Scientists design artificial sound crystal to bring heat under control

Written by Karmela Padavic-Callaghan 

Nature, at the scale of atoms, is governed by quantum mechanics. While we can peek into this realm with powerful microscopes, much of it remains obscured because of heat. Signature quantum features like superposition and entanglement are extremely delicate when it comes to temperature changes — they dissipate like water droplets on a hot summer day. For scientists, this presents a conundrum: devices that harness the elusive world of quantum could revolutionize technology but attempts to reach into that world are riddled with obstacles.

Scientists studying optomechanics, or the interactions between light and mechanical vibrations, live at the edge of the quantum realm. They build chips and structures that are often bigger than the typical quantum platform, which only makes their battle with heat more acute. In such devices the quantum versions of mechanical vibrations, known as phonons, are tiny but prevalent vehicles for heat.  

In the October issue of Nature Communications, a team of IQUIST researchers led by Kejie Fang, Assistant Professor in Electric and Computer Engineering, present a new method for efficiently trapping phonons inside a fabricated device. Their design reduces heating by keeping some phonons stuck in place while redirecting others, and the heat they carry, to a completely different part of the device. The researchers also showed that trapped phonons stayed stable for a long time. This work could provide a starting point for the development of new quantum tools that use phonons for storing information, converting mechanical motion into electrical signals, or detecting rare astrophysics events.

In optomechanics, fabricated structures vibrate and produce acoustic waves. These undulations are made up of discrete bundles of energy called phonons. From a quantum perspective, each phonon can be visualized as a tiny spring, rather than a wave spread out across space. And they can be affected by any particle that collides with them — just like a spring reacts to any force that pushes on it.  

“Phonons interact with almost everything because mechanical vibrations can happen in almost any physical system” Fang says. In optomechanical devices, uncontrolled phonons run around every which way, similar to how sound vibrations spread throughout a drumhead after it has been hit. During this rollicking, phonons spread energy wherever they go inside the device, causing warming that ultimately destroys the quantum nature of the whole system.

According to Fang, one way to keep things cold and quantum is to trap some phonons and redirect the heat of other free phonons to another part of the device. In their experiment, Fang’s team made a device that does exactly this — they layer a periodic pattern in an aluminum nitride slab on a base of silicon dioxide and silicon, like an open-face sandwich. The top is called a phononic, or sound, crystal and, similar to a real crystal which supports complex behaviors of electrons, this artificial periodic structure supports phonons. The team generated vibrations in the crystal by applying electrical voltage and then measuring how different phonons traveled through the device. By design, the artificial crystal allowed most phonons to move around and run away into the base and forced specific ones to stay in place as a standing wave. Measurements showed that whenever a phonon was trapped in this way, it was stable and long-lived. 

Artistic depiction of silicon dioxide and a sound crystal (Credit: E. Edwards).
Artistic depiction of a sound crystal (Credit: E. Edwards).

The team achieved this control over phonons with two design tricks. First, the bottom silicon dioxide and silicon acts much like a soft slice of bread, absorbing some of the

uncontrolled, unwanted phonons including heat. Secondly, they designed the sound crystal in a way that allowed for sweet spots at which phonons vibrating at certain frequencies were particularly resistant to any sort of disturbance. This is why only some phonons got trapped and could not carry their energy to other parts of the device.

Fang’s team wants to use this phonon trapping method to controllably study which particular quirks of quantum mechanics are still be present in optomechanical systems as they grow in size. 

“At 100 by 100 micrometer, our structure is already much massive than many that have been studied before, and it would not be difficult to scale it up,” says Fang. He notes that making a massive structure cold enough for quantum effects to dominate is challenging but says that their team could feasibly manipulate a quantum phonon in a structure up to a millimeter in size. With such a device, they could potentially create one of the most massive mechanical quantum systems that have been made so far.

Now that the team demonstrated control over the mechanical part, they can turn towards the optical part of optomechanics. Fang’s devices can already attract light particles, or photons, and send them to interact with phonons. Such an idea could be useful from an applied point of view: for example, researchers might use a laser to transfer information carried by light onto a trapped phonon, which could provide storage. “This is a standalone device, and we don’t need to make any further complicated structures. It can already couple with light, and our next step is basically just to turn on the light,” says Fang.