Spontaneous Parametric Down-conversion Process Research Articles

Theory of fourfold interference with photon pairs from spatially separated sources

We present a theory for fourfold quantum interference of photons generated from independent spontaneous parametric down-conversion processes. Closed-form expressions for fourfold quantum interference patterns and visibility are found. The theoretical result for fourfold quantum interference patterns is in good agreement with experimental data reported. Detailed numerical calculations for the dependence of fourfold quantum interference visibility on experimentally controllable parameters are carried out. It is found out that higher visibility can be achieved for small biphoton width, short pump pulse coherence time, and narrow bandwidth of spectral filters. The optimal condition for obtaining at the same time higher fourfold interference visibility and intensity is also discussed.

Polarisation-entangled photon-pair source at 1550 nm using 1 mm-long PPLN waveguide in fibre-loop configuration

A source of polarisation-entangled photons that uses the spontaneous parametric downconversion process in the telecommunication wavelength band is reported. A 1 mm-long periodically poled lithium niobate waveguide was placed in a fibre loop to realise spontaneous parametric downconversion, and it was operated at room temperature. Two-photon interferences having visibilities greater than 84% before subtraction of accidental coincidence counts were observed.

Two-photon quantum interference in the 1.5 μm telecommunication band

We report on two-photon quantum interference experiments in the standard telecommunication band. Two identical photons in the 1.5 mm wavelength band were generated in spatially separated modes by a type-I spontaneous parametric down-conversion process, and injected into a fiber-optic Hong-Ou-Mandel interferometer. Two-photon interference patterns of dip and spatial beating in the coincidence counting rate were observed by varying the difference in optical path lengths. The visibilities obtained in the net coincidences were close to the theoretical value of 100%. The raw visibilities were also well above the classical limit.

Bright narrowband source of photon pairs at optical telecommunication wavelengths using a type-II periodically poled lithium niobate waveguide

We report on the generation of narrowband photon pairs at telecommunication wavelengths using a periodically poled lithium niobate waveguide that utilizes the nonlinear tensor element d(24) for type-II quasi phase matching. The FWHM bandwidth of the spontaneous parametric downconversion was 1 nm. The brightness of the photon pair source was ~6x10(5)/s/GHz when the pump power was 1 mW. The indistinguishability of the signal and idler photons generated by the degenerate spontaneous parametric downconversion process was studied in a Hong-Ou-Mandel type interference experiment.

Entangled Photon Absorption in an Organic Porphyrin Dendrimer

Two-photon absorption spectroscopy is an intensity dependent nonlinear effect related to the excitation of virtual intermediate states. The classical two-photon absorption has an extremely low efficiency which is quantified by its cross-section (delta approximately 10(-48) cm4 s at 800 nm). To overcome this limitation, we demonstrate a novel effect of the two-photon absorption method utilizing the high degree of quantum optical correlation between photon pairs created by the process of spontaneous parametric downconversion. A large entangled two-photon absorption cross-section (delta(e) approximately 10(-17) cm2 at 800 nm) was measured in an organic porphyrin dendrimer. We also discuss the nonmonotonic behavior of variation of the entangled two-photon absorption cross-section by controlling the entanglement time. This novel effect may open new avenues for ultrasensitive detection in chemical and biological systems. TPA spectroscopy has been considered as a powerful tool in physics, chemistry, and biology. The inherent nonlinear process of the classical TPA is distinguishable from the single photon absorption (SPA) linear process. Although the benefits of greater penetration depth and better control and reduction of scattering, the TPA spectroscopy has been restricted by the necessity of a high power optical source due to the low efficiency of the TPA effect. The use of entangled photons from a correlated source for the purpose of the two-photon effect is promising in this regard as one may obtain two-photon effects with very small numbers of photons.

Effects of spatial transverse correlations in second-harmonic generation

Second-harmonic generation is studied for the case where the fundamental field is light produced in a spontaneous parametric down-conversion process. We show that second-harmonic generation is sensitive to the transverse correlations between signal and idler fields. In particular, when the fundamental is prepared in a state exhibitting spatial antibunching, the second-harmonic intensity may be zero, independent of the intensity of the fundamental field.

