It is commonly accepted that the isomerization process primarily occurs in lowly charged molecular states, while highly charged states typically favor a direct fragmentation pathway through Coulomb repulsion. In the current experiment with an intense 800-nm laser field, a three-body isomerization pathway leading to intermediate ${\mathrm{O}}_2^{2+}$ formation and subsequent dissociation is observed in highly excited states of ${\mathrm{CO}}_2^{3+}$. It is confirmed by the calculated potential energy curves that ${\mathrm{CO}}_2^{3+}$ molecular ions in highly excited states with a linear geometry could transition through conical intersections into low-lying states with a bond angle around ${70}^{\circ }$ (e.g., $6^{2}A1\to 1^{2}A1$, ${10}^{2}B1\to 1^{2}B1$), finally leading to oxygen molecule ions formation via isomerization. The present findings demonstrate that isomerization-induced change in molecular structures is a universal and nonnegligible phenomenon, even in triply charged molecular ions, providing insights into the photoinduced ultrafast processes in Earth's primitive atmosphere and benefit the understanding of ${\mathrm{CO}}_2$-dominated planetary atmospheres.

We theoretically study how the peculiar properties of the vacuum state of an ultrastrongly coupled system can affect basic light-matter interaction processes. In this unconventional electromagnetic environment, an additional emitter no longer couples to the bare cavity photons, but rather to the polariton modes emerging from the ultrastrong coupling. As such, the effective light-matter interaction strength is sensitive to the properties of the distorted vacuum state. Different interpretations of our predictions in terms of modified quantum fluctuations in the vacuum state and of radiative reaction in classical electromagnetism are critically discussed. Whereas our discussion is focused on the experimentally most relevant case of intersubband polaritons in semiconductor devices, our framework is fully general and applies to generic material systems.

The current quantum resource theory for Gaussian steering in continuous-variable systems is flawed and incomplete. Its primary shortcoming stems from an inadequate comprehension of the architecture of Gaussian channels transforming Gaussian unsteerable states into Gaussian unsteerable states, resulting in a restricted selection of free operations. In the present paper we explore in depth the structure of such $(m+n)$-mode Gaussian channels and introduce a class of Gaussian unsteerable channels and a class of maximal Gaussian unsteerable channels, both of which may be chosen as free operations, which completes the resource theory for Gaussian steering from $A$ to $B$ by using Alice's Gaussian measurements. We also propose two quantifications ${\mathcal{J}}_j$ ($j=1,2$) of $(m+n)$-mode Gaussian steering from $A$ to $B$. The computation of the value of ${\mathcal{J}}_j$ is straightforward and efficient, as it relies solely on the covariance matrices of Gaussian states, eliminating the need for any optimization procedures. Though the ${\mathcal{J}}_j$ are not genuine Gaussian steering measures, they have some nice properties such as being nonincreasing under certain Gaussian unsteerable channels. Additionally, we compare ${\mathcal{J}}_2$ with the Gaussian steering measure ${\mathcal{N}}_3$, which is based on the Uhlmann fidelity, revealing that ${\mathcal{J}}_2$ is an upper bound of ${\mathcal{N}}_3$ for a certain class of $(1+1)$-mode Gaussian pure states. As an illustration, we apply ${\mathcal{J}}_2$ to discuss the behavior of Gaussian steering for a special class of $(1+1)$-mode Gaussian states in Markovian environments, which uncovers the intriguing phenomenon of rapid decay in quantum steering.

We explore a discrete-time, coined quantum walk on a quantum network where the coherent superposition of walker moves originates from the unitary interaction of the walker coin with the qubit degrees of freedom in the quantum network. The walk dynamics leads to a growth of entanglement between the walker and the network on one hand and, on the other, between the network qubits among themselves. The initial entanglement among the network qubits plays a crucial role in determining the asymptotic values of these entanglement measures and the quantum walk statistics. Specifically, the entanglement entropy of the walker-network state and the negativity of the quantum network-qubit state saturate to values increasing with the initial network entanglement. The asymptotic time-averaged walker-position probability distribution shows increasing localization around the initial walker position with higher initial network entanglement. A potential application of these results as a characterization tool for quantum network properties is suggested.

