composite pulses

Robustness of dynamical decoupling sequences | Phys. Rev. A 87, 042309 (2013)

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Robustness of dynamical decoupling sequences

Mustafa Ahmed Ali Ahmed [1,2], Gonzalo A. Álvarez [1,3], and Dieter Suter [1]
1Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany
2Department of Physics, International University of Africa, Khartoum, Sudan
3Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel

Active protection of quantum states is an essential prerequisite for the implementation of quantum computing. Dynamical decoupling (DD) is a promising approach that applies sequences of control pulses to the system in order to reduce the adverse effect of system-environment interactions. Since every hardware device has finite precision, the errors of the DD control pulses can themselves destroy the stored information rather than protect it. We experimentally compare the performance of different DD sequences in the presence of an environment that was chosen such that all relevant DD sequences can equally suppress its effect on the system. Under these conditions, the remaining decay of the qubits under DD allows us to compare very precisely the robustness of the different DD sequences with respect to imperfections of the control pulses.

©2013 American Physical Society

via Phys. Rev. A 87, 042309 (2013): Robustness of dynamical decoupling sequences.

Average error per pulse for different DD sequences with delay τ =100μs. The average decay per pulse for different sequences is plotted against the number of pulses. The most conspicuous feature is that CP performs very badly and CPMG very well. The compensated sequences lie between these two extremes, and we find that the higher order sequences (XY8, KDD perform better than the lower order sequences (XY4). For unknown initial conditions, KDD shows the best performance. Under the present conditions, sequences that differ only with respect to time reversal symmetry perform quite similarly.
Average error per pulse for different DD sequences with delay τ =100μs.
The average decay per pulse for different sequences is plotted against the number of pulses. The most conspicuous feature is that CP performs very badly and CPMG very well. The compensated sequences lie between these two extremes, and we find that the higher order sequences (XY8, KDD perform better than the lower order sequences (XY4). For unknown initial conditions, KDD shows the best performance. Under the present conditions, sequences that differ only with respect to time reversal symmetry perform quite similarly.
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Experimental protection of quantum gates against decoherence and control errors | Phys. Rev. A 86, 050301(R) 2012

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 Experimental protection of quantum gates against decoherence and control errors

Alexandre M. Souza, Gonzalo A. Álvarez, and Dieter Suter
Fakultät Physik, Technische Universität Dortmund, D-44221, Dortmund, Germany

One of the biggest challenges for implementing quantum devices is the requirement to perform accurate quantum gates. The destructive effects of interactions with the environment present some of the most difficult obstacles that must be overcome for precise quantum control. In this work we implement a proof of principle experiment of quantum gates protected against a fluctuating environment and control pulse errors using dynamical decoupling techniques. We show that decoherence can be reduced during the application of quantum gates. High-fidelity quantum gates can be achieved even if the gate time exceeds the free evolution decoherence time by one order of magnitude and for protected operations consisting of up to 330 individual control pulses.

©2012 American Physical Society

via Phys. Rev. A 86, 050301 2012: Experimental protection of quantum gates against decoherence and control errors.

Gate fidelity as a function of gate time for different gate operations protected by different dynamical decoupling (DD) sequences. “Simple” indicates gates that were implemented by unprotected rotations. BB1 means that the gates are only protected by BB1 composite pulses which does not protect against decoherence. The delay between the pulses for the NOOP was ≈ 3μs.
Gate fidelity as a function of gate time for different gate operations protected by different dynamical decoupling (DD) sequences. “Simple” indicates gates that were implemented by unprotected rotations. BB1 means that the gates are only protected by BB1 composite pulses which does not protect against decoherence. The delay between the pulses for the NOOP was ≈ 3μs.

Robust dynamical decoupling | Review Article | Phil. Trans. R. Soc. A 370, 4748 (2012)

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Review article: Robust dynamical decoupling

Alexandre M. Souza, Gonzalo A. Álvarez and Dieter Suter
Fakultät Physik, Technische Universität Dortmund, 44221 Dortmund, Germany

Abstract

Quantum computers, which process information encoded in quantum mechanical systems, hold the potential to solve some of the hardest computational problems. A substantial obstacle for the further development of quantum computers is the fact that the lifetime of quantum information is usually too short to allow practical computation. A promising method for increasing the lifetime, known as dynamical decoupling (DD), consists of applying a periodic series of inversion pulses to the quantum bits. In the present review, we give an overview of this technique and compare different pulse sequences proposed earlier. We show that pulse imperfections, which are always present in experimental implementations, limit the performance of DD. The loss of coherence due to the accumulation of pulse errors can even exceed the perturbation from the environment. This effect can be largely eliminated by a judicious design of pulses and sequences. The corresponding sequences are largely immune to pulse imperfections and provide an increase of the coherence time of the system by several orders of magnitude.

via Robust dynamical decoupling: Phil. Trans. R. Soc. A 370, 4748 (2012).