In the Hong-Ou-Mandel interferometric scheme, two identical photons that illuminate a balanced beam splitter always leave through the same exit port. Similar effects have been predicted and (partially) experimentally confirmed for multiphoton Fock-number states. In the limit of large photon numbers, the output distribution follows a ${(1-x^2)}^{-1/2}$ law, where $x$ is the normalized imbalance in the output photon numbers at the two output ports. We derive an analytical formula that is also valid for imbalanced input photon numbers with a large total number of photons, and focus on the extent to which the hypothesis of perfect balanced input can be relaxed, discussing the robustness and universal features of the output distribution.

The quantum speed limit (QSL) time for open system characterizes the most efficient response of the system to the environmental influences. Previous results showed that the non-Markovianity governs the quantum speedup. Via studying the dynamics of a dissipative two-level system, we reveal that the non-Markovian effect is only the dynamical way of the quantum speedup, while the formation of the system-environment bound states is the essential reason for the quantum speedup. Our attribution of the quantum speedup to the energy-spectrum character can supply another vital path for experiments when the quantum speedup shows up without any dynamical calculations. The potential experimental observation of our quantum speedup mechanism in the circuit QED system is discussed. Our results may be of both theoretical and experimental interest in exploring the ultimate QSL in realistic environments, and may open new perspectives for devising active quantum speedup devices.

By using an exact analytical approach to the time evolution of decay we investigate the tunneling decay of ultracold single atoms to discuss the conditions for deviations of the exponential decay law. We find that $R$, given by the ratio of the energy of the decaying fragment ${\mathcal{E}}_r$ to its corresponding width ${\mathrm{\Gamma }}_r$, is the relevant quantity in this study. When $R$ is less than $0.3$, the decay of the atom goes to a good approximation for the first few lifetimes as $exp(-{\mathrm{\Gamma }}_{r}t/2\hslash )t^{-3/2}$. We also find that for values of $R\sim 1$, the nonexponential behavior occurs in a postexponential regime that goes as $t^{-3}$ after around a dozen lifetimes. The above conditions depend on suitably designed potential parameters and suggest that for values $R\lesssim 1$, the experimental verification of nonexponential decay might be possible.

We derive an extension to the quantum regression theorem which facilitates the calculation of two-time correlation functions and emission spectra for systems undergoing non-Markovian evolution. The derivation exploits projection operator techniques, with which we obtain explicit equations of motion for the correlation functions, making only a second-order expansion in the system-environment coupling strength and invoking the Born approximation at a fixed initial time. The results are used to investigate a driven semiconductor quantum dot coupled to an acoustic phonon bath, where we find the non-Markovian nature of the dynamics has observable signatures in the form of phonon sidebands in the resonance fluorescence emission spectrum. Furthermore, we use recently developed non-Markovianity measures to demonstrate an associated flow of information from the phonon bath back into the quantum dot exciton system.

We study the quantum adiabatic decoherence of a multispin array, coupled with an environment of harmonic phonons, in the framework of the theory of open quantum systems. We follow the basic formal guidelines of the well-known spin-boson model, since in this framework it is possible to derive the time dependence of the reduced density matrix in the adiabatic time scale, without resorting to coarse-graining procedures. However, instead of considering a set of uncoupled spins interacting individually with the boson field, the observed system in our model is a network of weakly interacting spin pairs; the bath corresponds to lattice phonons, and the system-environment interaction is generated by the variation of the dipole-dipole energy due to correlated shifts of the spin positions, produced by the phonons. We discuss the conditions that the model must meet in order to fit within the adiabatic regime. By identifying the coupling of the dipole-dipole spin interaction with the low-frequency acoustic modes as the source of decoherence, we calculate the decoherence function of the reduced spin density matrix in closed way, and estimate the decoherence rate of a typical element of the reduced density matrix in one- and three-dimensional models of the spin array. Using realistic values for the various parameters of the model we conclude that the dipole-phonon mechanism can be particularly efficient to degrade multispin coherences, when the number of active spins involved in a given coherence is high. The model provides insight into the microscopic irreversible spin dynamics involved in the buildup of quasiequilibrium states and in the coherence leakage during refocusing experiments in nuclear magnetic resonance of crystalline solids.

