We investigate quantum teleportation through noisy quantum channels by solving analytically and numerically a master equation in the Lindblad form. We calculate the fidelity as a function of decoherence rates and angles of a state to be teleported. It is found that the average fidelity and the range of states to be accurately teleported depend on types of noises acting on quantum channels. If the quantum channels are subject to isotropic noise, the average fidelity decays to 1/2, which is smaller than the best possible value of 2/3 obtained only by the classical communication. On the other hand, if the noisy quantum channel is modeled by a single Lindblad operator, the average fidelity is always greater than 2/3. Quantum teleportation ͓1,2͔ is a process by which a sender, called Alice, transmits an unknown quantum state to a remote recipient, called Bob, via dual classical and quantum channels. Here a pair of maximally entangled particles, forming a quantum channel, should be used for the perfect quantum teleportation. However, while being distributed and kept by Alice and Bob, an entangled state may lose its coherence and become a mixed state due to the interaction with its environment.Bennett et al. ͓1͔ noted that the quantum channel that is less entangled reduces the fidelity of teleportation, and/or the range of states that can be accurately teleported. Popescu ͓3͔ investigated the relations among teleportation, Bell's inequalities, and nonlocality. It was demonstrated that there are mixed states that do not violate any Bell-type inequality, but still can be used for teleportation. Horodecki et al. ͓4͔ showed that any two mixed spin-1 2 states that violate the Bell-CHSH inequality are useful for teleportation. Also Horodecki et al. ͓6͔ proved the relation between the optimal fidelity of teleportation and the maximal singlet fraction of the quantum channel. Banaszek ͓7͔ investigated the fidelity of quantum teleportation using nonmaximally entangled states. Ishizaka ͓8͔ studied the quantum channel subject to local interaction with two-level environment. Although the studies cited above reveal the important relations between the degree of entanglement of the quantum channel and quantum teleportation, there seem to be little studies on the direct connection between the quantum teleportation and decoherence rates. Thus it might be interesting to know how the type and strength of noise acting on quantum channels affect the fidelity of quantum teleportation.In this paper, we investigate quantum teleportation through noisy channels by solving analytically and numerically a master equation in the Lindblad form. We obtain the fidelity of quantum teleportation as a function of decoherence time and angles of an unknown state to be teleported. Thus we explicitly demonstrate Bennett et al.'s argument that noisy quantum channels reduce the range of states to be accurately teleported. We also examine the characteristic dependence of the average fidelity on types of noises acting on qubits at each stage of the teleportatio...

We show that the coherent superposition tâ râ † of photon subtraction and addition applied to each local mode of a two-mode entangled state can enhance the nonlocality manifested by the violation of a Bell inequality. A twomode squeezed state is used as an input state for this demonstration with four different Bell inequalities employed: Bell inequalities adopting displaced parity operator, pseudospin operator, homodyne measurement, and conditional entropy, respectively. We find that the coherent operation significantly enhances the nonlocality remarkably in the weak squeezing limit, compared with other possible non-Gaussian operations. It can also give a maximal Bell violation with a very small squeezing for the inequalities with pseudospin operator and conditional entropy.

For the efficient operation of a cavity ringdown spectroscopy (CRDS) system utilized with a continuous-wave (cw) laser, we numerically analyze the coupling efficiency of a cw laser to a ringdown cavity in terms of changes in the scanning rate, the laser linewidth, and the mirror reflectivity. We also demonstrate a new simple design for a CRDS system that can produce a CRDS signal with only a piezoelectric transducer (PZT), without the acousto-optic modulator that is usually adopted to switch off the cw laser beam that enters the cavity. Furthermore, we investigate the feasibility of the cw CRDS technique with a fast-scanning PZT by recording a CRDS spectrum of acetylene overtones. The detection sensitivity that corresponds to the noise-equivalent absorption is found to be approximately 3 x 10(-9)/cm.

The transmission of a plane-mirror Fabry-Perot (PFP) interferometer is theoretically modeled and investigated by treating the spatial and spectral features in a unified manner. A spatiospectral transfer function is formulated and utilized to describe the beam propagation and the multiple-beam interference occurring in an ideal one-dimensional strip PFP interferometer with no diffraction loss. The spatial-frequency filtration of a finite-size beam input not only determines the transmitted spatial beam profile but also plays a crucial role in affecting the overall spectral transmittance. The inherent deviations of the spectral transmittance from what we know as the standard Airy's formula are revealed in diverse aspects, including the less-than-unity peak transmittance, the displacement of a resonance peak frequency, and the asymmetric detuning profile. Our theoretical analysis extends to the misaligned PFP interferometers, such as the cases in which non-normal-incidence beams or wedge-aligned mirrors are used that could severely degrade the effective interferometer finesse.

We demonstrate a novel technique for measuring ultralow linear birefringence of supermirrors (high-reflectivity dielectric mirror coatings). The polarimetric cavity ringdown technique is used in conjunction with the differential detection scheme with circular polarization to enhance the measurement sensitivity. The technique could, in principle, provide the convenience and reliability of linear detection signals and a reasonable tolerance to experimental imperfections. Phase retardation and orientation of each cavity mirror can be determined separately without the influence of the other mirror. The minimum detectable phase retardation achieved experimentally with this technique is ~6 x 10(-8) rad.

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We derive two classes of multimode Bell inequalities under local realistic assumptions, which are violated only by the entangled states negative under partial transposition in accordance with the Peres conjecture. Remarkably, the failure of local realism can be manifested by exploiting wave and particle correlations of readily accessible continuous-variable states, with very large violation of inequalities insensitive to detector efficiency, which makes a strong case for a loophole-free test.

A numerical method is introduced that solves the optical Bloch equations describing a two-level atom interacting with a strong polychromatic pump field with an equidistant spectrum and an arbitrarily intense monochromatic probe field. The method involves a transformation of the optical Bloch equations into a system of equations with time-independent coefficients at steady state via double harmonic expansion of the densitymatrix elements, which is then solved by the method of matrix inversion. The solutions so obtained lead immediately to the determination of the polarization of the atomic medium and of the absorption and dispersion spectra. The method is applied to the case when the pump field is bichromatic and trichromatic, and the physical interpretation of the numerically computed spectra is given. ͓S1050-2947͑99͒10409-8͔