Squeezed hole spin qubits in Ge quantum dots with ultrafast gates at low power

Abstract

Hole spin qubits in planar Ge heterostructures are one of the frontrunner platforms for scalable quantum computers. In these systems, the spin-orbit interactions permit efficient all-electric qubit control. We propose a minimal design modification of planar devices that enhances these interactions by orders of magnitude and enables low power ultrafast qubit operations in the GHz range. Our approach is based on an asymmetric potential that strongly squeezes the quantum dot in one direction. This confinement-induced spin-orbit interaction does not rely on microscopic details of the device such as growth direction or strain, and could be turned on and off on demand in state-of-the-art qubits.

Two-body Wigner molecularization in asymmetric quantum dot spin qubits

“…These effects may be more or less critical depending on the actual design of the quantum dots. They are not only relevant for the manipulation and readout of spin qubits [41,42,65], but raise new opportunities and challenges, such as the detection of the fingerprints of correlations in the dynamics of quantum dots and quantum dot arrays, or when piling up additional electrons in one-dimensional channels [81].…”

“…To conclude, τ T /τ S is in general expected to be much larger when the particles tunnel along the weak confinement axis, and to reach unity in very anisotropic dots. In devices taking advantage of the anisotropy of the dots to enhance Rabi frequencies for example [40][41][42], the orientation of the weak confinement axis must, therefore, be carefully chosen in order not to hinder PSB readout and exchange interactions. A particularly important working point for exchange operations is the symmetric operating point (SOP) ε −U , which is protected against charge noise [19,80].…”

“…Interestingly, some architectures, such as dots confined along a one-dimensional (nanowire) channel [5,[37][38][39], can produce very anisotropic qubits. This anisotropy may be leveraged to enhance the qubit performances by, e.g., increasing the Rabi frequencies (as proposed in hole spin qubits [40][41][42]).…”

Anisotropic $g$-tensors in hole quantum dots: The role of the transverse confinement direction

“…The parameter β so is material-dependent and depends also in an intricate way on the exact shape of the transverse confining potential. By focusing solely on this Rashba term, we neglect the Dresselhaus contribution stemming from the lack of a crystallographic inversion center (which can contribute to the SOI in materials like GaAs and InAs) and we disregard the "direct" Rashba SOI due to HH-LH mixing [31][32][33][34]68]. With this choice, the analysis that follows is most relevant for quantum dots hosted in materials with a large Rashba coefficient.…”

“…However, since the orbitals that constitute the valence band are of p-type [29], the corresponding states have a total six-fold angular momentum degree of freedom, possibly leading to highly anisotropic dynamics. Compared to the valley mixing of the electronic states, however, these dynamics are relatively predictable, and the built-in mixing of orbital and spin degrees of freedom can yield strong effective spinorbit coupling that allows for fast qubit operation [30][31][32][33][34][35][36]. Moreover, the p-type orbital nature of the valence band has the additional advantage of weaker effective hyperfine coupling to any residual spinful nuclei, due to the wave function having a node at the atomic site [37].…”