  The nature of the nano-scale environment presents a major challenge for
solid-state implementation of spin-based qubits. In this work, a single
electron spin in an optically pumped nanometer-sized III-V semiconductor
quantum dot is used to control a macroscopic nuclear spin of several thousand
nuclei, freezing its decay and leading to spin life-times exceeding 100 seconds
at low temperatures. Few-millisecond-fast optical initialization of the nuclear
spin is followed by a slow decay exhibiting random telegraph signals at long
delay times, arising from low probability electron jumps out of the dot. The
remarkably long spin life-time in a dot surrounded by a densely-packed nuclear
spin environment arises from the Knight field created by the resident electron,
which leads to suppression of nuclear spin depolarization.
