We theoretically characterize interactions, energetics, and chemical reaction paths in ionic two-body and three-body systems of alkali-metal and alkaline-earth-metal atoms in the context of modern experiments with cold hybrid ion-atom mixtures. Using ab initio techniques of quantum chemistry such as the coupled-cluster method, we calculate ground-state electronic properties of all diatomic $A{B}^{+}$ and most of the triatomic $A_2{B}^{+}$ molecular ions consisting of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, and Yb atoms. Different geometries and wave-function symmetries of the ground state are found for different classes of molecular ions. We analyze intermolecular interactions in the investigated systems including additive two-body and nonadditive three-body ones. As an example we provide two-dimensional interaction potential-energy surfaces for ${\mathrm{KRb}}^{+}+\mathrm{K}$ and ${\mathrm{Rb}}^{+}+{\mathrm{Sr}}_2$ mixtures. We identify possible channels of chemical reactions based on the energetics of the reactants. The present results may be useful for investigating controlled chemical reactions and other applications of molecular ions formed in cold hybrid ion-atom systems.

We compute the ground-state electric dipole polarizability $\alpha$ of the group-13 ions using the perturbed relativistic coupled-cluster theory. To account for the relativistic effects and quantum electrodynamical corrections, we use the Dirac-Coulomb-Breit Hamiltonian with the corrections from the Uehling potential and the self-energy. The effects of triple excitations are considered perturbatively in the theory. Our results for polarizability are in good agreement with previous theoretical results for all the ions. From our results we find that the nonlinear terms in perturbed relativistic coupled-cluster theory have significant contributions and must be included to obtain accurate value of dipole polarizability for group-13 ions. For the correction from the Breit interaction, we find that it is largest for ${\mathrm{Al}}^{+}$ and decreases with increasing $Z$. The corrections from the vacuum polarization and the self-energy increases with increasing $Z$.

We have computed the thermally averaged total, elastic rate coefficient for the collision of a room-temperature helium atom with an ultracold lithium atom. This rate coefficient has been computed as part of the characterization of a cold-atom vacuum sensor based on laser-cooled ${}^6\mathrm{Li}$ or ${}^7\mathrm{Li}$ atoms that will operate in the ultrahigh-vacuum ($p<{10}^{-6}$ Pa) and extreme-high-vacuum ($p<{10}^{-10}$ Pa) regimes. The analysis involves computing the $X{}^2{\mathrm{\Sigma }}^{+}$ HeLi Born-Oppenheimer potential followed by the numerical solution of the relevant radial Schrödinger equation. The potential is computed using a single-reference-coupled-cluster electronic-structure method with basis sets of different completeness in order to characterize our uncertainty budget. We predict that the rate coefficient for a 300 K helium gas and a 1 $\mu \mathrm{K}$ Li gas is $1.467\left(13\right)\times {10}^{-9}$ ${\mathrm{cm}}^3$/s for ${}^4\mathrm{He}+{}^6\mathrm{Li}$ and $1.471\left(13\right)\times {10}^{-9}$ ${\mathrm{cm}}^3$/s for ${}^4\mathrm{He}+{}^7\mathrm{Li}$, where the numbers in parentheses are the one-standard-deviation uncertainties in the last two significant digits. We quantify the temperature dependence as well. Finally, we evaluate the $s$-wave scattering length and binding of the single van der Waals bound state of HeLi. We predict that this weakly bound level has a binding energy of $-0.0064\left(43\right)\times hc$ ${\mathrm{cm}}^{-1}$ and $-0.0122\left(67\right)\times hc$ ${\mathrm{cm}}^{-1}$ for ${}^4\mathrm{He}{}^6\mathrm{Li}$ and ${}^4\mathrm{He}{}^7\mathrm{Li}$, respectively. The calculated binding energy of ${}^4\mathrm{He}{}^7\mathrm{Li}$ is consistent with the sole experimental determination.

We provide a physically motivated definition for the binding energy (or bond dissociation) of a bipartite quantum system. We consider coherently applying an external field to cancel out the interaction between the subsystems, to break their bond and separate them as systems from which no work can be extracted coherently by any cyclic evolution. The minimum difference between the average energies of the initial and final states obtained this way is defined as the binding energy of the system. We show that the final optimal state is a passive state. We discuss how the required evolution can be realized through a sequence of control pulses. The utility of our definition is illustrated through two examples. In particular, we also show how quantum tunneling can assist or enhance a bond-breaking process. This extends our definition to probabilistic events.

We study the propagation of a density perturbation in a weakly interacting boson gas confined on a lattice and in the presence of square dimerized impurities. Such a two-dimensional dual random dimer model (2D DRDM), previously introduced [Capuzzi, Gattobigio, and Vignolo, Phys. Rev. A 92, 053622 (2015)], is the disorder transition from a single square lattice, where impurities are absent, to a bipartite square lattice, where the number of impurities is maximum and coincides with half the number of lattice sites. We show that disorder correlations can play a crucial role in the dynamics for a broad range of parameters by allowing density fluctuations to propagate in the 2D DRDM lattice, even in the limit of strong disorder. In such a regime, the propagation speed depends on the percentage of impurities, interpolating between the speed in a single monoperiodic lattice and that in a bipartite one.

