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American Journal of Modern Physics

The Test of Entanglement of Polarization States of a Semi-Classical Optical Parametric Oscillator

Gerald Mugaya Lisamadi

Published Jan 1, 2017• 0 citations
Publisher

Abstract

We study the dynamical continuous variable entanglement in a semi-classical Optical parametric oscillator (OPO). The general time evolving photon polarization state vectors arising from exact analytical solutions of Heisenberg’s equations of the system are used to obtain the photon polarization Bell state vectors. The reduced density matrices of photon polarization Bell state vectors of the semi-classical OPO produce a greater violation of CHSH Bell’s inequality beyond the Cirel’son’s inequality.

DOI
10.11648/j.ajmp.20170603.11

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We study the dynamical continuous variable entanglement in a semi-classical Optical parametric oscillator (OPO). The general time evolving photon polarization state vectors arising from exact analytical solutions of Heisenberg’s equations of the system are used to obtain the photon polarization Bell state vectors. The reduced density matrices of photon polarization Bell state vectors of the semi-classical OPO produce a greater violation of CHSH Bell’s inequality beyond the Cirel’son’s inequality.
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Optical parametric oscillators (OPO) have been extensively used in the continuous variable quantum optics community as a resource to produce non-classical states of light, including squeezed states or entangled beams. In this paper, general description of multimode OPOs is presented, show that they are able to produce a very wide variety of multimode quantum states of light, and give techniques to measure or separate them. The description applies equally to the spatial and temporal configurations, which are experimentally studied in lab.
In analogy with the spin-squeezing inequality of Wang and Sanders [Phys. Rev. A 68, 012101 (2003)], we find inequalities describing macroscopic polarization correlations that are obeyed by all classical fields and whose violation implies entanglement of the photons that make up the optical beam. We consider a realistic and exactly solvable experimental scenario employing polarization-squeezed light from an optical parametric oscillator (OPO) and find polarization entanglement for postselected photon pairs separated by less than the OPO coherence time. The entanglement is robust against losses and extremely bright: efficiency can exceed that of existing "ultrabright" pair sources by at least an order of magnitude. This translation of spin-squeezing inequalities to the optical domain will enable direct tests of discrete-variable entanglement in a squeezed state.
Entanglement is a key resource for fundamental tests of physics and emerging quantum technologies. In quantum optics, two perspectives on entanglement coexist. In the continuous-variable framework, entanglement is understood as holding between optical modes. In contrast, discrete-variable quantum optics focuses on quantum correlations in degrees of freedom such as polarization that label fixed numbers of photons. In this paper, we show that entanglement can transcend this separation. Spontaneous parametric down-conversion inherently generates correlations in optical phase space, photon number, and labelling degrees of freedom simultaneously. In polarization, this structure is traditionally described by macroscopic Bell states. Existing witnesses, however, fail to detect the genuine hybrid entanglement of these states, which goes beyond the continuous-discrete-variable categorization. Here, we lay the groundwork for a general framework unifying continuous- and discrete-variable notions of entanglement. In particular, we derive an operational witness providing a sufficient criterion for genuine hybrid number-polarization entanglement and outline its experimental implementation. Finally, we discuss exemplary states which, together with our results on macroscopic Bell states, motivate a broader classification of genuine hybrid quantum correlations.
We observed continuous-variable entanglement between the bright beams emitted above threshold by an ultrastable optical parametric oscillator (OPO), classically phase locked at a frequency difference of 161.8273240(5)MHz161.827\phantom{\rule{0.2em}{0ex}}324\phantom{\rule{0.2em}{0ex}}0(5)\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}161.8273240(5)MHz. The amplitude-difference squeezing is $\ensuremath{-}3\phantom{\rule{0.3em}{0ex}}\mathrm{dB}$ and the phase-sum one is $\ensuremath{-}1.35\phantom{\rule{0.3em}{0ex}}\mathrm{dB}$. Besides proving entanglement in a phase-locked OPO, such outstanding frequency-difference stability paves the way for transferring entanglement between different optical frequencies and densely implementing continuous-variable quantum information in the frequency domain.