Quantum correlation, or entanglement, is now believed to be an indispensable physical resource for certain tasks in quantum information processing, for which classically correlated states cannot be useful. Besides information processing, what kind of physical processes can exploit entanglement? In this paper, we show that there is indeed a more basic relationship between entanglement and its usefulness in thermodynamics. We derive an inequality showing that we can extract more work out of a heat bath via entangled systems than via classically correlated ones. We also analyze the work balance of the process as a heat engine, in connection with the second law of thermodynamics.

We study the decay process of an unstable quantum system, especially the deviation from the exponential decay law. We show that the exponential period no longer exists in the case of the $s$-wave decay with small $Q$ value, where the $Q$ value is the difference between the energy of the initially prepared state and the minimum energy of the continuous eigenstates in the system. We also derive the quantitative condition that this kind of decay process takes place and discuss what kind of system is suitable to observe the decay.

Non-Abelian holonomy in dynamical systems may arise in adiabatic transport of energetically degenerate sets of states. We examine such a holonomy structure for mixtures of energetically degenerate quantal states. We demonstrate that this structure has a natural interpretation in terms of the standard Wilczek-Zee holonomy associated with a certain class of Hamiltonians that couple the system to an ancilla. The mixed-state holonomy is analyzed for holonomic quantum computation using ion traps.

We derive a threshold result for fault-tolerant quantum computation for local non-Markovian noise models. The role of error amplitude in our analysis is played by the product of the elementary gate time $t_0$ and the spectral width of the interaction Hamiltonian between system and bath. We discuss extensions of our model and the applicability of our analysis.

It is well known that, in order to build the universal quantum circuit, one only needs one-qubit rotation gate and two-qubit controlled-NOT gate and until now quantum networks have been built from these gates. However, the minimum components of quantum networks in real experiments are not these quantum gates, so we develop a general method for realizing the conditional phase-shift gate in multiqubit ion-trap quantum computation which has the scalability to $N$ qubits $(N⩾3)$. The duration of the laser manipulations for the proposed conditional phase-shift gate is almost the same as that for the controlled-NOT gate in ion-trap quantum computation. Moreover, we simulate Grover’s algorithm taking into consideration the real laser fluctuations and analyze the effect of decoherence on the practical search.

Hyperfine-structure-resolved absorption spectra of the $6{}^{2}S_{1∕2}\to 5{}^{2}D_{5∕2}$ of cesium atoms have been observed with a temperature-controlled cell and a diode laser. From a comparison with the absorption of the $D_2$ transition at the same atom density range, the oscillator strength of the electric quadrupole transition was determined to be $(4.69\pm 0.05)\times {10}^{-7}$. Reflection spectra of the same transition have also been observed in the partial and total reflection regimes with a reflection cell. An increase in the apparent oscillator strength was observed for total reflection with an increase in the angle of incidence, by a factor up to 1.5 at ${\theta }_c+83.8\mathrm{mrad}$ and 2.4 at ${\theta }_c+107.5\mathrm{mrad}$ for $s$ and $p$ polarizations, respectively, where ${\theta }_c$ is the critical angle. The observed enhancement is ascribed to the incidence-angle-dependent wave vector and polarization vector of the evanescent wave.

The absorption line strengths of electric-dipole-forbidden (magnetic dipole and electric quadrupole) transitions have been calculated in the evanescent light field that accompanies the total reflection of light. Owing to the complex wave and polarization vectors of the evanescent light, the apparent oscillator strength is enhanced from that in the propagating light and the enhancement for the electric quadrupole transition depends on the polarization vector of the incident light. Separation of the contribution from each component of the wave and polarization vectors is proposed with a magnetic-sublevel-resolved measurement for the $\left(s\text{−}d\right)$ electric quadrupole transition.

We propose an approach for elimination of radiationless transitions in the excited states of polyatomic molecules. Our method is based on the concept of electronically localized eigenstates of coupled vibronic Hamiltonians—the occurrence of stationary eigenfunctions of essentially pure electronic character in the (otherwise strongly mixed) spectra of excited manifold Hamiltonians of systems undergoing nonradiative decay. The origin of this phenomenon and its generality are explained within the framework of the theory of scars of unstable periodic orbits. Optimal control theory is combined with the notion of electronically localized states to prepare the system in an excited superposition that is immune to nonradiative decay. The approach is applied to suppress ultrafast radiationless transitions in two model systems that differ widely in coupling strength and electronic structure.

