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We theoretically investigate the preparation of mid-infrared (MIR) spectrally-uncorrelated biphotons from a spontaneous parametric down-conversion process using doped LN crystals, including MgO doped LN, ZnO doped LN, and In2O3 doped ZnLN with doping ratio from 0 to 7 mol%. The tilt angle of the phase-matching function and the corresponding poling period are calculated under type-II, type-I, and type-0 phase-matching conditions. We also calculate the thermal properties of the doped LN crystals and their performance in Hong-Ou-Mandel interference. It is found that the doping ratio has a substantial impact on the group-velocity-matching (GVM) wavelengths. Especially, the GVM2 wavelength of co-doped InZnLN crystal has a tunable range of 678.7 nm, which is much broader than the tunable range of less than 100 nm achieved by the conventional method of adjusting the temperature. It can be concluded that the doping ratio can be utilized as a degree of freedom to manipulate the biphoton state. The spectrally uncorrelated biphotons can be used to prepare pure single-photon source and entangled photon source, which may have promising applications for quantum-enhanced sensing, imaging, and communications at the MIR range., 11 pages, 9 figures

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

We experimentally demonstrate a polarization-entangled photon source at 810 nm using a type-II phase-matched periodically poled potassium titanyl phosphate (PPKTP) crystal pumped by a low-cost, broadband laser diode with a central wavelength of 405 nm and a typical bandwidth of 0.53 nm. The PPKTP crystal is placed in a Sagnac loop to realize the compact size and high stability. The downconverted biphotons, the signal and the idler, have typical bandwidths of 5.57 and 7.32 nm. We prepare two Bell states | Ψ + ⟩ and | Ψ − ⟩ with the fidelities of 0.948 ± 0.004 and 0.963 ± 0.002 . In the polarization correlation measurement, the visibilities are all higher than 96.2%, and in the Bell inequality test, the S value can achieve 2.78 ± 0.01 . This high-quality and low-cost entangled photon source may have many practical applications in quantum information processing.

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

We report a simple scheme for direct generation of frequency-bin entangled photon pairs via spontaneous parametric downconversion. Our fabricated nonlinear optical crystal with two different poling periods can simultaneously satisfy two different, spectrally symmetric nondegenerate quasi-phase-matching conditions, enabling the direct generation of entanglement in two discrete frequency-bin modes. Our produced photon pairs exhibited Hong-Ou-Mandel interference with high-visibility beating oscillations, a signature of two-mode frequency-bin entanglement. Moreover, we demonstrate deterministic entanglement-mode conversion from frequency-bin to polarization modes, with which our source can be more versatile for various quantum applications. Our scheme can be extended to direct generation of high-dimensional frequency-bin entanglement, and thus will be a key technology for frequency-multiplexed optical quantum information processing.

In this work, we experimentally manipulate the spectrum and phase of a biphoton wave packet in a two-dimensional frequency space. The spectrum is shaped by adjusting the temperature of the crystal, and the phase is controlled by tilting the dispersive glass plate. The manipulating effects are confirmed by measuring the two-photon spectral intensity (TSI) and the Hong-Ou-Mandel (HOM) interference patterns. Unlike the previous independent manipulation schemes, here we perform joint manipulation on the biphoton spectrum. The technique in this work paves the way for arbitrary shaping of a multi-photon wave packet in a quantum manner., Comment: 4 pages, 4 figures

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.

The NOON state, and its experimental approximation the Holland-Burnett state, have important applications in phase sensing measurement with enhanced sensitivity. However, most of the previous Holland-Burnett state interference (HBSI) experiments only investigated the area of the interference pattern in the region immediately around zero optical path length difference, while the full HBSI pattern over a wide range of optical path length differences has not yet been well explored. In this work, we experimentally and theoretically demonstrate up to six-photon HBSI and study the properties of the interference patterns over a wide range of optical path length differences. It was found that the shape, the coherence time and the visibility of the interference patterns were strongly dependent on the detection schemes. This work paves the way for applications which are based on the envelope of the HBSI pattern, such as quantum spectroscopy and quantum metrology.

Hong-Ou-Mandel (HOM) interference between independent photon sources (HOMI-IPS) is the fundamental block for quantum information processing, such as quantum gate, Shor's algorithm, Boson sampling, etc. All the previous HOMI-IPS experiments were carried out in time-domain, however, the spectral information during the interference was lost, due to technical difficulties. Here, we investigate the HOMI-IPS in spectral domain using the recently developed fast fiber spectrometer, and demonstrate the spectral distribution during the HOM interference between two heralded single-photon sources, and two thermal sources. This experiment can not only deepen our understanding of HOMI-IPS in the spectral domain, but also be utilized to improve the visibility by post-processing spectral filtering., Comment: 8 pages, 7 figures

Free space optical (FSO) communications are enabling high-speed global wireless networks. Thanks to the highly directional nature of laser beam, they also yield a greater security advantage over radio frequency counterparts. When combined with a scheme of secret key agreement (SKA), FSO-SKA can establish at high speed a symmetric secret key which cannot be decrypted even by unbounded computer resources. Although there have been many theoretical studies on SKA, experimental investigations have been quite lacking, especially on quantifying eavesdropping risks and secret key rates in realistic environment. Here, we report the first full-field implementations of FSO-SKA in a 7.8-km terrestrial link with a probing station, enabling the estimation of eavesdropping risks. We attain the final key rates from 100 kbps to 7.77 Mbps under various atmospheric and beaming conditions even with total losses of 55dB or higher, in which known quantum key distribution schemes attain impractically low key rates.

