By combining a coherent state with fields from a number of parametric down-conversion processes, we are able to generate arbitrary $N$-photon entangled states, including the maximally entangled photon-number state or the so-called NOON state $(\mid N,0⟩+\mid 0,N⟩)$. This is achieved through destructive quantum interference for the cancellation of the unwanted states.

We construct a class of quantum channels in arbitrary dimensions for which entanglement improves the performance of the channel. The channels have correlated noise and when the level of correlation passes a critical value we see a sharp transition in the optimal input states (states which minimize the output entropy) from separable to maximally entangled states. We show that for a subclass of channels with some extra conditions, including the examples which we consider, the states which minimize the output entropy are the ones which maximize the mutual information.

We have used the combination of the coupling-constant integration procedure and the variational quantum Monte Carlo method to study the exchange-correlation (XC) interaction in small molecules: ${\mathrm{Si}}_2$, ${\mathrm{C}}_2{\mathrm{H}}_2$, ${\mathrm{C}}_2{\mathrm{H}}_4$, and ${\mathrm{C}}_2{\mathrm{H}}_6$. In this paper we report the calculated XC energy density, a central quantity in density functional theory, as deduced from the interaction between the electron and its XC hole integrated over the interaction strength. Comparing these “exact” XC energy densities with results using the local-density approximation (LDA), one can analyze the errors in this widely used approximation. Since the XC energy is an integrated quantity, error cancellation among the XC energy density in different regions is possible. Indeed we find a general error cancellation between the high-density and low-density regions. Moreover, the error distribution of the exchange contribution is out of phase with the error distribution of the correlation contribution. Similar to what is found for bulk silicon and an isolated silicon atom, the spatial variation of the errors of the LDA XC energy density in these molecules largely follows the sign and shape of the Laplacian of the electron density. Some noticeable deviations are found in ${\mathrm{Si}}_2$ in which the Laplacian peaks between the atoms, while the LDA error peaks in the regions “behind” atoms where a good portion of the charge density originates from an occupied $1{\sigma }_u$ antibonding orbital. Our results indicate that, although the functional form could be quite complex, an XC energy functional containing the Laplacian of the energy is a promising possibility for improving LDA.

We explore properties of atoms whose magnetic hyperfine sublevels are coupled by an external magnetic radio frequency (rf) field. We perform a thorough theoretical analysis of this driven system and present a number of systematic approximations which eventually give rise to dressed adiabatic radio frequency potentials. The predictions of this analytical investigation are compared to numerically exact results obtained by a wave packet propagation. We outline the versatility and flexibility of this class of potentials and demonstrate their potential use to build atom optical elements such as double wells, interferometers, and ringtraps. Moreover, we perform simulations of interference experiments carried out in rf induced double-well potentials. We discuss how the nature of the atom-field coupling mechanism gives rise to a decrease of the interference contrast.

We demonstrated coupling of rare-earth-metal ions in a crystal to a macroscopic cavity mode by observing optical bistability and normal-mode peaks due to sweeping-laser-induced population redistribution of the ions. The experimentally evaluated coupling constant between the individual ions and the single cavity mode is $15\mathrm{kHz}$, which is comparable with or larger than the dissipation of the ions and will exceed the cavity dissipation with a narrowing of the mode waist of the cavity to the wavelength. The results advance the application of a coupled system of rare-earth-metal ions in a crystal and an optical cavity for quantum information processing.

We propose a scheme that provides all-optically-controlled steering of light beam. The system is based on steep dispersion of a coherently driven medium. Using the eikonal equation, we study the steering angle, the spread of the optical beam, and the limits set by residual absorption of the medium under conditions of electromagnetically induced transparency. Implementation of another scheme for ultrashort pulses is also discussed.

We show that the solution of fully nonlinear lasing equations for stadium cavities exhibits a highly directional emission pattern. This directionality can be well explained by a ray-dynamical model, where the dominant ray-escape dynamics is governed by the unstable manifolds of the unstable short periodic orbits for the stadium cavity. Investigating the cold-cavity modes relevant for the lasing, we found that all of the high-$Q$ modes have the emission directionality corresponding to that of the ray-dynamical model.

