Harvard Researchers Develop Photon Router to Bridge Microwave and Optical Quantum Worlds

[kevinaggner]

Applied physicists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a photon router that could serve as a key building block for future quantum networks. Designed to create strong optical interfaces for microwave-based quantum computers, this innovation moves us closer to a modular, distributed quantum computing future powered by existing global telecommunications infrastructure.

Led by Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering and Applied Physics at SEAS, the team created a microwave-optical quantum transducer. This device is intended for use with quantum systems that operate using superconducting microwave qubits — nanoscale circuits that store quantum information similarly to how classical bits store 1s and 0s. The research appears in Nature Physics.

This new transducer essentially acts as a photon router, bridging the significant energy gap between microwave and optical photons. This breakthrough allows microwave qubits to be controlled by optical signals transmitted over long distances. Notably, it is the first device of its kind to demonstrate that a superconducting qubit can be manipulated using only light.

Hana Warner, a graduate student and first author of the paper, emphasized the potential of optics in quantum system design. “Building practical quantum networks requires scalable ways to interface various components,” Warner said. “Optical photons are ideal for this — they travel far with low loss and offer high bandwidth.”

Superconducting qubits are gaining traction as a promising platform for quantum computing due to their scalability, compatibility with current manufacturing technologies, and ability to maintain quantum states long enough for processing. However, they require extremely cold environments to function, using large and complex dilution refrigerators. This makes scaling microwave-only systems difficult. A more feasible approach would be to perform quantum operations using microwave qubits but connect and control them using optical photons.

The Harvard team’s compact device is only 2 millimeters long and shaped like a paperclip, resting on a chip about 2 centimeters wide. It links a microwave resonator to two optical resonators, allowing energy to move back and forth between them. This energy exchange, made possible by the unique properties of lithium niobate, eliminates the need for large, heat-generating microwave cables to control qubits.

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Beyond control, the same device could also read qubit states or convert fragile quantum data into robust light signals that can travel across optical fiber networks. This development marks a significant step toward superconducting quantum processors connected by low-loss, high-bandwidth optical systems.

“The next big step for this transducer would be enabling the reliable generation and distribution of entanglement between microwave qubits using light,” Lončar said.

The project brought together optical system experts at Harvard and quantum computing collaborators at Rigetti Computing, who provided the superconducting qubit platform for testing. Additional contributions came from researchers at the University of Chicago and the Massachusetts Institute of Technology.