A method to optimize the cost of a quantum channel is developed. The goal is to determine the cheapest channel that produces prescribed output states for a given set of input states. This is essentially a quantum version of optimal transport. To attach a clear conceptual meaning to the cost, channels are viewed in terms of what we call elementary transitions, which are analogous to point-to-point transitions between classical systems. The role of entanglement in optimization of cost is emphasized. We also show how our approach can be applied to theoretically search for channels performing a prescribed set of tasks on the states of a system, while otherwise disturbing the state as little as possible.

We discuss reduced-scaling strategies employing the recently introduced subsystem embedding subalgebra (SES) coupled-cluster (CC) formalism to describe quantum many-body systems. These strategies utilize properties of the SES CC formulations where the equations describing certain classes of subsystems can be integrated into computational flows composed of coupled eigenvalue problems of reduced dimensionality. Additionally, these flows can be determined at the level of the CC ansatz by the inclusion of selected classes of cluster amplitudes, which define the wave-function “memory” of possible partitioning of the many-body system into constituent subsystems. One of the possible ways of solving these coupled problems is through implementing procedures where the information is passed between the subsystems in a self-consistent manner. As a special case we consider local flow formulations where the local character of correlation effects can be closely related to the properties of subsystem embedding subalgebras employing a localized molecular basis. We also generalize flow equations to the time domain and to downfolding methods utilizing a double-exponential unitary CC ansatz, where the reduced dimensionality of constituent subproblems offers a possibility of efficient utilization of limited quantum resources in modeling realistic systems.

We reconsider the calculation of rovibrational transition frequencies in hydrogen molecular ions. Some previously neglected contributions, such as the deuteron polarizability, are included into consideration in comparison with our previous work. In particular, one-loop and two-loop QED corrections at $m{\alpha }^7$ and $m{\alpha }^8$ orders are recalculated in the framework of the adiabatic approximation, with systematic inclusion of corrections associated with vibrational motion. Improved theoretical transition frequencies are obtained and found to be in very good agreement with recent high-precision spectroscopy experiments in ${\mathrm{HD}}^{+}$. New values for the $m_p/m_e$ and $m_d/m_p$ mass ratios are determined.

The fragmentation dynamics of triply charged benzene [(${\mathrm{C}}_6{\mathrm{H}}_6{)}^{3+}$] induced by 260 eV electron-impact ionization are investigated using a multiparticle coincidence momentum spectrometer. By measuring three fragment ions and one outgoing electron in quadruple coincidence, we identify the complete three-body dissociation channels of ${{\mathrm{CH}}_2}^{+} + {\mathrm{C}}_2{{\mathrm{H}}_3}^{+} + {\mathrm{C}}_3{\mathrm{H}}^{+}, {{\mathrm{CH}}_3}^{+} + {\mathrm{C}}_2{{\mathrm{H}}_2}^{+} + {\mathrm{C}}_3{\mathrm{H}}^{+}$, and ${\mathrm{C}}_2{{\mathrm{H}}_2}^{+} + {\mathrm{C}}_2{{\mathrm{H}}_2}^{+} + {\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$. We determine the projectile-energy-loss spectra, Dalitz plots, Newton diagrams, momentum correlation maps, and kinetic-energy release for each fragmentation channel. The analysis of these spectra is supported by an ab initio molecular dynamics simulation, which provides a molecular movie of the dissociation process. Our study reveals sequential mechanisms for all three dissociation channels of (${\mathrm{C}}_6{\mathrm{H}}_6{)}^{3+}$ trication, i.e., an ultrafast ring-opening reaction followed by two subsequent Coulomb-explosion processes.

