Local Gate Research Articles

Mechanism of proton release during water oxidation in Photosystem II

Photosystem II (PSII) catalyzes light-driven water oxidation that releases dioxygen into our atmosphere and provides the electrons needed for the synthesis of biomass. The catalysis occurs in the oxygen-evolving oxo-manganese-calcium (Mn4O5Ca) cluster that drives the oxidation and deprotonation of substrate water molecules leading to the O2 formation. However, despite recent advances, the mechanism of these reactions remains unclear and much debated. Here, we show that the light-driven Tyr161D1 (Yz) oxidation adjacent to the Mn4O5Ca cluster, decreases the barrier for proton transfer from the putative substrate water molecule (W3/Wx) to Glu310D2, accessible to the luminal bulk. By combining hybrid quantum/classical (QM/MM) free energy calculations with atomistic molecular dynamics simulations, we probe the energetics of the proton transfer along the Cl1 pathway. We demonstrate that the proton transfer occurs via water molecules and a cluster of conserved carboxylates, driven by redox-triggered electric fields directed along the pathway. Glu65D1 establishes a local molecular gate that controls the proton transfer to the luminal bulk, while Glu312D2 acts as a local proton storage site. The identified gating region could be important in preventing backflow of protons to the Mn4O5Ca cluster. The structural changes, derived here based on the dark-state PSII structure, strongly support recent time-resolved X-ray free electron laser data of the S3 → S4 transition (Bhowmick et al. Nature 617, 2023) and reveal the mechanistic basis underlying deprotonation of the substrate water molecules. Our findings provide insight into the water oxidation mechanism of PSII and show how the interplay between redox-triggered electric fields, ion-pairs, and hydration effects control proton transport reactions.

Effect of Environmental Conditions on the Electrical Properties of MoTe2 Monolayers with hBN Cap Layer

We investigate effects of vacuum level on electrical properties of hBN-capped monolayer MoTe2 (1L-MoTe2) channel FETs. The 1L-MoTe2 transistors show ambipolar transfer characteristics, which enable both p- and n-type operation by gate bias tuning. The conductivity minimum point is shifted toward negative values by reducing vacuum level, demonstrating that charge transfer occurs by adsorption of some molecules in an atmosphere even for the device structures with hexagonal boron nitride (hBN) cap layer.A semiconducting molybdenum ditelluride (MoTe2) monolayer have attracted great research interest owing to the photon emission in the near-infrared wavelength and a strong spin-orbit coupling to result in the longer spin and valley relaxation, providing a pathway to novel optoelectronic device applications. Recently, it has been reported that electrostatic carrier density modulation by two separated local gate enables lateral PN junction formation. On the other hand, adsorption of atmospheric gases leads to charge transfer to the 1L-MoTe2, and the resulting carrier density changes lead to loss of the controllability and reproducibility of the lateral PN junction formation. The formation of hBN cap layer is reported to suppress degradation of optical properties of the 1L-MoTe2. However, there are few reports for the effect of adsorption of atmospheric gases on electrical properties of 1L-MoTe2 channel FETs with hBN cap layer. In this study, we focus on effects of vacuum level on current-voltage curves obtained from 1L-MoTe2 channel FETs with hBN cap layer.Firstly, 1L-MoTe2 was fabricated through the Au-mediated exfoliation process. Gold was deposited onto a bulk MoTe2 crystal by thermal evaporation. The gold film together with the topmost layer of the MoTe2 crystals was peeled off by using a thermal release tape owing to the stronger interaction of the topmost layer with the evaporated gold compared to the van der Waals interactions in the MoTe2 bulk crystals. The 1L-MoTe2/gold structures on the thermal release tape were transferred onto a 290-nm-thick SiO2/Si substrate, and subsequently, the surface gold film was selectively etched with a KI/I2 mixed solution. Graphite and hBN flakes mechanically exfoliated were prepared on a polydimethylsiloxane (PDMS), and these flakes were transferred on the 1L-MoTe2 using the standard dry transfer process for utilization as electrodes to measure the electrical properties and as a cap layer, respectively.Figure (a) shows the current-voltage (I d–V g) curves obtained from the fabricated 1L-MoTe2 channel FET under various vacuum levels. An increase in drain current I d is seen by increasing back-gate voltage V g for both positive and negative polarities in the 1L-MoTe2 channel, demonstrating the ambipolar operation of the FET. Figure (b) represents the conductivity minimum point V min plotted as a function of the pressure in the chamber. On decreasing the pressure, the V min is gradually shifted to the negative values, which correspond to the change of n-type behavior. It is found that the V min shifts by about -30 V at the pressure of 6.9×10-4 Pa. As previously reported, the V min shift should result from the charge transfer between the 1L-MoTe2 and some adsorbed molecules under the atmospheric conditions. Considering the existence of the hBN cap layer on the 1L-MoTe2 channel, some molecules physically absorbed in the interfaces between 1L-MoTe2 and the SiO2 without atomically flat surface may be desorbed by reducing the pressure, and thus the p-doping effect by adsorption of atmospheric gases disappears. It could be concluded that the changes of environmental conditions affect electrical properties of 1L-MoTe2 FETs even for the device structures with the hBN cap layer. Figure 1

