Nitrogen-vacancy (NV) centers in diamond are a promising platform for highly sensitive quantum sensors for magnetic fields and other physical quantities. The quest for high sensitivity combined with high spatial resolution naturally leads to dense ensembles of NV centers and hence to strong, long-range interactions between them. Hence, simulating strongly interacting NVs becomes essential. However, obtaining the exact dynamics for a many-spin system is a challenging task due to the exponential scaling of the Hilbert space dimension, a problem that is exacerbated when the system is modeled as an open quantum system. In this work, we employ the matrix product density operator (MPDO) method to represent the many-body mixed state and to simulate the dynamics of an ensemble of NVs in the presence of strong long-range couplings due to dipole-dipole forces. We benchmark different time-evolution algorithms in terms of numerical accuracy and stability against time evolution based on exact numerical diagonalization. Subsequently, we simulate the dynamics in the strong-interaction regime and study the impact of decoherence on the accuracy of the MPDO method. Last, we investigate the dynamics of quantum Fisher information and show that for small distances below about 5 nm the resulting strong dipole-dipole interaction can improve the sensitivity for magnetic field sensing when the ensemble is driven by pulses of constant amplitude.

We present a time-dependent Schrödinger equation approach within a nonrelativistic quantum electrodynamics framework to investigate resonant x-ray scattering driven by intense x-ray pulses. This method enables us to explore how coherent x-ray electron dynamics influence scattering signals from ${\mathrm{Ne}}^{+}$. We account for both resonance fluorescence and elastic scattering channels, while also considering competing photoionization and inner-shell decay processes. By computing the angular distribution and energy spectrum of scattered photons, we uncover interference effects between elastic scattering and resonance fluorescence pathways. Notably, this interference results in a small asymmetry in the energy spectrum. We discuss the experimental potential for detecting signatures of interference. Our findings demonstrate that the x-ray Rabi dynamics can be used to control scattering responses and provide insights into interference mechanisms and scattering efficiency in high-intensity x-ray regimes.

Chiral quantum state generation is an interesting topic in quantum physics and quantum information science. Here we propose an approach for generating chiral cat states in a spinning resonator supporting both the clockwise (CW) and counterclockwise (CCW) traveling modes, which are dispersively coupled to a two-level atom. The physical mechanism for the chiral-cat-state generation is based on the Sagnac-Fizeau effect. Concretely, when the resonator is rotating, the CW and CCW modes have different frequency detuning with respect to the atomic transition frequency and hence the atomic-state-dependent rotating angular velocities for the CW and CCW modes in phase space are different. This mode-dependent evolution leads to a chirality mechanism in the state generation. Based on the mode-dependent conditional rotation evolution and atomic projection measurement at proper time, we achieve the separate generation of cat states in the CW mode and coherent states in the CCW mode. We also investigate quantum coherence properties of the generated states by examining their Wigner functions. In addition, the influence of the system dissipations on the state generation in the open-system case is investigated. Our work will provide some insights into the development of chiral optical devices and nonreciprocal photonics.

As per the projection postulate, the evolution of the quantum state of a system upon measurement results in state update. In this work we investigate the characterization of updated states of multipartite entangled qubit states with nondestructive weak measurements, involving weakly coupling a pointer to each qubit while considering a relativistic setup involving different reference frames. As is well known, the updated state at intermediate times is frame dependent and outcome randomness, intrinsic to projective measurements, prevents any information from being acquired on the updated state. Here we will see that when weak measurements are implemented there is instead an interplay between randomness and interference, depending on the number of qubits, so as to maintain consistency between descriptions in arbitrary reference frames. Analytical results are given for a small or infinite number of qubits, while for a finite number of qubits we resort to Monte Carlo sampling in order to simulate numerically the results of weak measurements. As a by-product, we show how a single-shot measurement of the pointers' positions allows us to make quantitative predictions on the outcomes that can be obtained for any observable measured on a distant qubit of the multipartite state.

Bell inequalities represent a milestone for contemporary physics, both for quantum foundation investigations and technological applications (e.g., quantum communication and entanglement certification). Although loophole-free tests have been recently performed, a strong debate is still ongoing on the actual meaning of Bell inequality tests, for example, on the possible additional hypotheses and eventual loopholes to be included in Bell's theorem, as well as on the implications for certain interpretations of quantum mechanics. A recent work [S. Virzì et al., Quantum Sci. Technol. 9, 045027 (2024)] challenges some of the statements appeared in this debate, achieving an experimental estimation of the entire Bell-Clauser-Horne-Shimony-Holt (Bell-CHSH) parameter from a single entangled pair owing to a weak-interaction-based measurement approach. Here, we analyze the implications of this result for quantum mechanics foundations investigation, illustrating how it can tackle some of the aforementioned interpretations of Bell inequality tests and, more in general, of quantum mechanics itself.

