to directly examine waves of phase and amplitude without destroying the state.”

Engineering of acoustic metamaterials

The acoustic metamaterial constructed with EPFL is made up of lines of “acoustic atoms” and are essentially 16 small cubes connected together at openings to allow for the placement of multiple speakers or microphones. Speakers produce sound waves that propagate the sound waves in a controlled manner. The microphone measures the sound waves for feedback control. Cubes can be seen as building blocks for building more complex systems beyond simple lines.

“When you look at the organs of the ear, the cochlea, which causes hearing, it resembles an active acoustic metamaterial in its structure and function,” says Lissek. “Co-cows are made up of complete lines of cells that amplify different frequencies. Our metamaterials can work in the same way and may be tailored to study hearing problems like tinnitus.”

Towards quantum-style analog computing

Padlewski is also keen to investigate how to use metamaterial building blocks to build one of the first acoustic analog computers that can generate non-separable states. Inspired by the work of Pierre Damier at the University of Arizona, the computer is essentially equivalent to the sound equivalent of a quantum computer. Sound waves are not as vulnerable as quantum, allowing you to directly observe the superposition state without interfering with the system.

“Acoustic quantum analog computers look like crystal lattices, a periodic array of cells, just like atoms are placed in crystals,” adds Padlewski. “Acoustic approaches to quantum computation could provide another way to simultaneously process huge amounts of information.”