Dispersive readout of valley splittings in cavity-coupled silicon quantum dots

The bandstructure of bulk silicon has a six-fold valley degeneracy. Strain in the Si/SiGe quantum well system partially lifts the valley degeneracy, but the materials factors that set the splitting of the two lowest lying valleys are still under intense investigation. We propose a method for accurately determining the valley splitting in Si/SiGe double quantum dots embedded into a superconducting microwave resonator. We show that low lying valley states in the double quantum dot energy level spectrum lead to readily observable features in the cavity transmission. These features generate a "fingerprint" of the microscopic energy level structure of a semiconductor double quantum dot, providing useful information on valley splittings and intervalley coupling rates.Comment: 8 pages, 4 figure

Interplay of charge and spin coherence in Landau-Zener-St\"uckelberg-Majorana interferometry

We study Landau-Zener dynamics in a double quantum dot filled with two electrons, where the spin states can become correlated with charge states and the level velocity can be tuned in a time-dependent fashion. We show that a correct interpretation of experimental data is only possible when finite-time effects are taken into account. In addition, our formalism allows the study of partial adiabatic dynamics in the presence of phonon-mediated hyperfine relaxation and charge-noise-induced dephasing. Our findings demonstrate that charge noise severely impacts the visibility of Landau-Zener-St\"uckelberg-Majorana interference fringes. This indicates that charge coherence must be treated on an equal footing with spin coherence.Comment: 13 pages, 9 figure

Fast Room-Temperature Phase Gate on a Single Nuclear Spin in Diamond

Nuclear spins support long lived quantum coherence due to weak coupling to the environment, but are difficult to rapidly control using nuclear magnetic resonance (NMR) as a result of the small nuclear magnetic moment. We demonstrate a fast ~ 500 ns nuclear spin phase gate on a 14N nuclear spin qubit intrinsic to a nitrogen-vacancy (NV) center in diamond. The phase gate is enabled by the hyperfine interaction and off-resonance driving of electron spin transitions. Repeated applications of the phase gate bang-bang decouple the nuclear spin from the environment, locking the spin state for up to ~ 140 microseconds.Comment: Related papers at http://pettagroup.princeton.ed

MOCVD synthesis of compositionally tuned topological insulator nanowires

Device applications involving topological insulators (TIs) will require the development of scalable methods for fabricating TI samples with sub-micron dimensions, high quality surfaces, and controlled compositions. Here we use Bi-, Se-, and Te-bearing metalorganic precursors to synthesize TIs in the form of nanowires. Single crystal nanowires can be grown with compositions ranging from Bi2Se3 to Bi2Te3, including the ternary compound Bi2Te2Se. These high quality nanostructured TI compounds are suitable platforms for on-going searches for Majorana Fermions