Spatially resolving valley quantum interference of a donor in silicon

Area of Science:

• Quantum computing
• Condensed matter physics
• Materials science

Background:

• Silicon donor spins offer long coherence times but are affected by valley degrees of freedom near interfaces.
• Interface proximity modifies valley states, influencing qubit relaxation and exchange, critical for quantum operations.
• Previous spectroscopic methods indirectly probed wavefunctions, lacking direct access to valley population and donor environment.

Purpose of the Study:

• To directly probe the probability density of single quantum states of individual subsurface donors in silicon.
• To experimentally investigate valley interference patterns and valley population near interfaces.
• To determine the impact of interface proximity on spin relaxation and qubit operations.

Main Methods:

• Utilized scanning tunnelling spectroscopy (STS) to directly probe subsurface donor states.
• Performed measurements in both real space and reciprocal space.
• Analyzed quantum mechanical valley interference patterns in the donor ground state.

Main Results:

• Directly observed quantum mechanical valley interference patterns in subsurface donor states.
• Found valley population to be within 5% of bulk donors at depths greater than 2.85 ± 0.45 nm.
• Indicated negligible enhancement of spin relaxation for donors deeper than 3 nm from the interface.

Conclusions:

• Valley interference in subsurface donors is sensitive to atomic-scale position variations, impacting two-qubit gates.
• Spin relaxation enhancement is minimal for donors beyond 3 nm, supporting their use in quantum devices.
• Results are relevant for quantum computing schemes using valley polarization for information encoding.