Single-photon switches, which can turn physical processes on or off by using only a single packet of light, have far-reaching implications for quantum photonic technologies - and a new breakthrough makes it one step closer to realization.

Having a single-photon switch would far surpass any existing switching technologies. It could open new avenues in scalability or the potential for smaller devices or increased chip density for faster electronics. A new study from the City College of New York, led by physicist Vinod Menon, demonstrates the use of "Rydberg states" among solid-state materials. While the Rydberg states - electronically excited atoms or molecules following the Rydberg formula - have been observed in cold atom gases before, this is the first time it has been observed in solid-state materials. Researchers used the phenomenon to enhance nonlinear optical interactions to previously unachievable levels - marking the first step toward making chip-scale scalable single-photon switches possible.

Researchers present their study in the latest Nature Communications journal in a report titled "Enhanced nonlinear interaction of polaritons via excitonic Rydberg states in monolayer WSe2."

Cavity exciton polariton device
(Photo: Konstantinos Lagoudakis via Wikimedia Commons)
A diagram for a cavity exciton-polariton device, which generates an exciton-polariton.

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Exploiting the Rydberg States for Optical Interaction Applications

In solid-state systems, an exciton-polariton is a type of polariton, a hybrid between a light and a matter quasiparticle that is generated as a result of the strong coupling between the electromagnetic oscillations of both photons and excitons, or electronic excitations. These hybrid quasiparticles have become candidates for achieving nonlinearities approaching the quantum limit.

Vinod Menon, who also serves as the City College's Division of Science physics chair, explains in a university news release that their work demonstrates exciton-polaritons created using Rydberg excitons within 2D materials, or "atomically thin semiconductors." Their study was able to show that because of their larger sizes, excitons in their excited states demonstrate increased interactions, making them promising candidates for attaining the quantum domain of single-photon nonlinearities, consistent with Rydberg state behaviors in atomic systems.

Additionally, the demonstration of Rydberg realized exciton-polariton in the 2D semiconductors, and its nonlinear response opens the possibility of generating strong photon interactions for solid-state systems, which would lead to the next generation of quantum photonic technologies.

The team also includes Jie Gu, first author and graduate student under Menon's supervision, and members from the universities of Stanford, Aarhus, Columbia, and Montreal Polytechnic.

Overcoming the Scalability Limits in Computing

One of the largest hurdles towards developing faster, stronger computers is the physical limitations on how small we can make our electronic devices. Since the semiconductor transistor revolutionized the world a little over four decades ago, its development has been largely guided by the so-called "Moore's Law," which predicts the trend at which speed and processing power will grow about every two years.

It led researchers to explore other possibilities, one being quantum computers. Whereas the traditional computer stores data in sets of bits - with each unit having a value of one or zero - a quantum bit (or qubit) could store data in values from one, zero, or anywhere in between thanks to a phenomenon called superposition.


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