We report measurements of the intensity ratio of the $1s^{2}2s^{2}2p_{1/2}^{5}3d_{3/2}\to 1s^{2}2s^{2}2p^6$ resonance line to the $1s^{2}2s^{2}2p_{3/2}^{5}3d_{5/2}\to 1s^{2}2s^{2}2p^6$ intercombination line in neonlike ${\mathrm{Kr}}^{26+}$ and ${\mathrm{Mo}}^{32+}$. The measurements were performed at the EBIT-I electron beam ion trap facility at the Lawrence Livermore National Laboratory and utilized an x-ray microcalorimeter. The measured ratio for ${\mathrm{Mo}}^{32+}$ is in four times closer agreement with theoretical predictions than earlier measurements of ions with lower atomic number. Our measurement thus suggests a narrowing of the disagreement with atomic number, which had not been observed in the previously existing data. This implies that the disagreement with theory may be localized to ions within a range of atomic numbers in which intermediate coupling dominates.

We investigate multiphoton ionization of Kr for the formation of the two spin-orbit split states ${}^{2}P_{1/2}$ and ${}^{2}P_{3/2}$ of ${\mathrm{Kr}}^{+}$ in intense ultraviolet femtosecond laser fields ($\lambda \approx$ 398 nm, $\tau \approx$ 50 fs). As the laser intensity increases from 8 to 39 TW ${\mathrm{cm}}^{-2}$, the photoelectron angular distribution (PAD) exhibits the anomalous enhancement in the direction perpendicular to the laser polarization. With the support of the time-dependent density functional theory taking account of the spin-orbit interaction, the measured anomalous PAD is ascribed to the autoionization to ${}^{2}P_{3/2}$.

The long-range Coulomb effect (LRCE) is demonstrated experimentally and theoretically by investigating the pulse duration dependence of low-energy structure (LES) in above-threshold ionization of Ne. It is found experimentally that at 800 nm the LES shows itself as a double-hump structure (DHS) in momentum distribution of singly charged ion for Ne, and moreover, this structure is more prominent for multicycle laser fields than for few-cycle cases. This result can be reproduced and explained qualitatively with a semiclassical model and attributed to the paramount role of LRCE. That is to say, after the laser field vanishes, the electrons decelerate while flying away from the core by the long-range tail of Coulomb potential, which eventually makes DHS less notable.

In this work, we study many-body excitations of Bose-Einstein condensates trapped in periodic one-dimensional optical lattices. In particular, we investigate the impact of quantum depletion onto the structure of the low-energy spectrum and contrast the findings to the mean-field predictions of the Bogoliubov–de Gennes (BdG) equations. Accurate results for the many-body excited states are obtained by applying a linear-response theory atop the multiconfigurational time-dependent Hartree method for bosons equations of motion. We demonstrate for condensates in a triple well that even weak ground-state depletion of around $1\%$ leads to visible many-body effects in the low-energy spectrum, which deviates substantially from the corresponding BdG spectrum. We further show that these effects also appear in larger systems with more lattice sites and particles, indicating the general necessity of a full many-body treatment.

Precision measurements of ultrasmall linear velocities of one of the mirrors in a Michelson interferometer are performed using two different weak-value techniques. We show that the technique of almost-balanced weak values (ABWV) offers practical advantages over the technique of weak-value amplification, resulting in larger signal-to-noise ratios and the possibility of longer integration times due to robustness to slow drifts. As an example of the performance of the ABWV protocol we report a velocity sensitivity of 60 fm/s after 40 h of integration time. The sensitivity of the Doppler shift due to the moving mirror is 150 nHz.

The landscape of causal relations that can hold among a set of systems in quantum theory is richer than in classical physics. In particular, a pair of time-ordered systems can be related as cause and effect or as the effects of a common cause, and each of these causal mechanisms can be coherent or not. Furthermore, one can combine these mechanisms in different ways: by probabilistically realizing either one or the other or by having both act simultaneously (termed a physical mixture). In the latter case, it is possible for the two mechanisms to be combined quantum coherently. Previous work has shown how to experimentally realize one example of each class of possible causal relations. Here, we make a theoretical and experimental study of the transitions between these classes. In particular, for each of the two distinct types of coherence that can exist in mixtures of common-cause and cause-effect relations—coherence in the individual causal pathways and coherence in the way the causal relations are combined—we determine how it degrades under noise and we confirm these expectations in a quantum-optical experiment.

