We present a theoretical and experimental investigation of the postselection effect in precision spectroscopy of the $2{}^{3}S-2{}^{3}P$ transition in helium using an atomic beam. By employing the Monte Carlo wave function (MCWF) method, we simulate the time evolution of the ground and excited amplitudes in the presence of laser fields, revealing a systematic dependence of the measured transition frequency on the spatial parameters of the detection slit, including its position and width. Our results demonstrate that postselection—an inherent aspect of quantum projection in such measurements—can induce frequency shifts on the order of tens of kilohertz. By elucidating the underlying physical mechanisms, our study offers a practical framework for quantitatively correcting the postselection-induced shifts, thereby improving the accuracy of atomic beam spectroscopy.

We presently introduce three-body methods named boundary-corrected first-Born eikonal final state (CB1-EFS-3B) and boundary-corrected first-Born eikonal initial state (CB1-EIS-3B) for single charge exchange in ion-atom collisions. The perturbation potential in the CB1-EFS-3B approximation is the same as in the prior form of the three-body boundary-corrected first-Born (CB1-3B) approximation. However, unlike the CB1-3B method, the CB1-EFS-3B approach takes into account the eikonal form of the Coulomb continuum intermediate states of the captured electron in the exit channel. Similarly, in the case of the CB1-EIS-3B approach, the eikonal form of the Coulomb continuum intermediate states of the captured electron in the entrance channel is taken into account, whereas the perturbation potential remains the same as in the post form of the CB1-3B model. For the CB1-EFS-3B and CB1-EIS-3B methods, we have carried out an analytical reduction of the original six-dimensional integrals for the transition amplitudes, encompassing both symmetric and asymmetric collisions, to an efficient two-dimensional numerical quadrature over real variables. Using CB1-EFS-3B and CB1-EIS-3B models, we computed a large number of total cross sections for electron capture in the ${\mathrm{H}}^{+}-\mathrm{H}$ collisions as well as single-electron capture from the K shell of multielectron atomic targets (helium, nitrogen, and oxygen) by protons at intermediate and high impact energies. The independent particle model and the frozen core approximation are employed, considering only one target electron as active for multielectron targets. The initial state of the active electron in the multielectron targets is described by the hydrogenlike wave functions. The present results from the CB1-EFS/EIS-3B methods are found to be in good agreement with the existing experimental data on total cross sections. The provided cross-section database can find important applications in several investigative fields ranging from plasma physics, astrophysics, and heavy-ion transport physics through fusion research and technology to hadron therapy.

Ultrafast scattering techniques such as ultrafast electron diffraction and ultrafast x-ray diffraction have been utilized to elucidate the structural dynamics, reaction intermediates, and final products in molecular reactions following photoexcitation. The time-dependent structures are typically not directly retrieved from the experimental data, but they rely on comparison with calculations. The genetic algorithm (GA), a global optimization strategy, can be used to retrieve the molecular structures directly from diffraction patterns without any theoretical input. However, the robustness of the GA with respect to real experimental conditions such as a limited momentum transfer range, noise, and artifacts has not been studied in detail. In this work, we characterize the performance of the GA with simulated data that mimic realistic experimental conditions. We have developed and implemented a variant of the GA specific to diffraction measurements which performs better in the presence of imperfect data compared to the standard implementation of the GA. We demonstrate this method with both synthetic data and experimental ultrafast electron diffraction data on the UV-induced photodissociation of trifluoroiodomethane ($\mathrm{C}{\mathrm{F}}_3\mathrm{I}$) molecules.

We have measured the dissociative recombination (DR) of the deuterated triatomic hydrogen ions ${\mathrm{H}}_2{\mathrm{D}}^{+}$ and ${\mathrm{D}}_2{\mathrm{H}}^{+}$ as a function of storage time at the Cryogenic Storage Ring (CSR). Both molecular ions were stored for up to 1000 s inside the cryogenic vacuum of the CSR prior to the electron recombination measurements, allowing them to cool to their lowest rotational states. We implement a comprehensive model for all relevant processes to predict the internal state evolution of the ions during storage inside the CSR, employing calculated radiative transition strengths and state-selective rate coefficients for electron collisions. Our DR rate coefficient measurements with deuterated triatomic hydrogen ions in defined quantum states allow for meaningful comparisons with state-of-the-art theoretical calculations, paving the way for a better understanding of the complex DR process for polyatomic molecular ions.

