I point out the mathematical correspondence between an incoherent imaging model proposed by my group in the study of quantum-inspired superresolution [M. Tsang, R. Nair, and X.-M. Lu, Phys. Rev. X 6, 031033 (2016)] and a noise spectroscopy model also proposed by us [M. Tsang and R. Nair, Phys. Rev. A 86, 042115 (2012); S. Ng et al., Phys. Rev. A 93, 042121 (2016)]. Both can be regarded as random displacement models, where the probability measure for the random displacement depends on unknown parameters. The spatial-mode demultiplexing (SPADE) method proposed for imaging is analogous to the spectral photon counting method proposed by Ng et al. for optical phase noise spectroscopy: Both methods are discrete-variable measurements that are superior to direct displacement measurements (direct imaging or homodyne detection) and can achieve the respective quantum limits. Inspired by SPADE, I propose a modification of spectral photon counting when the input field is squeezed: The output field is simply unsqueezed before spectral photon counting. I show that this method is quantum optimal and far superior to homodyne detection for both parameter estimation and detection, thus solving the open problems in the work of Tsang and Nair and Ng et al.

Information reconciliation (IR) ensures the correctness of quantum key distribution systems by correcting the error bits that existed in the sifted keys. In this article, we propose a polar-code-based IR scheme with the frozen-bit erasure strategy, where an equivalent transmission of sifted keys is conducted so that each frozen bit in the decoding procedure is erased to zero. Thus, our IR scheme is no longer limited by the assumption of true random numbers and can be implemented simply and efficiently. Furthermore, we implement the proposed IR scheme with the fast simplified successive-cancellation list decoder and its throughput reaches $0.68\mathrm{Mbps}$ with the yield of 0.8333; when the decoder list size is 16, the block size is $1\mathrm{Mb}$ and the quantum bit error rate is 0.02.

We present results of calculations of several processes resulting from positronium (Ps) collisions with antiprotons: antihydrogen formation, Ps breakup, and $n_{\mathrm{Ps}}$-changing collisions. Calculations utilize the quantum convergent close-coupling (CCC) method and the classical trajectory Monte Carlo (CTMC) method. We identify a region of Ps principal quantum numbers $n_{\mathrm{Ps}}$ and Ps energies where the classical description is valid and where the CCC calculations become computationally too expensive. This allows us to present the most complete and reliable set of cross sections in a broad range of $n_{\mathrm{Ps}}$ and initial orbital momentum quantum numbers $l_{\mathrm{Ps}}$ which are necessary for experiments with antihydrogen at CERN.

We give a general derivation of the fourth-order DDC formalism (a formalism proposed by Dalibard, Dupont-Roc, and Cohen-Tannoudji) for calculating the interaction energy between two ground-state multilevel atoms which are coupled to electromagnetic fields in a thermal bath at temperature $T$. Both the contributions of the thermal field fluctuations and the radiation reaction of atoms are separately identified. As an application of the formalism, we revisit the interaction energy of two static ground-state two-level atoms in a free space and discover new behaviors such as the $\sim T{L}^{-2}$ behavior of the van der Waals interaction energy in the region where ${\lambda }^{3/4}{T}^{-1/4}\ll L\ll \lambda$, with $\lambda$ the transition wavelength of the atoms and $L$ the interatomic separation, and an oscillatory behavior of the Casimir-Polder interaction energy as $L$ varies which is superimposed on the monotonic $\sim T{L}^{-6}$ dependence when $L\gg \lambda \gg T^{-1}$.

We present a detailed study of the generation of photon pairs by spontaneous four-wave mixing in a structure composed of two linearly uncoupled resonators, where energy can be transferred from one resonator to another only through a nonlinear interaction. Specifically, we consider the case of two racetrack-shaped resonators connected by a coupler designed to guarantee that the resonance comb of each resonator can be tuned independently and to allow the nonlinear interaction between modes that belong to different combs. We show that such a coupler can be realized in at least two ways: A directional coupler or a Mach-Zehnder interferometer. For these two scenarios, we derive analytic expressions for the pair-generation rate via single-pump spontaneous four-wave mixing and compare these results with that achievable in a single-ring resonator.

We have studied the role of chiral symmetry in a periodically kicked Jaynes–Cummings (KJC) model by freezing an initial phase. We show that commensurate kicks ($2\pi k$ periodicity with integer $k$) conserve the chiral symmetry in the KJC model under the resonant condition, while incommensurate kicks break the symmetry. The chiral symmetry preserves the phase of an initial state against phase fluctuations during the dynamical evolution, but broken chiral symmetry erases the initial phase. The frozen phase is preserved within a finite evolution time for slight deviations of the kick period from an integer multiple of $2\pi$ and small variations of detuning from the resonant condition. The chiral symmetry-protected phase information is noteworthy as it provides various uses in quantum computation and information.

