Non-Hermitian operators are now routinely used to describe few-mode systems such as optical resonators and superconducting qubits, and exceptional points (EPs) are defective spectral singularities of such non-Hermitian operators. In contrast, the scattering-theoretic formulation of EP physics for unbounded Hamiltonians remains less settled. In this work, we formulate the geometric phase associated with encircling an EP when the underlying eigenstates are quantum resonances within a one-dimensional scattering model. To do this, we employ the complex-scaling method, where resonance poles of the S matrix are realized as discrete eigenvalues of the non-Hermitian dilated Hamiltonian, to construct situations in which resonant and scattering states coalesce into an EP in the complex energy plane, that is, the resonance pole is embedded into the continuum spectrum. We analyze the self-orthogonality in the vicinity of an EP, the Berry phase, and the Chern characteristic. Our results clarify how EP branch structure and geometric holonomy arise directly from resonance poles in scattering theory, thereby connecting non-Hermitian spectral topology with the traditional theory of quantum resonances.

In this work, we propose an efficient quantum-enhanced solid-state photocell based on GaN quantum dots (QDs). We exploit the strong built-in electric field in GaN QDs and the excitonic dipole-dipole interaction between adjacent QDs to break detailed balance, thereby enhancing device performance. This mechanism is significantly stronger than Fano interference, and our results demonstrate that such a photocell exhibits increased photovoltage and photocurrent compared to its noninteracting counterparts. Numerical calculations further show that the efficiency remains positive and saturates at a finite value for multi-QD systems. The proposed quantum photocell represents a promising step towards harnessing quantum effects in practical energy-harvesting devices.

An approach for analysis of effective interaction between pre- and postselected quantum particles is developed. It is argued that the cases of complete pre- and postselection of particles are more profound than the cases of partial pre- and postselection, since the former goes beyond modification of the average of interactions on an ensemble of experiments. Recently discussed paradoxical phenomena such as the pigeonhole paradox and the modification of the interaction from repulsion to attraction are analyzed within the introduced formalism, and a few surprising examples are presented.

The optical pumping process is a fundamental tool for quantum protocols using the electronic and nuclear spins in the nitrogen-vacancy (NV) center platform. In this work, we explore the process of electronic spin polarization through optical pumping from a thermodynamic perspective. We identify the sources of work and heat and show that the heat current is directly related to the experimentally accessible fluorescence of the NV center. We also show that the polarization of the electronic spin depends on the amount of energy that is provided to the system by the laser pump. Moreover, we study the von Neumann entropy change in the process. We demonstrate that the entropy can be separated into two contributions: one due to the heat produced and the other due to the energy provided by the laser, which is a consequence of the nonunitary nature of the pumping process. Finally, we show that increasing the power of the laser results in an increased entropy of the final state, which hinders the polarization efficiency.

Despite the intrinsic coupling between electric and magnetic fields in random stationary light, their polarization properties are not mutually determined. A complete second-order description thus necessitates a joint electromagnetic treatment. The $6\times 6$ electromagnetic polarization matrix introduced here generalizes the conventional electric $3\times 3$ matrix by incorporating both electric and magnetic contributions together with their mutual correlations. It consists of diagonal $3\times 3$ blocks, representing the electric and magnetic polarization matrices, and off-diagonal $3\times 3$ blocks that encode the full structure of electric-magnetic cross correlations. The information contained in this matrix can be interpreted through physically meaningful quantities such as active and reactive energy fluxes, in-phase and quadrature alignment matrices, and global indices describing electric-magnetic coupling. The formalism is applied to a field composed of two orthogonally propagating plane waves sharing a common linear electric polarization (a simple yet physically realizable configuration) that demonstrates the need for a general combined electric-magnetic representation even in free space. This approach provides a comprehensive and unified framework for characterizing electromagnetic polarization beyond the electric-field description alone, bridging classical statistical optics with quantumlike density-matrix interpretations.

