Researchers have now reported an enhancement of spin lifetime, or relaxation time, by more than two orders of magnitude in silicon quantum dots (QDs). They achieved this by tweaking the external magnetic field direction with respect to the silicon wafer’s crystallographic axis.
Reported in Physics Review Letters on June 23rd, 2020, the study titled “Cooling a Spin Relaxation Hot-Spot” was performed by a research group headed by academician GUO Guangcan from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS).
The paper was chosen as Editors’ Suggestion and selected for “Featured in Physics.”
Silicon QDs-based spin qubits have been a major obstacle in the advancement of large-scale quantum computation. This is due to their long coherence time and compatibility issues with modern semiconductor technology.
Spin qubits developed using silicon metal-oxide-semiconductor (MOS) and Si/SiGe heterostructure feature dephasing time and relaxation time that surpass hundreds of microseconds and hundreds of milliseconds, respectively. This aspect leads to a single-qubit control fidelity of more than 99.9% and a two-qubit gate fidelity of more than 98%.
The success in this front led to the active involvement of labs and companies, such as CEA-Leti, Intel, IMEC, etc., from the industry in this field.
But the occurrence of valley states (a state related to the dip in a specific electronic band) in silicon QDs could drastically minimize dephasing time and spin relaxation time through spin-valley mixing, limiting the control fidelity of qubits.
Researchers have reported that at a specific magnetic field, spin-valley mixing could reduce the spin relaxation time to less than 1 ms (even 1 μs under some conditions), which is known as spin relaxation “hot spot.”
When there is an increase in the number of qubits, the phenomenon will result in a higher number of “bad” qubits and inhibits further extension to more qubits. Conventionally, the negative effects of spin-valley mixing are suppressed by increasing the magnitude of valley splitting and pushing the qubit very far to prevent the mixing of the spin and valley states.
But the valley states are influenced by different factors from the material, which are not uniform in general. Therefore, it is challenging to control the valley splitting magnitude (specifically in Si/SiGe heterostructure).
An alternative method to achieve this is to directly regulate the magnitude of spin-valley mixing. Researchers have reported that in the case of GaAs QDs, it is possible to tune the strength of spin-orbit coupling by the in-plane magnetic field orientation, thus extending the spin relaxation time. However, until now, no reports exist on how the external magnetic field direction influences the strength of spin-valley mixing in silicon.
The researchers resolved this issue by fabricated high-quality silicon MOS QDs, thereby realizing single-shot readout of spin qubits. Using this reliable method, they analyzed the effect of the orientation and strength of the external magnetic field on spin relaxation rates.
The orientation of the in-plane external magnetic field at a specific angle enabled the spin relaxation “hot spot” to be “cooled down” by two orders of magnitude, thus increasing the relaxation time from less than 1 ms to more than 100 ms.
This great difference shows that spin-valley mixing has been effectively inhibited, laying a foundation for future studies on how to suppress spin qubits of spin-valley mixing.
Moreover, the team discovered that this anisotropy could still be more than two orders of magnitude upon changing the electric field. This indicates that the anisotropy magnitude is independent of a specific range of the electric field and could be applied to an array of qubits with different local electric fields. Thus, this should provide new means for optimizing control, readout, and multi-qubit extension of spin qubits based on silicon.
The study presented in this manuscript represents one of the few extensive studies realized for spin relaxation anisotropy in QDs and provides potential new ways to probe also the anisotropy properties of inter-valley and intra-valley spin mixing mechanisms.
Zhang, X., et al. (2020) Giant Anisotropy of Spin Relaxation and Spin-Valley Mixing in a Silicon Quantum Dot. Physical Review Letters. doi.org/10.1103/PhysRevLett.124.257701.