We present a no-go theorem for the distinguishability between quantum random numbers (i.e., random numbers generated quantum mechanically) and pseudorandom numbers (i.e., random numbers generated algorithmically). The theorem states that one cannot distinguish these two types of random numbers if the quantum random numbers are efficiently classically simulatable and the randomness measure used for the distinction is efficiently computable. We derive this theorem using the properties of cryptographic pseudorandom number generators, which are believed to exist in the field of cryptography. Our theorem is found to be consistent with the analyses on the actual data of quantum random numbers generated by IBM Quantum and also those obtained in the Innsbruck experiment for the Bell test, where the degrees of randomness of these two set of quantum random numbers turn out to be essentially indistinguishable from those of the corresponding pseudorandom numbers. Previous observations on the algorithmic randomness of quantum random numbers are also discussed and reinterpreted in terms of our theorems and data analyses.

We analyze the dynamics of metastable Markovian open quantum systems by unraveling their average dynamics into stochastic trajectories. We use quantum reset processes as examples to illustrate metastable phenomenology, including a simple three-state model whose metastability is of classical type, and a two-qubit model that features a metastable decoherence-free subspace. In the three-state model, the trajectories exhibit classical metastable phenomenology: fast relaxation into distinct phases and slow transitions between them. This extends the existing correspondence between classical and quantum metastability. It enables the computation of committors for the quantum phases, and the mechanisms of rare transitions between them. For the two-qubit model, the decoherence-free subspace appears in the unraveled trajectories as a slow manifold on which the quantum state has a continuous slow evolution. This provides a classical (nonmetastable) analog of this quantum effect. We discuss the general implications of these results, and we highlight the useful role of quantum reset processes for analysis of quantum trajectories in metastable systems.

We systematically characterize the dynamical evolution of time-parity ($\mathcal{PT}$)-symmetric two-level systems with spin-dependent dissipations. If the control parameters of the gap are linearly tuned with time, the dynamical evolution can be characterized with parabolic cylinder equations which can be analytically solved. We find that the asymptotic behaviors of particle probability on the two levels show initial-state-independent redistribution in the slow-tuning-speed limit as long as the system is nonadiabatically driven across exceptional points. Equal distributions appear when the nondissipative Hamiltonian shows gap closing. As long as the nondissipative Hamiltonian displays level anticrossing, the final distribution becomes unbalanced. The ratios between the occupation probabilities are given analytically. These results are confirmed with numerical simulations. The predicted equal-distribution phenomenon may be used to identify the closing of the energy gap from anticrossing between two energy bands.

Quantum thermodynamic relationships in emerging nanodevices are significant but often complex to deal with. The application of machine learning in quantum thermodynamics has provided a new perspective. This study employs reinforcement learning to output the optimal cycle of a quantum heat engine. Specifically, the soft actor-critic algorithm is adopted to optimize the cycle of a three-level coherent quantum heat engine with the aim of maximal average power. The results show that the optimal average output power of the coherent three-level heat engine is 1.28 times greater than the original cycle (steady limit). Meanwhile, the efficiency of the optimal cycle is greater than the Curzon-Ahlborn efficiency as well as efficiencies reported by other researchers. Notably, this optimal cycle can be fitted as an Otto-like cycle, which illustrates the effectiveness of the method.

We propose a method to dynamically decouple every magnetically sensitive hyperfine sublevel of a trapped ion from magnetic field noise, simultaneously using integrated circuits to adiabatically rotate its local quantization field. These integrated circuits allow passive adjustment of the effective polarization of any external (control or noise) field. By rotating the ion's quantization direction relative to this field's polarization, we can perform “passive” dynamical decoupling (PDD), inverting the linear Zeeman sensitivity of every hyperfine sublevel. This dynamically decouples the entire ion, rather than just a qubit subspace. Fundamentally, PDD drives the transition $m_F\to -m_F$ for every magnetic quantum number $m_F$ in the system—with only one operation—indicating it applies to qudits with constant overhead in the dimensionality of the qudit. We show how to perform pulsed and continuous PDD, weighing each technique's insensitivity to external magnetic fields versus their sensitivity to diabaticity and control errors. Finally, we show that we can tune the sinusoidal oscillation of the quantization axis to a motional mode of the crystal in order to perform a laser-free two-qubit gate that is insensitive to magnetic field noise.