The foundational work by Bell led to an interest in understanding nonlocal correlations that arise from entangled states shared between distinct, spacelike-separated parties, which formed a foundation for the theory of quantum information processing. We investigate the question of maximal correlations analogous to the maximally entangled states defined in the entanglement theory of multipartite systems. In this work, we define the maximality of nonlocal correlation as being analogous to the absolutely maximally entangled state. To formalize this, we employ the sheaf-theoretic framework for contextuality, which generalizes nonlocality. This provides a metric for correlations called the contextual fraction, which ranges from 0 (noncontextual) to 1 (maximally contextual). Using this, we have defined the absolutely maximal contextual correlations (AMCCs), which are maximally contextual and have maximal marginals. The Popescu-Rohrlich box serves as the bipartite example, and we construct various extensions of such correlations in the tripartite case. An infinite family of various forms of AMCC is constructed using the parity-check and the constraint-satisfaction-problem construction. We also demonstrate the existence of maximally contextual correlations, which do not exhibit maximal marginals, and refer to them as non-AMCC. The results are further applied to secret sharing and randomness extraction using AMCCs.

The emerging field of quantum thermodynamics is beginning to reveal the intriguing role that information can play in quantum thermal engines. Information enters as a resource when considering feedback-controlled thermal machines. While both a general theory of quantum feedback control as well as specific examples of quantum feedback-controlled engines have been presented, still lacking is a general framework for such machines. Here, we present a framework for a generic, two-stroke quantum heat engine interacting with $N$ thermal baths and Maxwell's demon. The demon performs projective measurements on the engine working substance, the outcome of which is recorded in a classical memory, embedded in its own thermal bath. To perform feedback control, the demon enacts unitary operations on the working substance, conditioned on the recorded outcome. By accounting for the thermodynamic cost of measurement, and hence considering the machine-memory compound as a hybrid (quantum-classical) standard thermal machine interacting with $N+1$ thermal baths, our framework puts the working substance and memory on equal footing, thereby enabling a comprehensible resolution to Maxwell's paradox and elucidating the intricate manner in which information impacts the performance of quantum engines. We illustrate the application of our framework with a two-qubit engine. A remarkable observation is that more information does not necessarily result in better thermodynamic performance: sometimes knowing less is better.

Spin defects in two-dimensional materials are a promising platform for quantum sensing. Simulating the defect's optical response and optically detected magnetic resonance (ODMR) contrast is key to identifying suitable candidates. However, existing simulation methods are typically unable to supply the required accuracy. Here, we propose two quantum algorithms to detect an imbalance in the triplet-to-singlet intersystem crossing (ISC) rates between excited states with the same and different spin projections, a necessary condition for nonzero ODMR response. The lowest-cost approach evaluates whether the evolution of an $S=0$ state under the spin-orbit coupling induces ISC to $S=1$, and also whether there is an imbalance in its intensity depending on the final-state spin projection. The second approach works by comparing the emission spectrum of a spin defect with and without the spin-orbit coupling operator, inferring ISC intensity for different spin transition channels from spectrum intensity changes. Additionally, we present an improved scheme to evaluate the defect's optical response, building upon previous work. We study these quantum algorithms in the context of the negatively charged boron vacancy in hexagonal boron nitride. We generate an embedded active space of 18 spatial orbitals using quantum defect embedding theory and show that the ISC rate imbalance can be detected with as few as 105 logical qubits and $4.41\times {10}^8$ Toffoli gates. By avoiding direct and costly rate calculations, our methods enable faster screening of candidate defects for ODMR activity, advancing the prospect of using quantum simulations to aid the development of high-performing sensing devices.

