Abstract: Entanglement and non-Gaussianity are physical resources that are essential for a large number of quantum-optics protocols. Non-Gaussian entanglement is indispensable for quantum-computing advantage and outperforms its Gaussian counterparts in a number of quantum-information protocols. The characterization of non-Gaussian entanglement is a critical matter as it is in general highly demanding in terms of resources. We propose a simple protocol based on the Fisher information for witnessing entanglement in an important class of non-Gaussian entangled states: photon-subtracted states. We demonstrate that our protocol is relevant for the detection of non-Gaussian entanglement generated by multiple photon-subtraction and that it is experimentally feasible through homodyne detection.

Abstract: We identify the multiparameter sensitivity of entangled spin states, such as spin-squeezed and Dicke states that are spatially distributed into several addressable spatial modes. Analytical expressions for the spin-squeezing matrix of families of states that are accessible by current atomic experiments reveal the quantum gain in multiparameter metrology, as well as the optimal strategies to maximize the sensitivity gain for the estimation of any linear combination of parameters. We further study the mode entanglement of these states by deriving a witness for genuine k-partite mode entanglement from the spin-squeezing matrix. Our results highlight the advantage of mode entanglement for distributed sensing, and outline optimal protocols for multiparameter estimation with nonclassical spatially-distributed spin ensembles. We illustrate our findings with the design of a protocol for gradient sensing with a Bose-Einstein condensate in an entangled spin state in two modes.

Abstract: Background: Positron Emission Tomography (PET) is a medical imaging modality that utilizes positron-emitting isotopes for many diagnostic purposes. The positron annihilates with an electron, creating two photons of 511 keV energy and opposite momenta, entangled in their orthogonal polarizations. If each photon undergoes a Compton scattering process, the difference of their azimuthal scattering angles reflects the initial orthogonality of the polarizations, peaking at +/- 90 degrees. This correlation, not yet utilized in conventional PET scanners, offers an additional, energy-independent method for background discrimination. Methods: The correlation can be measured using single-layer Compton polarimeters, compatible with conventional PET architecture. We assembled a demonstrator with two such modules comprising 3 & times; 3 & times; 20 mm3 GAGG:Ce scintillating pixels in 16 & times; 16 matrix, read by silicon photomultipliers, mounted on a rotating gantry with 430 mm diameter. Results: This paper reports on the study of random background in measurements with Ga-68 source with activities 200-380 MBq. We compare polarization-correlated Compton events, having a two-hit signature, to conventional single-pixel hits. The signal-to-random background ratio (SBR) obtained in the polarization-correlated events is larger than the one for the single-pixel hits, for all selected event samples. We also demonstrate a correlation between the SBR and the polarimetric modulation factor. Conclusion: The random background suppression in the measurements of the polarization-correlated annihilation quanta is higher than in the standard PET modality, which could be a valuable resource for PET imaging. Since there is a correlation between the SBR and the polarimetric modulation factor, it could serve as an estimator of the random background.