Dynamical decoupling multipulse sequences can be applied to solid-state spins for sensing weak oscillating fields from nearby single nuclear spins. By periodically reversing the probing system's evolution, other noises are counteracted and filtered out over the total evolution. However, the technique is subject to intricate interactions resulting in additional resonant responses, which can be misinterpreted with the actual signal intended to be measured. We experimentally characterize three of these effects present in single nitrogen-vacancy centers in diamond, where we also develop a numerical simulation model without rotating-wave approximation, showing robust correlation to the experimental data. Regarding centers with the ${}^{15}\mathrm{N}$ nitrogen isotope, we observe that a small misalignment in the bias magnetic field causes the precession of the nitrogen nuclear spin to be sensed by the electronic spin of the center. Another studied case of ambiguous resonances comes from the coupling with lattice ${}^{13}\mathrm{C}$ nuclei, where we use the echo modulation frequencies to obtain the interaction Hamiltonian and then utilize the latter to simulate multipulse sequences. Finally, we also measure and simulate the effects from the free evolution of the quantum system during finite pulse durations. Due to the large data volume and the strong dependence of these ambiguous resonances with specific experimental parameters, we provide a simulations data set with a user-friendly graphical interface, where users can compare simulations with their own experimental data for spectral disambiguation. Although focused on nitrogen-vacancy centers and dynamical decoupling sequences, these results and the developed model can potentially be applied to other solid-state spins and quantum sensing techniques.

We study static behavior and mechanical cooling of a dissipative optomechanical system by considering a phase-controlled coupling strength. We find that the static displacement of the mechanical oscillator vanishes when the dissipative coupling strength becomes purely imaginary, while a real coupling strength gives rise to a driving power-dependent static mechanical displacement. The absence of static displacement sustains the resonance linewidth of the effective Fabry-Pérot cavity and guarantees the approximation of a linear dissipative coupling. In this case, we show that the membrane can be cooled from the cryogenic temperature to its quantum ground state outside the resolved-sideband limit with a moderately strong driving operating at the red-detuned regime, where the optomechanical cooling rate can be competitive with typical schemes that utilize a real coupling strength and a blue-detuned laser drive. The dissipative optomechanical system with a phase-controlled coupling strength offers the flexibility to investigate fundamental optomechanical effects and may find potential applications in high-sensitive position transduction.

Polarized ultrashort high-energy sources of electromagnetic radiation are widely applied in nuclear physics, astrophysics, high-energy physics, etc. However, generation of a feasible and easy-to-deploy source, especially reaching the attosecond scale in the time-domain and MeV energy levels, is still a great challenge. Here, we put forward a method for producing ultracollimated high-brilliance polarized attosecond $\gamma$-rays via the interaction of an unpolarized electron beam with a solid-density plasma. As a relativistic electron beam enters a solid-density plasma, it can be modulated into high-density clusters via the self-modulation instability of itself and further into attosecond slices due to its own hosing instability. This is accompanied by the generation of similar pulse-width photon slices via synchrotron radiation. The severe hosing instability breaks symmetry of the excited electromagnetic fields, resulting in net linear polarization of the photon slices, which challenges the conventional perception that interaction of an axially symmetric unpolarized electron beam with a uniform plasma cannot generate radiation with net polarization. This paper provides new ideas for the generation of ultrashort, high-energy pulses with reduced dependence on the ultraintense peak power of the laser pulse.

The spiral phase plate is an essential optical component for the generation of Laguerre-Gaussian laser pulses with doughnut intensity profiles and helical wavefronts. This is due to its extraordinary ability to create and manipulate polarization distributions and orbital angular momentum of laser pulses. Here, we show that Gaussian laser pulses with near-relativistic intensity can acquire orbital angular momentum directly through a fixed-thickness plasma transmission spiral phase plate, whose density increases proportionally to the azimuthal angle around the plasma center. We propose the design for a plasma spiral phase plate and outline how the thickness and density distribution of the plasma determine the orbital angular momentum. We also use three-dimensional particle-in-cell simulations to examine the properties of plasma transmission spiral phase plates, such as orbital angular momentum generation and high-intensity laser pulses manipulation. The simulations suggest that Gaussian laser pulses could be turned into Laguerre-Gaussian laser pulses carrying orbital angular momentum through a $10\text{−}\mu \mathrm{m}$ plasma plate, with the conversion efficiency above 90% at high light intensity exceeding ${10}^{17}\mathrm{W}/{\mathrm{cm}}^2$. Our scheme offers a method for producing near-relativistic intensity vortex laser pulses with controlled orbital angular momentum, which could significantly advance the development of intense twisted laser pulses in high-field physics. Potential applications include the rotational dynamics of particle clumps, the generation of high-energy particles in plasma accelerators, and the radiation of attosecond pulses radiation with orbital angular momentum.

