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Physical Review A

Phase-space representation of amplitude-squared squeezing

Mark S. Hillery

Published Apr 1, 1992• 19 citations
Publisher

Abstract

Phase-space pictures have proven to be useful in analyzing standard squeezing. Here such pictures are developed for amplitude-squared squeezing, a form of higher-order squeezing. They are used to examine amplitude-squared squeezing of the squeezed vacuum state and to suggest a method of producing amplitude-squared squeezed states using a nonlinear interferometer. The phase-space representation of a variable which is a mixture of the usual amplitude-squared squeezing variables is also discussed.

DOI
10.1103/physreva.45.4944

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Phase-space pictures have proven to be useful in analyzing standard squeezing. Here such pictures are developed for amplitude-squared squeezing, a form of higher-order squeezing. They are used to examine amplitude-squared squeezing of the squeezed vacuum state and to suggest a method of producing amplitude-squared squeezed states using a nonlinear interferometer. The phase-space representation of a variable which is a mixture of the usual amplitude-squared squeezing variables is also discussed.
The real and imaginary parts of the square of the field amplitude obey an uncertainty relation. If the uncertainty in either of these variables is less than it would be in a minimum uncertainty state with equal uncertainties in both variables, the state is amplitude-squared squeezed. This is a nonclassical effect which can be produced by second harmonic generation, two-photon absorption, and degenerate parametric amplification, as well as by higher-order processes. Amplitude-squared squeezing can be converted into normal squeezing by interactions in which the square of the amplitude of one field is coupled to the amplitude of another. Second harmonic generation and some forms of four-wave mixing provide examples of such processes.
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.
In this paper the amplitude-squared squeezing in the multiphoton nonlinear optical processes is studied. It is examined that amplitude-squared squeezing can be created in the first order approximation by k-photon unitary operators Sk(z) (K 2) acting on the coherent state |α(|α|≠0) and one can not be created by Sk(z) (K 2, K≠4) acting on the vacuum state. It is found that amplitude-squared squeezing can be created by 4-photon unitary operator S4(z) acting on the vacuum state and this shows that the amplitude-squared squeezing is a new nonclassioal effect of light field which is independent of the second order squeezing, antibunohing and sub-Poisson statistics.
Operators are defined which correspond to the real and imaginary parts of the square of the field amplitude. These operators obey an uncertainty relation. Squeezing with respect to these operators is defined and minimum uncertainty states are discussed. A number of nonlinear optical processes are examined and shown to produce this effect. Interactions which couple one mode amplitude to the square of another convert this type of squeezing into normal squeezing. An example of such an interaction is given.
We define and study the properties of ``squeezed quantum multiplets''. Ordinary multiplets are sets of DDD-orthonormal quantum states formed by superpositions of states squeezed along DDD equally spaced directions in quadrature space. More generally, we also discuss superpositions of ``higher-order squeezed states'', including tri-squeezed and quad-squeezed states. All these states involve superpositions of multiples of ppp photons. We compare states in ordinary (p=2p=2p=2) multiplets and higher-order ones (p>2p>2p>2) in the most relevant cases, showing that ordinary squeezed multiplets and higher-order ones share some important similarities, as well as some differences. Finally, we present analytical expressions for phase-space distributions (Wigner and characteristic functions) representing ordinary squeezed multiplets. We use this to show that some squeezed multiplets are highly sensitive to perturbations in all phase-space directions, making them interesting for metrological applications.
We examine the phase-insensitive amplification of three different kinds of higher-order squeezing. In particular, we consider fourth-order squeezing, intrinsic fourth-order squeezing, and amplitude-squared squeezing. It is found that for any input state both fourth-order squeezing and intrinsic fourth-order squeezing disappear at the output if the intensity gain is greater than 2. Amplitude-squared squeezing, on the other hand, can survive amplification at gains slightly greater than 2. Therefore the photon-cloning limit (intensity gain equals 2) is not a fundamental limit to nonclassical behavior.