Modulation of Spin Qubit Frequency in Ge via Strain-Induced Spin-Orbit Interactions

Spin qubits in strained germanium quantum wells are rapidly emerging as a leading platform for semiconductor-based quantum computing. This progress is fueled by their strong spin-orbit interactions and the absence of valley-orbital degeneracies, enabling efficient all-electric qubit control. In recent experiments, we demonstrated that a pair of coupled quantum dots forming a hole singlet-triplet system achieves a rapid Rabi frequency of up to 100 MHz. Our findings reveal a pronounced dependence of the singlet-triplet qubit frequency on the voltage applied to the barrier gate shared by the quantum dots [1]. A mere 10 mV adjustment in the barrier gate voltage results in an approximately an order of magnitude change in the effective g-factor, underscoring both the promise of electrical control and the sensitivity of the qubit to small voltage variations. This behavior suggests a significant role of strain-induced spin-orbit interactions, arising from the movement of holes through an inhomogeneous strain environment—a characteristic feature of quantum dot devices. To corroborate this hypothesis, we conducted finite-element simulations [2] of the thermal contraction-induced strain profiles and their influence on the g-factor in our device. These simulations provide valuable insights into the strain landscape and its impact on spin-orbit coupling. Our work highlights both the opportunities for precise electrical tunability of spin qubits in germanium and the challenges posed by strain-induced variations. The implications of strain engineering for advancing Ge-based quantum technologies will be further discussed.

Collaborators: John Dean Rooney, Giordano Scappucci, Menno Veldhorst, Mu Niu, Johnathan Bryan, Chris Anderson, and Mark Gyure.

[1] John Rooney, Zhentao Luo, Lucas E. A. Stehouwer, Giordano Scappucci, Menno Veldhorst, and Hong-Wen Jiang. “Gate Modulation of the Hole Singlet-Triplet Qubit Frequency in Germanium.” arXiv:2311.10188v1 (2023).

[2] Abadillo-Uriel, José Carlos, Esteban A. Rodríguez-Mena, Biel Martinez, and Yann-Michel Niquet. “Hole-Spin Driving by Strain-Induced Spin-Orbit Interactions.” Physical Review Letters 131, 097002 (2023).