The [[7,1,3]] Steane code and [[23,1,7]] quantum Golay code have been identified as good candidates for fault-tolerant quantum computing via code concatenation. These two codes have transversal implementations of all Clifford gates but require some other scheme for fault-tolerant $T$ gates. Using magic states, Clifford operations, and measurements is one common scheme, but magic-state distillation can have a large overhead. Code conversion is one avenue for implementing a universal gate set fault tolerantly without the use of magic-state distillation. Analogously to how the [[7,1,3]] Steane code can be fault tolerantly converted to and from the [[15,1,3]] Reed-Muller code which has a transversal $T$ gate, the [[23,1,7]] Golay code can be converted to a [[95,1,7]] triorthogonal code with a transversal $T$ gate. A crucial ingredient of this procedure is the [[49,1,5]] triorthogonal code, which can itself be seen as being related to the self-dual [[17,1,5]] two-dimensional color code. Additionally, a method for code conversion based on a transversal cnot between the codes, rather than stabilizer measurements, is described.

We present a quantum thermometry method utilizing a probe-meter system and leveraging reinforcement learning (RL) to enhance sensitivity at low temperatures. Temperature information is exclusively obtained by coupling the probe qubit with the bath, facilitating efficient information extraction by measuring the meter system. Our evaluations, considering both dephasing and dissipative probe-bath couplings, demonstrate a significant enhancement in temperature sensitivity through meter-assisted thermometry, excluding the ultralow-temperature range. To further optimize performance, RL is employed to adjust both the interaction strength between the probe-meter system and the frequency of the meter system. Results showcase that RL effectively enhances temperature sensitivity across a broad low-temperature spectrum by improving average sensitivity within the specified temperature range. This optimization successfully expands the operational range of quantum thermometers. These findings highlight the promising potential of the RL approach for high-resolution, low-temperature quantum thermometry applications.

Hamiltonian simulation represents an important module in a large class of quantum algorithms and simulations such as quantum machine learning, quantum linear algebra methods, and modeling for physics, material science, and chemistry. One of the most prominent methods for realizing the time-evolution unitary is via the Trotter-Suzuki decomposition. However, there is a large class of possible decompositions for the infinitesimal time-evolution operator as the order in which the Hamiltonian terms are implemented is arbitrary. We introduce a perspective for generating a low-depth Trotter-Suzuki decomposition assuming the standard $\mathrm{Clifford}+$rz gate set by adapting ideas from quantum error correction. We map a given Trotter-Suzuki decomposition to a constrained path on a graph which we deem the Pauli frame graph (PFG). Each node of the PFG represents the set of possible Hamiltonian terms currently available to be applied, Clifford operations represent a move from one node to another, and so the graph distance represents the gate cost of implementing the decomposition. The problem of finding the optimal decomposition is then equivalent to solving a problem similar to the traveling salesman. Although this is an NP-hard problem, we demonstrate the simplest heuristic, greedy search, and compare the resulting two-qubit gate count and circuit depth to more standard methods for a large class of scientifically relevant Hamiltonians, both fermionic and bosonic, found in chemical, vibrational, and condensed-matter problems which naturally scale. We find, in nearly every case we study, the resulting depth and two-qubit gate counts are less than those provided by standard methods, by as much as an order of magnitude. We also find the method is efficient and amenable to parallelization, making it scalable for problems of real interest.

The phenomenon of dissociative electron attachment to ethyl acetate is investigated within the 1 to 13 eV energy region. The recorded yields of various fragment anions within the above-mentioned energy range reveal a diverse array of products with six different mass numbers. Examples include (${M-\mathrm{H})}^{-}$, ${\mathrm{CH}}_3{}^{-}$, ${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{O}}^{-}$, ${\mathrm{CH}}_3{\mathrm{CO}}^{-}$, ${\mathrm{CH}}_2{\mathrm{CHO}}^{-}$, and ${\mathrm{CH}}_3{\mathrm{COO}}^{-}$, formed through the fracture of single bonds. Interestingly, the generation of other fragments, such as ${\mathrm{HCCO}}^{-}$, suggests a more intricate structural rearrangement of the nuclei, adding a layer of complexity to the observed dissociation dynamics.