We study the directional dependence of the coupling between a nanofiber-guided light field and a two-level atom with an electric quadrupole transition. We examine the situation where the nanofiber is aligned along the $z$ axis, the atom lies on the fiber transverse $x$ axis, the quantization axis for the atomic internal states is the other orthogonal transverse $y$ axis, the atomic upper and lower levels are the magnetic sublevels $M^{\prime }$ and $M$ of hyperfine-structure levels of an alkali-metal atom, and the field is in a quasilinearly polarized fundamental guided mode ${\mathrm{HE}}_{11}$ with the polarization $\xi =x$ or $y$. We find that the absolute value of the quadrupole Rabi frequency depends on the propagation direction of the light field in the cases of ($M^{\prime }-M=\pm 1, \xi =y$) and ($M^{\prime }-M=\pm 2, \xi =x$). We show that the directional dependence of the coupling leads to the directional dependence of spontaneous emission into guided modes. We find that the directional dependence of the atom-field coupling in the case of quadrupole transitions is not entirely due to spin-orbit coupling of light: there are some other contributions resulting from the gradient of the spatial phase factor of the field.

We investigate the quantum phase transition (QPT) in the XXZ central spin model, which can be described as a spin-$\frac{1}{2}$ particle coupled to $N$ bath spins. In general, the QPT is supposed to occur only in the thermodynamical limit. In contrast, we present that the central spin model exhibits a normal-to-superradiant phase transition in the limit where the ratio of the transition frequency of the central spin to that of the bath spins and the number of bath spins tend to infinity. We give the low-energy effective Hamiltonian analytically in the normal phase and the superradiant phase, and we find that the longitudinal interaction $\mathrm{\Delta }$ can significantly influence the excitation number and the coherence of the ground state. These two quantities are remarkably enhanced for the negative longitudinal interaction while suppressed for the positive longitudinal interaction. In addition, the finite-size effect on the central spin model is also illustrated through the mean-field analysis. We further exploit the quantum Fisher information to characterize the QPT and propose a measurement scheme that can be applied in practice. This work builds a connection between the qubit-spin systems and the qubit-field systems, which provides a possibility for the realization of criticality-enhanced quantum sensing in central spin systems.

In this paper, we focus on a scheme in which three high-quality-factor mechanical modes of a hexagonal boron nitride (hBN) membrane monolayer are coupled to a common optically addressable spin defect present in the membrane via magnetic field interaction. We show that this coupling induces an effective phonon-phonon interaction in the dispersive regime and under appropriate magnetic field and microwave modulation. We derive the Langevin-Heisenberg equations of motion for vibrational modes to analyze optimal parameter regimes for reaching a physically stable system. We also investigate the effect of coupling strength on purity and entanglement. Our results demonstrate that bipartite and genuinely tripartite steady-state entanglement between different vibrational modes of hBN may be achieved in a broad spectrum of experimental parameters. This study has the potential to enable scalability to be implemented for the generation of two-dimensional continuous-variable cluster states for universal quantum computation.

The three-body fragmentation of ${\mathrm{C}}_2\mathrm{H}{{}_2}^{3+}$ to ${\mathrm{H}}^{+}+{\mathrm{C}}^{+}+{\mathrm{CH}}^{+}$ as a consequence of one CH and one CC bond breaking is investigated by 50-keV/u ${\mathrm{Ne}}^{8+}$ impact. All three fragments are detected in coincidence with a scattered projectile (either ${\mathrm{Ne}}^{7+}$ or ${\mathrm{Ne}}^{6+}$) employing a reaction microscope, and their momentum vectors as well as the kinetic energies were obtained. Four distinguished structures are observed in the energy correlation spectra, indicating that abundant fragmentation mechanisms contribute to the ${\mathrm{H}}^{+}+{\mathrm{C}}^{+}+{\mathrm{CH}}^{+}$ channel. The Newton diagrams and Dalitz plots are employed to trace fragmentation mechanisms. We found that both the concerted fragmentation and the sequential pathway with CH bond breaking prior to CC contribute to this channel. The possible electronic states of the ${\mathrm{C}}_2\mathrm{H}{{}_2}^{3+}$ precursor that may contribute to the identified fragmentation mechanisms are analyzed with the help of quantum chemical calculations. Furthermore, the influence of the collision dynamics between the projectile and the target to the dissociation mechanisms is discussed by comparing the contributions from the reaction channel with transferring one electron while ionizing the other two, i.e., T1I2, and the reversed channel T2I1. The T2I1 channel is observed to be more efficient to initiate fragmentation mechanisms leading to higher kinetic-energy release.

