We estimate the resource requirements, the total number of physical qubits and computational time, required to compute the ground-state energy of a one-dimensional quantum transverse Ising model (TIM) of $N$ spin-1/2 particles, as a function of the system size and the numerical precision. This estimate is based on analyzing the impact of fault-tolerant quantum error correction in the context of the quantum logic array architecture. Our results show that a significant amount of error correction is required to implement the TIM problem due to the exponential scaling of the computational time with the desired precision of the energy. Comparison of our results to the resource requirements for a fault-tolerant implementation of Shor’s quantum factoring algorithm reveals that the required logical qubit reliability is similar for both the TIM problem and the factoring problem.

We review known and discuss new signatures of high-intensity Compton scattering assuming a scenario where a high-power laser is brought into collision with an electron beam. At high intensities one expects to see a substantial redshift of the usual kinematic Compton edge of the photon spectrum caused by the large, intensity-dependent effective mass of the electrons within the laser beam. Emission rates acquire their global maximum at this edge while neighboring smaller peaks signal higher harmonics. In addition, we find that the notion of the center-of-mass frame for a given harmonic becomes intensity dependent. Tuning the intensity then effectively amounts to changing the frame of reference, going continuously from inverse to ordinary Compton scattering with the center-of-mass kinematics defining the transition point between the two.

We study fluctuations of the number of photocounts measured by an ideal photodetector illuminated by light scattered in an amplifying disordered medium, below the threshold for random lasing. We show that the variance of fluctuations and their correlation function carry information about fluctuating properties of the medium. A direct link is established between the fluctuations of the number of photocounts due to the amplified spontaneous emission (ASE) and the dimensionless conductance $g$ of the medium. Our results suggest a possibility of probing amplifying disordered media by analyzing statistics of their ASE, without illuminating them from outside by a probe beam.

We investigate the possibility to achieve quantum gates using atoms confined in low-$Q$ cavities by single-photon input-output process. Different from in high-$Q$ cavities with strong coupling, we may accomplish universal quantum gates on the atoms by Faraday rotations. Our idea could be applied to multiqubit gating on local and nonlocal atoms as well as on encoded qubits in decoherence-free subspace.

A fully relativistic restricted active space configuration-interaction method is employed to compute the $P$-odd electron-nucleus interaction constant $W_{\text{A}}$ for the ground $\left({{}^2\Sigma }_{1/2}\right)$ state of BaF, which yields the result of $W_{\text{A}}=160\text{Hz}$. Our present estimated result of the $P$-odd interaction constant $W_{\text{A}}$ is in reasonable agreement with previous calculations. This result will be useful for the interpretation of the nuclear-spin-dependent parity-nonconservation experiment on BaF.

We present a rigorous study of low-temperature collisions of polar molecules and atoms in a microwave laser field. Our calculations show that inelastic relaxation of molecules in a microwave cavity may occur through collision-induced absorption of photons stimulated by the anisotropy of the atom-molecule interaction potential. Our results indicate that molecular collisions at temperatures below 1 K can be efficiently manipulated with microwave laser fields of moderate strength.

Ultrafast dynamics of excited molecules is studied through time-resolved two-vacuum-ultraviolet (vuv)-photon ionization using a nonlinear volume autocorrelator unit. The two-vuv-photon process is induced by the intense fifth harmonic radiation of a femtosecond Ti:sapphire laser. In a proof-of-principle experiment, ultrafast dynamics of excited ethylene and oxygen molecules are investigated. Molecular decay times are deduced by comparing the experimental data with the results of a numerical model that accounts for the spatial and temporal characteristics of the harmonic field. The present experiments pave a convenient way for time domain investigations in the vuv-xuv spectral region in all states of matter at few femtosecond to subfemtosecond temporal scales.

This work presents a theory of the frequency-resolved light emission of active two-dimensional dielectric microresonators, which are characterized by a highly nonparaxial mode structure and frequently feature a position-dependent dielectric constant and nonuniform gain. The Lorentzian intensity profile is characterized by an appropriately generalized Petermann factor, a renormalized peak position, and the cold-cavity resonance lifetime $\Gamma$. The theory also delivers a relation of $\Gamma$ to the laser threshold that improves earlier phenomenological expressions even for the case of a homogeneous medium.

Einstein, Podolsky, and Rosen brought up a state with momenta of two particles anticorrelated, which introduced the concept of entanglement and excited many interesting studies. In contrast, we propose an experimental scheme to prepare and identify two-photon momentum positively correlated entanglement state (MPES) via Hong-Ou-Mandel interference and correlated imaging. Experimental data show that two photons are highly entangled with the transverse momenta of them positively correlated. Thus two-photon MPES has been achieved, and further investigations in properties and applications of two-photon momentum entanglement become possible.

