Polarization-entangled Photon Pairs Research Articles

Polarization-entangled Bell states generation based on birefringence in high nonlinear microstructure fiber at 15 μm: erratum

Polarization-entangled photon pair generation based on two scalar scattering processes of the vector four- photon scattering has been demonstrated experimentally in high nonlinear microstructure fiber with birefringence. By controlling pump polarization state, polarization-entangled Bell states can be realized. It provides a simple way to realize efficient and compact fiber-based polarization-entangled photon pair sources.

High-fidelity entanglement swapping at telecommunication wavelengths

We have demonstrated entanglement swapping with 1550 nm polarization-entangled photon pairs created by spontaneous parametric down conversion in two spatially separated type-II periodically poled lithium niobate bulk crystals. In a partial Bell-state measurement, entanglement was observed between two photons from separate sources. We employed sinusoidally gated InGaAs/InP avalanche photodiodes operated at a repetition frequency of 1.28 GHz, which increased the four-fold coincidence count rate without degrading the fidelity of entanglement swapping. The obtained fidelity exceeds 90%, which indicates that the entangled states obtained in this experiment are high enough for immediate further use in quantum communication protocols such as quantum teleportation or quantum cryptography.

Demonstration of active routing of entanglement in a multi-user network

We implement an entanglement distribution network based on wavelength-multiplexing and optical switching for quantum communication applications. Using a high-brightness source based on spontaneous parametric down-conversion in periodically-poled lithium niobate waveguides, we generate polarisation entangled photon pairs with a broad spectrum covering the telecom wavelengths around 1550 nm. The photon pairs have entanglement fidelities up to 99%, and are distributed via passive wavelength multiplexing in a static multi-user network. We furthermore demonstrate a possible network application in a scenario with a single centralised source dynamically allocating two-party entanglement to any pair of users by means of optical switches. The whole system, from the pump laser up to the receivers, is fibre and waveguide based, resulting in maximal stability, minimal losses and the advantage of readily integrable telecom components in the 1550 nm range.

Post-selection free, integrated optical source of non-degenerate, polarization entangled photon pairs

We present an integrated source of polarization entangled photon pairs in the telecom regime, which is based on type II-phasematched parametric down-conversion (PDC) in a Ti-indiffused waveguide in periodically poled lithium niobate. The domain grating - consisting of an interlaced bi-periodic structure - is engineered to provide simultaneous phase-matching of two PDC processes, and enables the direct generation of non-degenerate, polarization entangled photon pairs with a brightness of B = 7 × 10(3) pairs/(s×mW×GHz). The spatial separation of the photon pairs is accomplished by a fiber-optical multiplexer facilitating a high compactness of the overall source. Visibilities exceeding 95 % and a violation of the Bell inequality with S = 2.57±0.06 could be demonstrated.

Poled-fiber source of broadband polarization-entangled photon pairs

We demonstrate broadband polarization-entangled photon pair generation in a poled fiber phase matched for Type II downconversion in the 1.5 μm telecom band. Even with signal-idler separation greater than 100 nm, we observe fringe visibilities greater than 97% and tangle greater than 0.8. A Hong-Ou-Mandel interference experiment is also used to experimentally confirm the broadband nature of the entanglement.

Attractive photons in a quantum nonlinear medium

The fundamental properties of light derive from its constituent particles--massless quanta (photons) that do not interact with one another. However, it has long been known that the realization of coherent interactions between individual photons, akin to those associated with conventional massive particles, could enable a wide variety of novel scientific and engineering applications. Here we demonstrate a quantum nonlinear medium inside which individual photons travel as massive particles with strong mutual attraction, such that the propagation of photon pairs is dominated by a two-photon bound state. We achieve this through dispersive coupling of light to strongly interacting atoms in highly excited Rydberg states. We measure the dynamical evolution of the two-photon wavefunction using time-resolved quantum state tomography, and demonstrate a conditional phase shift exceeding one radian, resulting in polarization-entangled photon pairs. Particular applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light.

A polarization maintaining scheme for 1.5 μm polarization entangled photon pair generation in optical fibers

In this paper, the generation of polarization entangled photon pairs at 1.5 {\mu}m is experimentally demonstrated utilizing a polarization maintaining all-fiber loop, consisting of a piece of commercial polarization maintaining fiber and a polarization beam splitter/combiner with polarization maintaining fiber pigtails. A quantum state tomography measurement is performed to analyze the entanglement characteristic of the generated quantum state. In the experiment, a polarization entangled Bell state is generated with a entanglement fidelity of 0.97+/-0.03 and a purity of 0.94+/-0.03 demonstrating that the proposed scheme can realize polarization entangled photon pair generation with polarization maintaining property which is desired in applications of quantum communication and quantum information.

