We investigate the dynamics of longitudinal modes in quantum-well semiconductor ring lasers by means of a spatiotemporal traveling-wave model. We report the existence of a multimode instability in such a system that provokes a periodic deterministic directional reversal involving jumps between consecutive longitudinal modes. The switching sequence follows the modal frequencies from blue to red, and every modal jump is accompanied by a reversal of the direction of emission. We characterize and analyze this instability via a bifurcation analysis of the full traveling-wave model as well as by performing a linear stability analysis of the monochromatic solutions.

In recent experiments it has been demonstrated that wavelength-scale microdisk lasers with the shape of a limaçon can support high-quality whispering-gallery modes with a regular nodal line structure. Here, we show that these modes can be accurately described by a perturbation theory for slightly deformed microdisk cavities. Our results reveal that the performance of the perturbation theory can be considerably improved by a suitable choice of the origin around which the perturbation series is expanded.

We describe a measurement of the frequency of the ${}^2{S}_{1/2}(F=0)$-${}^2{D}_{3/2}(F^{\prime }=2)$ transition of Yb${}^{+}$ at the wavelength 436 nm (frequency 688 THz), using a single Yb${}^{+}$ ion confined in a Paul trap and two cesium fountains as references. In one of the fountains, the frequency of the microwave oscillator that interrogates the cesium atoms is stabilized by the laser that excites the Yb${}^{+}$ reference transition with a linewidth in the hertz range. The stability is transferred to the microwave oscillator with the use of a fiber laser-based optical frequency comb generator that also provides the frequency conversion for the absolute frequency measurement. The frequency comb generator is configured as a transfer oscillator so that fluctuations of the pulse repetition rate and of the carrier offset frequency do not degrade the stability of the frequency conversion. The phase noise level of the generated ultrastable microwave signal is comparable to that of a cryogenic sapphire oscillator. For fountain operation with optical molasses loaded from a laser-cooled atomic beam source, we obtain a stability corresponding to a fractional Allan deviation of 4.1 × 10${}^{-14}$ ${\left(\tau /\mathrm{s}\right)}^{-1/2}$. With the molasses loaded from thermal vapor and an averaging time of 65 h, we measure the frequency of the Yb${}^{+}$ transition with a relative statistical uncertainty of 2.8 × 10${}^{-16}$ and a systematic uncertainty of 5.9 × 10${}^{-16}$. The frequency was also simultaneously measured with the second fountain that uses a quartz-based interrogation oscillator. The unperturbed frequency of the Yb${}^{+}$ transition is realized with an uncertainty of 1.1 × 10${}^{-16}$ that mainly results from the uncertainty of the blackbody shift at the operating temperature near 300 K. The transition frequency of 688 358 979 309 307.82(36) Hz, measured with the two fountains, is in good agreement with previous results.

We report a wave generator of complex solutions of the nonlinear Schrödinger equation (NLSE) combining both intensity and phase spectral shaping of an initial optical frequency comb with subsequent nonlinear propagation in an optical fiber. We apply the explicit analytical form of the two-breather solutions of the NLSE as a linear spectral filter to shape ideal modulation of a continuous wave. The additional nonlinear propagation of the tailored wave provides experimental evidence of both the growth and decay of the fundamental second-order periodic breather solution. The temporal and spectral profiles of the higher-order breather are in excellent agreement with the corresponding analytical solution.

We perform a comparison study on strong-field ionization of the rare-gas dimers (Kr${}_2$ and Ar${}_2$) with their rare-gas monomers (Kr and Ar) in infrared laser fields at wavelengths of 795 and 1320 nm in the intensity range of (0.4–2)×10${}^{14}$ W/cm${}^2$. The photoelectron longitudinal momentum distributions of the rare-gas dimers reveal prominent intensity and wavelength dependence. Compared to that of the rare-gas monomers, photoelectrons obtain smaller drift momentum for multiphoton ionization of the dimers at the same laser intensity. However, in the tunneling regime, the ionization of the dimers shows similar photoelectron longitudinal momentum distributions with the monomers at the same laser intensity. We identify that, for multiphoton ionization of the dimers, the recaptured electrons during rescattering prefer to start charge oscillation in the dimer ions, which is coupled by the laser field. When the laser field decreases, the charge oscillated electrons will be released and will have less chance to obtain larger drift momentum from the laser field. The ionization of the dimers in the tunneling regime behaves much like the ionization of one of the two constituent, nearly unperturbed rare-gas atoms.

