quantum physics

Robustness of Spin-Chain State-Transfer Schemes – Springer

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Robustness of Spin-Chain State-Transfer Schemes

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

in Quantum State Transfer and Network Engineering, edited by G. M. Nikolopoulos and I. Jex (Springer Berlin Heidelberg, 2014), pp. 149–182.

Abstract

Spin chains are linear arrangements of qubits (spin-1/2 objects) with interactions between nearest or more distant neighbors. They have been considered for quantum information transfer between subunits of a quantum information processing device at short or intermediate distances. The most frequently studied task is the transfer of a single-qubit state. Several protocols have been developed to achieve this goal, broadly divisible into two classes, fully-engineered and boundary-controlled spin chains. We discuss state transfer induced by the natural dynamics of these two classes of systems, and the influence of deviations from the ideal system configuration, that is, manufacturing errors in the nearest-neighbor spin couplings. The fidelity of state transfer depends on the chain length and the disorder strength. We observe a power-law scaling of the fidelity deficit, i.e. the deviation from perfect transfer. Boundary-controlled chains can provide excellent fidelity under suitable circumstances and are potentially less difficult to manufacture and control than fully-engineered chains. We also review other existing theoretical work on the robustness of quantum state transfer as well as proposals for experimental implementation.

via Robustness of Spin-Chain State-Transfer Schemes – Springer.

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Coherent dynamical recoupling of diffusion-driven decoherence in magnetic resonance | Phys. Rev. Lett. 111, 080404 (2013)

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Coherent dynamical recoupling of diffusion-driven decoherence in magnetic resonance

Gonzalo A. Álvarez, Noam Shemesh, and Lucio Frydman
Department of Chemical Physics, Weizmann Institute of Science, Rehovot, 76100, Israel
Received 13 May 2013; published 20 August 2013

During recent years, dynamical decoupling (DD) has gained relevance as a tool for manipulating and interrogating quantum systems. This is particularly relevant for spins involved in nuclear magnetic resonance (NMR), where DD sequences can be used to prolong quantum coherences, or to selectively couple or decouple the effects imposed by random environmental fluctuations. In this Letter, we show that these concepts can be exploited to selectively recouple diffusion processes in restricted spaces. The ensuing method provides a novel tool to measure restriction lengths in confined systems such as capillaries, pores or cells. The principles of this method for selectively recoupling diffusion-driven decoherence, its standing within the context of diffusion NMR, extensions to the characterization of other kinds of quantum fluctuations, and corroborating experiments, are presented.

© 2013 American Physical Society

via Phys. Rev. Lett. 111, 080404 (2013): Coherent Dynamical Recoupling of Diffusion-Driven Decoherence in Magnetic Resonance.

arXiv: [1305.2794] Coherent dynamical recoupling of diffusion-driven decoherence in magnetic resonance.

Time evolution of the spin magnetization under CPMG (N = 8 pulses) and Hahn-echo sequences for spins diffusing in a restricted space (circles, triangles), and under free diffusion (crosses, dashes). The solid black lines show the time range where the restricted diffusion effects dominate; the difference ∆M_SDR between these lines gives a contrast, over which signals can be coherently modulated by a suitable Selective Dynamical Decoupling (SDR) filter function.
Time evolution of the spin magnetization under CPMG (N = 8 pulses) and Hahn-echo sequences for spins diffusing in a restricted space (circles, triangles), and under free diffusion (crosses, dashes). The solid black lines show the time range where the restricted diffusion effects dominate; the difference ∆M_SDR between these lines gives a contrast, over which signals can be coherently modulated by a suitable Selective Dynamical Decoupling (SDR) filter function.
Experimental SDR signals normalized with the first data point (symbols) as a function of the x delays of the SDR sequence. The solid lines are analytical fittings of our model to the experimental curve. By using the measured diffusion coefficient D0 ∼ 2.3 × 10−5 cm^2/s, the fitted diameter d given in the plots is in agreement with the nominal value d = 5 ± 1μm.
Experimental SDR signals normalized with the first data point (symbols) as a function of the x delays of the SDR sequence. The solid lines are analytical fittings of our model to the experimental curve. By using the measured diffusion coefficient D0 ∼ 2.3 × 10−5 cm^2/s, the fitted diameter d given in the plots is in agreement with the nominal value d = 5 ± 1μm. The behavior of the SDR curves resemble the root mean square displacement of the diffusing spins in a restricted space: in both cases curves plateu for times x larger than the correlation time characteristic of achieving a restricted diffusion regime, evidence a full sampling of the restricting space.

Random – but not quite: exploiting quantum decoherence as a tool for characterizing unknown systems | Seminar

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SEMINAR at IV Quantum Information Workshop – Paraty 2013

Wednesday, August 14th 2013

See comments of the talk at the Paraty 2013’s Blog: Decoupling system and environment.

