Using a 1 GW, 1 ps pump laser pulse in high-gain parametric down conversion allows us to detect sub-shot-noise spatial quantum correlation with up to 100 photoelectrons per mode by means of a high efficiency charge coupled device. The statistics is performed in single shot over independent spatial replica of the system. Evident quantum correlations were observed between symmetrical signal and idler spatial areas in the far field. In accordance with the predictions of numerical calculations, the observed transition from the quantum to the classical regime is interpreted as a consequence of the narrowing of the down-converted beams in the very high-gain regime.

It is known that entanglement swapping can be used to realize entanglement purifying. By this way, two particles belong to different non-maximally entangled pairs can be projected probabilisticly to a maximally entangled state or to a less entangled state. In this report, we show, when the less entangled state is obtained, if a unitary transformation is introduced locally, then a maximally entangled state can be obtained probabilisticly from this less entangled state.The total successful probability of our scheme is equal to the entanglement of a single pair purification (if two original pairs are in the same non-maximally entangled states) or to the smaller entanglement of a single pair purification of these two pairs ( if two original pairs are not in the same non-maximally entangled states). The advantage of our scheme is no continuous indefinite iterative procedure is needed to achieve optimal purifying. 03.67.-a Typeset using REVT E X

Constructing a quantum memory for a photonic entanglement is vital for realizing quantum communication and network [1][2][3][4]. Besides enabling the realization of high channel capacity communication [5], entangled photons of high-dimensional space are of great interest because of many extended applications in quantum information and fundamental physics fields [6][7][8][9]. Photons entangled in a two-dimensional space had been stored in different system [10][11][12][13], but there have been no any report on the storage of a photon pair entangled in a high-dimensional space. Here, we report the first experimental realization of storing an entangled orbital angular momentum (OAM) state through a far off-resonant two-photon transition (FORTPT) in a cold atomic ensemble. We reconstruct the matrix density of an OAM entangled state postselected in a two-dimensional subspace with a fidelity of 90.3%±0.8% and obtain the Clauser, Horne and Shimony and Holt inequality parameter S of 2.41±0.06 after a programmed storage time. All 2 results clearly show the preservation of entanglement during the storage. Besides, we also realize the storage of a true-single-photon via FORTPT for the first time.The establishment of quantum network in the future needs distribution of quantum entangled photons over channels between different nodes [14,15]. To overcome the exponential scaling of the error rate with the channel length, the concept of quantum repeater is introduced [16], which combines entanglement swapping and quantum memory to efficiently extend the achievable distance of quantum communication. During the last years, important progresses have been made towards the realization of an efficient and coherent quantum memory based on gas and solid atomic ensemble [17][18][19][20][21], photons encoded in a two-dimensional space spanned for example by orthogonal polarizations or different paths had been stored [10][11][12][13]. Moreover, many groups and researchers are active in storing light encoded using a high-dimensional space in different physical systems [22][23][24][25][26][27][28][29][30][31][32]. In quantum information and quantum optics fields, a photon encoded in a high-dimensional space [33][34][35][36] could carry 2 log d bits information, where d is the number of orthogonal basis vectors of the Hilbert space. In such a way, the transmission rate of quantum communications is increased greatly [37], and the capacity of channel could be also significantly improved [5]. Moreover, it affords quantum key distribution a more secure flux of information [38], etc. Because of the inherent infinite dimension of orbital angular momentum (OAM) space [39][40][41], a light is usually encoded in OAM space to offer the higher-information-density coding. Therefore, the preparation of a high-dimensional OAM entangled state plays a vital role inquantum information and communication fields, and usually was realized by using the spontaneous parametric down-conversion in a crystal [41] or spontaneous Raman scattering (SRS) in an atomic ens...

In this letter, two different probabilistic teleportations of a two-particle entangled state by pure entangled three-particle state are shown. Their successful probabilities are different.Comment: 7 pages, Revtex, no figure, to appear in Phys. Lett. A 268 (2000) 161-16