The article presents first experimental data on the Stark induced $e\text{−}f$ mixing in the $\left(4\right){}^1\Pi$ state of the ${}^{85}\mathrm{Rb}{}^{133}\mathrm{Cs}$ molecule, as well as the ab initio calculations of permanent electric dipole moments $\left(d\right)$ in the $(1,2,3,4){}^1\Pi$ states and $q$ factors in the $(2,4){}^1\Pi$ states. The appearance of the “forbidden” lines in the laser-induced $\left(4\right){}^1\Pi \to X{}^1\Pi ^{+}$ fluorescence spectrum in the presence of an electric field allowed us to obtain, for the rovibronic $v^{\prime }\left(J^{\prime }\right)=2\left(82\right)$ level of the $\left(4\right){}^1\Pi$ state correlating to the $\mathrm{Rb}\left(4d\right)+\mathrm{Cs}\left(6s\right)$ atomic limit, the ratio $q∕d=0.9\times {10}^{-7}{\mathrm{cm}}^{-1}∕\mathrm{D}$ with 20% experimental error. Electronic structure calculations on the ground and excited states of the RbCs molecule were performed in the framework of the Hund’s $a$-coupling scheme by means of the many-body multipartitioning perturbation theory. The ab initio $d\left(R\right)$ estimates revealed large values of about $d\approx -7\mathrm{D}$ for the $\left(4\right){}^1\Pi$ state. The calculated $q$ factor value for the $\left(2\right){}^1\Pi$ state is in excellent agreement with the literature data. The theoretical estimate of the $q∕d$ ratio for the $v^{\prime }\left(J^{\prime }\right)=2\left(82\right)$ level of the $\left(4\right){}^1\Pi$ state agrees satisfactory well with the present experimental value.

A refined theoretical approach has been developed to study the double-differential cross sections (DDCS’s) in proton-helium collisions as a function of the ratio of ionized electron velocity to the incident proton velocity. The refinement is done in the present coupled-channel calculation by introducing a continuum distorted wave in the final state coupled with discrete states including direct as well as charge transfer channels. It is confirmed that the electron-capture-to-the-continuum (ECC) peak is slightly shifted to a lower electron velocity than the equivelocity position. Comparing measurements and classical trajectory Monte Carlo (CTMC) calculations at 10 and 20 keV proton energies, excellent agreement of the ECC peak heights is achieved at both energies. However, a minor disagreement in the peak positions between the present calculation and the CTMC results is noted. A smooth behavior of the DDCS is found in the present calculation on both sides of the peak whereas the CTMC results show some oscillatory behavior particularly to the left of the peak, associated with the statistical nature of CTMC calculations.

We trace the evolution of the shape of the two-electron photoionization spectrum with photon energy growth using quite a precise two-electron wave function, obtained by the correlation function hyperspherical harmonic method. We obtain the values of ${\omega }_1$ and ${\omega }_2$ at which the spectrum curve changes its shape. At $\omega ={\omega }_1$ the $\mathsf{U}$ shape changes to a $\mathsf{W}$ shape. At $\omega ={\omega }_2$ the central $\mathsf{W}$ peak splits into two. We consider ground states of the helium atom and of heliumlike ions with the nuclear charge $Z$, the negative ion of hydrogen ${\mathrm{H}}^{-}$, and the excited $n^{1}S$ state of helium. The limiting laws for $Z⪢1$ and $n⪢1$ are obtained. The analysis is carried out without calculations of the particular energy distributions themselves.

A joint theoretical and experimental investigation is made of the influence of long-lived metastable levels on the electron-impact single ionization of ${\mathrm{C}}^{2+}$. It is expected that our electron cyclotron resonance ion source produces a beam with 40% of the ${\mathrm{C}}^{2+}$ ions in the $1s^{2}2s^2{}^{1}S_0$ ground level and 60% in the $1s^{2}2s2p{}^{3}P_{0,2}$ excited levels. The comparison of nonperturbative close-coupling calculations with previous single-pass crossed beams and with our multiple-pass storage-ring measurements for the electron-impact ionization of ${\mathrm{C}}^{2+}$ is consistent with the predicted large metastable fraction. Reasonable agreement is found between the present time-dependent close-coupling, $R$-matrix with pseudostates, and converged-close-coupling ionization cross-section calculations for the ground and first excited configuration, and experimental measurement, assuming a 60% metastable fraction in the ion beam. Distorted-wave calculations are found to overestimate the ionization cross section from both the ground and metastable terms, compared with nonperturbative calculations, resulting in an overestimation of the resultant total cross section when compared with experiment. It is clear that collisional-radiative modeling of the evolution of atomic plasmas through the Be-like ionization stage will need to take into account the role of both ground and metastable levels.