An effective theory of the excited states of positronium is derived and some of its consequences are explored. At large physical separation, the binding of the electron and positron is assumed to be described completely by QED, whereas all short-ranged phenomena, including those within and beyond QED, can be accounted for with energy-dependent quantum defects. This theory has at least two practical applications. First, it provides an accurate and economical, yet largely QED-independent, means to fit the positronium spectrum in order to predict and compare the outcome of experiments. Second, matching the spectrum in this effective theory to that predicted by QED reveals nontrivial relationships that exist within bound-state QED; some higher order contributions to the spectrum may be obtained from lower order contributions. These relations are verified up to order $m{\alpha }^6$, and predictions are made for the order $m{\alpha }^7$ and $m{\alpha }^8$ corrections. This theory and its extensions to other hydrogenic systems may provide a useful complement to bound-state QED.

The problem of time delay in tunneling ionization is revisited. The origin of time delay at the tunnel exit is analyzed, underlining the two faces of the concept of the tunneling time delay: the time delay around the tunnel exit and the asymptotic time delay at a detector. We show that the former time delay, in the sense of a delay in the peak of the wave function, exists as a matter of principle and arises due to the sub-barrier interference of the reflected and transmitted components of the tunneling electronic wave packet. We exemplify this by describing the tunneling ionization of an electron bound by a short-range potential within the strong-field approximation in a “deep tunneling” regime. If sub-barrier reflections are extracted from this wave function, then the time delay of the peak is shown to vanish. Thus, we assert that the disturbance of the tunneling wave packet by the reflection from the surface of the barrier causes a time delay in the neighborhood of the tunnel exit.

We report a robust delocalization transition of a pair of hard-core bosons moving in a one-dimensional quasicrystal with power-law hopping. We find that in the regime of strong interactions quasiperiodicity first suppresses the transport, as in the usual Anderson picture, and then, transport is enhanced when the quasiperiodic modulation is increased. By introducing an effective Hamiltonian, valid for strong interactions, we unveil the mechanism behind the delocalization transition. Stationary single-particle properties, as well as two-particle correlations, confirm all of our findings. Extensive numerical calculations lead us to establish the values of quasiperiodic modulation, interparticle interactions, and power hops for which the delocalization takes place. Our results are of direct relevance to current experiments of systems with long-range interactions.

We study the spectral signatures and coherence properties of radio-frequency-dressed hyperfine Zeeman sublevels of ${}^{87}\mathrm{Rb}$. Experimentally, we engineer combinations of static and RF magnetic fields to modify the response of the atomic spin states to environmental magnetic field noise. We demonstrate analytically and experimentally the existence of “magic” dressing conditions where decoherence due to electromagnetic field noise is strongly suppressed. Building on this result, we propose a bichromatic dressing configuration that reduces the global sensitivity of the atomic ground states to low-frequency noise, and enables the simultaneous protection of multiple transitions between the two ground hyperfine manifolds of atomic alkali species. Our methods produce protected transitions between any pair of hyperfine sublevels at arbitrary (low) DC-magnetic fields.

Motivated by recent experiments, we calculate particle emission from a Bose-Einstein condensate trapped in a single deep well of a one-dimensional lattice when the interaction strength is modulated. In addition to pair emission, which has been widely studied, we observe single-particle emission. Within linear response, we are able to write closed-form expressions for the single-particle emission rates and reduce the pair emission rates to one-dimensional integrals. The full nonlinear theory of single-particle emission is reduced to a single variable integrodifferential equation, which we numerically solve.

We study theoretically an optical cavity and a parity-time ($\mathcal{PT}$)-symmetric pair of mechanical resonators, where all oscillators are pairwise coupled, forming an optomechanical system with a closed-contour interaction. Due to the presence of both gain and feedback, we explore its stability and the root loci over a wide coupling range. Under the red-sideband pumping and for the so-called $\mathcal{PT}$-unbroken phase, it displays a double optomechanically induced transparency (OMIT) for an experimentally realizable parameter set. We show that both the transmission amplitude and the group delay can be continuously steered from the lower transmission window to the upper one by the loop coupling phase which breaks the time-reversal symmetry and introduces a static synthetic gauge field. In the $\mathcal{PT}$-unbroken phase both the gain-bandwidth and delay-bandwidth products remain constant over the full range of the controlling phase. Tunability in transmission and bandwidth still prevails in the $\mathcal{PT}$-broken phase, albeit over a reduced range. In essence, we suggest a simple scheme that grants coupling phase-dependent control of the single and double OMIT phenomena within an effective $\mathcal{PT}$-symmetric optomechanical system.

