We performed multiconfiguration Dirac-Hartree-Fock and relativistic configuration-interaction calculations for states of the $4d^{9}5s^2$ and $4d^{10}5s$ configurations in ${\mathrm{Pd}}^{-}$ and for states of the $5d^{9}6s^2$ and $5d^{10}6s$ configurations in ${\mathrm{Pt}}^{-}$. Valence-valence, core-valence, and core-core electron correlation effects are accounted for in the present studies. The calculated excited energy levels agree within 2.5% with the available experimental values for both ${\mathrm{Pd}}^{-}$ and ${\mathrm{Pt}}^{-}$ and we confirm that ${\mathrm{Pd}}^{-}$ only has one bound excited state. We report a more precise value for the radiative lifetime of $5d^{9}6s^2{}^2{D}_{3/2}$ in ${\mathrm{Pt}}^{-}$ ($67.0\pm 0.3$ ms), which agrees within the experimental range (50–200 ms), but the radiative lifetime of $5d^{10}6s{}^2{S}_{1/2}$ ($4.00\pm 0.16$ s) in ${\mathrm{Pt}}^{-}$ is 50% longer than the experimental measurement ($2.54\pm 0.10$ s). The radiative lifetimes for $4d^{9}5s^2{}^2{D}_{3/2,5/2}$ in ${\mathrm{Pd}}^{-}$ are also discussed in detail.

The total and differential cross sections of mutual neutralization in ${\mathrm{H}}^{+}+{\mathrm{H}}^{-}$ collisions are calculated ab initio and fully quantum mechanically for energies between 0.001 and 600 eV. Effects which have not previously been considered in studies on mutual neutralization (MN) for this system, such as inclusion of rotational couplings and autoionization, are investigated. Adiabatic potential curves corresponding to the relevant states of ${}^1\mathrm{\Sigma }_g^{+}$, ${}^1\mathrm{\Sigma }_u^{+}$, ${}^1\mathrm{\Pi }_g$ and ${}^1\mathrm{\Pi }_u$ symmetries as well as radial and rotational nonadiabatic couplings are computed ab initio. A quasidiabatic model is developed and applied in order to investigate the importance of higher excited states as well as the inclusion of autoionization. Molecular data for the lowest electronic resonant state in each symmetry are obtained by performing electron scattering calculations. It is shown that rotational couplings cause a significant increase of the total MN cross section while autoionization plays a minor role as a loss mechanism. Additionally, a differential cross section is obtained that is symmetric around $\theta ={90}^{\circ }$. This result is in disagreement with a previous theoretical calculation where it was found that the differential cross section is dominated by backwards scattering.

Retrieving atomic and molecular properties from entangling macroscopic measurements is an ambitious task. In particular, the phase difference between the adjacent attosecond bursts emitted from polar molecules is of interest and has been reconstructed from measured high-order harmonic generation (HHG) when molecules are exposed to an intense laser pulse. However, besides the ratio of intensities between the even and odd harmonic orders which can be measured directly, the suggested construction method requires nonmeasured parameters that can be supplied through theoretical calculations. The complexity or comprehensiveness of this theory clearly affects the accuracy of the retrieval procedure. In this study we propose a robust parameter-free method to retrieve the subtle molecular asymmetry, i.e., both the spectral phase difference and intensity ratio of two adjacent attosecond bursts, from purely experimental measurements of HHG. For this purpose, we first figure out that the phase difference is an intrinsic feature of polar molecules, almost independent of the external laser's parameters. After that, we design a detailed procedure to quantitatively extract these features via the time-frequency profile from the intensity and phase measurements of HHG emitted from partially oriented molecular samples. In this step, we develop the analytical formula to convert the phase difference and intensity ratio of a partially oriented sample to those of a single molecule. The accuracy and robustness of the methods are validated by numerical data.

We investigate postionization population transfer processes among the three low-lying electronic states—$\mathrm{X}{}^2{\mathrm{\Sigma }}_{\mathrm{g}}^{+}$, $\mathrm{A}{}^2{\mathrm{\Pi }}_{\mathrm{u}}$, and $\mathrm{B}{}^2{\mathrm{\Sigma }}_{\mathrm{u}}^{+}$—of ${\text{N}}_2^{+}$ created in an ultrashort-pulsed intense laser field using a sudden turn-on model by explicitly including the ionization timing effect throughout the entire laser pulse. By adopting the rovibronic model in which the electronic, vibrational, and rotational degrees of freedom of ${\text{N}}_2^{+}$ are included, we have revealed that the population inversion between the $\mathrm{X}{}^2{\mathrm{\Sigma }}_{\mathrm{g}}^{+}(v=0)$ state and $\mathrm{B}{}^2{\mathrm{\Sigma }}_{\mathrm{u}}^{+}(v=0)$ state can be realized in the specific rotational quantum number ranges in the R- and P branches when ionization occurs around the central peak of the temporal shape of the laser field intensity. Based on the quasistationary Floquet theory, we have examined the laser field intensity dependence of the final population in the upper Floquet state, corresponding to the $\mathrm{B}{}^2{\mathrm{\Sigma }}_{\mathrm{u}}^{+}(v=0)$ state of ${\text{N}}_2^{+}$, and revealed that the population transfer can be interpreted by the ionization at the ionization timing of $t=0$ as long as the laser field intensity is less than $4\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, while the ionization processes occurring in the entire range of the laser pulse need to be taken into account when the field intensity becomes larger.

