Dressed photon-orbital states in a quantum dot: Inter-valley spin resonance

TL;DR

This paper investigates microwave-induced transitions in silicon quantum dots, revealing complex dressed states, inter-valley spin transitions, and non-linear effects that impact qubit coherence and control.

Contribution

It introduces the concept of dressed photon-orbital states to unify understanding of multiple microwave resonances, including novel inter-valley spin transitions and non-linear phenomena.

Findings

Observation of six microwave resonance lines in Si/SiGe quantum dots.

Identification of inter-valley spin transitions with enhanced electric field sensitivity.

Detection of a dynamical anti-crossing caused by system non-linearity.

Abstract

The valley degree of freedom is intrinsic to spin qubits in Si/SiGe quantum dots. It has been viewed alternately as a hazard, especially when the lowest valley-orbit splitting is small compared to the thermal energy, or as an asset, most prominently in proposals to use the valley degree of freedom itself as a qubit. Here we present experiments in which microwave electric field driving induces transitions between both valley-orbit and spin states. We show that this system is highly nonlinear and can be understood through the use of dressed photon-orbital states, enabling a unified understanding of the six microwave resonance lines we observe. Some of these resonances are inter-valley spin transitions that arise from a non-adiabatic process in which both the valley and the spin degree of freedom are excited simultaneously. For these transitions, involving a change in valley-orbit state, we find a tenfold increase in sensitivity to electric fields and electrical noise compared to pure spin transitions, strongly reducing the phase coherence when changes in valley-orbit index are incurred. In contrast to this non-adiabatic transition, the pure spin transitions, whether arising from harmonic or subharmonic generation, are shown to be adiabatic in the orbital sector. The non-linearity of the system is most strikingly manifest in the observation of a dynamical anti-crossing between a spin-flip, inter-valley transition and a three-photon transition enabled by the strong nonlinearity we find in this seemly simple system.