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Conventional optical synthesis, the manipulation of the phase and amplitude of spectral components to produce an optical pulse in different temporal modes, is revolutionizing ultrafast optical science and metrology. These technologies rely on the Fourier transform of light fields between time and frequency domains in one-dimensional space. However, within this treatment it is impossible to incorporate the quantum correlation among photons. Here we expand the Fourier synthesis into high dimensional space to deal with the quantum correlation, and carry out an experimental demonstration by manipulating the two-photon probability distribution of a biphoton in two-dimensional time and frequency space. As a potential application, we show manipulation of a heralded single-photon wave packet, which is never explained by the conventional one-dimensional Fourier optics. Our approach opens up a new pathway to tailor the temporal characteristics of a biphoton wave packet with high dimensional quantum-mechanical treatment. We anticipate such high dimensional treatment of light in time and frequency domains could bridge the research fields between quantum optics and ultrafast optical measurements., Comment: 12 pages, 7 figures

Spectrally intrinsically uncorrelated biphoton states generated from nonlinear crystals are very important but rare resources for quantum photonics and quantum information applications. Previously, such biphoton states were generated from several kinds of crystals, however, their wavelength ranges and nonlinear efficiencies were still limited for various applications. In order to explore new crystal for wider wavelength range and higher nonlinear efficiency, here we theoretically study the generation of spectrally uncorrelated biphoton states from 14 crystals in the `BBO family', including BBO, CLBO, KABO, KBBF, RBBF, CBBF, BABF, BiBO, LBO, CBO, LRB4, LCB, YCOB, and GdCOB. They satisfy three kinds of group-velocity matching condition from near-infrared to telecom wavelengths. Furthermore, heralded single photons can be generated with a purity as high as 0.98, which is achieved without any narrow filtering. The indistinguishability of photons from independent sources is examined by the Hong-Ou-Mandel interference, which results in a visibility of 98% also without any further filtering, i.e., photons from different heralded single-photon sources are highly indistinguishable. Our study may provide single-photon sources with good performance for quantum information processing at near-infrared and telecom wavelengths., Comment: 10 pages, 6 figures

Light incident upon molecules trigger fundamental processes in diverse systems present in nature. However, under natural conditions, such as sunlight illumination, it is impossible to assign known times for photon arrival owing to continuous pumping, and therefore, the photo-induced processes cannot be easily investigated. In this work, we theoretically demonstrate that characteristics of sunlight photons such as photon number statistics and spectral distribution can be emulated through quantum entangled photon pair generated with the parametric down-conversion (PDC). We show that the average photon number of the sunlight in a specific frequency spectrum, e.g., the visible light, can be reconstructed by adjusting the PDC crystal length and pump frequency, and thereby molecular dynamics induced by the pseudo-sunlight can be investigated. The entanglement time, which is the hallmark of quantum entangled photons, can serve as a control knob to resolve the photon arrival times, enabling investigations on real-time dynamics triggered by the pseudo-sunlight photons., Comment: 7 pages

Recently, the free space optical (FSO) communications have been widely studied as an alternative for large capacity communications and its possible implementation in satellite and terrestrial laser links. In satellite communications, clouds can strongly attenuate the laser signal that would lead to high bit-error rates or temporal unavailability of the link. To overcome the cloud coverage effects, often site diversity technique is implemented. When using multiple ground stations though, simplified optical system is required to allow the usage of more flexible approaches. In terrestrial laser communications, several methods for optical system simplification by using a multimode fiber (MMF) have been proposed.

We investigate the detection of a single-photon source using highly efficient superconducting nanowire single-photon detectors (SNSPDs) at telecom wavelengths. Both the single-photon source and the detectors are characterized in detail. At a pump power of 100 mW (400 mW), the measured coincidence counts can achieve 400 kcps (1.17 Mcps), which is the highest ever reported at telecom wavelengths to the best of our knowledge. The multi-pair contributions, the experimental and theoretical second order coherence functions, and the saturation property of SNSPD are analyzed in detail. The experimental data and theoretical analysis should be useful for the future experiments to detect ultra-bright down-conversion sources with high-efficiency detectors.