Experimental Distillation of Quantum Nonlocality

We report the first experimental demonstration of distillation of quantum nonlocality, confirming the recent theoretical protocol [Phys. Rev. Lett. 102, 120401 (2009)]. Quantum nonlocality is described by a correlation box with binary inputs and outputs, and the nonlocal boxes are realized through appropriate measurements on polarization entangled photon pairs. We demonstrate that nonlocality is amplified by connecting two nonlocal boxes into a composite one through local operations and four-photon measurements.

Monolithic Integration of Polarization-entangled Photon Pair Source Using Silicon Photonics Technology

Monolithic Integration of Polarization-entangled Photon Pair Source Using Silicon Photonics Technology

Investigation of the collapse of quantum states using entangled photons

We propose a scheme to investigate the time scale of wave-function collapse by using polarization-entangled photon pairs. The setup is similar to those employed to study quantum correlations, but in the present case, synchronization is essential at all stages. We find that it is possible to discriminate between the scenarios of instantaneous collapse and finite-time reduction via a large number of double measurements of polarization. The quantities to be recorded present distinct behaviors in each scenario, the deviations being small but distinguishable from pure statistical fluctuations. The connection between the presented formalism and the theory of weak measurements is discussed as well as the possible consequences for experimental tests of nonlocality.

Flexible source of nondegenerate entangled photons based on a two-crystal Sagnac interferometer

Sources of entangled photon pairs are a key component in both fundamental tests of quantum theory and practical applications such as quantum key distribution and quantum computing. In this work, we describe and characterize a source of polarization entangled photon pairs based on two spontaneous parametric down-conversion (SPDC) crystals in a Sagnac interferometer. Our source is compact and produces high-quality entangled states in a very flexible manner. The wavelengths of the photon pairs, around 810 and 1550 nm, the phase between orthogonal components of the entangled state, and the tangle of the state are all independently adjustable. In addition to presenting basic characterization data, we present experimental violations of CHSH and Leggett inequalities, as well as an instance of the "beautiful" Bell inequality, which has not previously been tested experimentally.

Temperature-Controlled Symmetry of Linear Polarization of Photoluminescence from InGaAs-Buried InAs/GaAs Quantum Dots

We have succeeded in adjusting the symmetry of the linear polarization of exciton emission from self-assembled InAs/GaAs quantum dots by controlling the combination of temperature and composition of the InGaAs burying layer. The anisotropic shape of the Stranski–Krastanow-type quantum dot is a drawback to the generation of a polarization-entangled photon pair. We found that the polarization symmetry of the intensity and wavelength of photon emission depends on the sample temperature and the composition of the burying layer. The ground-state emission peaks in two linear polarization directions were tuned to overlap by lowering the temperature and using a high indium composition of the burying layer. Our results will aid in the development of an entangled-photon generator using the emission of exciton molecule in self-assembled quantum dots.

Robustness of quantum communication based on a decoherence-free subspace using a counter-propagating weak coherent light pulse

We study distribution schemes for a polarization entangled photon pair based on a decoherence-free subspace over lossy quantum channels and propose an efficient scheme that is robust against not only collective phase noises but also general collective noises for two qubits. While the proposed scheme employs two photons to build the decoherence-free subspace, the success probability is proportional to the channel transmittance of a single photon with the aid of a counter-propagating weak coherent light pulse. The key ingredient in the scheme is found to be the reciprocity of the channel. The proposed scheme shares the rest of the properties with the previously proposed schemes; i.e., it can be realized by linear optical elements and it is robust against the fluctuations in the optical circuits used by the two parties.

Direct Bell States Generation on a III-V Semiconductor Chip at Room Temperature

We demonstrate the direct generation of polarization-entangled photon pairs at room temperature and telecom wavelength in an AlGaAs semiconductor waveguide. The source is based on spontaneous parametric down-conversion with a counterpropagating phase-matching scheme. The quality of the two-photon state is assessed by the reconstruction of the density matrix giving a raw fidelity to a Bell state of 0.83; a theoretical model, taking into account the experimental parameters, provides ways to understand and control the amount of entanglement. Its compatibility with electrical injection, together with the high versatility of the generated two-photon state, make this source an attractive candidate for completely integrated quantum photonics devices.

