We discuss the concept of characteristic squeezing modes applied to a travelling-wave optical parametric amplifier pumped by an ultrashort pulse. The characteristic modes undergo decoupled single-mode squeezing transformations, and therefore they form a useful basis to describe the evolution of the entire multimode system. This provides an elegant and intuitive picture of quantum statistical properties of parametric fluorescence. We analyse the efficiency of detecting quadrature squeezing, and present results of numerical calculations for a realistic nonlinear medium.

The occurrence of more than 1 root canal in central and lateral mandibular incisors is approximately 40% (type III was the predominant canal type). In central and lateral incisors with 1 root canal, long oval canals are common.

We analyze and demonstrate the feasibility and superiority of linear optical single-qubit fingerprinting over its classical counterpart. For one-qubit fingerprinting of two-bit messages, we prepare 'tetrahedral' qubit states experimentally and show that they meet the requirements for quantum fingerprinting to exceed the classical capability. We prove that shared entanglement permits 100% reliable quantum fingerprinting, which will outperform classical fingerprinting even with arbitrary amounts of shared randomness. . Here we establish the feasibility of single-qubit optical quantum fingerprinting, by theoretical analysis and also by experimentally generating and assessing the appropriate quantum optical states for encoding. In particular we (i) develop an optical protocol for single-qubit fingerprinting, (ii) show that two-photon coincidence measurements suffice as the experimental test for comparing fingerprints, (iii) prove that one shared entangled bit between Alice and Bob allows zero-error quantum fingerprinting which outperforms classical fingerprinting even with unlimited shared randomness between Alice and Bob, and (iv) present experimental results on the supply of fingerprint states that demonstrates the feasibility of the protocol. Our results open the prospect of experimental quantum communication complexity; although here we focus on singlequbit fingerprinting and correlated photon pairs, scalability will become possible as multiphoton entanglement capabilities improve [4].

A dense sample of atomic Rb vapor excited by a 4 ps laser pulse two-photon resonant with the 5S-5D transition in a large Fresnel number geometry emits a delayed coherent omnidirectional ir pulse on the 5D-6P transition. This superfluorescence emission involves a coherent population transfer to the 6P level and results in simultaneous directional uv emission and coherent population transfer back to the ground 5S state. This uv emission is a manifestation of parametric time-delayed four-wave mixing. For a composite noncollinear, j k 1 j j k 2 j, excitation, the uv radiation pattern is conical and the cone angle depends on the angle between the excitation pulses. [S0031-9007 (99)09254-6] PACS numbers: 42.50.Fx, 42.65.HwAmplified spontaneous emission (ASE) occurs whenever a system is pumped so that there is gain aL . 1 along the sample length L [1]. For sufficiently high gain, aL ¿ 1, the fields developed become so strong that they modify the atomic state populations; the intensity of the emission becomes proportional to the square of the number of cooperating radiators and superfluorescence obtains. Classical superfluorescence experiments were performed in elongated samples which allowed development of two minimally competing superfluorescent modes, forward and backward [2][3][4]. In the present work, the sample is quasispherical and the gain is large in all directions. Now mode competition is significant and might be expected to either preclude the development of superfluorescence altogether or to at least restrict it to a few well isolated modes which would randomly fire from one shot to the next. Nevertheless, we find that superfluorescence develops and it is omnidirectional. This cannot arise from a spherical mode as the sample is large compared to the radiation wavelength. It must be that, in our sample, spontaneously emitted randomly directed photons are amplified and, in the process, generate separate antennas. These are macroscopic antennas (each consisting of a phased array of many oscillating atomic dipoles) which, if sufficiently excited, radiate a coherent burst of radiation in a time much shorter than the fluorescence lifetime T 1 . It is the classic case of radiation damping [5]. Thus, a three-dimensional distribution of randomly phased (with respect to each other) antennas (a kind of pin cushion array), individually directed throughout 4p, is produced and each antenna separately and simultaneously produces a short (compared to T 1 ) coherent burst.We work in Rb vapor and generate a high gain inversion on the 5D-6P transition by applying a short laser pulse two-photon resonant with the 5S-5D transition. This pulse is sufficiently short that (i) no appreciable superfluorescence takes place during its application and (ii) the atoms are left in a coherent superposition of states jS͘ and jD͘. Since the 5P state is not populated by the excitation pulse, both the 5D-6P and the 5D-5P transitions are inverted with the gain on the former dominating completely as it lies in the far infrared while the latter lies...