Quantum key distribution is unbreakable in theory but may be hacked via imperfections in its hardware implementations. While many imperfections have been mitigated by countermeasures and advanced security proofs, several remain unsolved. One of these is a superlinear behavior in single-photon detectors, when the click probability rises faster with the photon number of an incoming light pulse than expected from individual independent photon detections. Here we test an avalanche single-photon detector sinusoidally gated at 312.5 MHz for superlinearity. Its click probability is moderately superlinear. However, we notice that the click timing depends strongly on the incoming pulse energy. The click occurs progressively earlier, shifting more than 2 ns as the energy rises over a wide 50-dB range. An attacker might use this energy-time effect to conditionally toggle the click between adjacent key bit slots, violating an implicit assumption in the security proofs and rendering them inapplicable. We propose two attacks that exploit this flaw.

We propose a method to directly recover the degree of entanglement distributed by entanglement swapping in the presence of noise. Our approach introduces a reversing operation that probabilistically undoes the effect of amplitude damping or photon loss on a single entangled pair, enabling heralded recovery of entanglement. We demonstrate that entanglement can be substantially recovered even under strong noise, including parameter regimes where the distributed entanglement would otherwise vanish due to entanglement sudden death. We analyze the effectiveness of the protocol in two representative repeater models, i.e., two-way and one-way architectures, and identify the optimal reversing strategy. Due to its heralded and single-copy nature, our protocol is readily compatible with other entanglement recovery techniques such as entanglement purification and distillation. Our work provides a practical and experimentally feasible way toward robust entanglement distribution in current and near-term quantum repeater architectures.

Near-term quantum networks face a bottleneck due to low quantum communication rates. This degrades performance both by lowering operating speeds and increasing qubit storage time in noisy memories, making some quantum internet applications infeasible. One way to circumvent this bottleneck is multiplexing: combining multiple signals into a single signal to improve the overall rate. Standard multiplexing techniques are classical in that they do not make use of coherence between quantum channels nor account for decoherence rates that vary during a protocol's execution. In this paper, we first derive semiclassical limits to multiplexing for many-qubit protocols and then introduce two techniques: single click quantum multiplexing and multiserver multiplexing. These can enable beyond-classical multiplexing advantages. We illustrate these techniques through three example applications: (1) entanglement generation between two asymetric quantum network nodes (i.e., repeaters or quantum servers with unequal memories), (2) remote state preparation between many end user devices and a single quantum node, and (3) remote state preparation between one end user device and many internetworked quantum nodes. By utilizing many noisy internetworked quantum devices instead of fewer low-noise devices, our multiplexing strategies enable new paths towards achieving high-speed many-qubit quantum network applications.

In this work we investigate phase estimation of a two-qubit system under the action of the quantum cyclic $n$-switch in generalized amplitude-damping, depolarizing, and phase-damping channels. Our results show that the quantum cyclic $n$-switch can improve the precision of phase estimation in correlated generalized amplitude-damping channels and uncorrelated depolarizing channels. Increasing the dimension of the control system can significantly enhance the precision. For correlated generalized amplitude-damping channels under the quantum cyclic $n$-switch, both the environmental thermal noise and the classical correlation between channels can play a positive role in improving the estimation precision. In uncorrelated depolarizing channels, the quantum cyclic $n$-switch operation could suppress the decoherence, thereby enhancing the precision of phase estimation. In correlated phase-damping channels under the action of the quantum cyclic $n$-switch, the classical correlation can eliminate the negative effect of dephasing noise on coherence to improve phase estimation.

We propose a distributed quantum dense coding protocol that uses a control system to superpose two dense coding processes, allowing us to simultaneously and coherently encode and nonclassically route the sender's single-qubit system to two receiver laboratories. We find that dense coding with coherently controlled encoding and routing performs better than the standard dense coding protocol in both global and one-way local decoding strategies employed by receiver laboratories in weak entanglement regimes and with noisy mixed states. We extend the protocol to incorporate noisy channels and show that there exists a certain noise strength below which our protocol outperforms the noiseless standard dense coding protocol in presence of quantum dephasing noise. We extend our protocol to an arbitrary number of receivers and analyze its performance under a global decoding strategy.

Although only two quantum states of a physical system are often used to encode quantum information into qubits, in principle, qudits (a system with a number $d$ states) could be used to increase the information capacity of the system. In this work, three-state systems (qutrits) are studied using a parametrization of the group $U\left(3\right)$ that is relevant for controlling superconducting qutrits using fixed-frequency resonant control pulses. The parametrization uses a decomposition of a general $3\times 3$ unitary matrix into the exponential of a diagonal matrix and the exponential of an off-diagonal matrix. This decomposition is numerically confirmed to allow the parametrization of any element in $U\left(3\right)$. However, many different sets of parameters can give the same element of $U\left(3\right)$. Furthermore, different elements could have different numbers of decompositions. Why are there different numbers? Which decompositions are better and why? These two questions are answered so that the parametrization can be better utilized and searches can be made more efficient. Other parametrizations are derived and discussed and the methods used are applicable to these other parametrizations as well.