Localized-surface-plasmon (LSP)-enhancement effect on one-photon photochromic ring-opening reaction of two diarylethene (DE) derivatives was studied using absorption spectroscopy. DEs were doped in polymer films and those were coated onto Au-nanoparticles-integrated substrates. Acceleration of the one-photon photochromic reactions was observed in the presence of Au nanoparticles. The origin of the behavior was attributed to an enhancement effect of incident electromagnetic field by LSP. An alternative mechanism, i.e., photothermal effect was also experimentally examined and discussed.

Optical fibres have proved to be an important medium for manipulating and generating light in applications including soliton transmission1, light amplification2, all-optical switching3 and supercontinuum generation4. In the quantum regime, fibres may prove useful for ultralow-power all-optical signal processing5 and quantum information processing6. Here, we demonstrate the first experimental observation of optical nonlinearity at the single-photon level in an optical fibre. Taking advantage of the large nonlinearity and managed dispersion of photonic crystal fibres7,8, we report very small (1 × 10−7 to ∼1 × 10−8 rad) conditional phase shifts induced by weak coherent pulses that contain one or less than one photon per pulse on average. We discuss the feasibility of quantum information processing using optical fibres, taking into account the observed Kerr nonlinearity, accompanied by ultrafast response time and low induced loss. The tiny phase changes introduced by nonlinear optics performed at the single-photon level is reported in a photonic crystal fibre with carefully designed nonlinear and dispersion properties. The approach may prove useful in future quantum information processing schemes.

We report a high-efficiency source of cross-polarized photon pairs via type-II quasi-phase matched spontaneous parametric down-conversion using periodically poled lithium niobate (PPLN). The photon pairs are generated from the PPLN in a fifth-order quasi-phase matching condition around 800 nm, which matches the wavelength band of efficient Si avalanche photodiodes. We obtained the photon-pair production rate of 6.8×105 s-1 mW-1.

Entanglement swapping at telecom wavelengths is at the heart of quantum networking in optical fiber infrastructures. Although entanglement swapping has been demonstrated experimentally so far using various types of entangled photon sources both in near-infrared and telecom wavelength regions, the rate of swapping operation has been too low to be applied to practical quantum protocols, due to limited efficiency of entangled photon sources and photon detectors. Here we demonstrate drastic improvement of the efficiency at telecom wavelength by using two ultra-bright entangled photon sources and four highly efficient superconducting nanowire single photon detectors.We have attained a four-fold coincidence count rate of 108 counts per second, which is three orders higher than the previous experiments at telecom wavelengths. A raw (net) visibility in a Hong-Ou-Mandel interference between the two independent entangled sources was 73.3 $\pm$ 1.0% (85.1 $\pm$ 0.8%). We performed the teleportation and entanglement swapping, and obtained a fidelity of 76.3% in the swapping test.Our results on the coincidence count rates are comparable with the ones ever recorded in teleportation/swaping and multi-photon entanglement generation experiments at around 800\,nm wavelengths. Our setup opens the way to practical implementation of device-independent quantum key distribution and its distance extension by the entanglement swapping as well as multi-photon entangled state generation in telecom band infrastructures with both space and fiber links., Comment: 9 pages, 5 figures

We present two-photon diffraction and interference experiments utilizing parametric down-converted photon pairs (biphotons) and a transmission grating. The biphoton exhibits a diffraction-interference pattern equivalent to an effective single particle with half wavelength of the constituent photons.

