We demonstrated a practical gas production method for the ground-state carbon atoms in vacuum by pulsed laser ablation of graphite. The observation of ${}^{3}P\text{−}{}^{3}D$ two-photon excitation-induced fluorescence at 165.7 nm in the absence of a visible ablation plume proved the production of ground-state carbon atoms and enabled the evaluation of the velocity distribution and atomic density of the produced gas. With a low ablation fluence of 0.18 $\mathrm{J}/{\mathrm{cm}}^2$ at 266 nm, the number of atoms produced was $1.5\times {10}^8$ per pulse. The temperature of the gas was approximately 5600 K. Spectral measurements of the plumes at higher excitation fluences of 8, 25, and 50 $\mathrm{J}/{\mathrm{cm}}^2$ indicated that thermal sublimation at a low excitation fluence was essential to produce ground-state carbon atom gases. This method provides a promising source of atomic gas of carbon for laser cooling and precision spectroscopy when combined with precooling, such as using buffer-gas cooling.

Novel emitters of muonium ($\mathrm{Mu}={\mu }^{+}+e^{-}$) with high conversion efficiencies can enhance the precision of muonium spectroscopy experiments and enable next-generation searches for new physics. At the Paul Scherrer Institute (PSI), we investigate muonium production at room-temperature as well as in cryogenic environment using a superfluid helium converter. In this paper, we describe the development of compact detection schemes which resulted in the background-suppressed observation of atomic muonium in vacuum, and can be adapted for cryogenic measurements. Using these setups, we compared the emission characteristics of various muonium production targets at room temperature using low momentum ($p_{\mu }=11–13\mathrm{MeV}/c$) muons, and observed muonium emission from zeolite targets into vacuum. For a specific laser-ablated aerogel target, we determined a muon-to-vacuum-muonium conversion efficiency of $7.23\pm 0.05{\text{(stat)}}_{-0.76}^{+1.06}\text{(sys)}\%$, assuming thermal emission of muonium. Moreover, we investigated muonium-helium collisions and from it we determined an upper temperature limit of 0.3 K for the superfluid helium converter.

We measure the hyperfine structure of $n{P}_{1/2}$ Rydberg states for $n=$ 42, 43, 44, and 46 using mm-wave spectroscopy on an ensemble of laser-cooled ${}^{85}\mathrm{Rb}$ atoms. Systematic uncertainties in our measurement from the Zeeman splittings induced by stray magnetic fields and dipole-dipole interactions between two Rydberg atoms are factored in with the obtained statistical uncertainty. Our final measurement of the $n{P}_{1/2}$ hyperfine coupling constant is $A_{\text{HFS}}=1.443\left(31\right)$ GHz. This measurement is useful for studies of long-range Rydberg molecules, Rydberg electrometry, and quantum simulation with dipole-dipole interactions involving $n{P}_{1/2}$ atoms.

High-precision tests of fundamental symmetries are looking for the parity- ($\mathcal{P}$), time-reversal- ($\mathcal{T}$) violating electric dipole moment of the electron (eEDM) as proof of physics beyond the Standard Model. Particularly, in polyatomic molecules, the complex vibrational and rotational structure gives the possibility to reach high enhancement of the $\mathcal{P},\mathcal{T}$-odd effects in moderate electric fields, and with the possibility of increasing the statistical sensitivity by using laser cooling. In this work, we calculate the $\mathcal{P},\mathcal{T}$-odd molecular enhancement factor of the eEDM ($W_{\mathrm{d}}$) and of the scalar-pseudoscalar interaction ($W_{\mathrm{s}}$) necessary for the interpretation of future experiments on the promising candidates ${\mathrm{BaCH}}_3$ and ${\mathrm{YbCH}}_3$. We employ high-accuracy relativistic coupled cluster methods and systematically evaluate the uncertainties of our computational approach. Compared to other Ba- and Yb-containing molecules, ${\mathrm{BaCH}}_3$ and ${\mathrm{YbCH}}_3$ exhibit larger $W_{\mathrm{d}}$ and $W_{\mathrm{s}}$ associated to the increased covalent character of the $M$–C bond. The calculated values are $3.22\pm 0.12\times {10}^{24}\frac{h\text{Hz}}{e\text{cm}}$ and $13.80\pm 0.35\times {10}^{24}\frac{h\text{Hz}}{e\text{cm}}$ for $W_{\mathrm{d}}$, and $8.42\pm 0.29h\mathrm{kHz}$ and $50.16\pm 1.27h\mathrm{kHz}$ for $W_{\mathrm{s}}$, in ${\mathrm{BaCH}}_3$ and ${\mathrm{YbCH}}_3$, respectively. The robust, accurate, and cost-effective computational scheme reported in this work makes our results suitable for extracting the relevant fundamental properties from future measurements and also can be used to explore other polyatomic molecules sensitive to various violations of fundamental symmetries.

