After years of slow progress, researchers may finally be seeing a clear path forward in the quest to build powerful quantum computers. These machines are expected to dramatically shorten the time required for certain calculations, turning problems that would take classical computers thousands of years into tasks that could be completed in hours.
A team led by physicists at Stanford University has developed a new kind of optical cavity that can efficiently capture single photons, the basic particles of light, emitted by individual atoms. Those atoms serve as the core components of a quantum computer because they store qubits, which are the quantum equivalent of the zeros and ones used in traditional computing. For the first time, this approach allows information to be collected from all qubits at once.
Optical Cavities Enable Faster Qubit Readout
In research published in Nature, the team describes a system made up of 40 optical cavities, each holding a single atom qubit, along with a larger prototype that contains more than 500 cavities. The results point to a realistic route toward building quantum computing networks that could one day include as many as a million qubits.
“If we want to make a quantum computer, we need to be able to read information out of the quantum bits very quickly,” said Jon Simon, the study’s senior author and associate professor of physics and of applied physics in Stanford’s School of Humanities and Sciences. “Until now, there hasn’t been a practical way to do that at scale because atoms just don’t emit light fast enough, and on top of that, they spew it out in all directions. An optical cavity can efficiently guide emitted light toward a particular direction, and now we’ve found a way to equip each atom in a quantum computer within its own individual cavity.”
How Optical Cavities Control Light
An optical cavity works by trapping light between two or more reflective surfaces, causing it to bounce back and forth. The effect can be compared to standing between mirrors in a fun house, where reflections seem to stretch endlessly into the distance. In scientific settings, these cavities are far smaller and use repeated passes of a laser beam to extract information from atoms.
Although optical cavities have been studied for decades, they have been difficult to use with atoms because atoms are extremely small and nearly transparent. Getting light to interact with them strongly enough has been a persistent challenge.
A New Design Using Microlenses
Rather than relying on many repeated reflections, the Stanford team introduced microlenses inside each cavity to tightly focus light onto a single atom. Even with fewer light bounces, this method proved more effective at pulling quantum information from the atom.
“We have developed a new type of cavity architecture; it’s not just two mirrors anymore,” said Adam Shaw, a Stanford Science Fellow and first author on the study. “We hope this will enable us to build dramatically faster, distributed quantum computers that can talk to each other with much faster data rates.”
Beyond the Binary Limits of Classical Computing
Conventional computers process information using bits that represent either zero or one. Quantum computers operate using qubits, which are based on the quantum states of tiny particles. A qubit can represent zero, one, or both states at the same time, allowing quantum systems to handle certain calculations far more efficiently than classical machines.
“A classical computer has to churn through possibilities one by one, looking for the correct answer,” said Simon. “But a quantum computer acts like noise-canceling headphones that compare combinations of answers, amplifying the right ones while muffling the wrong ones.”
Scaling Toward Quantum Supercomputers
Scientists estimate that quantum computers will need millions of qubits to outperform today’s most powerful supercomputers. According to Simon, reaching that level will likely require connecting many quantum computers into large networks. The parallel light-based interface demonstrated in this study provides an efficient foundation for scaling up to those sizes.
The researchers showed a working 40-cavity array in the current study, along with a proof-of-concept system containing more than 500 cavities. Their next goal is to expand to tens of thousands. Looking further ahead, the team envisions quantum data centers in which individual quantum computers are linked through cavity-based network interfaces to form full-scale quantum supercomputers.
Broader Scientific and Technological Impact
Significant engineering hurdles remain, but the researchers believe the potential benefits are substantial. Large-scale quantum computers could lead to breakthroughs in materials design and chemical synthesis, including applications related to drug discovery, as well as advances in code breaking.
The ability to efficiently collect light also has implications beyond computing. Cavity arrays could improve biosensing and microscopy, supporting progress in medical and biological research. Quantum networks may even contribute to astronomy by enabling optical telescopes with enhanced resolution, potentially allowing scientists to directly observe planets orbiting stars beyond our solar system.
“As we understand more about how to manipulate light at a single particle level, I think it will transform our ability to see the world,” Shaw said.
Simon is also the Joan Reinhart Professor of Physics & Applied Physics. Shaw is also a Felix Bloch Fellow and an Urbanek-Chodorow Fellow.
Additional Stanford co-authors include David Schuster, the Joan Reinhart Professor of Applied Physics, and doctoral students Anna Soper, Danial Shadmany, and Da-Yeon Koh.
Other co-authors include researchers from Stony Brook University, the University of Chicago, Harvard University, and Montana State University.
This research received support from the National Science Foundation, Air Force Office of Scientific Research, Army Research Office, Hertz Foundation, and the U.S. Department of Defense.
Matt Jaffe of Montana State University and Simon act as consultants to and hold stock options in Atom Computing. Shadmany, Jaffe, Schuster, and Simon, as well as Aishwarya Kumar of Stony Brook, hold a patent on the resonator geometry demonstrated in this work.