Quantum weak measurements became extremely popular in classical optics to amplify small optical signals for fundamental interests and potential applications. Later, as a more general extension, joint weak measurement has been proposed to extract the weak value from a joint quantum measurement. However, unlike traditional weak measurements, the detection of a joint weak value in the realm of classical optics remains less explored. Here, using the polarization-dependent longitudinal and transverse optical beam shift as a platform, we experimentally realize a classical optical analog of the quantum joint weak measurement. Polarization states are cleverly pre- and postselected, and different single and joint canonical position-momentum observables of the beam are experimentally extracted and subsequently analyzed for successful detection of the complex joint weak value. We envision that this work will find usefulness for gaining fundamental insights on quantum measurements and to tackle analogous problems in optics.

Chirped-pulse interferometry (CPI) is a classical low-coherence interferometry technique with automatic dispersion cancellation and improved resolution over white-light interference. Previous work showed that CPI with linearly chirped Gaussian laser pulses achieves a $\sqrt{2}$ improvement in resolution over conventional white-light interferometry, but this is less than the factor of 2 improvement exhibited by a comparable quantum technique. In this work, we show how a particular class of nonlinearly chirped laser pulses can meet, and even exceed, the factor of 2 improvement in resolution. This enhanced-resolution CPI removes the resolution advantage of quantum interferometers in dispersion-cancelled interferometry.

Diabatic quantum annealing aims to mitigate the challenges posed by small energy gaps and decoherence in quantum optimization by exploiting nonadiabatic transitions. In this paper, we compare the performance of two diabatic protocols in a realistic, dissipative setting using the maximum weighted independent set problem as a test case, showing that the potential advantages of diabatic protocols are strongly reduced in the presence of dissipation. Our results highlight the importance of evaluating quantum annealing strategies under practical conditions.

Randomized measurements are useful for analyzing quantum systems, especially when quantum control is not fully perfect. However, their practical realization typically requires multiple rotations in the complex space due to the adoption of random unitaries. Here, we introduce two simplified randomized measurements that limit rotations in a subspace of the complex space. The first is real randomized measurements (RRMs) with orthogonal evolution and real local observables. The second is partial real randomized measurements (PRRMs) with orthogonal evolution and imaginary local observables. We show that these measurement protocols exhibit different abilities in capturing correlations of bipartite systems. We explore various applications of RRMs and PRRMs in different quantum information tasks such as characterizing high-dimensional entanglement, quantum imaginarity, and predicting properties of quantum states with classical shadow.

For the post-processing of the quantum-key-distribution (QKD) system, the error correction is the most important step, whose efficiency significantly influences the final key generation rate. Traditionally, the error-correction algorithm is developed based on the binary symmetrical channel model. However, this may not comply with the actual channel status. For example, for the fiber-based QKD system, differentiable statuses of the fiber (unstable or steady) should exist for some time and cause different bit error rates. If this channel feature can be exploited, it may considerably improve the error-correction efficiency. This paper proposes to introduce the Gilbert-Elliott (GE) model to analyze the QKD channel. The error-correction algorithm adopts the low-density parity-check (LDPC) code. The GE nodes are added into the traditional belief propagation (BP) algorithm of the LDPC decoder. Simulation results showed that the remaining frame error rate could be improved from ${10}^{-2}$ (or ${10}^{-3}$) to ${10}^{-4}$. An actual QKD system with the quantum channel state under manual control was adopted to collect the key bit string. By analyzing the occurrence probability of the $k$ consecutive "1," the key bit string showed closely matched distribution with the GE model prediction. After the error correction, the remaining bit error rate of the modified BP algorithm was significantly smaller than that of the standard BP algorithm with an improvement of more than 2 orders of magnitude. Both simulation and actual experimental results demonstrate that our proposed method can greatly improve the error-correction efficiency if the channel characteristic of the QKD system is accurately exploited, which provides an efficient way to generate a higher key rate.

We measured the thresholds for photodetachment from the first and second excited rotational levels of ${\mathrm{CH}}^{-}$ to the lowest vibrational, rotational, and fine-structure level of CH to be $E_1=1.213\pm 0.002\mathrm{eV}$ and $E_2=1.206\pm 0.002\mathrm{eV}$, respectively. Based on these measurements and the rigid rotor approximation, we arrive at an electron affinity of $\text{EA}=E_1+\frac{1}{2}(E_1-E_2)=1.217\pm 0.002\mathrm{eV}$. This value deviates from earlier experimental results but agrees with the calculation by Feller [J. Chem. Phys. 144, 014105 (2016)]. In the present experiment, we stored ensembles of initially hot ${\mathrm{CH}}^{-}$ in the cryogenic ion-beam storage ring Double ElectroStatic Ion-Ring ExpEriment (DESIREE) for tens of seconds such that the vast majority of the ions were in the few lowest excited rotational levels of the electronic and vibrational ground state. We identified the initial states for photodetachment channels with threshold energies $E_1$ and $E_2$ by comparing the time dependences of measured photodetachment signals with radiative rotational-cooling rates calculated using the literature values of the dipole moment of ${\mathrm{CH}}^{-}$. The conditions of a few occupied rotational levels are superior to those of previous studies of this system and an important step toward future studies with an all-rotational-ground-state ion beam.

