  The quiet quantum environment of holes in solid-state devices has been at the
core of increasingly reliable architectures for quantum processors and
memories.1-6 However, due to the lack of scalable materials to properly tailor
the valence band character and its energy offsets, the precise engineering of
light-hole (LH) states remains a serious obstacle toward coherent photon-spin
interfaces needed for a direct mapping of the quantum information encoded in
photon flying qubits to stationary spin processor.4-9 Herein, to alleviate this
long-standing limitation we demonstrate an all-group IV low-dimensional system
consisting of highly tensile strained germanium quantum well grown on silicon
allowing new degrees of freedom to control and manipulate the hole states.
Wafer-level, high bi-isotropic in-plane tensile strain ($>1\%$) is achieved
using strain-engineered, metastable germanium-tin alloyed buffer layers
yielding quantum wells with LH ground state, high $g$-factor anisotropy, and a
tunable splitting of the hole subbands. The epitaxial heterostructures display
sharp interfaces with sub-nanometer broadening and show room-temperature
excitonic transitions that are modulated and extended to the mid-wave infrared
by controlling strain and thickness. This ability to engineer quantum
structures with LH selective confinement and controllable optical response
enables manufacturable silicon-compatible platforms relevant to integrated
quantum communication and sensing technologies.
