We propose how to achieve significantly enhanced quantum refrigeration and entanglement by coupling a pumped auxiliary cavity to an optomechanical cavity. We obtain both analytical and numerical results and find optimal-refrigeration and -entanglement conditions under the auxiliary-cavity-assisted (ACA) mechanism. Our method leads to a significant amplification in the net refrigeration rate and reveals that the ACA entanglement has a much stronger noise robustness in comparison with the unassisted case. By appropriately designing the ACA mechanism, an effective mechanical susceptibility can be well adjusted, and a genuine tripartite entanglement of cooling-cavity photons, auxiliary-cavity photons, and phonons can be generated. Specifically, we show that both optomechanical refrigeration and entanglement can be greatly enhanced for the blue-detuned driving of the auxiliary cavity but suppressed for the red-detuned case. Our work paves a way towards further quantum control of macroscopic mechanical systems and the enhancement and protection of fragile quantum resources.

Quantum many-body systems that support high-order quantum phase transitions are quite rare. Here we consider an important generalization of the Dicke model in which an ensemble of multilevel atoms, instead of two-level atoms as in the conventional Dicke model, interact with a single photonic mode, and show that this generalized Dicke model can become multicritical. For a subclass of experimentally realizable schemes, multicritical conditions of arbitrary order can be expressed analytically in compact forms. As such, experiments can be readily designed to achieve quantum phase transition of desired order. We also calculate the atom-photon entanglement entropy for both critical and noncritical cases and find that the order of the criticality strongly affects the critical entanglement entropy: higher order yields stronger entanglement. Our work provides deep insight into quantum phase transitions and multicriticality.

We analyze decoherence and objectivization processes in spin-spin models for arbitrary spins without interactions between different spins in the environment. We first derive the most general analytic form of the decoherence factor in the measurement limit, where the interaction Hamiltonian dominates the rest. We then analyze thermal environments and derive exact, analytic formulas for both the decoherence factor and the state fidelity of postinteraction environment states, conditioned on different states of the system. This allows us to analyze the objectivization process of the state of the central spin during the interaction. We do so using so-called spectrum broadcast structures (SBS), which are specific multipartite quantum states encoding a certain operation notion of objectivity. We analyze analytically (for short times) and numerically how higher spin influences the efficiency of decoherence and objectivization processes. As expected, the higher the spin of the environment, the more efficient decoherence and objectivization become. This work is a generalization of previous studies, limited to spin-$1/2$ systems only, and we hope will be useful in future objectivity experiments.

Network Bell experiments reveal a form of nonlocality conceptually different from standard Bell nonlocality. Standard multiparty Bell experiments involve a single source shared by a set of observers. In contrast network Bell experiments feature multiple independent sources and each of them may distribute physical systems to a set of observers who perform randomly chosen measurements. The $n$-locality scenario in star-network configuration is an instance of network Bell experiment. Such a scenario involves $n$ number of edge observers (Alices) a central observer (Bob) and $n$ number of independent sources having no prior correlation. Each Alice shares an independent state with the central observer Bob. Usually in network Bell experiments one considers that each party measures only two observables. In this work we propose a nontrivial generalization of $n$-locality scenario in star-network configuration where each Alice performs some integer $m$ number of binary-outcome measurements and the central party Bob performs $2^{m-1}$ measurements. We derive a family of generalized $n$-locality inequalities for any arbitrary $m$. Using an elegant sum-of-squares approach we derive the optimal quantum violation of the aforementioned inequalities when each and every Alice measures $m$ number of mutually anticommuting observables. For $m=2$ and 3 one obtains the optimal quantum value with a two-qubit entangled state shared between each Alice and Bob. We further demonstrate that the optimal quantum violation of $n$-locality inequality for any arbitrary $m$ can be obtained when every Alice shares $\lfloor m/2\rfloor$ copies of two-qubit maximally entangled state with the central party Bob. We also argue that for $m>3$ a single copy of a two-qubit-entangled state may not be enough to exhibit the violation of $n$-locality inequality but multiple copies of it can activate the quantum violation. We discuss the implications of our study and raise some open questions.

Quantum teleportation (QT) lies at the heart of modern day quantum information science and technology. Despite extensive studies over past two decades, obtaining the necessary and/or sufficient criterion for QT with continuous-variable (CV) resources, besides entanglement, still remains an open concern. In this backdrop, here we analyze the role of a purely quantum optical (QO) attribute, known as quadrature squeezing, in CV teleportation. We first provide an analytic proof that for Gaussian resources quadrature squeezing is necessary for QT. However, for non-Gaussian resources we show a clear distinction between the pure and the mix states. For the pure states, quadrature squeezing appears to be necessary for QT, in the sense that there is no QT without quadrature squeezing. However, in the case of mix states we observe otherwise, i.e., QT could be achieved even without quadrature squeezing. Our results present the exotic character of the QO attributes of the CV resources and necessitate a deeper search for the necessary and/or sufficient criterion for CV QT.