A compact dual-axis $\mathrm{K}-\mathrm{Rb}-{}^{21}\mathrm{Ne}$ comagnetometer with one circularly polarized pump beam and two orthogonal linearly polarized probe beams is presented. It can be exploited for rotation sensing by operating in the spin-exchange relaxation-free regime. Due to the light shift arising from the pump laser, there is a cross-talk effect between the two sensitive axes, which limits the comagnetometer applications. To eliminate this effect, an external field parallel to the direction of the pump beam is used to compensate the light-shift field. It is validated theoretically and experimentally in the dual-axis $\mathrm{K}-\mathrm{Rb}-{}^{21}\mathrm{Ne}$ comagnetometer. With the cross-talk effect suppressed, the comagnetometer can carry out high-precision rotation sensing along two sensitive axes simultaneously and independently.

We consider the variation of von Neumann entropy of subsystem reduced states of general many-body lattice spin systems due to local quantum quenches. We obtain Lieb-Robinson-like bounds that are independent of the subsystem volume. The main assumptions are that the Hamiltonian satisfies a Lieb-Robinson bound and that the volume of spheres on the lattice grows at most exponentially with their radius. More specifically, the bound exponentially increases with time but exponentially decreases with the distance between the subsystem and the region where the quench takes place. The fact that the bound is independent of the subsystem volume leads to stronger constraints (than previously known) on the propagation of information throughout many-body systems. In particular, it shows that bipartite entanglement satisfies an effective “light cone,” regardless of system size. Further implications to $t$ density-matrix renormalization-group simulations of quantum spin chains and limitations to the propagation of information are discussed.

In this work we investigate how the presence of initial entanglement affects energy transport in a network. The network has sites dedicated to the incoherent input or output of energy and intermediate control sites where initial entanglement can be established. For short times, we find that the initial entanglement in the control sites provides a robust efficiency enhancer for energy transport. For longer times, dephasing considerably damps the quantum correlations and the advantage of having initial entanglement tends to disappear in favor of the well-known mechanism of noise-assisted transport. Our findings from the study of these two mechanisms may be useful for a better understanding of the relation between nonclassicality and transport, a topic of potential interest for quantum technologies.

Calculations of the correlations between the Rabi frequency on the $H{}^3\mathrm{\Delta }_1$ to $C{}^1\mathrm{\Pi }$ transition in the ThO molecule and experimental-setup parameters in the electron electric-dipole moment ($e\text{EDM}$) search experiment are performed. Calculations are required for estimations of systematic errors in the experiment due to imperfections in laser beams used to prepare the molecule and read out the $e\text{EDM}$ signal.

We investigate the time- and energy-minimum optimal solutions for the robust control of two-level quantum systems against offset or control-field uncertainties. Using the Pontryagin maximum principle, we derive the global optimal pulses for the first robustness orders. We show that the dimension of the control landscape is lower than or equal to $2N$ for a field robust to the $N\mathrm{th}$ order, which leads to an estimate of its complexity.

Stochastic unravelings represent a useful tool to describe the dynamics of open quantum systems, and standard methods, such as quantum state diffusion (QSD), call for the complete positivity of the open-system dynamics. Here, we present a generalization of QSD, which also applies to positive, but not completely positive evolutions. The rate and the action of the diffusive processes involved in the unraveling are obtained by applying a proper transformation to the operators which define the master equation. The unraveling is first defined for semigroup dynamics and then extended to a definite class of time-dependent generators. We test our approach on a prototypical model for the description of exciton transfer, keeping track of relevant phenomena, which are instead disregarded within the standard, completely positive framework.

Electron-ion radiative recombination assisted by a bichromatic (two-component) elliptically polarized laser field is analyzed in the frame of the $S\text{-matrix}$ theory. The second Born approximation is applied in the expansion of the $S\text{-matrix}$ element where the first term in the expansion corresponds to the direct recombination of electrons with ionic targets, while the second term corresponds to the recombination preceded by an electron-ion scattering. The latter process is possible in the presence of a laser field. If the electron scatters on an ionic target, it may be subsequently driven back by the laser field and recombine with the same ion. The photon emitted in this process may have a high energy. We have studied the dependence of the energy spectrum on various laser-field and incident electron parameters. The energy spectra obtained show plateaulike structures with abrupt cutoffs. These cutoffs are explained by a classical analysis.

