Source Of Polarization-entangled Photons 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.

Polarization-entangled photon-pair source with van der Waals 3R-WS2 crystal

Ultracompact entangled photon sources are pivotal to miniaturized quantum photonic devices. Van der Waals (vdW) nonlinear crystals promise efficient photon-pair generation and on-chip monolithic integration with nanophotonic circuitry. However, it remains challenging to generate maximally entangled Bell states of photon pairs with high purity, generation rate, and fidelity required for practical applications. Here, we realize a polarization-entangled photon-pair source based on spontaneous parametric down conversion in an ultrathin rhombohedral tungsten disulfide (3R-WS2) crystal. This vdW entangled photonic source exhibits a high photon-pair purity with a coincidence-to-accidental ratio of above 800, a generation rate of 31 Hz, and two maximally polarization-entangled Bell states with fidelities exceeding 0.93 and entanglement degree over 0.97. These results stem from scalable optical nonlinearity, enhanced second-order susceptibility by electronic transitions, and a well-defined symmetry-enabled selection rule inherent in 3R-WS2. Our polarization entangled photon source can be integrated with photonic structures for generating more complex entangled states, thus paving an avenue for advanced quantum photonic systems toward computation and metrology.

CMOS photonic integrated source of broadband polarization-entangled photons

We showcase a fully on-chip CMOS-fabricated silicon photonic integrated circuit employing a bidirectionally pumped microring and polarization splitter-rotators tailored for the generation of broadband (>9 THz), high-fidelity (90–98%) polarization-entangled photons. Spanning the optical C+L-band and producing over 116 frequency-bin pairs on a 38.4-GHz-spaced grid, this source is ideal for flex-grid wavelength-multiplexed entanglement distribution in multiuser networks.

Compact polarization-entangled photon source based on coexisting noncritically birefringent and quasi phase matching in a nonlinear crystal

Polarization-entangled photons are indispensable to numerous quantum technologies and fundamental studies. In this paper, we propose and demonstrate what we believe to be a novel source that generates collinear polarization-entangled photons by simultaneously achieving two distinct types of phase-matching conditions (noncritically birefringent and quasi phase matching) in a periodically poled nonlinear crystal with a large poling period of 2 mm. The photon pairs are generated in a polarization-entangled state with a fidelity and concurrence of 0.998 and 0.935, respectively, and violate the Clauser-Horne-Shimony-Holt inequality by 84 standard deviations. The compact source does not require interferometer, delicate domain structures, or post selection, and is advantageous for scalable quantum computing and communication, where many replicas or chip-scale devices are needed.

Visible-band polarization-entangled photon source by quantum frequency transduction

Visible-band polarization-entangled photon source by quantum frequency transduction

Machine-learning-assisted efficient reconstruction of the quantum states generated from the Sagnac polarization-entangled photon source

Machine-learning-assisted efficient reconstruction of the quantum states generated from the Sagnac polarization-entangled photon source

Narrowband telecom-band polarization-entangled photon source by superposed monolithic cavities

Narrowband telecom-band polarization-entangled photon source by superposed monolithic cavities

Bright source of narrowband polarization-entangled photons from a thick type-II ppKTP crystal.

We demonstrate a high brightness (∼2.36 × 105 pairs/s/mW) polarization-entangled photon-pair source at 800-nm via spontaneous parametric down-conversion (SPDC) in a 3-cm long type-II ppKTP crystal pumped unidirectionally in a single-pass geometry. A high coincidences-to-accidentals ratio (CAR ∼ 1200) depicted by our source indicates a strong temporal correlation between the generated photon pairs. This correlated photon source is tunable from collinear to non-collinear emission of the photons and over a range of signal/idler wavelengths ∼8 - 9nm corresponding to a temperature range of 20-60°C. We measure a quantum state fidelity F>95% with the singlet entangled state |ψ -⟩=12(|H V⟩-|V H⟩) along with a violation of the CHSH-Bell's inequality by ∼485 standard deviations (S = 2.68 ± 0.0014).

Polarization Engineering of Entangled Photons from a Lithium Niobate Nonlinear Metasurface.

Complex polarization states of photon pairs are indispensable in various quantum technologies. Conventional methods for preparing desired two-photon polarization states are realized through bulky nonlinear crystals, which can restrict the versatility and tunability of the generated quantum states due to the fixed crystal nonlinear susceptibility. Here we present a solution using a nonlinear metasurface incorporating multiplexed silica metagratings on a lithium niobate film of 300 nm thickness. We fabricate two orthogonal metagratings on a single substrate with an identical resonant wavelength, thereby enabling the spectral indistinguishability of the emitted photons, and we demonstrate in experiments that the two-photon polarization states can be shaped by the metagrating orientation. Leveraging this essential property, we formulate a theoretical approach for generating arbitrary polarization-entangled qutrit states by combining three metagratings on a single metasurface, allowing the encoding of the desired quantum states or information. Our findings enable miniaturized optically controlled quantum devices by using ultrathin metasurfaces as polarization-entangled photon sources.

Degenerate polarization entangled photon source based on a single Ti-diffusion lithium niobate waveguide in a polarization Sagnac interferometer

Combining a Ti-diffusion periodically poled lithium niobate (PPLN) waveguide with a Sagnac interferometer, two opposite directions type-II spontaneous parametric down conversions (SPDC) occur coherently and yield a high brightness, high stability polarization entanglement source. The source produces degenerate photon pairs at 1540.4 nm with a brightness of B = (1.36 ± 0.03) × 106 pairs/(s⋅nm⋅mW). We perform quantum state tomography to reconstruct the density matrix of the output state and obtain a fidelity of F = 0.983 ± 0.001. The high brightness and phase stability of our waveguide source enable a wide range of quantum information experiments operating at a low pump power as well as hold the advantage in mass production which can promote the practical applications of quantum technologies.