We derive a theoretical model with frequency-dependent parameters for describing the transmission of electromagnetic waves in $\mathcal{PT}$-symmetric metamaterials (MMs). Based on the derived theoretical model, we investigate the tunable band structure and eigenvalue problem in the negative or positive index region of MMs, and predict the existence and stability of fundamental, out-of-phase, and in-phase dipole gap solitons in defocusing or focusing nonlinear $\mathcal{PT}$-symmetric MMs. Furthermore, we discuss the enhanced localization of solitons under different formation conditions. The results reveal that there exist abundant gap solitons with frequency controllability in $\mathcal{PT}$-symmetric MMs, and the established theoretical model paves the way to explore more solitons in $\mathcal{PT}$-symmetric MMs.

A new generation mechanism of coherent extreme-ultraviolet (XUV) and x-ray radiation presents in the few-cycle laser interaction with solid density plasma. Two-dimensional simulations show that the XUV and x-rays intensities do not have power law or exponential law dependence on the frequency which is followed by the high harmonic spikes generated through the coherent-wake-emission or relativistically oscillating mirror processes. The XUV and x rays are actually nonlinearly scattered by the successively propagating nanometer electron layers formed in the combined effects of the ponderomotive forces of the incident laser pulse and the electric force due to the charge separation. The nanometer electron layers move in the laser field and the characteristic electrons have their directions bended several times due to the change of the laser's magnetic field. At every bending point, these electrons emit strong synchrotron radiation along a direction that deviates from the reflected laser, even if the normalized drive laser amplitude is at $a_0\approx 1$. The simulation results for two cases with and without the density profile truncation indicate that the efficiency of this mechanism strongly depends on the preplasma.

We present the development and characterization of a pulsed $243\text{-nm}$ laser designed for cooling positronium (Ps) to near the recoil limit. The laser, based on the recent chirped pulse-train generator (CPTG) demonstrated by Yamada et al. [K. Yamada et al., Phys. Rev. Appl. 16, 014009 (2021)], outputs a train of pulses with spectral widths of $10\mathrm{GHz}$. The center frequency of each pulse is shifted upward (up-chirped) in time by $4.9\times {10}^2\mathrm{GHz}\mu {\mathrm{s}}^{-1}$. These parameters are determined by the mechanism of chirp cooling, a suitable scheme for cooling numerous Ps atoms to the recoil temperature of laser cooling. To achieve the desired performance, we drove an optical phase modulator in the CPTG with a high modulation depth based on the operating principle of the cooling laser. Time-resolved spectroscopic measurements confirmed that the developed laser satisfies the chirp rate and instantaneous spectral width requirements for efficient chirp cooling. We believe that the experimental demonstration of Ps laser cooling with a pulse energy of hundreds of microjoules has become possible using realistic methods for the generation and velocity measurement of Ps.

In this work we show that the set of nonsignaling resources of a locally tomographic generalized probabilistic theory (GPT), such as quantum and classical theory, coincides with its set of GPT-common-cause realizable resources, where the common causes come from an associated GPT. From a causal perspective, this result provides a reason for, in the study of resource theories of common-cause processes, taking the nonsignaling channels as the resources of the enveloping theory. This answers a critical open question in Schmidt et al. [Quantum 5, 419 (2021)]. An immediate corollary of our result is that every nonsignaling assemblage is realizable in a GPT, answering in the affirmative the question posed in Cavalcanti et al. [npj Quantum Inf. 8, 76 (2022)].

We pinpoint the conditions necessary for discrete time crystal (DTC) formation in fully connected spin-cavity systems from the perspective of parametric resonance by mapping these systems onto oscillatorlike models. We elucidate the role of nonlinearity and dissipation by mapping the periodically driven open Dicke model onto effective linear and nonlinear oscillator models, while we analyze the effect of global symmetry breaking using the Lipkin-Meshkov-Glick model with tunable anisotropy. We show that the system's nonlinearity restrains the dynamics from becoming unbounded when driven resonantly. On the other hand, dissipation keeps the oscillation amplitude of the period-doubling instability fixed, which is a key feature of DTCs. The presence of global symmetry breaking in the absence of driving is found to be crucial in the parametric resonant activation of period-doubling response. We provide analytic predictions for the resonant frequencies and amplitudes leading to DTC formation for both systems using their respective oscillator models.