Losses are ubiquitous in physics and are usually regarded as harmful in quantum information processing. Here, we propose a loss-induced scheme to achieve nonreciprocity and nonreciprocal entanglement in a superconducting platform, where two remote superconducting transmon qubits are connected via two lossy auxiliary cavities. The nonreciprocity in our scheme originates from interference between multiple lossy coupling paths. The coherent phases associated with the qubit-resonator couplings reverse sign under propagation reversal, while the loss-induced phases remain direction independent. Their combined effect leads to different interference conditions in the opposite directions, resulting in unequal effective couplings. We show that this loss-induced scheme can generate nonreciprocal quantum entanglement, indicating that loss can be utilized as a resource. Moreover, the tunability of nonreciprocity and nonreciprocal entanglement in our scheme can be manipulated by the relative phase induced by loss, allowing to tailor both reciprocal and nonreciprocal behaviors. Our results establish a direct link between engineered loss and nonreciprocal entanglement in quantum information processing and offer potential applications in scalable quantum networks.

We report the theoretical prediction and measurement of a tune-out wavelength for the ground state of the thulium atom in a linearly polarized optical dipole trap with a wavelength of approximately 576 nm. The measurements were conducted using a combination of trap frequency and RF loss spectroscopy, thus making it possible to separate the scalar and tensor parts of the total polarizability without measurements in the range of negative total polarizability. The calculated tune-out wavelength is consistent with the measured one of $575.{646}_{-0.014}^{+0.016}$ nm in air. The existence of the zero in the polarizability for the Tm ground state was confirmed by the trap loss experiment, which also made it possible to refine the tune-out wavelength to $575.{646}_{-0.004}^{+0.004}$. Also, it was experimentally demonstrated that, with a proper choice of the dipole trap polarization, it was possible to achieve Bose-Einstein condensation of thulium atoms in the range from 575.348 to 575.689 nm, covering the tune-out wavelength, thus we infer that the imaginary part of polarizability, or scattering of trap photons is insignificant.

We show that a periodically driven two-leg flux ladder hosting interacting hardcore bosons exhibits a prethermal Meissner phase for large drive amplitudes and at special drive frequencies. Such a prethermal Meissner phase is characterized by a finite time-averaged chiral current. We find an analytic expression of these frequencies using Floquet perturbation theory. Our analysis reveals that the presence of the prethermal Meissner phase is tied to the emergence of strong Hilbert space fragmentation in these driven ladders. We support our analytical results by numerical study of finite-size flux ladders using exact diagonalization, and we discuss experiments using ultracold dipolar atom platforms that may test our theory.

We investigate the zero-temperature phase diagram of a one-dimensional constrained quantum spin chain realized in coherently driven Rydberg-atom arrays with competing local Rabi driving and dipole-dipole exchange interactions. Projecting onto the blockade-constrained Hilbert space yields an effective model in which local and nonlocal quantum fluctuations compete on equal footing. Combining exact diagonalization, the density-matrix renormalization group, and variational uniform matrix-product-state calculations, we establish a complete phase diagram comprising a Luttinger liquid, an antiferromagnetic ordered phase, and a polarized paramagnet. We identify two distinct mechanisms for the destruction of antiferromagnetic order: a conventional Ising transition at strong driving and a continuous quantum melting into the Luttinger liquid at weak driving, characterized using entanglement-based diagnostics and finite-entanglement scaling. In addition, we uncover an exact ground-state factorization line embedded within the ordered phase, providing an analytically tractable zero-entanglement reference point for experiments with programmable Rydberg quantum simulators.

In this paper, we study the propagation of structured light in fiber resonators that in effect are generalizations of the Möbius strip. In particular, we address two examples of such systems: ring resonators based on strongly elliptical twisted two-mode fibers and on bielliptical fibers composed of two consecutive sections of elliptical fibers with different orientations of deformation ellipses. We study formation of whispering gallery modes (WGMs) in such systems. By analyzing orbital Stokes parameters of such WGMs we prove that those modes may form in certain cross-section optical vortices (OVs) with modulus unity topological charges. We demonstrate that the ability of resonators with effective topology of a Möbius strip to possess of vortex WGMs can be used for generation of OV frequency combs from Laguerre-Gaussian modes. We emphasize that to guarantee generation of vortex comb in a Möbius ring resonator parameters should be such as to warrant the existence of vortex WGMs in the resonator. We also study the effect of the spin-orbit interaction (SOI) on generation of OV frequency combs by Möbius-strip-like resonators on base of elliptical twisted fibers. We study the influence of the SOI and show that its presence does not prevent implementation of vortex WGMs in orthogonal polarizations and generation of OV combs in those polarizations.

