We investigate the information distribution among different entities in the weak-measurement protocol. Focusing on multilevel, decaying systems under continuous (no-click) monitoring, we derive exact, conservation-type information relations that hold for each outcome in the record. Analogous relations hold when an explicit reversal is applied, with the reversal success probability entering the relation. We extend the framework to finite-count outcomes (arbitrary photon numbers), obtaining quantitative trade-offs that link information change in the weak-measurement process to the entities to which the information is distributed. These results provide a unified, outcome-resolved account of information flow in monitored open quantum dynamics and provide insight into a deeper understanding of open-system dynamics and its control.

Enhancing quantum illumination with highly entangled probes remains an active area of research. In this context, non-Gaussian operations provide an effective route for engineering probe states that can surpass the standard two-mode squeezed state (TMSS). In this work, we investigate a specific nonlocal non-Gaussian operation protocol and show that the engineered state using this protocol outperforms previously considered local non-Gaussian scenarios, engineered based on photon catalysis, addition, and subtraction under realistic conditions, including photon loss. Furthermore, by employing a 50:50 beam splitter with photon-number difference detection, we demonstrate a significant enhancement in the signal-to-noise ratio (SNR) for target detection relative to the TMSS. Thus, our protocol exhibits improved performance, highlighting a resource-efficient and experimentally feasible probe for enhanced quantum illumination.

We study trade-off relations in information extraction from quantum systems subject to null-result weak measurements, where the absence of a detected photon continuously updates the system state. We present a detailed analysis of qubit and qutrit systems and investigate a general framework for a multilevel quantum system. We develop a dynamical characterization of null-result weak measurements that quantifies the information extracted over time, revealing the amount of the obtained information and also the rate of the information accumulation. The characterizations are obtained by examining the time-dependent evolution of the information theoretic quantities. More specifically, we consider Shannon entropy, mutual information, fidelity and relative entropy to characterize the weak measurement dynamics. Our results provide an information theoretic analysis of the weak measurement process and highlight the dynamical nature of information extraction and reversibility in the weak measurement processes.

We propose PIFLUX, a single-molecule localization scheme combining deep-subwavelength plasmonic illumination with widefield detection. Interference between counter-propagating gap plasmons and a normally incident optical field generates an illumination pattern whose position can be tuned through the plasmon phase while preserving its spatial period. A Cramér-Rao analysis shows PIFLUX reaches few-nanometer precision matching MINFLUX while doubling that of SIMFLUX over a micrometer field of view, and a maximum-likelihood estimator confirms this on a synthetic nuclear pore complex.

Giant atoms enable precise control over single-photon dynamics in waveguide-based quantum circuits through spatially distributed interactions with the guided electromagnetic modes. Recent advancements in the realm of superconducting circuits allow for experimental implementation of giant atoms. In this work, we describe a microscopic model of a transmon qubit coupled to a transmission line and show that its Hamiltonian maps onto a generalized form for giant-atom-waveguide interactions. We demonstrate the generation of a frequency comb from a system comprising a single-photon Gaussian pulse incident on a cavity with giant-atom mirrors coupled to a one-dimensional waveguide. The finesse of this single-photon frequency comb can be significantly improved by using giant atoms instead of point atoms. Furthermore, this high-finesse cavity enables a substantial enhancement of the peaks in the comb spectra when the photon pulse is initially inside the giant-atom cavity. Our work provides a platform for the design of advanced quantum devices for high-resolution spectroscopy and quantum computation.

We present a scheme for achieving broadband complete reflection by constructing photonic bandgap via collective atom-atom interaction in a one-dimensional (1D) waveguide quantum electrodynamics (QED) system. Moreover, we propose several strategies to further expand the ultrahigh reflection windows, including increasing the number of atoms with separations near the Bragg distance and inducing gradient frequency modulation among the atoms. The center frequency and bandwidth of the ultrahigh reflection window are dynamically adjustable by applying external electromagnetic field. The results here can enrich the many-body physics of waveguide-QED system and offer a pathway for achieving broadened ultrahigh reflection in a controllable way, which can find important applications in the realms of chip-integrated band filter, quantum storage, optical switching, and wavelength-selective devices.

