Operation speed and coherence time are two core measures for the viability of a qubit. Strong spin-orbit interaction (SOI) and relatively weak hyperfine interaction make holes in germanium (Ge) intriguing candidates for spin qubits with rapid, all-electrical coherent control. Here we report ultrafast single-spin manipulation in a hole-based double quantum dot in a germanium hut wire (GHW). Mediated by the strong SOI, a Rabi frequency exceeding 540 MHz is observed at a magnetic field of 100 mT, setting a record for ultrafast spin qubit control in semiconductor systems. We demonstrate that the strong SOI of heavy holes (HHs) in our GHW, characterized by a very short spin-orbit length of 1.5 nm, enables the rapid gate operations we accomplish. Our results demonstrate the potential of ultrafast coherent control of hole spin qubits to meet the requirement of DiVincenzo’s criteria for a scalable quantum information processor.

Compact fiber-to-chip light coupling with low loss and large bandwidth, SMF-to-chip edge coupler in particular, is extensively demanded in integrated photonics. The inescapable challenge of edge coupler is the difficulty and complexity in fabrication and packaging. During the past decades, metamaterials have manifested marvelous talent in integrated photonics. Here, we experimentally demonstrate an ultracompact edge coupler via metamaterial for SMF with a mode field diameter of 10 μm, which is fully based on silicon-on-insulator material and a CMOS compatible fabrication process. In this work, we theoretically analyze the coupling performance and the fabrication difficulty. The experimental results show that this metamaterial-based coupler possesses low coupling loss and broad bandwidth simultaneously with the coupling length of only 90 μm. At 1550 nm, the coupling losses are 2.22/2.53 dB/facet for the fundamental TE/TM mode, while the minimum average loss could reach 1.81 dB/facet. The measured bandwidth with a loss below 3 dB is as broad as 120 nm, covering the entire C/L band. Moreover, this prominently eased fabrication process potentially exhibits significant superiority in both research and industrial applications.

Silicon photonic integration has gained great success in many application fields owing to the excellent optical device properties and complementary metal-oxide semiconductor (CMOS) compatibility. Realizing monolithic integration of III-V lasers and silicon photonic components on single silicon wafer is recognized as a long-standing obstacle for ultra-dense photonic integration, which can provide considerable economical, energy-efficient and foundry-scalable on-chip light sources, that has not been reported yet. Here, we demonstrate embedded InAs/GaAs quantum dot (QD) lasers directly grown on trenched silicon-on-insulator (SOI) substrate, enabling monolithic integration with butt-coupled silicon waveguides. By utilizing the patterned grating structures inside pre-defined SOI trenches and unique epitaxial method via hybrid molecular beam epitaxy (MBE), high-performance embedded InAs QD lasers with monolithically out-coupled silicon waveguide are achieved on such template. By resolving the epitaxy and fabrication challenges in such monolithic integrated architecture, embedded III-V lasers on SOI with continuous-wave lasing up to 85 °C are obtained. The maximum output power of 6.8 mW can be measured from the end tip of the butt-coupled silicon waveguides, with estimated coupling efficiency of approximately -6.7 dB. The results presented here provide a scalable and low-cost epitaxial method for the realization of on-chip light sources directly coupling to the silicon photonic components for future high-density photonic integration.

Using inelastic cotunneling spectroscopy we observe a zero field splitting within the spin triplet manifold of Ge hut wire quantum dots. The states with spin ±1 in the confinement direction are energetically favored by up to 55 μeV compared to the spin 0 triplet state because of the strong spin–orbit coupling. The reported effect should be observable in a broad class of strongly confined hole quantum-dot systems and might need to be considered when operating hole spin qubits.