The competition between unitary quantum dynamics and dissipative stochastic effects, as emerging from continuous-monitoring processes, can culminate in measurement-induced phase transitions. Here, a many-body system abruptly passes, when exceeding a critical measurement rate, from a highly entangled phase to a low-entanglement one. We consider a different perspective on entanglement phase transitions and explore whether these can emerge when the measurement process itself is modified, while keeping the measurement rate fixed. To illustrate this idea, we consider a noninteracting fermionic system and focus on diffusive detection processes. Through extensive numerical simulations, we show that, upon varying a suitable unraveling parameter—interpolating between measurements of different quadrature operators—the system displays a transition from a phase with area-law entanglement to one where entanglement scales logarithmically with the system size. Our findings may be relevant for tailoring quantum correlations in noisy quantum devices and for conceiving optimal classical simulation strategies.

The understanding of open quantum systems is crucial for the development of quantum technologies. Of particular relevance is the characterization of divisible quantum dynamics, seen as a generalization of Markovian processes to the quantum setting. Here, we propose a way to detect divisibility and quantify how nondivisible a quantum channel is through the concept of channel discrimination. We ask how well we can distinguish generic dynamics from divisible dynamics. We show that this question can be answered efficiently through semidefinite programming, which provides us with an operational and efficient way to quantify nondivisibility.

We present a fast and robust protocol for realizing topological quantum state transfer in a dimeric Su-Schrieffer-Heeger spin chain. By giving a more general condition sufficient for the resonant adiabatic evolution, we develop a method of constructing time-driving functions for topological quantum state transfer, which makes it possible to shorten the needed operation time via resonant adiabatic passage between topologically protected edge states. Compared with previous protocols, our protocol can shorten the operation time appreciably while maintaining robustness against disorder in the spin chain, thereby avoiding decoherence resulting from long-running evolution and increasing transmission efficiency. Numerical simulations demonstrate that our protocol possesses excellent robustness and scalability. Our results provide a promising route for the realization of fast and robust quantum state transfer in spin chain systems.

We present a modular error mitigation protocol for running bounded-error quantum polynomial time (BQP) computations on a quantum computer with time-dependent noise. Utilizing existing tools from quantum verification and measurement-based quantum computation, our protocol interleaves standard computation rounds alongside test rounds for noise sampling and inherits an exponential bound (in the number of circuit runs) on the probability that a returned classical output is correct. We introduce a postselection technique called basketing to address time-dependent noise and reduce overhead. The result is an error mitigation protocol which requires minimal noise assumptions, making it straightforwardly implementable on existing, noisy intermediate-scale quantum devices. We perform a demonstration of the protocol using classical noisy simulation, presenting a universal measurement pattern which directly maps to (and can be tiled on) the heavy-hex layout of current IBM hardware.

Serially improved Gaussian-type orbitals for molecular applications (SIGMA) basis sets have been developed for third-row elements, spanning from sodium to argon. This methodology, originally established for first- and second-row atoms, has been extended to produce both standard and augmented basis sets with sizes ranging from double-zeta to quadruple-zeta. The SIGMA basis sets yield atomic and molecular energies that are consistently lower than those obtained using Dunning basis sets with the same number of contracted functions. This advantage is observed for various computational methods, including Hartree-Fock, Møller-Plesset perturbation theory, single-double configuration interaction, and coupled-cluster approaches. Comparative analyses of SIGMA and other basis sets are presented, showing their performance in calculating atomic energies and a range of molecular properties.

We investigate a shift in the critical temperature of rotating Bose-Einstein condensates mediated by the melting of the vortex lattice. Numerical simulations reveal that this temperature exhibits contrasting behavior depending on the system configuration: a negative shift occurs for fixed trap potentials due to the expansion of the condensate, while a positive shift is observed for fixed volumes, where vortex-lattice rigidity suppresses thermal fluctuations. We introduce a vortex-energy model that captures the role of vortex interactions, the positional energy of the vortex lattice, as well as the phase transition and how the vortex lattice disappears. The findings provide insights into the thermodynamic properties of rotating condensates and the dynamics of vortex-lattice melting, offering potential parallels with other quantum systems such as type-II superconductors.

