Dispersive readout of spin-valley and pure valley qubits in silicon

Understanding the energy spectrum in silicon quantum dots is of prime importance for the realization of reliable spin qubits in such structures. In particular, controlling the interplay between spin and valley physics remains an active area of investigation, as valley states are expected to limit spin readout fidelity as well as qubit coherence and relaxation.

In this context, we present a unit cell comprising two electrons in a double quantum dot with in situ dispersive readout. We demonstrate control over both the exchange interaction and the valley spectrum by tuning the confinement and detuning potentials.

Using dispersive readout, we achieve high-fidelity single-shot readout of spin–valley states with a visibility exceeding 93%. Moreover, by adjusting the valley splitting close to zero at a particular detuning point, we realize a pure valley qubit with a single-shot readout fidelity above 70%. We further investigate the characteristics of this valley qubit and show that its performance is limited by relaxation at low detuning and by dephasing at large detuning.

In conclusion, we have used dispersive readout to probe the spin and valley physics of a two-electron system across several devices. We demonstrate consistently high visibility for the spin qubit despite a relatively small lever arm (<0.15 eV/V). We also exploit this method to reproducibly probe a pure valley qubit and report promising metrics for a charge-like qubit.