Accurate spectroscopic investigations of the heaviest elements are inherently challenging, due to their short lifetimes and low production yields. Success of such measurements requires both dedicated experimental techniques and strong theoretical support. Laser resonance chromatography (LRC) is a promising approach for heavy-ion spectroscopy, in particularly for metals with low vapor pressure, such as actinium. We have employed the state-of-the-art relativistic Fock space coupled-cluster approach as well as the configuration interaction with the many-body perturbation theory method to calculate the energy levels, the transition amplitudes, the branching ratios, and the hyperfine structure parameters of the lowest excited states in ${\mathrm{Ac}}^{+}$. Knowledge of these properties is required for the design of experiments. Our calculations are in close agreement with experimental transition energies, leading us to expect a similar level of accuracy for the calculated hyperfine structure parameters. Based on these predictions, two possible experimental schemes are proposed for the planned LRC measurements.

We study the polaritonic states and dynamics of multiple Jahn-Teller (JT) active molecules coupled to the modes of a Fabry-Perot cavity. We find that collective effects dramatically alter the interplay of electronic, vibrational, and cavity angular momenta, giving rise to markedly different polaritonic spectra and dynamics even when going from one to two JT molecules. Starting from the ground vibronic state, we find that JT molecules collectively coupled to a common cavity can access a cascade of high-angular-momentum vibronic states in the presence of a single-cavity photon, in sharp contrast to the single molecule case where the range of accessible vibronic angular momentum values are bounded. The observable consequences are a broadening of the cavity-molecular polariton spectrum and a suppression of photon polarization dynamics under broadband excitation of the system. Our results uncover rich pathways for vibronic angular momentum transfer unique to collective molecular polaritonics with potential implications for cavity-assisted photophysics and photochemistry.

Large-scale level-by-level calculations have been systematically performed to obtain the $L$-shell resonant electronic recombination cross sections of ${\mathrm{Fe}}^{15+}$ ions, using the relativistic configuration interaction method considering the complex electronic correlations, the Breit and QED corrections. The higher-order electron correlation effects between resonances associated with the double- and triple-excited channels are found to be very important, which reduced the recombination cross sections by nearly a factor of 2. The high-$n$ ($n\ge 6$) quasidegenerate Rydberg states contribute to the recombination spectra with resonant energies above 714.32 eV; the inclusion of core excitation ${\left(2s2p\right)}^{7}3l^2$ ($l$ = $s, p$, and $d$) autoionizing states is crucial to achieving accurate radiative branching ratios and cross sections, which are consequently reduced by nearly a factor of 1.5. The calculated results show excellent agreement with the available high-precision experimental measurements at heavy-ion test storage ring TSR. Based on this, the resonant transfer excitation (RTE) cross sections of Na-like ${\mathrm{Fe}}^{15+}$ ions incident on ${\mathrm{H}}_2$ molecules are obtained using the impulse approximation. It is found that the trielectronic excitation has a significant effect on the RTE cross sections, with the contribution being about $17.88\%$. It is expected that the present study will provide valuable insights into the spectral simulation of astrophysical and fusion plasmas.

Delta-kick cooling is a technique employed to achieve low kinetic temperatures by decreasing momentum width at the cost of increased position width. In an ideal implementation, this method uses a harmonic potential to deliver a single near-instantaneous momentum kick. In practice, potentials that are approximately harmonic near their center are commonly used. As a result, the breakdown of the harmonic approximation far from the center limits the cooling performance. Inspired by aberration cancellation in optics, we propose to use compound matter-wave lens systems for $\delta \text{−}\mathrm{kick}$ cooling with Gaussian potentials. By strategically combining attractive and repulsive kicks, we show that it is possible to mimic the effect of a harmonic potential. For a test case with reasonable experimental parameters, our method suggests a reduction in kinetic temperature by a factor of 2.5 using a two-pulse sequence and by a factor of 3.2 using a three-pulse sequence.

We report a theoretical and experimental investigation of autoionizing resonances from the $5d6p{}^3\mathrm{D}_1^o$ manifold in neutral barium for efficient loading of ion traps. Our calculations predict large resonant cross sections for many narrow autoionizing resonances, but we find experimentally that, for most of these, Doppler broadening during trap loading depresses the effective cross sections that can be achieved in practice. We identify and demonstrate a strong, broad transition at $530.8\mathrm{nm}$ and show that it furnishes an order-of-magnitude increase in trap loading efficiency compared to other demonstrated resonances.

