Metal-oxide-semiconductor junctions are central to most electronic and optoelectronic devices. Here, the element-specificity of broadband extreme ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and recombination kinetics in each layer of a Ni-TiO 2 -Si junction. After photoexcitation of silicon, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in spectral shifts in the Ni M 2,3 XUV edge that are characteristic of holes and the absence of holes initially in TiO 2 . Meanwhile, the electrons are observed to remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO 2 , shifting the Ti spectrum to higher oxidation state, followed by electron-hole recombination at the Si-TiO 2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO 2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.

Wavelengths in the telecommunication window (∼ 1.25 − 1.65 µm) are ideal for quantum communication due to low transmission loss in fiber networks. To realize quantum networks operating at these wavelengths, long-lived quantum memories that couple to telecom-band photons with high efficiency need to be developed. We propose coupling neutral ytterbium atoms, which have a strong telecom-wavelength transition, to a silicon photonic crystal cavity. Specifically, we consider the 3 P0 ↔ 3 D1 transition in neutral 171 Yb to interface its long-lived nuclear spin in the metastable 3 P0 'clock' state with a telecom-band photon at 1.4 µm. We show that Yb atoms can be trapped using a short wavelength (≈ 470 nm) tweezer at a distance of 350 nm from the silicon photonic crystal cavity. At this distance, due to the slowly decaying evanescent cavity field at a longer wavelength, we obtain a single-photon Rabi frequency of g/2π ≈ 100 MHz and a cooperativity of C ≈ 47 while maintaining a high photon collection efficiency into a single mode fiber. The combination of high system efficiency, telecom-band operation, and long coherence times makes this platform well suited for quantum optics on a silicon chip and long-distance quantum communication.

Optimisation problems typically involve finding the ground state (i.e. the minimum energy configuration) of a cost function with respect to many variables. If the variables are corrupted by noise then this maximises the likelihood that the solution is correct. The maximum entropy solution on the other hand takes the form of a Boltzmann distribution over the ground and excited states of the cost function to correct for noise. Here we use a programmable annealer for the information decoding problem which we simulate as a random Ising model in a field. We show experimentally that finite temperature maximum entropy decoding can give slightly better bit-error-rates than the maximum likelihood approach, confirming that useful information can be extracted from the excited states of the annealer. Furthermore we introduce a bit-by-bit analytical method which is agnostic to the specific application and use it to show that the annealer samples from a highly Boltzmann-like distribution. Machines of this kind are therefore candidates for use in a variety of machine learning applications which exploit maximum entropy inference, including language processing and image recognition.

Entangled photons exhibit non-classical light–matter interactions that create new opportunities in materials and molecular science.