Xiao-Jie Hao

A graphene quantum dot with a single electron transistor as an integrated charge sensor

A quantum dot (QD) with an integrated charge sensor is becoming a common architecture for a spin or charge based solid state qubit. To implement such a structure in graphene, we have fabricated a twin-dot structure in which the larger dot serves as a single electron transistor (SET) to read out the charge state of the nearby gate controlled small QD. A high SET sensitivity of 10−3e/Hz allowed us to probe Coulomb charging as well as excited state spectra of the QD, even in the regime where the current through the QD is too small to be measured by conventional transport means.

Strong and Tunable Spin−Orbit Coupling of One-Dimensional Holes in Ge/Si Core/Shell Nanowires

We investigate the low-temperature magneto-transport properties of individual Ge/Si core/shell nanowires. Negative magneto-conductance was observed, which is a signature of one-dimensional weak antilocalization of holes in the presence of strong spin--orbit coupling. The temperature and back gate dependences of phase coherence length, spin--orbit relaxation time, and background conductance were studied. Specifically, we show that the spin--orbit coupling strength can be modulated by more than five folds with an external electric field. These results suggest the Ge/Si nanowire system possesses strong and tunable spin--orbit interactions and may serve as a candidate for spintronics applications.

Electron spin resonance and spin-valley physics in a silicon double quantum dot

Silicon quantum dots are a leading approach for solid-state quantum bits. However, developing this technology is complicated by the multi-valley nature of silicon. Here we observe transport of individual electrons in a silicon CMOS-based double quantum dot under electron spin resonance. An anticrossing of the driven dot energy levels is observed when the Zeeman and valley splittings coincide. A detected anticrossing splitting of 60 MHz is interpreted as a direct measure of spin and valley mixing, facilitated by spin-orbit interaction in the presence of non-ideal interfaces. A lower bound of spin dephasing time of 63 ns is extracted. We also describe a possible experimental evidence of an unconventional spin-valley blockade, despite the assumption of non-ideal interfaces. This understanding of silicon spin-valley physics should enable better control and read-out techniques for the spin qubits in an all CMOS silicon approach.

Controllable tunnel coupling and molecular states in a graphene double quantum dot

We have measured a graphene double quantum dot device with multiple electrostatic gates that are used to enhance control to investigate it. At low temperatures, the transport measurements reveal honeycomb charge stability diagrams which can be tuned from weak to strong interdot tunnel coupling regimes. We precisely extract a large interdot tunnel coupling strength for this system allowing for the observation of tunnel-coupled molecular states extending over the whole double dot. This clean, highly controllable system serves as an essential building block for quantum devices in a nuclear-spin-free world.We have measured a graphene double quantum dot device with multiple electrostatic gates that are used to enhance control to investigate it. At low temperatures the transport measurements reveal honeycomb charge stability diagrams which can be tuned from weak to strong interdot tunnel coupling regimes. We precisely extract a large interdot tunnel coupling strength for this system allowing for the observation of tunnel-coupled molecular states extending over the whole double dot. This clean, highly controllable system serves as an essential building block for quantum devices in a nuclear-spin-free world. Electronic address: tutao@ustc.edu.cn Electronic address: gpguo@ustc.edu.cn

Probing a quantum Hall pseudospin ferromagnet by resistively detected nuclear magnetic resonance

Resistively detected nuclear magnetic resonance (RD-NMR) has been used to investigate a two-subband electron system in a regime where quantum Hall pseudospin ferromagnet (QHPF) states are prominently developed. It reveals that the easy-axis QHPF state around the total filling factor

Observation of an in-plane magnetic-field-driven phase transition in a quantum Hall system with SU(4) symmetry

\ensuremath{\nu}=4

Scaling behavior and variable hopping conductivity in the quantum Hall plateau transition

can be detected by the RD-NMR measurement. Approaching one of the Landau-level (LL) crossing points, the RD-NMR signal strength and the nuclear-spin-relaxation rate

Experimental studies of scaling behavior of a quantum Hall system with a tunable Landau level mixing

1/{T}_{1}

Phase diagram of a quantum Hall pseudospin ferromagnet in a two-subband electron system

enhance significantly, a signature of low-energy spin excitations. Furthermore, the RD-NMR signal at another identical LL crossing point is surprisingly missing which presents a puzzle. These observations demonstrate that the spin freedom may play an role in the understanding of the QHPF states.

Eliminating interactions between non-neighboring qubits in the preparation of cluster states in quantum molecules

In condensed matter physics, the study of electronic states with SU(N) symmetry has attracted considerable and growing attention in recent years, as systems with such a symmetry can often have a spontaneous symmetry-breaking effect giving rise to a novel ground state. For example, pseudospin quantum Hall ferromagnet of broken SU(2) symmetry has been realized by bringing two Landau levels close to degeneracy in a bilayer quantum Hall system. In the past several years, the exploration of collective states in other multi-component quantum Hall systems has emerged. Here we show the conventional pseudospin quantum Hall ferromagnetic states with broken SU(2) symmetry collapsed rapidly into an unexpected state with broken SU(4) symmetry, by in-plane magnetic field in a two-subband GaAs/AlGaAs two-dimensional electron system at filling factor around