The electron shelving effect significantly improves the control accuracy of medium energy level transitions, enabling high-sensitivity fluorescence signal detection in precision spectroscopy and quantum metrology and thus allowing a more precise investigation of the microstructure and dynamical behavior of the medium. Here, we investigate theoretically the fluorescence properties of bichromatically driven three-level atoms with electron shelving effect under strong coupling conditions. The physical process by which the electron shelving effect leads to the appearance of two types of peaks in the central peaks of multiple Mollow triplets is explored. This phenomenon is attributed to the different orientations of electric dipole moments in fluorescence emission process caused by a nonresonant weak field. Due to the disappearance of the central broad peak of the central Mollow triplet caused by the quantum interference, the pure sharp peak survives completely induced by the electron shelving effect. The physical picture of the two-mode squeezing effect in out-of-phase quadrature noise spectra originating from the asymmetrical fluorescence operators induced by the nonresonant weak field is revealed. Moreover, the frequency range and degree of two-mode squeezing can be improved by appropriately increasing the Rabi frequency of the weak field. These results not only are helpful for realizing the electron shelving effect experiment in polychromatic strong-coupling systems but also provide a scheme for developing nonclassical spectroscopy and quantum precision measurement based on the electron shelving effect.

We demonstrate that in a quantum-dot-nanowire system, excitations of surface plasmon modes can be used to engineer anisotropy-induced photon entanglement between orthogonal exciton states. When a single quantum dot is excited in close proximity to a metallic nanowire, photon emission from the exciton states couples directly to the subwavelength confinement of optical fields in the nanowire, causing the long-lived entangled exciton states. The spectral density of states is immensely enhanced in the vicinity of the nanowire, mediating a strong coupling regime and effective quantum interference where the excitation energy can coherently be transferred between the orthogonal quantum-dot exciton states and the nanowire. The Rabi oscillation with non-Markovian features in the decay profiles signifies the strong interaction between the exciton states in the quantum dot near the surface plasmon nanowire.

The direct coupling system between a magnon and a superconducting qubit exhibits a special advantage in the quantum information processing [Phys. Rev. Lett. 129, 037205 (2022) ]. In this paper, based on this direct coupling system, the magnonic squeezed Schrödinger cat states (MSSCSs) can be generated by the conditional displacement mechanism. Numerical simulation shows that our scheme is robust to the dissipation of the system. In particular, we find that the coherence amplitude of MSSCSs can be enhanced via a squeezed transformation. Our results have potential application in quantum information processing and quantum optics.

In quantum information, the Werner state is a benchmark to test the boundary between quantum mechanics and classical models. There have been three well-known critical values for the two-qubit Werner state, i.e., $V_{\mathrm{c}}^{\mathrm{E}}=1/3$ characterizing the boundary between the entanglement and separable model, $V_{\mathrm{c}}^{\mathrm{B}}=1/K_G\left(3\right)$ characterizing the boundary between Bell's nonlocality and the local-hidden-variable model, and $V_{\mathrm{c}}^{\mathrm{S}}=1/2$ characterizing the boundary between Einstein-Podolsky-Rosen (EPR) steering and the local-hidden-state model. So far, the problem of $V_{\mathrm{c}}^{\mathrm{E}}=1/3$ has been completely solved by an inequality involving in the positive-partial-transpose criterion, while the question of how to reveal the other two critical values by the inequality approach is still open. In this work, we focus on EPR steering, which is a form of quantum nonlocality intermediate between entanglement and Bell's nonlocality. By proposing the stronger $N$-setting linear EPR-steering inequalities, we have successfully obtained the desired value $V_{\mathrm{c}}^{\mathrm{S}}=1/2$ for the two-qubit Werner state, thus resolving the long-standing problem.

Previously, it was predicted that the parity-violating energy difference (PVED) between enantiomers and the molecular electron chirality (MEC), which is the integrated value of the electron chirality density over the entire molecule, are enhanced for ionization or electronic excitation. Following the confirmation of the enhancement of the PVED by electronic excitation in our previous papers, the present work confirms that electronic excitation enhances the MEC for ${\mathrm{H}}_2{\mathrm{Te}}_2$ and ${\mathrm{H}}_2{\mathrm{Se}}_2$. The PVED in the first excited state is explained by the contribution from the highest occupied molecular orbital if the PVED contribution from the highest occupied molecular orbital in the ground state dominates over any other contributions. In this paper, it is checked whether this explanation can also be applicable to the MEC. It is also reported that the MEC is not the simple sum of contributions from each atom and the mixing by the superposition between atomic orbitals of different atoms is important.