Vibronic coupling plays a crucial role in x-ray photoelectron spectra (XPS) of molecules. In a series of three papers, we present a comprehensive exploration of the $N$-heterocycles family, known for their diverse structures, to summarize the general rules of vibronic coupling in high-resolution vibrationally resolved XPS spectra at the N $1s$ edge. Building upon our previous studies on six-membered monocyclic azines [Phys. Rev. A 106, 022811 (2022)] and fused bicyclic compounds indoles with five and six members [Phys. Rev. A 108, 022816 (2023)], in this study, we focus on investigating a series of 12 five-membered $N$-heterocycles using Franck-Condon simulations, incorporating Duschinsky rotation effects and density functional theory. Our calculations reveal distinct spectral characteristics of amine and imine within these 12 systems in binding energies, spectral characteristics, structural changes, vibrational coupling strengths, and effects of hydrogenation. Furthermore, we expand our analysis to encompass all 35 $N$-heterocycles discussed in the three papers and consolidate these findings into the general rules. We find that $1s$ ionization in amine nitrogen induces more substantial geometrical changes, resulting in larger vibronic coupling strength compared to imine nitrogens. The spectra of imine nitrogens exhibit two distinct characteristic peaks originating from the 0-0 and 0-1 transitions, whereas the spectra of amine nitrogens are characterized by a broad peak with numerous weak fingerprints due to significant mixing of various 0-$n$ transitions. We observe that amine (imine) nitrogens generally cause a negative (positive) change in zero-point vibrational energy. This study provides valuable insights into vibronic coupling in $N$-heterocycles, shedding light on the distinguishing features and behavior of amine and imine nitrogens in vibrationally resolved XPS spectra.

The laser-induced recollision of electron trajectories is usually used to describe high-order harmonic generation (HHG), which also lays the foundation for high-order harmonic spectroscopy. Electron trajectory models based on the saddle-point approximation are conceptually similar to the light ray model in optics. Considering that ray optics is not complete to explain all wave optics phenomena, we revisit the validity of trajectory models when incorporating the Coulomb potential. Results show that, under commonly used gas HHG experimental conditions, both the classical three-step model and the quantum orbit model have dozens or hundreds of attoseconds of deviation in emission time. By using the Gaussian wave-packet method, we reveal that the time shift can be attributed to the deformation of the recollision electron wave packet. Wave-packet deformation is an inherent wavelike phenomenon caused by group delay dispersion and higher-order dispersion, and thereby is difficult to modify by particlelike trajectories. In addition, we reveal the notable influence of wave-packet deformation on observables in two-color field HHG experiments.

The first color photos of the parametric down-conversion (PDC) emission cone illustrate the correlation of longitudinal and transverse momentum in the process, i.e., wavelength-dependent emission angles of PDC photons. However, current experiments and applications are more conveniently described in terms of discrete mode sets, with the most suitable choice depending on the propagation symmetries of the experimental setting. Remarkably, despite the fact that experiments with PDC sources are becoming ever more demanding, e.g., in terms of brightness or state fidelity, a description of spectral-spatial coupling in parametric down-conversion for the case of discrete modal decompositions remains elusive. We present a comprehensive study, in theory and experiment, of the spectral dependence of the transverse Laguerre-Gauss modes in parametric down-conversion. Moreover, we show how the spectral and spatial coupling can be harnessed to tune the purity of the well-known orbital angular momentum entanglement. This paper has implications for efficient collection of entangled photons in a transverse single mode, quantum imaging, and engineering pure states for high-dimensional quantum information processing.

