Coherent Exchange Research Articles

Influence of finite size and image charge screening on the antiferromagnetic ordering of CoO ultrathin films

The surface antiferromagnetic (AFM) ordering of ultrathin CoO(001) films grown on Ag(001) has been investigated using low energy electron diffraction (LEED) and valence band photoemission spectroscopy. We have observed coherent exchange scattered half-order spots in LEED with a periodicity of the magnetic unit cell of the CoO(001) surface. These spots show very low intensities (2–3% of the integer-order spots) and are visible only below a critical temperature for low electron beam energies (<70 eV), revealing its AFM origin. Significant enhancement of surface Nèel temperature (TN) of CoO(001) for different film coverages has been observed and it is exceeding the bulk TN even in the ultrathin limit. The enhancement of surface TN has been explained by the combined effects of finite size, surface strain, and polarizability of the underlying substrate. The critical behaviour near TN shows a coverage dependent evolution as the magnetic critical exponent (β) varies from 0.16 to 0.48 upon increasing film coverage from 0.5 ML to 10 ML, suggesting crossovers in the critical behaviour. Furthermore, upon transition from paramagnetic (PM) to AFM phase, minor modifications of the valence band electronic structure have been observed such as enhancement of spectral weight in the Co 3d band as well as widening of an energy gap relative to EF by ~150 meV, in agreement with the theoretical predictions.

Coherent Multispin Exchange Coupling in a Quantum-Dot Spin Chain

Heisenberg exchange coupling between neighboring electron spins in semiconductor quantum dots provides a powerful tool for quantum information processing and simulation. Although so far unrealized, extended Heisenberg spin chains can enable long-distance quantum information transfer and the generation of non-equilibrium quantum states. In this work, we implement simultaneous, coherent exchange coupling between all nearest-neighbor pairs of spins in a quadruple quantum dot. The main challenge in implementing simultaneous exchange couplings is the nonlinear and nonlocal dependence of the exchange couplings on gate voltages. Through a combination of electrostatic simulation and theoretical modeling, we show that this challenge arises primarily due to lateral shifts of the quantum dots during gate pulses. Building on this insight, we develop two models, which can be used to predict the confinement gate voltages for a desired set of exchange couplings. Although the model parameters depend on the number of exchange couplings desired (suggesting that effects in addition to lateral wavefunction shifts are important), the models are sufficient to enable simultaneous and independent control of all three exchange couplings in a quadruple quantum dot. We demonstrate two-, three-, and four-spin exchange oscillations, and our data agree with simulations.

Quantum State Engineering by Shortcuts to Adiabaticity in Interacting Spin-Boson Systems.

We present a fast and robust framework to prepare nonclassical states of a bosonic mode exploiting a coherent exchange of excitations with a two-level system ruled by a Jaynes-Cummings interaction mechanism. Our protocol, which is built on shortcuts to adiabaticity, allows for the generation of arbitrary Fock states of the bosonic mode, as well as coherent quantum superpositions of a Schrödinger cat-like form. In addition, we show how to obtain a class of photon-shifted states where the vacuum population is removed, a result akin to photon addition, but displaying more nonclassicality than standard photon-added states. Owing to the ubiquity of the spin-boson interaction that we consider, our proposal is amenable for implementations in state-of-the-art experiments.

Reaction: Molecular Spins as Qubits

Floriana Tuna is a Leverhulme Trust Senior Research Fellow at the University of Manchester (UoM). She holds MSc and PhD degrees in chemistry from the University of Bucharest (1989) and National Academy of Romania (1997), respectively. She worked as a visiting DAAD fellow at the University of Heidelberg (Germany), a CNRS postdoctoral fellow at ICMCB Bordeaux (France), and a Marie Curie fellow at the University of Warwick (UK) before joining UoM in 2003. She received the Romanian Academy “Ilie Murgulescu” Prize for excellence in research in 2005 and has published over 300 high-impact papers. Her major interests are in coordination chemistry, molecular magnetism, and quantum information science.