Local Chemical Enhancement and Gating of Organic Coordinated Ionic-Electronic Transport.

Superior properties in organic mixed ionic-electronic conductors (OMIECs) over inorganic counterparts have inspired intense interest in biosensing, soft-robotics, neuromorphic computing, and smart medicine. However, slow ion transport relative to charge transport in these materials is a limiting factor. Here, it is demonstrated that hydrophilic molecules local to an interfacial OMIEC nanochannel can accelerate ion transport with ion mobilities surpassing electrophoretic transport by more than an order of magnitude. Furthermore, ion access to this interfacial channel can be gated through local surface energy. This mechanism is applied in a novel sensing device, which electronically detects and characterizes chemical reaction dynamics local to the buried channel. The ability to enhance ion transport at the nanoscale in OMIECs as well as govern ion transport through local chemical signaling enables new functionalities for printable, stretchable, and biocompatible mixed conduction devices.

Parallel Logic Operations in Electrically Tunable Two-Dimensional Homojunctions.

Two-dimensional materials show great potential for future electronics beyond silicon materials. Here, we report an exotic multiple-port device based on multiple electrically tunable planar p-n homojunctions formed in a two-dimensional (2D) ambipolar semiconductor, tungsten diselenide (WSe2). In this device, we prepare multiple gates consisting of a global gate and several local gates, by which electrostatically induced holes and electrons are simultaneously accumulated in a WSe2 channel, and furthermore, at the boundaries, p-n junctions are formed as directly visualized by Kelvin probe force microscopy. Therefore, in addition to the gate voltages in our device, the drain/source bias can also be used to switch the 2D WSe2 channel on/off due to the rectification effect of the formed p-n junctions. More importantly, when the voltage on the global gate electrode is altered, all p-n junctions are affected, which makes it possible to perform parallel logic operations.

On the stabilizer formalism and its generalization

Abstract The standard stabilizer formalism provides a setting to show that quantum computations restricted to operations within the Clifford group are classically efficiently simulable: this is the content of the well-known Gottesman–Knill theorem. This work analyzes the mathematical structure behind this theorem to find possible generalizations and derivation of constraints required for constructing a non-trivial generalized Clifford group. We prove that if the closure of the stabilizing set is dense in the set of SU(d) transformations, then the associated Clifford group is trivial, consisting only of local gates and permutations of subsystems. This result demonstrates the close relationship between the density of the stabilizing set and the simplicity of the corresponding Clifford group. We apply the analysis to investigate stabilization with binary observables for qubits and find that the formalism is equivalent to the standard stabilization for a low number of qubits. Based on the observed patterns, we conjecture that a large class of generalized stabilizer states are equivalent to the standard ones. Our results provide better insights into the structure of Gottesman–Knill-type results, consequently allowing us to draw a sharper line between quantum and classical computation.

Multifractality in monitored single-particle dynamics

We study multifractal properties in time evolution of a single particle subject to repeated measurements. For quantum systems, we consider circuit models consisting of local unitary gates and local projective measurements. For classical systems, we consider models for estimating the trajectory of a particle evolved under local transition processes by partially measuring particle occupations. In both cases, multifractal behaviors appear in the ensemble of wave functions or probability distributions conditioned on measurement outcomes after a sufficiently long time. While the nature of particle transport (diffusive or ballistic) qualitatively affects the multifractal properties, they are even quantitatively robust to the measurement rate or specific protocols. On the other hand, multifractality is generically lost by generalized measurements allowing erroneous outcomes or by postselection of the outcomes with no particle detection. We demonstrate these properties by numerical simulations and also propose several simplified models, which allow us to analytically obtain multifractal properties in the monitored single-particle systems. Published by the American Physical Society 2024