Decoy states have recently been proposed as a useful method for substantially improving the performance of quantum key distribution (QKD) protocols when a coherent-state source is used. Previously, data postprocessing schemes based on one-way classical communications were considered for use with decoy states. In this paper, we develop two data postprocessing schemes for the decoy-state method using two-way classical communications. Our numerical simulation (using parameters from a specific QKD experiment as an example) results show that our scheme is able to extend the maximal secure distance from $142\mathrm{km}$ (using only one-way classical communications with decoy states) to $181\mathrm{km}$. The second scheme is able to achieve a 10% greater key generation rate in the whole regime of distances. We conclude that decoy-state QKD with two-way classical postprocessing is of practical interest.

Realistic single-photon sources do not generate single photons with certainty. Instead they produce statistical mixtures of photons in Fock states $\mid 1⟩$ and vacuum (noise). We describe how to eliminate the noise in the output of the sources by means of another noisy source or a coherent state and cross-phase-modulation (XPM). We present a scheme that announces the production of pure single photons and thus eliminates the vacuum contribution. This is done by verifying a XPM-related phase shift with a Mach-Zehnder interferometer.

We study the thermodynamics of a two-species homogeneous and dilute Bose gas that are self-interacting and quadratically coupled to each other. We make use of field-theoretical functional integral techniques and evaluate the one-loop finite-temperature effective potential for this system considering the resummation of the leading-order temperature-dependent as well as infrared contributions. The symmetry breaking pattern associated with the model is then studied by considering different values of self-species and interspecies couplings. We pay special attention to the eventual appearance of reentrant phases and/or shifts in the observed critical temperatures as compared to the monoatomic (one-field Bose) case.

We introduce a general displacement operator to investigate the unconventional geometric quantum computation with dissipation under the model of many identical three-level atoms in a cavity, driven by a classical field. Our concrete calculation is made for the case of two atoms, based on a previous scheme [S.-B. Zheng, Phys. Rev. A 70, 052320 (2004)] for the large-detuning interaction of the atoms with the cavity mode. The analytical results we present will be helpful for experimental realization of geometric quantum computation in real cavities.

A scheme for the output of an Einstein-Podolsky-Rosen entangled photon pair is proposed in a cavity QED system composed of two high $Q$ cavities and a moving atom passing through them and with a final detection on the atomic ground states. The theoretical derivation shows that the production of multientangled photons can be achieved by passing the atom through additional cavities along the atom’s path.

Fitness landscapes based on a limited number of laser pulse shape parameters can elucidate reaction pathways and can help to find the underlying control mechanism of optimal pulses determined by adaptive femtosecond quantum control. In a first experiment, we employ colored double pulses and systematically scan both the temporal subpulse separation and the relative amplitude of the two subpulses to acquire fitness landscapes. Comparison with results obtained from a closed-loop experiment demonstrates the capability of fitness landscapes for the revelation of possible control mechanisms. In a second experiment, using transient absorption spectroscopy, we investigate and compare the dependence of the excitation efficiency of the solvated dye molecule ${\mathrm{5,5}}^{\prime }$-dichloro-11-diphenylamino-${\mathrm{3,3}}^{\prime }$-diethyl-10,12-ethylene thiatricarbocyanine perchlorate (IR140) on selected pulse shapes in two parametrizations. The results show that very different pulse profiles can be equivalently adequate to maximize a given control objective. Fitness landscapes thus provide valuable information about different pathways along which a molecular system can be controlled with shaped laser pulses.

We present a very simple model for realizing directed transport with cold atoms in a pair of periodically flashed optical lattices. The origin of this ratchet effect is explained and its robustness demonstrated under imperfections typical of cold atom experiments. We conclude that our model offers a clear-cut way to implement directed transport in an atom optical experiment.