An experimental work [W. J. Wan, Y. D. Chong, L. Ge, H. Noh, A. D. Stone, and H. Cao, Science 331, 889 (2011)] showed that a resonator that contains a loss medium instead of a gain medium enables coherent perfect absorption (CPA) by interference. Here, we propose an alternative approach to realize such a CPA and coherent perfect transmission (CPT) as well as the switching between CPA and CPT in a cavity quantum electrodynamics (CQED) system by applying an external magnetic field. The proposed CPA and CPT scheme is based on the magnetically induced transparency (MIT) configuration, instead of the conventional electromagnetically induced transparency configuration, which offers a sensitive method to tune CPA and CPT and also is conducive to the flexible control of the CPA and CPT scheme. Under the linear and nonlinear excitation regimes, we derive the approximate analytical solutions of the cavity-output power and the CPA criteria of the CQED system, and find that the CPA can occur at different frequencies in both regimes. In the nonlinear excitation regime, the relationship between the cavity-output power and the cavity-input power can show a bistable characteristic by the control of the static magnetic field, and the CPA point is near the turning point of the cavity-input power on the bistable return hysteresis curve. Furthermore, the line shape and the threshold value for optical bistability can be flexibly controlled by varying the magnetic field under experimentally accessible conditions, thus the magnetic field can well tune the CPA. Also, we compare the analytical and numerical results of the cavity-output power in the nonlinear excitation regime, and they are in good agreement. Our results show that the CQED system based on the MIT configuration can be used as a perfect absorber or nearly perfect transmitter, which may have practical applications in optical logic and optical communication devices.

Branching flow, a phenomenon known for steady wave propagation in two-dimensional weakly correlated random potential, is also present in the time-dependent Schrödinger equation for a single particle in one dimension, moving in a fluctuating random potential. We explore the two-dimensional parameter space of this model using numerical simulations and identify its classical regions, where just one classical parameter is sufficient for its specification, and its quantum region, where such a simplification is not possible. We also identify the region of the parameter space where known analytical results of a classical white noise model are relevant. Qualitative behavior of quantum and classical particle dynamics is discussed in terms of branching time scale and a new time scale related to a particle's kinetic energy.

Automatic differentiation is an invaluable feature of machine learning and quantum machine learning software libraries. In this work, it is shown how quantum automatic differentiation can be used to solve the condensed-matter problem of computing fidelity susceptibility, a quantity whose value may be indicative of a phase transition in a system. Results are presented using simulations including hardware noise for small instances of the transverse-field Ising model, and a number of optimizations that can be applied are highlighted. Error mitigation (zero-noise extrapolation) is applied within the autodifferentiation framework to a number of gradient values required for the computation of fidelity susceptibility and a related quantity, the second derivative of the energy. Such computations are found to be highly sensitive to the additional statistical noise incurred by the error mitigation method.

Whether noisy quantum devices without error correction can provide quantum advantage over classical computers is a critical issue of current quantum computation. In this work, the random quantum circuits, which are used as the paradigm model to demonstrate quantum advantage, are simulated with depolarizing noise on experiment-relevant two-dimensional architecture. With comprehensive numerical simulation and theoretical analysis, we find that the maximum achievable operator entanglement entropy, which indicates maximal simulation cost, has area law scaling with the system size for constant noise rate. On the other hand, we also find that the maximum achievable operator entanglement entropy has power-law scaling with the noise rate for fixed system size, and the volume law scaling can be obtained only if the noise rate decreases when system size increase.

Recently, J. M. Arrazola et al. [Phys. Rev. A 100, 032306 (2019)] proposed a quantum algorithm for solving nonhomogeneous linear partial differential equations of the form $A\psi \left(\mathbf{r}\right)=f\left(\mathbf{r}\right)$. Its nonhomogeneous solution is obtained by inverting the operator $\widehat{A}$ along with the preparation and measurement of special ancillary modes. In this work we suggest modifications in its structure to reduce the costs of preparing the initial ancillary states and improve the precision of the algorithm for semidefinite operators. These achievements enable easier experimental implementation of the quantum algorithm.

In many-body quantum systems, the quantum Fisher information (QFI) an observer can obtain is susceptible to decoherence. Consequently, the Heisenberg scaling in quantum-enhanced metrology cannot usually be achieved. We show, via two different approaches and under certain approximations, that by applying periodic quantum error corrections we can achieve the Heisenberg scaling both for time (i.e., QFI $\propto t^2$) and the number of atoms $s$ (i.e., QFI $\propto s^2$) for an extended period of time on measurement of detuning frequency of the Tavis-Cummings model, where initially maximally entangled two-level atoms interact with a single cavity mode.

