Generation Of Polarization-entangled Photon Pairs Research Articles

Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots

Semiconductor quantum dots have emerged as an especially promising platform for the generation of polarization-entangled photon pairs. However, it was demonstrated recently that the two-photon excitation scheme employed in state-of-the-art experiments limits the achievable degree of entanglement by introducing which-path information. In this work, the combined impact of two-photon excitation and longitudinal acoustic phonons on photon pairs emitted by strongly-confining quantum dots is investigated. It is found that phonons further reduce the achievable degree of entanglement even in the limit of vanishing temperature due to phonon-induced pure dephasing and phonon-assisted one-photon processes, which increase the reexcitation probability. In addition, the degree of entanglement, as measured by the concurrence, decreases with rising temperature and/or pulse duration, even if the excitonic fine-structure splitting is absent and when higher electronic states are out of reach. Furthermore, in the case of finite fine-structure splittings, phonons enlarge the discrepancy in concurrence for different laser polarizations.

Optical fiber polarization-entangled photon pair source using intermodal spontaneous four-wave mixing in the visible spectral band

The theoretical and experimental investigation results of the generation of the polarization-entangled photon pairs operating at visible wavelengths are reported. The generation of polarization-entangled photon pairs was based on intermodal spontaneous four-wave mixing (IM-SFWM) using standard step-index few-mode fiber. It was shown theoretically that several combinations of IM-SFWM processes could occur depending on spatial modes of a pump beam. A polarization-entangled photon pair source based on Sagnac loop incorporating a segment of few-mode fiber were then experimentally created. A two-photon interference fringe visibility of our implemented photon pair source were 91.7% and 88.8% in H/V bases and D/A bases, respectively. A quantum state tomography was also conducted to reconstruct the density matrix of the generated state with a fidelity to a maximum entangled state of 93.1%.

Silicon Photonic Wires for Broadband Polarization Entanglement at Telecommunication Wavelengths

Compact and scalable sources of broadband polarization entanglement at telecommunication wavelengths will pave the way for multiuser long-distance quantum communication at enhanced data rates, but progress toward this goal has been hindered due to large birefringence in conventional silicon-on-insulator nanowaveguides. The authors theoretically demonstrate, via dispersion engineering, the successful generation of polarization-entangled photon pairs over a broad range of wavelengths. This work also provides a strategy to avoid entanglement degradation due to polarization-mode dispersion. The proposed devices will be useful building blocks for large-scale quantum communication networks.

Probing Permanent Dipole Moments and Removing Exciton Fine Structures in Single Perovskite Nanocrystals by an Electric Field.

Single perovskite nanocrystals have emerged as a novel type of semiconductor nanostructure capable of emitting single photons with rich exciton species and fine energy-level structures. Here we focus on single excitons and biexcitons in single perovskite CsPbI_{3} nanocrystals to show, for the first time, how their optical properties are modulated by an external electric field at the cryogenic temperature. The electric field can cause a blueshift in the photoluminescence peak of single excitons, from which the existence of a permanent dipole moment can be deduced. Meanwhile, the fine energy-level structures of single excitons and biexcitons in a single CsPbI_{3} nanocrystal can be simultaneously eliminated, thus preparing a potent platform for the potential generation of polarization-entangled photon pairs.

Single-photon generation from self-assembled GaAs/InAlAs(111)A quantum dots with ultrasmall fine-structure splitting

We present a novel semiconductor single-photon source based on tensile-strained (111)-oriented GaAs/InAlAs quantum dots (QDs) exhibiting ultrasmall exciton fine-structure splitting (FSS) of ≤ 8 µeV. Using low-temperature micro-photoluminescence spectroscopy, we identify the biexciton-exciton radiative cascade from individual QDs, which, combined with small FSS, indicates these self-assembled GaAs(111) QDs are excellent candidates for polarization-entangled photon-pair generation.

Highly efficient polarization-entangled photon-pair generation in lithium niobate waveguides based on bound states in continuum.