$P$-divisibility is a central concept in both classical and quantum non-Markovian processes; in particular, it is strictly related to the notion of information backflow. When restricted to a fixed commutative algebra generated by a complete set of orthogonal projections, any quantum dynamics naturally provides a classical stochastic process. It is indeed well known that a quantum generator gives rise to a $P$-divisible quantum dynamics if and only if all its possible classical reductions give rise to divisible classical stochastic processes. However, this property does not hold if one operates a classical reduction of the quantum dynamical maps instead of their generators: As an example, for a unitary dynamics, $P$-divisibility of its classical reduction is inevitably lost and the latter thus exhibits information backflow. Instead, for some important classes of purely dissipative qubit evolutions, quantum $P$-divisibility always implies classical $P$-divisibility and therefore excludes information backflow in both the quantum and classical scenarios. On the contrary, for a wide class of orthogonally covariant qubit dynamics, we show that loss of classical $P$-divisibility originates from the classical reduction of a purely dissipative $P$-divisible quantum dynamics as in the unitary case. Moreover, such an effect can be interpreted in terms of information backflow due to the coherences developed by the quantumly evolving classical state.

In this paper, we consider a single-particle, spin-momentum entangled state and measure the effect of relativistic boost on quantum coherence. The effect of the relativistic boost on single-particle generalized Gaussian wave packets is studied. The coherence of the wave function as measured by the boosted observer is studied as a function of the momentum and the boost parameter. Using various formulations of coherence, it is shown that in general, the coherence decays with the increase in momentum of the state, as well as the boost applied to it. A more prominent loss of coherence due to relativistic boost is observed for a single-particle electron than that of a neutron. The analysis is carried out with a generalized Gaussian wave packet of the form $\mathcal{N}{p}^{n}exp(-\frac{p^2}{{\sigma }^2})$, with $n$ being the “generalization parameter” and $\mathcal{N}$ denoting the appropriate normalization constant. We also obtain a range for parameter $n$ appearing in the wave packet. The upper bound is found to have a dependence on the mass of the particle and the width of the Gaussian wave packet. We have obtained the Frobenius-norm measure of coherence, $l_1$- and $l_2$-norm measure of coherence, and relative entropy of coherence for a $(1+1)$- and $(3+1)$-dimensional analysis. Corresponding to each of the cases, we observe that the $l_1$-norm measure of coherence is equal to the Frobenius-norm measure of coherence. We have analyzed the scenario for which such a beautiful coincidence can occur. Finally, we have plotted different measures of coherence for the electron as well as the neutron for different values of the width of the wave function $\sigma$, boost parameter $\beta$, and generalization parameter $n$.

Sampling tasks are a natural class of problems for quantum computers due to the probabilistic nature of the Born rule. Sampling from useful distributions on noisy quantum hardware remains a challenging problem. A recent paper [D. Layden et al., Nature (London) 619, 282 (2023).] proposed a quantum-enhanced Markov-chain Monte Carlo algorithm where moves are generated by a quantum device and accepted or rejected by a classical algorithm. While this procedure is robust to noise and control imperfections, its potential for quantum advantage is unclear. Here we show that there is no speedup over classical sampling on a worst-case unstructured sampling problem. We present an upper bound to the Markov gap that rules out a speedup for any unital quantum proposal.

Quantum speed limits, establishing fundamental thresholds for the speed of quantum processes, are central to a wide spectrum of applications of quantum science, ranging from speeding up gate operations in quantum computations and optimizing communication channels to unraveling the rate at which correlations can propagate within many-body systems. In the quest to tailor optimal quantum dynamical speeds, the role of quantum entanglement has been showcased in specific instances. However, a robust universal link between system entanglement and its dynamical speed remains elusive. Here, we formulate the genuine contribution of quantum entanglement by developing universal criteria determining this speed enhancement within a quantum system. Specifically, we establish an upper limit that rigorously bounds the speed of the dynamical evolution of separable states, providing an unattainable speed threshold for these states. Our findings underscore the capacity of quantum entanglement to surpass this limit, demonstrating its capability as a resource for enhancing dynamical speed of the system.

