We consider Gaussian quantum circuits supplemented with non-Gaussian input states and derive sufficient conditions for efficient classical strong simulation of these circuits. In particular, we generalise the stellar representation of continuous-variable quantum states to the multimode setting and relate the stellar rank of the input non-Gaussian states, a recently introduced measure of non-Gaussianity, to the cost of evaluating classically the output probability densities of these circuits. Our results have consequences for the strong simulability of a large class of near-term continuous-variable quantum circuits.

Shockley surface states (SS) have attracted much attention due to their role in various physical phenomena occurring at surfaces. It is also clear from experiments that they can play an important role in electron transport. However, accurate incorporation of surface states in $\mathit{ab}\mathit{initio}$ quantum transport simulations remains still an unresolved problem. Here we go beyond the state-of-the-art nonequilibrium Green's function formalism through the evaluation of the self-energy in real-space, enabling electron transport without using artificial periodic in-plane conditions. We demonstrate the method on three representative examples based on Au(111): a clean surface, a metallic nanocontact, and a single-molecule junction. We show that SS can contribute more than 30% of the electron transport near the Fermi energy. A significant and robust transmission drop is observed at the SS band edge due to quantum interference in both metallic and molecular junctions, in good agreement with experimental measurements. The origin of this interference phenomenon is attributed to the coupling between bulk and SS transport channels and it is reproduced and understood by tight-binding model. Furthermore, our method predicts much better quantized conductance for metallic nanocontacts.

The interplay between nontrivial topological states of matter and strong electronic correlations is one of the most compelling open questions in condensed matter physics. Due to experimental challenges, there is an increasing desire to find more microscopic techniques to complement the results of more traditional experiments. In this work, we locally explore the Kondo insulator ${\mathrm{Sm}}_{1-x}{\mathrm{Gd}}_x{\mathrm{B}}_6$ by means of electron spin resonance (ESR) of ${\mathrm{Gd}}^{3+}$ ions at low temperatures. Our analysis reveals that the ${\mathrm{Gd}}^{3+}$ ESR line shape shows an anomalous evolution as a function of temperature, wherein for highly dilute samples $(x\approx 0.0002)$ the ${\mathrm{Gd}}^{3+}$ ESR line shape changes from a localized ESR local moment character to a diffusive-like character. Upon manipulating the sample surface with a focused ion beam we demonstrate, in combination with electrical resistivity measurements, that the localized character of the ${\mathrm{Gd}}^{3+}$ ESR line shape is recovered by increasing the penetration of the microwave in the sample. This provides compelling evidence for the contribution of surface or near-surface excitations to the relaxation mechanism in the ${\mathrm{Gd}}^{3+}$ spin dynamics. Our work brings new insights into the importance of nontrivial surface excitations in ESR, opening new routes to be explored both theoretically and experimentally.

Mesoscopic quantum dots (QDs) are ubiquitous in quantum devices as reliable sources of hot electrons. However, we have observed an unexpectedly significant energy relaxation of QD-emitted hot electrons up to $\approx 55\%$ of its excitation $\le 1.5\mathrm{meV}$ from the Fermi level. The energetics of hot electrons were obtained through transverse magnetic focusing over a few microns using both QD and quantum point contact (QPC) emitters. Unlike the QPC counterparts, QD emissions deviated substantially from Fermi gas predictions—the focusing peak appeared at lower magnetic fields, and excessive broadening was observed. The phenomenon was modeled by a capacitive interaction transferring energy from the hot electron to the QD. Model simulations reproduced the key experimental features, implying the presence of a strong yet overlooked relaxation mechanism that is intrinsic to QD emissions. Our observation calls for the prudent use of QDs as single electron sources.