Traditional optical gyroscopes rely on the Sagnac effect that the resonant frequency shift is linear with the rotation rate. Recently, optical gyroscopes operating near an exceptional point (EP) have been extensively investigated to improve the response to rotation. However, the high-performance implementation of this kind of gyroscope requires strict conditions, which restrict the practical application of these devices. In this paper, a non-Hermitian photonic configuration based on a single resonator is proposed to establish an exceptional surface (EP surface) constructed of numerous EPs. It is demonstrated theoretically that rotation-induced frequency splitting in our configuration is four orders of magnitude higher than that in the traditional gyroscope. We also investigate the shot-noise-limited minimum detectable rotation rate compared to the conventional gyroscope with the same radius and intrinsic loss, when the system is tuned around the EP. This proposed approach attempts to combine robustness with high sensitivity detection of the rotation.

We study the propagation of 0.05–1 TW power, ultrafast laser pulses in a 10-m-long rubidium vapor cell. The central wavelength of the laser is resonant with the $D_2$ line of rubidium, and the peak intensity is in the ${10}^{12}$–${10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ range, enough to create a plasma channel with single-electron ionization. We observe the absorption of the laser pulse for low energy, a regime of transverse confinement of the laser beam by the strong resonant nonlinearity for higher energies and the transverse broadening of the output beam when the resonant nonlinearity ceases due to the valence electrons all being removed during ionization. We compare experimental observations of the transmitted pulse energy and the transverse fluence profile with the results of computer simulations modeling pulse propagation. We find a qualitative agreement between theory and experiment that corroborates the validity of our propagation model. While the quantitative differences are substantial, the results show that the model can be used to interpret the observed phenomena in terms of self-focusing and channeling of the laser pulses by the saturable, resonant nonlinearity.

We theoretically show that by continuously varying the amplitude of a constituent beam in the superposition of two Bessel-Gaussian (BG) beams with topological charges (TCs) $m$ and $n$ the TC of the entire superposition can be controlled. Between the constituent beams, there is a topological competition. For some parameters of the BG beams, TC of the superposition equals $n$, while for other parameters, it is equal to $m$. In the center of such a superposition (on the optical axis), there is an $m$-fold degenerate intensity null (optical vortex with the TC of $m<n$), whereas $n–m$ optical vortices with the unitary TC are in the beam periphery. We demonstrate that when the amplitudes of the constituent beams tend to be equal, peripheral vortices “move” to infinity. In this case, superposition of two BG beams has a half-integer TC, and this TC is conserved on propagation in space. The fractional part of the TC is, however, “hidden” in the infinity (“hidden” phase). Conservation of the half-integer TC of an optical vortex on propagation is shown.

We exploit local quantum estimation theory to investigate the measurement of linear $\left(g_1\right)$ and quadratic $\left(g_2\right)$ coupling strengths in a driven-dissipative optomechanical system in the red-sideband regime. We consider model parameters inspired by recent experiments, in the regime $g_2<g_1$. We find that (i) $g_1$ is easier to estimate than $g_2$ at lower driving strengths, while (ii) strong enough driving allows the two parameters to be estimated with similar relative precision. Our analysis also reveals that the majority of information about $g_1$ and $g_2$ is encoded in the reduced state of the mechanical element, and that the best estimation strategy for both coupling parameters is well approximated by a direct measurement of the mechanical position quadrature. Interestingly, we also show that temperature does not always have a detrimental effect on the estimation precision.