We propose a heralded protocol for implementing nontrivial quantum gates on two stationary qubits coupled to spatially separated cavities. By dynamically controlling the evolution of the composite system, nonlocal two-qubit quantum (e.g., cphase and cnot) gates can be achieved without real excitations of either cavity modes or atoms. The success of our protocol is conditioned on projecting an auxiliary atom onto a postselected state, which simultaneously removes various detrimental effects of dissipation on the gate fidelity. In principle, the success probability of the gate can approach unity as the single-atom cooperativity becomes sufficiently large. Furthermore, we show its application for implementing single- and two-qubit gates within a decoherence-free subspace that is immune to a collective dephasing noise. This faithful, heralded, and nonlocal protocol could, therefore, be useful for distributed quantum computation and scalable quantum networks.

Quantum digital signatures (QDSs), which utilize correlated bit strings among sender and recipients, guarantee the authenticity, integrity, and nonrepudiation of classical messages based on quantum laws. Continuous-variable (CV) quantum protocol with heterodyne and homodyne measurement has obvious advantages of low-cost implementation and easy wavelength division multiplexing. However, security analyses in previous research are limited to the proof against collective attacks in finite-size scenarios. Moreover, existing multibit CV QDS schemes have primarily focused on adapting single-bit protocols for simplicity of security proof, often sacrificing signature efficiency. Here, we introduce a CV QDS protocol designed to withstand general coherent attacks through the use of a cutting-edge fidelity test function, while achieving high signature efficiency by employing a refined one-time universal hashing signing technique. Our protocol is proved to be robust against finite-size effects and excess noise in quantum channels. In simulation, results demonstrate a significant reduction of over six orders of magnitude in signature length for a megabit message signing task compared with existing CV QDS protocols and this advantage expands as the message size grows. Our work offers a solution with enhanced security and efficiency, paving the way for large-scale deployment of CV QDSs in future quantum networks.

Composite pulse segmentation has emerged as a promising error-mitigation technique for a wide range of physical systems. In recent years, composite schemes were applied as mitigation strategies for quantum information processing and quantum computing. However, most of these strategies assume full error correlation between segments, which can result in gates with worse fidelity performance compared to noncomposite gates. In our research, we investigate how error correlations impact the fidelity of quantum gates within the composite segmentation framework. In our study, we prove the existence of a critical correlation threshold, above which the composite pulse method significantly enhances both the mean value and variance of the fidelity. To gain deeper insights, we analyze various properties of the threshold in the realm of integrated photonics, including the effects of geometrical variations and the limit where the number of segments approaches infinity. We numerically explore diverse scenarios, showcasing different aspects of the critical threshold within the photonic quantum gates framework. These findings contribute to new pathways of error-mitigation strategies and their implications in quantum information processing.

We present relativistic many-body calculations of the total electron binding energy of neutral atoms up to the element with $Z=120$. The binding energy for ions may be found by subtracting known ionization potentials. The accuracy of the results for $17<Z\le 103$ significantly exceeds that in NIST tables (there are no data for $Z>103$ there). We fit the numerical results for binding energies by an analytical function of $Z$. We also calculate numerical values and determine the dependence on $Z$ of the correlation corrections, Dirac and Breit relativistic corrections, and quantum electrodynamics corrections.

Since their first theoretical proposal [Ngoko Djiokap et al., Phys. Rev. Lett. 115, 113004 (2015)] and experimental demonstration [Pengel et al., Phys. Rev. Lett. 118, 053003 (2017)], free electron vortices (FEVs) prepared in light-driven processes have attracted considerable attention. Recently, a new class of unusually shaped FEVs, termed “reversible electron spirals,” was proposed theoretically [Strandquist et al., Phys. Rev. A 106, 043110 (2022)] for atomic single-photon ionization by two oppositely chirped and counterrotating circularly polarized (CRCP) attosecond pulses. Building on this concept, we present an experimental demonstration of shaped FEVs created by atomic multiphoton ionization (MPI) using oppositely chirped CRCP femtosecond pulse pairs. In MPI by polarization-shaped pulses, several shaped FEVs of different rotational symmetry are superimposed resulting in the total photoelectron wave packet observed in the experiment. We employ velocity map imaging techniques to measure the photoelectron momentum distribution (PMD) and reconstruct the three-dimensional PMD by photoelectron tomography. Fourier analysis is used to decompose the measured PMD into the contributing FEVs with different rotational symmetries. The experimental results are supported by an analytical model and reproduced by numerical simulations, which provide insight into the role of resonant intermediate states. Our approach allows us to retrieve spectral information from the shaped laser field and the signatures of the MPI dynamics inscribed within the shaped FEVs.

