Researchers use light to create a one-way street for mechanical energy

Is it possible to create a “one-way street” for mechanical energy that only allows heat and sound to flow in one direction?

In most standard setups, this is impossible: If acoustic energy can flow in one direction, then it can also flow in the reverse direction. Finding new ways to break this basic symmetry has sparked the interest of many scientists and engineers in recent years. Such one-way streets could be extremely useful in a variety of applications ranging from thermal management to communications systems.

A new experiment involving researchers with the Institute for Molecular Engineering at the University of Chicago and Yale University now demonstrates that by using light to mediate the interaction between two mechanical systems, they can create a controllable one-way channel for the flow of vibrational energy and heat.

The study, published today in Nature, was based on a theory developed earlier by the University of Chicago team and provides proof that the basic theory works. It also shows that the ideas can be implemented in a simple, compact system that could be incorporated in new devices.  

“This is a really exciting resource that can be used in both classical and quantum contexts,” said Aashish Clerk, a professor in molecular engineering at the University of Chicago who developed the theory and is a co-author of the study. “This research could open the door for many new studies.”

Breaking symmetry by using light

The principle that says energy and information exchange between two systems via a two-way street is known as “reciprocity” and is a fundamental rule in most physical systems. Breaking this symmetry is crucial in a number of different applications. For example, by preventing a backward flow of energy, one could protect a delicate signal source from corruption, or cool a system by preventing unwanted heating.

Breaking reciprocity is especially important in quantum computation, where scientists harness quantum phenomena to enable powerful new kinds of information processing. Here, breaking this symmetry ensures delicate quantum processors are not destroyed during the readout process.

In their experiment, the Yale researchers and Clerk used a small vibrating membrane made of silicon nitride (about 1 millimeter square and 50 nanometers thick) as the mechanical system. Much like a drumhead, this membrane could vibrate in several distinct ways, each with a distinct resonant frequency.

The researchers’ goal was to engineer a one-way flow of energy between two of these vibrational modes. To do this, the membrane was placed in a structure (called an optical cavity) with two parallel mirrors designed to trap light. By shining light on the cavity using lasers, the researchers were able to use light as a medium for transferring mechanical energy between two vibrational modes. When the lasers were tuned carefully (in a way predicted by theory), this transfer mechanism was completely directional.

From theory to lab to the quantum level

The experiment was based on basic theoretical concepts developed by Clerk and his former postdoc Anja Metelmann (now at the Freie University in Berlin) published in Physical Review X in 2015.

“You can come up with a lot of ideas that are exciting in terms of the basic theory and concepts, but often there is a gap between these abstract ideas and what you can actually build and realize in the lab,” Clerk said. “To me, it is exciting that our proposal was realized, and that the experimentalists had enough control over their system to make it work.”

The approach used in the experiment to achieve a one-way interaction—mechanical vibrations interacting with light—could pave the way for designing new devices targeting a variety of applications, ranging from mitigating heat flow to new kinds of communication systems. These unusual one-way interactions also have interesting fundamental implications.

As a theoretical physicist who focuses on quantum systems, Clerk is particularly interested in studying arrays where many quantum systems interact with one another in a unidirectional manner. This could be a powerful way to generate the unusual kinds of quantum states that are needed for quantum communication and quantum computation.

Other authors on the paper include Jack Harris, Haitan Xu, and Luyao Jiang of Yale University.

Citation: "Nonreciprocal control and cooling of phonon modes in an optomechanical system," H. Xu, Luyao Jiang, A. A. Clerk, J. G. E. Harris. Nature, doi: 10.1038/s41586-019-1061-2 

Funding: Air Force Office of Scientific Research and the Simons Foundation