Phase-stable source of polarization-entangled photons using a polarization Sagnac interferometer

Abstract

We demonstrate a simple, robust, and ultrabright parametric down-conversion source of polarization-entangled photons based on a polarization Sagnac interferometer. Bidirectional pumping in type-II phase-matched periodically poled KTiOPO4 yields a measured flux of 5 000 polarization-entangled pairs/s per mW of pump power in a 1-nm bandwidth at 96.8% quantuminterference visibility. The common-path arrangement of the Sagnac interferometer eliminates the need for phase stabilization for the biphoton output state.PACS numbers: 42.65. Lm,03.65.Ud,03.67.Mn,42.50.Dv Polarization-entangled photons are essential quantum resources for many applications in quantum information processing, including quantum cryptography [1], teleportation [2], and linear optics quantum computing [3]. The standard technique for efficiently producing polarization entanglement is by means of spontaneous parametric down-conversion (SPDC) in a χ (2) nonlinear crystal such as beta barium borate (BBO) or periodically poled KTiOPO 4 (PPKTP). In SPDC a pump photon is converted into two subharmonic photons, and the SPDC outputs can be arranged in various configurations to yield polarization entanglement between the photon pair. For most practical applications, it is desirable to have a high flux of entangled photon pairs for a given spectral bandwidth, together with a high degree of entanglement. One can quantify the performance of a down-conversion source in terms of its spectral brightness, namely the detected pairs/s/mW of pump power per nm of optical bandwidth, and its quantum-interference visibility.A common approach uses a thin BBO crystal under type-II phase matching to generate non-collinearly propagating photon pairs that are polarization entangled [4]. This simple arrangement requires a small aperture to restrict the field of view in order to obtain a high degree of entanglement, and consequently the flux is generally low. A slightly different approach using a long PPKTP crystal with collinearly propagating outputs yields a higher spectral brightness of 300 pairs/s/mW/nm after postselection with a 50-50 beam splitter [5]. The increased flux is the result of a longer crystal and more efficient pair collection with the use of collinear propagation. Yet, because of spatial-mode distinguishability in both approaches, apertures must be used and most of the output photon pairs are not collected.We have recently demonstrated a bidirectionally pumped down-converter that eliminates the constraint of spatial-mode distinguishability and obtained a detected * Electronic address: thkim@mit.edu † Now at Hewlett-Packard Laboratories, 1501 Page Mill Road, Palo Alto, CA 90304, USA.flux of ∼12 000 pairs/s/mW in a 3-nm bandwidth with a quantum-interference visibility of 90% [6]. In this bidirectional pumping geometry, we used a single PPKTP crystal to implement a configuration of two coherentlydriven SPDC sources whose outputs were combined interferometrically with a Mach-Zehnder (MZ) interferometer. The output photon pairs are polarization entangled over th...

Bell violation using entangled photons without the fair-sampling assumption

“…The entangled photon pairs at 810 nm are produced in a Sagnac type source (15,23) pumped by a 405 nm laser. The source is based on type-II spontaneous parametric downconversion using a nonlinear crystal (ppKTP).…”

Pulsed Sagnac source of narrow-band polarization-entangled photons

“…The most widely used method for generating polarization-entangled photon pairs is spontaneous parametric down-conversion (SPDC), which can be realized in a number of configurations and by using different crystals [4,5,6,7,8,9,10,11]. A well optimized SPDC source with high flux and high entanglement quality can be utilized to develop advanced enabling quantum technology such as a random number generator [12].…”

“…We have previously developed a cw SPDC source of polarization-entangled photons using a polarization Sagnac interferometer (PSI) configuration that is highly efficient and yields a high visibility in two-photon quantum interference [7,8]. The Sagnac source configuration is a phase-stable, single-crystal implementation of interferometrically combined outputs of two identical, coherently-driven down-converters [13,14].…”

Higher quantum bound for the Vértesi-Bene-Bell inequality and the role of positive operator-valued measures regarding its threshold detection efficiency

“…Once we obtained a higher quantum bound for I CH3 , we investigated if there is an experimental setup that can be used for observing that POVMs give higher violations in Bell tests based on this inequality. We analyzed the maximum errors supported by the inequality and indicated a source of entangled photons that can be used for the test [29][30][31].…”

“…(20)], to demonstrate the relevance of POVMs over the projective measurements on the violation of the I CH3 inequality, is one order of magnitude below the standard errors of photonic Bell experiments [16][17][18][19][20]. However, there are recent experimental setups, based on a twin-photon ultra-bright source, that have reported violations of Bell inequalities with precisions of the order ∆ = 0.001 [29][30][31]. Therefore, this new kind of source can be used, in principle, for an experimental demonstration of POVMs relevance in quantum nonlocality tests.…”