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Journal of Modern Optics

Polarization dependence on downconversion emission angle: investigation of the ‘Migdall effect’

Radhika Rangarajan, Alfred B. U’Ren, Paul G. Kwiat

Published Sep 20, 2010• 8 citations
Publisher

Abstract

Photonic sources based on spontaneous parametric downconversion (SPDC) are now pervasively employed in optical quantum information processing protocols. The recent focus on engineering efficient photonic sources requires a thorough understanding of several of the subtler characteristics of SPDC, which had previously been negligible. Here, we experimentally investigate one such phenomenon – the dependence of the downconversion polarization on emission angle. Additionally, we discuss the significance of this effect from the perspective of better engineering SPDC sources. We also propose techniques, specifically in the context of a two-crystal scheme for generating polarization-entangled photons, to exploit the effect and drastically improve source quality in certain limiting cases.

DOI
10.1080/09500340.2010.515753

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Photonic sources based on spontaneous parametric downconversion (SPDC) are now pervasively employed in optical quantum information processing protocols. The recent focus on engineering efficient photonic sources requires a thorough understanding of several of the subtler characteristics of SPDC, which had previously been negligible. Here, we experimentally investigate one such phenomenon – the dependence of the downconversion polarization on emission angle. Additionally, we discuss the significance of this effect from the perspective of better engineering SPDC sources. We also propose techniques, specifically in the context of a two-crystal scheme for generating polarization-entangled photons, to exploit the effect and drastically improve source quality in certain limiting cases.
The generation of entangled photon pairs through spontaneous parametric down conversion (SPDC) is widely used and considered one of the key entanglement generation techniques in quantum optics. The quality of the entanglement produced depends significantly on the physical properties of the nonlinear crystal employed in the SPDC process. In this study, we investigate the influence of the crystal parameters such as phase matching configurations on the degree of entanglement. Our results demonstrate the importance of crystal selection for achieving highly entangled photon sources, which are essential for applications in quantum communication and quantum information.
Single-photon and correlated two-photon sources are important elements for optical information systems. Nonlinear downconversion light sources are robust and stable emitters of single photons and entangled photon pairs. However, the rate of downconverted light emission, dictated by the properties of low-symmetry nonlinear crystals, is typically very small, leading to significant constraints in device design and integration. In this Letter, we study principles of spontaneous emission control (i.e., the Purcell effect) generalized to describe the enhancement of nonlinear generation of quantum light through spontaneous parametric downconversion. We develop a theoretical framework based on eigenmode analysis to study quantum nonlinear emission in a general anisotropic, dispersive, and lossy media. Our theory provides an unprecedented insight into the emission process. We find that spontaneous parametric downconversion in a media with hyperbolic dispersion is broadband and phase-mismatch-free. We further predict a significant enhancement of the downconverted emission rate in experimentally realistic nanostructures. Our theoretical formalism and approach to Purcell enhancement of nonlinear optical processes provides a framework for description of quantum nonlinear optical phenomena in complex nanophotonic structures.
Spontaneous parametric down-conversion (SPDC) is one of the most important sources for entangled or correlated photon pairs, which are the foundation of the emerging fields of quantum-optical imaging and spectroscopy. Using periodically poled materials as an SPDC-source allows broad emission spectra in widely tunable wavelength ranges in an efficient and walk-off-free process. For applications that employ nonlinear interference setups, maximum interferometric contrast is only achieved when signal and idler modes are overlapped precisely in the second crystal. Thus, accurate knowledge of the spectral-angular characteristic of SPDC is crucial for designing such devices.
This portfolio of work contributes to the remit of the National Physical Laboratory (NPL) to develop the underpinning expertise and tools for validating nascent and future optical quantum technologies based on the discrete and quantum properties of photons. This requirement overlaps with the requirement to provide validation for devices operating in the photon-counting regime. A common theme running through the portfolio is photon pairs generated through spontaneous parametric downconversion (SPDC). \n \nA Hong-Ou-Mandel (HOM) interferometer sourced with visible wavelength photon pairs from an SPDC process in beta-barium borate (BBO) was designed, built and characterised. The visibility of the HOM interference is dependent on the indistinguishability of the interfering photons, but is also influenced by imperfections of the interferometer; therefore an investigation was carried out to quantify the effects of the interferometer imperfections on the measured visibility so that the true photon indistinguishability could be measured with a quantified uncertainty. \n \nA bright source of correlated pair photons in the telecoms band based upon a pump enhanced SPDC process in periodically-poled potassium titanyl phosphate (PPKTP) was designed, built and characterised. From the characterisation measurements the source brightness was estimated to be 6.2×10⁴ pairs/ s/ mw pump. The photon pairs were further characterised through their incorporation as a source in a HOM interference experiment. \n \nThe developed correlated photon pair source was at the heart of a novel scheme for the generation of polarisation entangled photon pairs, for which the design, build and characterisation work is presented. The source was demonstrated to produce two of the four maximally entangled Bell states with quantum interference visibilities of around 0.95. The generated states were also shown to break a form of Bell’s inequality by around six standard deviations. The polarisation entangled photon pair source was originally built at the University of St Andrews and was later transferred to the NPL where it will extend NPL’s capabilities to this key spectral region. \n \nFinally a study was carried out to investigate the possibility of a wavelength tuneable device for the absolute measurement of single photon detector quantum efficiencies based upon an established SPDC technique.
We present an experimental platform for polarization-entangled photon pairs generation and characterization. The photon pairs are generated using the double-crystal scheme in the presence of the Migdall effect and fully characterized with quantum state tomography. The platform is used to study the dependence of the SPDC polarization on the scattering direction (the Migdall effect). We demonstrate the Migdall effect can significantly decrease the entanglement of produced states. The compensation techniques to overcome the negative action of the Migdall effect and to generate photon pairs in a maximally entangled state are also developed, and 2.7 times entanglement increase is demonstrated.