The ground-state cooling of the Brillouin scattering optomechanical system is limited by defects in practical samples. In this paper, we present a compensatory cooling mechanism for a Brillouin scattering optomechanical system with exceptional points (EPs). The dual-EP system, described in this paper, can be induced by two defects with specific relative angles and is demonstrated to effectively compensate the limited cooling process. The system actively manipulates the coupling strength of not only the optical modes but also the Brillouin phonon modes. Our results provide tools to manipulate the optomechanical interaction in multimode systems and open possibilities for quantum state transfer and quantum interface protocols based on phonon cooling in quantum applications.

The theory of gauge-fixed Maxwell equations in linear isotropic dielectrics is developed using a generalization of the standard $R_{\xi }$ gauge-fixing term. In static space times, the theory can be quantized using the Gupta-Bleuler method, which is worked out explicitly for optical fibers either in flat space time or at a constant gravitational potential. This yields a consistent first-principles description of gravitational fiber-optic interferometry at the single-photon level within the framework of quantum field theory in curved space times.

Colliding bunches of high-energy electrons with intense laser pulses provides a basis for studying strong-field QED processes enabled by high values of quantum nonlinearity parameter $\chi$. Nevertheless, the signal deconvolution is intricate due to the probabilistic nature of the processes and shot-to-shot variation of the impact parameter, which disfavors the use of tight focusing. We propose a concept for distinguishing the signal of high-$\chi$ emissions that enables the use of optimal focusing to attain the highest $\chi \approx 5.25[\varepsilon /\left(\text{1}\text{GeV}\right)]{[P/\left(\text{1}\text{PW}\right)]}^{1/2}[(\text{1}\mu \text{m})/\lambda ]$ for a given electron energy $\varepsilon$, laser power $P$, and wavelength $\lambda$. Reaching such a $\chi$ with $f/2$ focusing requires more than 10 times higher power.

We examine the precision limits of Hong-Ou-Mandel (HOM) timing measurements, as well as precision limits applying to generalized two-photon measurements. As a special case we consider the use of two-photon measurements using photons with variable bandwidths and frequency correlations. When the photon bandwidths are not equal, maximizing the measurement precision involves a tradeoff between high interference visibility and strong frequency anticorrelations, with the optimal precision occurring when the photons share nonmaximal frequency anticorrelations. We show that a generalized measurement has precision limits that are qualitatively similar to those of the HOM measurement whenever the generalized measurement is insensitive to the net delay of both photons. By examining the performance of states with more general frequency distributions, our analysis allows for engineering of the joint spectral amplitude for use in realistic situations, in which both photons may not have ideal spectral properties.

Fourier transform spectroscopy with classical interferometry corresponds to the measurement of a single-photon intensity spectrum from the viewpoint of the particle nature of light. In contrast, the Fourier transform of two-photon quantum interference patterns provides the intensity spectrum of the two photons as a function of the sum- or difference frequency of the constituent photons. This unique feature of quantum interferometric spectroscopy offers a different type of spectral information from the classical measurement and may prove useful for nonlinear spectroscopy with two-photon emission. Here, we report the experimental demonstration of two-photon quantum interference of biphotons emitted via biexcitons in the semiconductor CuCl. Besides applying Fourier transform to quantum interference patterns, we reconstruct the intensity spectrum of the biexciton luminescence in the two-photon sum- or difference frequency. We discuss the connection between the reconstructed spectra and exciton states in CuCl as well as the capability of quantum interferometry in solid-state spectroscopy.

Quantum optics with giant emitters has shown a new route for the observation and manipulation of non-Markovian properties in waveguide QED. In this paper we extend the theory of giant atoms, hitherto restricted to the perturbative light-matter regime, to deal with the ultrastrong-coupling regime. Using static and dynamical polaron methods, we address the low-energy subspace of a giant atom coupled to an Ohmic waveguide beyond the standard rotating-wave approximation. We analyze the equilibrium properties of the system by computing the atomic frequency renormalization as a function of the coupling characterizing the localization-delocalization quantum phase transition for a giant atom. We show that virtual photons dressing the ground state are nonexponentially localized around the contact points but decay as a power law. The dynamics of an initially excited giant atom is studied, pointing out the effects of ultrastrong coupling on the Lamb shift and the spontaneous emission decay rate. Finally, we comment on the existence of the so-called oscillating bound states beyond the rotating-wave approximation.