We propose and experimentally demonstrate a method for noninvasive measurements of cavity parameters by injection of squeezed vacuum into an optical cavity. The principle behind this technique is the destruction of the correlation between upper and lower quantum sidebands with respect to the carrier frequency when the squeezed field is incident on the cavity. This method is especially useful for ultrahigh-$Q$ cavities, such as whispering-gallery-mode cavities, in which absorption and scattering by light-induced nonlinear processes inhibit precise measurements of the cavity parameters. We show that the linewidth of a test cavity is measured to be $\gamma =844\pm 40\mathrm{kHz}$, which agrees with the classically measured linewidth of the cavity within the uncertainty $(\gamma =856\pm 34\mathrm{kHz})$.

In this paper, we study the dynamics of triple-ionization-induced dissociation in a heteronuclear diatomic molecule $\mathrm{NO}$. Compared to homonuclear diatomic molecules, $\mathrm{NO}$ shows a greater complexity in its final states following triple ionization. By utilizing a well-established technique from the study of sequential versus nonsequential ionization, our study shows that both the ${\mathrm{N}}^{2+}+{\mathrm{O}}^{+}$ and ${\mathrm{N}}^{+}+{\mathrm{O}}^{2+}$ channels are predominately formed nonvertically through a relatively slowly dissociating ${\mathrm{N}}^{+}+{\mathrm{O}}^{+}$ state. Finally, we show that both the ${\mathrm{N}}^{2+}+{\mathrm{O}}^{+}$ and ${\mathrm{N}}^{+}+{\mathrm{O}}^{2+}$ channels are formed at nearly the same internuclear separation that is much smaller than the critical internuclear distance, indicating that dissociative ionization of high charge states in a molecule can occur as a molecule steadily expands from its equilibrium separation rather than always at the critical internuclear distance.

In the theory of quantum dynamical filtering, one of the biggest issues is that the underlying system dynamics represented by a quantum stochastic differential equation must be known exactly in order that the corresponding filter provides an optimal performance; however, this assumption is generally unrealistic. Therefore, in this paper, we consider a class of linear quantum systems subjected to time-varying norm-bounded parametric uncertainties and then propose a robust observer such that the variance of the estimation error is guaranteed to be within a certain bound. Although in the linear case much of classical control theory can be applied to quantum systems, the quantum robust observer obtained in this paper does not have a classical analog due to the system’s specific structure with respect to the uncertainties. Moreover, by considering a typical quantum control problem, we show that the proposed robust observer is fairly robust against a parametric uncertainty of the system even when the other estimators—the optimal Kalman filter and risk-sensitive observer—fail in the estimation.

An atom trapping technique for determining absolute, total ionization cross sections (TICS) out of an excited atom is presented. The unique feature of our method is in utilizing Doppler cooling of neutral atoms to determine ionization cross sections. This fluorescence-monitoring experiment, which is a variant of the “trap loss” technique, has enabled us to obtain the experimental electron impact ionization cross sections out of the Cs $6{}^2{P}_{3∕2}$ state between $7\mathrm{eV}$ and $400\mathrm{eV}$. CCC, RMPS, and Born theoretical results are also presented for both the ground and excited states of cesium and rubidium. In the low energy region $(<11\mathrm{eV})$ where best agreement between these excited state measurements and theory might be expected, a discrepancy of approximately a factor of five is observed. Above this energy there are significant contributions to the TICS from both autoionization and multiple ionization.

The explicit derivation of the ultrafast optical polarization of a single molecule is presented. It is shown that an orientational wave packet carried by the excited molecular electron can provide a sharp definition for the space configuration of the laser-molecule interaction.

We study the action of the force and torque induced by a guided light field on a cesium atom outside a nanofiber. We demonstrate that the evanescent light field in a circular fundamental guided mode can force the atom to rotate around the nanofiber for a macroscopic time. We find that, due to the action of the torque, the angular momentum of the atom increases with time.