Gaussian boson samplers aim to demonstrate quantum advantage by performing a sampling task believed to be classically hard. The probabilities of individual outcomes in the sampling experiment are determined by the Hafnian of an appropriately constructed symmetric matrix. For non-negative matrices, there is a family of randomized estimators of the Hafnian based on generating a particular random matrix and calculating its determinant. While these estimators are unbiased (the mean of the determinant is equal to the Hafnian of interest), their variance may be so high as to prevent an efficient estimation. Here we investigate the performance of two such estimators, which we call the Barvinok and Godsil-Gutman estimators. We find that, in general, both estimators perform well for adjacency matrices of random graphs, demonstrating a slow growth of variance with the size of the problem. Nonetheless, there are simple examples where both estimators show high variance, requiring an exponential number of samples. In addition, we calculate the asymptotic behavior of the variance for the complete graph. Finally, we simulate the Gaussian boson sampling using the Godsil-Gutman estimator and show that this technique can successfully reproduce low-order correlation functions.

Effective methods for characterizing the noise in quantum computing devices are essential for programming and debugging circuit performance. Existing approaches vary in the information obtained as well as the amount of quantum and classical resources required, with more information generally requiring more resources. Here we benchmark the characterization methods of gate set tomography, Pauli channel noise reconstruction, and empirical direct characterization for developing models that describe noisy quantum circuit performance on a 27-qubit superconducting transmon device. We evaluate these models by comparing the accuracy of noisy circuit simulations with the corresponding experimental observations. We find that the agreement of noise model to experiment does not correlate with the information gained by characterization and that the underlying circuit strongly influences the best choice of characterization approach. Empirical direct characterization scales best of the methods we tested and produced the most accurate characterizations across our benchmarks.

Scanning-tunneling-microscopy–induced luminescence can be used to probe the optical and electronic properties of molecules. Concerning the vibronic coupling, we model the molecule as a two-level system with vibrational degrees of freedom. Based on Bardeen's theory, we express the inelastic tunneling current in terms of the Huang-Rhys factor within the inelastic electron scattering mechanism. We find that the differential conductance, varying with the bias voltage, exhibits a distinct step structure with various vibronic coupling strengths. The second derivative of the inelastic tunneling current with respect to the bias voltage shows the characteristics of a vibrational-level structure with the Franck-Condon factor. Consequently, we propose a method to determine the Huang-Rhys factor of molecules, holding promising potential within the realm of solid-state physics.

A sample mixture of alkali-metal homonuclear dimers, ${\mathrm{Ak}}_2$ and ${\mathrm{Ak}}_2^{\prime }$, and heteronuclear dimers, ${\mathrm{AkAk}}^{\prime }$, residing on the surface of helium nanodroplets are Coulomb exploded into pairs of atomic alkali-metal cations, $({\text{Ak}}^{\text{+}},{\text{Ak}}^{\text{+}})$, $({\text{Ak}}^{\prime +},{\text{Ak}}^{\prime +})$, and $({\text{Ak}}^{\text{+}},{\text{Ak}}^{\prime +})$, following double ionization induced by an intense 50-fs laser pulse. The measured kinetic-energy distribution $P\left(E_{\text{kin}}\right)$ of both the ${\text{Ak}}^{\text{+}}$ and ${\text{Ak}}^{\prime +}$ fragment ions contains overlapping peaks due to contributions from Coulomb explosion of the homonuclear and heteronuclear dimers. From $P\left(E_{\text{kin}}\right)$, we determine the distribution of internuclear distances $P\left(R\right)$ via the Coulomb-explosion imaging principle. Using coincident filtering based on momentum division between the two fragment ions, we demonstrate that the individual $P\left(E_{\text{kin}}\right)$ distributions pertaining to ions from either hetero- or homonuclear dimers can be retrieved. This filtering method works through the concurrent detection of two-dimensional velocity images of the ${\text{Ak}}^{\text{+}}$ and the ${\text{Ak}}^{\prime +}$ ions implemented through the combination of a velocity-map-imaging spectrometer and a TPX3CAM detector. The key finding is that $P\left(R\right)$ can be measured for any specific dimer in the unavoidably mixed sample of hetero- and homonuclear alkali-metal dimers. We report results for $\mathrm{LiK}$ and $\mathrm{NaK}$, which were previously unmeasurable by the Coulomb-explosion technique. Our method should also work for other heteronuclear dimers and for differentiating between different isotopologues of any of the dimers.

We report a detailed study of high-order harmonic generation (HHG) in helium. When comparing predictions from a single-active-electron model with those from all-electron simulations, such as the attomesa code and $R$-matrix-with-time-dependence method, which can include different numbers of states in the close-coupling expansion, it seems imperative to generate absolute numbers for the HHG spectrum in a well-defined framework. While qualitative agreement in the overall frequency dependence of the spectrum, including the cutoff frequency predicted by a semiclassical model, can be achieved by many models in arbitrary units, only absolute numbers can be used for benchmark comparisons between different approaches.