Secret key agreement using physical properties of a wireless channel is becoming a promising scheme to establish a secret key between two users, especially in short-distance radio frequency (RF) communications. In this scheme, the existence of codes or key distillation that can make the leaked information to an eavesdropper arbitrarily small can be derived in an information theoretical way, given a priori knowledge on the channel linking a sender (Alice), a legitimate receiver (Bob), and an eavesdropper (Eve), which is called the wiretap channel. In practice, however, it is often difficult for Alice and Bob to get sufficient knowledge on Eve. In this study, we implement a free-space optical wiretap channel in a 7.8 km-terrestrial link and study how to estimate Eve's tapping ability, demonstrating high speed secret key agreement in the optical domain under a certain restricted condition of line-of-sight.

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.

We demonstrate a down-conversion-based twin photon source pumped by a 10 GHz repetition-rate-tunable comb laser at 1553 nm wavelength. We show high Hong-Ou-Mandel interference visibilities, which are free from the pump-power induced degradation.

We have developed a new tapping mode-scanning near-field optical Raman microscope (SNORM) with a caved and pyramidical probe, using resonant Raman scattering and measured the stress distribution of very-large-scale integration (VLSI) standards made of the silicon dioxide film and Si. It has been found that compressive stresses of about 0.69 GPa/cm2 are concentrated on the corner of the area uncovered by silicon dioxide. The SNORM we developed has at least the spatial resolution of less than 250 nm and is a useful tool to measure image of stresses in Si devices within a short time and with a penetration depth of 5 nm.

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.

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

We demonstrate a Hong-Ou-Mandel interference between two independent, intrinsically pure, heralded single photons from spontaneous parametric down-conversion (SPDC) at telecommunication wavelength. A visibility of $85.5\ifmmode\pm\else\textpm\fi{}8.3%$ was achieved without using any bandpass filter. Thanks to the group-velocity-matched SPDC and superconducting nanowire single-photon detectors (SNSPDs), the fourfold coincidence counts are one order higher than that in the previous experiments. The combination of bright single-photon sources and SNSPDs is a crucial step for future practical quantum infocommunication systems at telecommunication wavelength.

We theoretically and experimentally investigate the spectral tunability and purity of photon pairs generated from spontaneous parametric down conversion in periodically poled $\mathrm{KTiOPO_4}$ crystal with group-velocity matching condition. The numerical simulation predicts that the purity of joint spectral intensity ($P_{JSI}$) and the purity of joint spectral amplitude ($P_{JSA}$) can be kept higher than 0.98 and 0.81, respectively, when the wavelength is tuned from 1460 nm to 1675 nm, which covers the S-, C-, L-, and U-band in telecommunication wavelengths. We also directly measured the joint spectral intensity at 1565 nm, 1584 nm and 1565 nm, yielding $P_{JSI}$ of 0.989, 0.983 and 0.958, respectively. Such a photon source is useful for quantum information and communication systems., 8 pages, 5 figures

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 studied blue light-induced near-infrared absorption (BLINIRA) and its temperature dependence in ferroelectric crystals, i.e., MgO-doped stoichiometric-LiNbO3, congruent-LiNbO3, MgO-doped stoichiometric-LiTaO3, and stoichiometric-LiTaO3. We found the BLINIRA decreased with the increase of temperature.

We have developed a high-efficiency source of cross-polarized photon pairs in a 800-nm band via type-II quasi-phase matched spontaneous parametric down-conversion using periodically poled lithium niobate. We obtained 25,000 coincidence counts sec-1 mW-1 of the photon pairs.

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.

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.

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.

Cross-phase modulation(XPM) in a silicon wire waveguide at 1.55 μ m telecom band was studied down to ultralow light levels. In the low-power regime, we found that free-carrier dispersion as well as the optical Kerr effect contributes to the XPM. Possible mechanisms of the low-power XPM are discussed.

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.

We have developed a high-efficiency source of cross-polarized photon pairs generated via type-II spontaneous parametric down-conversion using periodically poled lithium niobate (PPLN). The photon pairs are generated from bulk PPLN under third-order quasi-phase-matched conditions around 800 nm. We achieved a photon-pair production rate of N=1.95×106 s-1mW-1.

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.