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...
Mechanical force plays an important role in the physiology of eukaryotic cells whose dominant structural constituent is the actin cytoskeleton composed mainly of actin and actin crosslinking proteins (ACPs). Thus, knowledge of rheological properties of actin networks is crucial for understanding the mechanics and processes of cells. We used Brownian dynamics simulations to study the viscoelasticity of crosslinked actin networks.Two methods were employed, bulk rheology and segment-tracking rheology where the former measures the stress in response to an applied shear strain, and the latter analyzes thermal fluctuations of individual actin segments of the network. Bulk rheology demonstrates that the storage shear modulus (G') increases more by the addition of ACPs that form orthogonal crosslinks than for those that form parallel bundles. In networks with orthogonal crosslinks, as crosslink density increases, the power law exponent of G' as a function of the oscillation frequency decreases from 0.75, which reflects the transverse thermal motion of actin filaments, to near zero at low frequency. Under increasing prestrain, the network becomes more elastic, and three regimes of behavior are observed, each dominated by different mechanisms: bending of actin filaments, bending of ACPs, and at the highest prestrain tested (55%), stretching of actin filaments and ACPs. In the last case, only a small portion of actin filaments connected via highly stressed ACPs support the strain. We thus introduce the concept of a 'supportive framework,' as a subset of the full network, which is responsible for high elasticity. Notably, entropic effects due to thermal fluctuations appear to be important only at relatively low prestrains and when the average crosslinking distance is comparable to or greater than the persistence length of the filament. Taken together, our results suggest that viscoelasticity of the actin network is attributable to different mechanisms depending on the amount of prestrain. Author SummaryThe actin cytoskeleton provides structural integrity to a cell, is highly dynamic, and plays a central role in a wide variety of essential biological phenomena. For years, researchers have studied the mechanics of the cytoskeleton by creating reconstituted actin gels with one or more of the crosslinking proteins found in cells. These reconstituted gels, however, failed to replicate many aspects of cell behavior. Recent studies have shown that tension, or prestrain, on the cytoskeleton due to actomyosin contractility contributes to the observed stiffness. Still, our understanding of cytoskeletal mechanics is incomplete, and 3 many observed phenomena cannot be explained by existing models. Here, we introduce a Brownian dynamics simulation of a three-dimensional network containing actin filaments and crosslinking proteins. By varying model parameters, we show relative contributions of thermal fluctuations and the stiffness of filaments and crosslinks. Under conditions that replicate those in a cell, properties of the cro...
A SWAP operation between different types of qubits of single photons is essential for manipulating hyperentangled photons for a variety of applications. We have implemented an efficient SWAP gate for the momentum and polarization degrees of freedom of single photons. The SWAP gate was utilized in a single-photon two-qubit quantum logic circuit to deterministically transfer momentum entanglement between a pair of down-converted photons to polarization entanglement. The polarization entanglement thus obtained violates Bell's inequality by more than 150 standard deviations.Comment: Changes in the body of the paper, one reference added, typos correcte
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