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arXiv:2607.11048

Spectroscopy of low-lying valley states in hot Si/SiGe quantum dots

Connor Nasseraddin, Heun Mo Yoo, Tanner M. Janda, Jason R. Petta

cond-mat.mes-hallquant-phSubmitted Jul 13, 2026• Announced Jul 14, 2026
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Abstract

The presence of low-lying valley states in Si may hinder the development of large-scale spin-based quantum processors. Rapid prototyping of novel Si/SiGe heterostructures and gate stacks will be central to identifying pathways that increase the valley splitting. We compare the performance of pulsed-gate spectroscopy (PGS) and detuning axis spectroscopy (DAPS) at temperatures up to 700 mK. We find that DAPS outperforms PGS, with DAPS resolving valley splittings as small as ~86 $μ$eV, while the energy resolution of PGS is only ~210~$μ$eV. Our work demonstrates that DAPS can be used to efficiently extract valley splittings at elevated temperatures in high throughput cryostats.

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cond-mat.mes-hallCross-listedarXiv:2607.09652•Jul 13, 2026
The presence of low-lying valley states in Si may hinder the development of large-scale spin-based quantum processors. Rapid prototyping of novel Si/SiGe heterostructures and gate stacks will be central to identifying pathways that increase the valley splitting. We compare the performance of pulsed-gate spectroscopy (PGS) and detuning axis spectroscopy (DAPS) at temperatures up to 700 mK. We find that DAPS outperforms PGS, with DAPS resolving valley splittings as small as ~86 μμμeV, while the energy resolution of PGS is only 210μμμeV. Our work demonstrates that DAPS can be used to efficiently extract valley splittings at elevated temperatures in high throughput cryostats.
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^{73}73Ge. Our work shows that Si/SiGe heterostructures with 3~--~4~nm wide quantum wells and 73^{73}73Ge and 29^{29}29Si concentrations of 50 ppm should support average valley splittings EvE_{v}Ev​~>>>~500~μμμeV and spin dephasing times T2∗T_2^*T2∗​ exceeding 15~μμμs assuming an effective quantum dot area of 700 nm2^22. In addition, sharper Si/SiGe interfaces in general result in larger valley splittings and longer spin dephasing times.
Singlet-triplet spin states of a quantum dot support promising semiconductor-based qubits, yet often suffer from poor state preparation and measurement, due to low-lying excited states. Engineering a large energy splitting is impeded by the inability to accurately measure both large and small energy splittings with the device biased to nominal operation. The authors present a measurement and fitting approach that accurately extracts both large and small splittings in this regime. They also find evidence that both orbital and valley degrees of freedom may set this energy separation, significantly affecting which paths to pursue in device design.
Valley excitations are considered one of the biggest challenges for a coherent spin shuttling in Si/SiGe heterostructures. In this work we theoretically investigate the valley dynamics during spin shuttling in a Si/SiGe heterostructure by considering a two-level system given by the two low-lying valley states of the mobile quantum dot. We assume that the electron is initially in the valley ground state and analyze the evolution of this state during shuttling. We find that, depending on the shuttling velocity, the system can be in three distinct regimes. In one of these regimes, the quantum information is preserved even in the presence of the valley excitations, allowing for coherent spin shuttling independently of the shuttling distance. This regime is obtained for a shuttling velocity of tens of m/s, a desired regime where dephasing mechanisms, such as spin-valley hotspots and spatially varying spin splitting, can be suppressed. This result is in accordance with recent experimental work that obtained a high spin shuttling fidelity after a fast and long shuttling. We also propose strategies based on the Partial Landau-Zener model to enhance the valley shuttling fidelity, which was found to be much more efficient than previously proposed strategies based on the standard Landau-Zener model.
While bulk silicon has long been understood to exhibit relatively weak spin-orbit coupling (SOC), confinement of electrons to quantum dots (QDs) at a silicon heterointerface results in significantly larger SOC. This is a concern for electron spin qubit performance, as intravalley and intervalley SOC can significantly perturb the operation of electron spin qubits. While these interactions can be harnessed to drive coherent rotations in a singlet-triplet qubit, coupling to low-lying excited valley states can lead to undesirable spin relaxation when valley splitting is on resonance with the Zeeman energy. In this work, we measure the angular dependence of the interfacial spin-orbit interaction as a function of the direction and magnitude of an applied external magnetic field in SiMOS and Si/SiGe heterostructures, two common material platforms for silicon spin qubits. We construct a physical model that accurately infers intra- and inter-valley SOC physics from fits to the data, allowing for a direct comparison between these two material systems. For the devices measured we find that, while the ggg-factor differences are comparable, the SiMOS QDs exhibit an order of magnitude larger spin-valley coupling than for Si/SiGe. Moreover, we find that the angular dependence of the spin-valley coupling is similar for both devices, with similar magnetic field orientations minimizing the spin-valley coupling. Our work points towards operational schemes for optimizing spin-valley coupling to avoid or exploit this mechanism for qubit operation.
We study an accumulation mode Si/SiGe double quantum dot (DQD) containing a single electron that is dipole coupled to microwave photons in a superconducting cavity. Measurements of the cavity transmission reveal dispersive features due to the DQD valley states in Si. The occupation of the valley states can be increased by raising the temperature or applying a finite source-drain bias across the DQD, resulting in an increased signal. Using the cavity input-output theory and a four-level model of the DQD, it is possible to efficiently extract valley splittings and the inter- and intravalley tunnel couplings.
Achieving valley splittings well in excess of the thermal energy of electrons and avoiding valley excitations is essential for the consistent initialization, operation and readout of gate-defined Si spin qubits. In this work, we present a device-level optimization strategy for pushing valley splittings to between 1 and 5 meV, well beyond values reported in nearly all previous theoretical studies. Using device-scale simulations that incorporate atomistic alloy disorder through a 1D tight-binding theory, we demonstrate that our proposed approach yields large valley splittings with a tight distribution across disorder realizations, a key requirement for reproducible qubit performance at scale. The approach rests on an unorthodox Si/SiGe heterostructure design combining a narrow quantum well, a small Ge spike, and a pure-Ge cap. We corroborate these predictions with targeted atomistic density functional theory calculations. These results offer a clear path forward for scalable Si/SiGe spin qubit devices and, if realized experimentally, effectively eliminate valley splitting as an existential problem for large scale SiGe-based quantum processors.