We present a mechanism contributing to the detection of photoassociated long-range Rydberg molecules via pulsed-field ionization: ionic products, created by the decay of a long-range Rydberg molecule, modify the excitation spectrum of surrounding ground-state atoms and facilitate the excitation of further atoms into Rydberg states by the photoassociation light. Such an ion-mediated excitation mechanism has been previously called a Coulomb antiblockade. Pulsed-field ionization typically does not discriminate between the ionization of a long-range Rydberg molecule and an isolated Rydberg atom, and thus the number of atomic ions detected by this mechanism is not proportional to the number of long-range Rydberg molecules present in the probe volume. By combining high-resolution UV and rf spectroscopy of a dense, ultracold gas of cesium atoms, theoretical modeling of the molecular-level structures of long-range Rydberg molecules bound below $n{}^{2}P_{3/2}$ Rydberg states of cesium, and a rate model of the photoassociation and decay processes, we unambiguously identify the signatures of this detection mechanism in the photoassociation of long-range Rydberg molecules bound below atomic asymptotes with negative Stark shifts.

We report the computational discovery of complex, topologically charged, and spectrally stable states in three-dimensional multicomponent nonlinear wave systems of nonlinear Schrödinger type. While our computations relate to two-component atomic Bose-Einstein condensates in parabolic traps, our methods can be broadly applied to high-dimensional nonlinear systems of partial differential equations. The combination of the so-called deflation technique with a careful selection of initial guesses enables the computation of an breadth of patterns, including ones combining vortex lines, rings, stars, and vortex labyrinths. Despite their complexity, they may be dynamically robust and amenable to experimental observation, as confirmed by Bogoliubov–de Gennes spectral analysis and numerical evolution simulations.

Achieving spin squeezing within solid-state devices is a long-standing research goal due to the promise of their particularities, for instance, their long coherence times, the possibility of low-temperature experiments, or the integration of entanglement-assisted sensors on-chip. In this work, we investigate an interferometer-free four-color scheme to achieve spin squeezing of rare-earth-ion-doped crystals. The proposal relies on an analytic derivation that starts from a Tavis-Cummings model for light-matter interaction, providing microscopic insights onto spin-squeezing generation. We evince the spin-squeezing signature in the light intensity variance. We consider the two particular cases of europium- and praseodymium-doped yttrium orthosilicates, workhorses of quantum technology developments. We show that up to 8 dB of spin squeezing can be obtained with readily accessible experimental resources, including noise due to photon scattering. Our results for rare-earth-ion-doped crystals add to the promising properties of these platforms for manipulating many-body entangled states and for high-precision measurements.

We study the dispersion relation of Kelvin waves propagating along single- and half-quantum vortices in miscible two-component Bose-Einstein condensates based on the analysis of the Bogoliubov–de Gennes equation. With the help of the interpolating formula connecting the dispersion relations in low- and high-wave-number regimes, we reveal the nontrivial dependence of the dispersion relation on the intercomponent interaction through the change in the vortex-core size of the vortical component. We also find the splitting of the Kelvin wave dispersion into gapless and gapped branches when both components have overlapping single-quantized vortices.

Phase plays an essential role in both classical and quantum wave dynamics, which has motivated many scientists to study the phases of many different nonlinear waves. Among those nonlinear waves, the ones described by rational solutions have been given much attention, partly because some of them can describe rogue wave dynamics. We revisit the phase of several well-known one-dimensional rational solutions and clarify that sudden phase inversion processes indeed exist for rogue waves, in contrast to the ones reported before. Moreover, we investigate the phase properties of rogue waves and rational W-shaped solitons in the Hirota and Sasa-Satsuma model, for which some high-order physical effects are considered, i.e., the third-order dispersion delayed nonlinear response and self-steeping effects. The quite distinctive phases are uncovered for the rational W-shaped solitons with similar density profiles in the two models, by limitation analysis on the underlying topological vector potentials of other related nonlinear waves. These results indicate that the underlying topological vector potentials can be used to identify the phases of nonlinear waves, especially for localized waves with abrupt phase jumps.

We predict a wide class of two-dimensional vector dissipative solitons in wide-aperture semiconductor vertical-cavity surface-emitting lasers with a saturable absorber. For them, polarization structure varies over the laser cross section and includes singularities: $\mathsf{V}$ points with zero total intensity, $\mathsf{C}$ points with purely circular polarization, and $L$ lines with purely linear polarization. We show that the transverse motion of these solitons obeys the Eulerian mechanics dictated by soliton symmetry, taking into account the vector nature of laser solitons; presented are scenarios of soliton transverse rotation and “lunar motion.”

“It is well-known to those who know it” that single-photon interference experiments can be modeled classically [S. Barnett, Phys. Scr. 97, 114004 (2022)]. When a single-photon light pulse was split by a biprism, good agreement with a classical fit was obtained and the photon was counted only once, consistent with a probabilistic interpretation [V. Jacques et al., Eur. Phys. J. D 35, 561 (2002)]. A justification for this “well know result of quantum optics” is implicit in the work by Hawton [Phys. Rev A 104, 052211 (2021)] where a real covariant field describing a single photon is first quantized. Here the theoretical basis of this result is reviewed and the theory is extended to multiphoton states. The crucial role of the charge-parity-time theorem in coupling to charged matter and resolution of the photon localization problem is discussed.