Elastic differential cross sections (DCSs) in angle of electron scattering by the representative lanthanide atoms La, Ce, Pr, Nd, Eu, Gd, Dy, and Tm have been calculated in the electron-impact energy range $0\le E\le 1\text{eV}$. Additionally, the DCSs in electron-impact energy are also presented at scattering angles $\theta =0°$, $90°$, and $180°$ for unambiguous identification of the binding energies (BEs) of the negative ions formed during the collisions as resonances. The shape resonances and the DCSs critical minima are identified as well. A Thomas-Fermi-type potential incorporating the vital core-polarization interaction is used for the calculations. Dramatically sharp resonances are found to characterize the near-threshold electron elastic DCSs, whose energy positions are identified with the BEs of the resultant negative ions. A procedure is suggested for measuring reliably the BEs of tenuously bound $\left(\text{BE}<0.1\text{eV}\right)$, weakly bound $\left(\text{BE}<1\text{eV}\right)$, and complicated open $d$- and $f$-subshell negative ions through the elastic DCSs both in scattering angle and electron-impact energy.

Electron loss from the four DNA nucleobases, namely, adenine, cytosine, thymine, and guanine as well as from RNA uracil base produced by high-energy impact of multiply charged bare ions are here theoretically investigated via a simple model combining two classical approximations—the classical trajectory Monte Carlo and the classical overbarrier—previously used with success to describe the ionization of water molecules by ions. We give in the present work an estimation of the single-electron ionization and single-electron capture cross sections for different incident projectiles, namely, ${\text{H}}^{+}$, ${\text{He}}^{2+}$, and ${\text{C}}^{6+}$ ions with impact energies ranging from 10 keV/amu to 10 MeV/amu. The obtained results are compared to the rare theoretical predictions available in the literature and therefore highlight binding effects whose existence has been already demonstrated in the case of atomic collisions but not for (so) large molecular systems.

Vibrational spectra of ${\text{Si}}_n\left(n=4–7\right)$ clusters are obtained by using Fourier transformation of the velocity autocorrelation function of the atoms calculated based on Car-Parrinello molecular dynamics. By comparing the calculated vibrational spectra with experimental results, the ground-state atomic structures of the clusters can be determined. Changes in the vibrational spectra and the bond-length root-mean-square fluctuation with temperature are obtained. It is shown that the characteristic structures of the vibrational spectra with ground-state configuration start to disappear before the melting start points indicated by sharp increase in the bond-length root-mean-square fluctuation with temperature. Therefore the vibrational spectra cannot be used to identify the existence of these clusters in gases before the temperature going down well below their actual formation temperatures. Melting of these small silicon clusters starts at a temperature lower than the bulk material and undergoes transition states over a finite temperature range. The trajectories of the atoms in configuration space are used to show that the structure changes during the melting process depend closely on the ground-state structures. The calculated electronic energy levels of ${\text{Si}}_4$ cluster, as an example, indicate that during melting process there are considerable energy-level broadenings caused by atomic thermal motions.

We present a theory of electromagnetically induced transparency (EIT) and slow light in a Doppler broadened three-level ladder (cascade) system incorporating residual Doppler broadening of two-photon coherence and velocity-changing and dephasing collision effects of a buffer gas. Both regimes of wave-vector mismatch occurring when either the probe frequency is higher than that of the (strong) control field or vice versa are considered. It is found that the velocity-changing collisions in general cause narrowing of EIT resonance linewidths which, in a particular wave-vector mismatch regime, can lead to large transparency and slow light generation at relatively low control field intensities. Large collisional dephasing of two-photon coherence in a ladder system, however, tends to mask these effects. A reduction in group velocity by more than 3 orders of magnitude is predicted in the regime where the probe frequency is lesser than that of the control field and in the absence of collisions.

Savichev’s modified adiabatic approximation [Sov. Phys. JETP 73, 803 (1991)] is used to obtain a general form of a quantum-mechanical amplitude of ionization by a low-frequency laser field. The method possesses only one requirement that the frequency of the laser field must be low. Connections of the obtained result with the quasiclassical approximation and other previous investigations are discussed.

Considering a Xe atom endohedrally confined in $C_{60}$, we predict the formation of another type of atom-fullerene hybrid states. These dimer-type states arise from the near degeneracy of inner levels in contrast to the known overlap-induced hybrid states around the Fermi level of smaller compounds. The photoionization cross sections of these hybrid states exhibit rich structures and are radically different from the cross sections of free atomic or fullerene states.

