Interesting problems in quantum computation take the form of finding low-energy states of (pseudo)spin systems with engineered Hamiltonians that encode the problem data. Motivated by the practical possibility of producing very low-temperature spin systems, we propose and exemplify the possibility to compute by coupling the computational spins to a quantum coherent bath that serves as a heat sink. The quantum tunneling effect provides additional cooling channels to accelerate the cooling process. We demonstrate both analytically and numerically that this strategy can achieve a quantum advantage in the unstructured search problem.

We present a precise fully relativistic numerical solution of the two-center Coulomb problem. The special case of unit nuclear charges is relevant for the accurate description of the $\mathrm{H}{{}_2}^{+}$ molecular ion and its isotopologues, systems that are an active experimental topic. The computation utilizes the 2-spinor minmax approach and the finite-element method. The computed total energies have estimated fractional uncertainties of a few times ${10}^{-20}$ for unit charges and a bond length of 2 atomic units. The fractional uncertainty of the purely relativistic contribution is $1\times {10}^{-17}$. The result is relevant for future precision experiments, whereas at present the uncertainties arising from the quantum electrodynamic treatment of the rovibrational transition frequencies are dominant.

We study the influence of electron screening on the Bohr-Weisskopf (BW) effect in many-electron atoms. The BW effect gives the finite-nucleus magnetization contribution to the hyperfine structure. Relativistic atomic many-body calculations are performed for $s$ and $p_{1/2}$ states of several systems of interest for studies of atomic parity violation and time-reversal-violating electric dipole moments—Rb, Cs, Fr, ${\mathrm{Ba}}^{+}, {\mathrm{Ra}}^{+}$, and Tl. For $s$ states, electron screening effects are small, and the relative BW correction for hydrogenlike ions and neutral atoms is approximately the same. We relate the ground-state BW effect in H-like ions, which may be cleanly extracted from experiments, to the BW effect in $s$ and $p_{1/2}$ states of neutral and near neutral atoms through an electronic screening factor. This allows the BW effect extracted from measurements with H-like ions to be used, with screening factors, in atomic calculations without recourse to modeled nuclear structure input. It opens the way for unprecedented accuracy in accounting for the BW effect in heavy atoms. The efficacy of this approach is demonstrated using available experimental data for H-like and neutral ${}^{203}\mathrm{Tl}$ and ${}^{205}\mathrm{Tl}$.

We employ the recently developed multiphoton $R$-matrix method for molecular above-threshold photoionization to obtain second-order ionization amplitudes that govern the interference in RABITT experiments. This allows us to extract RABITT time delays that are in better agreement with nonperturbative time-dependent simulations of this process than the typically used combination of first-order (Wigner) delays and asymptotic corrections. We calculate molecular-frame as well as orientation-averaged RABITT delays for ${\mathrm{H}}_2, {\mathrm{N}}_2, {\mathrm{CO}}_2, {\mathrm{H}}_2\mathrm{O}$, and ${\mathrm{N}}_2\mathrm{O}$ and analyze the origin of various structures in the time delays including the effects of partial-wave interference, shape resonances, and orientation averaging. Time delays for B and C states of ${\mathrm{CO}}_2{}^{+}$ are strongly affected by absorption of the second (IR) photon in the ion. This effect corresponds to an additional contribution ${\tau }_{\text{coupl}}$ to the asymptotic approximation for the RABITT delays $\tau \approx {\tau }_{\text{mol}}+{\tau }_{cc}+{\tau }_{\text{coupl}}$. Applicability of the asymptotic theory depends on the target and IR photon energy but typically starts at approximately 30–35 eV of XUV photon energy.

Extremely high charge states of atoms and molecules can be created when they are irradiated by intense x-ray pulses. At certain x-ray photon energies, electron ejection from atoms can be drastically enhanced by transient resonances created during the sequential ionization process. Here we report on the observation of such resonance effects in a molecule, ${\mathrm{CH}}_3\mathrm{I}$, and show the photon-energy-dependent shift of resonance-induced structures in ion charge state distributions. By comparing the ion charge state distribution of ${\mathrm{CH}}_3\mathrm{I}$ with that from ionization of atomic xenon, molecule-specific features are observed, which can be attributed to ultrafast intramolecular charge rearrangement. In addition, we experimentally demonstrate that the charge-rearrangement-enhanced x-ray ionization of molecules, previously found with hard x rays, also plays a role in the soft x-ray regime.

