We show that the recently proposed scheme of teleportation of continuous variables [S.L. Braunstein and H.J. Kimble, Phys. Rev. Lett. 80, 869 (1998)] can be improved by a conditional measurement in the preparation of the entangled state shared by the sender and the recipient. The conditional measurement subtracts photons from the original entangled twomode squeezed vacuum, by transmitting each mode through a low-reflectivity beam splitter and performing a joint photonnumber measurement on the reflected beams. In this way the degree of entanglement of the shared state is increased and so is the fidelity of the teleported state.

A scheme for generating Schrödinger cat-like states of a single-mode optical field by means of conditional measurement is proposed. Feeding into a beam splitter a squeezed vacuum and counting the photons in one of the output channels, the conditional states in the other output channel exhibit a number of properties that are very similar to those of superpositions of two coherent states with opposite phases. We present analytical and numerical results for the photon-number and quadrature-component distributions of the conditional states and their Wigner and Husimi functions. Further, we discuss the effect of realistic photocounting on the states.

Airborne contaminants, e.g., bacterial spores, are usually analyzed by time-consuming microscopic, chemical, and biological assays. Current research into real-time laser spectroscopic detectors of such contaminants is based on e.g., resonance fluorescence. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. However, generating and using maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is challenging. In particular, the short dephasing times and rapid internal conversion rates are major obstacles. However, adiabatic fast passage techniques and the ability to generate combs of phase-coherent femtosecond pulses provide tools for the generation and utilization of maximal quantum coherence in large molecules and biopolymers. We call this technique FAST CARS (femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman spectroscopy), and the present article proposes and analyses ways in which it could be used to rapidly identify preselected molecules in real time. There is an urgent need for the rapid assay of chemical and biological unknowns, such as bioaerosols. Substantial progress toward this goal has been made over the past decade. Techniques such as fluorescence spectroscopy (1, 2) and UV resonant Raman spectroscopy (3-7) have been successfully applied to the identification of biopolymers, bacteria, and bioaerosols.At present, field devices are being engineered (1) that will involve an optical preselection stage based on, e.g., fluorescence radiation as in Fig. 1. If the fluorescence measurement does not give the proper signature then that particle is ignored. Most of the time the particle will be an uninteresting dust particle; however, when a signature match is recorded, then the particle is selected for special biological assay (see Fig. 1b). The relatively simple fluorescence stage can very quickly sort out some of the uninteresting particles whereas the more time-consuming bio-tests will be used for only the ''suspects.''The good news about the resonance fluorescence technique is that it is fast and simple. The bad news is that although it can tell the difference between dust and bacterial spores it cannot differentiate between spores and many other organic bioaerosols (see Fig. 1c).However, despite the encouraging success of the abovementioned studies, there is still interest in other approaches to, and tools for, the rapid identification of chemical and biological substances. To quote from a recent study (8): ''Current [fluorescence-based] prototypes are a large improvement over earlier stand-off systems, but they cannot yet consistently identify specific organisms because of the similarity of their emission spectra. Advanced signal processing techniques may improve identification.'' Resonant Raman spectra hold promise for being spore specific as indicated in Fig. 2b. This is the good news, the bad news is that the Raman signal i...