ARQUIN: Architectures for Multinode Superconducting Quantum Computers

Many proposals to scale quantum technology rely on modular or distributed designs wherein individual quantum processors, called nodes, are linked together to form one large multinode quantum computer (MNQC). One scalable method to construct an MNQC is using superconducting quantum systems with optical interconnects. However, internode gates in these systems may be two to three orders of magnitude noisier and slower than local operations. Surmounting the limitations of internode gates will require improvements in entanglement generation, use of entanglement distillation, and optimized software and compilers. Still, it remains unclear what performance is possible with current hardware and what performance algorithms require. In this article, we employ a systems analysis approach to quantify overall MNQC performance in terms of hardware models of internode links, entanglement distillation, and local architecture. We show how to navigate tradeoffs in entanglement generation and distillation in the context of algorithm performance, lay out how compilers and software should balance between local and internode gates, and discuss when noisy quantum internode links have an advantage over purely classical links. We find that a factor of 10–100× better link performance is required and introduce a research roadmap for the co-design of hardware and software towards the realization of early MNQCs. While we focus on superconducting devices with optical interconnects, our approach is general across MNQC implementations.

Quantum Coding Transitions in the Presence of Boundary Dissipation

We investigate phase transitions in the encoding of quantum information in a quantum many-body system due to the competing effects of unitary scrambling and boundary dissipation. Specifically, we study the fate of quantum information in a one-dimensional qudit chain, subject to local unitary quantum circuit evolution in the presence of depolarizing noise at the boundary. If the qudit chain initially contains a finite amount of locally accessible quantum information, unitary evolution in the presence of boundary dissipation allows this information to remain partially protected when the dissipation is sufficiently weak, and up to timescales growing linearly in the system size L. In contrast, for strong enough dissipation, this information is completely lost to the dissipative environment. We analytically investigate this “quantum coding transition” by considering dynamics involving Haar-random, local unitary gates, and confirm our predictions in numerical simulations of Clifford quantum circuits. Scrambling the quantum information in the qudit chain with a unitary circuit of depth O(logL) before the onset of dissipation can perfectly protect the information until late times. The nature of the coding transition changes when the dynamics extend for times much longer than L. We further show that at weak dissipation, it is possible to code at a finite rate, i.e., a fraction of the many-body Hilbert space of the qudit chain can be used to encode quantum information. Published by the American Physical Society 2024

A High-Performance and Ultra-Low-Power Accelerator Design for Advanced Deep Learning Algorithms on an FPGA

This article addresses the growing need in resource-constrained edge computing scenarios for energy-efficient convolutional neural network (CNN) accelerators on mobile Field-Programmable Gate Array (FPGA) systems. In particular, we concentrate on register transfer level (RTL) design flow optimization to improve programming speed and power efficiency. We present a re-configurable accelerator design optimized for CNN-based object-detection applications, especially suitable for mobile FPGA platforms like the Xilinx PYNQ-Z2. By not only optimizing the MAC module using Enhanced clock gating (ECG), the accelerator can also use low-power techniques such as Local explicit clock gating (LECG) and Local explicit clock enable (LECE) in memory modules to efficiently minimize data access and memory utilization. The evaluation using ResNet-20 trained on the CIFAR-10 dataset demonstrated significant improvements in power efficiency consumption (up to 22%) and performance. The findings highlight the importance of using different optimization techniques across multiple hardware modules to achieve better results in real-world applications.

Approximate encoding of quantum states using shallow circuits

Quantum algorithms and simulations often require the preparation of complex states through sequences of 2-qubit gates. For a generic quantum state, the number of required gates grows exponentially with the number of qubits, becoming unfeasible on near-term quantum devices. Here, we aim at creating an approximate encoding of the target state using a limited number of gates. As a first step, we consider a quantum state that is efficiently represented classically, such as a one-dimensional matrix product state. Using tensor network techniques, we develop and implement an efficient optimization algorithm that approaches the optimal implementation, requiring a polynomial number of iterations. We, next, consider the implementation of the proposed optimization algorithm directly on a quantum computer and overcome inherent barren plateaus by employing a local cost function. Our work offers a universal method to prepare target states using local gates and represents a significant improvement over known strategies.