A dynamical simulation via a coupled-equation scheme that includes the ionization continua and field-induced effects describes a pump-probe experiment that monitors ultrafast electronic dynamics in LiH. The ionizing XUV attosecond pulse train that is included in the simulation is used as a frequency filter. By tuning the time interval between the attosecond pulses of the train to a beating frequency of the wave packet induced by the IR pump pulse we characterize the changing spatial localization from one end of the molecule to the other, reflecting the interferences of the nonstationary electronic density.

The practical construction of scalable quantum-computer hardware capable of executing nontrivial quantum algorithms will require the juxtaposition of different types of quantum systems. We analyze a modular ion trap quantum-computer architecture with a hierarchy of interactions that can scale to very large numbers of qubits. Local entangling quantum gates between qubit memories within a single register are accomplished using natural interactions between the qubits, and entanglement between separate registers is completed via a probabilistic photonic interface between qubits in different registers, even over large distances. We show that this architecture can be made fault tolerant, and demonstrate its viability for fault-tolerant execution of modest size quantum circuits.

Exposed in the nonzero temperature environment, a quantum system can both lose and gain excitations by interacting with the environment. In this work, we show how to optimally protect quantum states and quantum entanglement in such a situation based on measurement reversal from weak measurement. In particular, we present explicit formulas of protection. We find that this scheme can circumvent the entanglement sudden death in certain conditions.

In this paper, we present a hybrid scheme for topological quantum computation in a system of cold atoms trapped in an atomic lattice. A topological qubit subspace is defined using Majorana fermions which emerge in a network of atomic Kitaev one-dimensional wires. We show how braiding can be efficiently implemented in this setup and propose a direct way to demonstrate the non-Abelian nature of Majorana fermions via a single-parity measurement. We then introduce a proposal for the efficient, robust, and reversible mapping of the topological qubits to a conventional qubit stored in a single atom. There, well-controlled standard techniques can be used to implement the missing gates required for universal computation. Our setup is complemented with an efficient nondestructive protocol to check for errors in the mapping.

We investigate the time-dependent Casimir-Polder potential of a polarizable two-level atom placed near a surface of arbitrary material, after a sudden change in the parameters of the system. Different initial conditions are taken into account. For an initially bare ground-state atom, the time-dependent Casimir-Polder energy reveals how the atom is “being dressed” by virtual, matter-assisted photons. We also study the transient behavior of the Casimir-Polder interaction between the atom and the surface starting from a partially dressed state, after an externally induced change in the atomic level structure or transition dipoles. The Heisenberg equations are solved through an iterative technique for both atomic and field operators in the medium-assisted electromagnetic field quantization scheme. We analyze, in particular, how the time evolution of the interaction energy depends on the optical properties of the surface, in particular on the dispersion relation of surface plasmon polaritons. The physical significance and the limits of validity of the obtained results are discussed in detail.

We present an extension of the resolvent-operator method (ROM), originally designed for atomic systems, to extract differential photoelectron spectra (in photoelectron- and nuclear-kinetic energy) for diatomic molecules interacting with strong, ultrashort laser fields in the single active electron approximation. The method is applied to the study of H${}_2{}^{+}$ photodissociation and photoionization by femtosecond laser pulses in the XUV-IR frequency range. In particular, the method is tested (i) in the perturbative regime, for few-photon absorption and bound-bound electronic transitions, and (ii) in the strong-field regime, in which multiphoton absorption and tunneling are present. In the latter case, we show how the differential ROM allows one to track the transition between both regimes. We also analyze isotopic effects by comparing the dynamics of H${}_2{}^{+}$ and D${}_2{}^{+}$ ionization for different pulses.

The dynamics that take place in the optimal quantum control of atomic rubidium upon population transfer from state 5$S_{1/2}$ to state 5$D_{3/2}$ are investigated with Hamiltonian-encoding–observable-decoding (HE-OD). For modest laser powers two second-order pathways, 5$S_{1/2}$$\to$5$P_{3/2}$$\to$5$D_{3/2}$ (pathway 1) and 5$S_{1/2}$$\to$5$P_{1/2}$$\to$5$D_{3/2}$ (pathway 2), govern the population transfer process. Pathway 1 has larger transition dipoles than pathway 2. However, state 5$P_{3/2}$ along pathway 1 may also be excited to an undesired state 5$D_{5/2}$, which can result in population “leakage.” Thus, the two pathways may either cooperate or compete with each other in various dynamical regimes. An important feature in the case of cooperation is that the ratio between the amplitudes of pathways 1 and 2 oscillates over time with a frequency equal to the detuning between transitions 5$S_{1/2}$$\to$5$P_{3/2}$ and 5$P_{3/2}$$\to$5$D_{3/2}$. We also study the regime in which pathway 2 dominates the dynamics when the larger transition dipoles of pathway 1 can no longer compensate for its population leakage. The overall analysis illustrates the utility of HE-OD as a tool to reveal the quantum control mechanism.