Control of the coupling strength and linewidth of a cavity magnon-polariton

The full coherent control of hybridized systems such as strongly coupled cavity photon-magnon states is a crucial step to enable future information processing technologies. Thus, it is particularly interesting to engineer deliberate control mechanisms such as the full control of the coupling strength as a measure for coherent information exchange. In this work, we employ cavity resonator spectroscopy to demonstrate the complete control of the coupling strength of hybridized cavity photon-magnon states. For this, we use two driving microwave inputs which can be tuned at will. Here, only the first input couples directly to the cavity resonator photons, whilst the second tone exclusively acts as a direct input for the magnons. For these inputs, both the relative phase $\phi$ and amplitude $\delta_0$ can be independently controlled. We demonstrate that for specific quadratures between both tones, we can increase the coupling strength, close the anticrossing gap, and enter a regime of level merging. At the transition, the total amplitude is enhanced by a factor of 1000 and we observe an additional linewidth decrease of $13\%$ at resonance due to level merging. Such control of the coupling, and hence linewidth, open up an avenue to enable or suppress an exchange of information and bridging the gap between quantum information and spintronics applications.

Identification and time-resolved study of ferrimagnetic spin-wave modes in a microwave cavity in the strong-coupling regime

Recently, the hybridization of microwave-frequency cavity modes with collective spin excitations attracted large interest for the implementation of quantum computation protocols, which exploit the transfer of information among these two physical systems. Here, we investigate the interaction among the magnetization precession modes of a small YIG sphere and the MW electromagnetic modes, resonating in a tridimensional aluminum cavity. In the strong coupling regime, anti-crossing features were observed in correspondence of various magnetostatic modes, which were excited in a magnetically saturated sample. Time-resolved studies show evidence of Rabi oscillations, demonstrating coherent exchange of energy among photons and magnons modes. To facilitate the analysis of the standing spin-wave patterns, we propose here a new procedure, based on the introduction of a novel functional variable. The resulting easier identification of magnetostatic modes can be exploited to investigate, control and compare many-levels hybrid systems in cavity- and opto-magnonics research.

Information Exchange Between Administrative and Criminal Enforcement: The Case of the ECB and National Investigative Agencies

The banking system is often at the epicentre of large-scale financial crimes. Recent scandals involving major European credit institutions questioned the role of banking regulators in supporting law enforcement agencies and revealed the weakness of the current interaction between the European Central Bank (ECB) and national actors. Despite its ostensible coherence, the Single Supervisory Mechanism legal framework can prove problematic in terms of efficiency and adequacy in facing global financial crime. This article explores the specific case of information exchange between banking supervisors and criminal investigative authorities at national level. After a descriptive overview of the ECB reporting duties of potential criminal offences, we examine the possibility of a more coherent information exchange system, where a direct channel of communication between the ECB and criminal law enforcement agencies could serve as a better integrated strategy

Steering between level repulsion and attraction: broad tunability of two-port driven cavity magnon-polaritons

Cavity-magnon polaritons (CMPs) are the associated quasiparticles of the hybridization between cavity photons and magnons in a magnetic sample placed in a microwave resonator. In the strong coupling regime, where the macroscopic coupling strength exceeds the individual dissipation, there is a coherent exchange of information. This renders CMPs as promising candidates for future applications such as in information processing. Recent advances on the study of the CMP now allow not only for creation of CMPs on demand, but also for tuning of the coupling strength—this can be thought of as the enhancement or suppression of information exchange. Here, we go beyond standard single-port driven CMPs and employ a two-port driven CMP. We control the coupling strength by the relative phase ϕ and amplitude field ratio δ0 between both ports. Specifically, we derive a new expression from input–output theory for the study of the two-port driven CMP and discuss the implications on the coupling strength. Furthermore, we examine intermediate cases where the relative phase is tuned between its maximal and minimal value and, in particular, the high δ0 regime, which has not been yet explored.