Collective effects in the interaction of light with ensembles of identical scatterers play an important role in many fields of physics. However, often the term “identical” is not accurate due to the presence of hyperfine fields which induce inhomogeneous transition shifts and splittings. Here we develop a formalism based on the Green's-function method to model the linear response of such inhomogeneous ensembles in one-dimensional waveguides. We obtain a compact formula for the collective spectrum, which exhibits deviations from the uniform frequency shift and broadening expected of two-level systems. In particular, if the coherent contribution to the collective coupling is large, the effect of inhomogeneous broadening can be suppressed, with the linewidth approaching that of the superradiant value. We apply this formalism to describe collective effects in x-ray scattering off thin-film waveguides for inhomogeneous hyperfine parameters.

We present a study of photon statistics associated with high-order harmonic generation (HHG) involving one-mode and intermodal correlations of the high harmonic photons. The aim of the paper is to give insight into the nonclassical properties of high-order harmonic modes. To this end, we use a simplified model describing an elementary quantum emitter: the model of a two-level atom. While the material system is extremely simplified in this description, the conclusions and the methods may be generalized for more complex cases. Our primary interest is an effective model of HHG in which the exciting pulse is classical and the harmonics are quantized, although we touch upon the more generalized, fully quantized model as well. Evolution of the Mandel parameter, photon antibunching, squeezing, and cross correlations are calculated. Results imply that with respect to a single quantized emitter, nonclassicality of the harmonics is present: sub-Poissonian photon statistic and squeezing can characterize certain optical modes, while strong anticorrelation can also be present.

Collective multipartite Einstein-Podolsky-Rosen (EPR) steering is a type of quantum correlation shared among $n$ parties, where the EPR paradox of one party can be realized only by performing local measurements on all the remaining $n-1$ parties. Here, we propose an efficient method to produce collective multipartite EPR steering via symmetrically and asymmetrically cascading parametric amplification processes, i.e., four-wave mixing (FWM) of rubidium atoms. The simplified collective-steering criterion is introduced using the Coffman-Kundu-Wootters monogamy relation. Moreover, by actively adjusting the parametric gains, the collective EPR steerability is optimized in our schemes. We find that the scale of collective steering can be extended by cascading more FWMs; in particular, introducing optical loss is useful for generating collective steering with more parties only in the asymmetry structure. Our results pave the way for the construction of quantum networks and provide a promising candidate for one-sided device-independent quantum cryptography among multiple users.

The nonreciprocal photon transmission between two optical modes which are nonlinearly coupled in a cross-Kerr type is investigated by using a damped auxiliary optical mode as their common engineered reservoir. The optical nonreciprocity is created by quantum interference between the nonlinear coupling induced by the cross-Kerr nonlinearity and the dissipative coupling mediated by the adiabatic elimination of the auxiliary optical mode. The photon transmission probabilities in forward and backward directions for the relative phase $\phi$ between the two couplings are symmetric about the resonance with the probabilities for the phase $\pi -\phi$, and are exchange symmetric with those in the case of the phase $2\pi -\phi$. By tuning the dissipative coupling, in addition to the photon nonreciprocity being improved, the nonreciprocal photon amplification is produced. Meanwhile, it is found that not only can the cross-Kerr nonlinearity lead to a broadband optical nonreciprocity, but also it effectively produces the optical squeezing in the present scheme. The combination of the photon nonreciprocity with the optical squeezing will be useful in the realization of the quantum information processing with noise reduction.

We unravel the polaronic properties of impurities immersed in a correlated trapped one-dimensional Bose-Bose mixture. This setup allows the impurities to couple either attractively or repulsively to a specific host, thus offering a highly flexible platform for steering the emergent polaronic properties. Specifically, the impurity residue peak and strength of induced interactions can be controlled by varying the coupling of the impurities to the individual bosonic components. In particular, it is possible to maintain the quasiparticle character for larger interaction strengths as compared to the case of impurities immersed in a single bosonic species. We explicate a hierarchy of the polaron binding energies in terms of the impurity-medium interactions, thereby elucidating the identification of the polaronic resonances in recent experimental radio-frequency schemes. For strong attractive impurity-medium couplings, bipolaron formation is captured. Our findings pave the way for continuously changing the quasiparticle character, under the impact of trap effects, while exposing the role of correlations in triple-mixture settings.