SEMINAR at CBPF, Rio de Janeiro – Brazil, August 20th, 2013

SEMINAR at FaMAF, Córdoba – Argentina, August 27th, 2013

Abstract

The ability to understand and manipulate the dynamics of quantum systems that interact with external degrees of freedom is a major challenge for fundamental quantum physics and its diverse applications, e.g., quantum information processing (QIP) or precision measurements. Progress in dynamical decoupling has lead to new ways to “protect” quantum bits from external degrees of freedom. Sometimes, however, a little bit of “recoupling” –i.e., exposure to the unknowns of the surrounding medium– can help. In this seminar, I will present a series of experimental methods implemented in NMR where by varying the “protection” given to the quantum states of ½-spins (qubits) can lead to new tools for characterizing unknown systems. In particular, I will show how Dynamical Decoupling noise spectroscopy can probe the spectrum of the environmental noise in order to find optimal methods for protecting the qubits [1]. In a new twist, I will present a method termed Selective Dynamical Recoupling (SDR), where suitable designed pulse sequence applied to the spins can selectively address specific information from the probed systems. SDR can be used to measure coupling strengths to the environment via oscillatory modulations that can serve for example to probe chemical identities derived from chemical shifts [2]. Alternatively, SDR can be designed to selectively measure the correlation time of the environmental noise where its value can be useful to probe diffusion processes in restricted spaces to extract the sizes of pores or cells in a non-invasive manner [3]. Applications of this new and simple approach can be found in materials sciences and biology and in particular it can be useful for investigating the nature of tissue compartmentalization in vivo, in manners which eventually could be useful in human and clinical settings.

[1] G.A. Alvarez, and D. Suter. Phys. Rev. Lett. 107, 230501 (2011).

[2] P.E.S. Smith, G. Bensky, G.A. Alvarez, G. Kurizki, and L. Frydman. Proc. Natl. Acad. Sci. U. S. A. 109, 5958 (2012).

[3] G.A. Alvarez, N. Shemesh, and L. Frydman. Phys. Rev. Lett. 111, 080404 (2013).

Compartment size map of a ex-vivo mouse brain masked for the corpus callosum
Compartment size map of a ex-vivo mouse brain masked for the corpus callosum. The compartment sizes were measured by implementing Selective Dynamical Recoupling pulse sequences to spin-1/2 carrying molecules for selectively addressing the correlation time of their diffusion process.

Random – but not quite: exploiting quantum decoherence as a tool for characterizing unknown systems | Paraty 2013

Posted on Updated on

SEMINAR at IV Quantum Information Workshop – Paraty 2013

Wednesday, August 14th 2013

See comments of the talk at the Paraty 2013’s Blog: Decoupling system and environment.

Abstract

The ability to understand and manipulate the dynamics of quantum systems that interact with external degrees of freedom is a major challenge for fundamental quantum physics and its diverse applications, e.g., quantum information processing (QIP) or precision measurements. Progress in dynamical decoupling has lead to new ways to “protect” quantum bits from external degrees of freedom. Sometimes, however, a little bit of “recoupling” –i.e., exposure to the unknowns of the surrounding medium– can help. In this seminar, I will present a series of experimental methods implemented in NMR where by varying the “protection” given to the quantum states of ½-spins (qubits) can lead to new tools for characterizing unknown systems. In particular, I will show how Dynamical Decoupling noise spectroscopy can probe the spectrum of the environmental noise in order to find optimal methods for protecting the qubits [1]. In a new twist, I will present a method termed Selective Dynamical Recoupling (SDR), where suitable designed pulse sequence applied to the spins can selectively address specific information from the probed systems. SDR can be used to measure coupling strengths to the environment via oscillatory modulations that can serve for example to probe chemical identities derived from chemical shifts [2]. Alternatively, SDR can be designed to selectively measure the correlation time of the environmental noise where its value can be useful to probe diffusion processes in restricted spaces to extract the sizes of pores or cells in a non-invasive manner [3]. Applications of this new and simple approach can be found in materials sciences and biology and in particular it can be useful for investigating the nature of tissue compartmentalization in vivo, in manners which eventually could be useful in human and clinical settings.

[1] G.A. Alvarez, and D. Suter. Phys. Rev. Lett. 107, 230501 (2011).

[2] P.E.S. Smith, G. Bensky, G.A. Alvarez, G. Kurizki, and L. Frydman. Proc. Natl. Acad. Sci. U. S. A. 109, 5958 (2012).

[3] G.A. Alvarez, N. Shemesh, and L. Frydman. Phys. Rev. Lett. (2013) – in press. arXiv:1305.2794.

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.

Robustness of spin-chain state-transfer schemes

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Joachim Stolze, Gonzalo A. Álvarez, Omar Osenda, and Analia Zwick
To appear in Georgios M. Nikolopoulos and Igor Jex, editors:
Quantum State Transfer and Quantum Network Engineering. Springer Series in Quantum Science and Technology, Springer, Berlin 2013.

Spin chains are linear arrangements of qubits (spin-1/2 objects) with interactions between nearest or more distant neighbors. They have been considered for quantum information transfer between subunits of a quantum information processing device at short or intermediate distances. The most frequently studied task is the transfer of a single-qubit state. Several protocols have been developed to achieve this goal, broadly divisible into two classes, fully-engineered and boundary- controlled spin chains. We discuss state transfer induced by the natural dynamics of these two classes of systems, and the influence of deviations from the ideal system configuration, that is, manufacturing errors in the nearest-neighbor spin couplings. The fidelity of state transfer depends on the chain length and the disorder strength. We observe a power-law scaling of the fidelity deficit, i.e. the deviation from perfect transfer. Boundary-controlled chains can provide excellent fidelity under suitable circumstances and are potentially less difficult to manufacture and control than fully-engineered chains. We also review other existing theoretical work on the robustness of quantum state transfer as well as proposals for experimental implementation.