Nearly degenerate wavelength-multiplexed polarization entanglement by cascaded optical nonlinearities in a PPLN ridge waveguide device

In this paper we report the generation of wavelength-multiplexed polarization-entangled photon pairs in the 1.5-μm communication wavelength band by using cascaded optical second nonlinearities (sum-frequency generation and subsequent spontaneous parametric down-conversion, c-SFG/SPDC) in a periodically poled LiNbO(3) ridge waveguide device. The c-SFG/SPDC method makes it possible to fully use the broad spectral bandwidth of SPDC in nearly frequency-degenerate conditions, and can provide more than 50 pairs of wavelength channels for the entangled photon pairs in the 1.5-μm wavelength band, using only standard optical resources in the telecom field. Visibilities higher than 98% were clearly observed in two-photon interference fringes for all the wavelength channels under investigation (eight pairs). We further performed a detailed experimental investigation of the cross-talk characteristics and the impact of detuning the pump wavelengths.

Generation of Polarization-Entangled Photons in a Standard Polarization-Maintaining Fiber

We demonstrate the capability of standard, commercially available polarization-maintaining fibers (PMFs) to generate polarization-entangled photon-pairs by inserting a PMF source into a Sagnac interferometer. We perform a quantum state tomography to reconstruct the density matrix, yielding, without background subtraction or spectral filtering, a polarization-entangled photon-pair state with 92.37 ± 0.14% fidelity to the maximally entangled Bell state. With its high coupling efficiency into SMFs and ability to produce spectrally uncorrelated photons, we expect this source to be useful for free-space and fiber-based quantum information protocols.

Distilling one, two and entangled pairs of photons from a quantum dot with cavity QED effects and spectral filtering

A quantum dot can be used as a source of one- and two-photon states and of polarization entangled photon pairs. The emission of such states is investigated here from the point of view of frequency-resolved two-photon correlations. These follow from a spectral filtering of the dot emission, which can be achieved either by using a cavity or by placing a number of interference filters before the detectors. A combination of these various options is used to iteratively refine the emission in a ‘distillation’ process and arrive at highly correlated states with a high purity. The so-called ‘leapfrog processes’, where the system undergoes a direct transition from the biexciton state to the ground state by direct emission of two photons, are shown to be central to the quantum features of such sources. Optimum configurations are singled out in a global theoretical picture that unifies the various regimes of operation.

Bell inequalities for the simplest exclusivity graph

Which is the simplest logical structure for which there is quantum nonlocality? We show that there are only three bipartite Bell inequalities with quantum violation associated with the simplest graph of relationships of exclusivity with a quantum-classical gap. These are the most elementary logical Bell inequalities. We show that the quantum violation of some well-known Bell inequalities is related to them. We test the three Bell inequalities with pairs of polarization-entangled photons and report violations in good agreement with the quantum predictions. Unlike other experiments testing noncontextuality inequalities with pentagonal exclusivity, the ones reported here are free of the compatibility loophole.

Efficient generation of hyperentangled photon pairs with controllable waveforms from cold atoms

We propose a scheme to efficiently generate time-frequency and polarization-entangled photon pairs with cold atomic ensembles via spontaneous four-wave mixing through combining the photon pairs from two symmetrical spatial modes by polarization beam splitters. With a two-dimensional magneto-optical trap, polarization-entangled photon pairs with controllable temporal length (>100 ns) can be generated at the rate of about 105 per second after taking into account all losses. Therefore, it is a feasible photon source for scalable linear optical quantum computation and long-distance quantum communication.

A monolithically integrated polarization entangled photon pair source on a silicon chip

Integrated photonic circuits are one of the most promising platforms for large-scale photonic quantum information systems due to their small physical size and stable interferometers with near-perfect lateral-mode overlaps. Since many quantum information protocols are based on qubits defined by the polarization of photons, we must develop integrated building blocks to generate, manipulate, and measure the polarization-encoded quantum state on a chip. The generation unit is particularly important. Here we show the first integrated polarization-entangled photon pair source on a chip. We have implemented the source as a simple and stable silicon-on-insulator photonic circuit that generates an entangled state with 91 ± 2% fidelity. The source is equipped with versatile interfaces for silica-on-silicon or other types of waveguide platforms that accommodate the polarization manipulation and projection devices as well as pump light sources. Therefore, we are ready for the full-scale implementation of photonic quantum information systems on a chip.