A controlled-not (cnot) circuit is the key gadget for entangling qubits in quantum computing systems. However, the qubit connectivity of noisy intermediate-scale quantum (NISQ) devices is constrained by their limited-connectivity architecture. To improve the performance of cnot circuits on NISQ devices, we investigate the optimization of the size/depth of cnot circuits under the limited-connectivity architecture. We present a method that can optimize the size of any $n$-qubit cnot circuit $O\left(\frac{n^2}{log\delta }\right)$ on any connected graph with minimum degree $\delta$, and prove this bound is optimal for the regular graph. For the near-term sparsely connected structure, we additionally present a method that can optimize the size of any $n$-qubit cnot circuit to below $2n^2$. The numerical experiment shows that our method performs better than state-of-the-art results. Specifically, we present an example to illustrate the applicability of our algorithm. For the grid structure, which is commonly used in current quantum devices, we demonstrate that the depth of any $n$-qubit cnot circuit can be optimized to be linear in $n$ with certain ancillary qubits (ancillas). Experimental results indicate that this method has significant improvements compared with all of the existing methods. We additionally test our algorithms on the five-qubit IBMQ devices, and the experiments show that the measurement results of the optimized circuit with our algorithm are more robust to noise compared with the IBM mapping method.

Fluctuations in the number of nascent RNA accurately reflect transcriptional activity. However, mathematical models predicting their distributions are difficult to solve analytically due to their non-Markovian nature stemming from transcriptional elongation. Here we circumvent this problem by deriving an exact relationship between the steady-state distribution of nascent RNA and the distribution of initiation times, which can be computed for any general initiation mechanism described by a set of first-order reactions. We test our theory using simulations and live cell imaging data.

Beam parameter optimization in accelerators involves multiple, sometimes competing, objectives. Condensing these individual objectives into a single figure of merit unavoidably results in a bias towards particular outcomes, often in an undesired way in the absence of prior knowledge. Finding an optimal objective definition then requires operators to iterate over many possible objective weights and definitions, a process that can take many times longer than the optimization itself. A more versatile approach is multi-objective optimization, which establishes the trade-off curve or Pareto front between objectives. Here we present the first results on multi-objective Bayesian optimization of a simulated laser-plasma accelerator. We find that multi-objective optimization reaches comparable performance to its single-objective counterparts while allowing for instant evaluation of entirely new objectives. This dramatically reduces the time required to find appropriate objective definitions for new problems. Additionally, our multi-objective, multi-fidelity method reduces the time required for an optimization run by an order of magnitude. It does so by dynamically choosing simulation resolution and box size, requiring fewer slow and expensive simulations as it learns about the Pareto-optimal solutions from fast low-resolution runs. The techniques demonstrated in this paper can easily be translated into many different computational and experimental use cases beyond accelerator optimization.

For high repetition ultrahigh-intensity laser system, automatic alignment of structured target is key to achieving consistent particle acceleration and plasma heating. In this work, we demonstrate efficient electron acceleration with two sequential steps of laser processing using a high repetition rate, 30-fs ultrahigh-intensity laser. The first pulse does laser machining and creates a steep cylindrical crater on the surface of a flat stainless-steel target. The crater is formed by the hydrodynamic expansion of the heated surface and by spallation of the inner, deeper material by nonthermal relativistic electrons. The crater shape is well controlled and reproducible with 200 μm width and 350 μm depth. The second pulse irradiates deeply inside the crater and interacts with the crater wall, efficiently accelerating electrons via surface plasmonic, without need for target realignment. The laser absorption efficiency increases from 32.5 to 97.5% by the process.

The quantization of the PageRank algorithm is a promising tool for a future quantum internet. Here we present a modification of the quantum PageRank, introducing arbitrary phase rotations (APR) in the underlying Szegedy's quantum walk. We define three different APR schemes with only one phase as a degree of freedom. We have analyzed the behavior of these algorithms in a small generic graph, observing that a decrease of the phase reduces the standard deviation of the instantaneous PageRank, so the nodes of the network can be distinguished better. However, the algorithm takes more time to converge, so the phase cannot be decreased arbitrarily. With these results we choose a concrete value for the phase to later apply the algorithm to complex scale-free graphs. In these networks, the original quantum PageRank is able to break the degeneracy of the residual nodes and detect secondary hubs that the classical algorithm suppresses. Nevertheless, not all of the detected secondary hubs are real according to the PageRank's definition. Some APR schemes can overcome this problem, restoring the degeneration of the residual nodes and highlighting the truly secondary hubs of the networks. Finally, we have studied the stability of the new algorithms. The original quantum algorithm was known to be more stable than the classical. We have found that one of our algorithms, whose PageRank distribution resembles the classical one, has a stability similar to the original quantum algorithm.