Hydrodynamics of longitudinally discontinuous, vertically double layered and partially covered rigid vegetation patches in open channel flow

AbstractThe aim of this paper is to numerically investigate how the flow structures are affected through a longitudinally discontinuous and vertically two‐layered vegetation occupying half width of the channel, with steady flow rate and subcritical conditions. A three‐dimensional (3‐D) Reynolds stress turbulence model (RSM), incorporated by Computational Fluid Dynamics (CFD) code FLUENT, was first validated with the experimental data, and then used for simulation purpose. The results showed that the flow stream‐wise velocities within the gap regions are visibly slower than that in the vegetation patch regions. Along the cross section, the velocity in the vegetation region (VR) reduced significantly due to resistance offered by the vegetation, which affected the channel conveyance; as compared to the free (non‐vegetated) region. The flow instability in the lateral direction was triggered by the flow shear due to the presence of partly distributed vegetation, resulting in the formation of coherent vortices and exchange of momentum at the interface. The discharge percentage passing through the free region (FR) was found to be 144–525% larger than that passing through the VR. The flow resistance increased significantly with higher vegetation density, whereas it decreased when both the vegetation layers were submerged. Moreover, the flow characteristics profiles in large gaps were more stable than in small gaps. The turbulent kinetic energy (TKE) and turbulence intensity also increased significantly through the patch regions compared to that of the gap regions. The results indicated that the flow structures and the flow resistance are strongly influenced by partial and discontinuous vegetation.

Quantum electrodynamics with magnetic textures

Coherent exchange between photons and different matter excitations (like qubits, acoustic surface waves or spins) allows for the entanglement of light and matter and provides a toolbox for performing fundamental quantum physics. On top of that, coherent exchange is a basic ingredient in the majority of quantum information processors. In this work, we develop the theory for coupling between magnetic textures (vortices and skyrmions) stabilized in ferromagnetic nanodiscs and microwave photons generated in a superconducting circuit. Within this theory we show that this hybrid system serves for performing broadband spectroscopy of the magnetic textures. We also discuss the possibility of reaching the strong coupling regime between these texture excitations and a single photon residing in a microwave superconducting cavity.

Comparison between continuous- and discrete-mode coherent feedback for the Jaynes-Cummings model

Using the example of the Jaynes-Cummings model, we present a comparison between time-delayed coherent feedback mediated by reservoirs with continuous and discrete mode structures and work out their qualitative differences. In contrast to the discrete-mode case, the continuous-mode case results in the well-known single-delay dynamics which can, e.g., stabilize Rabi oscillations. The discrete-mode case, however, shows population trapping, not present in the continuous-mode model. Given these differences, we discuss the cavity output spectra and show how these characteristic properties are spectrally identifiable. This work demonstrates the fundamental difference between the continuous-mode case, which represents a truly dissipative mechanism, and the discrete-mode case that is in principle based on a coherent excitation exchange process.

BIM Data Exchange Standard for Hydro-Supported Structures

With the rapid adoption of building information modeling (BIM) across diverse industries, the need for a universal standard for BIM data exchanges has become critical for the success of BIM applications. Questions of the level of development and interoperability of BIM as methods to exchange data and integrate various model-based applications into an efficient work process have come to the forefront of BIM standardization to improve the design and construction of facilities. Current BIM software authoring tools center on conventional structural systems and are not yet adequate for modeling floating structures. Hydro-supported structures comprise a special type of structure that relies primarily on the hydraulic buoyancy of the supporting water body to maintain equilibrium and safety. It is an area that is increasingly drawing the attention of architects and engineers to resolve many issues in coastal areas and to deal with issues related to increased flooding and tropical storms, such as shoreline stabilization, hurricane response, alternative energy, and rainwater collection and utilization. The study centers on expanding BIM standardization for designing and analyzing floating and amphibious structures. It seeks to address the information delivery manual (IDM) and model view definitions (MVDs) as they provide the basis for data exchange specifications for the industry foundation classes (IFC) schema. This research aims to enable a proficient BIM workflow for designing hydro-supported structures characterized by the coherent, organized, and reproducible exchange of consistent data that is commonly recognized across the architecture, engineering, and construction (AEC) industry.