Neutral atoms trapped in microscopic optical tweezers have emerged as a growing platform for quantum science. Achieving homogeneity over the tweezers array is an important technical requirement, and our research focuses on improving it for holographic arrays generated with a spatial light modulator. We present a series of optimization methods to calculate better holograms, fueled by precise measurement schemes. These innovations enable to achieve intensity homogeneity with a relative standard deviation of 0.3%, shape variations below 0.5%, and positioning errors within 70 nm. Such ultraprecise holographic optical tweezers arrays allow for the most demanding applications in quantum science with atomic arrays.

Creating and manipulating quantum states of light requires nonlinear interactions. We present here a multimode theory for the transformation of an arbitrary quantum pulse by Hamiltonians that are quadratic in field creation and annihilation operators. We show, in particular, that any input quantum pulse will feed only one or two distinct output modes. Our result readily provides both the output modes and the quantum states after transformation of arbitrary input wave packets. While being central for applications in quantum information processing with traveling bosonic modes, e.g., in quantum networks, our theoretical method and its results are not anticipated by the conventional Bogoliubov diagonalization of the problem in many input and output eigenmodes.

We examine metrological scenarios where the parameter of interest is encoded onto a quantum state through the action of a noisy quantum gate and investigate the ultimate bound to precision by analyzing the behavior of the quantum Fisher information (QFI). We focus on qubit gates and consider the possibility of employing successive applications of the gate. We go beyond the trivial case of unitary gates and characterize the robustness of the metrological procedure introducing noise in the performed quantum operation, looking at how this affects the QFI at different steps (gate applications). We model the dephasing and tilting noise affecting qubit rotations as classical fluctuations governed by a von Mises–Fisher distribution. Compared to the noiseless case, in which the QFI grows quadratically with the number of steps, we observe a nonmonotonic behavior, and the appearance of a maximum in the QFI, which defines the ideal number of steps that should be performed in order to precisely characterize the action of the gate. Analyzing also the combination of different types of gate imperfections we defined a form of generic noise, finding also nontrivial regimes in which the combination of the two different noises leads to a more resilient QFI or, more remarkably, to an enhancement of the QFI with respect to the case in which only one type of noise is considered.

We consider the time evolution of a particle on a ring with a long solenoid through and show that due to the Aharonov-Bohm effect this system naturally makes up a physical implementation of the quantum phase estimation algorithm for a $\text{U}\left(1\right)$ unitary operator. The implementation of the full quantum phase estimation algorithm with a $\text{U}\left(N\right)$ unitary operator is realized through the non-Abelian Aharonov-Bohm effect. The implementation allows for a more physically intuitive understanding of the algorithm. As an example we use the path integral formulation of the implemented quantum phase estimation algorithm to analyze the classical limit $\hslash \to 0$.

Spin squeezing, a compelling manifestation of multiparticle entanglement, is considered one of the most promising quantum resources. Non-Markovian characteristics play a crucial role in many critical physical processes involving strong system-environment interactions. We investigate the generation of spin squeezing in a hybrid quantum system composed of silicon-vacancy color centers (SiV) and diamond acoustic waveguides in a finite-temperature non-Markovian environment. The results indicate that memory parameters, damping rates, environmental temperature, and the total spin number play critical roles in the generation of spin squeezing. By appropriately tuning these parameters, the spin-squeezing effect of the quantum system can be significantly enhanced. This study provides theoretical foundations and optimization strategies for the application of quantum systems in complex environments, such as quantum information processing and quantum precision measurement.