A slab made of a dielectric structurally chiral medium (DSCM) strongly reflects the cohanded circularly polarized plane wave, but not the crosshanded circularly polarized plane wave, in a spectral regime called the circular Bragg regime. The effect of inserting a central phase defect in a DSCM slab with a modest number of structural periods is a spectral reflection hole in the circular Bragg regime, for cohanded incidence only. However, if the incident plane wave is left-circularly polarized, the geometric phase of the transmitted plane wave contains evidence of both the circular Bragg regime and the spectral reflection hole, regardless of the structural handedness of the DSCM. This evidence is indicative of the type of phase defect. The effect of inserting a central phase defect in a DSCM slab with a large number of structural periods is a spectral transmission hole in the circular Bragg regime, for crosshanded incidence only. The spectral transmission hole may be difficult to observe experimentally because of absorption inside the DSCM slab, but it will still be evident in the geometric phase of the transmitted plane wave, if the incident plane wave is left-circularly polarized.

Accurate NMR shielding constants of As and Sb in ${\mathrm{AsF}}_6^{-}$, ${\mathrm{AsO}}_4^{3-}$, ${\mathrm{SbCl}}_6^{-}$, and ${\mathrm{SbF}}_6^{-}$ complexes were calculated using the nonrelativistic coupled-cluster method and relativistic four-component density functional theory (DFT). Magnetic dipole moments of ${}^{75}\mathrm{As}$, ${}^{121}\mathrm{Sb}$, and ${}^{123}\mathrm{Sb}$ nuclei were rederived and the new recommended reference nuclear magnetic dipole moments are $\mu \left({}^{75}\mathrm{As}\right)=1.43711\left(4\right){\mu }_N$, $\mu \left({}^{121}\mathrm{Sb}\right)=3.35540\left(33\right){\mu }_N$, and $\mu \left({}^{123}\mathrm{Sb}\right)=2.54389\left(25\right){\mu }_N$, correcting systematic errors of up to $0.008{\mu }_N$ in previous reference values.

We study the effect of geometric constraints on the formation of indirect excitonic complexes with excess charge by considering the problem of two identical electrons moving in half-space subject to mutual Coulomb repulsion and the Coulomb attraction to a heavy hole of charge $Z$ residing on a flat impenetrable surface. We find that $Z=1$ is the critical charge necessary for binding, as compared to $Z_c\approx 0.911$ in the unconstrained system. This suggests that interlayer electron-hole systems may serve as a flexible and experimentally accessible platform to study electronic behavior close to the critical nuclear charge necessary for binding.

Classical simulation of quantum systems plays an important role in the study of many-body phenomena and in the benchmarking and verification of quantum technologies. Exact simulation is often limited to small systems because the dimension of the Hilbert space increases exponentially with the size of the system. For systems that possess a high degree of symmetry, however, classical simulation can reach much larger sizes. Here we consider an ensemble of strongly interacting atoms with permutation symmetry, enabling the computation of certain collective observables for hundreds of atoms at arbitrarily long evolution times. The system is realized by an ensemble of three-level atoms, where one of the levels corresponds to a highly excited Rydberg state. In the limit of all-to-all Rydberg blockade, the Hamiltonian is invariant under permutation of the atoms. Using techniques from representation theory, we construct a block-diagonal form of the Hamiltonian, where the size of the largest block increases only linearly with the system size. We apply this formalism to derive efficient pulse sequences to prepare arbitrary permutation-invariant quantum states. Moreover, we study the quantum dynamics following a quench, uncovering a parameter regime in which the system thermalizes slowly and exhibits pronounced revivals. Our results create opportunities for the experimental and theoretical study of large interacting and nonintegrable quantum systems.

We study a system of trapped two-component Fermi gases with a zero-range interaction in two dimensions or one dimension. We calculate the one-particle density matrices of these systems at small displacements, from which we show that the $N$-body energies are linear functionals of the occupation probabilities of single-particle energy eigenstates. A universal energy functional was first derived in 2011 [S. Tan, Phys. Rev. Lett. 107, 145302 (2011)] for trapped zero-range interacting two-component Fermi gases in three dimensions. We also calculate the asymptotic behaviors of the occupation probabilities of single-particle energy eigenstates at high energies.