A deeper understanding of the differences between quantum and classical dynamics promises great potential for emerging technologies. Nevertheless, some aspects remain poorly understood, particularly concerning the role of quantum coherence in open quantum systems. On the one hand, coherence leads to entanglement and even nonlocality. On the other, it may lead to a suppression of fluctuations, causing violations of thermokinetic uncertainty relations (TUR and KUR) that are valid for classical processes. These represent two different manifestations of coherence, one depending only on the state of the system (static) and one depending on two-time correlation functions (dynamical). Here we employ these manifestations of coherence to determine when mesoscopic quantum transport can be captured by a classical model based on stochastic jumps, and when such a model breaks down, implying nonclassical behavior. To this end, we focus on a minimal model of a double quantum dot coupled to two thermal reservoirs. In this system, quantum tunneling induces Rabi oscillations and results in both entanglement and nonlocality, as well as TUR and KUR violations. These effects, which describe the breakdown of a classical description, are accompanied by a peak in coherence. Our results provide guiding principles for the design of out-of-equilibrium devices that exhibit nonclassical behavior.

We present a unified treatment of the Aharonov-Bohm (AB) effect for two-dimensional multiband electronic systems possessing isotropic band structures. We propose an integral representation of the AB scattering state of an electron scattered by an infinitely thin solenoid. Moreover, we derive the asymptotic form of the AB scattering state and obtain the differential cross section from that. We found a remarkable result, namely, that this cross section is the same for all isotropic systems and agrees with that obtained first by Aharonov and Bohm for spinless free-particle systems. To demonstrate the generality of our theory, we consider several specific multiband systems relevant to condensed matter physics.

It is well known that two-dimensional fermionic systems with a nonzero Chern number must break the time-reversal symmetry, manifested by the appearance of chiral edge modes on an open boundary. Such an incompatibility between topology and symmetry can occur more generally. We will refer to this phenomenon as enforced symmetry breaking (ESB) by topological orders. In this work, we systematically study ESB of a finite symmetry group $G_f$ by fermionic invertible topological orders. Mathematically, the group $G_f$ is a central extension over a bosonic symmetry group $G$ by the fermion parity group ${\mathbb{Z}}_2^f$, characterized by a 2-cocycle $\lambda \in {\mathcal{H}}^2(G,{\mathbb{Z}}_2)$. For given $G$ and $\lambda$, we are able to obtain a series of criteria on the existence or nonexistence of ESB by the corresponding fermionic invertible topological orders. For 2D systems, we define a set of physical quantities to describe symmetry-enriched invertible topological orders and derive obstruction functions using both fermionic and bosonic languages. The study in the bosonic language is performed after gauging the fermion parity, and we find that some obstruction functions are consequences of conditional anomalies of the bosonic symmetry-enriched topological states, with the conditions inherited from the original fermionic system. The obstruction functions are crucial components of the ESB criteria that we derive. With these criteria, we discover many new ESB examples, e.g., we find that the quaternion group $Q_8$ is incompatible with two copies of $p+ip$ superconductors. We also obtain explicit results on the ESB phenomena of the continuous group ${\mathrm{SU}}_f\left(N\right)$ by 2D invertible topological orders through a different argument.

We have performed micro-/nanofocused angle-resolved photoemission spectroscopy (ARPES) on cleaved single-crystal surfaces of bismuth to clarify the spatially resolved electronic states. While the dominant area of the cleaved surface was found to display the well-known Rashba-spin-split surface state with the (111)-surface origin, the steplike region with a typical width of $\sim 10\text{–}20$ $\mu \mathrm{m}$ shows distinctly different band structure and fermiology originating from the hard-to-cleave (100) surface. This unexpected mixture of the (100)-derived electronic states in a tiny area of the cleaved (111) surface is attributed to the crystal planes separated by a twin boundary, as supported by laser microscopy and electron backscatter diffraction measurements. The present study paves a pathway toward investigating electronic states associated with inhomogeneities and coexisting phases of hard-to-cleave crystal planes and complex materials by spatially resolved ARPES, making this technique a powerful method to investigate the interplay between local electronic states and crystal structures when combined with structural characterization techniques.

