Entanglement Visibility Research Articles

Simple but efficient polarization-entangled photon sources

We report on the design and characterization of polarization-entangled photon sources at NIR and telecom wavelengths, based on spontaneous parametric downconversion in a linear interferometric arrangement with beam displacers. The sources demonstrate state-of-the-art performance with respect to detected brightness, heralding efficiency, and entanglement visibility. Featuring a stable and compact profile with simple alignment, they hold great promise for future development of field- and space-ready prototypes, alongside serving as a practical guide for researchers interested in designing efficient entangled photon sources.

Model for optimizing the visibility of time-bin entanglement

Model for optimizing the visibility of time-bin entanglement

Visible-wavelength polarization-entangled photon source for quantum communication and imaging

We present a polarization-entangled photon pair source operating in the visible light range around 532 nm. Employing a collinear crossed-crystal scheme with type-I degenerate phase matching in barium borate (BBO), our source achieves a brightness of 9.5 k pairs/s/mW and a quantum state fidelity of 98.3%, making it a candidate for integration in microscopes and make use of the advantages of mid-visible optimized single-photon detection technologies. In order to study potential applications, we present a trade-off between source brightness and polarization entanglement visibility and propose use cases for different filtering configurations of the source, capable of a brightness up to 1.23 M pairs/s/mW.

Approaching the Tsirelson bound with a Sagnac source of polarization-entangled photons

High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. The Bell-CHSH inequality S\leq2S≤2 is violated by bipartite entanglement and only maximally entangled states can achieve S=2\sqrt{2}S=22, the Tsirelson bound. Spontaneous parametric down-conversion sources can produce entangled photons with correlations close to the Tsirelson bound. Sagnac configurations offer intrinsic stability, compact footprint and high collection efficiency, however, there is often a trade off between source brightness and entanglement visibility. Here, we present a Sagnac polarization-entangled source with 2\sqrt{2}-S=(5.65\pm0.57\times10^{-3})22−S=(5.65±0.57×10−3), on-par with the highest SS parameters recorded, while generating and detecting (4660\pm70)pairs/s/mW(4660±70)pairs/s/mW, which is a substantially higher brightness than previously reported for Sagnac sources and around two orders of magnitude brighter than for traditional cone sources with the highest SS parameters. Our source records 0.9953\pm0.00030.9953±0.0003 concurrence and 0.99743\pm0.000140.99743±0.00014 fidelity to an ideal Bell state. By studying systematic errors in Sagnac sources, we identify that the precision of the collection focal point inside the crystal plays the largest role in reducing the SS parameter in our experiment. We provide a pathway that could enable the highest SS parameter recorded with a Sagnac source to-date while maintaining very high brightness.

Demonstration of a polarization-entangled photon-pair source based on phase-modulated PPLN

We develop and demonstrate a source of polarization-entangled photon pairs using spontaneous parametric down-conversion (SPDC) in domain-engineered, periodically poled lithium niobate (PPLN) at telecom wavelengths. Pumped at 775 nm, this domain-engineered type-II SPDC source produces non-degenerate signal and idler pairs at 1530 nm and 1569 nm. Because of birefringence, the photon pair with horizontally polarized signal and vertically polarized idler has a different phasematching condition than the pair with vertically polarized signal and horizontally polarized idler. Using phase-modulation of the domain structure, we produced a crystal that can simultaneously generate both states in a distributed fashion throughout a single crystal. Performing SPDC using this aperiodically poled crystal, we observed polarization entanglement visibility above 93%. We compare the phase-modulated crystal to other aperiodic structures, including dual-periodically-poled and interlaced biperiodic structures.

Entanglement in macroscopic systems

We present a theoretical study of entanglement in ensembles consisting of an arbitrary number of particles. Multipartite entanglement criteria in terms of observables are formulated for a fixed number of particles as well as for systems with a fluctuating particle number. To access the quality of the verified entanglement, the operational measure of the entanglement visibility is introduced. As an example, we perform an analytical characterization of quantum systems composed of interacting harmonic oscillators and witness the entanglement via energy measurements. Our analysis shows that the detectable entanglement decays for macroscopic particle numbers without the need for decoherence processes and for all considered coupling regimes. We further study thermal states of the given correlated system together with the temperature dependence of entanglement.

Quality of polarization entanglement in spontaneous parametric down conversion

We experimentally demonstrated a high degree of polarization entanglement known as entanglement visibility through spontaneous parametric down conversion process pumped by a femtosecond laser. The entangled-photon pair was obtained using two type-I BBO crystal. The down-converted photons from these crystals demonstrates a high visibility of 98.7% (θ2 = 0°) and 90% (θ2 = 22.5°). These results are in agreement with the theory which expects high visibility from such arrangement.

Spectral correlation and interference in non-degenerate photon pairs at telecom wavelengths.

We characterize an entangled-photon-pair source that produces signal and idler photons at 1533nm and 1567nm using fiber-assisted signal-photon spectroscopy. By erasing the polarization distinguishability, we observe interference between the two down-conversion paths. The observed interference signature is closely related to the spectral correlations between photons in a Hong-Ou-Mandel interferometer. These measurements suggest good indistinguishability between the two down-conversion paths, which is required for high entanglement visibility.

Entanglement, energy transfer and coherence visibility in small molecular systems

We study the dynamical entanglement of vibrations, intramolecular energy transfer and coherence properties in triatomic molecular systems based on discrete self-trapping theory. O3 and SO2 samples are employed as typical local-mode (LM) and normal-mode molecules, respectively. It is demonstrated that the LM molecule prepared in a LM characteristic state is much more suitable to realize quantum computation. In addition, by introducing a section of entanglement and energy transfer, we investigate the relationship between the two quantities generally. The dynamics between entanglement and energy transfer can reveal a good synchronism under some conditions. Moreover, the intramolecular coherence properties presented by the coherence visibility are discussed in some cases.