We present efficient methods to interpolate data with a quantum computer to complement uploading techniques and quantum postprocessing. The quantum algorithms make use of the efficient quantum Fourier transform (QFT) as well as classical signal and imaging processing techniques, and may facilitate quantum advantages for some relevant families of data. We showcase a QFT interpolation method, a quantum cosine transform (QCT) interpolation geared towards natural data, and we improve upon them by utilizing a quantum circuit's capabilities of processing data in superposition. A circuit for the QCT is presented. We illustrate the methods on probability distributions and quantum encoded images, and discuss the precision of the resulting interpolations.

Surface codes are generally studied based on the assumption that each of the qubits that make up the surface code lattice suffers noise that is independent and identically distributed (i.i.d.). However, real benchmarks of the individual relaxation $\left(T_1\right)$ and dephasing $\left(T_2\right)$ times of the constituent qubits of state-of-the-art quantum processors have recently shown that the decoherence effects suffered by each particular qubit actually vary in intensity. In consequence, in this paper we introduce the independent nonidentically distributed (i.n.i.d.) noise model, a decoherence model that accounts for the nonuniform behavior of the decoherence parameters of qubits. Additionally, we use the i.n.i.d. model to study how it affects the performance of a specific family of quantum error correction codes known as planar codes. For this purpose we employ data from four state-of-the-art superconducting processors: ibmq_brooklyn, ibm_washington, Zuchongzhi, and Rigetti Aspen-M-1. Our results show that the i.i.d. noise assumption overestimates the performance of surface codes, which can suffer up to $95\%$ performance decrements in terms of the code pseudothreshold when they are subjected to the i.n.i.d. noise model. Furthermore, we consider and describe two methods which enhance the performance of planar codes under i.n.i.d. noise. The first method involves a so-called reweighting process of the conventional minimum weight perfect matching (MWPM) decoder, while the second one exploits the relationship that exists between code performance and qubit arrangement in the surface code lattice. The optimum qubit configuration derived through the combination of the previous two methods can yield planar code pseudothreshold values that are up to $650\%$ higher than for the traditional MWPM decoder under i.n.i.d. noise.

Quantum algorithms can outperform their classical counterparts in various tasks, the most prominent example being Shor's algorithm for efficient prime factorization on a quantum computer. It is clear that one of the reasons for the speedup is the superposition principle of quantum mechanics, which allows a quantum processor to be in a superposition of different states at the same time. While such a superposition can lead to entanglement across different qubits of the processors, there also exist quantum algorithms that outperform classical ones using superpositions of individual qubits without entangling them. As an example, the Bernstein-Vazirani algorithm allows one to determine a bit string encoded into an oracle. While the classical version of the algorithm requires multiple calls of the oracle to learn the bit string, a single query of the oracle is enough in the quantum case. In this article, we analyze in detail the quantum resources in the Bernstein-Vazirani algorithm. For this, we introduce and study its probabilistic version, where the goal is to guess the bit string after a single call of the oracle. We show that in the absence of entanglement, the performance of the algorithm is directly related to the amount of quantum coherence in the initial state. We further demonstrate that a large amount of entanglement in the initial state prevents the algorithm from achieving optimal performance. We also apply our methods to quantum computation with mixed states, proving that pseudopure states achieve optimal performance for a given purity in the Bernstein-Vazirani algorithm. We further investigate quantum resources in the one clean qubit model, showing that the model can exhibit speedup over any known classical algorithm even with an arbitrarily little amount of multipartite entanglement, general quantum correlations, and coherence.

We report frequency-comb-based measurements of Ca Rydberg energy levels. Counterpropagating laser beams at 390 and 423 nm excite Ca atoms from the $4s^2{}^1{S}_0$ ground state to $4sns{}^1{S}_0$ Rydberg levels with $n$ ranging from 40 to 110. Near-resonant two-photon two-color excitation of atoms in a thermal beam makes it possible to eliminate the first-order Doppler shift. The resulting line shapes are symmetric and Gaussian. We verify laser metrology and absolute accuracy by reproducing measurements of well-known transitions in Cs, close to the fundamental wavelengths of our frequency-doubled Ti:sapphire lasers. From the measured transition energies we derive the ionization potential of Ca, $E_{\mathrm{IP}}=1478154283.42\pm 0.08\text{(statistical)}\pm 0.07\text{(systematic)}$ MHz, improving the previous determinations by a factor of 11.

