We conduct a systematic investigation of the oscillations of the Rydberg-state populations as a function of intensity arising from strong-field multiphoton excitation of Rydberg atoms. By numerically solving the time-dependent Schrödinger equation and by constructing a quantum model based on Freeman resonances, we elucidate the underlying physical mechanisms governing the intensity-dependent oscillatory behavior of the Rydberg-state population dynamics. Our analysis shows that these oscillations originate from the synergistic interplay of three distinct quantum-interference mechanisms: (1) interference between excitation pathways at the rising and falling edges of the laser pulse (long-time interference), (2) interference between different optical cycles within the same side of the laser pulse (short-time interference), and (3) interference within individual optical cycles (intracycle interference). Additionally, we find that ground-state depletion has varying degrees of influence on the oscillation patterns formed by the three interference mechanisms under different laser-intensity conditions.

First-principles modeling of dense hydrogen is crucial in materials and planetary sciences. Despite its apparent simplicity, predicting the ionic and electronic structure of hydrogen is a formidable challenge, and it is connected with the insulator-to-metal transition, a century-old problem in condensed matter. Accurate simulations of liquid hydrogen are also essential for modeling gas giant planets. Here, we perform an exhaustive study of the equation of state of hydrogen using density functional theory (DFT) and quantum Monte Carlo simulations. We find that the pressure predicted by DFT may vary qualitatively when using different functionals. The predictive power of first-principles simulations is restored by validating each functional against higher-level wavefunction theories, represented by computationally intensive variational and diffusion Monte Carlo calculations. Our simulations provide evidence that hydrogen is denser at planetary conditions, compared to currently used equations of state. For Jupiter, this implies a lower bulk metallicity (i.e., a smaller mass of heavy elements). Our results further amplify the inconsistency between Jupiter’s atmospheric metallicity measured by the Galileo probe and the envelope metallicity inferred from interior models.

Rydberg atoms in dc electric fields acquire static dipole moments. When the atoms are close to a surface producing an inhomogeneous electric field, such as by the adsorbates on an atom chip, depending on the sign of the dipole moment of the Rydberg-Stark eigenstate, the atoms may experience a force toward or away from the surface. We show that by applying a bias electric field and coupling a desired Rydberg state by a microwave field of proper frequency to another Rydberg state with opposite sign of the dipole moment, we can create a trapping potential for the atom at a prescribed distance from the surface. Perfectly overlapping trapping potentials for several Rydberg states can also be created by multicomponent microwave fields. A pair of such trapped Rydberg states of an atom can represent a qubit. Finally, we discuss an optimal realization of the $\mathrm{SWAP}$ gate between pairs of such atomic Rydberg qubits separated by a large distance but interacting with a common mode of a planar microwave resonator at finite temperature.

${\mathrm{KTaO}}_3$ (KTO) is an incipient ferroelectric, characterized by a softening of the lowest transverse optical (TO) mode with decreasing temperature. Cooper pairing in the recently discovered KTO-based heterostructures has been proposed to be mediated by the soft TO mode. Here, we study the electron coupling to the zone-center odd-parity modes of bulk KTO by means of relativistic Density Functional Perturbation Theory (DFPT). The coupling to the soft TO mode is by far the largest, with comparable contributions from both intraband and interband processes. Remarkably, we find that for this mode, spin nonconserving matrix elements are particularly relevant. We develop a three-band microscopic model with spin-orbit-coupled $t_{2g}$ orbitals that reproduces the main features of the ab initio results. For the highest energy band, the coupling can be understood as a “dynamical” isotropic Rashba effect. In contrast, for the two lowest bands, the Rashba-like coupling becomes strongly anisotropic. The DFPT protocol implemented here enables the calculation of the full electron-phonon coupling matrix projected onto any mode of interest, and it is easily applicable to other systems.

