Robert J. Sewell

Bio: Robert J. Sewell is an academic researcher from ICFO – The Institute of Photonic Sciences. The author has contributed to research in topics: Quantum entanglement & Spin-½. The author has an hindex of 19, co-authored 52 publications receiving 1069 citations. Previous affiliations of Robert J. Sewell include Imperial College London.

Magnetic sensitivity beyond the projection noise limit by spin squeezing.

Abstract: We report the generation of spin squeezing and entanglement in a magnetically sensitive atomic ensemble, and entanglement-enhanced field measurements with this system. A maximal m(f) = ± 1 Raman coherence is prepared in an ensemble of 8.5 × 10(5) laser-cooled (87)Rb atoms in the f = 1 hyperfine ground state, and the collective spin is squeezed by synthesized optical quantum nondemolition measurement. This prepares a state with large spin alignment and noise below the projection-noise level in a mixed alignment-orientation variable. 3.2 dB of noise reduction is observed and 2.0 dB of squeezing by the Wineland criterion, implying both entanglement and metrological advantage. Enhanced sensitivity is demonstrated in field measurements using alignment-to-orientation conversion.

Interaction-based quantum metrology showing scaling beyond the Heisenberg limit

Abstract: Quantum metrology uses entanglement and other quantum resources to improve precision measurement, resulting in sensitivity limited by the Heisenberg uncertainty principle. But in theory, interactions among particles may allow scaling beyond this limit into 'super-Heisenberg' territory. Napolitano et al. prove experimentally that this can indeed occur in a nonlinear, non-destructive measurement of the magnetization of an atomic ensemble. The work shows that inter-particle interactions could be a useful resource for quantum metrology, although the relative performance of nonlinear versus linear measurements has yet to be explored more generally. Quantum metrology aims to use entanglement and other quantum resources to improve precision measurement, resulting in Heisenberg limited sensitivity. However, theory suggests that interactions among particles may allow scaling beyond this limit. This study proves experimentally that this can occur in a nonlinear, non-destructive measurement of the magnetization of an atomic ensemble. The work shows that interparticle interactions could be a useful resource for quantum metrology, although the relative performance of nonlinear versus linear measurements has yet to be explored more generally. Quantum metrology aims to use entanglement and other quantum resources to improve precision measurement1. An interferometer using N independent particles to measure a parameter can achieve at best the standard quantum limit of sensitivity, δ ∝ N−1/2. However, using N entangled particles and exotic states2, such an interferometer3 can in principle achieve the Heisenberg limit, δ ∝ N−1. Recent theoretical work4,5,6 has argued that interactions among particles may be a valuable resource for quantum metrology, allowing scaling beyond the Heisenberg limit. Specifically, a k-particle interaction will produce sensitivity δ ∝ N−k with appropriate entangled states and δ ∝ N−(k−1/2) even without entanglement7. Here we demonstrate ‘super-Heisenberg’ scaling of δ ∝ N−3/2 in a nonlinear, non-destructive8,9 measurement of the magnetization10,11 of an atomic ensemble12. We use fast optical nonlinearities to generate a pairwise photon–photon interaction13 (corresponding to k = 2) while preserving quantum-noise-limited performance7,14. We observe super-Heisenberg scaling over two orders of magnitude in N, limited at large numbers by higher-order nonlinear effects, in good agreement with theory13. For a measurement of limited duration, super-Heisenberg scaling allows the nonlinear measurement to overtake in sensitivity a comparable linear measurement with the same number of photons. In other situations, however, higher-order nonlinearities prevent this crossover from occurring, reflecting the subtle relationship between scaling and sensitivity in nonlinear systems. Our work shows that interparticle interactions can improve sensitivity in a quantum-limited measurement, and experimentally demonstrates a new resource for quantum metrology.

Generation of macroscopic singlet states in a cold atomic ensemble.

Abstract: A spin entangled system of more than a million laser cooled atoms has been created using spin measurements along three orthogonal axes -- the first measurement-induced spin squeezing to a singlet state.

Measuring Energy Differences by BEC Interferometry on a Chip

Abstract: We investigate the use of a Bose-Einstein condensate trapped on an atom chip for making interferometric measurements of small energy differences. We measure and explain the noise in the energy difference of the split condensates, which derives from statistical noise in the number difference. We also consider systematic errors. A leading effect is the variation of the rf magnetic field in the trap with distance from the wires on the chip surface. This can produce energy differences that are comparable with those due to gravity.

Interaction-based quantum metrology showing scaling beyond the Heisenberg limit

Abstract: Atom-mediated optical nonlinearities, within an atom-light quantum interface, allow spin measurement with sensitivity scaling better than the Heisenberg limit. This demonstrates the use of interactions as a new resource for quantum metrology

Advances in quantum metrology

Abstract: The statistical error in any estimation can be reduced by repeating the measurement and averaging the results. The central limit theorem implies that the reduction is proportional to the square root of the number of repetitions. Quantum metrology is the use of quantum techniques such as entanglement to yield higher statistical precision than purely classical approaches. In this Review, we analyse some of the most promising recent developments of this research field and point out some of the new experiments. We then look at one of the major new trends of the field: analyses of the effects of noise and experimental imperfections.

Quantum sensing

Abstract: "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

Quantum metrology with nonclassical states of atomic ensembles

Abstract: Quantum technologies exploit entanglement to revolutionize computing, measurements, and communications. This has stimulated the research in different areas of physics to engineer and manipulate fragile many-particle entangled states. Progress has been particularly rapid for atoms. Thanks to the large and tunable nonlinearities and the well-developed techniques for trapping, controlling, and counting, many groundbreaking experiments have demonstrated the generation of entangled states of trapped ions, cold, and ultracold gases of neutral atoms. Moreover, atoms can strongly couple to external forces and fields, which makes them ideal for ultraprecise sensing and time keeping. All these factors call for generating nonclassical atomic states designed for phase estimation in atomic clocks and atom interferometers, exploiting many-body entanglement to increase the sensitivity of precision measurements. The goal of this article is to review and illustrate the theory and the experiments with atomic ensembles that have demonstrated many-particle entanglement and quantum-enhanced metrology.

The elusive Heisenberg limit in quantum-enhanced metrology

Abstract: Quantum metrology employs the properties of quantum states to further enhance the accuracy of some of the most precise measurement schemes to date. Here, a method for estimating the upper bounds to achievable precision in quantum-enhanced metrology protocols in the presence of decoherence is presented.

Quantum metrology from a quantum information science perspective

Abstract: We summarize important recent advances in quantum metrology, in connection to experiments in cold gases, trapped cold atoms and photons. First we review simple metrological setups, such as quantum metrology with spin squeezed states, with Greenberger–Horne–Zeilinger states, Dicke states and singlet states. We calculate the highest precision achievable in these schemes. Then, we present the fundamental notions of quantum metrology, such as shot-noise scaling, Heisenberg scaling, the quantum Fisher information and the Cramer–Rao bound. Using these, we demonstrate that entanglement is needed to surpass the shot-noise scaling in very general metrological tasks with a linear interferometer. We discuss some applications of the quantum Fisher information, such as how it can be used to obtain a criterion for a quantum state to be a macroscopic superposition. We show how it is related to the speed of a quantum evolution, and how it appears in the theory of the quantum Zeno effect. Finally, we explain how uncorrelated noise limits the highest achievable precision in very general metrological tasks.This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to '50 years of Bell's theorem'.