Hasil untuk "Atomic physics. Constitution and properties of matter"

Timon Schapeler, Isabell Mischke, Fabian Schlue et al.

Superconducting nanowire single-photon detectors (SNSPDs) can enable photon-number resolution (PNR) based on accurate measurements of the detector’s response time to few-photon optical pulses. In this work, we investigate the impact of the optical pulse shape and duration on the accuracy of this method. We find that Gaussian temporal pulse shapes yield cleaner arrival-time histograms and, thus, more accurate PNR, compared to bandpass-filtered pulses of equal bandwidth. For low system jitter and an optical pulse duration comparable to the other jitter contributions, photon numbers can be discriminated in our system with a commercial SNSPD. At 60 ps optical pulse duration, photon-number discrimination is significantly reduced. Furthermore, we highlight the importance of using the correct arrival-time histogram model when analyzing photon-number assignment. Using exponentially modified Gaussian distributions, instead of the commonly used Gaussian distributions, we can more accurately determine photon-number misidentification probabilities. Finally, we reconstruct the positive operator-valued measures of the detector, revealing sharp features that indicate the intrinsic PNR capabilities.

Dawei Jiao, Mahdi Bayanifar, Alexei Ashikhmin et al.

In this article, we consider second-generation (2G) quantum repeaters (QRs) for creating long-distance entanglement in quantum networks. Combining a distance-dependent depolarizing error model with the nonlocal Bell state purification procedure required by 2G QRs leads to an error model consisting of correlated and biased errors. To correct correlated errors, nonsymmetric Calderbank&#x2013;Steane&#x2013;Shor (CSS) codes with joint decoding between stations can be used. The dominating errors are biased, such that different repeater stations suffer from different types of errors. To mitigate this, different quantum codes can be used at the stations, optimized for the specific error model of the station. To comply with the 2G QR procedure, the codes used in neighboring stations must allow for the transversal implementation of nonlocal logical <sc>cnot</sc> gates across the two stations or, alternatively, nonlocal <sc>cz</sc> gates combined with logical Hadamard gates. We provide a complete characterization of pairs of CSS codes that allow <sc>cnot</sc> or <sc>cz</sc> transversality, and examine an explicit family of mirrored CSS codes allowing <sc>cz</sc> transversality. We verify Hadamard gate transversality using our framework and show the importance of the logical qubit mapping matrix. Also, we conclude that using different QECCs does not lead to universal computation with the Clifford + <inline-formula><tex-math notation="LaTeX">$T$</tex-math></inline-formula> gate set. Finally, we study the entanglement generation rate (EGR) in 2G QRs with limited quantum memory, minimizing the number of intermediate stations for a given fidelity and EGR. By simulation, we observe that nonsymmetric and mirrored structure QECCs outperform the conventional approach of using symmetric CSS codes at the repeater stations.

Shuyao Chen, Mingqian Yuan, Qixun Guo et al.

Abstract Yttrium iron garnet (YIG) film, especially with perpendicular magnetic anisotropy (PMA), is a promising material for energy-efficient spintronic devices due to its extremely low damping constant. However, a poorly crystallized layer tends to form on the top surface of the YIG film during the annealing process, which severely hinders the interfacial spin transport. To overcome this limitation, we developed a surface treatment method using soft phosphoric acid. After the surface wet-etching treatment, both the spin mixing conductance and interfacial thermal conductance between the PMA-YIG film and post-deposited Pt layer can be increased by ~70% and ~100%, respectively. These PMA-YIG films with wet-etched surfaces hold promise for ultrahigh-density spintronic device applications.

Robert de Mello Koch, Bo-Qiang Lu, Pedro Ornelas et al.

Quantum skyrmions as topologically structured entangled states have the potential to be a pathway toward robustness against external perturbations, but so far no theoretical framework exists to validate this. Here, we introduce the notion of a new entanglement observable based on such topologies and develop a theoretical framework for studying its evolution in general quantum channels. Using photons entangled in orbital angular momentum and polarization as an example, we show that the noise affecting both photons can be recast as a position-dependent perturbation affecting only the photon in the polarization state, restricting the noise to a finite-dimensional Hilbert space. From this we predict complete resilience for both non-depolarizing and depolarizing noise, the former by rigorous arguments based on homotopic maps and the latter by numerical simulation. Finally, we identify sources of local noise that can destabilize the topology and suggest why this may be ignored in practical situations. Our work sets a solid foundational framework for understanding how and why topology enhances the resilience of such entanglement observables, with immediate relevance to the distribution of information through entanglement in noisy environments, such as quantum computers and quantum networks.