Spin-exchange collisions have been widely studied in recent years, and various quantum-mechanical scattering approaches have been developed to calculate the rates. However, these methods based on global knowledge of wave functions can be computationally demanding, are limited to small systems, and do not offer a mechanistic interpretation. Here, we present a semiclassical transition-state theory (SCTST) derived from first principles to describe the nonadiabatic transition between two states that differ only in their spins, where classical TST and Landau-Zener theory fail. We apply our theory to describe the spin-exchange collision between the nuclear spin of ${}^3\mathrm{He}$ and the electronic spin of ${}^{23}\mathrm{Na}$. SCTST reveals that the reaction proceeds via temperature-dependent transition states, determined by an intricate compromise between minimizing the activation energy and maximizing the hyperfine coupling. It further demonstrates the importance of quantum delocalization effects prevalent in spin exchange even when tunneling is suppressed and successfully explains the observed weak temperature dependence of the rate. Finally, we showcase the power of SCTST by making predictions for the ${}^3\mathrm{He}+{\mathrm{CH}}_3$ spin-exchange collision in full dimensionality, where we find that the internal degrees of freedom induce symmetry-broken transition states.

We demonstrate that the control protocols of quantum information devices can be simulated by assuming a low-rank ansatz for the density matrix. The rationale underlying this assumption is that quantum information protocols, by design, operate in a regime of nearly pure quantum states. Within the low-rank assumption, the simulation of these protocols is considerably faster than solving the full Lindblad master equation. This advantage can be used to increase the accuracy of the simulation by avoiding uncontrolled approximations and to streamline protocol optimization. We benchmark our approach on the optimization of the transmon qubit dispersive readout in a realistic transmon-resonator-filter model. With a Hilbert space dimension $N=2000$, assuming a rank as low as $M=20$, we achieve a nearly 100-fold speedup compared to full master equation integration while accurately reproducing all relevant observables. By combining the low-rank approximation with a compact pulse parametrization and gradient-free optimization, we obtain state-of-the-art readout assignment errors ${\varepsilon }_a\approx 1.2\times {10}^{-3}$ for a 40 ns readout pulse schedule, while comfortably running on a laptop and not relying on the rotating-wave approximation. Our approach is broadly applicable to most quantum control protocols, including quantum gates, state preparation, and fast reset operations. This establishes low-rank methods as a general tool for optimal control across diverse quantum platforms.

We explore the analogy between following the motion of a reflected wave packet and a quantum measurement of the spatial delay imposed on the particle by the scattering potential. It is shown that converting such delays into temporal durations can lead to “negative times,” and best be avoided. It is also demonstrated that a “soft” potential can be replaced by a superposition of hard walls. This representation is used for calculating the Goos-Hänchen shift of a Gaussian beam incident on a potential step or a barrier at an oblique angle.

Polymer solutions are widely used in hydraulic fracturing operations to enhance proppant transport, suppress turbulence, and reduce fluid loss. However, how their properties influence the local flow field and fracture dynamics, especially in the rapidly evolving region near the fracture tip, remains poorly understood. This study investigates the coupled fluid flow and fracture behavior near the tip of a propagating hydrofracture driven by polymer solutions using high-speed imaging within a transparent, three-dimensional-printed polymethyl methacrylate model. Solutions of high-molecular-weight polymers induce flow and fracture behavior that closely resemble those of water, while viscous Newtonian fluids produce different behavior. This is attributed to pronounced shear thinning, which significantly lowers the effective viscosity under high shear and hence alters the local flow field and stress distribution near the fracture tip. These findings underscore the importance of fluid rheology in shaping both flow patterns and fracture development during hydraulic fracturing.

Linear cross-entropy benchmarking (LXEB) with random quantum circuits is a standard method for evaluating quantum computers. However, LXEB requires classically simulating the ideal output distribution of a given quantum circuit with high numerical precision, which becomes infeasible beyond approximately 50 qubits, even on state-of-the-art supercomputers. As a result, LXEB cannot be directly applied to evaluate large-scale quantum devices, which now exceed 100 qubits and continue to grow rapidly in size. To address this limitation, we introduce a constraint known as particle-number conservation into the random quantum circuits used for benchmarking. This restriction significantly reduces the size of the Hilbert space for a fixed particle number, enabling classical simulations of circuits with over 100 qubits when the particle number is $O\left(1\right)$. Furthermore, we propose a modified version of LXEB, called MLXEB, which enables fidelity estimation under particle-number-conserving dynamics. Through numerical simulations, we investigate the conditions under which MLXEB provides accurate fidelity estimates.

