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arXiv:1410.6658 · Physical Review A

Schmidt modes in the angular spectrum of bright squeezed vacuum

Polina R. Sharapova, Angela M. Pérez, Olga V. Tikhonova, Maria V. Chekhova

Published Apr 10, 2015• 93 citations
arXivHTMLPDF

Abstract

We investigate both theoretically and experimentally strong correlations in macroscopic (bright) quantum states of light generated via unseeded parametric down-conversion and four-wave mixing. The states generated this way contain only quantum noise, without a classical component, and are referred to as bright squeezed vacuum (BSV). Their important advantage is the multimode structure, which offers a larger capacity for the encoding of quantum information. For the theoretical description of these states and their correlation features we introduce a generalized fully analytical approach, based on the concept of independent collective (Schmidt) modes and valid for the cases of both weak and strong nonlinear interaction. In experiment, we generate states of macroscopic BSV with up to ${10}^{10}$ photons per mode and examine large photon-number spatial correlations that are found to be very well described by our theoretical approach.

DOI
10.1103/physreva.91.043816

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Control over the spectral properties of the bright squeezed vacuum (BSV), a highly multimode nonclassical macroscopic state of light that can be generated through high-gain parametric down conversion, is crucial for many applications. In particular, in several recent experiments BSV is generated in a strongly pumped SU(1,1) interferometer to achieve phase supersensitivity, perform broadband homodyne detection, or tailor the frequency spectrum of squeezed light. In this work, we present an analytical approach to the theoretical description of BSV in the frequency domain based on the Bloch-Messiah reduction and the Schmidt-mode formalism. As a special case we consider a strongly pumped SU(1,1) interferometer. We show that different moments of the radiation at its output depend on the phase, dispersion, and the parametric gain in a nontrivial way, thereby providing additional insights on the capabilities of nonlinear interferometers. In particular, a dramatic change in the spectrum occurs as the parametric gain increases.
The bright squeezed vacuum state of light is a macroscopic nonclassical state found at the output of a strongly pumped unseeded travelling-wave optical parametric amplifier. It has been applied to quantum imaging, quantum communication, and phase supersensitivity, to name a few. Bright squeezed states are in general highly multimode, while most applications require a single mode. We separated two nonlinear crystals in the direction of propagation of the pump in order to narrow the angular spectrum down to a nearly-single angular mode. We observed noise reduction in the photon number difference between the two down-converted channels, which constitutes of proof of nonclassicality. By introducing a dispersive medium between the two nonlinear crystals, we were able to tailor the frequency spectrum of bright squeezed vacuum and to dramatically reduce the number of frequency modes down to 1.82 ± 0.02, bringing us closer to truly single-mode bright squeezed vacuum.
Multimode bright squeezed vacuum is a non-classical state of light hosting a macroscopic photon number while offering promising capacity for encoding quantum information in its spectral degree of freedom. Here, we employ an accurate model for parametric down-conversion in the high-gain regime and use nonlinear holography to design quantum correlations of bright squeezed vacuum in the frequency domain. We propose the design of quantum correlations over two-dimensional lattice geometries that are all-optically controlled, paving the way toward continuous-variable cluster state generation on an ultrafast timescale. Specifically, we investigate the generation of a square cluster state in the frequency domain and calculate its covariance matrix and the quantum nullifier uncertainties, that exhibit squeezing below the vacuum noise level.
Squeezed states of light are referred to as non-classical field states and are known to be characterized by strong correlations between photons both in the spectral and spatial domain. For these reasons they are very perspective for many important and interesting practical applications in modern quantum optics including storage and transfer of quantum information. From this point of view multiphoton or bright squeezed vacuum states of light generated during stimulated parametric down conversion process with intensive pump seem to be the most attractive.
Abstract We theoretically investigate the nonlinear spectral broadening of bright squeezed vacuum (BSV) driven by Kerr nonlinearity in bulk sapphire and demonstrate a route toward generating ultrafast broadband BSV-based light. By analyzing the spectral evolution and frequency correlations during nonlinear propagation, we show that the intrinsic photon-number fluctuations sampled from the BSV input significantly enhance ensemble-averaged spectral broadening compared with coherent pulses of the same mean intensity. Quantum statistics enter through Husimi-Q sampling of the incident squeezed-vacuum state, whereas each realization is subsequently propagated as a semiclassical complex envelope. The generated broadband field exhibits strong photon-number and intensity correlations across a wide spectral range together with excellent temporal compressibility. After a common dispersion compensation, the source supports single-shot pulse durations ranging from approximately 7 fs to 70 fs. This work combines ultrafast temporal characteristics with quantum-originated input statistics in a single optical platform and may provide new opportunities for strong-field quantum optics, including high-harmonic generation and attosecond science.