Vectorial near-field coupling.

The coherent exchange of optical near fields between two neighbouring dipoles plays an essential role in the optical properties, quantum dynamics and thus the function of many naturally occurring and artificial nanosystems. These interactions are challenging to quantify experimentally. They extend over only a few nanometres and depend sensitively on the detuning, dephasing and relative orientation (that is, the vectorial properties) of the coupled dipoles. Here, we introduce plasmonic nanofocusing spectroscopy to record coherent light scattering spectra with 5 nm spatial resolution from the apex of a conical gold nanotaper. The apex is excited solely by evanescent fields and coupled to plasmon resonances in a single gold nanorod. We resolve resonance energy shifts and line broadenings as a function of dipole distance and relative orientation. We demonstrate how these phenomena arise from mode couplings between different vectorial components of the interacting optical near fields, specifically from the coupling of the nanorod to both transverse and longitudinal polarizabilities of the taper apex.

Transient response of the cavity magnon-polariton

The Rabi oscillations of a coupled spin-photon system are experimentally studied using time domain microwave transmission measurements. An external magnetic bias field is applied to control the amplitude and frequency of Rabi oscillations, which are a measure of the coherent spin-photon energy exchange and coupling strength respectively. The amplitude and frequency control is described in the time domain as the transient response of a two-level spin-photon system. This approach reveals the link between steady state and transient behaviour of the cavity-magnon-polariton, which is necessary to interpret time domain cavity-spintronic experiments. This transient model also allows us to understand and quantify the polariton mode composition, which can be controlled by the bias field.

Strong interaction of quantum emitters with a WS2 layer enhanced by a gold substrate.

We investigate the spontaneous emission (SE) effects of a two-level quantum emitter (QE) near a WS2 layer. The QE is placed above the WS2 layer in a host medium with constant dielectric permittivity. The material below the WS2 layer is either the same as the host medium or Au. We find that the Purcell factor of the QE near the dielectric/WS2 takes values up to 104. When the value of the dielectric permittivity of the host medium is increased, the enhancement of the Purcell factor diminishes. For a dielectric/WS2/dielectric structure, we obtain a Rabi splitting in the SE spectrum of the QE up to 75meV at room temperature. This splitting is more pronounced for a WS2 layer of improved material quality which can be achieved by improved fabrication methods or operating at lower temperatures. When the WS2 layer is placed on top of an Au substrate, the hybrid exciton-surface plasmon polariton modes strongly interact with the QE, inducing coherent energy exchange between the two, as manifested by a pronounced Rabi splitting in the emission spectrum which is increased to 100meV.

Real-Time Tunable Strong Coupling: From Individual Nanocavities to Metasurfaces

Strong light–matter coupling, characterized by a coherent exchange of energy between an emitter and cavity, plays an important role in, for example, quantum information science and thresholdless lasing. To achieve strong coupling, precise spatial and spectral overlap between the emitter and cavity is required, presenting a significant challenge to move from individually strongly coupled cavities to a large number of cavity-coupled systems, as required for future practical applications. Here we demonstrate a versatile platform for realizing strong coupling that scales uniformly from individual nanocavities up to millimeter-scale metasurfaces, while the coupling strength can be tuned dynamically. Fluorescent dye molecules are sandwiched between silver nanocubes and a metallic film creating a plasmonic cavity with a mode volume of only ∼0.002 (λ/n)3. A prominent anticrossing behavior is observed which corresponds to a large Rabi splitting energy of 152 meV. The plasmon resonance can be tuned up to 45 nm (∼21...

