The signals and idlers from a pair of matched, doubly resonant optical parametric amplifiers (OPAs) can be combined to yield polarization-entangled vector signal and idler beams. Within any coincidence interval much longer than the OPA cavity lifetime, every signal photon of polarization is accompanied by an idler photon of the conjugate polarization ´, and vice versa. For periodically poled lithium tantalate OPAs, the pair production rate is estimated to be approximately 1.5 × 10 6 s -1 , within a 30 MHz bandwidth at a 795 nm centre wavelength and a 70 ns duration coincidence interval, using only 0.7 mW of pump power per OPA.
Summary form only given. A type-II phase-marched parametric downconverter can be used to produce polarization-entangled photon pairs. We propose a production scheme for polarization-entangled photon pairs by using a pair of optical parametric amplifiers (OPAs) whose narrowband generation rate is high enough to make it a practical source for a trapped-atom quantum memory. For a type-II system using periodically-poled potassium titanyl phosphate (PPKTP) (KTiOPO/sub 4/) optical parametric amplifiers OPAs.
A 532 nm pumped type-II phase-matched, doubly resonant KTP optical parametric amplifier (OPA) was operated near frequency degeneracy to yield an inferred downconverted photon pair production rate of 1.7×10 6 s -1 at a pump power of 100 µW. The OPA output consisted of three components: narrowband doubly resonant mode pairs; narrowband singly resonant mode pairs for which either the signal or idler was resonant with the cavity and broadband nonresonant mode pairs. Under frequency-degenerate operation, the broadband nonresonant mode pairs were polarization triplet states. We observed quantum interference between the orthogonally polarized photons of the triplet states when they were analysed with a polarizer set at 45° relative to the OPA's output polarizations, leading to reduced coincidence counts.
In this paper, we address the issue of the generation of nondegenerate cross-polarization-entangled photon pairs using type-II periodically poled lithium niobate. We show that, by an appropriate engineering of the quasi-phase-matching grating, it is possible to simultaneously satisfy the conditions for two spontaneous parametric down-conversion processes, namely, ordinary pump photon down conversion to either extraordinary signal and ordinary idler paired photons or to ordinary signal and extraordinary idler paired photons. In contrast to single type-II phase matching, these two processes, when enabled together, can lead to the direct production of cross-polarization-entangled states for nondegenerate signal and idler wavelengths. Such a scheme should be of great interest in applications requiring polarization-entangled nondegenerate paired photons with, for instance, one of the entangled photons at an appropriate wavelength being used for local operation or for quantum storage in an atomic ensemble and the other one at the typical wavelength of 1550 nm for propagation through an optical fiber.
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.
In this paper, we present a compact source of narrow-band energy–time-entangled photon pairs in the telecom regime based on a Ti-indiffused periodically poled lithium niobate (PPLN) waveguide resonator, i.e. a waveguide with end-face dielectric multi-layer mirrors. This is a monolithic doubly resonant optical parametric oscillator (OPO) far below threshold, which generates photon pairs by spontaneous parametric down-conversion (SPDC) at around 1560 nm with a 117 MHz (0.91 pm)-bandwidth. A coherence time of 2.7 ns is estimated by a time correlation measurement and a high quality of the entangled states is confirmed by a Bell-type experiment. Since highly coherent energy–time-entangled photon pairs in the telecom regime are suitable for long distance transmission and manipulation, this source is well suited to the requirements of quantum communication.