Stephan Rachel, Roland Wiesendanger

For the past decade, Majorana quasiparticles have become one of the hot topics in condensed matter research. Besides the fundamental interest in the realization of particles being their own antiparticles, going back to basic concepts of elementary particle physics, Majorana quasiparticles in condensed matter systems offer exciting potential applications in topological quantum computation due to their non-Abelian quantum exchange statistics. Motivated by theoretical predictions about possible realizations of Majorana quasiparticles as zero-energy modes at boundaries of topological superconductors, experimental efforts have focussed in particular on quasi-one-dimensional semiconductor-superconductor and magnet-superconductor hybrid systems. However, an unambiguous proof of the existence of Majorana quasiparticles is still challenging and requires considerable improvements in materials science, atomic-scale characterization and control of interface quality, as well as complementary approaches of detecting various facets of Majorana quasiparticles. Bottom-up atom-by-atom fabrication of disorder-free atomic spin chains on atomically clean superconducting substrates has recently allowed deep insight into the emergence of topological sub-gap Shiba bands and associated Majorana states from the level of individual atoms up to extended chains, thereby offering the possibility for critical tests of Majorana physics in disorder-free model-type 1D hybrid systems.

Ian Pannemarsh

In recent decades the field of quantum computation has seen remarkable development. While much progress has been made toward the realization of a fully digital, scalable, and fault tolerant quantum computer, there are still many essential challenges to overcome. In the interim, direct emulation of quantum systems of interest can fill an important gap not only for exploring fundamental questions about many-body physics and the quantum to classical transition, but also for potentially providing alternative methods to verify results from quantum simulations. In this work we will demonstrate a method utilizing closed loop control of the collective magnetic moment of an ensemble of cold neutral atoms via non-destructive measurements to emulate various spin system Hamiltonians. By modifying the feedback control law appropriately we are able to generate nonlinear dynamical behavior in the ensemble, allowing us to explore the physics of collective spin systems at mesoscopic scales. Moreover, controlling the number of atoms in the collective spin can potentially allow us to investigate these dynamics in the transition from fully quantum to the classical limit. In particular, we emulate two models: the Lipkin-Meshkov-Glick (LMG) Hamiltonian, and a closely related model, the Kicked Top. In the former case, we show that our system undergoes a symmetry-breaking phase transition in the expected parameter regime. In the latter, we explore two interesting aspects: the formation of chaos, and a dynamically driven time crystal phase. We will then discuss the advantages and limits of this approach.

Ben Bartlett, Olivia Y. Long, Avik Dutt et al.

Synthetic dimensions have generated great interest for studying many types of topological, quantum, and many-body physics, and they offer a flexible platform for simulation of interesting physical systems, especially in high dimensions. In this paper, we describe a programmable photonic device capable of emulating the dynamics of a broad class of Hamiltonians in lattices with arbitrary topologies and dimensions. We derive a correspondence between the physics of the device and the Hamiltonians of interest, and we simulate the physics of the device to observe a wide variety of physical phenomena, including chiral states in a Hall ladder, effective gauge potentials, and oscillations in high-dimensional lattices. Our proposed device opens new possibilities for studying topological and many-body physics in near-term experimental platforms.

Yanyan Zhao, Jijun Zhao, Yu Guo et al.

Benjamin Geisler, James J. Hamlin, Gregory R. Stewart et al.

Abstract Motivated by the recent observation of superconductivity with T c  ~ 80 K in pressurized La3Ni2O7 1, we explore the structural and electronic properties of A 3Ni2O7 bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La3Ni2O7 at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations between T c and the in-plane Ni-O-Ni bond angles for La3Ni2O7. Moreover, two structural phases with significant c + octahedral rotations and in-plane bond disproportionations are uncovered for A = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Ni e g manifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb3Ni2O7 as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism.