The surge of interest in nonperturbative high-harmonic generation in solids has been driven by the appeal of compact solid-state extreme ultraviolet sources and the prospect of untangling the material properties through high-harmonic generation response to strong fields. The traditional assumption is that the brighter, lower-order harmonics are purely perturbative in nature. However, the border between the perturbative and nonperturbative regimes often remains unclear. Here, we show that third-harmonic generation (THG) using 800-nm, 40-fs pulses displays a nonperturbative response in a wide-band-gap insulator MgO. Furthermore, we show that with the introduction of dopants, the nonperturbative THG reverts to the perturbative behavior. We attribute this to the blocking of intraband oscillations and the increased linear absorption pathways introduced by the dopant energy levels.

Dipole interaction between neighbor systems is of importance in the behavior of atoms and molecules as it produces distortion in the electronic structure of the system. In this work, we study the dipole processes in a $\mathrm{He}{\mathrm{H}}^{+}$ molecule induced by an initially excited lithium atom placed at an $R_0$ distance from the center of mass of the molecule. The electronic and nuclear degrees of freedom are treated by the electron-nuclear dynamics approach as it allows a time-dependent description of the electronic and nuclear dynamics. The energy transferred from the neighbor excited lithium atom to the $\mathrm{He}{\mathrm{H}}^{+}$ molecule is distributed into several channels depending on the initial vibrational state of the $\mathrm{He}{\mathrm{H}}^{+}$ and initial $R_0$ separation. We find that several processes are induced by the dipole interaction. Among these, we find that the charge-transfer channel from the Li onto the ionic molecule $\mathrm{He}{\mathrm{H}}^{+}$ is the dominant outcome. Also, we find that a virtual photon dissociation process takes place via a dipole interaction that induces nuclear motion of the molecule through an electronic relaxation of the initial lithium $2p$ electron to the $2s$ state or to the $1s$ state of the He or H atom of the neighbor system, as well as vibrational intermolecular energy transfer. We report dissociation of the $\mathrm{He}{\mathrm{H}}^{+}$ molecule followed by chemical rearrangement leading to the formation of LiH and LiHe molecules and their respective charged ions. We find that deexcitation occurs on femtosecond while the molecular dissociation on picosecond time scales. Consequently, the dipole interaction between neighbors induces a richer dynamics.

Phase conjugation with stimulated Brillouin scattering can be used to correct wave-front aberrations in high-power laser systems. Here, we consider such a process with cylindrical vector lasers beams. By using a vectorial approach and a modal decomposition, we obtain a differential matrix equation that enables the calculation of the structure of Stokes vector eigenmodes and their Brillouin gain. The emphasis is put on the mode with the largest gain, which contributes the most to the Stokes beam. We show that the phase conjugation of orbital angular momentum in cylindrical vector beams is prevented from happening almost everywhere on the higher-order Poincaré sphere. This phenomenon is traced to the tendency of the Stokes beam to acquire a polarization structure similar to the incident pump beam, which constrains the possible combinations of topological charges of the two orbital angular momentum modes making up the Stokes cylindrical vector beams. However, near the poles of the higher-order Poincaré sphere, the Stokes mode with the largest gain acquires an additional helical phase factor that produces the conjugated topological charge for the stronger circularly polarized component without affecting the polarization structure. Experimental results are found to agree with our theoretical predictions.

We address the behavior of squeezing in two movable mirrors coupled to a pumped correlated emission laser inside a doubly resonant optomechanical driven cavity in the presence of biased noise fluctuations. Aiming at generating controllable and robust mechanical squeezing that can be utilized in making the quantum features of radiation in the cavity accessible for application, we explore mechanical squeezing that can be induced as a result of the transfer of coherent superposition. We found that a coupled mechanical oscillator mode exhibits squeezing in the good cavity limit and adiabatic regime. It is also shown that the degree of mechanical squeezing is robust for large amplitudes of atomic pumping until maximum squeezing is achieved but weakens afterwards. The squeezing turns out to be powerful mainly for strong atom-field coupling, large atomic injection rates, and intense biased noise fluctuations. In light of the observed possibility of controlling the realizable degree of squeezing, we hope and expect that the considered system could be employed in applications such as quantum metrology.