The Ne-like resonance $2p^6{}^{1}S\text{−}2p^{5}3d{}^{1}P\left(3C\right)$ to intercombination $2p^6{}^{1}S\text{−}2p^{5}3d{}^{3}D\left(3D\right)$ line-intensity ratio, $R(3C∕3D)$, has been extensively studied through high-accuracy measurements and calculations of atomic structure and collision cross sections. However, even state-of-the-art relativistic multiconfiguration atomic-physics codes generally predict values of $R(3C∕3D)$ that are significantly larger than those observed in coronal-density experiments. In this paper, predictions of $R(3C∕3D)$ across the Ne-like isoelectronic sequence from chromium to silver are brought into agreement with coronal-density experimental measurements in two steps: first by including a semiempirical correction in the collisional atomic data due to configuration-interaction effects, and next by including the effects of cascades on the upper level populations. The configuration-interaction correction is inspired by the observation that nearly all data-production codes fail to predict $R(3C∕3D)$ correctly and is justified theoretically by a careful analysis of configuration-interaction contributions to level energies. The dependence of $R(3C∕3D)$ with electron density and its agreement with moderate-density measurements are shown. A brief description of the application of our technique to line ratios in He- and Ni-like x-ray spectra is also given.

The influence of small electric fields on the x-ray spectrum emitted by highly excited Rydberg ions formed in charge capture by bare and hydrogenic ions on Rydberg atoms is studied systematically. The branching ratio to direct decay to the $1S$ ground state of the H-like and He-like product ions is observed to saturate at electric fields which are on the order of $10\mathrm{V}∕\mathrm{cm}$ and which scale approximately as ${(Q∕n)}^5$ where $Q$ is the product ion’s core charge and $n$ is the principal quantum number of the emitting levels. The limiting value of this branching ratio is sensitive to the degree of population alignment, and is found to be in good agreement with models that begin with classical trajectory Monte Carlo estimates of the alignment. The data indicate that multiple collisions with the Rydberg target are a significant factor and may alter the degree of alignment after the initial capture.

We consider the general problem of the quantum noise in a multipixel measurement of an optical image. We first give a precise criterion in order to characterize intrinsic single-mode and multimode light. Then, using a transverse mode decomposition, for each type of possible linear combination of the pixels’ outputs we give the exact expression of the detection mode, i.e., the mode carrying the noise. We give also the only way to reduce the noise in one or several simultaneous measurements.

We propose a scheme of employing quantum interference and coherence in an optical medium with coupled electromagnetic fields to create a photonic band gap. A variable photonic band gap is achieved by the cross-phase-modulation of two counterpropagating coupling fields on a weak probe pulse. The proposed photonic band gap has potential applications for the dynamic control of group velocity dispersion compensation in fiber-optic communications.

In a recent paper [Phys. Rev. A 68, 033803 (2003)] experimental results on a double-slit configuration with two entangled bosons are presented. The authors argue that their data contradicts the de Broglie–Bohm interpretation of quantum mechanics. In this Comment we show that this conclusion is incorrect.

We show that in a two-mode, three-level, double-$\Lambda$ system an efficient multiphoton destructive interference involving one- and three-photon pumping pathways occurs, leading to a unique type of induced transparency. Unlike the conventional electromagnetically induced transparency achieved with a three-state $\Lambda$ system, the induced transparency is critically dependent upon two distinctive relaxation processes involving the production and propagation of an internally generated field. When a two-mode probe field is injected under suitable conditions, we show that the two probe pulses, after a characteristic propagation distance, evolve into a pair of temporal, amplitude, and group velocity matched pulse, traveling loss free in a highly dispersive medium.

We show that a specific implementation of a unitary map on multiple qubits in an ion trap is physically equivalent to a Hamiltonian evolution that belongs to the same universality class as the transverse Ising Hamiltonian. We suggest experimental signatures, and present numerical simulations for the case of four qubits.