The previously applied independent-atom model (IAM) for highly charged ion-molecule collisions which implemented the suppression of multiple ionization and capture on the basis of geometric overlaps of cross-sectional areas representing ion-atom cross sections using a pixel-counting method (PCM) [H. J. Lüdde, T. Kalkbrenner, M. Horbatsch, and T. Kirchner, Phys. Rev. A 101, 062709 (2020)] is extended to incorporate the possibility of multiple collisions within the molecule. This is accomplished on the basis of estimated mean free paths for sequential projectile-atom collisions. The IAM-PCM was demonstrated to be successful in describing proton-molecule collisions and moderately-charged ion impact at high collision energies. The new model does agree with these results, but has important consequences for highly charged projectiles, providing cross sections larger than those of IAM-PCM, but still well below the simple additivity rule results.

High-order harmonic generation from solids involving deep-lying valence bands is numerically simulated based on the periodical potential by solving the time-dependent Schrödinger equation. By introducing ultraviolet pulses, a distinct resonant peak appears on the harmonic spectrum, which indicates the participation of the deep-lying valence band in harmonic emission. Considering the non-negligible contribution of the deep-lying valence band, the resonance-assisted multiphoton injection (RAMI) mechanism is established. Moreover, we evaluate the interference of harmonics originating from different valence bands.

We investigate the coherence and correlations of many-body states appearing in topological pumping in a one-dimensional Bose gas. By analyzing the system at zero and infinite interaction strengths, we reveal a rescaling of momentum distributions accompanied by a self-similar behavior in first- and second-order correlation functions. In the excited states of noninteracting bosons, the momentum distribution shows a comblike structure similar to that of noninteracting fermions but at a higher density. This is mirrored by Friedel oscillations in the one-body density matrix. At the same time, the density-density correlations still exhibit a bosonic enhancement. Our work illustrates how topological pumping induces a nontrivial mapping between bosonic and fermionic correlations.

Terahertz (THz) pulses from the two-color laser-field scheme in air have been widely investigated due to THz broadband and the simplicity of the generation technology. The traditional unidirectional pulse propagation equation has usually been adopted to investigate THz generation and laser propagation. However, this simulation method needs to be examined when THz radiation propagation is considered since the THz frequency is close to the characteristic frequency of the plasma ionized by the two-color laser field. Here, we propose using the time-domain nonlinear wave equation (NLWE-t) to calculate THz propagation accurately and develop a hybrid algorithm to improve the computation speed. With the developed hybrid simulation and the NLWE-t, we study the temporal evolution of THz waveforms and observe that a typical THz waveform can be considered a combination of a main body and an oscillatory tail. The main body contains mainly lower-frequency components, while the tail contains higher-frequency components. This tail originates from the interactions between the main body and the plasma density gradient, and new higher-frequency components can be excited during this process. This process is essentially the cross-phase modulation of the THz wave, which is different from the self-phase modulation responsible for blueshifting in ultrafast pulses. Finally, we propose a method to modulate THz waveforms by a preformed plasma, which is similar to the backward-propagating THz radiation emitted from two-color plasma filamentation. With the plasma frequency being higher than the THz frequency, the plasma can be used as a phase modulator to adjust the spatial position relationship of different THz frequency components. The lower-frequency THz wave and the higher-frequency THz wave can be separated through the reflection at the boundary. In this way, one could modulate THz waveforms and frequencies by adjusting the plasma density and length.

We developed an algorithm that constructs a model of a reconfigurable optical interferometer, independent of specific architectural constraints. The programming of unitary transformations on the interferometer's optical modes relies on either an analytical method for deriving the unitary matrix from a set of phase shifts or an optimization routine when such decomposition is not available. Our algorithm employs a supervised learning approach, aligning the interferometer model with a training set derived from the device being studied. A straightforward optimization procedure leverages this trained model to determine the phase shifts of the interferometer with a specific architecture, obtaining the required unitary transformation. This approach enables the effective tuning of interferometers without requiring a precise analytical solution, paving the way for the exploration of new interferometric circuit architectures.

We develop a Lyapunov-based framework to model the evolution of entangled biphotons interacting with cavity and material modes. Using Gaussian-preserving dynamics and Møller operators, we map input joint spectral amplitudes to experimentally measurable joint spectral intensities. Our model reproduces key features of observed spectra and reveals off-diagonal correlations arising from cavity decay, providing a scalable and tractable tool for quantum spectroscopic analysis.

A two-level system coupled by a coherent field is a ubiquitous model in atomic and molecular physics. While the resulting avoided crossing of light-dressed states has been extensively probed spectroscopically, such measurements typically reveal only the eigenenergies of the Hamiltonian. Here we present a mapping that also resolves the state populations, thus providing direct access to the amplitude coefficients of the eigenvectors. We demonstrate this in Rydberg states of cold rubidium atoms, combining population transfer spectroscopy with selective field ionization to map both the eigenvalues and the eigenvector amplitude coefficients of the coupled two-level system.

