Abstract Quantum dots in silicon are a promising architecture for semiconductor quantum computing due to a high degree of electric control and compatibility with existing silicon fabrication processes. Although electron charge and spin are prominent methods for encoding the qubit state, valley states in silicon can also store quantum information via valley-orbit coupling with protection against charge noise. By observing coherent oscillations between valley states in a Si/SiGe double quantum dot device tuned to the two-electron charge configuration, we measure the valley energy splitting in both quantum dots individually. We further demonstrate two-axis quantum control of the valley qubit using gated pulse sequences with X and Z rotations occurring within a fast operation time of 300 ps. This control is used to completely map out the surface of the Bloch sphere in a single phase-space plot that is subsequently used for state and process tomography.Abstract Quantum dots in silicon are a promising architecture for semiconductor quantum computing due to a high degree of electric control and compatibility with existing silicon fabrication processes. Although electron charge and spin are prominent methods for encoding the qubit state, valley states in silicon can also store quantum information via valley-orbit coupling with protection against charge noise. By observing coherent oscillations between valley states in a Si/SiGe double quantum dot device tuned to the two-electron charge configuration, we measure the valley energy splitting in both quantum dots individually. We further demonstrate two-axis quantum control of the valley qubit using gated pulse sequences with X and Z rotations occurring within a fast operation time of 300 ps. This control is used to completely map out the surface of the Bloch sphere in a single phase-space plot that is subsequently used for state and process tomography.

npj Quantum InformationNov 6, 201936 citations

Control strategies for semiconductor-quantum-dot-based single and double qubits

Robert Roloff, Markus Wenin, W. Pötz

Journal of Computational ElectronicsMar 1, 20098 citations

Co-optimization of spin coherence and valley splitting in Si/SiGe heterostructures

Peihong Zhang, Xuedong Hu, Saif Ullah +1 more

Single electron spins can be used to encode and process information in semiconductor quantum devices. Progress has been hindered by materials challenges, such as the small energy splitting between low-lying valley states and hyperfine coupling to nuclear spins. Here we use density functional theory to optimize the valley splitting and spin dephasing time in realistic Si/SiGe heterostructures. Reductions in the Si quantum well width generally increase the valley splitting. However, in narrow quantum wells, a larger fraction of the electronic wavefunction resides in the SiGe buffer layers, which increases the hyperfine coupling with spinful $^{73}$ Ge. Our work shows that Si/SiGe heterostructures with 3~--~4~nm wide quantum wells and $^{73}$ Ge and $^{29}$ Si concentrations of 50 ppm should support average valley splittings $E_{v}$ ~ $>$ ~500~ $μ$ eV and spin dephasing times $T_2^*$ exceeding 15~ $μ$ s assuming an effective quantum dot area of 700 nm $^2$ . In addition, sharper Si/SiGe interfaces in general result in larger valley splittings and longer spin dephasing times.Single electron spins can be used to encode and process information in semiconductor quantum devices. Progress has been hindered by materials challenges, such as the small energy splitting between low-lying valley states and hyperfine coupling to nuclear spins. Here we use density functional theory to optimize the valley splitting and spin dephasing time in realistic Si/SiGe heterostructures. Reductions in the Si quantum well width generally increase the valley splitting. However, in narrow quantum wells, a larger fraction of the electronic wavefunction resides in the SiGe buffer layers, which increases the hyperfine coupling with spinful $^{73}$Ge. Our work shows that Si/SiGe heterostructures with 3~--~4~nm wide quantum wells and $^{73}$Ge and $^{29}$Si concentrations of 50 ppm should support average valley splittings $E_{v}$~$>$~500~$μ$eV and spin dephasing times $T_2^*$ exceeding 15~$μ$s assuming an effective quantum dot area of 700 nm$^2$. In addition, sharper Si/SiGe interfaces in general result in larger valley splittings and longer spin dephasing times.

cond-mat.mtrl-sciCross-listedarXiv:2605.31342Jun 1, 2026

Valley-Based Noise-Resistant Quantum Computation Using Si Quantum Dots

Dimitrie Culcer, André Saraiva, Belita Koiller +2 more

We devise a platform for noise-resistant quantum computing using the valley degree of freedom of Si quantum dots. The qubit is encoded in two polarized (1,1) spin-triplet states with different valley compositions in a double quantum dot, with a Zeeman field enabling unambiguous initialization. A top gate gives a difference in the valley splitting between the dots, allowing controllable interdot tunneling between opposite valley eigenstates, which enables one-qubit rotations. Two-qubit operations rely on a stripline resonator, and readout on charge sensing. Sensitivity to charge and spin fluctuations is determined by intervalley processes and is greatly reduced as compared to conventional spin and charge qubits. We describe a valley echo for further noise suppression.We devise a platform for noise-resistant quantum computing using the valley degree of freedom of Si quantum dots. The qubit is encoded in two polarized (1,1) spin-triplet states with different valley compositions in a double quantum dot, with a Zeeman field enabling unambiguous initialization. A top gate gives a difference in the valley splitting between the dots, allowing controllable interdot tunneling between opposite valley eigenstates, which enables one-qubit rotations. Two-qubit operations rely on a stripline resonator, and readout on charge sensing. Sensitivity to charge and spin fluctuations is determined by intervalley processes and is greatly reduced as compared to conventional spin and charge qubits. We describe a valley echo for further noise suppression.

arXiv:1107.0003Physical Review LettersMar 20, 2012101 citations

Coherent control of the quantum state of a single quantum dot

N. H. Bonadeo, John Erland Østergaard, Dan Gammon +3 more

ScienceJan 1, 19980 citations

^{73}

Ge. Our work shows that Si/SiGe heterostructures with 3~--~4~nm wide quantum wells and

^{73}

Ge and

^{29}

Si concentrations of 50 ppm should support average valley splittings

E_{v}

>

~500~

μ

eV and spin dephasing times

T_2^*

exceeding 15~

μ

s assuming an effective quantum dot area of 700 nm

^2

. In addition, sharper Si/SiGe interfaces in general result in larger valley splittings and longer spin dephasing times.