Search for: All records

Trapped ions are a well-developed platform to estimate a phase imprinted on their internal quantum states. However, an optimal quantum-enhanced protocol to robustly read out the phase under practical conditions in trapped ions has not been explored. Here, we study such a protocol via interaction-based readout in trapped ions with a one-axis twisting Hamiltonian considering a practical experimental situation. We start from a more fundamental spin-motion coupling in trapped ions, mediated by laser beams, to explore the dependence of the phase sensitivity on the specific interaction. We evaluate the decoherence effects of both Rayleigh scattering and Raman scattering due to spontaneous emission of the laser beams. We find that the phase sensitivity is mostly robust to decoherence when the system is operated in the so-called “squeezing regime”. Interestingly, we find that larger Raman scattering leads to slightly better phase sensitivity for a fixed total decoherence rate in this regime. We further analyze the effects of the frequency fluctuations from the center-of-mass (COM) mode in the phase sensing protocol and find that they are negligible under practical experimental parameters. Our work lays the theoretical foundation for estimating a collective phase rotation using entangled spin states in trapped ions. 

Gaussian states with nonclassical properties such as squeezing and entanglement serve as crucial resources for quantum information processing. Accurately quantifying these properties within multimode Gaussian states has posed some challenges. To address this, we introduce a unified quantification: the “classical-nonclassical polarity,” represented by P. For a single mode, a positive value of P captures the reduced minimum quadrature uncertainty below the vacuum noise, while a negative value represents an enlarged uncertainty due to classical mixtures. For multimode systems, a positive P indicates bipartite quantum entanglement. We show that the sum of the total classical-nonclassical polarity is conserved under arbitrary linear optical transformations for any two-mode and three-mode Gaussian states. For any pure multimode Gaussian state, the total classical-nonclassical polarity equals the sum of the mean photon number from single-mode squeezing and two-mode squeezing. Our results provide a new perspective on the quantitative relation between single-mode nonclassicality and entanglement, which may find applications in a unified resource theory of nonclassical features. 

Abstract It is well-known that the precision of a phase measurement with a Mach-Zehnder interferometer employing strong classic light can be greatly enhanced with the addition of weak nonclassical light. In the context of quantifying nonclassicality, the amount by which a nonclassical state can enhance precision in this way has been termed its ’metrological power’. To-date, the enhancement provided by weak nonclassical states has been calculated only for specific measurement configurations. Here we are able to optimize over all measurement configurations to obtain the maximum enhancement that can be achieved by any single or multi-mode nonclassical state together with strong classical states, for local and distributed quantum metrology employing any linear or nonlinear single-mode unitary transformation. Our analysis reveals that the quantum Fisher information for quadrature-displacement sensing is the sole property that determines the maximum achievable enhancement in all of these different scenarios, providing a unified quantification of the metrological power.