The precision of the mixed configuration-interaction plus many-body-perturbation-theory (CI+MBPT) method is limited in multivalence atoms by the large size of valence CI space. Previously, to study this problem, the CI+MBPT method was applied to calculations of energies in a four-valence electron atom, Si i. It was found that by using a relatively small cavity of 30 a.u. and by choosing carefully the configuration space, quite accurate agreement between theory and experiment at the level of $100 {\text{cm}}^{-1}$ can be obtained, especially after subtraction of systematic shifts for groups of states of the same $J$ and parity. However, other properties are also important to investigate. In this work, the CI+MBPT method is applied to studies of transition probabilities, oscillator strengths, and lifetimes. A close agreement with accurate experimental measurements and other elaborate theories is obtained. The long-term goal is to extend the CI+MBPT approach to applications in more complex atoms, such as lantanides and actinides.

Photoelectron momentum distributions (PMDs) of the hydrogen atom driven by multicycle elliptically polarized strong laser fields are studied in detail, based on the numerical solution of the time-dependent Schrödinger equation and the Volkov wave propagation. Both short and long driving pulses of the 800-nm field are considered, as well as the ellipticity dependence, to describe the mechanism of symmetry breaking in the hydrogen-atom PMD. Moreover, we demonstrate that the value of a retardation angle in the longitudinal PMD can depend on the order of above-threshold ionization spectra.

We have developed a technique that uses the tensor light shift to measure and cancel the frequency shift produced by the quadrupolar anisotropy of a vapor cell. We demonstrate the technique on the $6S_{1/2},F=4$ level of Cs using the $D_1$ transition. The method extends our ability to study quadrupolar wall interactions beyond diamagnetic atoms. We have deduced the twist angle per wall adhesion for cesium on an alkene coating to be ${\theta }_{\mathrm{Cs}-\mathrm{alkene}}=1.4\mathrm{mrad}$. This value is about 37 times larger than the twist angle observed in ${}^{131}\text{Xe}$, suggesting that it is not produced by the interaction of the nuclear quadrupole moment with a collisional electric-field gradient. Alternative mechanisms that may be responsible for the observed quadrupolar frequency shifts are discussed. By canceling the cell-induced quadrupole shift we have extended our cells’ effective spin-relaxation times by as much as a factor of 2. This cancellation improves magnetometer sensitivity in highly anisotropic cells and could reduce systematic uncertainties in some precision measurements.

The dynamics of closed quantum systems may be manipulated by using an applied field to achieve a control objective value for a physical goal. The functional relationship between the applied field and the objective value forms a quantum control landscape, and the optimization process consists of a guided climb up the landscape from the bottom to the top. Two classes of landscape features are important for understanding the ease of finding an optimal control field. The first class of topological landscape features has been proven to be especially simple in that no suboptimal local maxima exist (upon satisfaction of certain assumptions), which partially accounts for the ease of finding optimal fields. Complementary to the topology, the second class of features entails the landscape structure, characterizing the sinuous nature of the paths leading to an optimal control field. Previous work found that the landscape structure is also particularly simple, as excursions up the landscape guided by a gradient algorithm correspond to nearly straight paths through the space of control fields. In this paper we take an alternative approach to examining landscape structure by constructing, and then following, exactly straight trajectories in control space. Each trajectory starts at a corresponding point on the bottom of the landscape and ends at an associated point on the top, with the observable values taken either as the state-to-state transition probability, the expectation value of a general observable, or the distance from a desired unitary transformation. In some cases the starting point is at a suboptimal critical-point saddle, with the goal, again, of following a straight field path to the optimal objective yield or another suboptimal critical point. We find that the objective value almost always rises monotonically upon following a straight control path from one critical point to another, which shows that landscape structure is very simple, being devoid of rough bumps and gnarled “twists and turns”. An analysis reveals that the generally featureless nature of quantum control landscapes can be understood in terms of the occurrence of many interfering quantum pathways contributing while traversing the landscape, essentially smoothing out the terrain. These results also provide a basis for further studies to seek a new efficient algorithm to discover optimal fields by means of taking into account the inherently smooth landscape structure.