Integrated optics provides a platform for the experimental implementation of highly complex and compact circuits for practical applications as well as for advances in the fundamental science of quantum optics. The lithium niobate (LN) waveguide is an important candidate for the construction of integrated optical circuits. Based on the bound state in the continuum (BIC) in a LN waveguide, we propose an efficient way to produce polarization-entangled photon pairs. The implementation of this method is simple and does not require the polarization process needed for periodically poled LN. The generation rate of the entangled photon pairs increases linearly with the length of the waveguide. For visible light, the generation efficiency can be improved by more than five orders of magnitude with waveguides having the length of only a few millimeters, compared with the corresponding case without BICs. The phenomena can appear in a very wide spectrum range from the visible to THz regions. This study is of great significance for the development of active integrated quantum chips in various wavelength ranges.

Study on a polarization-entangled photon-pair source based on niobium-doped potassium titanyl phosphate

We investigate the high-purity polarization-entangled photon-pair generation in niobium-doped potassium titanyl phosphate (Nb:KTP). The technique is based on Type II spontaneous parametric down-conversion (SPDC). Two kinds of Nb:KTPs with Nb-doping concentrations of 3.4 mol% and 7.5 mol% will be considered in each periodically poled and non-poled case. The properties of Type II SPDC, including the range of the beam propagation direction (or the spectral range of photons) and the corresponding effective nonlinearities and beam walk-offs, will be discussed in detail both theoretically and numerically. This is followed by a joint spectral analysis is followed to quantify the heralded-state spectral purities of the photon pairs generated by Type II SPDC. The results show that photon-pairs can be generated with high purities of 0.995–0.997 with proper pump filtering. We will also show that the spectral positions of photon pairs can be chosen from a wide range, including 1520.0 nm, 1582.4 nm, 1811.6–1837.4 nm, and 1998.6–2026.8 nm.

Sagnac-type entangled photon source using only conventional polarization optics

We designed and implemented a novel combination of a Sagnac-interferometer with a Mach–Zehnder interferometer for a source of polarization-entangled photons. The new versatile configuration does not require multi-wavelength polarization optics, yet it performs with a good polarization quality and phase-stability over a wide wavelength range. We demonstrate the interferometer using only standard commercial optics to experimentally realize the pulsed generation of polarization-entangled photon-pairs at wavelengths of 764 nm and 1221 nm via type-I spontaneous four-wave mixing in a polarization-maintaining fiber. Polarization entanglement was verified by a polarization-correlation measurement with a visibility of 95.5% from raw coincidence counts and the violation of the Clauser–Horne–Shimony–Holt (CHSH) inequality with S = 2.70 ± 0.04. The long-term phase-stability was characterized by an Allan deviation of 8° over an integration time of about 1 h with no active phase-stabilization.

Numerical Investigation of High-Purity Polarization-Entangled Photon-Pair Generation in Non-Poled KTP Isomorphs

We investigated the high-purity entangled photon-pair generation in five kinds of “non-poled” potassium titanyl phosphate (KTP) isomorphs (i.e., KTiOPO4, RbTiOPO4, KTiOAsO4, RbTiOAsO4, and CsTiOAsO4). The technique is based on the spontaneous parametric down-conversion (SPDC) under Type II extended phase matching (EPM), where the phase matching and the group velocity matching are simultaneously achieved between the interacting photons in non-poled crystals rather than periodically poled (PP) KTPs that are widely used for quantum experiments. We discussed both theoretically and numerically all aspects required to generate photon pairs in non-poled KTP isomorphs, in terms of the range of the beam propagation direction (or the spectral range of photons) and the corresponding effective nonlinearities and beam walk-offs. We showed that the SPDC efficiency can be increased in non-poled KTP isomorphs by 29% to 77% compared to PPKTP cases. The joint spectral analyses showed that photon pairs can be generated with high purities of 0.995–0.997 with proper pump filtering. In contrast to the PPKTP case, where the EPM is achieved only at one specific wavelength, the spectral position of photon pairs in the non-poled KTP isomorphs can be chosen over the wide range of 1883.8–2068.1 nm.