The fragmentation of carbon monoxide dimers induced by collisions with low-energy ${\mathrm{Ar}}^{9+}$ ions is investigated using the cold target recoil ion momentum spectroscopy technique. The presence of a neighbor molecule in the dimer serves here as a diagnostic tool to probe the lifetimes of the $\mathrm{C}{\mathrm{O}}^{2+}$ molecular dications resulting from the collision. The existence of metastable states with lifetimes ranging from 2 ps to 200 ns is clearly evidenced experimentally through a sequential three-body fragmentation of the dimer, whereas fast dissociation channels are observed in a so-called concerted three-body fragmentation process. The fast fragmentation process leads to a kinetic energy release distribution also observed in collisions with monomer CO targets. This is found in contradiction with the conclusions of a previous study attributing this fast process to the perturbation induced by the neighbor molecular ion.

An explicitly correlated Gaussian basis is used to calculate the energies and wave functions of one-dimensional few-electron systems in confinement potentials created by external potentials or coupling to light in cavity. The appearance and properties of electron density peaks as the function of the relative strength of the confinement and the Coulomb interaction are studied. It is shown that similar Wigner crystal-like structures can be formed by coupling electrons to light due to the dipole self-interaction term in the light-matter Hamiltonian, provided an additional extremely weak confining potential is present. The relation of these systems to Wigner crystals is discussed.

A simple and efficient method to create gap solitons is proposed in a spin-orbit-coupled spin-1 Bose-Einstein condensate. We find that a free expansion along the spin-orbit coupling dimension can generate two moving gap solitons, which are identified from a generalized massive Thirring model. The dynamics of gap solitons can be controlled by adjusting spin-orbit coupling parameters.

The resonant nonlinear refraction of $4\text{–}5\text{−}\mu \text{m}$ light in CO and $\mathrm{C}{\mathrm{O}}_2$ gas at a peak intensity of $15\mathrm{MW}/\mathrm{c}{\mathrm{m}}^2$ was demonstrated using time- and frequency-resolved measurements of self-focusing and self-defocusing. The nonlinearity of these molecular gases exhibits intensity-dependent sign reversals and a $<4\text{−}\mathrm{ns}$ response time. A change from self-focusing to self-defocusing or vice versa was observed to occur for Rabi frequencies that are comparable to the collisional linewidth. A density matrix model for the nonlinear susceptibility of a strongly driven two-level system provides a qualitative explanation for these results.

Interferometers provide a highly sensitive means to investigate and exploit the coherence properties of light in metrology applications. However, interferometers come in various forms and exploit different properties of the optical states within. In this paper, we introduce a classification scheme that characterizes any interferometer based on the number of involved nonlinear elements by studying their influence on single-photon and photon-pair states. Several examples of specific interferometers from these more general classes are discussed, and the theory describing the expected first-order and second-order coherence measurements for single-photon and single-photon-pair input states is summarized and compared. These theoretical predictions are then tested in an innovative experimental setup that is easily able to switch between implementing an interferometer consisting of only one or two nonlinear elements. The resulting singles and coincidence rates are measured in both configurations and the results are seen to fit well with the presented theory. The measured results of coherence are tied back to the presented classification scheme, revealing that our experimental design can be useful in gaining insight into the properties of the various interferometeric setups containing different degrees of nonlinearity.

Correlations in the open quantum systems manifest a rich phenomenology under the influence of various physical sources of noise. We theoretically study the dynamics of quantum correlations and their classical counterparts in a two-qubit system, measured by one-norm geometric quantifiers. We consider both qubits are initially prepared in the arbitrary $X$-type states and locally subjected to nonequilibrium dephasing environments with non-Markovian and nonstationary statistical features. We then explore the effects of nonequilibrium on the abrupt changes in the evolution of geometric correlations in the weak- and strong-coupling regimes. Particularly, in the weak-coupling regime, we show that the environmental nonequilibrium feature can efficiently control the appearance of double-sudden transitions and the decay rate in geometric quantum correlation dynamics. However, in the strong-coupling case, we show that the nonequilibrium nature can prolong the interval for the frozen quantum correlation and suppresses the number of multiple sudden changes in its decay rate. Moreover, the abrupt change from decay to a constant regime in classical correlation evolution reveals the emergence of the pointer-state basis. We show that the pointer-state basis appearance can be retarded in the weak-coupling case when the environment deviates from equilibrium. However, in a strong-coupling regime, we observe the metastable pointer-states basis characterized by multiple, successive sudden transitions in classical correlation dynamics. Remarkably, we unveil that the nonequilibrium feature may suppress the apparition of the metastable pointer-states basis.

