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Many next-generation quantum technologies, such as ultra-secure quantum communications and possibly game-changing quantum computers, rely on photons.

Because these light particles may be entangled or placed in a superposition—two quantum states that enable quantum technologies—they can be entangled or placed in a superposition.

However, in order to achieve these states, scientists must deal with very non-classical types of light with a limited number of photons, or even only one. Because conventional sources of light (such as a laser) produce situations where there is always the chance of a large number of photons, this may be a challenging undertaking requiring a sophisticated setup.

Theorists at the University of Chicago's Pritzker School of Molecular Engineering (PME) have devised a new method for trapping single photons in a cavity. Their approach permits two sources to emit a certain quantity of photons into a cavity before destructive interference cancels out both sources, thereby forming a "wall" that stops any further photons from entering.

This novel technique might make it easier to generate quantum light without the need for the complex materials and systems that are often required.

Prof. Aashish Clerk and graduate students Andrew Lingenfelter and David Roberts conducted the study, which was published in Science Advances on Nov. 26.

Creating an 'interference wall'

Typical methods for trapping single photons in a cavity use materials with a high optical nonlinearity, which drives photons in the cavity to interact intensely with one another. By adding just one photon, the cavity's resonance frequency may be substantially altered in such systems. When a laser is shone into the cavity, one photon can enter but not a second (because of the frequency shift produced by the first photon).

The issue with this process is that it necessitates extremely high optical nonlinearities and very low dissipation, which are difficult, if not impossible, to obtain in most systems.

Clerk's research team proposes a method that employs two distinct sources to concurrently emit photons into a cavity with a very mild nonlinearity (far too weak for conventional approaches to work). Once the desired number of photons are caught in the cavity, these sources cancel each other out with destructive interference, forming a "wall" that blocks photons.

There are several applications that might be used. Using destructive interference in this way eliminates the need for specific optically nonlinear materials, allowing the system to be used on a variety of platforms, including quantum simulation.

Not only can the fundamental principle be applied to visible light, but it may also be used for other types of electromagnetic radiation. Using it to create and regulate microwave-frequency photons in a superconducting circuit is one fascinating potential. This might open up new possibilities for quantum data storage and processing. Clerk's group is currently collaborating with experimentalists to put this plan into action.

He and his team are also looking at using the technique to entangle photons, in which observing one photon automatically conveys information about the photon it is entangled with, regardless of how far apart they are.

"We think this scheme could work in a lot of different systems," Clerk said. "If you don't need special materials, it really expands the potential of light-based quantum technologies."