A team from University of Science and Technology of China (USTC) and Origin Quantum Computing Company Limited, reported an improvement of spin lifetime (relaxation time) by over two orders of magnitude in silicon Quantum Dots (QDs) by tuning the direction of the external magnetic field with respect to the crystallographic axis of the silicon wafer.
Recently, the relaxation time and dephasing time of spin qubits developed in Si MOS (Metal-Oxide-Semiconductor) and Si/SiGe heterostructure have surpassed hundreds of milliseconds and hundreds of microseconds, respectively, resulting in a single-qubit control fidelity over 99.9% and a two-qubit gate fidelity over 98%.
However, the existence of valley states in silicon quantum dots could reduce spin relaxation time and dephasing time seriously via spin-valley mixing and limit the control fidelity of qubits. It was reported that at a certain magnetic field, spin-valley mixing could decrease the spin relaxation time to shorter than one millisecond (even one microsecond under certain conditions), called a spin relaxation hot spot. When the number of qubits increase, this phenomenon will cause a great number of “bad” qubits and impedes further extension to more qubits.
The researchers fabricated high quality Si MOS quantum dot and achieved single-shot readout of spin qubits and investigated the effect of both the strength and orientation of the external magnetic field on spin relaxation rates. They found when the in-plane external magnetic field is oriented at a certain angle, the spin relaxation hot spot could be “cooled down” by two orders of magnitude, increasing the relaxation time from below one millisecond to over one hundred milliseconds. This great variation indicates that spin-valley mixing is effectively suppressed, and it lays a foundation for future research on how to rid spin qubits of spin-valley mixing. (Phys.org)
The paper has published in Physical Review Letters.