Parafermionic zero modes, ${\mathbb{Z}}_n$-symmetric generalizations of the well-known ${\mathbb{Z}}_2$ Majorana zero modes, can emerge as edge states in topologically nontrivial strongly correlated systems displaying fractionalized excitations. In this paper, we investigate how signatures of parafermionic zero modes can be detected by its effects on the properties of a quantum dot tunnel-coupled to a system hosting such states. Concretely, we consider a strongly correlated one-dimensional fermionic model supporting ${\mathbb{Z}}_4$ parafermionic zero modes coupled to an interacting quantum dot at one of its ends. By using a combination of density matrix renormalization group calculations and analytical approaches, we show that the dot's zero-energy spectral function and average occupation numbers can be used to distinguish between trivial, ${\mathbb{Z}}_4$ and $2\times {\mathbb{Z}}_2$ phases of the system. The present work opens the prospect of using quantum dots as detection tools to probe nontrivial topological phases in strongly correlated systems.

Understanding collisions between ultracold molecules is crucial for making stable molecular quantum gases and harnessing their rich internal degrees of freedom for quantum engineering. Transient complexes can strongly influence collisional physics, but in the ultracold regime, key aspects of their behavior have remained unknown. To explain experimentally observed loss of ground-state molecules from optical dipole traps, it was recently proposed that molecular complexes can be lost due to photoexcitation. By trapping molecules in a repulsive box potential using laser light near a narrow molecular transition, we are able to test this hypothesis with light intensities three orders of magnitude lower than what is typical in red-detuned dipole traps. This allows us to investigate light-induced collisional loss in a gas of nonreactive fermionic ${}^{23}\mathrm{Na}{}^{40}\mathrm{K}$ molecules. Even for the lowest intensities available in our experiment, our results are consistent with universal loss, meaning unit loss probability inside the short-range interaction potential. Our findings disagree by at least two orders of magnitude with latest theoretical predictions, showing that crucial aspects of molecular collisions are not yet understood and provide a benchmark for the development of new theories.

We numerically investigate the topological phase transition induced purely by disorder in a spring-mass chain. We employ two types of disorders—chiral and random types—to explore the interplay between topology and disorder. By tracking the evolution of real-space topological invariants, we obtain the topological phase diagrams and demonstrate the bilateral capacity of disorder to drive topological transitions, from topologically nontrivial to trivial and vice versa. The corresponding transition is accompanied by the realization of a mechanical topological Anderson insulator. The findings from this study hint that the combination of disorder and topology can serve as an efficient control knob to manipulate the transfer of mechanical energy.

We consider the problem of estimating multiple analytic functions of a set of local parameters via qubit sensors in a quantum sensor network. To address this problem, we highlight a generalization of the sensor symmetric performance bounds of Rubio et al., [J. Phys. A 53, 344001 (2020)] and develop an optimized sequential protocol for measuring such functions. We compare the performance of both approaches to one another and to local protocols that do not utilize quantum entanglement, emphasizing the geometric significance of the coefficient vectors of the measured functions in determining the best choice of measurement protocol. We show that, in many cases, especially for a large number of sensors, the optimized sequential protocol results in more accurate measurements than the other strategies. In addition, in contrast to the sensor symmetric approach, the sequential protocol is known to always be explicitly implementable. The sequential protocol is very general and has a wide range of metrological applications.

We propose a geometric quantum computation (GQC) scheme, called Floquet GQC (FGQC), where error-resilient geometric gates based on periodically driven two-level systems can be constructed via a non-Abelian geometric phase proposed in a recent study [V. Novičenko and G. Juzeliūnas, Phys. Rev. A 100, 012127 (2019)]. Based on Rydberg atoms, we give possible implementations of universal FGQC single-qubit gates and a nontrivial FGQC two-qubit gate. By using numerical simulation, we evaluate the performance of the FGQC Z and X gates in the presence of both decoherence and a certain kind of systematic control error. For the currently available coherence time of the Rydberg state, $T_2\approx 32\mu \mathrm{s}$, the numerical results show that the X and Z gate fidelities are about $0.900$ and $0.899$, respectively. In addition, we find that FGQC is robust against global control error; both analytical demonstration and numerical evidence are given. As the coherence time of various qubits grows, FGQC may provide a promising error-resilient quantum computation scheme in the future.

