Applied physicists at Harvard John A. Paulson School of Engineering and Applied Sciences have accomplished a notable feat in the realm of quantum computing. They have engineered a groundbreaking photon router capable of integrating with quantum networks, thereby providing robust optical interfaces that are essential for noise-sensitive microwave quantum computers. This innovation represents a vital advancement toward the eventual realization of modular, distributed quantum computing networks that can effectively utilize the existing infrastructure of telecommunications, particularly the extensive fiber-optic networks that span the globe.
Contemporary fiber-optic networks operate by facilitating the transmission of information via pulses of light, or photons. These networks are already capable of swiftly sending data across millions of miles, underpinning the digital communication backbone of our society. The team, led by Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering and Applied Physics at Harvard, has developed a microwave-optical quantum transducer. This device serves a crucial role in quantum processing systems, leveraging superconducting microwave qubits—akin to the classical bits represented as binary 1s and 0s.
The transducer operates by effectively acting as a router for photons, bridging the substantial energy gap that exists between microwave and optical photons. This feature allows for the control of microwave qubits utilizing optical signals generated from remote locations. Notably, this marks the first instance of successfully controlling a superconducting qubit solely through optical means, thus establishing a new pathway for the manipulation of quantum states.
Hana Warner, the first author of the paper and a graduate student engaged in this groundbreaking research, suggests that this transducer offers an avenue to harness the advantages of optics in the development of future quantum networks. She emphasizes the significance of scalability and the necessity of creating practical interfaces among various components in quantum computing. Optical photons, recognized as exceptional carriers of information due to their high bandwidth and low loss characteristics, stand at the forefront of this technological evolution.
Superconducting qubits are recognized as a developing platform within quantum computing, offering notable scalability and compatibility with existing manufacturing systems. Their ability to maintain quantum superposition, a critical state for performing calculations, makes them particularly appealing for future endeavors. However, the field faces significant hurdles, particularly regarding the extremely low operational temperatures required for superconducting microwave qubit platforms. This necessitates complex cooling systems, such as dilution refrigerators, that could pose challenges as the quantum computing landscape evolves to require millions of functional qubits.
Herein lies the promise of the newly developed transducer; it facilitates quantum operations via microwave qubits while utilizing optical photons as efficient and scalable interfaces, thus bypassing some of the limitations posed by traditional microwave-frequency signals. The optical device, measuring just 2 millimeters and resembling a paper clip, is integrated on a chip that approximately spans 2 centimeters. It achieves this functionality through the coupling of a microwave resonator with dual optical resonators, thereby enabling energy exchange that leverages the innate properties of its base material, lithium niobate.
The innovative approach adopted by the Harvard research team negates the need for cumbersome and hot microwave cables traditionally employed for controlling qubit states. Instead, the same devices that facilitate control could also serve to read out qubit states or form direct links that convert delicate quantum information into robust packets of light capable of transmitting between quantum computing nodes. This evolution in design thus brings the scientific community closer to a future where superconducting quantum processors are interconnected through high-powered optical networks characterized by minimal losses.
As an extension of this exciting development, Marko Lončar articulates hopes for the next phase of their research. This phase could focus on the reliable generation and distribution of entanglement among microwave qubits through the use of light, thus expanding the transformative potential of this technology. To achieve this, the Harvard team has synergized their optical expertise with collaborators from Rigetti Computing, leveraging their superconducting qubit platform to effectively test and map various experimental conditions for the transducer.
Fabrication of the advanced chips was executed at Harvard’s Center for Nanoscale Systems, part of the National Nanotechnology Coordinated Infrastructure Network. This initiative is backed by the National Science Foundation, further exemplifying the collaborative effort driving forward quantum technological advancements. Additional support has stemmed from a variety of funding sources, including various government research departments, emphasizing the broad interest in and potential impact of this work.
The implications of this research extend far beyond mere academic interest; they lay the groundwork for pivotal breakthroughs in the engineering of quantum computers and the development of future communication networks. The ability to control qubits optically and connect them through fiber optics could drastically enhance the processing power and reliability of quantum systems, thus preparing the way for a new era in computing that harnesses the peculiar properties of quantum mechanics.
This milestone embodies not only the ingenuity of the Harvard research team but also represents a significant stride towards overcoming the current limitations faced by quantum computing platforms. Researchers across the globe are watching closely as these advancements unfold, which could eventually culminate in new quantum technologies that redefine our understanding of computational power and information transmission.
As the network of quantum technologies continues to evolve, the cross-disciplinary approach of integrating optics with quantum systems will likely produce unforeseen innovations, establishing a robust framework for the future of quantum information science. The journey toward realizing a fully interconnected quantum network is just beginning, and with each breakthrough, the field becomes one step closer to unlocking the true potential of quantum computing.
In summary, the successful creation and implementation of the microwave-optical quantum transducer introduce an exciting prospect for future quantum computing architectures and communication systems. The research spearheaded by the team at Harvard is poised to alter the trajectory of quantum technology, making a lasting contribution that may one day lead to a global quantum network powered by unique optical interfaces.
Subject of Research: Microwave-optical quantum transducer
Article Title: Coherent control of a superconducting qubit using light
News Publication Date: 2-Apr-2025
Web References: Harvard SEAS
References: Nature Physics, DOI: 10.1038/s41567-025-02812-0
Image Credits: Credit: Lončar group / Harvard SEAS
Keywords: Quantum computing, superconducting qubits, optical transducer, quantum networks, microwave signals, optical fibers, entanglement, lithium niobate, Massachusetts Institute of Technology, Rigetti Computing, Harvard SEAS, quantum information science