Theory and experiment of entanglement in a quasi-phase-matched two-crystal source

We report results regarding a source of polarization entangled photon pairs created by the process of spontaneous parametric downconversion in two orthogonally oriented, periodically poled, bulk ${\mathrm{KTiOPO}}_{4}$ crystals. The source emits light colinearly at the nondegenerate wavelengths of 810 and $1550\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and is optimized for a single-mode optical fiber collection and long-distance quantum communication. The configuration favors long crystals, which promote a high photon-pair production rate at a narrow bandwidth, together with a high pair probability in fibers. The quality of entanglement is limited by chromatic dispersion, which we analyze by determining the output state. We find that such a decoherence effect is strongly material dependent, providing for long crystals an upper bound on the visibility of the coincidence fringes of 41% for ${\mathrm{KTiOPO}}_{4}$, and zero for ${\mathrm{LiNbO}}_{3}$. The best obtained raw visibility, when canceling decoherence with an extra piece of crystal, was $91\ifmmode\pm\else\textpm\fi{}0.2%$, including background counts. We confirm by a violation of the CHSH-inequality ($S=2.679\ifmmode\pm\else\textpm\fi{}0.004$ at $55\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1∕2}$ standard deviations) and by complete quantum state tomography that the fibers carry high-quality entangled pairs at a maximum rate of $55\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{THz}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{mW}}^{\ensuremath{-}1}$.

On the distribution of 1550-nm photon pairs efficiently generated using a periodically poled lithium niobate waveguide

We report on efficient generation of 1550-nm photon pairs in a periodically poled lithium niobate waveguide using the spontaneous parametric down-conversion process. Such photon pairs are expected to find applications in fiber-based long-distance quantum communication. Pumping the waveguide with a pulsed semiconductor laser with a pulse rate of 800 kHz and a maximum average pump power of 50 μW, we obtain a coincidence rate of 600 s −1. Despite only two single-photon detectors are used, we gain some information about the photon-number distribution. Our measurements are found to be in agreement with a Poissonian photon-pair distribution, but clearly differ from the expected outcomes for both conventional and two-mode squeezed states, the latter corresponding to a thermal photon-pair distribution. The Poissonian photon-pair distribution is also explained by comparing the coherence time of the pump light and of the detected photons. An average of 0.9 generated photon pairs per pulse can thus be inferred.

Noncollinear correlated photon pair source in the 1550 nm telecommunication band

We present a source of noncollinear correlated photon pairs in the standard 1550 nm telecommunication band. They are generated by a spontaneous parametric down-conversion process and emitted in a cone because of type-I noncollinear phase matching. Within the band, the source gives a completely flexible choice of the frequencies of the photon pairs, and correlation properties related to spatial momentum as well as energy and time can easily be utilized.We characterize the source by measuring the spatial intensity distribution of the down-converted light and by performing coincidence counting.

Design of an integrated optical source of twin photons

The phase matching for the nonlinear process of frequency-degenerated spontaneous parametric downconversion in tunneling-coupled channel waveguides is analyzed. The use of type II nonlinear interaction with individual photons in each pair appearing in different output channels allows for constructing polarization entangled states for quantum information processing applications.

Ultrabright femtosecond source of biphotons based on a spatial mode inverter

A method of enhancing the efficiency of entangled biphoton sources based on a type II femtosecond spontaneous parametric downconversion (SPDC) process is proposed and implemented experimentally. Enhancement is obtained by mode inversion of one of the SPDC output beams, which allows the beams to overlap completely, thus maximizing the number of SPDC photon pairs with optimum spatiotemporal overlap. By use of this method, biphoton count rates as high as 16 kHz from a single 0.5-mm-long beta-barium borate crystal pumped by second-harmonic radiation from a Ti:sapphire laser were obtained.

Generation of high-flux ultra-broadband light by bandwidth amplification in spontaneous parametric down conversion

We present a method for generating an ultra-broadband spectrum of spontaneous parametric down conversion (SPDC) that is independent of the thickness of the nonlinear crystal, thereby yielding down-converted photons of high flux. We show that the bandwidth amplification inherent in the SPDC process is maximized at a wavelength where the group-velocity dispersion is minimal. Experimental results demonstrating the broad spectrum produced by this technique are presented.

Theory of spontaneous parametric down-conversion from photonic crystals

An effective-medium approach is used to derive a set of coupled-mode equations describing quadratic parametric interactions in a one-dimensional inhomogeneous medium of finite length. The solutions for both copropagating and counterpropagating interactions are used to study the quantum-mechanical process of spontaneous parametric down-conversion (SPDC). An example is studied in which the inhomogeneous medium is a photonic crystal. The effect that device geometry has on both the efficiency and bandwidth of SPDC is explored. With appropriate design, we find that it is possible not only to enhance the efficiency of SPDC, but also to generate broadband copropagating and narrowband counterpropagating down-converted light. A setup is proposed with the potential to act as a source of entangled photon pairs.

Quantum ellipsometry using correlated-photon beams

We report ellipsometric measurements made on semiconductor samples using photon-correlated beams produced by the process of spontaneous parametric down-conversion. Such a source yields higher accuracy than its quantum-limited conventional counterpart. We also show that our approach has the added advantage of not requiring an external reference sample for calibration.

