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Simmons, C. B.

Publications and source records attributed to Simmons, C. B..

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Signatures of atomic-scale structure in the energy dispersion and coherence of a Si quantum-dot qubit

In this paper, we report anomalous behavior in the energy dispersion of a three-electron double-quantum-dot hybrid qubit and argue that it is caused by atomic-scale disorder at the quantum-well interface. By employing tight-binding simulations, we identify potential disorder profiles that induce behavior consistent with the experiments. The results indicate that disorder can give rise to “sweet spots” where the decoherence caused by charge noise is suppressed, even in a parameter regime where true sweet spots are unexpected. Conversely, “hot spots” where the decoherence is enhanced can also occur. Our results suggest that interfacial atomic structure can be used in particular cases as a tool to enhance the fidelity of Si double-dot qubits.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Extending the coherence of a quantum dot hybrid qubit

Identifying and ameliorating dominant sources of decoherence are important steps in understanding and improving quantum systems. Here we show that the free induction decay time (T$*\atop{2}$) and the Rabi decay rate (Γ Rabi ) of the quantum dot hybrid qubit can be increased by more than an order of magnitude by appropriate tuning of the qubit parameters and operating points. Thus, by operating in the spin-like regime of this qubit, and choosing parameters that increase the qubit’s resilience to charge noise (which we show is presently the limiting noise source for this qubit), we achieve a Ramsey decay time T$*\atop{2}$ of 177 ns and a Rabi decay time 1/Γ Rabi exceeding 1 µs. Furthermore, we find that the slowest Γ Rabi is limited by fluctuations in the Rabi frequency induced by charge noise and not by fluctuations in the qubit energy itself.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Combining experiment and optical simulation in coherent X-ray nanobeam characterization of Si/SiGe semiconductor heterostructures

The highly coherent and tightly focused x-ray beams produced by hard x-ray light sources enable the nanoscale characterization of the structure of electronic materials but are accompanied by significant challenges in the interpretation of diffraction and scattering patterns. X-ray nanobeams exhibit optical coherence combined with a large angular divergence introduced by the x-ray focusing optics. The scattering of nanofocused x-ray beams from intricate semiconductor heterostructures produces a complex distribution of scattered intensity. We report here an extension of coherent x-ray optical simulations of convergent x-ray beam diffraction patterns to arbitrary x-ray incident angles to allow the nanobeam diffraction patterns of complex heterostructures to be simulated faithfully. These methods are used to extract the misorientation of lattice planes and the strain of individual layers from synchrotron x-ray nanobeam diffraction patterns of Si/SiGe heterostructures relevant to applications in quantum electronic devices. The systematic interpretation of nanobeam diffraction patterns from semiconductor heterostructures presents a new opportunity in characterizing and ultimately designing electronic materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

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

An intuitive realization of a qubit is an electron charge at two well-defined positions of a double quantum dot. The qubit is simple and has the potential for high-speed operation because of its strong coupling to electric fields. But, 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. Moreover, both methods consistently yield process fidelities greater than 86% with respect to a universal set of unitary single-qubit operations.

77 NANOSCIENCE AND NANOTECHNOLOGY↗