Exceptional points, where two or more eigenstates of a non-Hermitian system coalesce, are now of interest across many fields of physics, from the perspective of open-system dynamics, sensing, nonreciprocal transport, and topological phase transitions. In this work, we investigate exceptional points in cavity optomechanics, a platform of interest to diverse communities working on gravitational-wave detection, macroscopic quantum mechanics, quantum transduction, etc. Specifically, we clarify the role of quantum jumps in making a clear distinction between Liouvillian and Hamiltonian exceptional points in optomechanical systems. While the Liouvillian exceptional point arises from the unconditional Lindblad dynamics and is independent of the phonon-bath temperature, the Hamiltonian exceptional point emerges from the conditional no-jump evolution and acquires a thermal shift due to an enhanced conditional damping. Employing the thermofield formalism, we derive a unified spectral framework that interpolates between these regimes via an analytical hybrid-Liouvillian description. Remarkably, in the weak-quantum-jump regime, the exceptional point is perturbed only at the second order, highlighting the robustness of the Hamiltonian exceptional point under small hybrid perturbations. Our work reveals a continuous family of hybrid exceptional points, clarifies the operational and physical differences between the conditional and unconditional dissipative dynamics in optomechanical systems, and provides a probe for thermal baths.

In this paper, we construct a one-dimensional photonic quasicrystal by combining two incommensurate spatial harmonics, where the ratio of their periods is the irrational number $\beta$. We evaluate the photonic quasicrystal accurately by a generalized spectral method that embeds the quasiperiodic structure into a higher-dimensional periodic system. We study the spectral distribution of one-dimensional photonic quasicrystals and find some interesting phenomena. As the computational resolution $N$ increases, there are more eigenvalues within finite frequency bandwidths, and the maximum localization always occurs at spectral gap edges for states near the index $N+1$. By varying $\beta$ within the range of $(0,1)$, we present a butterfly shaped spectral structure with abundant band gaps. We find that the spectral structure factor $Q$ (defined as $I_{mg}/N$, where $I_{mg}$ is the maximum gap index) exhibits different linear patterns as $\beta$ changes: $Q=1-\beta$ when $\beta <{\beta }_c$, while $Q=\beta$ when $\beta >{\beta }_c$, where ${\beta }_c\approx 0.424$ is the transition point. This linear relationship holds robustly in the strong quasiperiodic regime ($\beta$ away from 0 or 1) and is independent of the specific type of irrational number used. The relationship disappears (weak quasiperiodic regime) near $\beta =0$ or $\beta =1$. It demonstrates that the intrinsic spectral properties of one-dimensional photonic quasicrystals are fundamentally governed by the magnitude of the irrational parameter $\beta$.

We present a general framework for the Wigner distribution function (WDF) of Helmholtz–Gauss (HzG) wave fields. The WDF is written as a rapidly convergent series of modified Bessel functions and is completely determined by the complex Fourier coefficients of the spectral function of the underlying nondiffracting Helmholtz beam. Using these same coefficients, we also obtain the normalization constant of any HzG wave as a rapidly convergent series, and show how symmetry constraints on the Fourier coefficients induce corresponding symmetry relations of the WDF in phase space. We prove that the HzG WDF has the universal structure of a four-dimensional Gaussian in phase space, multiplied by a modulation that, in general, depends on the Hamiltonian, orbital angular momentum, and two angles, all expressed in terms of phase-space variables. We apply this formalism to the four fundamental HzG families, presenting explicit expressions for the WDFs of the Mathieu–Gauss, parabolic-Gauss, and cosine- and sine-Gauss beams, and confirming previous results for Bessel–Gauss beams. The formalism can be readily applied to any superposition of HzG waves. Finally, we derive a secondary representation that yields an equivalent expansion in products of Laguerre–Gauss functions, connecting the general HzG beam to the eigenmodes of the paraxial wave equation.

