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Physical Review A

Minimum uncertainty states for amplitude-squared squeezing: Hermite polynomial states

János A. Bergou, Mark S. Hillery, Daoqi Yu

Published Jan 1, 1991• 84 citations
Publisher

Abstract

The real and imaginary parts of the square of the field amplitude are the variables that describe amplitude-squared squeezing. These quantities obey an uncertainty relation. Here we find a particularly simple subset of the states that satisfy the uncertainty relation as an equality. These states are constructed by applying a squeeze operator to a state that consists of a Hermite polynomial, whose argument is the mode creation operator multiplied by a constant, acting on the vacuum. The squeezed vacuum is such a state. These states may or may not be squeezed in the normal sense, and may or may not have sub-Poissonian photon statistics.

DOI
10.1103/physreva.43.515

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The real and imaginary parts of the square of the field amplitude are the variables that describe amplitude-squared squeezing. These quantities obey an uncertainty relation. Here we find a particularly simple subset of the states that satisfy the uncertainty relation as an equality. These states are constructed by applying a squeeze operator to a state that consists of a Hermite polynomial, whose argument is the mode creation operator multiplied by a constant, acting on the vacuum. The squeezed vacuum is such a state. These states may or may not be squeezed in the normal sense, and may or may not have sub-Poissonian photon statistics.
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.
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.
The amplitude of the electric field of a mode of the electromagnetic field is not a fixed quantity: there are always quantum mechanical fluctuations. The amplitude, having both a magnitude and a phase, is a complex number and is described by the mode annihilation operator a. It is also possible to characterize the amplitude by its real and imaginary parts which correspond to the Hermitian and anti-Hermitian parts of a, X sub 1 = 1/2(a(sup +) + a) and X sub 2 = i/2(a(sup +) - a), respectively. These operators do not commute and, as a result, obey the uncertainty relation (h = 1) delta X sub 1(delta X sub 2) greater than or = 1/4. From this relation we see that the amplitude fluctuates within an 'error box' in the complex plane whose area is at least 1/4. Coherent states, among them the vacuum state, are minimum uncertainty states with delta X sub 1 = delta X sub 2 = 1/2. A squeezed state, squeezed in the X sub 1 direction, has the property that delta X sub 1 is less than 1/2. A squeezed state need not be a minimum uncertainty state, but those that are can be obtained by applying the squeeze operator.
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.
Based on the two-mode squeezing operator which is the quantum version of the symplectic transformation, we find that the corresponding squeezed two-mode number state is just a two-variable Hermite polynomial excited state, which possesses well-behaved features, e.g. its Wigner function is a direct product of two Laguerre polynomials; its Husimi function, a Gaussian broaden version of the Wigner function, is related to two-variable Hermite polynomials. Moreover, its quantum statistical features, such as squeezing properties and the inter-mode photon bunching, are discussed.
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'.