Efficient generation and detection of indistinguishable twin photons are at the core of quantum information and communications technology (Q-ICT). These photons are conventionally generated by spontaneous parametric down conversion (SPDC), which is a probabilistic process and hence occurs at a limited rate, which restricts wider applications of Q-ICT. To increase the rate, one had to excite SPDC by higher pump power, while it inevitably produced more unwanted multi-photon components, harmfully degrading quantum interference visibility. Here we solve this problem by using recently developed 10 GHz repetition-rate-tunable comb laser, combined with a group-velocity-matched nonlinear crystal and superconducting nanowire single photon detectors. They operate at telecom wavelengths more efficiently with less noises than conventional schemes, those typically operate at visible and near infrared wavelengths generated by a 76 MHz Ti Sapphire laser and detected by Si detectors. We could show high interference visibilities, which are free from the pump-power induced degradation. Our laser, nonlinear crystal and detectors constitute a powerful tool box, which will pave a way to implementing quantum photonics circuits with variety of good and low-cost telecom components and will eventually realize scalable Q-ICT in optical infra-structures.

We demonstrate pulsed polarization-entangled photons generated from a periodically poled $\mathrm{KTiOPO_4}$ (PPKTP) crystal in a Sagnac interferometer configuration at telecom wavelength. Since the group-velocity-matching (GVM) condition is satisfied, the intrinsic spectral purity of the photons is much higher than in the previous scheme at around 800 nm wavelength. The combination of a Sagnac interferometer and the GVM-PPKTP crystal makes our entangled source compact, stable, highly entangled, spectrally pure and ultra-bright. The photons were detected by two superconducting nanowire single photon detectors (SNSPDs) with detection efficiencies of 70% and 68% at dark counts of less than 1 kcps. We achieved fidelities of 0.981 $\pm$ 0.0002 for $\left| {\psi ^ -} \right\rangle$ and 0.980 $\pm$ 0.001 for $\left| {\psi ^ +} \right\rangle$ respectively. This GVM-PPKTP-Sagnac scheme is directly applicable to quantum communication experiments at telecom wavelength, especially in free space., Comment: 5 pages, 5 figures http://dx.doi.org/10.1364/OE.22.011498

Photon source is one of the fundamental bricks for quantum information and computation systems. A widely-tunable, highly efficient and spectrally pure photon source would be very useful for such applications. In this paper, we present such a source based on spontaneous parametric down conversion (SPDC) in a periodically poled KTiOPO4 (PPKTP) crystal.

We proposed and demonstrated a simple but deterministic scheme for generating polarization-entangled photon pairs at telecommunication wavelengths with type-II quasi-phase-matched spontaneous parametric down-conversion (QPM-SPDC) having two poling periods. We fabricated a LiNbO3 crystal having two poling periods so as to generate entangled photons at two wavelengths, i.e., 1506 nm and 1594 nm. We characterized the two-photon polarization state with state tomography and confirmed that the state was highly entangled.

The reduction of de Broglie wavelength is one of the fascinating characteristics of multiphoton quantum interference. It will be applicable to, for instance, super-resolution imaging beyond the classical diffraction limit and ultrasensitive phase measurement beyond the standard quantum limit. Thus far, the reduced de Broglie wavelength has been experimentally demonstrated using visible or near infrared photons [1–3]. However, it is still a challenging task to demonstrate multiphoton quantum interference in a telecom-wavelength region, because of insufficient quantum efficiencies of single-photon detectors working at telecom wavelength. Also, efficient photon sources of multiple photons that are mutually indistinguishable are essential to carry out various experiments including multiphoton quantum interference and quantum information protocols. In this context, we have developed an efficient photon-pair source designed for such multiphoton experiments at telecom wavelength. Making use of this source, we demonstrate the first measurement of reduced de Broglie wavelength of the four-photon state at the telecom wavelength.

We report the experimental demonstration of four-photon quantum interference using telecom-wavelength photons. Realization of multi-photon quantum interference is essential to linear optics quantum information processing and measurement-based quantum computing. We have developed a source that efficiently emits photon pairs in a pure spectrotemporal mode at a telecom wavelength region, and have demonstrated the quantum interference exhibiting the reduced fringe intervals that correspond to the reduced de Broglie wavelength of up to the four photon `NOON' state. Our result should open a path to practical quantum information processing using telecom-wavelength photons., Comment: 4 pages, 4 figures

We demonstrate the first observation of the cross‐Kerr nonlinearity induced by a single‐photon‐level coherent optical pulse in a photonic crystal fiber. Taking advantage of large nonlinearity and managed dispersion of the optical fiber, we have successfully measured very small (10−7) conditional phase shifts.