We investigate the dynamics of relativistic electrons interacting with intense laser fields in a linear or circular polarization. First, we study the momentum distributions of a single spatially localized wave packet. We find that these distributions are squeezed in the polarization plane ($y\text{−}z$) as well as along the laser propagation ($x$) direction. In a chosen gauge, the squeezing direction is controlled by the laser vector potential $\mathbf{A}$ and the electron initial momentum. For the case when the electron initial momentum is zero the squeezing occurs directly along the direction of $\mathbf{A}$. We obtain analytical expressions within linear momentum approximation that explain the squeezing features very well by defining a squeezing vector and rotational angle of the squeezed momentum distribution. We analyze the symmetric properties of the momentum distributions viewed in different momentum planes and discuss the effects of different helicity of circular laser polarizations and the direction of the spin quantization. An unexpected feature of bending of momentum distribution is found for very intense laser fields. We extend our investigation to the momentum distribution of two spatially separated wave packets, particularly the orientations of two crossing distributions. It is found that the absolute phase of the initial laser field affects the orientation of the electron momentum distributions while quantum superposition of two states with the same spin gives interference in the momentum distributions that depends on the quantum phase of the electron.

An all-optical method to retrieve the temporal intensity profile of an extreme ultraviolet (XUV) attosecond pulse is proposed based on XUV-assisted high-order harmonic generation (HHG) by an intense infrared (IR) pulse. For a harmonic located on the XUV-induced high-energy plateau (beyond the IR HHG plateau), the measured harmonic yield as a function of the time delay between the XUV and IR pulses is shown to accurately map the temporal intensity profile of the XUV pulse. Single-color and two-color orthogonal, linearly polarized IR pulses are used to demonstrate the method.

We study synchronization of a room-temperature optomechanical system formed by two resonators coupled via radiation pressure to the same driven optical cavity mode. By using stochastic Langevin equations and effective slowly varying amplitude equations, we explore the long-time dynamics of the system. We see that thermal noise can induce significant non-Gaussian dynamical properties, including the coexistence of multistable synchronized limit cycles and phase diffusion. Synchronization in this optomechanical system is very robust with respect to thermal noise: in fact, even though each oscillator phase progressively diffuses over the whole limit cycle, its phase difference is locked, and such a phase correlation remains strong in the presence of thermal noise.

We analyze the dephasing dynamics of an impurity coupled to an anharmonic environment. We show that a strong anharmonicity produces two different effects depending on the environment temperature: For high temperatures, the system suffers a strong dephasing, while for low temperatures there is a strong information backflow (as measured by the Breuer-Laine-Piilo non-Markovianity measure). Both dephasing and backflow are particularly significant when the anharmonic potential allows environment states very close to the dissociation limit. In contrast, the information backflow is suppressed when assuming the environment to be Gaussian. In this regard, we find that the Gaussian approximation is particularly poor at low temperatures.

Entanglement is an essential resource in many quantum information tasks and the entanglement witness is a widely used tool for its detection. In experiments the prepared state generally deviates from the target state due to some noise. Normally the white-noise model is applied to quantifying such a derivation and at the same time reveals the robustness of the witness. However, there may exist other kinds of noise, in which the coherent noise can dramatically “rotate” the prepared state. In this way, coherent noise is likely to lead to a failure of detection, even though the underlying state is actually entangled. In this work, we propose an efficient entanglement detection protocol for $N$-partite Greenberger-Horne-Zeilinger (GHZ)–like states. The protocol can eliminate the effect of coherent noise and at the same time feedback the corresponding noise parameters, which are beneficial to further improvements of the experiment system. In particular, we consider two experiment-relevant coherent noise models; one is from the unconscious phase accumulation on $N$ qubits, and the other is from the rotation on the control qubit. The protocol effectively realizes a family of entanglement witnesses by postprocessing the measurement results from $N+2$ local measurement settings, which adds only one more setting than the original witness specialized for the GHZ state. Moreover, by considering the trade-off between the detection efficiency and the white-noise robustness, we further reduce the number of local measurements to three without altering the performance under the coherent noise. Our protocol can enhance entanglement detection under coherent noises and act as a benchmark for state-of-the-art quantum devices.

We report measurements of shifts in the frequencies of $1{}^{3}S_1\to 2{}^{3}P_J$ and $2{}^{3}P_J\to n{}^{3}D/n{}^{3}S$ transitions optically driven in positronium atoms while they are inside the open volumes of MgO smoke powder. The observed intervals are larger than the corresponding vacuum excitations, but, surprisingly, the transitions to Rydberg states are less strongly affected, and the energy shifts exhibit no dependence on the principal quantum number $n$ of the final state. We attribute these shifts to resonant interactions between Ps atoms and MgO surfaces, mediated via spectrally overlapping MgO ultra violet (UV) photo-luminescent absorption bands. Since many insulating materials suitable for Ps confinement exhibit similar broadband UV absorption characteristics, the observed phenomena have implications for optical diagnostics and laser cooling schemes of relevance to studies of high-density Ps ensembles in insulating cavities, including the production of a Ps Bose-Einstein condensate.