Measuring optical activity with unpolarized light: Ghost polarimetry

Quantifying the optical activity of a sample requires the precise measurement of the rotation of the plane of linear polarization of the transmitted light. Central to this notion is that the sample needs to be exposed to light of a defined polarization state. We show that by using a polarization-entangled photon source we can measure optical activity whilst illuminating a sample with unpolarized light. This not only allows for low light measurement of optical activity but also allows for the analysis of samples that would otherwise be perturbed if subjected to polarized light.

Toward a Photonic Demonstration of Device-Independent Quantum Key Distribution.

The security of quantum key distribution (QKD) usually relies on that the users' devices are well characterized according to the security models made in the security proofs. In contrast, device-independent QKD-an entanglement-based protocol-permits the security even without any knowledge of the underlying quantum devices. Despite its beauty in theory, device-independent QKD is elusive to realize with current technologies. Especially in photonic implementations, the requirements for detection efficiency are far beyond the performance of any reported device-independent experiments. In this Letter, we report a proof-of-principle experiment of device-independent QKD based on a photonic setup in the asymptotic limit. On the theoretical side, we enhance the loss tolerance for real device imperfections by combining different approaches, namely, random postselection, noisy preprocessing, and developed numerical methods to estimate the key rate via the von Neumann entropy. On the experimental side, we develop a high-quality polarization-entangled photon source achieving a state-of-the-art (heralded) detection efficiency about 87.5%. Although our experiment does not include random basis switching, the achieved efficiency outperforms previous photonic experiments involving loophole-free Bell tests. Together, we show that the measured quantum correlations are strong enough to ensure a positive key rate under the fiber length up to 220m. Our photonic platform can generate entangled photons at a high rate and in the telecom wavelength, which is desirable for high-speed generation over long distances. The results present an important step toward a full demonstration of photonic device-independent QKD.

Statistical limits for entanglement swapping with semiconductor entangled photon sources

Semiconductor quantum dots are promising building blocks for quantum communication applications. Although deterministic, efficient, and coherent emission of entangled photons has been realized, implementing a practical quantum repeater remains outstanding. Here we explore the statistical limits for entanglement swapping with sources of polarization-entangled photons from the commonly used biexciton-exciton cascade. We stress the necessity of tuning the exciton fine structure, and explain why the often observed time evolution of photonic entanglement in quantum dots is not applicable for large quantum networks. We identify the critical, statistically distributed device parameters for entanglement swapping based on two sources. A numerical model for benchmarking the consequences of device fabrication, dynamic tuning techniques, and statistical effects is developed, in order to bring the realization of semiconductor-based quantum networks one step closer to reality.

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.

Broadband-laser-diode pumped periodically poled potassium titanyl phosphate-Sagnac polarization-entangled photon source

We experimentally demonstrate a polarization-entangled photon source at 810 nm using a type-II phase-matched periodically poled potassium titanyl phosphate (PPKTP) crystal pumped by a low-cost, broadband laser diode with a central wavelength of 405 nm and a typical bandwidth of 0.53 nm. The PPKTP crystal is placed in a Sagnac loop to realize the compact size and high stability. The downconverted biphotons, the signal and the idler, have typical bandwidths of 5.57 and 7.32 nm. We prepare two Bell states | Ψ + ⟩ and | Ψ − ⟩ with the fidelities of 0.948 ± 0.004 and 0.963 ± 0.002 . In the polarization correlation measurement, the visibilities are all higher than 96.2%, and in the Bell inequality test, the S value can achieve 2.78 ± 0.01 . This high-quality and low-cost entangled photon source may have many practical applications in quantum information processing.

Improving application performance with biased distributions of quantum states

We consider the properties of a specific distribution of mixed quantum states of arbitrary dimension that can be biased towards a specific mean purity. In particular, we analyze mixtures of Haar-random pure states with Dirichlet-distributed coefficients. We analytically derive the concentration parameters required to match the mean purity of the Bures and Hilbert--Schmidt distributions in any dimension. Numerical simulations suggest that this value recovers the Hilbert--Schmidt distribution exactly, offering an alternative and intuitive physical interpretation for ensembles of Hilbert--Schmidt-distributed random quantum states. We then demonstrate how substituting these Dirichlet-weighted Haar mixtures in place of the Bures and Hilbert--Schmidt distributions results in measurable performance advantages in machine-learning-based quantum state tomography systems and Bayesian quantum state reconstruction. Finally, we experimentally characterize the distribution of quantum states generated by both a cloud-accessed IBM quantum computer and an in-house source of polarization-entangled photons. In each case, our method can more closely match the underlying distribution than either Bures or Hilbert--Schmidt distributed states for various experimental conditions.

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.

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.

Simultaneous Measurement of Surface Shape and Film Thickness Using Polarization Entangled Photon Source

Simultaneous Measurement of Surface Shape and Film Thickness Using Polarization Entangled Photon Source

Metasurface enabled quantum edge detection.

Metasurfaces consisting of engineered dielectric or metallic structures provide unique solutions to realize exotic phenomena including negative refraction, achromatic focusing, electromagnetic cloaking, and so on. The intersection of metasurface and quantum optics may lead to new opportunities but is much less explored. Here, we propose and experimentally demonstrate that a polarization-entangled photon source can be used to switch ON or OFF the optical edge detection mode in an imaging system based on a high-efficiency dielectric metasurface. This experiment enriches both fields of metasurface and quantum optics, representing a promising direction toward quantum edge detection and image processing with remarkable signal-to-noise ratio.