Anderson localization is related to exponential localization of a particle in the configuration space in the presence of a disorder potential. Anderson localization can be also observed in the momentum space and corresponds to quantum suppression of classical diffusion in systems that are classically chaotic. Another kind of Anderson localization has been recently proposed, i.e., localization in the time domain due to the presence of disorder in time. That is, the probability density for the detection of a system at a fixed position in the configuration space is localized exponentially around a certain moment of time if a system is driven by a force that fluctuates in time. We show that an electron in a Rydberg atom, perturbed by a fluctuating microwave field, Anderson localizes along a classical periodic orbit. In other words the probability density for the detection of an electron at a fixed position on an orbit is exponentially localized around a certain time moment. This phenomenon can be experimentally observed.

We investigate the ground-state properties of ultracold atoms with long-range interactions trapped in a two-leg ladder configuration in the presence of an artificial magnetic field. Using a Gross-Pitaevskii approach and a mean-field Gutzwiller variational method, we explore both the weakly interacting and strongly interacting regimes, respectively. We calculate the boundaries between the density-wave–supersolid and the Mott-insulator–superfluid phases as a function of magnetic flux and uncover regions of supersolidity. The mean-field results are confirmed by numerical simulations using a cluster mean-field approach.

Atmospheric channels are a promising candidate to establish secure quantum communication on a global scale. However, due to their turbulent nature, it is crucial to understand the impact of the atmosphere on the quantum properties of light and examine it experimentally. In this paper, we introduce a method to probe atmospheric free-space links with quantum light on a laboratory scale. In contrast to previous works, our method models arbitrary intensity losses caused by turbulence to emulate general atmospheric conditions. This allows us to characterize turbulent quantum channels in a well-controlled manner. To implement this technique, we perform a series of measurements with different constant attenuations and simulate the fluctuating losses by combining the obtained data. We directly test the proposed method with an on-chip source of nonclassical light and a time-bin-multiplexed detection system. With the obtained data, we characterize the nonclassicality of the generated states for different atmospheric noise models and analyze a postselection protocol. This general technique in atmospheric quantum optics allows for studying turbulent quantum channels and predicting their properties for future applications.

To date, investigations of carrier-envelope-phase (CEP)-dependent effects have been limited to optical pulses with few cycles and high intensity and have not been reported for other types of pulses. Optomechanical systems are shown to have the potential to go beyond these limits. We present an approach using optomechanics to extend the concept of the traditional CEP in the few-cycle regime to mechanical pulses and develop a two-step model to give a physical insight. By adding an auxiliary continuous optical field, we show that a CEP-dependent effect appears even in the multicycle regime of mechanical pulses. We obtain the approximated analytical solutions providing full understanding for these optomechanically induced CEP-dependent effects. In addition, our findings show that one can draw on the optomechanical interaction to revive the CEP-dependent effects on optical pulses with an arbitrary number of cycles and without specific intensity requirements. The effects of CEP, broadly extended to encompass few- and multicycle optical and mechanical pulses, may stimulate a variety of applications in the preparation of a CEP-stabilized pulse, the generation of ultrasonic pulses with a desired shape, the linear manipulation of optical combs, and more.

We present a frequency-renormalized multipolaron expansion method to explore the ground state of the quantum Rabi model (QRM). The main idea is to take polaron as starting point to expand the ground state of QRM. The polarons are deformed and displaced oscillator states with variationally determined frequency-renormalization and displacement parameters. This method is an extension of the previously proposed polaron concept and the coherent state expansion used in the literature, which shows high efficiency in describing the physics of the QRM. The proposed method is expected to be useful for solving other more complicated light-matter interaction models.

Multimode quantum resources or states, in which quantum correlations are shared and distributed among multiple parties, are important not only for fundamental tests of quantum effects but also for their numerous possible applications in quantum technologies, such as quantum imaging and quantum metrology. Here we demonstrate the single-step fabrication of a multimode quantum resource from four-wave mixing (FWM) process in hot Rb vapor using a spatially structured pump, which consists of a coherent combination of two tilted pump beams. During this FWM process, one probe beam is amplified, three conjugate and two new probe beams are generated. The measured degrees of the intensity squeezing for the four-beam case and six-beam case are around $-4.1\pm 0.1$ dB and $-4.7\pm 0.1$ dB, respectively. The generated multiple quantum correlated beams are naturally separated with distinct directions, which is crucial for sending them out to quantum nodes at different locations in quantum communication. Our scheme is compact, simple, phase insensitive, and easily scalable to larger number of quantum-correlated modes.

We study the interplay of control and parameter estimation on a quantum spin chain. A single qubit probe is attached to one end of the chain, while we wish to estimate a parameter on the other end. We find that control on the probe qubit can substantially improve the estimation performance and discover some interesting connections to quantum state transfer.