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Physical Review A

Minimum-uncertainty states, a two-photon system, and quantum-group symmetry

Zhe Chang

Published Nov 1, 1992• 5 citations
Publisher

Abstract

We provide a model for a two-photon system that possesses quantum-group symmetry. The quadrature-phase amplitudes of the model are defined in terms of deformed oscillator operators. We emphasize the investigation of the most general minimum-uncertainty states, i.e., coherent states and squeezed states, for the quadrature-phase amplitudes ${\mathrm{\ensuremath{\alpha}}}_{1}$ and ${\mathrm{\ensuremath{\alpha}}}_{2}$. We show that the squeezed states of the two-photon quantum optics are simultaneous eigenvectors of the operators ${\mathit{A}}_{+}$ and ${\mathit{A}}_{\mathrm{\ensuremath{-}}}$ with eigenvalues ${\mathit{a}}_{+}$ and ${\mathit{a}}_{\mathrm{\ensuremath{-}}}$, respectively. For an important subset of the squeezed states, the expectation values and variances of relevant operators are given explicitly.

DOI
10.1103/physreva.46.5860

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We provide a model for a two-photon system that possesses quantum-group symmetry. The quadrature-phase amplitudes of the model are defined in terms of deformed oscillator operators. We emphasize the investigation of the most general minimum-uncertainty states, i.e., coherent states and squeezed states, for the quadrature-phase amplitudes \ensuremathα1{\mathrm{\ensuremath{\alpha}}}_{1}\ensuremathα1​ and \ensuremathα2{\mathrm{\ensuremath{\alpha}}}_{2}\ensuremathα2​. We show that the squeezed states of the two-photon quantum optics are simultaneous eigenvectors of the operators A+{\mathit{A}}_{+}A+​ and A\ensuremath−{\mathit{A}}_{\mathrm{\ensuremath{-}}}A\ensuremath−​ with eigenvalues a+{\mathit{a}}_{+}a+​ and a\ensuremath−{\mathit{a}}_{\mathrm{\ensuremath{-}}}a\ensuremath−​, respectively. For an important subset of the squeezed states, the expectation values and variances of relevant operators are given explicitly.
We describe a multi-parameter family of the minimum-uncertainty squeezed states for the harmonic oscillator in nonrelativistic quantum mechanics. They are derived by the action of the corresponding maximal kinematical invariance group on the standard ground state solution. We show that the product of the variances attains the required minimum value 1/4 only at the instances that one variance is a minimum and the other is a maximum, when the squeezing of one of the variances occurs. The generalized coherent states are explicitly constructed and their Wigner function is studied. The overlap coefficients between the squeezed, or generalized harmonic, and the Fock states are explicitly evaluated in terms of hypergeometric functions and the corresponding photon statistics are discussed. Some applications to quantum optics, cavity quantum electrodynamics and superfocusing in channelling scattering are mentioned. Explicit solutions of the Heisenberg equations for radiation field operators with squeezing are found.
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{)}^{2})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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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'.
A new formalism for analyzing two-photon devices, such as parametric amplifiers and phase-conjugate mirrors, is proposed in part I, focusing on the properties and the significance of the quadrature-phase amplitudes and two-mode squeezed states. Time-stationary quasi-probability noise is also detailed for the case of Gaussian noise, and uncertainty principles for the quadrature-phase amplitudes are outlined, as well as some important properties of the two-mode states. Part II establishes a mathematical foundation for the formalism, with introduction of a vector notation for compact representation of two-mode properties. Fundamental unitary operators and special quantum states are also examined with an emphasis on the two-mode squeezed states. The results are applied to a previously studied degenerate limit (epsilon = 0).
Certain superposition (linear combination) of energy eigenfunctions of linear harmonic oscillators are called coherent states because they are of significance in optics for the representation of coherent light waves. A hallmark of coherent states is that the variances (squares of the uncertainties) of x and px are constant in time and further their product becomes the minimum value allowed by the Heisenberg uncertainty principle. On the other hand, some linear combinations of harmonic oscillator energy functions give rise to squeezed states . A curious property of squeezed states is that the variances of x and px oscillate in time 180o out of phase with one another with the frequency twice of the oscillator. The wave packet of a coherent state possesses a minimum uncertainty. 〈x〉 and 〈px〉 have the same oscillatory forms as in the classical case. The coherent states have also been called minimum uncertainty coherent states, the Schrödinger coherent states or the Glauber coherent states [1-6]. The squeezed states are sometimes referred to in the literature as two-photon coherent states and generalized coherent states.