We investigate spin and heat currents in the nonequilibrium steady state of inhomogeneous $XX$-spin chains, which act as interfaces between two bosonic heat baths. Using special functions that diagonalize the single-particle Hamiltonian, we derive closed-form expressions for these currents. For small temperature differences between the baths, we show that inhomogeneities breaking the mirror symmetry of the chain significantly reduce both heat and spin conductivities. Connections with perfect state transfer are discussed.

Quantum maps are fundamental to quantum information theory and open quantum systems. Covariant or weakly symmetric quantum maps, in particular, play a key role in defining quantum evolutions that respect thermodynamics, establish free operations in resource theories, and are consistent with transformations of quantum reference frames. To implement quantum maps in the laboratory, one typically engineers a physical dilation, which corresponds to a unitary evolution entangling the system with an environment. This work systematically explores how weak symmetries of quantum maps manifest in their dilations. We demonstrate that for various classes of physical dilations, including Hamiltonian-driven dilations and short-time collision models that simulate Markovian open quantum dynamics, weak symmetries always lead to strong symmetries in the dilated evolution, resulting in conserved quantities in the system-environment space. We also characterize the subspace where these symmetries arise using Krylov subspaces. Moreover, we show that some different types of physical dilations have no constraints on the dilated evolution, requiring no strong symmetry. Finally, we complement our findings with a variety of illustrative and pedagogical examples. Our results provide essential guidelines for constructing physical dilations of quantum maps, offering a comprehensive understanding of how symmetries shape their implementations in a laboratory or on a quantum computer.

System-bath correlations play a pivotal role in the study of open quantum systems and have emerged as an important feature in distinguishing between quantum and classical behavior in energy transfer and spectroscopy. A variety of classical models have been constructed to test the limits of purely classical mechanics to capture such effects and, thus, by elimination to identify features that are unique to quantum systems. Yet numerical modeling in such classical frameworks has been largely limited to perturbative approximations, leaving some doubt regarding the extent to which these models are capable of capturing system-bath correlation effects. To overcome this gap, we here apply the numerically exact hierarchical equations of motion (HEOM) framework to examine dynamics under a “classical density matrix” framework that follows strictly Newtonian dynamics. Under the assumption of a Drude-Lorenz spectral density, we show that the resulting HEOM framework becomes identical to the stochastic Liouville equation. By comparing numerically exact dynamical simulations under this model with various “flavors” of the quantum HEOM formalism and with experimental absorption and fluorescence data, we examine the various roles that system-bath interactions play in thermalization and spectroscopic line-shape functions. Although the numerically exact classical simulations do reveal transient system-bath correlations, these are generally weaker than their quantum counterparts and vanish as the system approaches thermal equilibrium to produce a factorized equipartition state. For experimentally relevant parameters, we find that the classical treatment provides a fairly accurate treatment of absorption spectroscopy but that loss of the quantum detailed balance condition leads to a (sometimes dramatic) failure in describing fluorescence spectra.

Complex numbers are widely used in quantum physics and are indispensable components for describing quantum systems and their dynamical behavior. The resource theory of imaginarity has been built recently, enabling a systematic research of complex numbers in quantum information theory. In this work, we develop two theoretical methods for quantifying imaginarity, motivated by recent progress within resource theories of entanglement and coherence. We provide quantifiers of imaginarity by the convex roof construction and quantifiers of the imaginarity by the least imaginarity of the input pure states under real operations. We also apply these tools to study the state conversion problem in the resource theory of imaginarity.