An algorithm for a particular problem may find some instances of the problem easier and others harder to solve, even for a fixed input size. We numerically analyze the relative hardness of MAX 2-SAT problem instances for various continuous-time quantum algorithms and a comparable classical algorithm. This has two motivations: To investigate whether small-sized problem instances, which are commonly used in numerical simulations of quantum algorithms for benchmarking purposes, are a good representation of larger instances in terms of their hardness to solve, and to determine the applicability of continuous-time quantum algorithms in a portfolio approach, where we take advantage of the variation in the hardness of instances between different algorithms by running them in parallel. We find that, while there are correlations in instance hardness between all of the algorithms considered, they appear weak enough that a portfolio approach would likely be desirable in practice. Our results also show a widening range of hardness of randomly generated instances as the problem size is increased, which demonstrates both the difference in the distribution of hardness at small sizes and the value of a portfolio approach that can reduce the number of extremely hard instances. We identify specific weaknesses of these quantum algorithms that can be overcome with a portfolio approach, such their inability to efficiently solve satisfiable instances (which is easy classically).

Discrete-time quantum walks, quantum generalizations of classical random walks, provide a framework for quantum information processing, quantum algorithms, and quantum simulation of condensed-matter systems. The key property of quantum walks, which lies at the heart of their quantum information applications, is the possibility for a parametric quantum speedup in propagation compared to classical random walks. In this work we study propagation of quantum walks on percolation-generated two-dimensional random lattices. In large-scale simulations of topological and trivial split-step walks, we identify distinct prediffusive and diffusive behaviors at different timescales. Importantly, we show that even arbitrarily weak concentrations of randomly removed lattice sites give rise to a complete breakdown of the superdiffusive quantum speedup, reducing the motion to ordinary diffusion. By increasing the randomness, quantum walks eventually stop spreading due to Anderson localization. Near the localization threshold, we find that the quantum walks become subdiffusive. The fragility of quantum speedup implies dramatic limitations for quantum information applications of quantum walks on random geometries and graphs.

We report on photoassociation spectroscopy probing the $c^3{\mathrm{\Sigma }}_1^{+}$ potential of the bialkali NaCs molecule, identifying 11 vibrational lines between $v^{\prime }=0$ and $v^{\prime }=25$ of the excited $c^3{\mathrm{\Sigma }}_1^{+}$ potential and fitting their rotational and hyperfine structure. The observed lines are assigned by fitting to an effective Hamiltonian model of the excited-state structure with rotational and hyperfine constants as free parameters. We discuss unexpected broadening of select vibrational lines and its possible link to strong spin-orbit coupling of the $c^3{\mathrm{\Sigma }}_1^{+}$ potential with the nearby $b^3{\mathrm{\Pi }}_1$ and $B^1{\mathrm{\Pi }}_1$ manifolds. Finally, we report use of the $v^{\prime }=22$ line as an intermediate state for two-photon transfer of weakly bound Feshbach molecules to the rovibrational ground state of the $X^1{\mathrm{\Sigma }}^{+}$ manifold.

The photon blockade effect is commonly exploited in the development of single-photon sources. While the photon blockade effect could be used to prepare high-fidelity single-photon states in idealized regimes, practical implementations in optomechanical systems suffer from an interplay of competing processes. Here we derive a control scheme that exploits destructive interference of Fock state amplitudes of more than one photon. The resulting preparation time for photon-blockaded quantum states is limited only by the optomechanical interaction strength and can thus be orders of magnitude shorter than in existing schemes that achieve photon blockade in the steady state.

The quantum approximate optimization algorithm (QAOA) is a promising method for solving certain classical combinatorial optimization problems on near-term quantum devices. When employing the QAOA to 3-SAT and Max-3-SAT problems, the quantum cost exhibits an easy-hard-easy or easy-hard pattern, respectively, as the clause density is changed. The quantum resources needed in the hard-region problems are out of reach for current noisy intermediate-scale quantum (NISQ) devices. We show by numerical simulations with up to 14 variables and analytical arguments that the adaptive-bias QAOA (ab-QAOA) greatly improves performance in the hard region of the 3-SAT problems and hard region of the Max-3-SAT problems. For similar accuracy, on average, ab-QAOA needs 3 levels for 10-variable 3-SAT problems as compared to 22 for QAOA. For 10-variable Max-3-SAT problems, the numbers are 7 levels and 62 levels. The improvement comes from a more targeted and more limited generation of entanglement during the evolution. We demonstrate that classical optimization is not strictly necessary in the ab-QAOA since local fields are used to guide the evolution. This leads us to propose an optimization-free ab-QAOA that can solve the hard-region 3-SAT and Max-3-SAT problems effectively with significantly fewer quantum gates as compared to the original ab-QAOA. Our work paves the way for realizing quantum advantages for optimization problems on NISQ devices.

