Probing the valley phase of Si quantum dot qubits

In Si quantum dot qubits, the valley phase — the relative phase between the +z and -z valley states — is a key parameter governing valley splitting statistics, readout fidelity, and exchange coupling in double quantum dots. Alloy disorder induces large spatial fluctuations of the valley phase, degrading qubit control and scalability. Mitigating these effects in experiments that are sensitive to the valley phase requires knowledge of the valley phase landscape.

Motivated by recent advances in valley splitting mapping, we propose a method to extract the valley phase by combining measurements of valley splitting and spin-valley coupling. Accounting for realistic experimental constraints, we show that spin-valley maps can be obtained using existing techniques and that they are robust to charge noise and alloy disorder. By exploiting the relations between the spin-valley and valley splitting, we are able to reconstruct valley phase landscapes along 1D channels, up to a π/2 ambiguity. This reconstruction involves an initial value problem, which we show can be solved by accessing regions of near-zero valley splitting. Simulations including alloy disorder show that this also markedly improves the accuracy of the phase characterization. Our approach provides an experimentally feasible path for mapping the valley phase, an essential step toward scalable silicon qubits.