The two-center wave-packet convergent close-coupling method is generalized to multiply charged ion collisions with helium and applied to the ${\mathrm{He}}^{2+}\text{−}\mathrm{He}$ scattering problem. The approach is applicable in a wide range of collision energies including low and intermediate energies, where coupling between various reaction channels and electron exchange between the fragments in the rearrangement channel are important. The target structure is treated using the configuration-interaction method within the frozen-core approximation, where one of the electrons of the target is assumed to stay in the ground state throughout the collision. This accounts for the indistinguishability and the correlations between the electrons of the target. We also use a simpler alternative method that is based on an effective one-electron target description that neglects the electron-electron correlations. In both methods, the continuum of the target atom and the hydrogenlike atom formed after electron capture by the projectile is discretized using the wave-packet approach. We present cross sections for total and state-selective electron capture, excitation, and single ionization of the target. The results are provided for the incident energies from 10 keV/u to 5 MeV/u, where one-electron processes are expected to dominate. Particular attention is focused on the intermediate-energy region where substantial deviations between various theoretical results are found. However, overall, the present results are in good agreement with experimental data and other calculations, where available. We demonstrate that both effective single-electron and two-electron methods can provide a realistic picture of all single-electron scattering processes taking place in ${\mathrm{He}}^{2+}\text{−}\mathrm{He}$ collisions in terms of the total cross sections. This creates a reliable platform for modeling differential scattering and ionization in this four-body system.

We investigate the attosecond transient absorption spectrum (ATAS) of graphene by numerically solving four-band density-matrix equations, which demonstrates apparent “fishbone” resonance structures. To gain insight into these interesting structures, we exploit a simplified model that considers only the electrons of $\mathrm{\Gamma }$ and $\mathrm{M}$ points in the Brillouin zone. With the help of this model, we can analytically express the ATAS as the sum of zeroth- and first-order Bessel functions in the variables of the strength and frequency of the infrared pump field as well as the effective mass of electrons at the $\mathrm{\Gamma }$ and $\mathrm{M}$ points. Lorentzian and Fano line shapes in the absorption spectrum are addressed. The fishbone structure consists of a periodic V-shaped structure that can be explained by first-order Bessel functions, and its tilt angle is solely determined by the frequency of the pump laser. The periodicity of the V-shaped structure in the fishbone originates from the periodic dependence of the Lorentzian and Fano line shapes of the absorption spectrum on the time delay between the pump and probe lasers. Compared with the numerical results, our analytical theory can qualitatively or even quantitatively predict the zeroth- and first-order fringes in the fishbone structures of the ATAS.

We investigate the effects of Efimov states on the postquench dynamics of a system of three identical bosons with contact interactions in a spherically symmetric three-dimensional harmonic trap. The quench we consider is in the $s$-wave contact interaction, and we focus on quenches from the noninteracting to strongly interacting regimes and vice versa. The calculations use the hyperspherical solutions of the three-body problem and enable us to evaluate the semianalytical results of the Ramsey and particle separation, postquench. In the case where the interactions are quenched from the noninteracting to strongly interacting regime we find convergent aperiodic solutions for both the Ramsey signal and the particle separation. In contrast, for quenches from the strongly interacting regime to the noninteracting regime both the Ramsey signal and particle separation are periodic functions. However, in this case we find that the solutions for the particle separation diverge, indicating that in such a system large oscillations may be observable.

We continue to study frequency-dependent complex bulk viscosities of one-dimensional Bose and Fermi gases with contact interactions, which exhibit the weak-strong duality according to our recent work. Here we show that they are contributed to by Drude peaks divergent at zero frequency as typical for transport coefficients of quantum integrable systems in one dimension. In particular, their Drude weights are evaluated based on the Kubo formula in the high-temperature limit at arbitrary coupling as well as in the weak-coupling and strong-coupling limits at arbitrary temperature, where systematic expansions in terms of small parameters are available. In all three limits, the Drude peaks are found at higher orders compared to the finite regular parts.

We consider two bottlenecks in quantum computing: limited memory size and noise caused by heat dissipation. Trying to optimize both, we investigate “on-the-go erasure” of quantum registers that are no longer needed for a given algorithm: freeing up auxiliary qubits as they stop being useful would facilitate the parallelization of computations. We study the minimal thermodynamic cost of erasure in these scenarios, applying results on the Landauer erasure of entangled quantum registers. For the class of algorithms solving the Abelian hidden subgroup problem, we find optimal on-the-go erasure protocols. We conclude that there is a trade-off: if we have enough partial information about a problem to build efficient on-the-go erasure, we can use it to instead simplify the algorithm, so that fewer qubits are needed to run the computation in the first place. We provide explicit protocols for these two approaches.