We report on the direct observation of recurrent fluorescence (RF) from initially cold ${\mathrm{C}}_4{}^{-}$ anions obtained at specific photoexcitations from the electronic ground state ($\mathrm{X}{}^2{\mathrm{\Pi }}_g$) under conditions mimicking the environment in interstellar space. Using a combination of buffer gas cooling in a (small) radio-frequency ion trap and laser excitation in an electrostatic ion storage ring, we explicitly illustrate the cooling of a molecular system due to RF and meet an experimental challenge posed almost 40 years ago. In the effort to directly observe RF, we also reexamine the spectroscopy of the $\mathrm{C}{}^2{\mathrm{\Pi }}_u\leftarrow \mathrm{X}{}^2{\mathrm{\Pi }}_g$ transition in ${\mathrm{C}}_4{}^{-}$ as probed by two-photon photodetachment spectroscopy and assign all observed transitions from cold ${\mathrm{C}}_4{}^{-}$. Our assignment and modeling of the observed photoabsorption spectrum allow us to determine spin-orbit coupling constants for both the ground ($\mathrm{X}{}^2{\mathrm{\Pi }}_g$) and third excited ($\mathrm{C}{}^2{\mathrm{\Pi }}_u$) state of ${\mathrm{C}}_4{}^{-}$ and reveal their variation with vibrational excitation energy for the ${\nu }_{2.\mathrm{C}}$ symmetric stretch mode and the ${\nu }_{3.\mathrm{C}}$ asymmetric stretch mode of the excited state.

We report the numerical observation of two-electron nondispersive wave packets in fully three-dimensional helium subjected to a linearly polarized monochromatic field. These localized quantum states follow periodic trajectories of the classical three-body Coulomb system without dispersion. The system is modeled using a spectral configuration interaction approach based on Coulomb-Sturmian functions, combined with Floquet theory and complex scaling. The resulting quasienergy spectrum reveals the existence of long-lived Floquet states arising from near-resonant coupling between frozen planet configurations.

Searches for the nuclear magnetic quadrupole moment (MQM) and nuclear Schiff moment (NSM) have high discovery potential for violations of time- $\left(T\right)$ and parity- $\left(P\right)$ reversal symmetries beyond the standard model. Molecules containing heavy nuclei are typically used to enhance the sensitivity to MQMs and NSMs due to their strong internal electric fields and potential octupole deformation. To extract these effects in the laboratory frame, a bias electric field is required to polarize the molecule by mixing states of opposite parity (parity doublets). Typical heavy nuclei that are sensitive to symmetry violation also possess large nuclear electric quadrupole moments (EQMs) when the nuclear spin is $I\ge 1$. We show that EQMs can significantly modify the energy splitting between parity-doublet states and thus change the required polarizing electric field. As a result, the EQM-induced energy splitting must be taken into account in designing such experiments. We provide qualitative estimates of parity doubling from EQMs and supporting ab initio calculations, along with implications for candidate molecules in symmetry-violation searches.

We investigate laser-induced nonsequential double ionization with linearly polarized bichromatic fields, focusing on the recollision-excitation with subsequent ionization (RESI) mechanism. Using the strong-field approximation, we assess how the symmetries of the field influence the dominant events. Furthermore, we show that, by manipulating the field parameters such as the field frequencies and relative phase between the two driving waves, one can influence the correlated electron-momentum distributions. Specific features of a linearly polarized bichromatic field are that the momentum distributions of the second electron are no longer centered around vanishing momenta and that there may be more than one ionization event per half-cycle. This can be used to confine the RESI distributions to specific momentum regions and to determine a hierarchy of parameters that make an event dominant.