We report an optical method of generating arbitrary polarization states by manipulating the thicknesses of a pair of uniaxial birefringent plates, the optical axes of which are set at a crossing angle of π/4. The method has the remarkable feature of being able to generate a distribution of arbitrary polarization states in a group of highly discrete spectra without spatially separating the individual spectral components. The target polarization-state distribution is obtained as an optimal solution through an exploration. Within a realistic exploration range, a sufficient number of near-optimal solutions are found. This property is also reproduced well by a concise model based on a distribution of exploration points on a Poincaré sphere, showing that the number of near-optimal solutions behaves exponentially with respect to the number of spectral components of concern. As a typical example of an application, by applying this method to a set of phase-locked highly discrete spectra, we numerically demonstrate the continuous generation of a vectorlike optical electric field waveform, the helicity of which is alternated within a single optical cycle in the time domain.

There exist two families of superradiance phenomena: relying on free electrons or on correlated emission by ensembles of atoms. Here we investigate emission from ensembles of atoms driven by coherently shaped free electrons. This interaction creates superradiance emerging from both the atoms’ and the electrons’ coherence—with emission intensity that scales quadratically in both the number of atoms and number of electrons. This phenomenon enables electrons to become probes of quantum correlations in matter, with high temporal and spatial resolution.

Multipartite generalizations of spin coherent states are introduced and analyzed. These are the spin analogs of multimode optical coherent states as used in continuous variable quantum information, but generalized to possess full spin symmetry. Two possible generalizations are given, one of which is a simple tensor product of a given multipartite quantum state. The second generalization uses the bosonic formulation in the Jordan-Schwinger map, which we call spinor states. In the unipartite case, spinor states are equivalent to spin coherent states; however, in the multipartite case, they are no longer equivalent. Some fundamental properties of these states are discussed, such as their observables and covariances with respect to symmetric operators, form-preserving transformations, and entanglement. We discuss the utility of such multipartite spin coherent and spinor states as a way of storing quantum information.

Quantum transduction is an important building block for quantum networking. Although various platforms have been proposed, the efficiency of state-of-the-art systems is still way below the required threshold to provide robust quantum information transduction via a direct conversion approach. Increasing the efficiency via merely increasing the power of the pulsed pump will lead to heating that substantially reduces the repetition rate thus hinders the quantum information rate. In [Phys. Rev. Appl. 16, 064044 (2021)], we proposed a transduction paradigm based on continuous-variable quantum teleportation that shows a much higher rate in the low cooperativity region. While more recently [Phys. Rev. Res. 4, L042013 (2022)] proposed to utilize microwave squeezing to assist direct conversion. In this work, we explore the role of squeezing in the teleportation-based transduction protocol via evaluating quantum capacity lower and upper bounds. Our analyses include both microwave squeezing and optical squeezing, and provide a systematical benchmark between the continuous variable teleportation-based approach and continuous variable direct conversion approach. We show that the enhancement from single-mode squeezing on the teleportation-based approach is limited, in contrast to the direct conversion case. In the low cooperativity region required by reasonable repetition rate, the teleportation-based protocol provides larger quantum information rate and fidelity of state transfer. At the same time, it takes substantial squeezing for direct conversion to get close to the teleportation-based approach, at the risk of potentially introducing additional heating from the pump.

The three-body fragmentation dynamics of cyclopropane $\left({\text{C}}_3{\text{H}}_6\right)$ induced by ${\text{Ni}}^{19+}$ ions at an impact energy of 5.8 MeV/u are studied using a cold target recoil ion momentum spectroscopy (COLTRIMS) reaction microscope. Two completely measured three-body fragmentation channels of ${\text{C}}_3{\text{H}}_6^{3+}$ were identified definitely, i.e., ${\text{C}}_3{\text{H}}_6^{3+}\to {\text{H}}^{+}+\text{C}{\text{H}}_2^{+}+{\text{C}}_2{\text{H}}_3^{+}$ and ${\text{C}}_3{\text{H}}_6^{3+}\to {\text{H}}^{+}+\text{C}{\text{H}}_3^{+}+{\text{C}}_2{\text{H}}_2^{+}$. It is found that sequential fragmentation processes are dominant for the channels considered. For each channel, two distinct sequential mechanisms are distinguished, depending on whether the CH or CC bond breaks first. The observation is further supported by the characteristic kinetic energy release (KER) distributions associated with different mechanisms. Additionally, the reason for the prevalence of sequential fragmentation processes and their varying degrees of importance for the two channels are briefly discussed.