We report broadband entangled photon sources that have control capability of the frequency entanglement. Directly observing the frequency correlation between the constituent photons, we demonstrate the biphoton sources consisting of frequency-correlated or frequency-anticorrelated photons.

The purpose of our study is to establish the technique to measure such weak cross-Kerr nonlinearity induced by a single photon in solid states. The experimental set-up is based on a pump-probe measurement.

We have developed a high-efficiency source of cross-polarized photon pairs via type-II quasi-phase matched spontaneous parametric down-conversion using periodically poled lithium niobate (PPLN). The photon pairs are generated from the PPLN in fifth-order quasi-phase matched condition around 800 nm, which matches the wavelength band of efficient Si avalanche photodiodes. We obtained 25,000 coincidence counts sec-1mW-1of the photon pairs.

We report the spectral properties of broadband biphotons generated by pulsed type-II spontaneous parametric down-conversion based on a periodically poled MgO-doped stoichiometric LiTaO3 (PPMgSLT). Spectroscopic and interferometric measurements were used to characterize the biphoton source. For the spectroscopic measurements, we used conventional spectroscopy and joint spectral intensity measurements. For the interferometric measurements, sum- and difference-frequency two-photon interference experiments were performed to estimate the spectral correlation and to ensure the broadband nature of biphotons, respectively. When using a picosecond Ti:sapphire as a pump source, the spectral width of biphotons was over 40 nm. Finally, we demonstrated the relationship between the two-photon interference visibility and the symmetricity of the joint spectral distribution.

Characterization of small nonlinear phase shifts will be a key to the optical quantum information processing. We propose and demonstrate a novel technique to measure the nonlinear phase shifts in a waveguide-type Kerr medium.

We have investigated the polarization entanglement between photon pairs generated from a biexciton in a CuCl single crystal via resonant hyper parametric scattering. The pulses of a high repetition pump are seen to provide improved statistical accuracy and the ability to test Bell's inequality. Our results clearly violate the inequality and thus manifest the quantum entanglement and nonlocality of the photon pairs. We also analyzed the quantum state of our photon pairs using quantum state tomography., 4 pages, 5 figures

We present one- and two-photon diffraction and interference experiments involving parametric down-converted photon pairs. By controlling the divergence of the pump beam in parametric down-conversion, the diffraction-interference pattern produced by an object changes from a quantum (perfectly correlated) case to a classical (uncorrelated) one. The observed diffraction and interference patterns are accurately reproduced by Fourier-optical analysis taking into account the quantum spatial correlation. We show that the relation between the spatial correlation and the object size plays a crucial role in the formation of both one- and two-photon diffraction-interference patterns., 10 pages, 13 figures, rev.2

Entanglement is one of the key features of quantum information and communications technology. The method that has been used most frequently to generate highly entangled pairs of photons1,2 is parametric down-conversion. Short-wavelength entangled photons are desirable for generating further entanglement between three or four photons, but it is difficult to use parametric down-conversion to generate suitably energetic entangled photon pairs. One method that is expected to be applicable for the generation of such photons3 is resonant hyper-parametric scattering (RHPS): a pair of entangled photons is generated in a semiconductor via an electronically resonant third-order nonlinear optical process. Semiconductor-based sources of entangled photons would also be advantageous for practical quantum technologies, but attempts to generate entangled photons in semiconductors have not yet been successful4,5. Here we report experimental evidence for the generation of ultraviolet entangled photon pairs by means of biexciton resonant RHPS in a single crystal of the semiconductor CuCl. We anticipate that our results will open the way to the generation of entangled photons by current injection, analogous to current-driven single photon sources6,7.