We prove that as a one-dimensional Fermi gas is brought across the resonance adiabatically from large repulsion to large attraction, the singlet ground state will give way to the maximum spin state, which is the lowest energy state among the states accessible to the system in this process. In the presence of tiny symmetry-breaking fields that destroy spin conservation, the singlet ground state can evolve to the ferromagnetic state or a spin segregated state. We have demonstrated these effects by exact calculations on fermion cluster relevant to current experiments, and have worked out the quantum-mechanical wave function that exhibits phase separation.

We consider a polaronic model in which impurity fermions interact with background bosons in a dipolar condensate. The polaron in this model emerges as an impurity dressed with a cloud of phonons of the dipolar condensate, which, due to the competition between the attractive and repulsive parts of the dipole-dipole interaction, obey an anisotropic dispersion spectrum. We study how this anisotropy affects the Čerenkov-like emission of Bogoliubov phonon modes, which can be directly verified by experiments in which a dipolar Bose-Einstein condensate moves against an obstacle. We also study the spectral function of impurity fermions, which is directly accessible to the momentum-resolved rf spectroscopy in cold atoms.

We develop a procedure to modify the correlations of a speckle potential. This procedure, that is suitable for spatial light modulator devices, allows one to increase the localization efficiency of the speckle in a narrow energy region whose position can be easily tuned. This peculiar energy-dependent localization behavior is explored by pulling the potential through a cigar-shaped Bose-Einstein condensate. We show that the percentage of dragged atoms as a function of the pulling velocity depends on the potential correlations below a threshold of the disorder strength. Above this threshold, interference effects are no longer clearly observable during the condensate drag.

We consider an atomic Bose-Einstein condensate trapped in a symmetric one-dimensional double-well potential in the four-mode approximation and show that the semiclassical dynamics of the two ground-state modes can be strongly influenced by a macroscopic occupation of the two excited modes. In particular, the addition of the two excited modes already unveils features related to the effect of dissipation on the condensate. In general, we find a rich dynamics that includes Rabi oscillations, a mixed Josephson-Rabi regime, self-trapping, chaotic behavior, and the existence of fixed points. We investigate how the dynamics of the atoms in the excited modes can be manipulated by controlling the atomic populations of the ground states.

When the light interacts with matters in a lossy cavity, in the standard cavity quantum electrodynamics, the dissipation of cavity fields is characterized simply by the strengths of the two couplings: the light-matter interaction and the system-environment coupling through the cavity mirror. However, in the ultrastrong-light-matter-interaction regime, the dissipation depends also on whether the two couplings are mediated by the electric field or the magnetic one (capacitive or inductive in superconducting circuits). Even if we know correctly the microscopic mechanism (Lagrangian) of the system-environment coupling, the coupling Hamiltonian itself is in principle modified due to the ultrastrong interaction in the cavity. In this paper, we show a recipe for deriving a general expression of the Hamiltonian of the system-environment coupling, which is applicable even in the ultrastrong-light-matter-interaction regime in the good-cavity and independent-transition limits.

The question “Where was a quantum particle between pre- and postselection measurements?” is analyzed in view of a recent proposal that the quantum particle was in the overlap of the forward and backward evolving wave functions. It is argued that this proposal corresponds not only to the criterion of where the particle leaves a weak trace but also to the criterion of where the local interactions can affect the probability of postselection and where finding the particle in a strong nondemolition measurement is possible. The concept of a “secondary presence” of a pre- and postselected particle where local interactions affect the weak trace in the overlap region is introduced.

The recent demonstration of electron vortex beams has opened up the new possibility of studying orbital angular momentum (OAM) in the interaction between electron beams and matter. To this aim, methods to analyze the OAM of an electron beam are fundamentally important and a necessary next step. We demonstrate the measurement of electron beam OAM through a variety of techniques. The use of forked holographic masks, diffraction from geometric apertures, and diffraction from a knife edge and the application of an astigmatic lens are all experimentally demonstrated. The viability and limitations of each are discussed with supporting numerical simulations.

Recent experiments and theory have further illuminated the concept of “quantum contextuality.” In this paper we take an inequality—the pentagram (or KCBS) inequality, which is violated by an unentangled spin-1 system—and given a relaxed assumption of noncontextuality show that a hidden-variable model may be constructed that replicates exactly the quantum bound. This is in contrast to the case of an entangled pair of spin-1/2 particles where a similar relaxation of the assumption of locality does not lead to a replication of the quantum bound. Some reasons are proposed for why this may be the case.