Rydberg atoms possess long coherence time and inherent scalability, which makes it promising to implement quantum algorithms. An exact and robust quantum search algorithm (SA) is essential to some practical applications. Here we propose a multisolution three-qubit SA by employing quantum circuit and geometric operations, in which the target states can be successfully searched with the fidelity of at least 99.8$\%$ and the geometric operators guarantee robustness against systematic errors. In particular, the geometric three-qubit gate operators employed reduce the implementation time and help to resist against the detrimental influence of decoherence. Moreover, our scheme can be straightforwardly extended to multiqubit cases. We have carried out numerical simulations based on the master equation to illustrate the superiority of our scheme. We consider that our study provides an alternative method for executing quantum SAs with high success probability.

Photoionization is one of the most fundamental processes in laser-matter interaction. It plays a crucial role also from the practical point of view since the electron-density dynamics in a gaseous medium is a process that affects the field propagation. Photoionization has been addressed in different regimes: linear, multiphoton, and tunneling. The ideal tool that allows for the description of ionization, in all aforementioned regimes, is the numerical evaluation of the time-dependent Schrödinger equation. The determination of the electron density needs the computation of the time-dependent ionization probability which unfortunately is an ambiguous quantity due to the gauge dependence of the latter. In this paper, we show how to overcome this difficulty by properly defining the time-dependent ionization probability in the context of the resolvent operator method. We show in particular that the velocity gauge allows for a definition of adiabatic states that is suitable to define an ionization threshold at all times during the interaction to compute ionization probability. Applications to linear, multiphoton, and tunneling regimes are presented for the one-dimensional problem. The extension to the nondipole case is discussed and we show that time-dependent ionization probability cannot be defined unambiguously due to the introduction of the magnetic-field component. We also discuss the case of gauge invariance in a subspace of the eigenbasis defined by the Hamiltonian.

High-order-harmonic spectroscopy has become an essential ingredient in probing various ultrafast electronic processes in solids with subcycle temporal resolution. Despite its immense importance, the sensitivity of high-order-harmonic spectroscopy to phonon dynamics in solids is not well known. This work addresses this critical question and demonstrates the potential of high-order-harmonic spectroscopy to probe the impact of coherent phonons on electron dynamics in solids. A pump pulse excites in-plane optical phonon modes in monolayer graphene, and a circularly polarized pulse is employed to probe the excited phonon dynamics that generates higher-order harmonics. We show that the coherent phonon dynamics alters the dynamical symmetry of graphene with the probe pulse and leads to the generation of symmetry-forbidden harmonics. Moreover, sidebands associated with the prominent harmonic peaks are generated as a result of the coherent dynamics. It is found that the symmetries and the characteristic timescale of the excited phonon mode determine the polarization and positions of these sidebands. This paper opens an avenue in time-resolved probing of phonon-driven dynamical symmetries in solids with subcycle temporal resolution.

Motivated by the realization of a dissipative quantum point contact in ultracold atomic gases, we investigate a two-terminal mesoscopic transport system in which a single-particle loss is locally present in a one-dimensional chain. By means of the Dyson equation approach in the Keldysh formalism that can incorporate dissipative effects, we reveal analytic structures of the particle and energy currents whose formal expressions correspond to ones in certain three-terminal systems where particle loss is absent. The obtained formulas are also consistent with non-Hermitian and three-terminal Landauer-Büttiker analyses. The universality of the current expressions holds regardless of the quantum statistics, and may be useful for understanding lossy two-terminal transport in terms of three-terminal transport and vice versa.

In the study of quantum transport, much has been known about dynamics near thermal equilibrium. However, quantum transport far away from equilibrium is much less well understood—the linear response approximation does not hold for physics far out of equilibrium in general. In this work, motivated by recent cold atom experiments on probing quantum many-body dynamics of a one-dimensional XXZ spin chain, where a transition from ballistic to diffusive dynamics has been established by increasing the interaction strengths, we study the strong interaction limit of the one-dimensional spinless fermion model, which is dual to the XXZ spin chain. We develop a highly efficient computation algorithm for simulating the nonequilibrium dynamics of this system exactly, and examine the nonequilibrium dynamics starting from a density modulation quantum state. We find ballistic transport in this strongly correlated setting and show that a plane-wave description emerges at long-time evolution. We also observe a sharp distinction between transport velocities in short and long times as induced by interaction effects and provide a quantitative interpretation for the long-time transport velocity. We expect our results to shed light on the understanding of the dynamics of the XXZ spin chain in the strong interaction regime.

