high fidelity

Rev. Mod. Phys.:Protecting quantum information against environmental noise

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Colloquium: Protecting quantum information against environmental noise

Dieter Suter and Gonzalo A. Álvarez
Rev. Mod. Phys. 88, 041001 (2016)
Published 10 October 2016

RevModPhys.88.041001Quantum-mechanical systems retain their properties so long as the phase of quantum superpositions evolve stably over time. Contact with an environment can disrupt this phase evolution. But for environments that do not exchange energy with the quantum system, strategies exist where the controlled driving of the system can recover or maintain the quantum phase. This Colloquium surveys the host of techniques that are available to “refocus” the phase when disturbed by various forms of classical or quantum environment. While the first such techniques were developed long ago, ideas from quantum information theory have introduced new strategies for accomplishing this goal.

Source: Reviews of Modern Physics – Volume 88 Issue 4

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Quantum state transfer in disordered spin chains: How much engineering is reasonable? | Quant. Inf. Comm. (2015)

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Analia Zwick, Gonzalo A. Álvarez, Joachim Stolze, and Omar Osenda

Quant. Inf. Comput. 15, 582-600 (2015).

The transmission of quantum states through spin chains is an important element in the im- plementation of quantum information technologies. Speed and fidelity of transfer are the main objectives which have to be achieved by the devices even in the presence of imperfections which are unavoidable in any manufacturing process. To reach these goals, several kinds of spin chains have been suggested, which differ in the degree of fine-tuning, or engineering, of the system parameters. In this work we present a systematic study of two important classes of such chains. In one class only the spin couplings at the ends of the chain have to be adjusted to a value different from the bulk coupling constant, while in the other class every coupling has to have a specific value. We demonstrate that configurations from the two different classes may perform similarly when subjected to the same kind of disorder in spite of the large difference in the engineering effort necessary to prepare the system. We identify the system features responsible for these similarities and we perform a detailed study of the transfer fidelity as a function of chain length and disorder strength, yielding empirical scaling laws for the fidelity which are similar for all kinds of chain and all dis- order models. These results are helpful in identifying the optimal spin chain for a given quantum information transfer task. In particular, they help in judging whether it is worthwhile to engineer all couplings in the chain as compared to adjusting only the boundary couplings.

via [1306.1695] Quantum state transfer in disordered spin chains: How much engineering is reasonable?.

Comparison of the averaged state transfer fidelity for different quantum state transfer channels. The left hand side panels are boundary controlled spin-chain channels and the right hand side panels are fully engineered perfect state transfer channels. Two kinds of disorder are considered in the plot: Absolute disorder with a perturbation strength proportional to the maximum coupling strength of the spin-chain or relative disorder when the perturbation strength in each spin-spin coupling is relative to its optimal value. For the boundary controlled spin channels, both types of disorder are equivalent since the bulk of the chains contains homogeneous couplings, while for the fully engineered spin-channels they provide different effects on the transfer fidelity. The average is calculated over 1000 disorder realizations. The black contour lines belong to fidelities F = 0.99, 0.95, 0.9, 0.8, 0.7, respectively. The colored symbols show the crossovers between the different systems.
Comparison of the averaged state transfer fidelity for different quantum state transfer channels. The left hand side panels are boundary controlled spin-chain channels and the right hand side panels are fully engineered perfect state transfer channels. Two kinds of disorder are considered in the plot: Absolute disorder with a perturbation strength proportional to the maximum coupling strength of the spin-chain or relative disorder when the perturbation strength in each spin-spin coupling is relative to its optimal value. For the boundary controlled spin channels, both types of disorder are equivalent since the bulk of the chains contains homogeneous couplings, while for the fully engineered spin-channels they provide different effects on the transfer fidelity. The average is calculated over 1000 disorder realizations. The black contour lines belong to fidelities F = 0.99, 0.95, 0.9, 0.8, 0.7, respectively. The colored symbols show the crossovers between the different systems.

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.

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.