We perform a systematic analysis of intrachannel quantum interference in laser-induced nonsequential double ionization with linearly polarized bichromatic fields, focusing on the recollision-excitation with subsequent ionization (RESI) mechanism and employing the strong-field approximation. We generalize and elaborate several analytic interference conditions for RESI in arbitrary driving fields, with a focus on the interference arising from the specific symmetries of bichromatic fields. For example, for waves of comparable strengths, multiple events per half-cycle for the direct electron must be considered. Furthermore, interference breaks some of the symmetries arising from the field. We detangle the superimposed interference fringes originating from phase differences related to symmetrization due to electron exchange, temporal shifts, and a combination of exchange and event interference. We show that the hierarchy between exchange-only and exchange-temporal interference is fluid and can be manipulated by an appropriate choice of driving-field parameters. This is enabled by different types of interference occupying specific regions of the plane spanned by the electron momentum components parallel to the driving-field polarization.

This work presents a method for achieving complete, robust, and efficient population transfer between the two ground states in a three-level loop quantum system. The approach utilizes composite pulse sequences by effectively mapping the three-state system onto an equivalent two-level system. This transformation allows the use of broadband composite pulses designed initially for conventional two-state quantum systems. Unlike traditional implementations, the composite pulses in the three-level system are not controlled through phase adjustments; instead, they are realized via the amplitude ratio of the Rabi frequencies.

We propose a molecular cavity optomechanical (MCOM) framework that exploits the collective vibrational modes of a molecular ensemble coupled to a plasmonic nanocavity. Our approach replaces traditional mechanical elements in cavity optomechanics with molecular vibrations, allowing optomechanically induced transparency (OMIT) to emerge even in regimes with ultralow optical quality factor ($\mathcal{Q}$) that are typically inaccessible to standard cavity optomechanics. We observe that the OMIT window exhibits gain at very low $\mathcal{Q}$ factor and with a large number of molecular ensembles. Specifically, we demonstrate dynamic control of the photonic spin Hall effect (PSHE) by tuning the optical susceptibility through parameters such as molecular resonance, collective coupling strength, and the $\mathcal{Q}$ factor. Our method first computes the coefficients of the transverse electric (TE) and transverse magnetic (TM) components of reflected light using the Fresnel formula, demonstrating the transverse shift of reflected light from positive to negative at the Brewster angle. Our findings suggest that molecular ensembles are a viable platform for room-temperature nanophotonics, facilitating spin-controlled signal processing and quantum-enhanced sensing through integrated optomechanics and spin-orbit coupling.

Longitudinally resolved terahertz radiations from tip-electrodes dc-electric-field-biased laser filament were generated. The terahertz waveform undergoes a shifting of 0.43 ps and broadening of 0.46 ps in the time domain during its propagation in the 40-mm-long filament. Both experimental and numerical results have proved that the propagation of THz in a dc-biased filament waveguide lead to the terahertz waveform shifting and broadening.

Subwavelength atomic arrays feature strong light-induced dipole-dipole interactions, resulting in subradiant collective resonances characterized by narrowed linewidths. In this work, we present a sideband cooling scheme for atoms trapped in subwavelength arrays that utilizes these narrow collective resonances. Working in the Lamb-Dicke regime, we derive an effective master equation for the atomic motion by adiabatically eliminating the internal degrees of freedom of the atoms and validate its prediction with numerical simulations of the full system. Our results demonstrate that subradiant resonances enable the cooling of ensembles of atoms to temperatures lower than those achievable without dipole interactions, provided the atoms have different trap frequencies. Remarkably, narrow collective resonances can be sideband-resolved even when the individual atomic transition is not. In such scenarios, ground-state cooling becomes feasible solely due to light-induced dipole-dipole interactions. This approach could be utilized for future quantum technologies based on dense ensembles of emitters, and paves the way towards harnessing many-body cooperative decay for enhanced motional control.

We present a protocol for generating nonclassical states of atomic spin ensembles through the backaction induced by a hybrid measurement of light that is entangled with atoms, combining both homodyne and single-photon detection. In phase I of the protocol we create a spin-squeezed state by measuring the light's polarization rotation due to the Faraday effect in a balanced polarimeter, equivalent to a homodyne measurement. In phase II we send a second probe beam through the sample and detect single photons scattered into the signal mode. Before doing so, we rotate the uncertainty bubble to increase the projection fluctuations of the measured spin component. This increases the coupling strength between the atoms and photons and thus the rate of scattering of single photons into the signal mode. In the ideal case, the result is a squeezed Dicke state, with substantial quantum advantage for sensing spin rotations. We benchmark the protocol's utility in the presence of inevitable decoherence due to optical pumping using the Fisher information as a measure of quantum advantage. We show that in the presence of decoherence, the quantum Fisher information associated with the non-Gaussian mixed state we prepare is substantially larger than the classical Fisher information obtained from the standard measurement of spin rotations. We deduce a measurement basis that is close to optimal for achieving the quantum Cramér-Rao bound in the presence of decoherence.