We rigorously prove a no-go theorem that, in sharp contrast to the case of compiling unitary gates, it is impossible to compile an arbitrary channel to arbitrary precision with any given finite elementary channel set, regardless of the length of the decomposition sequence. However, for a fixed error bound $\epsilon$, we find a general and systematic strategy to compile arbitrary quantum channels. We construct a universal set with a constant number of $\epsilon$-dependent elementary channels, such that an arbitrary quantum channel can be decomposed into a sequence of these elementary channels followed by a unitary gate, with the sequence length bounded by $O(\frac{1}{\epsilon }log\frac{1}{\epsilon })$ in the worst case. We further optimize this approach by exploiting proximal policy optimization—a powerful deep reinforcement learning algorithm for the gate compilation. We numerically evaluate the performance of our algorithm concerning topological compiling of Majorana fermions, and we show that our algorithm can conveniently and effectively reduce the use of expensive elementary operations.

Nonadiabatic holonomic quantum computation (NHQC) provides an essential way to construct robust and high-fidelity quantum gates due to its geometric features. However, NHQC is more sensitive to decay and dephasing errors than a conventional dynamical gate since it requires an ancillary intermediate state. Here, we utilize the Hamiltonian reverse engineering technique to study the influence of intermediate-state decoherence on NHQC gate fidelity, and propose schemes to construct an arbitrary single-qubit holonomic gate and nontrivial two-qubit holonomic gate with high fidelity and robustness to the decoherence. Although the proposed method is generic and can be applied to many experimental platforms, such as superconducting qubits, trapped ions, and quantum dots, here we take a nitrogen-vacancy center as an example to show that the gate fidelity can be significantly enhanced from 89% to 99.6% in contrast to recent experimental NHQC schemes [Phys. Rev. Lett. 119, 140503 (2017); Nat. Photonics 11, 309 (2017); Opt. Lett. 43, 2380 (2018)], and the robustness against decoherence can also be significantly improved. All in all, our scheme provides a promising way for fault-tolerant geometric quantum computation.

We report on a neutron diffraction study of single-crystal ${\mathrm{CePtAl}}_3$ complemented by measurements of the specific heat under applied magnetic field. Below $T_{\mathrm{N}}\approx 3$ K, ${\mathrm{CePtAl}}_3$ develops incommensurate antiferromagnetic order with a single modulation vector $\mathbf{k}$ = $\left(0.67600\right)$. Residual magnetic scattering intensity above $T_{\mathrm{N}}$ and a broadening of the specific heat anomaly at $T_{\mathrm{N}}$ may be consistently described in terms of a Gaussian distribution of transition temperatures with a standard deviation $\sigma \approx 0.5$ K. The distribution of $T_{\mathrm{N}}$ may be attributed to the observation of occupational and positional disorder between the Pt and Al sites consistent with structural information inferred from neutron diffraction. Measurements under magnetic field reveal changes of the magnetic domain populations when the field is applied along the $\left[010\right]$ direction consistent with a transition from cycloidal to amplitude-modulated magnetic order $\sim$2.5 T.

Intensity interferometry based on Hanbury Brown and Twiss's seminal experiment for determining the radius of the star Sirius formed the basis for developing the quantum theory of light. To date, the principle of this experiment is used in various forms across different fields of quantum optics, imaging, and astronomy. Although the technique is powerful, it has not been generalized for objects at different temperatures. Here, we address this problem using a generating functional formalism by employing the $P$-function representation of quantum-thermal light. Specifically, we investigate the photon coincidences of a system of two extended objects at different temperatures using this theoretical framework. We show two unique aspects in the second-order quantum coherence function: interference oscillations and a long-baseline asymptotic value that depends on the observation frequency, temperatures, and size of both objects. We apply our approach to the case of binary stars and discuss the advantages of measuring these two features in an experiment. In addition to the estimation of the radii of each star and the distance between them, we also show that the present approach is suitable for the estimation of temperatures as well. To this end, we apply it to the practical case of binary stars Luhman 16 and Spica $\alpha$ Vir. We find that for currently available telescopes, an experimental demonstration is feasible in the near term. Our work contributes to the fundamental understanding of intensity interferometry of quantum-thermal light and can be used as a tool for studying two-body thermal emitters, from binary stars to extended objects.

Large atomic ensembles coupled to a single optical resonator mode can be steered to strongly enhanced or suppressed collective emission via phase controlled excitation. Employing the Tavis-Cummings model we find so far unreported phenomena. Using a second order cumulant expansion we predict that a homogeneously excited ensemble equally distributed between odd and even sites along the cavity mode is extremely subradiant as long as the average excitation remains below 50%, but shows pulsed emission for inversion. The combination of these two properties enables the implementation of an efficient cavity-enhanced Ramsey probing featuring a fast readout and minimal heating with particular advantages for atomic clock transitions. For continuous illumination the nonlinear atom-field interaction induces regular superradiant self-pulsing. Additionally, we observe an increased pulse delay time in comparison to an excitation through the cavity.