Quantum simulation can help us study poorly understood topics such as high-temperature superconductivity and drug design. However, existing quantum simulation algorithms for current quantum computers often have drawbacks that impede their application. Here, we provide a hybrid quantum-classical algorithm for simulating the dynamics of quantum systems. Our approach takes the ansatz wave function as a linear combination of quantum states. The quantum states are fixed, and the combination parameters are variationally adjusted. Unlike existing variational quantum simulation algorithms, our algorithm does not require any classical-quantum feedback loop and by construction bypasses the barren plateau problem. Moreover, our algorithm does not require any complicated measurements such as the Hadamard test. The entire framework is compatible with existing experimental capabilities and thus can be implemented immediately.

The action of qubit channels on projective measurements on a qubit state is used to establish an equivalence between channels and properties of generalized measurements characterized by bias and sharpness parameters. This can be interpreted as shifting the description of measurement dynamics from the Schrodinger to the Heisenberg picture. In particular, unital quantum channels are shown to induce unbiased measurements. The Markovian channels are found to be equivalent to measurements for which sharpness is a monotonically decreasing function of time. These results are illustrated by considering various noise channels. The effect of bias and sharpness parameters on various quantum correlations and on the energy cost of measurements is discussed. Further, we illustrate the quantum circuit implementation of the two element positive operator-valued measures characterized by the bias and sharpness parameters.

We study phase-imprinted solitons of ultracold bosons in an optical lattice with a harmonic trap exhibiting superfluid (SF) and Mott-insulator shell structures. An earlier study [K. V. Krutitsky, J. Larson, and M. Lewenstein, Phys. Rev. A 82, 033618 (2010)] reported three types of phase-imprinted solitons in the Bose-Hubbard model: in-phase soliton, out-of-phase soliton, and wavelet. In this paper, we uncover the dynamical phase diagram of these phase-imprinted solitons. We also reveal another type of phase-imprinted soliton, the hybrid soliton. In a harmonically trapped system, solitonic excitations created at the SF core cannot penetrate the outer SF shell. This repulsion at the surface of the outer SF shell can be counteracted by imposing a repulsive potential at the center of the trap. These results can be interpreted as a kind of impedance matching of excitations in Bose-Einstein condensates in terms of the effective chemical potentials or local particle numbers in the shell, and analogous results can be observed in sound waves created by a local single-shot pulse potential.

In this paper, we investigate the effect of different optical field initial states on the performance of the Tavis-Cummings (TC) quantum battery. In solving the dynamical evolution of the system, we found a fast way to solve the Bethe ansatz equation. We find that the stored energy and the average charging power of the TC quantum battery are closely related to the probability distribution of the optical field initial state in the number states. We define a quantity called the number-state stored energy. With this prescribed quantity, we only need to know the probability distribution of the optical field initial state in the number states to obtain the stored energy and the average charging power of the TC quantum battery at any time. We propose an equal probability and equal expected value splitting method by which we can obtain two inequalities, and the two inequalities can be reduced to Jensen's inequalities. By this method, we found the optimal initial state of the optical field. We found that the maximum stored energy and the maximum average charging power of the TC quantum battery are proportional to the initial average photon number, and the quantum battery can be fully charged when the initial average photon number is large enough. We found two phenomena, which can be described by two empirical inequalities. These two phenomena imply the hypersensitivity of the stored energy of the TC quantum battery to the number-state cavity field. Finally, we discuss the impact of decoherence on battery performance.

We report absolute frequency measurements on the $6D_{5/2}$ level of neutral ${}^{133}\mathrm{Cs}$ using sub-Doppler two-photon spectroscopy. The absolute center-of-gravity energy for the $6D_{5/2}$ level is determined to be $22631.683877\left(6\right){\mathrm{cm}}^{-1}$, a factor of 170 times improvement over the previous measurement from 1964 of $22631.6863\left(10\right){\mathrm{cm}}^{-1}$. This measurement also corrects a $2.4\sigma$ discrepancy with the previously measured value. The hyperfine coefficients were found to be $A=-4.629\left(14\right)$ and $B=-0.10\left(15\right)\mathrm{MHz}$, which are consistent with previous results.

We theoretically investigate the one-dimensional dynamics of a dark soliton in a two-component immiscible mixture of Bose-Einstein condensates with repulsive interactions. We analyze the reflection and transmission of a soliton when it propagates through the domain wall, and we show that a dark-bright soliton can be dynamically generated by the interaction of the dark soliton with the domain wall, outside the regime of parameters where stationary solutions are known to exist. The dynamics of this dark-bright soliton is harmoniclike, with a numerical frequency that is in good agreement with the predictions of a semianalytical model.