Weakly Nonlinear Density-Velocity Relation

We rigorously derive weakly nonlinear relation between cosmic density and velocity fields up to third order in perturbation theory. The density field is described by the mass density contrast, \de. The velocity field is described by the variable \te proportional to the velocity divergence, \te = - f(\Omega)^{-1} H_0^{-1} \nabla\cdot\bfv, where $f(\Omega) \simeq \Omega^{0.6}$, $\Omega$ is the cosmological density parameter and $H_0$ is the Hubble constant. Our calculations show that mean \de given \te is a third order polynomial in \te, \lan \de \ran|_{\te} = a_1 \te + a_2 (\te^2 - \s_\te^2) + a_3 \te^3. This result constitutes an extension of the formula \lan \de \ran|_{\te} = \te + a_2 (\te^2 - \s_\te^2), found by Bernardeau~(1992) which involved second order perturbative solutions. Third order perturbative corrections introduce the cubic term. They also, however, cause the coefficient $a_1$ to depart from unity, in contrast with the linear theory prediction. We compute the values of the coefficients $a_p$ for scale-free power spectra, as well as for standard CDM, for Gaussian smoothing. The coefficients obey a hierarchy $a_3 \ll a_2 \ll a_1$, meaning that the perturbative series converges very fast. Their dependence on $\Omega$ is expected to be very weak. The values of the coefficients for CDM spectrum are in qualitative agreement with the results of N-body simulations by Ganon et al. (1996). The results provide a method for breaking the $\Omega$-bias degeneracy in comparisons of cosmic density and velocity fields such as IRAS-POTENT.Comment: 34 pages, figures included, minor changes, a few references added, accepted for publication in MNRA

Quantum state estimation with unknown measurements

Improved measurement techniques are central to technological development and foundational scientific exploration. Quantum optics relies upon detectors sensitive to non-classical features of light, enabling precise tests of physical laws and quantum-enhanced technologies such as precision measurement and secure communications. Accurate detector response calibration for quantum-scale inputs is key to future research and development in these cognate areas. To address this requirement quantum detector tomography (QDT) has been recently introduced. However, the QDT approach becomes increasingly challenging as the complexity of the detector response and input space grows. Here we present the first experimental implementation of a versatile alternative characterization technique to address many-outcome quantum detectors by limiting the input calibration region. To demonstrate the applicability of this approach the calibrated detector is subsequently used to estimate non-classical photon number states.Comment: 7 pages, 3 figure