The role of interatomic and intermolecular energy and charge-transfer processes in weakly bound matter is currently lively debated due to emerging destructive low-energy electrons and radicals. Here, we discuss two mechanisms of single-photon two-site double ionization occurring competitively or subsequently to resonant interatomic Coulombic decay (rICD) in inner-valence ($2s\to np$) excited Ne dimers and clusters. The first mechanism is photoelectron-impact ionization which is, in general, not related to resonant excitation, but in the present case strongly enhanced and, thus, observable due to resonant excitation. Studying this mechanism at its energetic threshold enables addressing a subset of Ne dimers with selected bond lengths. The second mechanism is collisional ionization of energetic Rydberg atoms, which are produced by rICD in Ne clusters and may be ionized by collisions with neutrals on their way through the medium. Both mechanisms are identified by the coincident detection of charged products and, for the case of collisional ionization, confirmed by calculations. These mechanisms produce one more low-energy electron and ion than conventional rICD and, thus, should be considered in the discussion of the biochemical impact of photoinduced rICD processes.

Magnetic responses in nonmagnetic transition metal dichalcogenides (TMDs) via reduced dimension and defect engineering continue to bring forth emergent properties and has revolutionized important technologies such as data storage and magnetic sensing. Using the first-principles calculation, we investigate the zigzag-edged ${\mathrm{PtSe}}_2$ (zz-${\mathrm{PtSe}}_2$) nanoribbon with a sizable magnetization solely at the zigzag edges and a remarkable magnetic-nonmagnetic switching induced by a small compressive uniaxial strain of about −2%. We further interpret the sudden disappearance of the magnetization by a simple but clear model simulation considering the localized quasi-one-dimensional magnetic channels mediated by metallic backgrounds along the edges. Such newly proposed strain-controlled magnetic transition through band structure engineering highlights zz-${\mathrm{PtSe}}_2$ as a unique candidate for controllable TMD-based one-dimensional magnets with promising advances in fundamental physics and technological opportunities in spintronics, computing, and sensors.

Using a nonperturbative classical approach, we study the dynamics of a mobile particle interacting with an infinite one-dimensional (1D) chain of harmonic oscillators. This minimal system is an effective model for many 1D transport phenomena, such as molecular motion in nanotubes and ionic conduction through solid-state materials. As expected, coupling between the mobile particle and the chain induces dissipation of the mobile particle's energy. However, both numerical and analytic results demonstrate an unconventional nonmonotonic dependence of the drag on particle speed. In addition, when this system is subjected to a constant bias, it supports multiple steady-state drift velocities.

It is well known that scattering from acoustic fields can produce forces on single particles; however, they can also induce interparticle forces due to multiple scattering events. This multiparticle force—here referred to as acoustic binding—is comparable to other acoustic forces when the particles are of order wavelength in diameter. In principle, this force could be used as a tunable method for directing the assembly of particles of mm scales, but has not been extensively explored in previous work. Here, we use a numerical method to compute binding interactions between strongly scattering bodies and find that they can produce stable clusters of particles with approximately wavelength separation. Moreover, we also observe that—depending on the level of viscous damping—these structures can produce driven linear, rotational, or vibrational motion. These effects are a result of the nonconservative and nonpairwise nature of the acoustic binding force and represent contactless manipulation and transport methods with a variety of potential applications.

In the field of quantum chaos, spectral statistics is one of the most extensively investigated characteristics. Despite a large body of existing literature, the effects of many-body interactions on the spectral statistics of relativistic quantum systems remain poorly understood. Treating electron-electron interactions through the one-orbital mean-field Hubbard model, we address this fundamental issue using graphene billiards with the geometric shape of a circular sector as prototypical systems. Our approach is to consider the two characteristically different cases where the statistics are Poisson and Gaussian orthogonal ensemble (GOE) so the corresponding classical dynamics are typically integrable and chaotic, respectively, and to systematically investigate how the statistics change as the Hubbard interaction strength increases from zero. We find that, for energies near the Dirac point, the Hubbard interactions have a significant effect on the spectral statistics. Regardless of the type of spectral statistics to begin with, increasing the Hubbard interaction strength up to a critical value causes the statistics to approach GOE, rendering more applicable the random matrix theory. As the interaction strength increases beyond the critical value, the statistics evolve toward Poisson, due to the emergence of an energy gap rendering the quasiparticles massive. We also find that the energy levels above and below the Dirac point can exhibit different statistics, and the many-body interactions have little effect on the statistics for levels far from the Dirac point. These results reveal the intriguing interplay between many-body interactions and the spectral statistics, which we develop a physical picture to understand.

The mean ionization state (MIS) is a critical property in dense plasma and warm dense matter research, for example, as an input to hydrodynamics simulations and Monte Carlo simulations. Unfortunately, however, the best way to compute the MIS remains an open question. Average-atom (AA) models are widely used in this context due to their computational efficiency, but as we show here, the canonical approach for calculating the MIS in AA models is typically insufficient. We therefore explore three alternative approaches to compute the MIS. First, we modify the canonical approach to change the way electrons are partitioned into bound and free states; second, we develop a novel approach using the electron localization function; finally, we extend a method, which uses the Kubo-Greenwood conductivity to our average-atom model. Through comparisons with higher-fidelity simulations and experimental data, we find that any of the three new methods usually outperforms the canonical approach, with the electron localization function and Kubo-Greenwood methods showing particular promise.