Quantum reflection occurs when ultracold atoms are incident on a material surface with sufficiently low velocity. The reflecting matter wave can interfere with the incident wave to form a detectable pattern, and this pattern contains information about atom-surface interactions at micrometer scales. We discuss how such an interferometer could be used to probe for anomalous short-range forces that are predicted by some beyond-standard model theories. We compare a simple analytical model for the anomalous phase to numerical solution of both the Schrödinger and Gross-Pitaevskii equations, finding good agreement. With interactions, the phase does depend on the atomic density, which can be a source of noise. We nonetheless predict that under realistic conditions, the reflection technique can reach sensitivities approaching those obtained with macroscopic objects, and significantly improve the limits on anomalous coupling to atoms.

We study the ground-state and low-lying metastable phases of repulsive binary Bose-Einstein condensates confined in twisted, spin-dependent periodic optical lattices. For balanced mixtures, weak intercomponent interactions yield a fourfold momentum-space symmetry dictated by the lattice geometry. Increasing the coupling strength leads to the emergence of additional momentum peaks that combine with the lattice-induced structure to produce an eightfold rotationally symmetric pattern, signaling quasiperiodic order. At intermediate interactions, global phase separation suppresses this quasicrystalline state; however, at stronger coupling, local phase separation gives rise to a long-lived metastable phase in which the eightfold symmetry is restored. In this regime, a secondary ring of dominant momentum peaks appears at smaller wave vectors, indicating longer-wavelength density modulations and a crossover from lattice-dominated to interaction-driven quasicrystalline order. In contrast, imbalanced mixtures form partially miscible density clusters with eightfold-symmetric quasiperiodic patterns only at intermediate coupling, while stronger interactions drive global phase separation and permanently destroy quasicrystalline order. Real-time simulations demonstrate that these quasiperiodic structures are dynamically stable and experimentally accessible. Our results show that quasicrystalline order can emerge in binary condensates without explicitly quasiperiodic lattices and reveal population balance as a key ingredient for stabilizing quantum quasicrystals.

We investigate the low-energy dynamics of small-amplitude surface oscillations of spherical superfluid droplets in vacuum. Starting from the effective field theory of superfluid phonons, we derive an effective action governing the surface oscillations under a fixed particle-number constraint. The normal-mode eigenfrequencies ${\omega }_{\ell }$ for each angular momentum quantum number $\ell$ are determined and shown to depend on a dimensionless parameter measuring the ratio of surface tension to bulk compressibility energy. We identify a critical value of this parameter at which the breathing mode ($\ell =0$) becomes mechanically unstable, and show that all multipole surface modes with $\ell \ge 2$ enter the low-energy regime when the surface tension is sufficiently small. Within this regime, we further quantize the surface oscillations, whose quanta correspond to ripplons, allowing the construction of general multiripplon states obeying angular momentum selection rules. We also apply our formalism to a concrete example: a weakly interacting two-component Bose mixture realizing a self-bound superfluid droplet. The resulting description is universal in the sense that it applies to surface dynamics of generic nonrelativistic superfluids with a free interface, independent of microscopic details.

In this work, we investigate the nonequilibrium dynamics in a one-dimensional two-component anyon-Hubbard model, which can be mapped to an extended Bose-Hubbard ladder with density-dependent hopping phase and synthetic gauge flux. Through numerical simulations of few-particle dynamics and the analytical symmetry analysis without particle-number constraints, we reveal the asymmetric and imbalance transports with broken inversion symmetry and find two dynamical symmetries in the expansion dynamics unique to the two-component anyon-Hubbard chain. For any initial state that preserves both the time-reversal and inversion symmetries, the expansion is always dynamically symmetric under spatial inversion and component flip, when the sign of the anyonic statistics phase or the signs of both the gauge flux and interaction are changed. For vanishing Hubbard interaction, we show the dynamical suppression induced by both the statistics phase and the gauge flux. For finite Hubbard interaction, we further uncover tunable chiral-antichiral dynamics for two-component anyons under the gauge flux. We demonstrate that both chiral and antichiral dynamics can be exhibited and tuned by the statistics phase and the gauge flux. The dynamical phase regimes with respect to the chiral-antichiral dynamics for different system parameters are obtained. These findings highlight the dynamical phenomena arising from the interplay of anyonic exchange statistics, synthetic gauge fields, and Hubbard interactions in multicomponent anyons.