Preparation of a pulsed polarization entangled photon pair via interference

In this paper, we report on a scheme to generate a pulsed polarization entangled photon pair by coherently combining two type-I spontaneous parametric down-conversion (SPDC) processes via a Michelson interferometer. We use the interferometric technique to engineer the indistinguishability and the entanglement of a photon pair produced by an ultra-short pulse. Experimentally, the two photons created exhibit high-visibility quantum interference for both space-time and polarization variables. Such a source of highly entangled photon pair is useful in quantum information field.

Quantum entanglement swapping with spontaneous parametric down-conversion

Two remote parties that have never interacted each other can be entangled through entanglement swapping operation done by a third party. Currently existing entanglement swapping experiments are done probabilistically by postselection, i.e., once a successful swapping is verified, the resultant entanglement is destructed. We propose a simple nonpostselection scheme to demonstrate the high-quality quantum entanglement swapping with the spontaneous parametric down-conversion process. Our scheme only requires the normal photon detectors which only distinguish the vacuum and nonvacuum Fock states.

Quantum information processing and precise optical measurement with entangled-photon pairs

Two photons in a pair generated in the nonlinear optical process of spontaneous parametric down-conversion are, in general, strongly quantum entangled. Accordingly, they contain extremely strong energy, time, polarization and momentum quantum correlations. This entanglement involves more than one quantum variable and has served as a powerful tool in fundamental studies of quantum theory. It is now playing a large role in the development of novel information processing techniques and new optical measurement technologies. Here we review some of these technologies and their origins.

Experimental Test of the Kochen-Specker Theorem with Single Photons

Using the spontaneous parametric down-conversion process in a type-I phase matching beta-barium-borate crystal as a single photon source, we perform an all-or-nothing-type Kochen-Specker experiment proposed by Simon et al. [Phys. Rev. Lett. 85, 1783 (2000)]] to verify whether noncontextual hidden variables or quantum mechanics is right. The results strongly agree with quantum mechanics.

진동수가 서로 다른 얽힘상태 광자쌍의 4차 간섭

매개하향변환과정에서 동시에 발생한 서로 다른 진동수의 광자쌍을 이용한 4차 간섭실험에서 비고전적인 맥놀이 효과를 측정하였다. 각진동수가 각각 <TEX>$\omega1$</TEX>과 <TEX>$\omega2$</TEX>인 광자쌍을 빔분할기에서 중첩시키고, 빔분할기의 두 출구에 놓인 두 검출기로 동시계수 할 때, 두 광자가 빔분할기에 도달하는 시간 간격에 따라 cos(<TEX>$\omega1$</TEX>-<TEX>$\omega2$</TEX>)의 함수로 동시계수의 변화를 나타내었다. 두 광자의 파장 차이가 41㎚일 때 측정된 맥놀이 진동수 (<TEX>$\omega1$</TEX>-<TEX>$\omega2$</TEX>)/2<TEX>$\pi$</TEX>는 0.29<TEX>${\times}10^{14}$</TEX>㎐이었고, 진동수 차이에 해당하는 두 광자의 시간간격은 34.82fs로 측정되었다. One of the nonclassical effects in two-photon interference experiments, spatial quantum beating, is observed in fourth-order interference with pairs of photons produced by a spontaneous parametric down-conversion process. When photon pairs in different frequencies <TEX>$\omega1$</TEX> and <TEX>$\omega2$</TEX> are mixed together, and directed to two detectors, the coincidence counts exhibit a cosine modulation with difference frequency | <TEX>$\omega1$</TEX>- <TEX>$\omega2$</TEX>|. The measured coincidence counts turned out to have an interference pattern with periodicity of 10.45 ㎛ in position or 34.82fs in time delay, which corresponds to the period 2<TEX>$\pi$</TEX>/| <TEX>$\omega1$</TEX>- <TEX>$\omega2$</TEX>| for the beat frequency of 0.29<TEX>${\times}10^{14}$</TEX>Hz.

Conditional-Phase Switch at the Single-Photon Level

We present an experimental realization of a two-photon conditional phase switch, related to the " c-phis;" gate of quantum computation. This gate relies on quantum interference between photon pairs and generates entanglement between two optical modes through the process of spontaneous parametric down-conversion (SPDC). The interference effect serves to enhance the effective nonlinearity by many orders of magnitude, so it is significant at the quantum (single-photon) level. By adjusting the relative optical phase between the classical pump for SPDC and the pair of input modes, one can impress a large phase shift on one beam which depends on the presence or absence of a single photon in a control mode.