We examine the transport of useful energy, i.e., extractable work as quantified by the ergotropy, along a spin chain with tunable exchange couplings between the sites. We focus on, and interpolate between, the two physically relevant limits of uniform interaction strengths and engineered couplings which achieve perfect state transfer (PST). By modeling the individual constituents as quantum batteries, we consider how the manner in which the extractable work appears in the initial state of the first site impacts the chain's ability to transport ergotropy to the final site. For non-PST couplings, we establish that there is a clear quantum advantage when the ergotropy is initially endowed in quantum coherences and demonstrate that this ergotropy is more efficiently transferred. For extractable work encoded in a population inverted state, we show that this considerably limits the length of chain over which any ergotropy can be faithfully transported. For PST couplings, we consider the robustness to disorder and again demonstrate a quantum advantage for coherently endowed ergotropy. Finally, we examine the work probability distribution associated with quenching on the interactions, which provides insight into the work cost in switching on the couplings. We show that PST couplings lead to smaller fluctuations in this work cost, indicating that they are more stable.

Magic-angle twisted bilayer graphene (MATBG) exhibits remarkable electronic properties under external magnetic fields, notably the emergence of flat Landau levels. In this study, we present a comprehensive analysis of MATBG's operational phase diagram under three distinct quantum thermodynamic cycles, i.e., the quantum Otto cycle (QOC), quantum Carnot cycle (QCC), and quantum Stirling cycle (QSC). Employing the continuum eight-band model, we evaluate the thermodynamic performance of MATBG across multiple operational modes—heat engine, refrigerator, cold pump, and Joule pump—and benchmark it against other graphene systems such as monolayer graphene, $\mathit{AB}$-Bernal stacked bilayer graphene, and nonmagic-angle twisted bilayer graphene. Our findings reveal that MATBG demonstrates superior heat engine performance in QSC, while achieving high efficiency albeit with reduced work output in QOC. Even though the performance of MATBG as a cold pump or refrigerator is modest in QOC and QSC, it shows notable improvement as a refrigerator in QCC. Additionally, we identify a highly reversible Joule pump mode in both QSC and QOC under strict adiabaticity, underscoring the unique thermodynamic behavior of MATBG.

Phase estimation is a major mission in quantum metrology, especially in quantum interferometry. A full phase estimation scheme usually includes the optimal probe state and measurement. For the finite-dimensional states in Fock basis, the NOON state ceases to be optimal when the average particle number is fixed yet not equal to the Fock dimension (Fock number of the highest occupied Fock state of one mode), and what is the true optimal finite-dimensional probe state in this case is still undiscovered. Hereby we present several theorems to answer this question and provide a complete optimal scheme to realize the ultimate precision limit in practice. These optimal finite-dimensional probe states reveal an important fact that the Fock dimension could be treated as a metrological resource, and the given scheme is particularly useful in scenarios where weak light or limited particle number is demanded.

We introduce a quantum circuit that “transcribes” the elements of the process matrix $\chi$ of an arbitrary quantum channel onto the state of a pair of ancillary systems. The circuit works for an arbitrary quantum channel acting on a $d$-dimensional target system and returns as output not only the $\chi$ matrix encoded in the $d^2$-dimensional state of the ancilla systems but also the initial state of the target system. We compare the circuit to the well-known Choi circuit that outputs the Choi-Jamiołkowski representation of the channel and the lesser-known Choi-Bell circuit that outputs the $\chi$ matrix. We show that the Choi-Bell circuit can be modified to transcribe quantum channels and also return the initial target state but requires more nonlocal gates. We briefly discuss some interesting features of the circuit and its eventual application in quantum process tomography and other computational tasks.

The quantum-phase-estimation algorithm (QPEA) is widely used to find estimates of unknown phases. The original algorithm relied on an input state in a uniform superposition of all possible bit strings. However, it is known that other input states can reduce certain Bayesian risks of the final estimate. Here, we derive a method to find the risk-minimizing input state for any risk. These states are represented by an eigenvector of a Toeplitz matrix with elements given by the Fourier coefficients of the loss function of interest. We show that, while the true optimal state does not have a closed form for a general loss function, it is well approximated by a state with a cosine form. When the cosine frequency is chosen appropriately, these states outperform the original QPEA and achieve the optimal theoretical quantum-advantage scaling for three common risks. Furthermore, we prove that the uniform input state is suboptimal for any reasonable loss function. Finally, we design methods to mitigate the impact of depolarizing noise on the performance of QPEA.