magnetic resonance spectroscopy
Experimental protection of quantum gates against decoherence and control errors | Phys. Rev. A 86, 050301(R) 2012
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
Image from “Iterative rotation scheme for robust dynamical decoupling.” [Gonzalo A. Álvarez, Alexandre M. Souza, and Dieter Suter, Phys. Rev. A 85, 052324 (2012)]
Gonzalo A. Álvarez, Alexandre M. Souza, and Dieter Suter
Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany
Received 1 March 2012; published 29 May 2012
The loss of quantum information due to interactions with external degrees of freedom, which is known as decoherence, remains one of the main obstacles for large-scale implementations of quantum computing. Accordingly, different measures are being explored for reducing its effect. One of them is dynamical decoupling DD which offers a practical solution because it only requires the application of control pulses to the system qubits. Starting from basic DD sequences, more sophisticated schemes were developed that eliminate higher-order terms of the system-environment interaction and are also more robust against experimental imperfections. A particularly successful scheme, called concatenated DD CDD, gives a recipe for generating higher-order sequences by inserting lower-order sequences into the delays of a generating sequence. Here, we show how this scheme can be improved further by converting some of the pulses to virtual and thus ideal pulses. The resulting scheme, called (XY4)^n, results in lower power deposition and is more robust against pulse imperfections than the original CDD scheme.
©2012 American Physical Society
Pieter E. S. Smith, Guy Bensky, Gonzalo A. Álvarez, Gershon Kurizki, and Lucio Frydman
Since the pioneering works of Carr-Purcell and Meiboom-Gill [Carr HY, Purcell EM (1954) Phys Rev 94:630; Meiboom S, Gill D (1985) Rev Sci Instrum 29:688], trains of π-pulses have featured amongst the main tools of quantum control. Echo trains find widespread use in nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI), thanks to their ability to free the evolution of a spin-1/2 from several sources of decoherence. Spin echoes have also been researched in dynamic decoupling scenarios, for prolonging the lifetimes of quantum states or coherences. Inspired by this search we introduce a family of spin-echo sequences, which can still detect site-specific interactions like the chemical shift. This is achieved thanks to the presence of weak environmental fluctuations of common occurrence in high-field NMR—such as homonuclear spin-spin couplings or chemical/biochemical exchanges. Both intuitive and rigorous derivations of the resulting “selective dynamical recoupling” sequences are provided. Applications of these novel experiments are given for a variety of NMR scenarios including determinations of shift effects under inhomogeneities overwhelming individual chemical identities, and model-free characterizations of chemically exchanging partners.chemical exchange dynamic decoupling magnetic field inhomogeneity magnetic resonance quantum control