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We study an inverse scattering problem in which the far-field spectral cross-correlation functions of scattered fields are used to determine the unknown dielectric susceptibility of the scattering object. One-photon states for the incident field can resolve (at $100\%$ visibility) twice more Fourier components of the susceptibility compared to the (naive) Rayleigh estimate, provided that the measurement is performed in the back-scattering regime. Coherent states are not capable of reaching this optimal resolution (or do so with negligible visibility). Using two-photon states improves upon the one-photon resolution, but the improvement (at $100\%$ visibility) is smaller than twice, and it demands prior information on the object. This improvement can also be realized via two independent laser fields. The dependence on the prior information can be decreased (but not eliminated completely) upon using entangled states of two photons.

Using a phenomenological approach to field quantization, an expression for the Keldysh function of photons between two planar interfaces (Casimir geometry) is found for any stationary quantum state of the two bodies. The case of one interface sliding against the other is considered in detail.

In this article we show that Einstein covariance principle provides a wide opportunity in the solutions of different problems of theoretical physics. Here we apply covariance principle in some problems of classical electrodynamics and kinetics. Extension of this approach in the other fields is obvious., Comment: 16 pages

We show that the Einstein covariance principle provides an opportunity to generate infinitely many solutions of given covariant equation from a known one. With use of this statement we derive exact expressions for charge and current densities in a medium in an arbitrary external field. We obtain that in a homogeneous nondispersive medium the field of a point particle differs at short-distances from the well known expression. We also demonstrate that in a linear medium second harmonic generation and optical detection become possible in the field of radially polarized radiation., Comment: 10 pages