Hamiltonian simulation is a promising application for quantum computers to achieve a quantum advantage. We present classical algorithms based on tensor network methods to optimize quantum circuits for this task. We show that, compared to Trotter product formulas, the classically optimized circuits can be orders of magnitude more accurate and significantly extend the total simulation time.

Ground-based optical astronomical observations supported by or in the vicinity of laser guide-star systems can be contaminated by Raman-scattered laser photons. Anticipating, alleviating, and correcting for the impact of this self-inflicted contamination requires a detailed knowledge of the pure-rotational and rotational-vibrational spectrum of the molecules in our atmosphere. We present a 15.3-hr-deep combined spectrum of the 4LGSF's 589 $\mathrm{nm}\approx 509$ THz sodium laser beams of Paranal observatory, acquired with the ESPRESSO spectrograph at a resolution $\lambda /\mathrm{\Delta }\lambda \cong 140000\approx 0.12$ ${\mathrm{cm}}^{-1}$ and an altitude of 23 km above mean sea level. We identify 865 Raman lines over the spectral range of [3770; 7900] Å$\approx$ [+9540; –4315] ${\mathrm{cm}}^{-1}$, with relative intensities spanning $\sim 5$ orders of magnitudes. These lines are associated to the most abundant molecules of dry air, including their isotopes: ${}^{14}\mathrm{N}{}^{14}\mathrm{N}, {}^{14}\mathrm{N}{}^{15}\mathrm{N}, {}^{16}\mathrm{O}{}^{16}\mathrm{O}, {}^{16}\mathrm{O}{}^{17}\mathrm{O}, {}^{16}\mathrm{O}{}^{18}\mathrm{O}$, and ${}^{12}\mathrm{C}{}^{16}\mathrm{O}{}^{16}\mathrm{O}$. The signal-to-noise of these observations implies that professional observatories can treat the resulting catalog of Raman lines as exhaustive (for the detected molecules, over the observed Raman shift range) for the purpose of predicting/correcting/exploiting Raman lines in astronomical data. Our observations also reveal that the four laser units of the 4LGSF do not all lase at the same central wavelength. We measure a blueshift of $+43\pm 10$ MHz $\cong -50\mp 10$ fm with respect to ${\lambda }_{*}=5891.59120$ Å for LGSU1/2, and $+94\pm 10$ MHz $\cong -109\mp 10$ fm for LGSU3/4. These offsets, including the difference of $\sim 50$ MHz between LGSU1/2 and LGSU3/4, are larger than the observed 4LGSF spectral stability of $\pm 3$ MHz over hours. They remain well within the operational requirements for creating artificial laser guide-stars, but hinder the assessment of the radial velocity accuracy of ESPRESSO at the required level of 10 m ${\mathrm{s}}^{-1}$. Altogether, our observations demonstrate how Raman lines can be exploited by professional observatories as highly-accurate, on-sky wavelength references.

Anomalous diffusion in various complex systems abounds in nature and spans multiple space and time scales. Canonical characterization techniques that rely upon mean squared displacement break down for nonergodic processes, making it challenging to characterize anomalous diffusion from an individual time-series measurement. Nonergodicity reigns when the time-averaged mean square displacement differs from the ensemble-averaged mean squared displacement even in the limit of long measurement series. In these cases, the typical theoretical results for ensemble averages cannot be used to understand and interpret data acquired from time averages. The difficulty then lies in obtaining statistical descriptors of the measured diffusion process that are not nonergodic. We show that linear descriptors such as the standard deviation, coefficient of variation, and root mean square break ergodicity in proportion to nonergodicity in the diffusion process. In contrast, time series of descriptors addressing sequential structure and its potential nonlinearity: multifractality change in a time-independent way and fulfill the ergodic assumption, largely independent of the time series' nonergodicity. We show that these findings follow the multiplicative cascades underlying these diffusion processes. Adding fractal and multifractal descriptors to typical linear descriptors would improve the characterization of anomalous diffusion processes. Two particular points bear emphasis here. First, as an appropriate formalism for encoding the nonlinearity that might generate nonergodicity, multifractal modeling offers descriptors that can behave ergodically enough to meet the needs of linear modeling. Second, this capacity to describe nonergodic processes in ergodic terms offers the possibility that multifractal modeling could unify several disparate nonergodic diffusion processes into a common framework.