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Physical Review A

Second-order squeezed states

Paulina Marian

Published Apr 1, 1997• 14 citations
Publisher

Abstract

We examine the generalized squeezed states defined as eigenstates of a linear combination of the lowering and raising operators ${\mathrm{a}}^{2}$ and (${\mathrm{a}}^{\mathrm{\ifmmode\dagger\else\textdagger\fi{}}}$${)}^{2}$, respectively. This approach is entirely equivalent to the minimum-uncertainty method applied to the amplitude-squared operators. We solve the eigenvalue equation in Glauber's coherent-state representation and find two independent solutions. Their Fock-state expansions, one containing only even and the other only odd number states, reveal a strong nonclassical character. We show that the calculation of the mean photon number is sufficient to obtain the expectation values of interest. Consequently, photon statistics is investigated in both cases by using the generating function of the photon-number distribution. We find the conditions under which the second-order squeezed states display photon antibunching and quadrature squeezing. Also discussed is the preservation of their amplitude-squared squeezing by linear amplification at gains exceeding 2. Analytically, our results are simple formulas in terms of Kummer and Gauss hypergeometric functions that allow straightforward numerical calculations.

DOI
10.1103/physreva.55.3051

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We examine the generalized squeezed states defined as eigenstates of a linear combination of the lowering and raising operators a2{\mathrm{a}}^{2}a2 and ({\mathrm{a}}^{\mathrm{\ifmmode\dagger\else\textdagger\fi{}}}$${)}^{2}, respectively. This approach is entirely equivalent to the minimum-uncertainty method applied to the amplitude-squared operators. We solve the eigenvalue equation in Glauber's coherent-state representation and find two independent solutions. Their Fock-state expansions, one containing only even and the other only odd number states, reveal a strong nonclassical character. We show that the calculation of the mean photon number is sufficient to obtain the expectation values of interest. Consequently, photon statistics is investigated in both cases by using the generating function of the photon-number distribution. We find the conditions under which the second-order squeezed states display photon antibunching and quadrature squeezing. Also discussed is the preservation of their amplitude-squared squeezing by linear amplification at gains exceeding 2. Analytically, our results are simple formulas in terms of Kummer and Gauss hypergeometric functions that allow straightforward numerical calculations.
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Normal squeezing is described in terms of variables which are linear in the field creation and annihilation operators. Amplitude-squared squeezing, on the other hand, is defined in terms of variables which are quadratic in these operators. As a result processes, such as second-harmonic generation, which couple one mode amplitude to the square of another convert this kind of squeezing into normal squeezing. In particular, the variables one considers are the hermitian part and one over i times the antihermitian part of the square of the annihilation operator. The variances of these observables obey an uncertainty relation. In order to better understand the properties of amplitude-squared squeezing it is useful to find the minimum uncertainty states corresponding to this uncertainty relation. These states are described by two parameters; the first,, gives the expectation of the square of the annihilation operator and the other λ, describes the amount of amplitude-squared squeezing. AH of these states have a mean amplitude of zero. These states for which =0 have unusual noise properties in that their Q functions have a four-fold rotational symmetry. We also find that a squeezed vacuum state is an amplitude-squared minimum uncertainty state.
General solutions to the eigenvalue equation for amplitude-squared squeezed minimum uncertainty states have been found and investigated. An expression for the average photon number in these states is derived. The Q-function as well as the photocount distribution of these states has been investigated. Plots of Q-functions indicate that the 'manner' of amplitude-squared squeezing is different from that of normal squeezing. It is shown that twofold and fourfold symmetry of Q-function in phase space is related to the suppression of certain terms of the photocount probability for a quantum state. Finally, oscillation of the photocount probability in highly squeezed minimum uncertainty states for amplitude-squared squeezing has been found and interpreted in terms of 'interference in phase space'.
We present a detailed discussion of a type of four-photon coherent state defined as right eigenstates of the operator â 4 where â is the usual annihilation operator. There are actually four sets of states that need to be considered, namely those containing as the lowest number states ¦0>, ¦1>, ¦2>, or ¦3>. These correspond to the possible unique superpositions of the ordinary coherent states ¦±α> and ¦±iα>. We discuss the nonclassical properties of these states such as photon antibunching and squeezing. The usual second order squeezing does not exist for these states but higher order squeezing and square field amplitude squeezing do exist. Also discussed are the quasiprobability distributions, namely the P-function, the Q-function and the Wigner function. Finally, a method of generating these states based on the competition between a four-photon parametric process and incoherent losses from four-photon absorption is presented.
An analysis of the second-order and amplitude-squared (AS) squeezing is presented for the two atoms prepared in a two-atom squeezed state using coherent and squeezed field inputs. The system produce strong sqeezing and superposition angle (θ) is more effective for small values of average photon number [Formula: see text] as compared to that of large one. The degree of squeezing of both types increases as θ increases for coherent field input with small [Formula: see text]. Some values of θ for the initial field in a squeezed-vacuum state with any value of input squeezing can make one of the quadrature of the both types of squeezing permanently squeezed for T≥0. For the initial field in a more general squeezed state, the second-order and AS squeezing behave randomly for different values of θ.