Light with helical phase structures, carrying quantized orbital angular momentum (OAM), has many applications in both classical and quantum optics, such as high-capacity optical communications and quantum information processing. Frequency conversion is a basic technique to expand the frequency range of fundamental light. The frequency conversion of OAM-carrying light gives rise to new physics and applications such as up-conversion detection of images and high dimensional OAM entanglements.Quasi-phase matching (QPM) nonlinear crystals are good candidates for frequency conversion, particularly for their high-valued effective nonlinear coefficients and no walk-off effect. Here we report the first experimental second-harmonic generation (SHG) of OAM light with a QPM crystal, where a UV light with OAM of 100  is generated. OAM conservation is verified using a specially designed interferometer. With a pump beam carrying an OAM superposition of opposite sign, we observed interesting interference phenomena in the SHG light; specifically, a photonics gear-like structure is obtained that gives direct evidence of OAM conservation, which will be very useful for ultra-sensitive angular measurements. We also develop a theory to reveal the underlying physics of the phenomena.The methods and theoretical analysis shown here are also applicable to other frequency conversion processes, such as sum frequency generation and difference-frequency generation, and may also be generalized to the quantum regime for single photons.PACS numbers: 42.65. Ky,42.50.Tx, 42.25.Hz, 42.70.Mp Orbital angular momentum (OAM) in light is a very useful degree of freedom that has no dimensional limitation, and has been widely studied in both classical and quantum optics fields since The interaction of OAM light with matter, such as nonlinear crystals [12,[20][21][22] and atomic vapors [23][24][25], produces many interesting phenomena in contrast to those obtained using Gaussian beams.Allen [20,21] has demonstrated the OAM transformation and conservation in frequency conversion in LBO crystal. Zeilinger's group [12] has realized high-dimensional OAM entanglement in the spontaneous parametric down-conversion processes. In all these nonlinear interaction processes, the total OAM conservation of light plays a very important role. The frequency conversion of OAM lights will be very useful in up-conversing detection of images [26] and generating of OAM light from a fundamental OAM light at special wavelengths (in the UV or mid-infrared frequency domains), which are hard to produce them with traditional method. For nonlinear processes with crystals, the benefits from quasi-phase matching (QPM) when compared with birefringence phase matching make QPM crystals good candidates for frequency conversion of OAM light, particularly for their high-valued effective nonlinear coefficients and no walk-off effect. Then some important questions are coming naturally: can we use QPM crystals for nonlinear frequency conversion of OAM light? Whether the total OAM of light i...

A novel multiplex chemiluminescent mycotoxin immunoassay suspension array system was developed by combining the silica photonic crystal microspheres (SPCMs) encoding technique and a chemiluminescent immunoassay (CLIA) method. The SPCMs were used as a carrier of the suspension array and encoded by their reflectance peak positions, which overcome fluorescence photobleaching, and the potential interference between the encoding fluorescence and detection fluorescence. Aflatoxin B1 (AFB1), fumonisin B1 (FB1) and ochratoxin A (OTA) artificial antigens were immobilized on the surfaces of SPCMs by using 3-glycidoxypropyltrimethoxysilane as a linker. Horseradish peroxidase (HRP) was used as a labeling enzyme for the secondary antibody in the enzyme-catalyze H2O2-luminol chemiluminescence system. The CLIA detection system was easily integrated with a multifunctional microplate reader and displayed a two to three orders of magnitude dynamic linear detection range from 0.001 to 1, 0.001 to 1, and 0.01 to 1 ng mL(-1) for AFB1, FB1 and OTA with 50% inhibitory concentrations (IC50) of 0.01, 0.036, and 0.04 ng mL(-1), respectively. The recovery rates are in the range of 63.5 to 121.6% for the three mycotoxins in three kinds of spiked cereal samples. The results of detection in 12 naturally contaminated cereal samples were consistent with that of the classic enzyme-linked immunosorbent assay (ELISA) method. This proposed system is simple, rapid, low cost and high throughput for multiplex mycotoxin assay.

In this paper, we present a scheme for quantum key distribution, in which different-frequency photons are used to encode the key. Thses different-frequency photons are produced by an acoustic-optical modulator and two kinds of narrow filters . This scheme may be implementable in practice.Comment: 3 pages, to appear in Appl.Phys. B 70, 415-417 (2000

Quantum memories have been realized in different physical systems, such as atomic ensembles or solid systems. To date, all quantum memories have realized the storage and retrieval of photons encoded using only a two-dimensional space spanned, for example, by the orthogonal polarization states, and hence can only store a quantum bit. Using electromagnetically induced transparency in a cold atomic ensemble, we report the experimental realization of a quantum memory storing a photon encoded in a three-dimensional space spanned by orbital angular momentum (OAM) states. We experimentally reconstruct the storage process density matrix with a fidelity of 85.3% ± 1.8% using a 4-f imaging system. We also realize storage of two special photonic qutrit states as examples. Toward storing a higher-dimensional state encoded in OAM space, the efficiency difference between different OAM states should be considered according to the experimental results. The capability to store high-dimensional quantum states with high fidelity is a key step towards building high-dimensional quantum networks.