Sweet Spot Combining Qubit Coherence and Speed*

Strong spin-orbit interaction (SOI) has attracted major interest in quantum dot spin qubits, as it enables ultrafast, scalable and compact qubit devices. However, the SOI not only facilitates all electrical driving, but at the same time couples the qubit to charge noise, leading to dephasing. This seems to suggest that stronger SOI leads to faster qubit operation but only at the cost of coherence.

Here, we provide experimental evidence in a Ge/Si core/shell nanowire demonstrating a qubit which reaches its most coherent point of operation exactly when its speed is the fastest, coinciding with vanishing derivatives of its g-factor with respect to all gates. Such a compromise-free sweet spot combining qubit speed and coherence requires a strong SOI with a maximum in coupling strength. Here, this is likely provided by strong and gate tunable direct-Rashba SOI in the 1D confinement of the Ge/Si nanowire employing iso-Zeeman driving from a remote gate. Our results overturn the conventional paradigm that fast operation speeds imply reduced qubit lifetimes.

Additionally, were were able to find a charge configuration of the double dot for which both spins can be individually addressed at different microwave frequencies, allowing the implementation of exchange-based conditional spin rotations at 1.5 K.