Synthetic spin-orbit coupling in cold atoms couples the pseudospin and spatial degrees of freedom, and therefore, the inherent spin symmetry of the system plays an important role. In systems of two pseudospin degrees, two particles contain symmetric states and antisymmetric states, but the spin symmetry can be mixed for more particles. This mixed spin symmetry has been overlooked and has not been investigated thoroughly. We study the role of mixed spin symmetry in the presence of spin-orbit coupling and consider a system of three bosons with two hyperfine states trapped in a harmonic potential. We investigate the ground state and the energy spectrum by implementing exact diagonalization. Similar to that in two-particle systems, the interplay between spin-orbit coupling and repulsive interactions between antialigned pseudospins increases the population of the unaligned spin components in the ground state. Also, the emergence of mixed-spin symmetric states compensates for the rise of the interaction energy. In contrast to two-particle systems, the pair correlation of the ground state is analogous to the Tonks-Girardeau gas even for relatively small contact interactions, and this feature is enhanced by the spin-orbit coupling.

Bose-Einstein condensates are suitable systems for studying fundamental aspects of quantum backreaction. Here, the backreaction problem in one-dimensional condensates is considered from the perspective of energy and momentum conservation. By assuming the validity of Bogoliubov theory, the backreaction equations are used to identify the contributions to the system energy and momentum coming from quantum fluctuations and condensate corrections. The backreaction is solved for a condensate trapped in a ring configuration such that particle interactions are gradually switched on. It is shown that the energy in the condensate cannot be addressed without taking into account how the system entered the interacting regime, and even for homogeneous condensates the power transferred to the condensate by quantum fluctuations showcases an intricate nonmonotonic pattern.

Several dissipative solitons or soliton attractors have been investigated in dissipative nonlinear systems, but the quantitative theoretical frame for multisoliton attractors is still lacking, which hinders the quantitative investigation of soliton attractors. We provide a method to obtain a theoretical frame for multisoliton attractors in mode-locked fiber lasers with various initial states. These results provide possibilities for realizing controllable generation of soliton attractors. We further propose that the properties of soliton attractors can be used to calculate dissipation strengths. We perform experiments to observe the generation of multisoliton attractors in a mode-locked fiber laser and demonstrate the effectiveness of the calculation method for the dissipation strengths. Our results provide possibilities for studying the dynamics of localized waves quantitatively in nonlinear non-Hermitian systems.

Broadband squeezed light is a valuable resource in the quantum information science community. Here, we propose an appealing method for the directional generation of broadband microwave squeezing based on hybrid magnonics, where multiple chiral cavity-magnonic subsystems are connected together through a bidirectional waveguide in a cascading way. The chirality arises from the selective coupling between the Kittel mode in the ferrimagnetic yttrium iron garnet sphere and one of the two degenerate counter-rotating microwave modes in a ring-shaped cavity. By introducing Floquet engineering to simultaneously induce red and blue sidebands in the photon-magnon coupling, the chiral squeezing-type interaction can be engineered in each cavity-magnonic subsystem. Considering all the photon-magnon couplings in the cascaded system with the same chirality, we show that a broadband squeezed microwave field can be emitted on demand to either of the two output ports along the waveguide. Compared with a single chiral cavity-magnonic system, both the squeezing amount and the bandwidth can be significantly enhanced through the cumulative effect of the cascaded system.

Sequential measurements present a novel framework for constructing large-scale quantum networks, particularly in quantum correlation sharing. In this paper, we design projective measurements and combination strategies to achieve nonlocality sharing in star-shaped networks by leveraging shared classical randomness. This network consists of $n$ branches, each containing one or two sequential observers. By investigating two-sided nonlocality sharing within a 2-local network, we identify the optimal projective measurement combination strategy based on the double violation of the 2-locality inequality. Utilizing this optimal strategy, we demonstrate that four inequalities can be violated simultaneously in the 2-local scenario. Furthermore, in an $n$-local scenario involving $m$ ($1\le m\le n$) branches with two sequential observers, our optimal strategy shows that non-$n$-locality sharing among all observers can be achieved through the simultaneous violation of $2^m n$-locality inequality. Our findings provide a distinctive perspective on quantum correlation sharing in networks.