Harvard scientists have achieved a significant breakthrough in quantum computing by developing metasurfaces that could transform how quantum information is processed and transmitted.
The research team at Harvard's John A. Paulson School of Engineering and Applied Sciences, led by Professor Federico Capasso, has created specially designed metasurfaces—flat devices etched with nanoscale light-manipulating patterns—that function as ultra-thin replacements for bulky quantum optical setups. Their findings were published in Science on July 24, 2025, in a paper titled "Metasurface quantum graphs for generalized Hong-Ou-Mandel interference."
"We're introducing a major technological advantage when it comes to solving the scalability problem," explains graduate student Kerolos M.A. Yousef, the paper's first author. "Now we can miniaturize an entire optical setup into a single metasurface that is very stable and robust."
Conventional quantum photonic systems rely on complex networks of lenses, mirrors, and beam splitters to manipulate photons and create the entangled states necessary for quantum computing. These systems become increasingly unwieldy as more components are added, making practical quantum computers difficult to build. The Harvard team's innovation collapses all these components into a single, flat array of subwavelength elements that control light with remarkable precision.
A key innovation was the team's application of graph theory—a branch of mathematics that uses points and lines to represent connections—to design metasurfaces capable of controlling properties like brightness, phase, and polarization of photons. This approach allowed them to visually map how photons interfere with each other and predict experimental outcomes, making the design of complex quantum states more intuitive.
"With the graph approach, metasurface design and the optical quantum state become two sides of the same coin," notes research scientist Neal Sinclair, who collaborated on the project.
The resulting metasurfaces offer numerous advantages over conventional setups: they don't require intricate alignments, are robust against environmental disturbances, can be fabricated using standard semiconductor techniques, and minimize optical loss—a critical factor for maintaining quantum information integrity.
Beyond quantum computing, this technology could advance quantum sensing and enable "lab-on-a-chip" capabilities for fundamental science research. The work represents a significant step toward practical room-temperature quantum computers and networks, which have long been challenging to implement compared to other quantum platforms.
The research was funded by the Air Force Office of Scientific Research and performed at Harvard's Center for Nanoscale Systems, with crucial collaboration from Professor Marko Lončar's quantum optics and integrated photonics team.