What happens when a quantum physicist is frustrated by the limitations of quantum mechanics when trying to study densely packed atoms? At EPFL, you get a metamaterial, an engineered material that exhibits exotic properties.
That frustrated physicist is PhD student Mathieu Padlewski. In collaboration with Hervé Lissek and Romain Fleury at EPFL’s Laboratory of Wave Engineering, Padlewski has built a novel acoustic system for exploring condensed matter and their macroscopic properties, all the while circumventing the extremely sensitive nature that is inherent to quantum phenomena. Moreover, the acoustic system can be tweaked to study properties that go beyond solid-state physics. The results are published in Physical Review B.
“We’ve essentially built a playground inspired by quantum mechanics that can be adjusted to study various systems. Our metamaterial consists of highly tunable active elements, allowing us to synthesize phenomena that extend beyond the realm of nature,” says Padlewski. “Potential applications include manipulating waves and guiding energy for telecommunications, and the setup may one day provide clues for harvesting energy from waves for instance.”

Schrödinger’s cat, the quantum conundrum
In quantum mechanics, the cat is both dead and alive inside of the box until you interfere with the system by measuring it, which is done in this case by opening the box. From a purely quantum perspective, the cat is in a superposition of two probable states: a probable state of being dead and a probable state of being alive, until you open the box only to observe if the cat is actually dead or alive. A cat cannot be both dead and alive at the same time, and that’s the essence of Schrödinger cat, a thought experiment devised by Erwin Schrödinger in 1935 that illustrates the complexities of quantum concepts when imagined beyond the quantum scale, like the scale of a cat.
The sensitive nature of quantum physics that makes observation of solid states so difficult comes from the act of measuring the system, which forces the quantum system into a state, instead of allowing the system to exist – uninterrupted – in a superposition of probability states. That said, physicists know how to probe the electronic states indirectly and infer their corresponding properties.

Modeling quantum phenomena with sound waves
But there is another phenomenon for which Schrödinger’s cat makes perfect sense in the macroscopic world, and it’s one that we can interact with: sound.
If we take the sound of one’s voice for instance, we know that the reason why someone’s voice is unique and rich is because we hear the whole spectrum of frequencies. The frequency spectrum is characteristic for a given voice, but it also explains why the piano has its unique timbre, or why the trumpet sounds differently from the trombone. In principle, we can simultaneously hear the fundamental frequency, aka the fundamental state, plus all of the higher frequencies known as the harmonics. Borrowing language from quantum physics, we are actually hearing a superposition of many states at once. Or by analogy with Schrödinger’s cat, the cat is both dead and alive, and we can hear it!
“Quantum probability waves are waves after all – why not model them with sound?” says Padlewski. “Probing the electronic states of a solid state, directly without perturbation, would be like having a blind person tread through a busy street without a cane. But in acoustics, we can probe waves directly, in phase and in amplitude without destroying the state – which is nice.”

Engineering an acoustic metamaterial
The acoustic metamaterial built at EPFL consists of a line of “acoustic atoms”, essentially 16 small cubes connected to one another with openings to allow for the placement of multiple speakers or microphones. Speakers generate sound waves that are to propagate through the line of acoustic atoms in a controlled way, microphones measure sound waves for feedback control. The cubes can be viewed as building blocks for building more complex systems that go beyond a simple line.
“When you see the cochlea, the ear’s organ responsible for hearing, it resembles our active acoustic metamaterial in its structure and functionality,” says Lissek. “The cochlea consists of a perfect line of cells that amplify different frequencies. Our metamaterial could potentially be tuned to function the same way and study hearing problems like tinnitus.”

Towards a quantum inspired analog computing
Padlewski is also keen to use the metamaterial building blocks to investigate ways to build one of the first acoustic analog computers capable of generating non-separable states. Inspired by the work of Pierre Deymier of Arizona University, this computer would essentially be an acoustic equivalent of a quantum computer. It would allow for the direct observation of superposed states without interfering with the system, because acoustic waves are not as fragile as quantum ones.
“An acoustic quantum analog computer would be more like a crystal lattice – a periodic array of cells just as atoms are arranged in crystals,” adds Padlewski. “The acoustic approach to quantum computation has the potential to offer an alternate way of processing vast amounts of information simultaneously.”