A new quantum computer breakthrough has been reported by Harvard physicists using a new photon router, a powerful optical interface for quantum networks that require low-loss and low-noise transmission of entangled states.
The work of the physicists from Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) led by Larko Lončar paves the way for using the existing telecommunications infrastructure to distribute modular quantum computing networks. Currently, millions of miles of global fiber optic cable carry photon light pulses to power computer communications, which in the future may be harnessed for quantum computing.
Quantum Transducer for Optical Communications
The team’s microwave-optical quantum transducer allows local computers operating on binary microwave qubits to communicate across long distances, translating the microwave energy into optical photons that can pass through fiber optic cables.
In the past, superconducting microwave qubits have never been controlled using only light. Now, during laboratory tests, the team successfully recorded a 1.18% conversion efficiency with low added microwave noise when operating the photon router.
Rigetti Computing partnered on the project by providing the superconducting qubit platform the SEAS team used to test their device. University of Chicago and Massachusetts Institute of Technology researchers also worked on the project. The team fabricated the chips used during the research in-house at Harvard using the university’s Center for Nanoscale Systems.
Microwave Quantum Computing
Using microwave superconducting qubits offers some significant advantages as a quantum solution, including scalability, stability, and compatibility with existing manufacturing processes. However, microwave qubits have the major drawback of requiring large cooling systems for the nanofabricated circuits to maintain their extremely low operating temperatures.
When systems require many of these circuits to run, the amount of refrigeration equipment required quickly becomes unmanageable. Scaling microwave signals becomes a major bottleneck, which optical photon communications may finally clear. Additionally, the higher energy of optical photons compared to microwaves makes them much less sensitive to thermal noise and fiber optics inability to effectively transfer thermal energy minimizes heat transfer in and out.
The Harvard team’s device is tiny—only two millimeters long—and sit on a two-centimeter chip. It’s lithium niobite base material allows a microwave resonator and two optical resonators to exchange energy. This exchange allowed for miniaturization from much larger microwave cables that normally control qubit states. In addition to the practical advantages of a smaller form factor resulting from not requiring exterior cooling, fiber optics mitigate the high signal loss and noise sensitivity of microwave communication.
Making Optical Networks A Real World Solution
“The realization of these systems is still a ways out, but in order to get there, we need to figure out practical ways to scale and interface with the different components,” said first author Hana Warner. “Optical photons are one of the best ways you can do that, because they’re very good carriers of information, with low loss, and high bandwidth.”
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The novel device aims to create powerful optical quantum networks. Beyond controlling qubits, the device could translate them into light for robust node communication.
“The next step for our transducer could be reliable generation and distribution of entanglement between microwave qubits using light,” Lončar said.
The paper “Coherent Control of a Superconducting Qubit Using Light” appeared on April 2, 2025 in Nature Physics.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted atryan@thedebrief.org, and follow him on Twitter@mdntwvlf.