Superfluid vortices are quantum excitations carrying a quantized amount of orbital angular momentum in a phase where global symmetry is spontaneously broken. We address a question whether magnetic vortices in superconductors with dynamical gauge fields carry nonzero orbital angular momentum or not. We provide the answer in view of the angular momentum conservation in several distinct classes of examples from interdisciplinary fields of physics across condensed matter, dense nuclear systems, and cosmology. The angular momentum carried by gauge field configuration around the magnetic vortex plays a crucial role in satisfying the principle of the angular momentum conservation. Based on various ways on how the angular momentum conservation is realized, we provide a general scheme of classifying magnetic vortices in different phases of matter.

We investigate proximity effects in a spin-triplet superconductor (TSC) interfaced with a ferromagnet (FM), assuming different types of magnetic profiles and chiral or helical pairings. The region of the coexistence of spin-triplet superconductivity and magnetism is significantly influenced by the orientation and spatial extension of the magnetization with respect to the spin configuration of the Cooper pairs, resulting in clear-cut anisotropy signatures. A characteristic mark of the inverse proximity effect arises in the induced spin polarization at the TSC-FM interface. This is unexpectedly stronger when the magnetic proximity is weaker, thus unveiling immediate detection signatures for spin-triplet pairs. We show that an anomalous magnetic proximity can occur at the interface between the itinerant ferromagnet $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ and the unconventional superconductor ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$. Such scenario indicates the potential to design characteristic inverse proximity effects in experimentally available $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3\text{−}{\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$ heterostructures and to assess the occurrence of spin-triplet pairs in the highly debated superconducting phase of ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$.

The rare earth nickelates $\mathrm{R}\mathrm{Ni}{\mathrm{O}}_3$ are metallic at high temperatures and insulating and magnetically ordered at low temperatures. The low temperature phase has been predicted to be type II multiferroic, i.e., ferroelectric and magnetic order are coupled and occur simultaneously. Confirmation of those ideas has been inhibited by the absence of experimental data on single crystals. Here we report on Raman spectroscopic data of $\mathrm{R}\mathrm{Ni}{\mathrm{O}}_3$ single crystals (R = Y, Er, Ho, Dy, Sm, Nd) for temperatures between 10 and 1000 K. Entering the magnetically ordered phase we observe the appearance of a large number of additional vibrational modes, implying a breaking of inversion symmetry expected for multiferroic order.

Code-division multiple-access (CDMA) multiuser detection is a kind of signal recovery problem. The main problem of CDMA multiuser detection is to estimate the original signal from the degraded information. In CDMA multiuser detection, the first-order phase transition happens. The first-order phase transition degrades the estimation performance. To avoid or mitigate the first-order phase transition, we apply adiabatic reverse annealing (ARA) to CDMA multiuser detection. In ARA, we introduce the initial Hamiltonian, which corresponds to the prior information of the original signal into quantum annealing (QA) formulation. The ground state of the initial Hamiltonian is the initial candidate solution. By using the prior information of the original signal, ARA enhances the performance of QA for CDMA multiuser detection. We evaluate the typical ARA performance of CDMA multiuser detection by means of statistical mechanics using the replica method. At first, we consider the oracle cases where the initial candidate solution is randomly generated with a fixed fraction of the original signal in the initial state. In the oracle cases, the first-order phase transition can be avoided or mitigated by ARA if we prepare for the proper initial candidate solution. We validate our theoretical analysis with quantum Monte Carlo simulations. The theoretical results to avoid the first-order phase transition are consistent with the numerical results. Next, we consider the practical cases where we prepare for the initial candidate solution obtained by commonly used algorithms. We show that the practical algorithms can exceed the threshold to avoid the first-order phase transition. Finally, we test the performance of ARA with the initial candidate solution obtained by the practical algorithm. In this case, ARA cannot avoid the first-order phase transition even if the initial candidate solution exceeds the threshold to avoid the first-order phase transition.

