This innovation could pave the way for quantum computers that are not confined to a single location but instead operate as interconnected systems through optical fiber infrastructure already deployed across the globe. These networks, which transmit light pulses at high speeds, could serve as the backbone for distributed quantum systems.
The team, led by Marko Loncar, the Tiantsai Lin Professor of Electrical Engineering and Applied Physics, has built a microwave-optical quantum transducer capable of linking superconducting microwave qubits with optical signals. Their work, published in Nature Physics, marks the first demonstration of controlling a superconducting qubit using light alone.
"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 paper first author and graduate student 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."
Superconducting qubits are promising for quantum computing thanks to their scalability and compatibility with current fabrication methods. However, these devices require ultra-cold environments maintained by dilution refrigerators, creating barriers to broader deployment. Scaling up microwave-based systems becomes increasingly difficult without a more efficient method of interfacing with them.
The newly developed transducer aims to resolve this by allowing quantum operations to remain in the microwave domain while shifting communication and control to the optical domain, which is more scalable and efficient.
The device, which is about 2 millimeters long and shaped like a paper clip, is mounted on a 2-centimeter chip. It connects one microwave resonator and two optical resonators using lithium niobate, a material that supports energy transfer between microwave and optical modes. This setup removes the need for traditional microwave control lines, which can be bulky and thermally disruptive.
This same configuration can also be adapted for reading out qubit states or establishing links between distant quantum computing nodes, transforming fragile quantum information into stable light-based signals for transmission.
"The next step for our transducer could be reliable generation and distribution of entanglement between microwave qubits using light," said Loncar.
Harvard researchers collaborated with Rigetti Computing, which supplied the superconducting qubit systems used to evaluate the device, as well as partners at the University of Chicago and MIT.
Research Report:Coherent control of a superconducting qubit using light
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