United States Air Force Research Laboratory: introduction to the focus issue.

This focus issue on the United States Air Force Research Laboratory (AFRL) spans the latest trends in imaging and detectors, atmospheric characterization, laser sources and propagation, optics and optical assemblies, optical characterization of materials, photonics, optical processing, and machine learning for applications that cover everything from stellar interferometry to studying damage to the plasma membranes of living cells.

Low connector-to-connector loss through silicon photonic chips using ultra-low loss splicing of SMF-28 to high numerical aperture fibers.

Here we present extremely low connector-to-connector loss (≤3 dB) through silicon photonic chips using ultra-low loss (≤0.15 dB) splicing between SMF-28 and ultra-high numerical aperture (UHNA) fibers. The small MFD from the UHNA fibers enables strong coupling to hybrid TE/TM edge couplers achieving TM (TE) losses of 1.25 (2.35) dB per coupler and low polarization-dependent loss. Mode coupling simulations and tolerance are investigated to understand performance.

Quantifying entanglement in a 68-billion-dimensional quantum state space.

Entanglement is the powerful and enigmatic resource central to quantum information processing, which promises capabilities in computing, simulation, secure communication, and metrology beyond what is possible for classical devices. Exactly quantifying the entanglement of an unknown system requires completely determining its quantum state, a task which demands an intractable number of measurements even for modestly-sized systems. Here we demonstrate a method for rigorously quantifying high-dimensional entanglement from extremely limited data. We improve an entropic, quantitative entanglement witness to operate directly on compressed experimental data acquired via an adaptive, multilevel sampling procedure. Only 6,456 measurements are needed to certify an entanglement-of-formation of 7.11 ± .04 ebits shared by two spatially-entangled photons. With a Hilbert space exceeding 68 billion dimensions, we need 20-million-times fewer measurements than the uncompressed approach and 1018-times fewer measurements than tomography. Our technique offers a universal method for quantifying entanglement in any large quantum system shared by two parties.

Aluminum nitride integrated photonics platform for the ultraviolet to visible spectrum.

We demonstrate a wide-bandgap semiconductor photonics platform based on nanocrystalline aluminum nitride (AlN) on sapphire. This photonics platform guides light at low loss from the ultraviolet (UV) to the visible spectrum. We measure ring resonators with intrinsic quality factor (Q) exceeding 170,000 at 638 nm and Q >20,000 down to 369.5 nm, which shows a promising path for low-loss integrated photonics in UV and visible spectrum. This platform opens up new possibilities in integrated quantum optics with trapped ions or atom-like color centers in solids, as well as classical applications including nonlinear optics and on-chip UV-spectroscopy.

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.