Quantum phase estimation (QPE) is a crucial subroutine in many quantum algorithms. It is used to estimate the eigenvalues of a unitary operator acting on a target quantum system. Conventional QPE is often expensive to implement because it requires a large number of ancilla qubits and the ability to perform quantum Fourier transform. Recent developments in iterative QPE reduce the implementation cost by repetitive uses of a single ancilla and classical postprocessing. However, both conventional and iterative schemes often require preparation of the target system in an eigenstate of the unitary operator, while it remains ambiguous to achieve QPE of multiple eigenvalues with no need of initial-state preparation. Here we clarify this issue by developing a theoretical framework based on sequential quantum channels for iterative QPE. We find that QPE of multiple eigenvalues can be efficiently realized for arbitrary initial target system state by actively utilizing the measurement backaction of iterative QPE on the target system with a long coherence time. Specifically, we investigate two iterative QPE schemes based on sequential Ramsey interferometry measurements (RIMs) of an ancilla qubit: (a) the repetitive scheme, which conducts repetitive RIMs to achieve the standard quantum limit in estimating the eigenvalues; (b) the adaptive scheme, which adjusts the parameters of each RIM based on prior measurement outcomes to attain the Heisenberg limit. In both schemes, sequential ancilla measurements generate sequential quantum channels on the target system, gradually steering it to the eigenstates of the estimated unitary operator, while the measurement statistics of the ancilla can reveal the embedded information about its eigenvalues with proper postprocessing. We demonstrate the analysis by simulating a central spin model and evaluate the performance and noise resilience of both schemes.

The energy levels and transition rates of promising laser-cooling candidate thorium anion ($\mathrm{T}{\mathrm{h}}^{-}$) are experimentally measured in the current work. Three excited states between the lower and upper states for the bound-bound electric dipole (E1) transition ${}^2{S}_{1/2}^o\leftrightarrow {}^4{F}_{3/2}^e$ of $\mathrm{T}{\mathrm{h}}^{-}$ are observed: ${}^4{G}_{5/2}^o, {}^4{F}_{9/2}^e$, and ${}^4{F}_{7/2}^o$, and their energy levels are determined to be 1847(13), 3166.8(59), and 3666(12) $\mathrm{c}{\mathrm{m}}^{-1}$, respectively. Meanwhile, the lifetime of the upper state ${}^2{S}_{1/2}^o$ of the E1 transition is experimentally determined to be 30(2) µs, substantially shorter than the previously calculated result, which makes $\mathrm{T}{\mathrm{h}}^{-}$ the most promising candidate for laser cooling of negative ions. Furthermore, the lifetimes of two other short-lived odd-parity excited states of $\mathrm{T}{\mathrm{h}}^{-}$ are also measured to be 62(5) and 53(3) µs, respectively.

Ionization of atoms by a strong laser field can be described using the improved strong-field approximation. The corresponding transition amplitude of high-order above-threshold ionization is presented in the form of a two-dimensional integral over the electron ionization time $t_0$ and the rescattering time $t$. This integral can be solved using the saddle-point (SP) method and the resulting $T$-matrix element is expressed as a sum (over the SP times $t_0$ and $t$) of the partial transition amplitudes. We address the problem of finding the solutions of the system of SP equations for the times $t_0$ and $t$. For a bichromatic linearly polarized laser field with the frequencies $r\omega$ and $s\omega$ ($r$ and $s$ are integers, $s>r$, and $\omega$ is the fundamental frequency) we found that there are $8s^2$ SP solutions per optical cycle. For one half of them the velocity of the electron emitted in the laser field polarization direction changes the sign at the rescattering time (we call such solutions backward-scattering solutions), while for the other half this velocity remains unchanged (these solutions we call forward-scattering SP solutions). For very short (or even negative) electron travel time we call these solutions backward-like and forward-like scattering SP solutions. For these solutions the imaginary parts of the times $t_0$ and $t$ become large so that the concept of real electron trajectories becomes questionable. Having such a classification, we found additional SP solutions even for the simplest case of a monochromatic linearly polarized laser field. For a bichromatic linearly polarized laser field with $s=2$ and equal component intensities we presented a detailed analysis of all 32 solutions per optical cycle, showing how the SP times $t_0$ and $t$ and the corresponding differential ionization rates depend on the photoelectron energy. We have also analyzed the case where the intensity of the second component decreases while the sum of the component intensities remains fixed.