Operator entanglement is a well-established measure of operator complexity across a system bipartition. In this work, we introduce a measure of the ability of a unitary channel to generate operator entanglement, representing an operator-level generalization of the state-space entangling power. This operator space entangling power is demonstrated to be linked to the scrambling properties of the unitary channel via the recently introduced concept of mutual averaged noncommutativity of quantum operator algebras. An upper bound for the operator space entangling power is identified that corresponds to unitary channels with scrambling properties akin to those of typical unitaries. Additionally, for Hamiltonian dynamics, we find that the short-time growth rate of the operator space entangling power matches the Gaussian scrambling rate of the bipartite out-of-time-order correlator, establishing a direct link between information scrambling and operator entanglement generation for short timescales. Finally, we examine the average growth of local operator entanglement across a symmetric bipartition of a spin chain. For dual-unitary circuits, a combination of analytical and numerical investigations demonstrates that the average growth of local operator entanglement exhibits two distinct regimes in relation to the operator space entangling power of the building-block gate.

We propose a deep-neural-network-based method for acquiring the magnetic field using the nitrogen-vacancy centers in diamond. The nitrogen-vacancy ensembles have been recognized as a promising quantum sensor for detecting microscale magnetic fields, and the sensed magnetic field vectors can be obtained according to the resonance frequencies of the nitrogen-vacancy centers along four crystallographic orientations in the diamond. The proposed method is used to realize the conversion from the resonance frequencies to magnetic field vectors in sensor's coordinate system. A subnanotesla accuracy can be obtained in simulation data set for measuring the magnetic field strength, which is equivalent to geomagnetic field. The proposed method can have a considerable advantage in acquiring high-precision magnetic field vectors comparing to existing solutions as well as the analytical solution. The method is a potential technical supplement for nitrogen-vacancy-center-based magnetometry, and can push forward the development for nitrogen-vacancy magnetometer in applications that require large dynamic range and high accuracy.

Worldwide efforts aim at the realization of advanced quantum simulators and processors. However, despite the development of intricate hardware and pulse control systems, it may still not be generally known which effective quantum dynamics, or channels, are implemented on these devices. To systematically infer those, we propose a neural-network algorithm approximating generic discrete-time dynamics through the repeated action of an effective quantum channel. We test our approach considering time-periodic Lindblad dynamics as well as nonunitary subsystem dynamics in many-body unitary circuits. Moreover, we exploit it to investigate cross-talk effects on the ibmq_ehningen quantum processor, which showcases our method as a practically applicable tool for inferring quantum channels when the exact nature of the underlying dynamics on the physical device is not known a priori. While the present approach is tailored for learning Markovian dynamics, we discuss how it can be adapted to also capture generic non-Markovian discrete-time evolutions.

We demonstrate the production of cold atomic strontium (Sr) and strontium-containing molecules (SrOH) in a cryogenic buffer-gas beam source via direct heating of strontium oxide (SrO) with 30 mJ laser pulses several milliseconds long. Here, $3.7\left(2\right)\times {10}^{14}$ Sr atoms are released, which represents a factor of 7 increase in atomic production per pulse compared to nanosecond-scale ablation laser pulses. A peak atomic density of $1.93\left(6\right)\times {10}^{12}$ atoms/${\mathrm{cm}}^3$ is achieved, which corresponds to a factor of 2 increase relative to ablation. We further propose extensions of this method to other atomic and molecular species.