High-order-harmonic generation in benzene is studied using a mixed quantum-classical approach in which the electrons are described using time-dependent density-functional theory while the ions move classically. The interaction with both linearly and circularly polarized infrared $(\lambda =800$ nm) laser pulses of duration of ten cycles (26.7 fs) is considered. The effect of allowing the ions to move is investigated as is the effect of including self-interaction corrections to the exchange-correlation functional. Our results for circularly polarized pulses are compared with previous calculations in which the ions were kept fixed and self-interaction corrections were not included, while our results for linearly polarized pulses are compared with both previous calculations and experiment. We find that even for the short-duration pulses considered here, the ionic motion greatly influences the harmonic spectra. While ionization and ionic displacements are greatest when linearly polarized pulses are used, the response to circularly polarized pulses is almost comparable, in agreement with previous experimental results.

We present an analytical model capable of describing two-photon ionization of atoms with attosecond pulses in the presence of intermediate and final isolated autoionizing states. The model is based on the finite-pulse formulation of second-order time-dependent perturbation theory. It approximates the intermediate and final states with Fano's theory for resonant continua, and it depends on a small set of atomic parameters that can either be obtained from separate ab initio calculations or be extracted from a few selected experiments. We use the model to compute the two-photon resonant photoelectron spectrum of helium below the $N=2$ threshold for the RABITT (reconstruction of attosecond beating by interference of two-photon transitions) pump-probe scheme, in which an XUV attosecond pulse train is used in association with a weak IR probe, obtaining results in quantitative agreement with those from accurate ab initio simulations. In particular, we show that (i) the use of finite pulses results in a homogeneous redshift of the RABITT beating frequency, as well as a resonant modulation of the beating frequency in proximity to intermediate autoionizing states; (ii) the phase of resonant two-photon amplitudes generally experiences a continuous excursion as a function of the intermediate detuning, with either zero or $2\pi$ overall variation.

We demonstrate numerically a method of focusing two-photon fields inside one-dimensional random media. The approach is based on coherent control of backscattering achieved by adaptive spectral pulse shaping. The spectral phases of a femtosecond laser pulse are adjusted for the constructive interference of its backward-traveling components, resulting in an enhanced reflection from within the random system. A delayed forward-propagating second pulse overlaps with the controlled reflection, increasing the interpulse multiphoton field at a location determined by the delay between the two pulses. The technique is shown to be robust against the variations of the disorder and to work with realistic pulse-shaping parameters, hence enabling applications in controlling random lasing and multiphoton imaging in scattering materials.

Plasmonic nanoatennas are a versatile tool for coherently manipulating light on a nanoscale by confining electric fields of the driving laser into subwavelength volumes, thereby significantly enhancing electric near fields. It is normally assumed that the time-dependent spectral properties of these near fields are independent of the duration of the driving laser pulse. Here we show that when a few-cycle laser pulse shines on a bow-tie nanoantenna, its spectral properties are dramatically modified, as evidenced by a large shift of the center wavelength of the near field, relative to the driving laser. In addition, for certain geometries, a second color appears in the near field, creating conditions for generation of an isolated attosecond pulse at megahertz repetition rate. Our results open the door to frequency-tunable ultrafast sources at nanometer scale without tuning the frequency of the driving laser.

We construct measures for the non-Markovianity of quantum evolution with a physically meaningful interpretation. We first provide a general setting in the framework of channel capacities and propose two families of meaningful quantitative measures, based on the largest revival of a channel capacity, avoiding some drawbacks of other non-Markovianity measures. We relate the proposed measures to the task of information screening. This shows that the non-Markovianity of a quantum process may be used as a resource. Under these considerations, we analyze two paradigmatic examples, a qubit in a quantum environment with classically mixed dynamics and the Jaynes-Cummings model.

