A light has emerged at the end of the tunnel in the long pursuit of developing quantum computers, which are expected to radically reduce the time needed to perform some complex calculations from thousands of years down to a matter of hours.
A team led by Stanford physicists has developed a new type of "optical cavity" that can efficiently collect single photons, the fundamental particle of light, from single atoms. These atoms act as the building blocks of a quantum computer by storing "qubits" – the quantum version of a normal computer's bits of zeros and ones. This work enables that process for all qubits simultaneously, for the first time.
In a study published in Nature , the researchers describe an array of 40 cavities containing 40 individual atom qubits as well as a prototype with more than 500 cavities. The findings indicate a way to ultimately create a million-qubit quantum computer network.
"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."
An optical cavity is created when two or more reflective surfaces cause light to bounce back and forth, as happens when a person steps between a set of fun house mirrors and sees thousands of their image repeated into the distance. Unlike the fun house, these optical cavities are much smaller and use the many bounces of a laser beam to get extra visual information from atoms. Researchers have been experimenting with optical cavities for decades, trying to get enough light to bounce back and forth enough times to interact with the tiny, nearly translucent atoms.
The research team led by Simon's lab took a different approach and used microlenses inside each cavity to focus the light more tightly on a single atom. This creates fewer bounces of light but is still more effective at getting 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
While conventional computers use a series of zeros and ones to represent bits of information, quantum computers rely on quantum states of small particles. These qubits can be zero or one, or a combination of both at the same time. This allows a quantum computer to complete certain complex calculations much more quickly than the traditional binary model.
"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."
Researchers estimate a quantum computer will require millions of qubits to outperform classical supercomputers. Reaching that number will likely mean networking many quantum computers together, Simon said. The achievement in this study of using cavities to create a parallel interface makes a highly efficient platform for scaling to these large sizes.
The team demonstrated a 40-cavity array containing atoms in this paper with a proof-of-concept array with more than 500, and the researchers are aiming toward tens of thousands. Looking ahead, they imagine quantum data centers where individual quantum computers each have a network interface consisting of a cavity array, enabling large-scale integration into quantum supercomputers.
Reaching that goal will require solving some major engineering challenges, but the potential is there, the researchers contend, and with it, all the promise of quantum computing. This could mean major advances in materials design and chemical synthesis, such as that used for drug discovery, as well as in code breaking. More broadly, the light-collection capabilities of the cavity arrays hold great promise for biosensing and microscopy, which could advance medical and biological research. Quantum networks could even help better understand space, by enabling optical telescopes with enhanced resolution that would allow direct observation of planets outside 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.