We investigate microwave interference between a spin ensemble and its mirror image in a one-dimensional (1D) waveguide. Away from the mirror, the resonance frequency of the Kittel mode (KM) inside a ferrimagnetic spin ensemble displays sinusoidal shifts (compared to a setup without the mirror) as a function of the distance to the mirror. Furthermore, the radiative decay of the KM into the waveguide shows a cosine-squared oscillation as the distance to the mirror is varied. These shifting frequencies and decay rates are a consequence of the KM's interaction with its own image, mediated by the waveguide. The decay is enhanced twofold when the spin ensemble sits at the magnetic antinode of the corresponding eigenmode in the waveguide; conversely, we measure an extension of the KM lifetime by a factor of 8 when the spin ensemble is approximately at a node. We can finely tune the KM decay rate to achieve maximal absorption of input photons at a critical coupling point. Moreover, by pumping the spin ensemble to generate a pump-induced magnon mode in a two-tone experiment, we achieve a pump-induced phase shift of up to about 90° for a coherent microwave probe field.

We present experimental results demonstrating the quantum interference of two photons distinguishable in their transverse momenta, each entering the input ports of a balanced beam splitter. This counterintuitive interference effect is made possible through spatially resolved measurements in the near field, i.e., by resolving the conjugate variable in which the photons are distinguishable. Our experimental findings agree with theoretical predictions. We contrast our results with a nonspatially resolved measurement where averaging over the photons' positions washes out the quantum interference observed in spatially resolved measurements.

Interactions between charged particles and light occur in real space and time, yet quantum field theory usually describes them in momentum space. While this approach is well-suited for calculating emission probabilities and cross sections, it is insensitive to spatial and temporal phenomena such as radiation formation, quantum coherence, and wave-packet spreading. These effects are becoming increasingly important for experiments involving electrons, photons, atoms, and ions, particularly with the advent of attosecond spectroscopy and metrology. Here, we propose a general method for describing the emission of photons in phase space via a Wigner function. Several effects for Cherenkov radiation are predicted, absent in the classical realm or in quantum theory in momentum space, such as a finite spreading time of the photon, a finite duration of the flash, and a quantum shift of the photon arrival time. The photon spreading time turns out to be negative near the Cherenkov angle, the flash duration is defined by the electron packet size, and the temporal shift can be both positive and negative. The characteristic timescales of these effects lie in the atto- and femtosecond ranges, thereby illustrating atomic origins of these macroscopic phenomena. The near-field distribution of the photon field resembles the electron packet shape, thus making “snapshots” of the emitter wave function. Our approach can easily be generalized to the other types of radiation and extended to scattering, decay, and annihilation processes, bringing tomographic methods of quantum optics to particle physics.

We investigate the generation of antibunched photon-magnon pairs in a hybrid quantum system that integrates a yttrium iron garnet (YIG) sphere and two microwave cavities. Through a two-mode two-photon drive, strongly correlated photon pairs are initially excited in the two Kerr-nonlinear cavities. One photon mode then coherently couples to the magnon mode in the YIG sphere, enabling the conversion of photon pairs into hybrid photon-magnon pairs, each composed of one photon and one magnon. The introduction of Kerr nonlinearity induces frequency shifts in the energy levels, facilitating a quantum blockade effect. Analysis of the second-order correlation functions for the photon and magnon modes demonstrates not only a conventional blockade induced by energy-level shifts, but also an unconventional blockade arising from destructive quantum interference. The Kerr nonlinearity required to achieve the latter can be weaker. Such strongly correlated photon-magnon pairs exhibit potential applications in quantum networks and quantum information processing.