Zero Spin Silicon, page-94

Further to my previous post, I have been doing some more research on this!

Seems to me that they will be using, isotopically pure planar silicon-on-insulator (SOI) wafers? that technology came from Translucent Inc and was sold to IQE? and are they talking about laser cooling also using ex Translucent Inc technology as well?

https://aip.scitation.org/doi/10.1063/5.0049372

1. Isotopically pure Si and SOI

In order to realize a large spin coherence time, it is important to eliminate the magnetic noise from neighboring crystal lattice defects as much as possible. It is therefore fortunate that silicon can be grown with a high degree of isotopic purity. Natural silicon is comprised of three stable isotopes: 28Si, 29Si, and 30Si with natural isotopic abundances of 92.22%, 4.69%, and 3.09%, respectively. Besides their different masses, these three isotopes also differ in their nuclear spin: 28Si and 30Si are spin-free, while 29Si has a nuclear spin of 1/2
As a result, the physical properties of a Si crystal can differ greatly depending on its isotopic composition. Enriched 28Si has a thermal conductivity up to ten times higher than natural Si. With regard to spin-photon interfaces, isotopically pure 28Si offers two major advantages over natural silicon.
First, inhomogeneous broadening of optical transitions is essentially eliminated due to the absence of local variations in the bandgap and binding energy.93,138–140 High-resolution photoluminescence excitation spectra of T-center ensembles in a 28Si host imply a residual inhomogeneous broadening of <0.14 μeV (<33 MHz).93 For single emitters, this can lead to optical linewidths limited only by their excited state lifetime. Hyperfine-resolved optical transitions can be observed—a feature that had been once reserved for isolated atoms and ions in vacuum. It should be noted that in nanophotonic devices, fabrication-induced strain, surface charges, or co-doping for charge carrier compensation are likely to increase the inhomogeneity, at least at the prototype stage. The extent to which emitters in Si will be subject to these additional broadening mechanisms will need to be determined in future studies. In this context, it should be noted that the inhomogeneity and spectral diffusion of NV centers in micrometer-thin etched diamond membranes has gradually been improved to values <0.41 μeV (<100 MHz), which suggests that for optimized fabrication of 28Si structures, even less inhomogeneous broadening might be expected.
Second, due to the absence of nuclear spins in 28Si, a main source of spin decoherence is no longer present. Isolated in such a semiconductor vacuum, some spin qubits have demonstrated coherence times exceeding several hours. Note that these experiments were conducted on the nuclear spin of the singly ionized P, which is not optically addressable.
Most of the 28Si studied for fundamental academic or metrologic purposes originates from the International Avogadro Project.
These bulk crystals are of no use for integrated photonics; however, there has been significant recent progress in growing the isotopically pure 28Si thin films required to implement integrated photonic components. UHV-compatible direct beam Penning ion sources have been used to create films with isotopic purity beyond 99.9998% 28Si, and mono-crystalline isotopically purified epilayers have been grown by chemical vapor deposition (CVD) on 300 mm substrates in the state-of-the-art industrial CMOS foundries using isotopically enriched silane (28SiH4).

integrated silicon photonic platform that assumes the availability of isotopically pure planar silicon-on-insulator (SOI) wafers

however the thermo-optic coefficient of silicon at cryogenic temperatures is negligible.
The deposition of a gas such as xenon onto a cold microcavity has successfully been used to tune λC in cryogenic conditions without drastically reducing Q.
However, this method by itself is not scalable as every cavity requires an individual amount of tuning, but gas condensation happens on a global scale. Individual tuning could be achieved in a subsequent step via local sublimation of the condensed gas (e.g., using a laser). Are they talking about laser cooling here?