Polarization-entangled Photon Pairs Research Articles

Generation of different Bell states within the spontaneous parametric down-conversion phase-matching bandwidth

We study the frequency-angular lineshape for a phase-matched nonlinear process producing entangled states and show that there is a continuous variety of maximally-entangled states generated for different mismatch values within the natural bandwidth. Detailed considerations are made for two specific methods of polarization entanglement preparation, based on type-II spontaneous parametric down-conversion (SPDC) and on SPDC in two subsequent type-I crystals producing orthogonally polarized photon pairs. It turns out that different Bell states are produced at the center of the SPDC line and on its slopes, corresponding to about half-maximum intensity level. These Bell states can be filtered out by either frequency selection or angular selection, or both. Our theoretical calculations are confirmed by a series of experiments, performed for the two above-mentioned schemes of producing polarization-entangled photon pairs and with two kinds of measurements: frequency-selective and angular-selective.

Bright phase-stable broadband fiber-based source of polarization-entangled photon pairs

We describe an achromatic, phase-stable, broadband source of polarization-entangled photon pairs with high spectral brightness that uses four-wave mixing in a fiber Sagnac interferometer. We achieved a polarization-entangled two-photon coincidence rate of $7\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$ per $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ $(0.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ of bandwidth per $300\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{W}$ of average pump power. At this rate, we observed two-photon fringe interference visibilities greater than 97%, over a $10\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ $(\ensuremath{\approx}21\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ spectral range. We measured violations of Bell's inequality by more than 22 standard deviations for each of the four Bell states in less than $3\phantom{\rule{0.3em}{0ex}}\text{minutes}$ per state. The high spectral brightness $(26\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}\phantom{\rule{0.2em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{mW}}^{\ensuremath{-}1})$, large tunable wavelength range, single spatial mode, and phase stability make this source a promising candidate for a wide range of quantum-information applications.

Cavity-QED-based entangled photon pair gun

We describe a cavity-QED scheme to deterministically generate polarization entangled photon pairs by using a single atom successively coupled to two single longitudinal mode cavities presenting polarization degeneracy. The cavities are initially prepared either in the vacuum state or in a single photon Fock state for each orthogonal polarization. Sharing the same basic elements, the source can operate on different physical processes. For a V-type three-level atom initially prepared in the ground state two implementations of the source are possible using either: i) two truncated Rabi Oscillations, or ii) a counterintuitive Stimulated Raman Adiabatic Passage process. Although slower than the former implementation, this second one is very efficient and robust under fluctuations of the experimental parameters and, particularly interesting, almost insensitive to atomic decay. For a four-level atom in a diamond configuration initially prepared in the upper state, the source can produce entangled photon pairs even in the bad cavity limit via an adiabatic passage process. We have performed Monte Carlo wave function simulations to characterize these sources by means of: i) the success probability P of producing the desired entangled state, ii) the fidelity F in the reduced space of two emitted cavity photons, and iii) the S parameter of the Clauser-Horne-Shimony-Holt (CHSH) inequality to quantify the entanglement capability.

Prediction of multichannel polarization-entangled photon pairs in a single periodically poled lithium niobate with a monochromatic pump

A method is presented for the first time to our knowledge for generating multiple-channel polarization-entangled photon pairs simultaneously based on type II quasi-phase-matched spontaneous parametric downconversion pumped by monochromatic light in a single periodically poled lithium niobate (PPLN) crystal. The expression for the count of the polarization-entangled photon pairs is analytically obtained. It is predicted that one-, two-, and even three-channel polarization-entangled photon pairs can be simultaneously generated just by suitably choosing the PPLN grating period and the pump frequency.

Triggered polarization-entangled photon pairs from a single quantum dot up to 30 K

The radiative biexciton–exciton decay in a semiconductor quantum dot (QD) has the potential of being a source of triggered polarization-entangled photon pairs. However, in most cases the anisotropy-induced exciton fine structure splitting destroys this entanglement. Here, we present measurements on improved QD structures, providing both significantly reduced inhomogeneous emission linewidths and near-zero fine structure splittings. A high-resolution detection technique is introduced which allows us to accurately determine the fine structure in the photoluminescence emission and therefore select appropriate QDs for quantum state tomography. We were able to verify the conditions of entangled or classically correlated photon pairs in full consistence with observed fine structure properties. Furthermore, we demonstrate reliable polarization-entanglement for elevated temperatures up to 30 K. The fidelity of the maximally entangled state decreases only a little from 72% at 4 K to 68% at 30 K. This is especially encouraging for future implementations in practical devices.