Single-atom magnets, the ultimate limit of high-density magnetic storage, have attracted widespread attention over recent decades. However, they are far from being applicable as a storage medium because of their low magnetic stability. In this paper, we describe a strategy to induce huge magnetic anisotropy in “bimetal magnets” on a benzene (Bz) substrate based on the electron filling of $d$ orbitals. Our first-principles calculations reveal that OsX-Bz ($X=\mathrm{Fe}$, Ru, and Os) exhibits high structural stability, large unquenched orbital moments ($1.55–1.59{\mu }_B$), and huge perpendicular magnetic anisotropy energy (MAE) above 54.8 meV. The synergistic effect of two transition metal components toward this huge MAE and preserved orbital moment is discussed in depth, which is mainly attributed to the d-d coupling induced energy level rearrangement. Our work may provide insights into the underlying physical mechanism and bimetal molecule magnet design.

Driving quantum systems periodically in time plays an essential role in the coherent control of quantum states. The rotating-wave approximation (RWA) is a good approximation technique for weak and nearly resonance driven fields. However, these experiments sometimes require large detuning and strong driving fields, for which the RWA may not hold. In this work, we experimentally, numerically, and analytically explore strongly driven two-mode Josephson circuits in the regime of strong driving and large detuning. Specifically, we investigate beam-splitter and two-mode squeezing interaction between the two modes induced by driving a two-photon sideband transition. Using numerical simulations, we observe that the RWA is unable to correctly capture the amplitude of the sideband transition rates. We verify this finding using an analytical model that is based on perturbative corrections. We find that the breakdown of the RWA in the regime studied does not lead to qualitatively different dynamics, but gives the same results as the RWA theory at higher drive strengths, enhancing the coupling rates compared to what one would predict. This is an interesting consequence compared to the carrier transition case, where the breakdown of the RWA results in qualitatively different time evolution of the quantum state. Our work provides an insight into the behavior of time-periodically driven systems beyond the RWA. We also provide a robust theoretical framework for including these findings in the calculation and calibration of quantum protocols in circuit quantum electrodynamics.

The ratio between the shear viscosity and the entropy $\eta /s$ is considered a universal measure of the strength of interactions in quantum systems. This quantity was conjectured to have a universal lower bound $(1/4\pi )\hslash /k_B$, which indicates a very strongly correlated quantum fluid. By solving the quantum kinetic theory for a nodal-line semimetal in the hydrodynamic regime, we show that $\eta /s\propto T$ violates the universal lower bound, scaling toward zero with decreasing temperature $T$ in the perturbative limit. We find that the hydrodynamic scattering time between collisions is nearly temperature independent, up to logarithmic scaling corrections, and can be extremely short for large nodal lines, near the Mott-Ragel-Ioffe limit. Our finding suggests that nodal-line semimetals can be very strongly correlated quantum systems.

We present a suite of “holographic” quantum algorithms for efficient ground-state preparation and dynamical evolution of correlated spin systems, which require far fewer qubits than the number of spins being simulated. The algorithms exploit the equivalence between matrix-product states (MPS) and quantum channels, along with partial measurement and qubit reuse, in order to simulate a $D$-dimensional spin system using only a $(D-1)$-dimensional subset of qubits along with an ancillary qubit register whose size scales logarithmically in the amount of entanglement present in the simulated state. Ground states can either be directly prepared from a known MPS representation or obtained via a holographic variational quantum eigensolver (holoVQE). Dynamics of MPS under local Hamiltonians for time $t$ can also be simulated with an additional (multiplicative) $\mathrm{poly}\left(t\right)$ overhead in qubit resources. These techniques open the door to efficient quantum simulation of MPS with exponentially large bond dimension, including ground states of two- and three-dimensional systems, or thermalizing dynamics with rapid entanglement growth. As a demonstration of the potential resource savings, we implement a holoVQE simulation of the antiferromagnetic Heisenberg chain on a trapped-ion quantum computer, achieving within $10\left(3\right)\%$ of the exact ground-state energy of an infinite chain using only a pair of qubits.