Disentangling sequential and concerted three-body fragmentation has been a longstanding endeavor in studies of molecular dynamics. To accomplish this goal, we recently introduced a novel method to separate sequential from concerted breakup, where the sequential events are analyzed in their native frames [J. Rajput et al., Phys. Rev. Lett. 120, 103001 (2018)]. The essence of this method is the use of the Jacobi coordinates' conjugate momenta to reduce the dimensionality of the multibody fragmentation in combination with a clear signature for sequential breakup. To demonstrate this method, we employ coincidence momentum imaging to study the strong-field dissociative ionization of OCS into ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{+}$, which typically undergoes concerted fragmentation or sequential breakup involving either a metastable ${\mathrm{CO}}^{2+}$ or ${\mathrm{CS}}^{2+}$ molecule. We identify sequential breakup using a uniform distribution as a function of the angle between the conjugate momenta, associated with the first and second fragmentation steps, which is due to the rotation of the intermediate molecule in the fragmentation plane. By exploiting this uniform distribution, we separate the sequential and concerted distributions in any plot created from the measured momenta.

The full-dimensional quantum mechanical description of double ionization in strong laser fields is still computationally very demanding. We demonstrate the versatility of a simplified ($1+1$)-dimensional model of the He atom, which allows the study of nonsequential double ionization in strong laser fields over a wide range of frequencies and intensities. The ionization yield, longitudinal ion momentum distributions, and two-electron momentum distributions obtained by using the applied approach are in good agreement with the experimental and semi-classical data available in the literature, thus complementing them with a quantum mechanical description. The methodology can be extended to other atomic species or to study higher ionization.

The Vlasov-equation approach to the theory of dynamic polarizability of multielectron atoms and molecules in a weak external electric field in the the one-dimensional case is developed. It is shown that all dynamic properties of the system in the linear response regime are completely determined by its eigenmodes. The problem of finding the eigenmodes is reduced to solving an algebraic eigenvalue problem. This enables us to obtain the exact solution of the linearized Vlasov equations from which the polarizability is found. The theory is illustrated by calculations for an atom and a diatomic molecule. To demonstrate the capabilities of the Vlasov-equation approach beyond the linear response approximation, we also present calculations of the total energy absorbed by the system from a pulse of a strong external field.

We employ a Bose-Einstein-condensate–based atom-optic kicked rotor to generate an asymmetrically localized momentum distribution that depends upon the initial velocity of the Bose-Einstein condensate. Asymmetric features are shown to arise from the early-time dynamics induced by the broken parity symmetry and asymptotically freeze as the dynamical localization stabilizes. The asymmetry in the momentum distribution critically depends upon the initial launch velocity and is sensitive to very small initial velocities (micromotion) of the Bose-Einstein condensate. We also perform a precise measurement of the micromotion. By utilizing the technique of measuring the early-time asymmetry of the momentum distribution, we report measurement of the micromotion down to $230\pm 17\mu \mathrm{m}$/s.

We present an optical aberration correction technique for ultracold quantum gas experiments which directly utilizes the quantum gas as a wave-front sensor. The direct use of the quantum gas enables correcting aberrations that are otherwise impractical to measure, e.g., introduced by vacuum windows. We report a root-mean-square precision and accuracy of $0.01\lambda$ and $0.03\lambda$, respectively, and also show independently the reduction of aberrations through measurement of the trap frequency of our optical tweezer. These improvements were achieved for a tweezer size that is well below our imaging resolution. The present work is in particular intended to serve as a tutorial for experimentalists interested in implementing similar methods in their experiment.

The leading experimental determinations of the fine-structure constant $\alpha$ currently rely on atomic photon-recoil measurements from Ramsey-Bordé atom interferometry with large-momentum transfer to provide an absolute mass measurement. We propose an experimental scheme for an intermediate-scale differential atom interferometer to measure the photon recoil of neutral atomic species with a single-photon optical clock transition. We calculate trajectories for our scheme that optimize the recoil phase while nullifying the undesired gravity-gradient phase by considering independently launching two clouds of ultracold atoms with the appropriate initial conditions. For Sr and Yb, we find an atom interferometer of height 3 m to be sufficient for an absolute mass measurement precision of $\mathrm{\Delta }m/m\sim 1\times {10}^{-11}$ with current technology. Such a precise measurement would halve the current uncertainty in $\alpha$, an uncertainty that would no longer be limited by an absolute mass measurement. The removal of this limitation would allow the current uncertainty in $\alpha$ to be reduced by a factor of 10 by corresponding improvements in relative mass measurements, thus paving the way for higher-precision tests of the standard model of particle physics.