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Optimized pulses for the control of uncertain qubits

Physical Review A - Atomic, Molecular, and Optical Physics

Carroll, Malcolm; Witzel, Wayne W.

The construction of high-fidelity control fields that are robust to control, system, and/or surrounding environment uncertainties is a crucial objective for quantum information processing. Using the two-state Landau-Zener model for illustrative simulations of a controlled qubit, we generate optimal controls for π/2 and π pulses and investigate their inherent robustness to uncertainty in the magnitude of the drift Hamiltonian. Next, we construct a quantum-control protocol to improve system-drift robustness by combining environment-decoupling pulse criteria and optimal control theory for unitary operations. By perturbatively expanding the unitary time-evolution operator for an open quantum system, previous analysis of environment-decoupling control pulses has calculated explicit control-field criteria to suppress environment-induced errors up to (but not including) third order from π/2 and π pulses. We systematically integrate this criteria with optimal control theory, incorporating an estimate of the uncertain parameter to produce improvements in gate fidelity and robustness, demonstrated via a numerical example based on double quantum dot qubits. For the qubit model used in this work, postfacto analysis of the resulting controls suggests that realistic control-field fluctuations and noise may contribute just as significantly to gate errors as system and environment fluctuations.

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TYPE Journal Article YEAR 2012
OSTIDOI

The QCAD framework for quantum device modeling

Computational Electronics (IWCE), 2012 15th International Workshop on

Gao, Xujiao G.; Nielsen, Erik N.; Muller, Richard P.; Young, Ralph W.; Salinger, Andrew G.; Carroll, Malcolm

We present the Quantum Computer Aided Design (QCAD) simulator that targets modeling quantum devices, particularly Si double quantum dots (DQDs) developed for quantum computing. The simulator core includes Poisson, Schrodinger, and Configuration Interaction solvers which can be run individually or combined self-consistently. The simulator is built upon Sandia-developed Trilinos and Albany components, and is interfaced with the Dakota optimization tool. It is being developed for seamless integration, high flexibility and throughput, and is intended to be open source. The QCAD tool has been used to simulate a large number of fabricated silicon DQDs and has provided fast feedback for design comparison and optimization.

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TYPE Journal Article YEAR 2012
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Spectroscopy and capacitance measurements of tunneling resonances in an Sb-implanted point contact

Bishop, Nathaniel B.; Stevens, Jeffrey S.; Childs, Kenton D.; Ohlhausen, J.A.; Lilly, Michael L.; Carroll, Malcolm; Young, Ralph W.; Bielejec, Edward S.; Ten Eyck, Gregory A.; Wendt, J.R.; Rahman, Rajib R.; Grubbs, Robert K.

We fabricated a split-gate defined point contact in a double gate enhancement mode Si-MOS device, and implanted Sb donor atoms using a self-aligned process. E-beam lithography in combination with a timed implant gives us excellent control over the placement of dopant atoms, and acts as a stepping stone to focused ion beam implantation of single donors. Our approach allows us considerable latitude in experimental design in-situ. We have identified two resonance conditions in the point contact conductance as a function of split gate voltage. Using tunneling spectroscopy, we probed their electronic structure as a function of temperature and magnetic field. We also determine the capacitive coupling between the resonant feature and several gates. Comparison between experimental values and extensive quasi-classical simulations constrain the location and energy of the resonant level. We discuss our results and how they may apply to resonant tunneling through a single donor.

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TYPE Conference YEAR 2010
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Results 76–100 of 109
Results 76–100 of 109