The Eckhaus instability is a secondary instability of nonlinear spatiotemporal patterns in which high-wave-number periodic solutions become unstable against small-wave-number perturbations. Here we show that this instability can take place in Kerr combs corresponding to subcritical Turing patterns upon changes in the laser detuning. The development of the Eckhaus instability leads to the cracking of patterns and a long-lived transient where the peaks of the pattern rearrange in space due to spatial interactions. In the spectral domain, this results in a metastable Kerr comb dynamics with timescales that can be larger than 1 min. This time is, at least, seven orders of magnitude larger than the intracavity photon lifetime and is in sharp contrast with all the transient behaviors reported so far in cavity nonlinear optics that are typically only a few photon lifetimes long (i.e., in the picosecond to the microsecond range). This phenomenology, studied theoretically in the Lugiato-Lefever model and the observed dynamics is compatible with experimental observations in Kerr combs generated in ultra-high-$Q$ whispering-gallery mode resonators.

We report on a speckle field that is non-Rayleigh, nondiffracting, and an exact solution to the scalar Helmholtz equation. We have shown that these fields can be directly optimized to a continuous range of contrast values corresponding to the sub-Rayleigh or super-Rayleigh statistics maintaining its nondiffracting characteristics. Besides, this work extends the range of non-Rayleigh statistics of nondiffracting fields, which may find many applications in the areas of solid-state physics, cold atoms, and optical imaging.

Table-top coherent vacuum-ultraviolet (VUV) sources are important in many researches. Dispersive wave (DW) emission in a gas-filled hollow-core fiber (HCF) is an efficient way to obtain tunable VUV source with microjoule-level pulse energy. However, the shortest VUV wavelength is limited above 110 nm. This work investigated a method using a piecewise HCF in order to push the shortest VUV DW emission below 110 nm efficiently. The rich and complex spatiotemporal soliton dynamics in the HCF is revealed through rigorous numerical simulations and quantitative analysis. It is found that both the generation of VUV light below 110 nm and the overall conversion efficiencies below 150 nm can be significantly improved if the piecewise gas-filled HCF system is properly designed. As a result, the whole VUV spectrum (200–100 nm) can now be accessed efficiently, with single pulse energy of microjoule level. Different from other widely used intense VUV sources, this source has broad spectrum along with femtosecond time width. These unique characteristics in the VUV spectrum can lead to new research opportunities in various areas.

Ultrafast x-ray diffraction imaging provides an opportunity to realize x-ray nanoimaging of biomolecules before radiation damage, while the image resolution is still restricted by the photon flux. Fourier-transform ghost imaging based on the temporal intensity correlation can achieve diffraction-limited imaging. However, a large number of temporal samplings are inevitable, which makes it almost impossible to be implemented in the ultrafast x-ray imaging. Here, we propose an x-ray single-exposure Fourier-transform ghost imaging (SFGI) approach. The Fourier information of an unknown sample can be obtained by measuring the spatial intensity correlation between two speckle fields, and the sample needs to be exposed only once. In our demonstration experiment of SFGI, the Fourier-transform diffraction pattern of a two-dimensional sample is achieved, and its face-centered-cubic feature in the spatial domain is retrieved successfully. The simulation results of the DNA origami and rice dwarf virus indicate that a spatial resolution of 10 nm may be reached, and x-ray ghost imaging with 0.1 photon/pixel speckle detection can be expected. Our research paves the way for the future application of ultrafast x-ray ghost imaging.

Underwater ghost imaging (GI) plays an important role in the marine research, marine environment protection, and engineering applications. However, underwater GI encounters the challenges of numerous measurements and noise interference caused by the scattering lights. To solve these problems, we propose a sub-Nyquist denoising GI method to acquire high-quality images of the underwater objects. The proposed method first uses a Coiflet-wavelet decomposition method to create an index order and then utilizes the order to reorder the Hadamard pattern sequence. Then, a total variation regularization algorithm is designed to restore the object images, and a nuclear-norm-minimization algorithm is developed to remove the noises from the restored images. Finally, an experimental setup is built to simulate the complicated underwater environment that includes the turbulence and bubbles. The numerical and experimental results show that the denoising capability of the proposed method is strong, and the imaging performance of the proposed method is similar (slightly better in some cases) to the recently reported state-of-the-art GI methods in the complicated underwater environment and sub-Nyquist sampling ratio condition (e.g., 0.03). The proposed method may find applications in marine underwater imaging areas.