Properties of the quantum state arising after the L-photon state has passed trough a linear quantum amplifier

We consider the system of N two-level atoms, of which N0 atoms are unexcited and N1 are excited. This system of N two-level atoms, which forms a linear quantum amplifier, interacts with a single-mode electromagnetic field. The problem of amplification of the L-photon states using such an amplifier is studied. The evolution of the electromagnetic field density matrix is described by the master equation for the field under amplification. The dynamics of this process is such that it can be described as the transformation of the scale of the phase space. The exact solution of the master equation is expressed using the transformed Husimi function of the L-quantum state of the harmonic oscillator. The properties of this function are studied and using it the average photon number and its fluctuations in the amplified state are found. © 2021, Editura Academiei Romane. All rights reserved

T-duality in the weakly curved background

We consider the closed string propagating in the weakly curved background which consists of constant metric and Kalb-Ramond field with infinitesimally small coordinate dependent part. We propose the procedure for constructing the T-dual theory, performing T-duality transformations along coordinates on which the Kalb-Ramond field depends. The obtained theory is defined in the non-geometric double space, described by the Lagrange multiplier $y_\mu$ and its $T$-dual $\tilde{y}_\mu$. We apply the proposed T-duality procedure to the T-dual theory and obtain the initial one. We discuss the standard relations between T-dual theories that the equations of motion and momenta modes of one theory are the Bianchi identities and the winding modes of the other

Linear Light Amplifier and Amplification of N-Photon States

We consider a linear quantum amplifier consisting of NA two-level atoms and study the problem of amplification of N-photon states. The N-photon states are associated with N-quantum states of the harmonic oscillator. We show that the process of interaction of the electromagnetic field with atoms can be associated with some transformation of the phase space and functions defined on this phase space. We consider the Husimi functions QN(q, p) of N-quantum states of the harmonic oscillator, which are defined on the phase space, investigate transformation of these functions, and find an explicit form of the density matrix of the amplified N-photon state. © 2019, Springer Science+Business Media, LLC, part of Springer Nature

Husimi function for time-frequency analysis in optical, microwave and plasmonics aplications

Many real-world signals, occurring in everyday engineering practice are non-stationary, and as a result, their frequency components may change gradually or abruptly over time. Such signals are typically analyzed using Fourier transform, however, this type of analysis is often not sufficient to reveal the true nature of localized (in time) frequency content. This is where time-frequency analysis (TFA) can be of great help. Several approaches of TFA exist, and in this paper we use Husimi function (Gaussian smoothed Wigner function) for this purpose [1,2,3]. Both the Wigner and Husimi functions are the phase space quasidistributions in quantum mechanics [4,5]. In quantum mechanics, Husimi function of a quantum mechanical state arises when simultaneous measurement of quantum conjugated observables - coordinate and momentum, is performed. Similarly, in signal analysis, conjugated variables are time and frequency. If the measurement has the highest physically possible accuracy (as dictated by the Heisenberg uncertainty relations), then the product of standard deviations of conjugated observables equals 2 and Gaussian smoothed Wigner function for in such a way chosen parameters is known as a Husimi function (HF) [2,5]. In this paper, characteristic signals which describe behavior of several devices used in optics, microwave engineering and plasmonics were obtained via 3D electromagnetic numerical simulations. These signals, and their time and frequency evolution, were then analyzed using specifically tailored HF.VI International School and Conference on Photonics and COST actions: MP1406 and MP1402 : PHOTONICA2017 : program and the book of abstracts; August 23 - September 1, 2017; Belgrad

Scaling Transform and Stretched States in Quantum Mechanics

We consider the Husimi Q(q, p)-functions which are quantum quasiprobability distributions on the phase space. It is known that, under a scaling transform (q; p) - GT (aiiq; aiip), the Husimi function of any physical state is converted into a function which is also the Husimi function of some physical state. More precisely, it has been proved that, if Q(q, p) is the Husimi function, the function aii(2) Q(aiiq; aiip) is also the Husimi function. We call a state with the Husimi function aii(2) Q(aiiq; aiip) the stretched state and investigate the properties of the stretched Fock states. These states can be obtained as a result of applying the scaling transform to the Fock states of the harmonic oscillator. The harmonic-oscillator Fock states are pure states, but the stretched Fock states are mixed states. We find the density matrices of stretched Fock states in an explicit form. Their structure can be described with the help of negative binomial distributions. We present the graphs of distributions of negative binomial coefficients for different stretched Fock states and show the von Neumann entropy of the simplest stretched Fock state

Derivation of the Husimi symbols without antinormal ordering, scale transformation and uncertainty relations

We propose a new method for the derivation of Husimi symbols, for operators that are given in the form of products of an arbitrary number of coordinates, and momentum operators, in an arbitrary order. For such an operator, in the standard approach, one expresses coordinate and momentum operators as a linear combination of the creation and annihilation operators, and then uses the antinormal ordering to obtain the final form of the symbol. In our method, one obtains the Husimi symbol in a much more straightforward fashion, departing directly from operator explicit form without transforming it through creation and annihilation operators. With this method the mean values of some operators are found. It is shown how the Heisenberg and the Schrodinger-Robertson uncertainty relations, for position and momentum, are transformed under scale transformation (q; p) - GT (lambda q; lambda p). The physical sense of some states which can be constructed with this transformation is also discussed

Subtle inconsistencies in the straightforward definition of the logarithmic function of annihilation and creation operators and a way to avoid them

Using the resolution of identity spanned by coherent states of the harmonic oscillator, any entire function of the creation and the annihilation operators and its action on a vector in Hilbert space can be defined directly and simply. We show that such a direct approach applied to non-entire functions ln (a) over cap and ln (a) over cap (dagger), present in the literature, may lead to errors and contradictions. We elucidate their roots and propose a way to avoid them. We discuss the obtained results