  Paramagnetic point defects in silicon provide qubits that could open up
pathways towards silicon-technology based, low-cost, room-temperature (RT)
quantum sensing. The silicon dangling bond (db) is a natural candidate, given
its sub-nanometer localization and direct involvement in spin-dependent
charge-carrier recombination, allowing for electrical spin readout. In
crystalline silicon, however, rapid loss of db spin-coherence at RT due to
free-electron trapping, strongly limits quantum applications. In this work, by
combining density-functional theory and multifrequency (100 MHz-263 GHz) pulsed
electrically detected magnetic resonance spectroscopy, we show that upon
electron capture, dbs in a hydrogenated amorphous silicon matrix form
metastable spin pairs in a well-defined quasi two-dimensional (2D)
configuration. Although highly localized, these entangled spin pairs exhibit
nearly vanishing intrinsic dipolar and exchange coupling. The formation of this
magic-angle-like configuration involves a >0.3 eV energy relaxation of a
trapped electron, stabilizing the pair. This extends RT spin coherence times
into the microsecond range in silicon required for a future spin-based quantum
sensing technology.
