Silicon is undoubtedly one of the most promising semiconductor materials for
spin-based information processing devices. Its highly advanced fabrication
technology facilitates the transition from individual devices to large-scale
processors, and the availability of an isotopically-purified 28Si form
with no magnetic nuclei overcomes what is a main source of spin decoherence in
many other materials. Nevertheless, the coherence lifetimes of electron spins
in the solid state have typically remained several orders of magnitude lower
than what can be achieved in isolated high-vacuum systems such as trapped ions.
Here we examine electron spin coherence of donors in very pure 28Si
material, with a residual 29Si concentration of less than 50 ppm and donor
densities of 1014−15 per cm3. We elucidate three separate mechanisms
for spin decoherence, active at different temperatures, and extract a coherence
lifetime T2​ up to 2 seconds. In this regime, we find the electron spin is
sensitive to interactions with other donor electron spins separated by ~200 nm.
We apply a magnetic field gradient in order to suppress such interactions and
obtain an extrapolated electron spin T2​ of 10 seconds at 1.8 K. These
coherence lifetimes are without peer in the solid state by several orders of
magnitude and comparable with high-vacuum qubits, making electron spins of
donors in silicon ideal components of a quantum computer, or quantum memories
for systems such as superconducting qubits.Comment: 18 pages, 4 figures, supplementary informatio