Harvard researchers have developed a metasurface capable of replacing complex optical components in quantum computing, aiming to enhance scalability, stability, and compactness of quantum networks. This innovation utilizes graph theory to simplify the design of quantum metasurfaces, enabling entangled photon generation and quantum operations on a single, ultra-thin chip.
Photons, as fundamental light particles, offer possibilities for information carriage in quantum computers and networks. Current methods involve waveguides on microchips or bulky optical devices like lenses and beam splitters to entangle photons for quantum information processing. However, scaling these systems presents challenges due to the large number of components and their imperfections.
Optics researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), led by Robert L. Wallace Professor of Applied Physics, Federico Capasso, created specialized metasurfaces. These flat devices, etched with nanoscale patterns, act as thin upgrades for quantum-optical chips.
The research, funded by the Air Force Office of Scientific Research (AFOSR), was published in Science. Capasso’s team demonstrated that a metasurface could generate complex, entangled photon states for quantum operations, replicating functions of larger optical devices.
“We’re introducing a major technological advantage when it comes to solving the scalability problem,” stated graduate student and first author Kerolos M.A. Yousef. “Now we can miniaturize an entire optical setup into a single metasurface that is very stable and robust.”
The results indicate that optical quantum devices could be based on error-resistant metasurfaces instead of conventional components. Advantages include simpler designs without intricate alignments, robustness to perturbations, cost-effectiveness, fabrication simplicity, and low optical loss. This approach aids room-temperature quantum computing, networking, quantum sensing, and “lab-on-a-chip” capabilities.
Designing a metasurface to control properties like brightness, phase, and polarization becomes mathematically complex as photon and qubit numbers increase. Each added photon creates new interference pathways, traditionally requiring a growing number of beam splitters and output ports.
Researchers employed graph theory, a mathematical branch using points and lines to represent connections, to manage this complexity. By representing entangled photon states as interconnected lines and points, they visualized photon interference and predicted experimental effects. Graph theory is common in some quantum computing and error correction applications, but not typically in metasurface design.
The paper resulted from a collaboration with Marko Loncar’s lab, which contributed expertise and equipment in quantum optics and integrated photonics.
Research scientist Neal Sinclair commented, “I’m excited about this approach, because it could efficiently scale optical quantum computers and networks — which has long been their biggest challenge compared to other platforms like superconductors or atoms.” Sinclair added, “It also offers fresh insight into the understanding, design, and application of metasurfaces, especially for generating and controlling quantum light. With the graph approach, in a way, metasurface design and the optical quantum state become two sides of the same coin.”