Quantum dot lasers on silicon have gained significant interest over the past decade due to their great potential as an on-chip silicon photonic light source. Here, we demonstrate multi-wavelength injection locking of InAs/GaAs quantum dot Fabry–Perot (FP) lasers both on GaAs and silicon substrates by optical self-injection via an external cavity. The number of locked laser modes can be adjusted from a single peak to multiple peaks by tuning wavelength dependent phase and mode spacing of back-injected light through a Lyot filter. The multi-wavelength injection locked laser modes exhibit average optical linewidth of ∼ 20    kHz , which are narrowed by approximately three orders of magnitude from their free-running condition. Furthermore, multi-wavelength self-injection locking via an external cavity exhibits flat-top optical spectral properties with approximately 30 stably locked channels under stable operation over time, where the frequency detuning is less than 700 MHz within 40 min. Particularly, FP lasers by direct epitaxial growth on silicon substrates are self-injection locked as a flat-top comb source with tunable free spectral range from approximately 25 to 700 GHz. The reported results emphasize the great potential of multi-wavelength injection locked lasers as tunable on-chip multi-wavelength light sources.

In recent years, GaSb-on-Si direct heteroepitaxy has been highly desirable to extend the operating wavelength range into mid-infrared and high-mobility applications, such as free-space communications, gas sensing, and hyperspectral imaging. High-quality GaSb films on Si remain challenging due to the high density of defects generated during the growth. For this purpose, epitaxial GaSb films were grown by molecular beam epitaxy on on-axis Si(001). Due to the large lattice mismatch (12.2%) between GaSb and Si, here, we proposed a radical design and growth strategy with the primary objective of achieving the annihilation of antiphase boundaries (APBs) and the reduction of threading dislocation density (TDD). Benefitting from a V-grooved Si hollow structure, we demonstrated the growth of emerging-APB-free GaSb film on Si(001) with low mosaicity. Moreover, by introducing InGaSb/GaSb dislocation filtering layers, the atomically flat surface root mean square roughness is improved to 0.34 (on Si) and 0.14 nm (on GaAs/Si). Moreover, the corresponding TDD can be reduced to 3.5 × 107 and 2 × 107 cm−2, respectively, one order of magnitude lower than the minimum value found in the literature. These reported results are a powerful lever to improve the overall quality of epitaxial Si-based antimonide, which is of high interest for various devices and critical applications, such as laser diodes, photo-detectors, and solar cells.

Direct epitaxial growth of III-V quantum dot (QD) lasers on Si (001) substrates is recognized as a promising and low-cost method for realizing high-performance on-chip light sources in silicon photonic integrated circuits (PICs). Recently, the CMOS-compatible patterned Si (001) substrates with sawtooth structures have been widely implemented to suppress the lattice mismatch induced defects and antiphase boundaries (APBs) for heteroepitaxial growth of high-quality III-V materials on Si. Considerable progresses have been made on high-performance 1300 nm InAs/GaAs QD lasers on Si (001). Here, we report a thermal stress-relaxed (111)-faceted silicon hollow structures by homoepitaxial method for reliable InAs/GaAs QD lasers growing on Si (001) substrates. Both simulation analysis and experimental results indicate that the voids buried below the sawtooth structures can release about 9% of the accumulative thermal stress of the III-V/Si system during the cooling process. Furthermore, electrically pumped InAs/GaAs QD narrow ridge lasers are grown and fabricated on the specially designed Si (001) platforms with a maximum operation temperature up to 90 ℃ under continuous-wave (CW) operation mode. Additionally, an extrapolated lifetime of over 5300 hours is calculated from the reliability test at 65 ℃. These results lead toward high-yield, scalable, and reliable III-V lasers on Si (001) substrates for PICs.

Hole spin qubits based on germanium (Ge) have strong tunable spin−orbit interaction (SOI) and ultrafast qubit operation speed. Here we report that the Rabi frequency (f Rabi ) of a hole spin qubit in a Ge hut wire (HW) double quantum dot (DQD) is electrically tuned through the detuning energy (ϵ) and middle gate voltage (V M ). f Rabi gradually decreases with increasing ϵ; on the contrary, f Rabi is positively correlated with V M . We attribute our results to the change of electric field on SOI and the contribution of the excited state in quantum dots to f Rabi . We further demonstrate an ultrafast f Rabi exceeding 1.2 GHz, which indicates the strong SOI in our device. The discovery of an ultrafast and electrically tunable f Rabi in a hole spin qubit has potential applications in semiconductor quantum computing.