Thermal radiation played a pivotal role in the preliminary development of quantum physics where it helped resolve the apparent incongruity of the ultraviolet catastrophe. In contemporary physics, thermal state generation and manipulation finds new application in fields such as quantum imaging and quantum illumination and as a practical realization of Maxwell's demon. These applications often go hand in hand with photon subtraction operations which probabilistically amplify the mean photon number (MPN) of thermal light as a result of its super-Poissonian photon statistics. In this article, we introduce an operation for thermal states of light based on a generalized photon subtraction scheme. Displaced-photon subtraction (DPS) makes use of coherent state displacement followed by a subsequent anti-displacement in combination with single-photon detection to probe the MPN of a thermal state. We find regimes in which the output of a successful DPS is amplified, unchanged, or attenuated relative to the unconditioned output state. The regime of operation is controlled via the magnitude of the coherent displacement. A theoretical description of generalized photon subtraction of a displaced thermal state is derived via a two-mode moment-generating function (MGF) and used to describe generalized DPS. We perform a proof of principle experimental implementation of DPS for the case of a balanced beam splitter for which results demonstrate good agreement with the model.

Optimal control theory, also known as Pontryagin's maximum principle, is applied to the quantum parameter estimation in the presence of decoherence. An efficient procedure is devised to compute the gradient of quantum Fisher information with respect to the control parameters and is used to construct the optimal control protocol. The proposed procedure keeps the control problem in the time-invariant form so that both first-order and second-order optimality conditions derived from Pontryagin's maximum principle apply; the second-order condition turns out to be crucial when the optimal control contains singular arcs. Concretely we look for the optimal control that maximizes quantum Fisher information for the “twist and turn” problem. We find that the optimal control is singular without dissipation but can become unbounded once the quantum decoherence is introduced. An amplitude constraint is needed to guarantee a bounded solution. With quantum decoherence, the maximum quantum Fisher information happens at a finite time due to the decoherence, and the asymptotic value depends on the specific decoherence channel and the control of consideration.

Many photonic quantum information tasks employ single photons and linear transformations to create and manipulate quantum states of light to process information. Integrated optical systems have become useful platforms to perform these tasks. New technologies introduce new possible processes available for multimode quantum operations. Here we explore one such setup, motivated by current experiments, which is a nonlinear-waveguide array system operating in a regime of nondegenerate-frequency down-converted photons. This allows one photon to act as a herald for the other photon, which is created in a superposition of the individual waveguide channels. We demonstrate this setup's ability to generate highly nonclassical states, such as $N$-photon Fock states and NOON states.

High-bandwidth, fiber-based optical cavities are a promising building block for future quantum networks. They are used to resonantly couple stationary qubits such as single or multiple atoms with photons routing quantum information into a fiber network at high rates. In high-bandwidth cavities, standard fluorescence imaging on the atom-cavity resonance line for controlling atom positions is impaired since the Purcell effect strongly suppresses all-directional fluorescence. Here, we restore imaging of ${}^{87}\mathrm{Rb}$ atoms strongly coupled to such a fiber Fabry-Pérot cavity by detecting the repumper fluorescence which is generated by continuous and three-dimensional Raman sideband cooling. We have carried out a detailed spectroscopic investigation of the repumper-induced differential light shifts affecting the Raman resonance, dependent on intensity and detuning. Our analysis identifies a compromise regime between imaging signal-to-noise ratio and survival rate, where physical insight into the role of dipole-force fluctuations in the heating dynamics of trapped atoms is gained.

We study the dynamics of Kuznetsov-Ma solitons (KMSs) in the framework of vector nonlinear Schrödinger (Manakov) equations. An exact multiparameter family of solutions for such KMSs is derived. This family of solutions includes the known results as well as the previously unknown solutions in the form of nondegenerate KMSs. We present the existence diagram of such KMSs that follows from the exact solutions. These nondegenerate KMSs are formed by nonlinear superposition of two fundamental KMSs that have the same propagation period but different eigenvalues. We present the amplitude profiles of solutions, their exact physical spectra, and their link to ordinary vector solitons and offer easy ways for their excitation using numerical simulations.

We present the exact theory of quantum engines whose working medium is a network of driven oscillators performing an arbitrary cyclic process while coupled to thermal and nonthermal reservoirs. We show that when coupled to a single reservoir work cannot be extracted unless there is population inversion, and prove that the ratio between the heat flowing out and into the working medium cannot be arbitrarily small, satisfying a form of Clausius inequality. We use such identity to prove that the efficiency of linear quantum engines satisfies a generalized bound, which coincides with the Carnot limit for thermal reservoirs. The previous results enable us to estimate the cost of preparing nonthermal reservoirs, which, if available, could be used to violate the Carnot limit.

