Cheng Zhou

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.

Ultrafast universal quantum control of a quantum-dot charge qubit using Landau–Zener–Stückelberg interference

A basic requirement for quantum information processing is the ability to universally control the state of a single qubit on timescales much shorter than the coherence time. Although ultrafast optical control of a single spin has been achieved in quantum dots, scaling up such methods remains a challenge. Here we demonstrate complete control of the quantum-dot charge qubit on the picosecond scale, orders of magnitude faster than the previously measured electrically controlled charge- or spin-based qubits. We observe tunable qubit dynamics in a charge-stability diagram, in a time domain, and in a pulse amplitude space of the driven pulse. The observations are well described by Landau–Zener–Stückelberg interference. These results establish the feasibility of a full set of all-electrical single-qubit operations. Although our experiment is carried out in a solid-state architecture, the technique is independent of the physical encoding of the quantum information and has the potential for wider applications.

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.

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

Back-action-induced non-equilibrium effect in electron charge counting statistics

We study real-time charge-counting statistics measured by a quantum point contact (QPC) coupled to a single quantum dot (QD) subject to different back-action strengths. By tuning the QD-QPC coupling or the QPC bias, we control the QPC back-action, which drives the QD electrons out of thermal equilibrium. The random telegraph signal (RTS) statistics show strong and tunable non-thermal-equilibrium saturation effect, which can be quantitatively characterized as a back-action-induced tunneling-out rate. We find that the QD-QPC coupling and QPC bias voltage play different roles in determining the back-action strength and the cut-off energy.

Quantum simulation of the Kibble-Zurek mechanism using a semiconductor electron charge qubit

The Kibble-Zurek mechanism provides a description of the topological structure occurring in the symmetry breaking phase transitions, which may manifest as the cosmological strings in the early universe or vortex lines in the superfulid. A particularly intriguing analogy between Kibble-Zurek mechanism and a text book quantum phenomenon, Landau-Zener transition has been discovered, but is difficult to observe up to now. In recent years, there has been broad interest in quantum simulations using different well-controlled physical setups, in which full tunability allows access to unexplored parameter regimes. Here we demonstrate a proof-of-principle quantum simulation of Kibble-Zurek mechanism using a single electron charge qubit in double quantum dot, set to behave as Landau-Zener dynamics. We measure the qubit states as a function of driven pulse velocity and successfully reproduce Kibble-Zurek like dependence of topological defect density on the quench rate. The high-level controllability of semiconductor two-level system make it a platform to test the key elements of topological defect formation process and shed a new insight on the aspect of non-equilibrium phase transitions.

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

Experimental realization of non-adiabatic universal quantum gates using geometric Landau-Zener-Stückelberg interferometry

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Back-action-driven electron spin excitation in a single quantum dot

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

Single-Electron Transistor and Quantum Dots on Graphene

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