Atomically precise control of a coupled donor-quantum dot system in silicon

Single donors in silicon have long spin coherence times of interest for quantum computing. While quantum dots are promising candidates to couple donor spin qubits[1,2], the experimental tunability of the interactions and the influence of valley degrees of freedom (recently measured in donors[3]) in donor-quantum dot coupling remain open questions. We have directly measured the spectrum and wavefunctions of a donor interacting with a single-electron quantum dot that follows a movable scanned probe tip with sub-nm accuracy. Analysis of the two-electron energy as a function of the quantum dot position reveals no detectable lattice-incommensurate ``exchange oscillations'', even though donor state and its valley population is robust to quantum dot potential. This contrasts direct donor-donor exchange, which is predicted to depend sensitively on donor position due to valley interference. This result has important implications for obtaining robust interactions among donors in a surface code quantum computer. [1] G Pica et al, Phys. Rev. B. 93, 035306 (2016). [2] Gonzalez-Zalba, Phys. Rev. X 5 031024 (2015). [3] Salfi et al, Nature Materials, 13, 606 (2014). [4] B. Koiller et al, Phys. Rev. Lett. 88, 027903 (2001).