Alberto Tarable, Rudi Paolo Paganelli, Marco Ferrari

Information reconciliation (IR) is a key step in quantum key distribution (QKD). In recent years, blind reconciliation based on low-density parity-check (LDPC) codes has replaced Cascade as a standard de facto since it guarantees efficient IR without a priori quantum bit error rate estimation and with limited interactivity between the parties, which is essential in high key-rate and long-distance QKD links. In this article, a novel blind reconciliation scheme based on rateless protograph LDPC codes is proposed. The rate adaptivity, essential for blind reconciliation, is obtained by progressively splitting LDPC check nodes, which ensures a number of degrees of freedom larger than puncturing in code design. The protograph nature of the LDPC codes allows us to use the same designed codes with a large variety of sifted-key lengths, enabling block length flexibility, which is important in largely varying key-rate link conditions. The code design is based on a new protograph discretized density evolution tool.

Huey-Wen Lin

We present the first lattice-QCD $x$-dependent pion valence-quark generalized parton distribution (GPD) calculated directly at physical pion mass using the Large-Momentum Effective Theory (LaMET) with next-to-next-to-leading order perturbative matching correction. We use clover fermions for the valence action on $2+1+1$ flavors of highly improved staggered quarks (HISQ), generated by MILC Collaboration, with lattice spacing $a \approx 0.09$fm and box size $L \approx 5.5$fm; the pion two-point measurements number up to $O(10^6)$ with boost momentum 1.73GeV. The pion valence distribution is renormalized in hybrid scheme with Wilson-line mass subtraction at large distances in coordinate space, followed by a procedure to match it to the $\overline{\text{MS}}$ scheme. We focus on the zero-skewness limit, where the GPD has a probability-density interpretation in the longitudinal Bjorken $x$ and the transverse impact-parameter distributions. We take the integral of our GPD functions to generate leading moment so that we can make comparisons with past lattice-QCD and experimental determinations of the pion form factors and found consistent agreement among them. We predict the higher GPD moments and reveal $x$-dependent tomography of the pion for the first time using lattice QCD.

Linh Le, Tu N. Nguyen

Quantum routing plays a key role in the development of the next-generation network system. In particular, an entangled routing path can be constructed with the help of quantum entanglement and swapping among particles (e.g., photons) associated with nodes in the network. From another side of computing, machine learning has achieved numerous breakthrough successes in various application domains, including networking. Despite its advantages and capabilities, machine learning is not as much utilized in quantum networking as in other areas. To bridge this gap, in this article, we propose <italic>a novel quantum routing model</italic> for quantum networks that employs machine learning architectures to construct the routing path for the maximum number of demands (source&#x2013;destination pairs) within a time window. Specifically, we present a deep reinforcement routing scheme that is called Deep Quantum Routing Agent (DQRA). In short, DQRA utilizes an empirically designed deep neural network that observes the current network states to accommodate the network&#x2019;s demands, which are then connected by a qubit-preserved shortest path algorithm. The training process of DQRA is guided by a reward function that aims toward maximizing the number of accommodated requests in each routing window. Our experiment study shows that, on average, DQRA is able to maintain a rate of successfully routed requests at above 80&#x0025; in a qubit-limited grid network and approximately 60&#x0025; in extreme conditions, i.e., each node can be repeater exactly once in a window. Furthermore, we show that the model complexity and the computational time of DQRA are polynomial in terms of the sizes of the quantum networks.

Mikkel Have Eriksen, Jakob E. Olsen, Christian Wolff et al.

We explore the emergence and active control of optical bistability in a two-level atom near a graphene sheet. Our theory incorporates self-interaction of the optically-driven atom and its coupling to electromagnetic vacuum modes, both of which are sensitive to the electrically-tunable interband transition threshold in graphene. We show that electro-optical bistability and hysteresis can manifest in the intensity, spectrum, and quantum statistics of the light emitted by the atom, which undergoes critical slow-down to steady-state. The optically-driven atom-graphene interaction constitutes a platform for active control of driven atomic systems in quantum coherent control and atomic physics.

Xinling Yu, José E. C. Serrallés, Ilias I. Giannakopoulos et al.