We report on high-resolution microwave spectroscopy of cesium Rydberg $(n+2)D_{5/2}\to n{F}_J$ transitions in a cold atomic gas. Atoms laser-cooled and trapped in a magnetic-optical trap are prepared in the $D$ Rydberg state using a two-step laser excitation scheme. A microwave field transmitted into the chamber with a microwave horn drives the Rydberg transitions, which are probed via state selective field ionization. Varying duration and power of the microwave pulse, we observe Fourier sideband spectra as well as damped, on-resonant Rabi oscillations with pulse areas up to $\gtrsim 3\pi$. Furthermore, we investigate the Zeeman effect of the clearly resolved $n{F}_J$ fine-structure levels in fields up to 120 mG, where the transition into $n{F}_{7/2}$ displays a thee-peak Zeeman pattern, while $n{F}_{5/2}$ shows a two-peak pattern. Our theoretical models explain all observed spectral characteristics, showing good agreement with the experiment. Our measurements provide a pathway for the study of high-angular-momentum Rydberg states, initialization and coherent manipulation of such states, Rydberg-atom macrodimers, and other Rydberg-atom interactions. Furthermore, the presented methods are suitable for calibration of microwave radiation as well as for nulling and calibration of dc magnetic fields in experimental chambers for cold atoms.

We present an analytical calculation of the complete ${\alpha }^6$ correction to energies of $nP$ levels of two-body systems consisting of the spin-0 or -$1/2$ extended-size particles with arbitrary masses and magnetic moments. The obtained results apply to a wide class of two-body systems such as hydrogen, positronium, muonium, and pionic or aniprotonic helium ion. We found an additional ${\alpha }^6$ correction for $nP$ levels of positronium, which was previously overlooked. Our results are also relevant for light muonic atoms, whose accurate theoretical predictions are required for extracting the nuclear charge radii.

We present experimental observation of above-threshold ionization (ATI) electrons produced by ionization of the neon K shell in a laser field where intensity exceeds ${10}^{20}\mathrm{W}/{\mathrm{cm}}^2$. An array of plastic scintillating calorimeter detectors was used to measure the high-energy electrons at four angles in the laser forward direction. Coarse energy resolution was obtained using aluminum filters of several thicknesses to block lower-energy electrons. A threshold intensity around $2\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$ is observed for production of energetic ATI electrons in the laser forward direction, with maximum electron energy exceeding 10 MeV. L-shell electrons with energies $<1.4$ MeV are scattered farther forward along the laser direction than expected. We present comparisons of the measured total electron energies to the predictions of Monte Carlo models employing the ADK-PPT ionization model and the Augst barrier suppression ionization model.

We present results of numerical simulations of the time-dependent Schrödinger equation and theoretical analysis concerning the interaction of a rare gas atom with circularly and elliptically polarized laser pulses. In agreement with recent observation in circularly polarized fields, the photoelectron energy spectra for counterrotating electrons are peaked at lower kinetic energy than those for corotating electrons. We show that this difference can be interpreted as being due to the additional pathways to ionization that are available for the counterrotating electrons only. Furthermore, our results show, in agreement with earlier work, that the offset angle by which the emission of electrons is rotated in an elliptically polarized field increases with each successive above-threshold ionization (ATI) order and is larger for the emission from counterrotating states as compared to that from corotating states. A simple model based on the interference and relative phase difference between just three continuum states provides remarkable agreement and once again emphasizes the importance of the additional ionization pathways available for the counterrotating electrons.