We demonstrate pulsed operation of a bidirectionally pumped polarization Sagnac interferometric down-conversion source and its generation of narrow-band, high-visibility polarization-entangled photons. Driven by a narrow-band, mode-locked pump at 390.35 nm, the phase-stable Sagnac source with a type-II phase-matched periodically poled
KTiOPO4 crystal is capable of producing 0.01 entangled pair per pulse in a 0.15-nm bandwidth centered at 780.7 nm with 1 mW of average pump power at a repetition rate of 31.1 MHz. We have achieved a mean photon-pair generation rate of as high as 0.7 pair per pulse, at which multipair events dominate and significantly reduce the two-photon quantum-interference visibility. For low generation probability $\ensuremath{\alpha}$, the reduced visibility $V=1\ensuremath{-}\ensuremath{\alpha}$ is independent of the throughput efficiency and of the polarization analysis basis, which can be utilized to yield an accurate estimate of the generation rate $\ensuremath{\alpha}$. At low $\ensuremath{\alpha}$ we have characterized the source entanglement quality in three different ways: average quantum-interference visibility of 99%, the Clauser-Horne-Shimony-Holt
S parameter of $2.739\ifmmode\pm\else\textpm\fi{}0.119$, and quantum state tomography with 98.85% singlet-state fidelity. The narrow-band pulsed Sagnac source of entangled photons is suitable for use in quantum information processing applications such as free-space quantum key distribution.
We review the use of bidirectional pumping and collinear propagation in type-II phase-matched periodically poled KTiOPO 4 to achieve efficient generation of polarization-entangled photons via spontaneous parametric downconversion. The advantages of this approach are fully exploited in our optimized polarization Sagnac interferometric source. Its measured flux of 4700 polarization-entangled photon pairs/s per milliwatt of pump power in a 1-nm bandwidth exhibited 99% quantum-interference visibility. We also present a plane-wave Gaussian state theory for continuous-wave spontaneous parametric downconversion. This formulation affords the complete description of multiple-pair emissions that is needed for quantum communication analyses while reducing to the usual biphoton-state description at low generation rates.
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.
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.
This thesis describes the design, development, and implementation of a pulsed source of polarization-entangled photons using spontaneous parametric down-conversion in a Sagnac interferometer. A tangle of 0.9286 ± 0.0015, fidelity to the state (|10〉 + |01〉)/√2 of \n0.9798 ± 0.0004 and a brightness of 597 pairs/s/mW were demonstrated. \n \nSpontaneous parametric down-conversion is a nonlinear optical process in which one photon is split into two lower-frequency photons while conserving momentum and energy, in this \nexperiment nearly degenerate photons are produced. These photons are then interfered at the output beamsplitter of the interferometer, exchanging path entanglement for polarization entanglement and generating the desired polarization-entangled photon pairs.