The development of efficient algorithms that generate robust quantum controls is crucial for the realization of quantum technologies. The commonly used gradient-based optimization algorithms are limited by their sensitivity to the initial guess, which affects their performance. Here we propose combining the gradient method with the simulated annealing technique to formulate a hybrid algorithm. Our numerical analysis confirms its superior convergence rate. Using the hybrid algorithm, we generate spin-selective $\pi$ pulses and employ them for experimental measurement of local noise spectra in a three-qubit nuclear magnetic resonance system. Moreover, here we describe a general method to construct noise-resilient quantum controls by incorporating noisy fields within the optimization routine of the hybrid algorithm. Upon experimental comparison with similar sequences obtained from standard algorithms, we find remarkable robustness of the hybrid sequences against dephasing errors.

We present an alternative protocol allowing for the preparation of critical states that instead of suffering from the critical slowing down benefits from the critical speeding up. Paradoxically, we prepare these states by going away from the critical point which allows for the speed-up. We apply the protocol to the paradigmatic quantum Rabi model and its classical oscillator limit as well as the Lipkin-Meshkov-Glick model. Subsequently, we discuss the application of the adiabatic speed-up protocol in quantum metrology and compare its performance with critical quantum metrology. We show that critical quantum metrology with the Lipkin-Meshkov-Glick model cannot even overcome the standard quantum limit, and we argue that, even though critical metrology protocols can overcome it in some cases, critical metrology is a suboptimal metrological strategy. Finally, we conclude that systems exhibiting a phase transition are indeed interesting from the viewpoint of quantum technologies; however, it may not be the critical point that should attract the most attention.

We study the Casimir-Polder potential of a multilevel alkali-metal atom near an optical nanofiber. We calculate the mean potential of the atom in a fine-structure state. We perform numerical calculations for the Casimir-Polder potentials of the ground state and a few low-lying excited states of a rubidium atom. We show that, unlike the potential of the ground state, which is negative and attractive, the potential of a low-lying excited state may take positive values, oscillate around the zero value with a decaying amplitude, and become repulsive in some regions of atom-to-surface distances. We observe that, for a nanofiber with a radius of 200 nm, the potential for the state $8S_{1/2}$ of a rubidium atom achieves a positive peak value of about $17\mu \mathrm{K}$ at a distance of about 150 nm from the fiber surface and becomes strongly repulsive in the region of distances from 150 to 400 nm. We also calculate the nanofiber-induced shifts of the transition frequencies of the atomic rubidium $D_2$ and $D_1$ lines. We find that the shifts are negative in the region of short distances, become positive, and oscillate around the zero value with a decaying amplitude in the region of large distances.

The electronic stopping power of a low-energy proton in a crystalline TiN film is investigated by computer simulation based on the time-dependent density-functional theory. The calculated results show that the electronic stopping power of the small impact parameter coincides with the experimental data. By evaluating the channeling electronic density and the stopping force in different channels, the direction dependence of the electronic stopping power is explored and revealed. Along the $\langle 110\rangle$ channel, the electronic stopping power of the proton is lower than that of the $\langle 100\rangle$ and $\langle 111\rangle$ channels; this trend is mainly attributed to the discrepancy in the channeling electronic density. In addition, our research gives evidence that the threshold velocity $v_{th}=0.1294\text{a.u.}$ is closely related to the indirect band gap. That result suggests that only the valence band's electrons across the band gap can contribute to the electronic energy dissipation; therefore, we discuss that the defect state induced in the gap by the passing ion serves like an elevator ferrying the valence-band electrons across the indirect band gap. Last, we investigate the charge accumulation and depletion as an energy-loss model by intuitive charge-density difference. Furthermore, the intensity of the chemical bond between the projectile and the host atoms around the channel is indirectly investigated and analyzed by obtaining the amplitude of the electron localization function. The shortening and elongation of the chemical bond are deemed a mechanism of the electronic energy loss in the process of the ion-solid interaction, which can contribute to the electronic stopping power. These results supply reference data for the application of the excellent material TiN in ion-beam irradiation.