Preparing matrix product states (MPSs) on quantum computers is an essential routine in the simulation of many-body physics. However, widely used schemes based on staircase circuits are often too deep to execute on current hardware. Here, we demonstrate that MPSs with short-range correlations can be prepared with shallow circuits by leveraging heuristics from approximate quantum compiling (AQC). We achieve this with ADAPT-AQC, an adaptive-ansatz preparation algorithm, and introduce a generalized initialization procedure for the existing AQC-Tensor algorithm. We first compare these methods for the task of preparing a molecular electronic structure ground state. We then use them to prepare an antiferromagnetic (AFM) ground state of the 50-site Heisenberg XXZ spin chain near the AFM-XY phase boundary. Through the execution of circuits with up to 59 cz depth and 1251 cz gates, we perform a global quench and observe the relaxation of magnetic ordering in a parameter regime previously inaccessible due to deep ground-state preparation circuits. Our results demonstrate how the integration of quantum and classical resources can push the boundary of what can be studied on quantum computers.

Both monolayer and bilayer ${\mathrm{Nb}}_3{\mathrm{I}}_8$ were theoretically predicted to be magnetic, whereas bulk ${\mathrm{Nb}}_3{\mathrm{I}}_8$ was experimentally observed to exhibit a nonmagnetic (NM) ground state. In this work, we pursue the origin of bulk NM behavior and the evolution of magnetism in ${\mathrm{Nb}}_3{\mathrm{I}}_8$ nanofilms using first-principles calculations. We identified three distinct stacking types in bilayer ${\mathrm{Nb}}_3{\mathrm{I}}_8$, further categorized into nine specific configurations. Strikingly, the HH-AA stacking configuration adopts an NM ground state, while the other eight configurations stabilize as antiferromagnetic (AFM) states. The magnetization loss in HH-AA stacking originates from charge transfer from interfacial I atoms to Nb atoms, driving the ${\mathrm{Nb}}_3$ trimer's transition from ${[\mathrm{Nb}}_3{]}^{8+}$ ($S$ = 1/2) to ${[\mathrm{Nb}}_3{]}^{7+}$ ($S$ = 0). Furthermore, multilayer ${\mathrm{Nb}}_3{\mathrm{I}}_8$ films with bulk stacking exhibit layer-dependent oscillations between ferromagnetic (FM) and NM states, alternating with odd and even layer counts. The NM ground state of bulk ${\mathrm{Nb}}_3{\mathrm{I}}_8$, which consists of alternating HH-AA and TT-AC stacking sequences, results from the moment quenching in HH-AA stacking over the AFM coupling in TT-AC stacking. Our findings establish ${\mathrm{Nb}}_3{\mathrm{I}}_8$ as a promising platform for stacking-engineered functionalities. Its stacking diversity not only broadens the functional landscape of two-dimensional materials but also reveals hidden correlations between structural degrees of freedom and electronic/magnetic responses.

We report the observation of discrete supercurrent suppression steps arising from quantum Cooper-pair tunneling in superconductor-graphene-superconductor (SGS) Josephson junctions under near-infrared illumination. This quantum phenomenon remains robust against variations in photon energy, optical power, bath temperature, and dc bias. The nearly identical step spacing observed in SGS Josephson junctions with different graphene channel lengths indicates the dominant role of the superconductor-graphene interface in governing Cooper-pair transport. In contrast, the discrete steps vanish in the negative differential resistance regime, where quasiparticle transport dominates, confirming their microscopic Cooper-pair tunneling origin.