Enhanced Strong Interaction between Nanocavities and p-shell Excitons Beyond the Dipole Approximation.

Large coupling strengths in exciton-photon interactions are important for the quantum photonic network, while strong cavity-quantum dot interactions have been focused on s-shell excitons with small coupling strengths. Here we demonstrate strong interactions between cavities and p-shell excitons with a great enhancement by the insitu wave-function control. The p-shell excitons are demonstrated with much larger wave-function extents and nonlocal interactions beyond the dipole approximation. Then the interaction is tuned from the nonlocal to the local regime by the wave function shrinking, during which the enhancement is obtained. A large coupling strength of 210 μeV has been achieved, indicating the great potential of p-shell excitons for coherent information exchange. Furthermore, we propose a distributed delay model to quantitatively explain the coupling strength variation, revealing the intertwining of excitons and photons beyond the dipole approximation.

Direct observation of ultrafast plasmonic hot electron transfer in the strong coupling regime

Achieving strong coupling between plasmonic oscillators can significantly modulate their intrinsic optical properties. Here, we report the direct observation of ultrafast plasmonic hot electron transfer from an Au grating array to an MoS2 monolayer in the strong coupling regime between localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs). By means of femtosecond pump-probe spectroscopy, the measured hot electron transfer time is approximately 40 fs with a maximum external quantum yield of 1.65%. Our results suggest that strong coupling between LSPs and SPPs has synergetic effects on the generation of plasmonic hot carriers, where SPPs with a unique nonradiative feature can act as an ‘energy recycle bin’ to reuse the radiative energy of LSPs and contribute to hot carrier generation. Coherent energy exchange between plasmonic modes in the strong coupling regime can further enhance the vertical electric field and promote the transfer of hot electrons between the Au grating and the MoS2 monolayer. Our proposed plasmonic strong coupling configuration overcomes the challenge associated with utilizing hot carriers and is instructive in terms of improving the performance of plasmonic opto-electronic devices.

Quantum Stochastic Model for Spin Dynamics in Magnetic Systems

We develop a numerical model that reproduces the thermal equilibrium and the spin transfer mechanisms in magnetic nanomaterials. We analyze the coherent two-particle spin exchange interaction and the electron-electron collisions. Our study is based on a quantum atomistic approach and the particle dynamics is performed by using a Monte Carlo technique. The coherent quantum evolution of the atoms is interrupted by instantaneous collisions with itinerant electrons. The collision processes are associated to the quantum collapse of the local atomic wave function. We show that particle-particle interactions beyond the molecular field approximation can be included in this framework. Our model is able to reproduce the thermal equilibrium and strongly out-of-equilibrium phenomena such as the ultrafast dynamics of the magnetization in nanomatrials.

Impact of interface structure on magnetic exchange coupling in MnBi/FexCo1−x bilayers

In this study we investigate the exchange coupling between the hard magnetic compound MnBi and the soft magnetic alloy FeCo including the interface structure between the two phases. Exchange spring MnBi-Fe$_x$Co$_{1-x}$ (x = 0.65 and 0.35) bilayers with various thicknesses of the soft magnetic layer were deposited onto quartz glass substrates in a DC magnetron sputtering unit from alloy targets. Magnetic measurements and density functional theory (DFT) calculations reveal that a Co-rich FeCo layer leads to more coherent exchange coupling. The optimum soft layer thickness is about 1 nm. In order to take into account the effect of incoherent interfaces with finite roughness, we have combined a cross-sectional High Resolution Transmission Electron Microscopy (HR-TEM) analysis with DFT calculations and micromagnetic simulations. The experimental results can be consistently described by modeling assuming a polycrystalline FeCo layer consisting of crystalline (110) and amorphous grains as confirmed by HR-TEM. The micromagnetic simulations show in general how the thickness of the FeCo layer and the interface roughness between the hard and soft magnetic phases both control the effectiveness of exchange coupling in an exchange spring system.