Short answer: When a single quantum dot is placed in a very small, low-loss photonic crystal cavity, light and the dot's exciton can exchange energy faster than either decays, which is called strong coupling. Three papers from 2010 to 2020 report it in three cavity designs: a waveguide cavity with a vacuum Rabi splitting of about 140 micro-electronvolts, a nanobeam cavity with 226 micro-electronvolts, and an L4/3 cavity with a quality factor of 33,000. These are low-temperature laboratory demonstrations.
What strong coupling means here
Each paper shows an anticrossing, where the dot and cavity resonances repel instead of crossing as one is tuned toward the other. The size of the gap, the vacuum Rabi splitting, shows how strong the coupling is. The cavity must have a high quality factor, meaning low loss, and a small mode volume, meaning the light is concentrated in a tiny space.
2010: a cavity in a photonic crystal waveguide
The Applied Physics Letters abstract says cavities embedded in photonic crystal waveguides offer a promising route toward large-scale integration of coupled resonators for quantum electrodynamics. The authors demonstrate a strongly coupled system formed by a single quantum dot and such a cavity. They identify the cavity resonance from photoluminescence mapping of the out-of-plane scattered signal along the waveguide. The dot's exciton is tuned toward the cavity mode by temperature control, and they observe a vacuum Rabi splitting of about 140 micro-electronvolts at resonance.
2011: a nanobeam cavity
A second Applied Physics Letters paper demonstrates strong coupling between a one-dimensional photonic crystal nanobeam cavity and a single quantum dot. The cavity has a quality factor of about 25,000 and a small mode volume. The authors observe an anticrossing with a vacuum Rabi splitting of 226 micro-electronvolts. They estimate the ratio of the dot-cavity coupling strength to the cavity decay rate, a figure of merit for quantum optical applications, at 2.1, which they say is the highest value among quantum-dot-based cavity quantum electrodynamics systems reported so far. That superlative was valid as of their 2011 paper and may not hold now.
2020: the L4/3 cavity
The Applied Physics Express abstract reports strong coupling between a single quantum dot and a GaAs-based L4/3-type photonic crystal nanocavity. The authors say the design supports a high theoretical quality factor of about 8 million, a small mode volume, and an electric field maximum lying inside the host dielectric, which helps coupling with a dot. They fabricated the cavities and observed a quality factor over 80,000 by photoluminescence. They confirmed strong coupling with a quality factor of 33,000 by observing a clear anticrossing in the spectra. The gap between the theoretical and measured quality factors is the authors' own figure.
Limits
The three papers use different cavity designs and quote different figures, so the splittings should not be ranked across them, particularly since the 2020 abstract reports quality factors and not a splitting. The abstracts describe strong coupling as a physics result, not a working single-photon source or quantum gate. These experiments are performed at low temperature, since the dot exciton is tuned by temperature control.
FAQ
What is a vacuum Rabi splitting?
The gap between two coupled resonances that appears when a dot and a cavity exchange energy faster than they lose it.
Which cavity gave the largest splitting?
The 2011 nanobeam paper reports 226 micro-electronvolts, against about 140 in the 2010 waveguide cavity.
Why does cavity quality matter?
A higher quality factor means lower loss, which is needed to reach strong coupling.
Are these single-photon sources?
The abstracts report strong coupling, not device performance as a source.
How was the dot tuned into resonance?
By temperature control in the 2010 experiment.
All field notes