The non-Bloch topology leads to the emergence of various counterintuitive phenomena in non-Hermitian systems under the open boundary condition, which can not find a counterpart in Hermitian systems. However, in the non-Hermitian system without chiral symmetry, being ubiquitous in nature, exploring its non-Bloch topology has so far eluded experimental effort. Here, by introducing the concept of nonchiral non-Bloch invariants, we theoretically predict and experimentally identify the non-Bloch topological phase diagram of a one-dimensional (1D) non-Hermitian system without chiral symmetry in discrete-time nonunitary quantum walks of single photons. Interestingly, we find that such topological invariants not only can distinguish topologically distinct gapped phases, but they also faithfully capture the corresponding gap closing in open-boundary spectrum at the phase boundary. Different topological regions are experimentally identified by measuring the featured discontinuities of the higher moments of the walker's displacement, which amazingly match excellently with our defined non-Bloch invariants. Our work provides a useful platform to study the interplay among topology, symmetries, and the non-Hermiticity.

Giant atoms introduce promising avenues for enhancing light-matter interaction in waveguides, which plays a crucial role in quantum information processing and quantum state control. In this work, we study the dynamics of a system comprising a single-photon Gaussian pulse initially inside a cavity with giant-atom mirrors coupled to a one-dimensional waveguide. The decay rate of the photon pulse can be significantly minimized by using giant atoms instead of point atoms and by carefully selecting the spacing between the coupling points of the giant atom to the waveguide. The quality factor for the cavity is analyzed by varying the number of atoms, coupling points, and coupling point spacing in each mirror to find the optimal parameters for a high $Q$-factor cavity. Our work contributes to the design of more efficient, high-quality quantum devices such as integrated quantum photonic circuits.

Gaussian states with nonclassical properties such as squeezing and entanglement serve as crucial resources for quantum information processing. Accurately quantifying these properties within multi-mode Gaussian states has posed some challenges. To address this, we introduce a unified quantification: the 'classical-nonclassical polarity', represented by $\mathcal{P}$. For a single mode, a positive value of $\mathcal{P}$ captures the reduced minimum quadrature uncertainty below the vacuum noise, while a negative value represents an enlarged uncertainty due to classical mixtures. For multi-mode systems, a positive $\mathcal{P}$ indicates bipartite quantum entanglement. We show that the sum of the total classical-nonclassical polarity is conserved under arbitrary linear optical transformations for any two-mode and three-mode Gaussian states. For any pure multi-mode Gaussian state, the total classical-nonclassical polarity equals the sum of the mean photon number from single-mode squeezing and two-mode squeezing. Our results provide a new perspective on the quantitative relation between single-mode nonclassicality and entanglement, which may find applications in a unified resource theory of nonclassical features.

Hydrogen is the most dominant atom in the universe and is considered the main component of baryonic matter. Thus far, the quantum features of the unbounded hydrogen atoms in the background of the universe and the possibility of emerging unique quantum effects, such as entanglement on the cosmological scale, have not been considered. In this work, we demonstrate that the dynamical expansion of the universe leads to the emergence of natural entanglement in the hyperfine structure of atomic hydrogen. Our findings reveal that there exists a critical age for the universe where hydrogen atoms naturally build up entanglement, resulting from the expansion of the universe. More precisely, when the universe reaches the age of about 2.5 x 1018 seconds (about 80 billion years old), the hyperfine structure entanglement in hydrogen atoms naturally takes off, demonstrating a peculiar quantum phenomenon known as entanglement sudden birth. This expansion-induced entanglement becomes maximum at about 3.6 x 1018 seconds (about 115 billion years), after the Big Bang. By analyzing the fate of seed atoms formed in the early universe, this study underscores the significance of unique quantum mechanical features, such as entanglement, on cosmological scales.

AbstractCat states, as an important resource in the study of macroscopic quantum superposition and quantum information applications, have garnered widespread attention. To date, preparing large-sized optical cat states has remained challenging. We demonstrate that, by utilizing interaction-free measurement and the quantum Zeno effect, even a fragile quantum microscopic system can deterministically control and become entangled with strong light fields, thereby generating large-amplitude optical cat states. During the entire preparation process, our method ensures that the microscopic system functions within a weak field environment, so that its quantum property can be protected. Furthermore, we show that the preparation of cat states is possible even when the quantum microsystem suffers from significant photon loss, provided that optical losses from classical devices are kept low, which implies that the fidelity of the cat state can be enhanced by improvements to and the perfection of the classical optical system.

