Douglas McClure

Shot noise in graphene.

We report measurements of current noise in single-layer and multilayer graphene devices. In four single-layer devices, including a p-n junction, the Fano factor remains constant to within +/-10% upon varying carrier type and density, and averages between 0.35 and 0.38. The Fano factor in a multilayer device is found to decrease from a maximal value of 0.33 at the charge-neutrality point to 0.25 at high carrier density. These results are compared to theories for shot noise in ballistic and disordered graphene.

Tunable noise cross correlations in a double quantum dot.

We report measurements of the cross correlation between temporal current fluctuations in two capacitively coupled quantum dots in the Coulomb blockade regime. The sign of the cross-spectral density is found to be tunable by gate voltage and source-drain bias. We find good agreement with the data by including an interdot Coulomb interaction in a sequential-tunneling model.

Distinct Signatures for Coulomb Blockade and Aharonov-Bohm Interference in Electronic Fabry-Perot Interferometers

Two distinct types of magnetoresistance oscillations are observed in two electronic Fabry-Perot interferometers of different sizes in the integer quantum Hall regime. Measuring these oscillations as a function of magnetic field and gate voltages, we describe three signatures that distinguish the two types. The oscillations observed in a

System for measuring auto- and cross correlation of current noise at low temperatures

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Edge-State Velocity and Coherence in a Quantum Hall Fabry-Pérot Interferometer

device are understood to arise from a Coulomb blockade mechanism and those observed in an

Shot-noise signatures of 0.7 structure and spin in a quantum point contact.

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Fabry-Perot interferometry with fractional charges.

device from an Aharonov-Bohm mechanism. This work clarifies, provides ways to distinguish, and demonstrates control over these distinct mechanisms of oscillations seen in electronic Fabry-Perot interferometers.

Noise correlations in a Coulomb-blockaded quantum dot.

We describe the construction and operation of a two-channel noise detection system for measuring power and cross spectral densities of current fluctuations near 2MHz in electronic devices at low temperatures. The system employs cryogenic amplification and fast Fourier transform based spectral measurement. The gain and electron temperature are calibrated using Johnson noise thermometry. Full shot noise of 100pA can be resolved with an integration time of 10s. We report a demonstration measurement of bias-dependent current noise in a gate defined GaAs∕AlGaAs quantum point contact.

Bulk and surface loss in superconducting transmon qubits

We investigate nonlinear transport in electronic Fabry-Pérot interferometers in the integer quantum Hall regime. For interferometers sufficiently large that Coulomb blockade effects are absent, a checkerboardlike pattern of conductance oscillations as a function of dc bias and perpendicular magnetic field is observed. Edge-state velocities extracted from the checkerboard data are compared to model calculations and found to be consistent with a crossover from skipping orbits at low fields to E-vector x B-vector drift at high fields. Suppression of visibility as a function of bias and magnetic field is accounted for by including energy- and field-dependent dephasing of edge electrons.

Investigating Surface Loss Effects in Superconducting Transmon Qubits

We report simultaneous measurement of shot noise and dc transport in a quantum point contact as a function of source-drain bias, gate voltage, and in-plane magnetic field. Shot noise at zero field exhibits an asymmetry related to the 0.7 structure in conductance. The asymmetry in noise evolves smoothly into the symmetric signature of spin-resolved electron transmission at high field. Comparison to a phenomenological model with density-dependent level splitting yields good quantitative agreement.