We investigate the effects of weak value amplification on the nonclassical properties of the two-mode squeezed vacuum state. We calculate the quadrature squeezing and second-order cross-correlation functions to demonstrate the advantages of postselective weak measurement (WM). We also examine the entanglement between the two modes by evaluating various entanglement criteria, including the Cauchy-Schwartz inequality, Hillery-Zubairy correlation, and Einstein-Podolsky-Rosen correlations. Our results show that postselected von Neumann measurements significantly optimize the two-mode squeezed vacuum state. Notably, the amplification effect of weak values plays a critical role in this process. We demonstrated that in the WM regime, as the weak values increase, the nonclassical properties of the two-mode squeezed vacuum state such as entanglement and correlations between the two modes become more pronounced compared to the original state. We also propose possible implementation methods for our approach using trapped ion systems and optical platforms. Our enhanced two-mode squeezed vacuum state may have potential applications in quantum information processing scenarios, such as single-photon generation, advanced imaging techniques, quantum computation, and long-distance quantum communication.

We develop a general theory for multiphoton qubit-resonator interactions enhanced by a qubit drive. The interactions generate qubit-conditional operations in the resonator when the driving is near $n$-photon cross resonance, i.e., when the qubit drive is $n$ times the resonator frequency. We pay special attention to the strong driving regime, where the resulting effective interactions are conditioned on the qubit dressed states. Next, we investigate the use of a two-tone drive to engineer an effective $n$-photon Rabi Hamiltonian with widely tunable effective system parameters, which could enable the realization of new regimes that have so far been inaccessible. Then, we discuss applications for the specific case where $n=2$, which results in qubit-conditional squeezing (QCS). We show that the QCS protocol can be used to generate a superposition of orthogonally squeezed states following a properly chosen qubit measurement. We outline quantum information processing applications for these states, including encoding a qubit in a resonator via the superposition of orthogonally squeezed states. We show how the QCS operation can be used to realize a controlled-squeeze gate and its use in bosonic phase estimation. The QCS protocol can also be utilized to achieve faster unitary operator synthesis on the joint qubit-resonator Hilbert space. Finally, we propose a multiphoton circuit QED implementation based on a transmon qubit coupled to a resonator via an asymmetric SQUID. We provide realistic parameter estimates for the two-photon operation regime that can host the aforementioned two-photon protocols. We use numerical simulations to show that, even in the presence of spurious terms and decoherence, our analytical predictions are robust.

Using standard results from statistics, we show that for any continuous quantum system (Gaussian or otherwise) and any observable $\widehat{A}$ (position or otherwise), the distribution ${\pi }_a\left(t\right)$ of time measurement at a fixed state $a$ can be inferred from the distribution ${\rho }_t\left(a\right)$ of a state measurement at a fixed time $t$ via the transformation ${\pi }_a\left(t\right)\propto \left|\frac{\partial }{\partial t}{\int }_{-\infty }^a{\rho }_t\left(u\right)du\right|$. This finding suggests that the answer to the long-lasting time-of-arrival problem is in fact secretly hidden within the Born rule and therefore does not require the introduction of a time operator or a commitment to a specific (e.g., Bohmian) ontology. The generality and versatility of the result are illustrated by applications to the time of arrival at a given location for a free particle in a superposed state and to the time required to reach a given velocity for a free-falling quantum particle. Our approach also offers a potentially promising new avenue toward the design of an experimental protocol for the yet-to-be-performed observation of the phenomenon of quantum backflow.

We investigate the properties of Floquet states in the vicinity of a conical intersection of quasienergies and work out the consequences of the underlying spatio-temporal symmetries for a driven two-level system coupled to an ohmic heat bath. We find that on manifolds with constant quasienergy splitting, the mean energies of the Floquet states are continuously interchanged. In the presence of dissipation, the parameter dependence of the stationary populations generally resembles that of the mean energies. In turn, the mean energies are an indicator for the qualitative behavior of the density operator in the long-time limit. A further consequence of the symmetries is that for specific driving parameters, the stationary state may be fully mixed even at arbitrarily low temperatures. For large driving frequencies, such states with maximal entropy are found in the whole vicinity of the intersection, which can be explained by a chirality emerging in this limit. Analytical results beyond a high-frequency approximation are illustrated by numerical data.