Schemes for fibre-based entanglement generation in the telecom band

We investigate schemes for generating polarization-entangled photon pairs in standard optical fibres. The advantages of a double-loop scheme are explored through comparison with two other schemes, namely, the Sagnac-loop scheme and the counter-propagating scheme. Experimental measurements with the double-loop scheme verify the predicted advantages.

Four-wave mixing dynamics of excitons in InGaAs self-assembled quantum dots

In this paper we summarize our experimental work on the dephasing of excitons inself-assembled InAs/GaAs quantum dots, measured using a sensitive four-wave mixingtechnique in heterodyne detection. Transient four-wave mixing is a powerful technique tomeasure not only exciton dephasing times but also fine-structure splitting energies andexciton–biexciton binding energies which are important physical parameters for theimplementation of quantum dots as source of polarization-entangled photon pairs and inthe design of quantum algorithms. The scope of this paper is to review and discuss, in thecontext of present theories and in comparison with other experiments in the literature, themost significant results of our study on a series of thermally annealed quantumdots exhibiting systematic differences in the excitonic recombination energiesand quantum confinement potentials. We will highlight the main outcomes ofthese measurements and give a perspective in terms of open questions that stillneed to be investigated, possible new experiments and potential areas of interest.

Proposal and analysis of a silica fiber with large and thermodynamically stable second order nonlinearity

The authors propose a general approach that can produce a silica fiber with large and thermodynamically stable second order nonlinearity. They estimate that it is possible to achieve second harmonic generation efficiency of more than 10% in such a nonlinear fiber with a length of a few hundred microns. By maintaining a high degree of spatial symmetry, the proposed nonlinear fiber can naturally generate polarization entangled photon pairs, a physical object of enormous significance for quantum information technology.

Joint spectrum mapping of polarization entanglement in spontaneous parametric down-conversion

Polarization-entangled photon pairs can be efficiently prepared into pure Bell states with a high fidelity via type-II spontaneous parametric down-conversion (SPDC) of narrow-band pump light. However, the use of femtosecond pump pulses to generate multiphoton states with precise timing often requires spectral filtering to maintain a high quality of polarization entanglement. This typically reduces the efficiency of photon pair collection. We experimentally map the polarization correlations of photon pairs from such a source over a range of down-converted wavelengths with a high spectral resolution and find strong polarization correlations everywhere. A spectrally dependent imbalance between contributions from the two possible decay paths of SPDC is identified as the reason for a reduction in entanglement quality observed with femtosecond pump pulses. Our spectral measurements allow us to predict the polarization correlations for arbitrary filter profiles when the frequency degree of freedom of the photon pairs is ignored.

Entangled photons from the biexciton cascade of quantum dots

Single InAs quantum dots embedded in a planar cavity, formed by mismatched sets of GaAs/AlAs distributed Bragg reflectors, can be a useful source of triggered polarization-entangled photon pairs. We demonstrate this with a fidelity exceeding 70% for the expected entangled state. Quantum dot based devices may therefore be of great use in quantum communications and quantum information processing.

Entangled mixed-state generation by twin-photon scattering

We report experimental results on mixed-state generation by multiple scattering of polarization-entangled photon pairs created from parametric down-conversion. By using a large variety of scattering optical systems we have experimentally obtained entangled mixed states that lie upon and below the Werner curve in the linear entropy-tangle plane. We have also introduced a simple phenomenological model built on the analogy between classical polarization optics and quantum maps. Theoretical predictions from such a model are in full agreement with our experimental findings.

The generation of polarization-entangled photon pairs using periodically poled lithium niobate waveguides in a fibre loop

We report the generation of polarization-entangled photon pairs at a telecom band using two cascade periodically poled lithium niobate waveguides in a fibre loop. The phase of an entangled photon state was stabilized automatically in our setup. The average visibility was about 93.5 ± 2.6% for the entangled photon pairs after subtracting accidental coincidences in our measurement.