Many important quantum systems are characterized by Hamiltonians expressible as a linear combination of time-independent generators of a finite Lie algebra, $\widehat{H}\left(t\right)={\sum }_{l=1}^L{\eta }_l\left(t\right){\widehat{g}}_l$. The Wei-Norman framework provides a powerful analytical method for computing the exact time-evolution operator in a factorized form, $\widehat{U}\left(t\right)={\prod }_{l=1}^L{e}^{{\mathrm{\Lambda }}_l\left(t\right){\widehat{g}}_l}$. By exploiting the underlying algebraic structure, the method yields a system of differential equations—the Wei-Norman equations—which determine the time-evolution operator coefficients ${\mathrm{\Lambda }}_l\left(t\right)$ from the Hamiltonian ones ${\eta }_l\left(t\right)$. Although robust and formally extendable, applying the method by hand becomes increasingly difficult or even intractable for high-order systems ($L\ge 6$). In this work, we introduce symdyn, a Python library that automates the application of this method. Given the commutators between generators, the library symbolically derives the Wei-Norman equations and efficiently computes nested commutators, similarity transformations, and the differential equations that yield Baker-Campbell-Hausdorff-like relations. We validate the library by recovering known results for quantum systems associated with the Lie algebras $\mathfrak{su}(1,1), \mathfrak{su}\left(2\right), \mathfrak{sl}\left(2\right)$, and $\mathfrak{so}(2,1)$, and further demonstrate its capabilities by deriving the Wei-Norman equations for a system of two coupled time-dependent harmonic oscillators involving 11 generators. Additionally, we specialize the library to handle the Lie group $\mathit{SU}\left(N\right)$ using a generic Cartan-Weyl basis for its algebra, and demonstrate its versatility by deriving the Wei-Norman equations for $\mathfrak{su}\left(2\right), \mathfrak{su}\left(3\right)$, and $\mathfrak{su}\left(4\right)$, including the explicit construction of a set of quantum gates for universal quantum computation.

Tightly focused laser beams, or optical tweezers, are essential for analog and digital quantum simulation with neutral atoms and trapped ions. Despite this, most of the current intuition and theoretical treatment utilizes the paraxial approximation, which is an insufficient description at the focus of tightly focused optical tweezers. We develop an analytic model, which we use in tandem with numerical simulations, to quantify how nonparaxial effects will manifest in the next generation of scalable quantum hardware, where tightly focused beams are used for individual qubit control. In particular, we calculate the light potentials of Gaussian and Laguerre-Gaussian beams driving the quadrupole ${}^2\mathrm{S}_{1/2}\to {}^2\mathrm{D}_{5/2}$ transition in ${}^{40}\mathrm{Ca}^{+}$. Longitudinal field components in the beam center cause spatially dependent Rabi frequencies and ac Stark shifts, leading to unexpected qubit-motion coupling. We characterize single- and two-qubit gate infidelities due to this effect with an analytic model and numerical simulation. We identify regimes where nonparaxial effects should be taken into account for high-precision quantum control, and highlight that the effects are potentially more severe for the case of neutral atom and molecule addressing.

We study a type of solitonic Andreev bound state (ABS) in a Corbino-geometry Josephson junction created on a two-dimensional electron gas (2DEG). The Josephson junction is subjected to a weak magnetic flux that induces a fluxoid mismatch between the inner disk and outer ring superconductors. The mismatch produces a Josephson vortex (phase soliton) that binds unconventional spinful but chargeless ABSs, analogous to Jackiw-Rebbi solitonic states. The position around the Josephson junction of the trapped ABSs can be controlled externally by a junction phase bias. As the solitonic ABSs are shuttled around the Josephson junction, the 2DEG spin-orbit coupling induces a geometric precession of their spin. We argue that these solitonic ABSs constitute a natural candidate for a type of superconducting Andreev spin qubit, dubbed solitonic Andreev spin qubit (SASQ), that combines features of Andreev spin qubits and geometric spin qubits. Holonomic single-qubit SASQ operations are induced through soliton shuttling, with the resulting SU(2) trajectories densely covering the qubit Bloch sphere. Effects of disorder, nonholonomic SASQ dynamics, and other aspects of qubit operation are also analyzed.