We study the feasibility of extracting a pure entangled state of noncomplementary and potentially well-separated regions of a quantum many-body system. It is shown that this can indeed be accomplished in nonequilibrium scenarios as well as the ground state of the considered spin chain models when one locally measures observables such as the magnetization of separated blocks of spins. A general procedure which can search for the optimal way to extract a pure entangled state through local projections is presented. Our results indicate a connection of the projective extraction of entanglement to good quantum numbers of the underlying Hamiltonian.

We carry out quantum inelastic scattering calculations of collisions of Rb atoms with inverting ${\text{NH}}_3$ and ${\text{ND}}_3$ molecules in the energy range between 0 and $100{\text{cm}}^{-1}$, which are important for experiments using velocity-controlled molecular beams to probe scattering resonances. We focus on molecules initially in the upper level of the ammonia inversion doublet for $j=1$, $k=1$, which is low-field seeking and can be controlled in a Stark decelerator. We calculate the integral elastic and state-to-state inelastic cross sections in the coupled-states approximation. We demonstrate the presence of both shape and Feshbach resonances in the elastic and inelastic cross sections at low collision energies and discuss their origin in terms of the bound states of the Rb-${\text{ND}}_3$ complex. We also consider elastic and inelastic cross sections in the ultracold regime, using close-coupling calculations, in order to assess the viability of sympathetic cooling of ${\text{ND}}_3$ by Rb. The inelastic cross section for relaxation to the lower level of the inversion doublet is smaller than expected for such a strongly coupled system but is still likely to be too large to allow sympathetic cooling for ${\text{ND}}_3$ in low-field-seeking states. However, there is a good prospect that sympathetic cooling will be possible for molecules in high-field-seeking states even when the collision partner is a magnetically trapped atom in a low-field-seeking state.

We study above threshold ionization, energy-resolved photoelectron angular distributions and photoelectron momentum distributions for the initial ${{}^2\Sigma }_g^{+}\left(1s{\sigma }_g\right)$ ground and the ${{}^2\Sigma }_u^{+}\left(2p{\sigma }_u\right)$ first-excited state in $\text{H}_2{}^{+}$, driven by an arbitrary orientated linearly polarized 800 nm laser pulse of different intensities and temporal durations. When the pulse duration is reduced, multiphoton structure is erased, and continuum states of gerade $\left(g\right)$ and ungerade $\left(u\right)$ inversion symmetry start to overlap. A strong localization of the photoelectron emission occurs as a consequence of the $g\text{−}u$ mixing. Furthermore, as the pulse length is reduced, effects of the carrier-envelope phase show up, and a number of complicated interference patterns in the momentum distribution radially as well angularly are predicted. Such patterns were previously reported in the atomic case. In the molecules, additional structures appear due to the interference between electronic waves emerging from different nuclear centers.

We show theoretically and experimentally that scattered light by thermal phonons inside a second-order nonlinear crystal is the source of additional phase noise observed in optical parametric oscillators. This additional phase noise reduces the quantum correlations and has hitherto hindered the direct production of multipartite entanglement in a single nonlinear optical system. We cooled the nonlinear crystal and observed a reduction in the extra noise. Our treatment of this noise can be successfully applied to different systems in the literature.

We investigate quantum interference and classical-interference effects when three-level atoms in the “$\Lambda$” or “V” configuration interact with both a cavity-field mode and an external driving field mode. We describe electromagnetically induced transparency ($\Lambda$ configuration), as well as a “field-induced transparency” (V configuration), and we go beyond the weak cavity-field studies of previous papers by utilizing a Heisenberg-picture density matrix approach. We find that under certain circumstances the cavity field evolves to be equal in magnitude to, but $180°$ out of phase with, the external pump field when the pump-field frequency equals the cavity frequency, a surprising effect found in two-level atoms in a cavity excited by an external pump field that persists in three-level atoms. The better the cavity, the quicker this buildup occurs. When the cavity field reaches this “out-of-phase” condition, the resonance fluorescence from the atom in the cavity goes to zero. This is a purely classical-interference effect between the two out-of-phase fields, with the resonance fluorescence going to zero at the same time as the two excited state populations go to zero. This is quite different from the quantum interference that occurs under the right circumstances when the state populations are coherently driven into a linear combination that is decoupled from any applied field and population is trapped in the excited states (“coherent population trapping”). When the cavity frequency is tuned to the frequency at which quantum interference would occur, the two interference effects compete with each other. The cavity field begins to evolve toward the $180°$ out-of-phase condition, thereby approaching a classical destructive interference condition with the pump field. Then at some point before 100% classical interference occurs, depending on the cavity damping rate and atom/cavity coupling strength, the atom builds up its coherence between state populations to produce the quantum interference effect. At that point, the atom becomes transparent and resonance fluorescence disappears, including fluorescence into the cavity, thereby causing the cavity field to die away.