Single is better than double: theoretical and experimental comparison between two thermal poling configurations of optical fibers.

Thermal poling, a technique to create permanently effective second-order susceptibility in silica optical fibers, has a suite of applications including frequency conversion and mixing for high harmonic generation and phase sensitive amplification, optical switching and modulation, and polarization-entangled photon pair generation. In this work, we compare both theoretically and experimentally two different electrode configurations for poling optical fibers, namely double-anode and single-anode, for two different geometries of the cladding holes. This analysis reveals that the single-anode configuration is optimal, both for the absolute value of effective χ (2) created in the fiber core, and for the simplification of the fiber fabrication process.

Benefit of birefringence for the direct generation of polarization-entangled photon pairs.

Generating polarization-entangled photon pairs on chip is generally complicated by the birefringence of waveguides. In this work, we propose a technique that uses waveguide birefringence and lends itself to simple device designs. The technique relies on two orthogonal spontaneous four-wave mixing processes. We employ the full quantum optics theory and dispersion analysis, and show that the technique can produce highly entangled states, with concurrence as high as 0.976 and covering the entire C-band.

Solid-state ensemble of highly entangled photon sources at rubidium atomic transitions

Semiconductor InAs/GaAs quantum dots grown by the Stranski–Krastanov method are among the leading candidates for the deterministic generation of polarization-entangled photon pairs. Despite remarkable progress in the past 20 years, many challenges still remain for this material, such as the extremely low yield, the low degree of entanglement and the large wavelength distribution. Here, we show that with an emerging family of GaAs/AlGaAs quantum dots grown by droplet etching and nanohole infilling, it is possible to obtain a large ensemble of polarization-entangled photon emitters on a wafer without any post-growth tuning. Under pulsed resonant two-photon excitation, all measured quantum dots emit single pairs of entangled photons with ultra-high purity, high degree of entanglement and ultra-narrow wavelength distribution at rubidium transitions. Therefore, this material system is an attractive candidate for the realization of a solid-state quantum repeater—among many other key enabling quantum photonic elements.

Extended phase-matching properties of periodically poled potassium niobate crystals for mid-infrared polarization-entangled photon-pair generation.

We report the extended phase-matching (EPM) properties of two kinds of periodically poled potassium niobate (KNbO3 or KN) crystals (i.e., periodic 180°- and 90°-domain structures) that are highly useful for the generation of polarization-entangled photon pairs in the mid-infrared (IR) spectral region. Under the degenerate Type II spontaneous parametric downconversion process satisfying the EPM condition, an input single photon with a frequency of 2ω generates a pair of synchronized photons with identical frequencies of ω that are orthogonally polarized with respect to each other (i.e., the frequency-coincident, polarization-entangled biphoton states). Our simulation results illustrate that the EPM is achievable in the mid-IR spectral region: at the wavelengths of 3.80 μm and 4.03 μm for periodic 90°- and 180°-domain structures, respectively. We will describe in detail the EPM properties of both cases in terms of interaction types and the corresponding nonlinear optic coefficients, phase-matching bandwidths, and domain poling periods. The calculated EPM bandwidths are much broader than 200 nm in the mid-IR for both cases, exhibiting a great potential for nonlinear-optic signal processing in quantum communication systems operating in the mid-IR bands.

Generation of Polarization-Entangled Photon Pairs in the Spontaneous Parametric Down Conversion Process in Periodically-Poled Potassium Titanyl Phosphate

Generation of Polarization-Entangled Photon Pairs in the Spontaneous Parametric Down Conversion Process in Periodically-Poled Potassium Titanyl Phosphate

Generation of polarization-entangled photon pairs in a cold atomic ensemble

We report an experimental generation of polarization-entangled photon pairs in a cold atomic ensemble. A single Stokes photon and one spin-wave excitation are simultaneously created via spontaneous Raman scattering. The spin-wave excitation is then converted into an anti-Stokes photon via an electromagnetic-induced-transparency reading process. The measured cross-correlation functions between the Stokes and anti-Stokes photons for two orthogonal polarizations are ~75 and 74, respectively, at a generation rate of the photon pair of ~60/s. Based on such correlations, we obtain polarization-entangled photon pairs, whose Bell parameter is S = 2.77 ± 0.01, violating Bell-CHSH inequality by ~77 standard deviations. The presented polarization-entangled photon source has high entanglement degree and fast generation rate, which will promise us to apply it in future quantum repeater.

