Using the process of spontaneous parametric down-conversion in a two-crystal geometry, we have generated a source of polarization-entangled photon pairs that is more than ten times brighter, per unit of pump power, than previous sources, with another factor of 30 to 75 expected to be readily achievable. We have measured a high level of entanglement between photons emitted over a relatively large collection angle, and over a 10-nm bandwidth. As a demonstration of the source capabilities, we obtained a 242-\ensuremath{\sigma} violation of Bell's inequalities in less than three minutes, and observed near-perfect photon correlations when the collection efficiency was reduced. In addition, both the degree of entanglement and the state purity should be readily tunable.
We develop a Sagnac-type source of ultrabright, non-degenerate, polarization-entangled photon pair that is highly stable and compact simultaneously. We use a
20 mm long PPLN bulk crystal which, upon pumping continuously with
532 nm wavelength, produces polarization-entangled photon-pairs of
785 nm signal and
1651 nm idler wavelengths via the type-0 spontaneous parametric down-conversion (SPDC) process. All optical components used in the setup are off-the-shelf components, readily available commercially; hence, no custom-designed or costly multi-wavelength polarization optics are required. At the same time, long-term phase stability is achieved without any additional active stabilization; due to the geometry of our Sagnac-type design. We also report one of the highest brightness of non-degenerate polarization-entangled photon pairs available in the literature. Even with a very low pump power of
0.034 mW, we detect a coincidence rate of
(6.96±0.03)×104 entangled pairs/sec/mW (averaged over three polarization-basis measurements: H/V, D/A, and R/L basis). The source's brightness is calculated to be
(6.17±0.04)×106 entangled pairs/sec/mW for the signal line width of
0.4 nm. From the raw coincidence counts (without any background coincidence correction), the fidelity of the entangled state is measured to be
(96.72±0.01)% with a concurrence of
(94.68±0.20)%. Bell-CHSH inequality violation is reported as
S=2.71±0.06.
We present an ultrabright polarization-entangled photon source that is optimally coupled into single-mode fibers (SMFs). This study theoretically and experimentally examines the characteristics of spontaneous parametric down-conversion (SPDC) photons, including their spectrum, bandwidth, emission angle, and intensity, as functions of crystal length, temperature and beam waist condition. Notably, we measure the collinear spatial modes of photon-pairs and collection optics under various beam waist conditions and analyze them using a collinear Gaussian approximation model. By employing a simple mode-matching optical setup, we optimize the SMF coupling and heralding efficiencies of the photon-pairs. Consequently, we achieve a spectral brightness exceeding 2.0 MHz/mW/nm from a fiber-coupled entangled photon source, utilizing a 30-mm ppKTP crystal inside a polarization Sagnac interferometer. This represents the highest spectral brightness of SPDC photons generated using a CW laser pumped bulk crystal to date. Polarization entanglement was verified by a quantum state tomography and a polarization-correlation measurement. The fidelity of the entangled state is measured to be 97.8 % and the Bell-CHSH value S = 2.782 +- 0.04. The results obtained here provide practical insights for designing high-performance SPDC sources for satellite-based communication and long-distance optical links with extremely high-photon loss.
We report on an ultrabright beamlike source of polarization-entangled photon pairs that is suitable for the task of multiphoton quantum information processing. The photon pairs are generated from a beamlike type-II parametric downconversion process in two adjacently located but 180 degrees rotated beta-barium borate crystals. Approximately 30,000 s(-1) entangled photon pairs are recorded experimentally with only 100 mW pump power. The fidelity of the generated entangled state as compared with a Bell state is measured to be 0.974 with the method of quantum state tomography. With this source, we also obtain a violation of Bell's inequality by 61 standard deviations in just a few seconds.
The need for spatial and spectral filtering in the generation of polarization entanglement is eliminated by combining two coherently driven type-II spontaneous parametric down-converters. The resulting ultrabright source emits photon pairs that are polarization entangled over the entire spatial cone and spectrum of emission. We detect a flux of $\ensuremath{\sim}12 000$ polarization-entangled pairs/s per mW of pump power at
90 quantum-interference visibility, and the source can be temperature tuned for
5nm around frequency degeneracy. The output state is actively controlled by precisely adjusting the relative phase of the two coherent pumps.
Quantum computation and quantum communication require sources of entangled photons, especially from low-cost diode lasers. This study considered a polarization entangled photons pulse source produced via a spontaneous parametric down-conversion process in the type-I phase-matching of two nonlinear crystals, incorporating a continuous wave laser diode operating in pulse mode as a pump. The generated photons were measured based on visibility as a function of the pump frequency and collection apertures. A polarization correlation between photon pairs of approximately 93% and a Bell parameter S value of 2.56 ± 0.05 were obtained using the source described here.
High-fidelity polarization-entangled photons are a powerful resource for quantum communication, distributing entanglement and quantum teleportation. The Bell-CHSH inequality S\leq2 S ≤ 2 is violated by bipartite entanglement and only maximally entangled states can achieve S=2\sqrt{2} S = 2 2 , 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}) 2 2 − S = ( 5.65 ± 0.57 × 10 − 3 ) , on-par with the highest S S parameters recorded, while generating and detecting (4660\pm70)pairs/s/mW ( 4660 ± 70 ) p a i r s / s / m W , 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 S S parameters. Our source records 0.9953\pm0.0003 0.9953 ± 0.0003 concurrence and 0.99743\pm0.00014 0.99743 ± 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 S S parameter in our experiment. We provide a pathway that could enable the highest S S parameter recorded with a Sagnac source to-date while maintaining very high brightness.