We investigate vortex nucleation and transport in a rotating dipolar supersolid arranged in a triangular droplet lattice, exploiting its description as an array of weakly linked condensates. By considering both Josephson and macroscopic self-trapping dynamics, we show that local phase differences between droplets provide a compact and highly predictive framework to explore a wide range of vortex behaviors. In particular, Josephson oscillations can be devised to induce vortex nucleation and motion near the vertices of the low-density hexagonal lattice (between droplets), while self-trapping dynamics induces running phases that enable directed vortex transport, which may be accompanied by vortex-antivortex pair creation and annihilation over finite timescales. Comparison with simulations based on the extended Gross-Pitaevskii equation demonstrates that a three-droplet description is essential to capture vortex motion near hexagon vertices. Together, Josephson and self-trapping dynamics provide a tunable protocol to trigger and track vortex nucleation, transport, and vortex-antivortex pair annihilation, revealing the microscopic topological mechanisms underlying phase slips in rotating dipolar supersolids.

We propose an atom beam splitter that enables the manipulation of the internal spin state of the atoms in the output beams using a spin-dependent optical potential. The utility of such an atom beam splitter is demonstrated theoretically through its application in measuring the Aharonov-Casher phase of atoms subjected to a constant homogeneous electric field.

Analog coprocessors for neural networks are an intensively developing field. They provide approximate results of computations for relatively low energy cost and at high speed. We show that a set of ring resonators with Brillouin interaction between photons and phonons, being coupled to a waveguide, can be used to implement matrix-vector multiplication. The input vector is formed by occupancies of the anti-Stokes optical modes pumped via spontaneous Brillouin scattering, i.e., scattering on thermal phonons. Brillouin scattering rates and coupling constants between ring resonators and the waveguide form the matrix.

Temporal modulation of photonic media generally induces effective gain and loss and thus non-Hermitian wave dynamics. Here, we show that under a temporally $\mathcal{PT}$-symmetric modulation, such media can nevertheless exhibit Hermiticity and a temporal $\mathcal{PT}$ phase transition. We prove that, for a homogeneously modulated photonic medium with complex-valued modulation $\epsilon \left(t\right)$, the temporal $\mathcal{PT}$ symmetry $\epsilon (-t)={\epsilon }^{*}\left(t\right)$ is a necessary but insufficient condition for obtaining a real eigenvalue spectrum, giving rise to $\mathcal{PT}$-symmetry phase transition. Specifically, the $\mathcal{PT}$ phase transition critically depends on the contrast between the modulation depth of the real and imaginary parts of permittivity when they are sinusoidally modulated with a $\pi /2$ phase difference—a minimal modulation scheme satisfying the temporal $\mathcal{PT}$ symmetry. We generalize the discretized temporal-interface transfer matrix method to a continuous differential operator framework, confirming the phase transition condition via Magnus expansion and Petermann analysis. Full-wave simulations and analytical calculations jointly confirm the occurrence of $\mathcal{PT}$ transition by examining the scattering behavior of a propagating pulse in such a type of modulated medium. The findings provide a temporal $\mathcal{PT}$-symmetric paradigm for controlling Hermiticity and non-Hermiticity in spatiotemporal photonic systems.

We present a Gaussian-beam theory of the spin Hall effect (SHE) of light for smoothly inhomogeneous media, extending the complex geometrical optics (CGO) framework to incorporate spin-orbit coupling through a linear eikonal correction term ${\mathsf{q}}^T{\mathsf{b}}_{\sigma }$, where $\mathsf{q}$ is the column vector of transverse coordinates relative to the central ray in a certain frame and ${\mathsf{b}}_{\sigma }$ is a spin-dependent coefficient vector in the same frame. This correction introduces a helicity-dependent transverse force, yielding the spin-Hall displacement ${\mathsf{d}}_{\sigma }=-{\left(\mathrm{Im}\mathsf{M}\right)}^{-1}\mathrm{Im}{\mathsf{b}}_{\sigma }$, where $\mathsf{M}$ is the complex curvature matrix governing beam diffraction in CGO. Applied to a helical-ray geometry in a gradient-index medium, the displacement is, in the Frenet-Serret frame, found to be predominantly binormal. Numerical results show persistent shear oscillations in $\mathrm{Re}{\mathsf{M}}^F\left(s\right)$, dynamically transferring spin-orbit drive from normal to binormal. Simultaneously, $\mathrm{Im}{\mathsf{M}}^F\left(s\right)$ develops strong anisotropy, locking the beam ellipse to the Frenet-Serret axes. Although $\mathrm{Im}{\mathsf{b}}_{\sigma }^F$ is predominantly normal, the extreme anisotropy of ${\left(\mathrm{Im}{\mathsf{M}}^F\right)}^{-1}$ amplifies the binormal contribution.