Short answer: In a Rydberg superatom, strong interactions between highly excited atoms let a whole atomic ensemble act like a single two-level emitter, which can control light at the few-photon level. Experiments in 2014 and 2016 reported a single-photon switch and a single-photon transistor with gain above 100. These are laboratory demonstrations, not working photonic quantum computers.
What is a Rydberg superatom?
A review article in an IOP journal, published in 2023, explains the idea. Rydberg atoms are highly excited atoms, and the Rydberg blockade, resulting from the strong interaction between individual Rydberg atoms, can turn a large ensemble into a system that collectively resembles a single two-level emitter. The review says the coupling of this artificial emitter to a driving photonic mode is collectively enhanced by Rydberg interactions, enabling strong coherent coupling at the few-photon level in free space. It also draws a similarity to waveguide quantum electrodynamics with emitters coupled to photonic waveguides, and briefly reviews single-photon generation and single-photon subtraction.
What did the 2014 switch experiment do?
A 2014 Physical Review Letters paper by Simon Baur, Daniel Tiarks, Gerhard Rempe and Stephan Dürr, at the Max-Planck-Institut fuer Quantenoptik, took all-optical switching into the quantum regime. The gate pulse contains only one photon on average. It is stored as a Rydberg excitation in an ultracold atomic gas using electromagnetically induced transparency, and Rydberg blockade suppresses transmission of the subsequent target pulse. The stored gate photon can then be retrieved, and a retrieved photon heralds successful storage. For that postselected subensemble the authors report an extinction of 0.05. They describe the switch as offering perspectives from quantum communication to quantum information processing.
What did the transistor experiment add?
A Nature Communications paper reports that Stark-tuned Foerster resonances can substantially increase the effective interaction between individual photons, which are coupled to Rydberg atoms through electromagnetically induced transparency. The authors report a single-photon transistor gain of over 100, a fidelity beyond 0.8 for non-destructive detection of single Rydberg atoms, and high-precision spectroscopy on Rydberg pair states. They also report a gain larger than 2 with gate photon read-out after the transistor operation. Their theory models agree with the data and identify the main decoherence mechanism of the transistor. The authors describe this as paving the way towards photonic quantum gates, which is a direction and not an achieved gate.
How should this be read?
Switches and transistors for single photons are building blocks, and the abstracts describe them as steps. The numbers above are the authors' own, from specific experiments in ultracold atomic gases. The review's statement that Rydberg control has already shown utility for quantum computing and information processing is the reviewers' characterization. This post is based on abstracts and visible summary text, and the full papers were not checked. It makes no claim about products or deployments.
FAQ
What does the Rydberg blockade do?
It suppresses further excitation near a Rydberg atom, so an ensemble collectively resembles a single two-level emitter, according to the 2023 review.
How good was the 2014 switch?
The authors report an extinction of 0.05 in the postselected subensemble where a retrieved photon heralded storage.
What is the single-photon transistor gain?
The Nature Communications authors report a gain of over 100 using Stark-tuned Foerster resonances.
Is this a photonic quantum gate?
No. The authors describe their transistor as paving the way towards photonic quantum gates.
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