>Symmetry permeates throughout science, underlying people's understanding on intriguing physical properties. These include conserved quantities, critical phenomena and selection rules. Dynamical symmetry, the counterpart of geometrical symmetry or static symmetry referring to the geometrical shape or structure of a physical system, is present in the motion behavior.

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.

In this work we investigate quantum interference in a four-level atom coupled to a negative index meta-material (NIMM) anisotropic plasmonic environment that supports both TE and TM polarized surface plasmons (SP). The analysis confirms the creation of the anisotropic environment and two dipoles can interfere with each other even if they are orthogonal by sharing such SP modes. The NIMM/plasmonic environment provides more options to control SP interaction with emitters and their spontaneous emission decays and spectrum. The spectrum depends critically on structure parameters, mode frequency, frequency dependent electric permittivity and magnetic permeability, and the location of the atom etc. We observe orders of magnitudes enhancement in the plasmonic-modified decays and spectrum compared to free space case.

The spin angular momentums of surface plasmon polaritons (SPPs) on chiral material interfaces and the Imbert-Fedorov shifts of linearly polarized light beams are investigated. Compared to a traditional TM-polarized SPP having a transverse spin, the SPP on a chiral material interface also has a longitudinal spin component, resulting from the nature that this new kind of SPP is a hybrid of TE and TM-polarized evanescent waves. When a light beam is incident on a sandwich structure composed of chiral material, prisms, and metal layers, in which the SPP is supported, the reflection and transmission processes can be analogous to the transport of a photon in a waveguide QED system. The SPP with longitudinal spin can be excited by the incident wave and the reflected and transmitted beams carry the spin features of the SPP. Moreover, the beams exhibit large Imbert-Fedorov shifts stemming from the spin-orbit coupling even for a linearly polarized incident beam. The shifts are determined by the longitudinal spin angular momentum and excitation coefficient of the SPP. This present work extends the study of photonic spin-orbit coupling and provides an important platform to investigate the plasmonic spin.

In this study, we explore the breaking of time-reversal symmetry in a scalable cavity quantum electrodynamics (QED) lattice. Such a lattice consists of triangular cells of three cavities coupled to a two-level atom. We synthesize artificial magnetic fields to enable the chiral transfer of photons by sinusoidally modulating the cavity frequencies. Considering various configurations with different numbers of cells in the system, we analyze the circulation of photons and investigate the effect of the frequency modulation on the control of the state transfer in the system. We show the breaking and sustaining of time-reversal symmetry in certain system geometries and consider the scalability of the generating synthetic magnetic field in the system. Our study highlights the potential of scalable cavity QED lattices with synthetic magnetic fields as a versatile tool for investigating quantum phenomena and shows the utility of these systems to serve as test beds for the simulation of condensed matter systems in quantum optical settings.

We study the emission of a single-photon by a two-level emitter inside an atomic cavity consisting of two atomic mirrors coupled to a one-dimensional waveguide. With proper atomic separations, we realize a frequency comb as well as spectrum narrowing in the waveguide with the symmetric setting, and also unidirectional spectrum narrowing with asymmetric parameters. Within a suitable range, we can control the central frequency of the narrowed spectra by modulating atomic separations. Due to the collective interaction among the emitters, entanglement sudden birth and revival are observed between the mirrors.

The notion of wave–particle duality remains one of the most debated subjects in the history of quantum physics. The most famous debate on the subject occurred between Bohr and Einstein. In this work, we revisit the wave–particle duality in the Bohr–Einstein debate from the viewpoint of the recently established duality-entanglement relation. We show that the duality-entanglement relation can provide a valuable framework for quantitative analysis of the Einstein's gedanken double-slit experiment and clarify some of its fundamental aspects.

It is well-known that the precision of a phase measurement with a Mach-Zehnder interferometer employing strong classic light can be greatly enhanced with the addition of weak nonclassical light. In the context of quantifying nonclassicality, the amount by which a nonclassical state can enhance precision in this way has been termed its ’metrological power’. To-date, the enhancement provided by weak nonclassical states has been calculated only for specific measurement configurations. Here we are able to optimize over all measurement configurations to obtain the maximum enhancement that can be achieved by any single or multi-mode nonclassical state together with strong classical states, for local and distributed quantum metrology employing any linear or nonlinear single-mode unitary transformation. Our analysis reveals that the quantum Fisher information for quadrature-displacement sensing is the sole property that determines the maximum achievable enhancement in all of these different scenarios, providing a unified quantification of the metrological power.