Coherent spin qubit shuttling through germanium quantum dots

Abstract

Quantum links can interconnect qubit registers and are therefore essential in networked quantum computing. Semiconductor quantum dot qubits have seen significant progress in the high-fidelity operation of small qubit registers but establishing a compelling quantum link remains a challenge. Here, we show that a spin qubit can be shuttled through multiple quantum dots while preserving its quantum information. Remarkably, we achieve these results using hole spin qubits in germanium, despite the presence of strong spin-orbit interaction. In a minimal quantum dot chain, we accomplish the shuttling of spin basis states over effective lengths beyond 300 microns and demonstrate the coherent shuttling of superposition states over effective lengths corresponding to 9 microns, which we can extend to 49 microns by incorporating dynamical decoupling. These findings indicate qubit shuttling as an effective approach to route qubits within registers and to establish quantum links between registers.

Operating semiconductor quantum processors with hopping spins

“…Consequently, a small tilt of the applied magnetic field from the in-plane g -tensor will lead to a strong reorientation of the spin quantization axis in the out-of-plane direction. Subsequently, when an in-plane magnetic field is applied, the orientation of the spin quantization axis is highly sensitive to the local g -tensor, and thus to confinement, strain, and electric fields, thus becoming a site-dependent property ( 21 , 24 , 28 , 29 ). Here, we exploited this aspect to establish hopping-based quantum operations in two different devices: a four-quantum dot array ( 30 ) arranged in a 2 × 2 configuration and a 10–quantum dot system arranged in a 3-4-3 configuration.…”

“…The relatively small magnetic fields ensured that the maximum qubit frequency (140 MHz) and its corresponding precession period (7 ns) were within the bandwidth of the arbitrary waveform generators used. In combination with engineered voltage pulses with subnanosecond resolution ( 21 ) [( 31 ), section 1], we were able to shuttle a spin qubit to an empty quantum dot and thereby accurately change the qubit precession direction several times within one precession period. Altogether, this enables efficient single-qubit control through discrete voltage pulses (Fig.…”

“…We let it precess for a time t D n , after which the spin was shuttled back and read out. The misalignment between the spin quantization axes gives rise to spin rotations with the Larmor frequency f D n ( 21 ). The resulting oscillations are shown as a function of waiting time in D6, t D6 , and magnetic field (Fig.…”

“…We used hole spins in germanium quantum dots, in which the strong spin-orbit interaction gives rise to an anisotropic g -tensor that is strongly dependent on the electrostatic and strain environment ( 20 ). We harnessed the resulting differences in the spin quantization axis between quantum dots ( 21 , 22 ) to achieve high-fidelity single-qubit control using discrete pulses by shuttling the spin between quantum dot sites. A key advantage in such a hopping-based operation is that the spin rotation frequency is given by the Larmor precession.…”