Electrical properties (EP), namely permittivity and electric conductivity, dictate the interactions between electromagnetic waves and biological tissue. EP can be potential biomarkers for pathology characterization, such as cancer, and improve therapeutic modalities, such radiofrequency hyperthermia and ablation. MR-based electrical properties tomography (MR-EPT) uses MR measurements to reconstruct the EP maps. Using the homogeneous Helmholtz equation, EP can be directly computed through calculations of second order spatial derivatives of the measured magnetic transmit or receive fields $(B_{1}^{+}, B_{1}^{-})$. However, the numerical approximation of derivatives leads to noise amplifications in the measurements and thus erroneous reconstructions. Recently, a noise-robust supervised learning-based method (DL-EPT) was introduced for EP reconstruction. However, the pattern-matching nature of such network does not allow it to generalize for new samples since the network's training is done on a limited number of simulated data. In this work, we leverage recent developments on physics-informed deep learning to solve the Helmholtz equation for the EP reconstruction. We develop deep neural network (NN) algorithms that are constrained by the Helmholtz equation to effectively de-noise the $B_{1}^{+}$ measurements and reconstruct EP directly at an arbitrarily high spatial resolution without requiring any known $B_{1}^{+}$ and EP distribution pairs.

Pierre Villars, Evgeny Blokhin, Shuichi Iwata

The fundamental relationship of the atomic structure (represented by its atomic property parameters, APPs) and its physical properties of a specific inorganic substance can be realized in the bottom-up data-centric and the top-down knowledge physics-centric ways. Nowadays these two approaches compete and enhance one another qualitatively and quantitatively. We present our own holistic method and implementation, based on the PAULING FILE peer-reviewed inorganic substances database, the world largest materials database containing under one shelter crystallographic structures, phase diagrams and large variety of physical properties of single-phase inorganic substances. In addition we present generated machine-learning data, as well as simulated DFT physics-centered data, which are in close connection and comparison with the PAULING FILE peer-reviewed reference data.

Floris van der Tak, François Lique, Alex Faure et al.

The Leiden Atomic and Molecular Database (LAMDA) collects spectroscopic information and collisional rate coefficients for molecules, atoms, and ions of astrophysical and astrochemical interest. We describe the developments of the database since its inception in 2005, and outline our plans for the near future. Such a database is constrained both by the nature of its uses and by the availability of accurate data: we suggest ways to improve the synergies among users and suppliers of data. We summarize some recent developments in computation of collisional cross sections and rate coefficients. We consider atomic and molecular data that are needed to support astrophysics and astrochemistry with upcoming instruments that operate in the mid- and far-infrared parts of the spectrum.

A. A. Kurteva, V. E. Mitroshin

Results of calculations in the framework of the dynamic collective model of probabilities of beta-transitions for more than 20 nuclei with mass numbers from А = 31 to А = 231 with one universal renormalization of weak interaction constants are presented. The quasi-particle and many-phonon (up to 10 phonons) states, as well as vacuum fluctuations of quasi-particles were taken into account. The calculated values lg ft show good agreement with the experimental data. In the aggregate the results of the calculations show that suppression of weak forces in nuclei does not depend on mass number and energy of states excited in beta-decay.

L. Wang, C. Yang, M. T. Dove et al.

A hallmark of a thermodynamic phase transition is the qualitative change of system thermodynamic properties such as energy and heat capacity. On the other hand, no phase transition is thought to operate in the supercritical state of matter and, for this reason, it was believed that supercritical thermodynamic properties vary smoothly and without any qualitative changes. Here, we perform extensive molecular dynamics simulations in a wide temperature range and find that a deeply supercritical state is thermodynamically heterogeneous, as witnessed by different temperature dependence of energy, heat capacity and its derivatives at low and high temperature. The evidence comes from three different methods of analysis, two of which are model-independent. We propose a new definition of the relative width of the thermodynamic crossover and calculate it to be in the fairly narrow relative range of 13-20\%. On the basis of our results, we relate the crossover to the supercritical Frenkel line.

Inga Tolstikhina, Makoto Imai, Nicolas Winckler et al.

I. M. Kadenko, V. A. Plujko, B. M. Bondar et al.

The cross-sections of prompt gamma-ray production from natSn and natC elements induced by 14.1-MeV neutrons were measured. The time-of-flight technique was used for n-γ discrimination. The experimental results were compared with theoretical calculations performed by Empire 3.2 and Talys 1.6 codes using different models for photon strength function and nuclear level density.