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arXiv:quant-ph/0309069

Generation of entangled 3D localized quantum wave-packets via optical parametric amplification

Claudio Conti

Published Sep 8, 2003• 1 citation
arXivHTMLPDF

Abstract

In the framework of the paraxial and of the slowly varying envelope approximations, with reference to a normally dispersive medium or to vacuum, the electromagnetic field is given as a continuous quantum superposition of non-dispersive and non-diffracting wave-packets (namely X-waves). Entangled states as pairs of elementary excitations traveling at (approximately) the same velocity are found in optical parametric amplification.

DOI
10.48550/arxiv.quant-ph/0309069

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In the framework of the paraxial and of the slowly varying envelope approximations, with reference to a normally dispersive medium or to vacuum, the electromagnetic field is given as a continuous quantum superposition of non-dispersive and non-diffracting wave-packets (namely X-waves). Entangled states as pairs of elementary excitations traveling at (approximately) the same velocity are found in optical parametric amplification.
The classical concept of a paraxial wave is extended to quantum optics as a condition on the state vector, and the family of vectors obeying this condition defines a paraxial Hilbert space. Field operators acting on this space are approximated by the product of a slowly-varying envelope operator and a classical carrier wave. This permits the discussion of Gaussian pulses and paraxial wave packets. This treatment is extended to obtain an expansion of state vectors and field operators in powers of the opening angle of the cone of paraxial rays. The leading term in the expansion yields the component of the angular momentum along the propagation axis as a sum of well defined orbital- and spin-parts. With the same technique, it is shown that approximate localization of photons is possible for localization volumes much larger that the mean wavelength in the paraxial state.
A quantum theory of parametric amplification and frequency conversion of an optical image in coupled nonlinear optical processes that include one parametric amplification process at high-frequency pumping and two up-conversion processes in the same pump field is developed. The field momentum operator that takes into account the diffraction and group velocities of the waves is used to derive the quantum equations related to the spatial dynamics of the images during the interaction. An optical scheme for the amplification and conversion of a close image is considered. The mean photon number density and signal-to-noise ratio are calculated in the fixed-pump-field approximation for images at various frequencies. It has been established that the signal-to-noise ratio decreases with increasing interaction length in the amplified image and increases in the images at the generated frequencies, tending to asymptotic values for all interacting waves. The variance of the difference of the numbers of photons is calculated for various pairs of frequencies. The quantum entanglement of the optical images formed in a high-frequency pump field is shown to be converted to higher frequencies during the generation of sum frequencies. Thus, two pairs of entangled optical images are produced in the process considered.
The new process of quantum-injection into an optical parametric amplifier operating in entangled configuration is adopted to amplify into a large dimensionality spin 1/2 Hilbert space the quantum entanglement and superposition properties of the photon-couples generated by parametric down-conversion. The structure of the Wigner function and of the field's correlation functions shows a decoherence-free, multiphoton Schroedinger-cat behaviour of the emitted field which is largely detectable against the squeezed-vacuum noise. Furthermore, owing to its entanglement character, the system is found to exhibit multi-particle quantum nonseparability and Bell-type nonlocality properties. These relevant quantum features are analyzed for several travelling-wave optical configurations implying different input quantum-injection schemes
The new process of quantum injection into an optical parametric amplifier operating in entangled configuration is adopted to ``amplify'' into a large dimensionality spin- 12\frac{1}{2}21​ Hilbert space the quantum entanglement and superposition properties of the photon couples generated by parametric down-conversion. The structure of the Wigner function and of the field's correlation functions shows a decoherence-free multiphoton Schroedinger-cat behavior of the emitted field which is largely detectable against the squeezed-vacuum noise.
We consider the quantization of slowly varying optical fields in a dispersive nonlinear medium and the application of phase-space methods to the resulting quantum field equations. A pragmatic approach to the quantization of the electromagnetic field is adopted whereby we apply canonical quantization to the Hamiltonian expressed in terms of the slowly varying electric field envelope, all approximations (quasimonochromatic and paraxial) having been made at the classical level. This approach allows us to include material dispersion, diffraction, and nonlinearity. Using phase-space methods we then develop a c-number functional Fokker-Planck equation from which the quantum statistical properties of propagating optical fields can be deduced.