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| Physicists have found a way to store photons in an ultracold atomic gas and make them interact. |
Photons — particles of light — normally pass right through each other without any interaction. That's why you can cross two laser beams and nothing happens at the crossing point. But making photons interact with one another is one of the holy grails of quantum information science, because it would allow for the construction of all-optical logic gates and quantum computing devices that use photons instead of electrons. A new experiment has brought that goal meaningfully closer.
A team of physicists has demonstrated that by storing individual photons inside a cloud of ultracold rubidium atoms — held in an optical trap — they can make those photons interact strongly with one another. The trick involves using Rydberg states: highly excited atomic energy levels in which the electrons are very far from the nucleus. Rydberg atoms have enormous interaction radii, meaning one excited atom can influence its neighbors over long distances.
How the Photon Storage Works
When a photon enters the atom cloud, it can be converted into an excitation of the atomic gas — a quasiparticle called a "dark-state polariton" — through a process called electromagnetically induced transparency (EIT). In this state, the photon is essentially stored in the atoms. When the Rydberg level is involved, an excitation in one part of the cloud blocks any other photon from being stored nearby, because the Rydberg interaction shifts the energy levels in surrounding atoms out of resonance. This is called the Rydberg blockade.
The result is that photons stored in the cloud effectively push each other away — they interact repulsively. When the photons are released, they emerge having exchanged a quantum phase with one another — exactly the kind of controlled interaction needed to build photon-based logic gates.
Why This Matters for Quantum Computing
All-photon quantum computing has some attractive properties: photons travel fast, carry quantum information at room temperature, and don't easily lose their quantum state to the environment. The main obstacle has always been that photons don't naturally talk to each other. This experiment shows that using Rydberg-atom intermediaries, you can make them do exactly that. It's a significant step toward photonic quantum gates that might one day form the basis of practical quantum computers or ultra-secure quantum communication networks.
Source: Physics World
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