Diffraction-interference patterns of parametric down-converted photons through a transmission grating by both one- and two-photon detection schemes are presented. The measured patterns are explained by the Fourier analysis of the two-photon wave packet, considering the quantum spatial correlation between the constituent photons. Spatial diffraction-interference patterns were recorded by a two-photon detector consisting of two avalanche photodiodes. For comparison, diffraction-interference patterns of Ti:sapphire laser are also observed using the same apparatus (a uncorrelated case).

Summary form only given. Recently, Jacobson et al. ( see Phys. Rev. Lett., vol.74, p. 4835 (1995)) proposed the concept of "photonic de Broglie wave" in multiphoton states. They argued that the photonic de Broglie wavelength of an ensemble of photons with wavelength /spl lambda//sub 0/ and average number of photons N can be measured using a special interferometer with "effective beam splitters" that do not split the multiphoton states into constituent photons. Fonseca et al. see Phys. Rev. Lett., vol. 82, p. 2868 (1999)) measured the photonic de Broglie wavelength of a two-photon state ("biphoton") using a kind of Young's interferometer. We propose and demonstrate the measurement of photonic de Broglie wavelength of biphotons generated by parametric down-conversion utilizing the usual Mach-Zehnder interferometer.

In terms of the concept of "photonic de Broglie wave" in multiphoton states, we demonstrate biphoton interference experiments using a Mach-Zehnder interferometer. We have observed interferometric oscillations corresponding to the photonic de Broglie wavelength of the biphoton.

Using a basic Mach-Zehnder interferometer, we demonstrate experimentally the measurement of the photonic de Broglie wavelength of entangled photon pairs (biphotons) generated by spontaneous parametric down-conversion. The observed interference manifests the concept of the photonic de Broglie wavelength. We also discuss the phase uncertainty obtained from the experiment.

We demonstrate one- and two-photon diffraction and interference experiments utilizing parametric down-converted photon pairs (biphotons) and a transmission grating. With two-photon detection, the biphoton exhibits a diffraction-interference pattern equivalent to that of an effective single particle that is associated with half the wavelength of the constituent photons. With one-photon detection, however no diffraction-interference pattern is observed. We show that these phenomena originate from the spatial quantum correlation between the down-converted photons., 4 pages, 5 figures

We demonstrate efficient up-conversion detection of photons at 1550 nm utilizing bulk periodically-poled LiNbO3 (PPLN) and nanosecond-pulsed pumping. An internal conversion efficiency of 96% and an overall photon detection efficiency of 19% including the efficiency of a visible photon detector were achieved with a low dark count rate.

We report the high-flux and broadband generation of biphotons with controlled frequency entanglement. For the generation of the entangled state consisting of frequency-anticorrelated photons, we use PPMgSLT pumped by a continuous-wave (cw) laser. Meanwhile, the state consisting of frequency-correlated photons is produced from PPKTP under the extended phase-matching condition. Both states exhibited interference patterns with over 90% visibilities in two-photon interference experiments.

The up-conversion dynamics of two photons in a three-level system is investigated theoretically. The analysis is based on a rigorous formalism that incorporates the multimode nature of the photon field, enabling quantitative handling of the spatial profiles of the photon pulses. We have derived an analytic formula that connects the wavefunction of two input photons to that of an up-converted output photon. The up-conversion efficiency and the output pulse profile are numerically calculated using this formula.

We propose an apparatus that realizes direct observation of nonlinear phase shifts induced by cross-phase modulation. The apparatus is based on a polarization-division Sagnac interferometer incorporating Faraday rotators, which, in principle, enables phase robustness, flexibility, and lossless operation. Here, we present the measurement of nonlinear phase shifts in a photonic crystal fiber, demonstrating the advantage of our system in its application to waveguide-type Kerr media. This apparatus is applicable to lossless all-optical phase gates and switches.

Utilization of weak cross-Kerr interaction will be a key to the photonic quantum information processing. We propose a novel technique to measure tiny cross-Kerr phase shifts and demonstrate it using a photonic crystal fiber.