The knowledge and thus characterization of the temporal modes of quantum light fields is important in many areas of quantum physics ranging from experimental setup diagnosis to fundamental-physics investigations. Recent results showed how the autocorrelation function computed from continuous-wave homodyne measurements can be a powerful way to access the temporal mode structure. Here, we push forward this method by providing a deeper understanding and by showing how to extract the amplitude and phase of the temporal mode function with reduced experimental resources. Moreover, a quantitative analysis allows us to identify a regime of parameters where the method provides a trustworthy reconstruction, which we illustrate experimentally.

Using criteria based on superselection rules that apply to massive bosons but not to photons, we analyze the quantum correlations between the two condensate modes of a Bose-Einstein condensate interferometer [described in Egorov et al., Phys. Rev. A 84, 021605 (2011)]. We show that the observation of interference fringes can only be explained by Einstein-Podolsky-Rosen steering correlations between the two modes. Moreover, the size of the two-mode correlation linked to the fringe visibility will place a lower bound on the mean number of atoms of the (pure) two-mode steerable state. In order to determine the two-mode correlations, we develop a multimode theory describing the dynamics of the condensate atoms and the thermal fraction through the interferometer sequence, in agreement with the experimentally measured fringe visibility. We thus present experimental evidence for two-mode entangled states genuinely involving 40 000 ${}^{87}\mathrm{Rb}$ atoms, and for two-way steerability between two groups of 20 000 indistinguishable atoms.

The experimental realization of balanced gain and loss in a quantum system has been a long-standing goal in quantum mechanics since the introduction of the concept of $\mathcal{PT}$ symmetry and has only recently been achieved. In this paper we analyze balanced gain and loss in Gaussian multiwell potentials with either only gain or loss in each well. By means of symmetrization via matrix models we can construct asymmetric extended potentials with partially real or complex conjugate spectra. This will be demonstrated explicitly for double-well and triple-well systems. Such systems can be realized with Bose-Einstein condensates in optical trapping potentials in the presence of localized particle gain and loss. The usage of asymmetric potentials in the process is more versatile and is considered beneficial in real experimental implementations.

Highly resonant photonic structures, such as cavities and metasurfaces, can dramatically enhance the efficiency of nonlinear processes by utilizing strong optical field enhancement at the resonance. The latter, however, comes at the expense of the bandwidth. Here, we overcome such a tradeoff by utilizing time-varying resonant structures. Using harmonics generation as an example, we show that the amplitude and phase format of the excitation, as well as the time evolution of the resonator, can be optimized to yield the strongest nonlinear response. We find the conditions for an efficient synthesis of electromagnetic signals that surpass the cavity bandwidth, and discuss a potential experimental realization of this concept.

The violation of Leggett-Garg-type inequalities (LGtIs) is studied on a two-level atom, driven by an external field in the presence of a squeezed thermal reservoir. The violations are observed in the underdamped regime where the spontaneous transition rate is much smaller compared to the Rabi frequency. An increase in thermal effects is found to decrease the extent of violation as well as the time over which the violation lasts. With the increase in the value of the squeezing parameter the extent of violation of LGtIs is seen to reduce. The violation of LGtIs is favored by an increase in the driving frequency. Further, the interplay of the degree of violation and strength of the measurements is studied. It is found that the maximum violation occurs for ideal projective measurements.

We introduce a scheme for a self-testing quantum random-number generator and demonstrate it experimentally. Compared to the fully device-independent model, our scheme requires an extra natural assumption, namely, that the mean number of photons of the signal optical modes is bounded. The scheme is self-testing, as it allows the user to verify in real time the correct functioning of the setup, hence guaranteeing the continuous generation of certified random bits. The scheme is based on a prepare-and-measure setup, which we implement in a proof-of-principle experiment using only off-the-shelf optical components. The randomness generation rate is 1.25 Mbits/s, comparable to commercial solutions. Overall, we believe that this scheme achieves a promising trade-off between the required assumptions, ease of implementation and performance.

This paper addresses the problem of designing universal quantum circuits to transform $k$ uses of a $d$-dimensional unitary input operation into a unitary output operation in a probabilistic heralded manner. Three classes of protocols are considered, parallel circuits, where the input operations can be performed simultaneously, adaptive circuits, where sequential uses of the input operations are allowed, and general protocols, where the use of the input operations may be performed without a definite causal order. For these three classes, we develop a systematic semidefinite programming approach that finds a circuit which obtains the desired transformation with the maximal success probability. We then analyze in detail three particular transformations: unitary transposition, unitary complex conjugation, and unitary inversion. For unitary transposition and unitary inverse, we prove that for any fixed dimension $d$, adaptive circuits have an exponential improvement in terms of uses $k$ when compared to parallel ones. For unitary complex conjugation and unitary inversion we prove that if the number of uses $k$ is strictly smaller than $d-1$, the probability of success is necessarily zero. We also discuss the advantage of indefinite causal order protocols over causal ones and introduce the concept of delayed input-state quantum circuits.