Spectral Diffusion in Solid-State Quantum Emitters

Short answer: Spectral diffusion is the random jumping of an emitter's wavelength, and it broadens the emission lines of solid-state single-photon sources. Papers from 2013 and 2015 studied it in nitrogen-vacancy centres in nanodiamonds and in gallium nitride nanowire quantum dots, and a 2023 paper title concerns gallium nitride defect emitters. These are materials-physics studies of mechanisms, not device products.

What is spectral diffusion?

The 2013 Physical Review Letters abstract defines spectral diffusion as random jumps in the emission wavelength of narrow lines. The authors call it a major hurdle for applications of solid-state quantum emitters, such as quantum dots, molecules or diamond defect centres, in an integrated quantum optical technology. It matters because photons from the same source need to be spectrally indistinguishable.

What did the 2013 nanodiamond study do?

The authors study the zero-phonon line of single nitrogen-vacancy centres in nano-size diamond with a new method based on photon correlation interferometry. They say the method works even though the spectral diffusion rate is several orders of magnitude higher than the photon detection rate, and improves the time resolution of previous experiments with nano-size diamond by 6 orders of magnitude. They study how the diffusion rate depends on excitation power, temperature and excitation wavelength under off-resonant excitation. They say the results give insight into the mechanism and suggest a strategy to increase the number of spectrally indistinguishable photons emitted by diamond nanocrystals.

What did the 2015 nanowire study do?

A 2015 Physical Review B paper uses Fourier transform spectroscopy to determine and control the degree of spectral diffusion in high-quality site-controlled gallium nitride nanowire quantum dots. A statistical model and Monte Carlo simulations provide evidence, according to the authors, that the broadening is caused by photoinduced excitation of defects. Under weak excitation they estimate the homogeneous linewidth at 135 microelectronvolts, discuss its origins, infer the existence of an alternate dephasing mechanism, and discuss a limit on the emission linewidth at zero excitation.

What about the 2023 paper?

A 2023 Applied Physics Letters article is titled "Ultrafast spectral diffusion of GaN defect single photon emitters" and is marked as an Editor's Pick. The page we could read showed no abstract, so this post claims nothing about its method or results beyond the title.

What to keep in mind

The two readable studies concern different emitters, diamond defects and nitride quantum dots, with different excitation conditions, and their numbers cannot be compared. The 6 orders of magnitude figure is a time-resolution improvement over earlier nano-diamond experiments and not a linewidth result. This post is based on abstracts and visible summary text, and full texts were not checked. It makes no claim about products.

FAQ

Why does spectral diffusion hurt?

It makes the emission wavelength jump, which the 2013 authors call a major hurdle for integrated quantum optics because photons become less indistinguishable.

What causes it in the nanowire dots?

The 2015 authors report evidence that photoinduced excitation of defects causes the broadening.

How was it measured in nanodiamonds?

With photon correlation interferometry, which the authors say improved time resolution by 6 orders of magnitude.

Can it be reduced?

The 2013 abstract says the results suggest a strategy to increase the number of indistinguishable photons. It does not report that it was achieved.