High-fidelity transmission of polarization encoded qubits from an entangled source over 100 km of fiber

We demonstrate non-degenerate down-conversion at 810 and 1550 nm for long-distance fiber based quantum communication using polarization entangled photon pairs. Measurements of the two-photon visibility, without dark count subtraction, have shown that the quantum correlations (raw visibility 89%) allow secure quantum cryptography after 100 km of non-zero dispersion shifted fiber using commercially available single photon detectors. In addition, quantum state tomography has revealed little degradation of state negativity, decreasing from 0.99 at the source to 0.93 after 100 km, indicating minimal loss in fidelity during the transmission.

Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks

We present a bright, narrowband, portable, quasi-phase-matched two-crystal source generating polarization-entangled photon pairs at 809 nm and 1555 nm at a maximum rate of 1.2 x 10(6) s(-1) THz-1 mW(-1) after coupling to single-mode fiber. The quantum channel at 1555 nm and the synchronization signal gating the single photon detector are multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This implementation makes quantum communication applications compatible with current high-speed optical networks.

Two-photon four-qubit cluster state generation based on a polarization-entangled photon pair

We propose and experimentally demonstrate a two-photon four-qubit cluster state generator using linear optics and a single polarization-entangled photon-pair source based on spontaneous parametric down-conversion (SPDC). Our novel scheme provides greater design flexibility compared to previous schemes that rely on hyperentanglement.

Characterization of fiber-generated entangled photon pairs with superconducting single-photon detectors

We demonstrate the suitability of fiber-generated entangled photon pairs for practical quantum communications in the telecom band by measuring their properties with superconducting single-photon detectors that produce negligible dark counts. The photon pairs are created in approximately 5-ps duration windows at 50 MHz rate while the detectors are operated in ungated free running mode. We obtain a coincidence to accidental-coincidence ratio >80 with raw photon-counting data, i.e., without making any post-measurement corrections. Using a previously demonstrated counter-propagating scheme we also produce polarization-entangled photon pairs at 50-MHz rate, which in coincidence detection directly yield two-photon interference with a fringe visibility >98%.

Control of fine-structure splitting of individual InAs quantum dots by rapid thermal annealing

Degeneracy of the bright single exciton spin state is a prerequisite for the production of triggered polarization-entangled photon pairs from the biexciton decay of a quantum dot. Normally, however, the exciton spin states are split due to in-plane asymmetries. Here the authors demonstrate that the exciton splitting of an individual dot can be tuned through zero by thermal annealing. Repeated annealing blueshifts the exciton emission line of the dot, accompanied by a reduction and inversion in polarization splitting. Annealing is also demonstrated to control the detuning between the exciton and biexciton transitions in any selected dot.

Efficient source of high purity polarization-entangled photon pairs in the 1550nm telecommunication band

The authors present an efficient source of high purity polarization-entangled photon pairs in the standard 1550nm telecommunication band using a type-II degenerate parametric downconversion process. They first measured the spatial intensity distributions of the downconverted light to verify the beamlike, collinear, and noncollinear generations at the 1550nm wavelength. All four Bell states could easily be generated by selecting the intersection of two downconverted light cones in the noncollinear phase matching. They observed two-photon fringe visibilities of up to 98±3% and violations of Bell’s inequality by up to nine standard deviations when the accidental coincidences were subtracted.

Quantum state tomography of a fiber-based source of polarization-entangled photon pairs

We report an experimental demonstration of a bright high-fidelity single-mode-optical-fiber source of polarization-entangled photon pairs. The source takes advantage of single-mode fiber optics, highly nonlinear microstructure fiber, judicious phase-matching, and the inherent stability provided by a Sagnac interferometer. With a modest average pump power (300 microW), we create all four Bell states with a detected two-photon coincidence rate of 7 kHz per bandwidth of 0.9 nm, in a spectral range of more than 20 nm. To characterize the purity of the states produced by this source, we use quantum-state tomography to reconstruct the corresponding density matrices, with fidelities of 95 % or more for each Bell state.

A 2-D photonic crystal based source of polarization entangled photon pairs with high nonlinear conversion efficiency and without walk-off compensation

Spontaneous parametric down-conversion in 2-dimensional photonic crystal made of semiconductor material with large nonlinear susceptibility is proposed to generate entangled photon pairs with high nonlinear conversion efficiency. In particular, the walk-off between the down-converted photons with orthogonal polarization states can be minimized through appropriate structure parameter design, leading to elimination of compensation measures to mitigate labeling effect on polarization entangled photon pairs.