We investigate the photoelectron momentum distributions (PMDs) resulting from ionization of Ar atoms by bicircular two-color (800- and 1600-nm) laser fields in the laser propagation plane. Our results demonstrate that the PMDs along the laser propagation direction reveal interference structures at a certain amplitude ratio of counter-rotating two-color laser fields. We attribute the observed photoelectron interference patterns along the laser propagation direction to temporal triple-slit interference. Furthermore, we show that the temporal triple-slit interference can be controlled by tuning the amplitude ratio. These findings have significant implications for high-resolution imaging of atomic and molecular structures, as well as quantum control using temporal triple-slit interference.

We investigate a generalized antiferromagnetic cluster $\mathit{XY}$ model in a transverse magnetic field, where long-range interactions decay algebraically with distance. This model can be exactly solvable within a free fermion framework. By analyzing the gap, we explicitly derive the critical exponents $\nu$ and $z$, finding that the relationship $\nu z=1$ still holds. However, the values of $\nu$ and $z$ depend on the decaying exponent $\alpha$, in contrast to those for the quantum long-range antiferromagnetic Ising chain. To optimize scaling behavior, we verify these critical exponents using correlation functions and fidelity susceptibility, achieving excellent data collapse across various system sizes by adjusting fitting parameters. Finally, we compute the entanglement entropy at the critical point to determine the central charge $c$, and find it also varies with $\alpha$. This study provides insights into the unique effect of long-range cluster interactions on the critical properties of quantum spin systems.

Transmitting energy and information are two essential aspects of nature. Recent findings suggest they are closely related, while a quantitative equivalence between them is still unknown. This thus motivates us to ask the following: Can information transmission tasks equal certain energy transmission tasks? We answer this question positively by bounding various one-shot classical capacities via different energy transmission tasks. Such bounds provide the physical implication that in the one-shot regime, transmitting $n$ bits of classical information is equivalent to $n\times k_{B}Tln2$ transmitted energy. Unexpectedly, these bounds further uncover a dynamical version of Landauer's principle, showing the strong link between transmitting (rather than erasing) information and energy. Finally, in the asymptotic regime, our findings further provide thermodynamic meanings for the Holevo-Schumacher-Westmoreland Theorem and a series of strong converse properties as well as no-go theorems.

This paper introduces a novel class of upper and lower bounds for a set of Hermitian operators with $K$ elements. These bounds, derived from the Löwner partial ordering, represent the maximal lower and minimal upper bounds for a pair of Hermitian operators. Additionally, the paper redefines the minimum Bayes cost and minimum error quantum state discrimination problems by identifying the maximal lower and minimal upper bounds for quantum density operators. These proposed bounds are then utilized to establish a tight bound for the error probability of quantum state discrimination problem. Furthermore, the paper provides a geometric interpretation of these bounds, offering valuable insights into the quantum state discrimination problem and aiding in the understanding of the optimal measurement strategy, especially for qubits.

Quantum secret sharing (QSS) is set to revolutionize secure communication. However, achieving practical implementations that balance efficiency, security, and flexibility remains a significant challenge. To address this issue, we propose an innovative QSS scheme that integrates a verification mechanism for the distribution process by leveraging quasicausal cones within the multiscale entanglement renormalization ansatz framework. This approach ensures the secure dissemination of entangled photons across a multiuser network while enhancing the integrity of the quantum state against disturbances. A key feature of our scheme is the application of quasicausal cones, which improve the system's fault tolerance and recovery precision, preserving quantum state coherence and stability even in the presence of errors or losses. The recursive and hierarchical design of our framework enables it to dynamically adapt to fluctuations in network size and the number of participants, making it well suited for large-scale quantum networks. Furthermore, our method reduces the costs associated with photon transmission and storage, enhancing resource allocation within expansive quantum networks. The strategic incorporation of quasicausal cones represents a major advancement, streamlining the flow of quantum information and minimizing the overall resource footprint. By flexibly accommodating varying network sizes and configurations, our scheme marks a significant step towards efficient large-scale QSS, underscoring the critical role of quasicausal cones in advancing quantum communication technologies.

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.