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.
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