Absolute measurements of elastic differential cross sections for positron scattering from argon at energies from 2–50 eV are presented as well as total elastic and total inelastic cross sections from 2–20 eV. Comparisons of the current data, previous experimental data, and theoretical models using the convergent close-coupling and relativistic optical potential methods are made.

By using a three-dimensional time-dependent Schrödinger equation, we calculate alignment-dependent ionization of model diatomic molecules subjected to an intense laser field with different internuclear distances. It is found that the ionization probability shows oscillatory behavior with respect to the angle between the laser polarization direction and the molecular axis for large internuclear distances. Comparing with the results obtained by Ammosov–Delone–Krainov (ADK) theory and $S$-matrix theory, the intriguing phenomenon can be attributed to the two-center interference effect of low-energy photoelectrons. Furthermore, it is shown that ADK theory is not applicable for the investigation of molecular ionization for large internuclear distances.

An atom interferometer with a milliradian phase resolution was realized using a subhertz linewidth diode laser. The Allan deviation of the phase fluctuation decreased according to $\sigma /\mathrm{mrad}=70/\sqrt{\tau /\mathrm{s}}$ and reached 1 mrad at an integration time of 5000 s. With the transparent electrodes with a transmittance of 97%, the visibility of the interference was 8.5%, which was almost the same size as that with zero electric field. The dependence of the phase on the laser frequency was 0.1 μrad/Hz and that on the electric field was 1 mrad for 0.1 V/cm. Using the transition between the ${}^3{P}_1, m=0$ and ${}^1{S}_0, m=0$ states of Ca, the Röntgen phase was measured to be 3.0 ± 2.1 mrad for a measurement time of $4\times {10}^4\mathrm{s}$, which coincides with the expected one of 1.1 mrad within the measured uncertainty under an electric field of 1.92 kV/cm, a magnetic field of 20 mT, and an interference distance of 17 mm.

Each step in a quantum random walk is typically understood to have two basic components: a “coin toss” which produces a random superposition of two states, and a displacement which moves each component of the superposition by different amounts. Here we suggest the realization of a walk in momentum space with a spinor Bose-Einstein condensate subject to a quantum ratchet realized with a pulsed, off-resonant optical lattice. By an appropriate choice of the lattice detuning, we show how the atomic momentum can be entangled with the internal spin states of the atoms. For the coin toss, we propose to use a microwave pulse to mix these internal states. We present experimental results showing an optimized quantum ratchet, and through a series of simulations, demonstrate how our proposal gives extraordinary control of the quantum walk. This should allow for the investigation of possible biases, and classical-to-quantum dynamics in the presence of natural and engineered noise.

Motivated by the physics of coherently coupled, ultracold atom-molecule mixtures, we investigate a classical model possessing the same symmetry, namely a $\text{U}\left(1\right)\times {\mathbb{Z}}_2$ symmetry, associated with the mass conservation in the mixture [U(1) symmetry], times the ${\mathbb{Z}}_2$ symmetry in the phase relationship between atoms and molecules. In two spatial dimensions the latter symmetry can lead to a finite-temperature Ising transition, associated with (quasi) phase locking between the atoms and the molecules. On the other hand, the U(1) symmetry has an associated Berezinskii-Kosterlitz-Thouless (BKT) transition towards quasicondensation of atoms or molecules. The existence of the two transitions is found to depend crucially on the population imbalance (or detuning) between atoms and molecules: When the molecules are the majority in the system, their BKT quasicondensation transition occurs at a higher temperature than that of the atoms; the latter has the unconventional nature of an Ising (quasi) phase-locking transition, lacking a finite local order parameter below the critical temperature. When the balance is gradually biased towards the atoms, the two transitions merge together to leave out a unique BKT transition, at which both atoms and molecules acquire quasi-long-range correlations, but only atoms exhibit conventional BKT criticality, with binding of vortex-antivortex pairs into short-range dipoles. The molecular vortex-antivortex excitations bind as well, but undergo a marked crossover from a high-temperature regime in which they are weakly bound, to a low-temperature regime of strong binding, reminiscent of their transition in the absence of atom-molecule coupling.