Enhancing the robustness of dynamical decoupling sequences with correlated random phases

We show that the addition of correlated phases to the recently developed method of randomized dynamical decoupling pulse sequences [Physical Review Letters 122, 200403 (2019)] can improve its performance in quantum sensing. In particular, by correlating the relative phases of basic pulse units in dynamical decoupling sequences, we are able to improve the suppression of the signal distortion due to $\pi$ pulse imperfections and spurious responses due to finite-width $\pi$ pulses. This enhances selectivity of quantum sensors such as those based on NV centers in diamond

Robust Trapped-Ion Quantum Logic Gates by Continuous Dynamical Decoupling

We introduce a novel scheme that combines phonon-mediated quantum logic gates in trapped ions with the benefits of continuous dynamical decoupling. We demonstrate theoretically that a strong driving of the qubit decouples it from external magnetic-field noise, enhancing the fidelity of two-qubit quantum gates. Moreover, the scheme does not require ground-state cooling, and is inherently robust to undesired ac-Stark shifts. The underlying mechanism can be extended to a variety of other systems where a strong driving protects the quantum coherence of the qubits without compromising the two-qubit couplings.Comment: Slightly longer than the published versio

Quantum sensing

"Quantum sensing" describes the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity. Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors, or atomic clocks. More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits. The field is expected to provide new opportunities - especially with regard to high sensitivity and precision - in applied physics and other areas of science. In this review, we provide an introduction to the basic principles, methods and concepts of quantum sensing from the viewpoint of the interested experimentalist.Comment: 45 pages, 13 figures. Submitted to Rev. Mod. Phy

Synchronization in complex networks

Synchronization processes in populations of locally interacting elements are in the focus of intense research in physical, biological, chemical, technological and social systems. The many efforts devoted to understand synchronization phenomena in natural systems take now advantage of the recent theory of complex networks. In this review, we report the advances in the comprehension of synchronization phenomena when oscillating elements are constrained to interact in a complex network topology. We also overview the new emergent features coming out from the interplay between the structure and the function of the underlying pattern of connections. Extensive numerical work as well as analytical approaches to the problem are presented. Finally, we review several applications of synchronization in complex networks to different disciplines: biological systems and neuroscience, engineering and computer science, and economy and social sciences.Comment: Final version published in Physics Reports. More information available at http://synchronets.googlepages.com

Dynamical decoupling based quantum sensing: Floquet analysis and finite-duration-pulse effects

A spin qubit can be protected from a dephasing spin bath using dynamical decoupling (DD). Microwave pi-pulses are repeatedly applied to the spin qubit to invert its state and average out any dephasing. Importantly, this protection fails when the DD pulse spacing is resonant with nuclear spins in the bath and characteristic dips appear in coherence traces forming the basis for nanoscale NMR and MRI. This emerging quantum technology has been demonstrated with the nitrogen vacancy center in diamond. Most DD protocols apply periodic repetitions of a basic pulse unit. This repetition motivates us to model the experiments using Floquet analysis. The characteristic coherence dips are found to be associated with avoided crossings in an underlying Floquet spectrum. The width and shape of these crossings determines the contrast and sharpness of the coherence dips. We derive analytic expressions for the coherence dips in terms of the Floquet quasienergies and Floquet modes. Typically, the DD microwave pulses are modelled as being instantaneous; however, real pulses have some finite duration and it was recently demonstrated that this pulse duration can cause extra dips to appear in coherence traces. We apply Floquet analysis to accurately model the complete system dynamics in the presence of these finite duration pulses and derive analytic expressions for the complete coherence response. We interpret the arrival of extra coherence dips as the opening of previously closed avoided crossings. We use this new understanding to propose protocols to exploit (for increased resolution) or suppress these extra coherence dips. Finally, we model the interplay between finite-duration-pulse effects and microwave detuning errors - an important problem as the detuning error is completely removed by instantaneous pulses so is not captured by most analytic models. We observe drastic effects including the splitting and suppression of the expected DD signal