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Microwave-driven coherent operation of a semiconductor quantum dot charge qubit

Nature Nanotechnology

Laros, James H.; Kim, Dohun; Ward, D.R.; Simmons, C.B.; Blume-Kohout, Robin J.; Nielsen, Erik N.; Savage, D.E.; Lagally, M.G.; Friesen, Mark; Coppersmith, S.N.; Eriksson, M.A.

An intuitive realization of a qubit is an electron charge at two well-defined positions of a double quantum dot. This qubit is simple and has the potential for high-speed operation because of its strong coupling to electric fields. However, charge noise also couples strongly to this qubit, resulting in rapid dephasing at all but one special operating point called the 'sweet spot'. In previous studies d.c. voltage pulses have been used to manipulate semiconductor charge qubits but did not achieve high-fidelity control, because d.c. gating requires excursions away from the sweet spot. Here, by using resonant a.c. microwave driving we achieve fast (greater than gigahertz) and universal single qubit rotations of a semiconductor charge qubit. The Z-axis rotations of the qubit are well protected at the sweet spot, and we demonstrate the same protection for rotations about arbitrary axes in the X-Y plane of the qubit Bloch sphere. We characterize the qubit operation using two tomographic approaches: standard process tomography and gate set tomography. Both methods consistently yield process fidelities greater than 86% with respect to a universal set of unitary single-qubit operations.

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TYPE Journal Article YEAR 2015
DOIOSTIScopus

Thermal flux limited electron Kapitza conductance in copper-niobium multilayers

Applied Physics Letters

Cheaito, Ramez; Hattar, Khalid M.; Gaskins, John T.; Yadav, Ajay K.; Duda, John C.; Laros, James H.; Ihlefeld, Jon I.; Piekos, Edward S.; Baldwin, Jon K.; Misra, Amit; Hopkins, Patrick E.

We study the interplay between the contributions of electron thermal flux and interface scattering to the Kapitza conductance across metal-metal interfaces through measurements of thermal conductivity of copper-niobium multilayers. Thermal conductivities of copper-niobium multilayer films of period thicknesses ranging from 5.4 to 96.2 nm and sample thicknesses ranging from 962 to 2677 nm are measured by time-domain thermoreflectance over a range of temperatures from 78 to 500 K. The Kapitza conductances between the Cu and Nb interfaces in multilayer films are determined from the thermal conductivities using a series resistor model and are in good agreement with the electron diffuse mismatch model. Our results for the thermal boundary conductance between Cu and Nb are compared to literature values for the thermal boundary conductance across Al-Cu and Pd-Ir interfaces, and demonstrate that the interface conductance in metallic systems is dictated by the temperature derivative of the electron energy flux in the metallic layers, rather than electron mean free path or scattering processes at the interface.

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TYPE Journal Article YEAR 2015
DOIOSTIScopus

Photoacoustic Sounds from Meteors

Sandia journal manuscript; Not yet accepted for publication

Spalding, Richard E.; Tencer, John T.; Sweatt, W.C.; Laros, James H.; Boslough, Mark B.; Gonzales, Gi G.

High-speed photometric observations of meteor fireballs have shown that they often produce high-amplitude light oscillations with frequency components in the kHz range, and in some cases exhibit strong millisecond flares. We built a light source with similar characteristics and illuminated various materials in the laboratory, generating audible sounds. Models suggest that light oscillations and pulses can radiatively heat dielectric materials, which in turn conductively heats the surrounding air on millisecond timescales. The sound waves can be heard if the illuminated material is sufficiently close to the observer’s ears. The mechanism described herein may explain many reports of meteors that appear to be audible while they are concurrently visible in the sky and too far away for sound to have propagated to the observer. This photoacoustic (PA) explanation provides an alternative to electrophonic (EP) sounds hypothesized to arise from electromagnetic coupling of plasma oscillation in the meteor wake to natural antennas in the vicinity of an observer.

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TYPE Journal Article YEAR 2015
OSTIOSTI
Results 2151–2175 of 2,290
Results 2151–2175 of 2,290