Optimal pair-generation rate for entanglement-based quantum key distribution

In entanglement-based quantum key distribution (QKD), the generation and detection of multi-photon modes leads to a trade-off between entanglement visibility and two-fold coincidence events when maximizing the secure key rate (SKR). We produce a predictive model for the optimal two-fold coincidence probability per coincidence window given the channel efficiency and detector dark count rate of a given system. This model is experimentally validated and used in simulations for QKD with satellites as well as optical fibers.

A comprehensive design and performance analysis of low Earth orbit satellite quantum communication

Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.

Quantum entanglement distribution with 810 nm photons through telecom fibers

We demonstrate the distribution of polarization entangled photons of wavelength 810 nm through standard telecom fibers. This technique allows quantum communication protocols to be performed over established fiber infrastructure, and makes use of the smaller and better performing setups available around 800 nm, as compared to those which use telecom wavelengths around 1550 nm. We examine the excitation and subsequent quenching of higher-order spatial modes in telecom fibers up to 6 km in length, and perform a distribution of high quality entanglement (visibility 95.6%). Finally, we demonstrate quantum key distribution using entangled 810 nm photons over a 4.4 km long installed telecom fiber link.

A fully automated entanglement-based quantum cryptography system for telecom fiber networks

We present in this paper a quantum key distribution (QKD) system based on polarization entanglement for use in telecom fibers. A QKD exchange up to 50 km was demonstrated in the laboratory with a secure key rate of 550 bits s−1. The system is compact and portable with a fully automated start-up, and stabilization modules for polarization, synchronization and photon coupling allow hands-off operation. Stable and reliable key exchange in a deployed optical fiber of 16 km length was demonstrated. In this fiber network, we achieved over 2 weeks an automatic key generation with an average key rate of 2000 bits s−1 without manual intervention. During this period, the system had an average entanglement visibility of 93%, highlighting the technical level and stability achieved for entanglement-based quantum cryptography.

Optimizing H1 cavities for the generation of entangled photon pairs

We report on the theoretical investigation of photonic crystal cavities etched on a suspended membrane for the generation of polarization entangled photon pairs using the biexciton cascade in a single quantum dot. The implementation of the spontaneous emission enhancement effect increases the entanglement visibility, while the concomitant preferential funneling of the emission in the cavity mode increases the collection of both entangled photons. We demonstrate and quantify that standard cavity designs present a polarization-dependent emission diagram, detrimental to entanglement. The optimization of H1 cavities allows us to obtain both high collection efficiencies and polarization-independent emission, while keeping the high Purcell factors necessary for high-quality entangled photon sources.

Entangling single photons on a beamsplitter

We report on a scheme for the creation of time-bin entangled states out of two subsequent single photons. Both photons arrive on the same input port of a beamsplitter and the situation in which the photons leave the beamsplitter on different output ports is post-selected. We derive a full quantum mechanical analysis of such time-bin entanglement for emitters subject to uncorrelated dephasing processes and apply this model to sequential single photons emerging from a single semiconductor quantum dot. Our results indicate that the visibility of entanglement is degraded by decoherence effects in the quantum dot, but can be restored by use of CQED effects, namely the Purcell effect.

The design of entangled source for free space quantum communications

We design and build an entanglement source suitable for free space quantum communications experiment. The coincidence counting rate after single mode fiber is 20000 cps. The entanglement visibility is more than 95% in horizontal vertical base and more than 90% in diagonal base. The Bell inequality was broken by more than 100 standard deviations in local experiment. The entanglement source satisfies the requirements for 20 km free space quantum communications experiment.

Quantum Nondemolition Circuit for Testing Bipartite Complementarity

We present a quantum circuit that implements a nondemolition measurement of complementary single- and bipartite properties of a two-qubit system: entanglement and single-partite visibility and predictability. The system must be in a pure state with real coefficients in the computational basis, which allows a direct operational interpretation of those properties. The circuit can be realized in many systems of interest to quantum information.

COMPLEMENTARITY OF ENTANGLEMENT AND INTERFERENCE

A complementarity relation is shown between the visibility of interference and bipartite entanglement in a two qubit interferometric system when the parameters of the quantum operation change for a given input state. The entanglement measure is a decreasing function of the visibility of interference. The implications for quantum computation are briefly discussed.

Polarization entanglement visibility of photon pairs emitted by a quantum dot embedded in a microcavity

We study the photon emission from a quantum dot embedded in a microcavity. Incoherent pumping of its excitons and biexciton provokes the emission of leaky and cavity modes. By solving a master equation we obtain the correlation functions required to compute the spectrum and the relative efficiency among the emission of pairs and single photons. A quantum regime appears for low pumping and large rate of emission. By means of a post-selection process, a two beams experiment with different linear polarizations could be performed producing a large polarization entanglement visibility precisely in the quantum regime.

Artificial atoms in microcavities

An artificial atom can be built up by means of nanostructures of either semiconductors or superconductors. They can be embedded in solid-state optical cavities in order to get strong coupling between the artificial atom excitations and the discrete spectrum of electromagnetic modes. We present here an overview of the dynamics of such a system for the cases of two- and four-level artificial atoms in the presence of an incoherent continuous pumping and the emission of cavity and leaky modes. From a master equation for the density matrix and the quantum regression theorem, one obtains the correlation functions required to compute both the spectrum and the characteristics of the emission of pairs and single photons. A quantum regime appears for low pumping and large rate of emission of cavity photons. In this regime, a large polarization entanglement visibility could be obtained in a two beams experiment with different linear polarizations.