CW-pumped telecom band polarization entangled photon pair generation in a Sagnac interferometer.

Polarization entangled photon pair source is widely used in many quantum information processing applications such as teleportation, quantum communications, quantum computation and high precision quantum metrology. We report on the generation of a continuous-wave pumped 1550 nm polarization entangled photon pair source at telecom wavelength using a type-II periodically poled KTiOPO(4) (PPKTP) crystal in a Sagnac interferometer. Hong-Ou-Mandel (HOM) interference measurement yields signal and idler photon bandwidth of 2.4 nm. High quality of entanglement is verified by various kinds of measurements, for example two-photon interference fringes, Bell inequality and quantum states tomography. The source can be tuned over a broad range against temperature or pump power without loss of visibilities. This source will be used in our future experiments such as generation of orbital angular momentum entangled source at telecom wavelength for quantum frequency up-conversion, entanglement based quantum key distributions and many other quantum optics experiments at telecom wavelengths.

Deterministic generation of a quantum-dot-confined triexciton and its radiative decay via three-photon cascade

Semiconductor quantum dots (QDs) have potential applications in quantum information processing due to the fact that they are potential on-demand sources of single and entangled photons. Generation of polarization-entangled photon pairs was demonstrated using the biexciton-exciton radiative cascade. One obvious way to increase the number of quantum correlated photons that the QDs emit is to use higher-order multiexcitons, in particular the triexciton. Towards achieving this goal, we first demonstrate deterministic generation of the QD-confined triexciton in a well-definedcoherent state and then spectrally identify and directly measure a three-photon radiative cascade resulting from the sequential triexciton-biexciton-exciton radiative recombination.

On-demand generation of indistinguishable polarization-entangled photon pairs

An on-demand source of indistinguishable and entangled photon pairs is a fundamental component for different quantum information applications such as optical quantum computing, quantum repeaters, quantum teleportation and quantum communication. Parametric down-conversion and four-wave mixing sources of entangled photons have shown high degrees of entanglement and indistinguishability but the probabilistic nature of their generation process also creates zero or multiple photon pairs following a Poissonian distribution. This limits their use in complex algorithms where many qubits and gate operations are required. Here we show simultaneously ultra-high purity (g^(2)(0) < 0.004), high entanglement fidelity (0.81 +/- 0.02), high two-photon interference non-post selective visibilities (0.86 +/- 0.03 and 0.71 +/- 0.04) and on-demand generation of polarization-entangled photon pairs from a single semiconductor quantum dot. Through a two-photon resonant excitation scheme, the biexciton population is deterministically prepared by a Pi-pulse. Applied on a quantum dot showing no exciton fine structure splitting, this results in the deterministic generation of indistinguishable entangled photon pairs.

Polarization-entangled photon-pair generation in commercial-grade polarization-maintaining fiber

We demonstrate a fiber-based source of polarization-entangled photon pairs at visible wavelengths suitable for integration with local quantum processing schemes. The photons are created through birefringent phase-matching in spontaneous four-wave mixing inside a Sagnac interferometer. We address entanglement degradation due to temporal distinguishability of the photons to enable the generation of a spectrally unfiltered polarization-entangled photon-pair state with $95.86\pm0.10%$ fidelity to a maximally entangled Bell state, evaluated with a tomographic state reconstruction without applying any corrections or background subtractions. Owing to the large birefringence of the fiber, photons are created far detuned from the pump, where Raman contamination is negligible. This source's spatial mode and ability to produce spectrally uncorrelated photons make it suitable for implementing quantum information protocols over free-space and fiber-based networks.

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.