Wafer-scale CMOS-compatible graphene Josephson field-effect transistors

Electrostatically tunable Josephson field-effect transistors (JoFETs) are one of the most desired building blocks of quantum electronics. Applications of JoFETs range from parametric amplifiers and superconducting qubits to a variety of integrated superconducting circuits. Here, we report on graphene JoFET devices fabricated with wafer-scale complementary metal-oxide-semiconductor (CMOS)-compatible processing based on chemical-vapor-deposited monolayer graphene encapsulated with atomic-layer-deposited Al2O3 gate oxide, lithographically defined top gate, and evaporated superconducting Ti/Al source, drain, and gate contacts. By optimizing the contact resistance down to ∼170 Ω μm, we observe proximity-induced superconductivity in the JoFET channels with different gate lengths of 150–350 nm. The Josephson junction devices show reproducible critical current Ic tunablity with the local top gate. Our JoFETs are in the short diffusive limit with the Ic reaching up to ∼3 µA for a 50 µm channel width. Overall, our demonstration of CMOS-compatible two-dimensional (2D) material-based JoFET fabrication process is an important step toward graphene-based integrated quantum circuits.

Local Gate Enhanced Correlated Phases in Twisted Monolayer-Bilayer Graphene.

Manipulating the flat band degeneracy and thus getting the correlated insulating phases has been an ideal thread for realizing the exotic quantum phenomenon in the moiré system. To achieve this goal, the delicately tuned twist angle and a substantial displacement field (D) are rigorously requested. Here, we report our scanning tunneling microscope (STM) work on reaching these correlated insulating states in twisted monolayer-bilayer graphene through a decorated tip. It acts as a local top gate, leading to an enhanced local D, and enables us to fully lift the 8-fold degeneracy of the flat bands. With the aid of this technique, we further expand the correlated insulating states into a more tolerant twist angle that is down to 0.92°. Moreover, the correlated insulating phases in the hole-doping regime are realized. Our tip decoration method allows us to integrate the STM study with the high displacement field for the correlated phases in the twisted moiré systems.

Cross-Cap Defects and Fault-Tolerant Logical Gates in the Surface Code and the Honeycomb Floquet Code

We consider the Z2 toric code, surface code, and Floquet code defined on a nonorientable surface, which can be considered as families of codes extending Shor’s nine-qubit code. We investigate the fault-tolerant logical gates of the Z2 toric code in this setup, which corresponds to e↔m exchanging symmetry of the underlying Z2 gauge theory. We find that nonorientable geometry provides a new way for the emergent symmetry to act on the code space, and discover the new realization of the fault-tolerant Hadamard gate of the two-dimensional surface code with a single cross cap connecting the vertices nonlocally along a slit, dubbed a nonorientable surface code. This Hadamard gate can be realized by a constant-depth local unitary circuit modulo nonlocality caused by a cross cap. Via folding, the nonorientable surface code can be turned into a bilayer local quantum code, where the folded cross cap is equivalent to a bilayer twist terminated on a gapped boundary and the logical Hadamard only contains local gates with intralayer couplings when being away from the cross cap, as opposed to the interlayer couplings on each site needed in the case of the folded surface code. We further obtain the complete logical Clifford gate set for a stack of nonorientable surface codes and similarly for codes defined on Klein-bottle geometries. We then construct the honeycomb Floquet code in the presence of a single cross cap, and find that the period of the sequential Pauli measurements acts as a HZ logical gate on the single logical qubit, where the cross cap enriches the dynamics compared with the orientable case. We find that the dynamics of the honeycomb Floquet code is precisely described by a condensation operator of the Z2 gauge theory, and illustrate the exotic dynamics of our code in terms of a condensation operator supported at a nonorientable surface. Published by the American Physical Society 2024

All-Oxide Metasurfaces Formed by Synchronized Local Ionic Gating.

Ionic gating of oxide thin films has emerged as a novel way of manipulating the properties of thin films. Most studies are carried out on single devices with a three-terminal configuration,but, by exploring the electrokinetics during the ionic gating, such a configuration with initially insulating films leads to a highly non-uniform gating response of individual devices within large arrays of the devices. It is shown that such an issue can be circumvented by the formation of a uniform charge potential by the use of a thin conducting underlayer. This synchronized local ionic gating allows for the simultaneous manipulation of the electrical, magnetic, and/or optical properties of large arrays of devices. Designer metasurfaces formed in this way from SrCoO2.5 thin films display an anomalous optical reflection of light that relies on the uniform and coherent response of all the devices. Beyond oxides, almost any material whose properties can be controlled by the addition or removal of ions via gating can form novel metasurfaces using this technique. These findings provide insights into the electrokinetics of ionic gating and a wide range of applications using synchronized local ionic gating.

Performance of Rotation‐Symmetric Bosonic Codes in a Quantum Repeater Network

Performance of Rotation‐Symmetric Bosonic Codes in a Quantum Repeater Network

Valley filtering using magnetic proximity effect in monolayer WS[formula omitted]

A quantum system implanted with monolayer transition metal dichalcogenide (TMDC) is proposed as a valley filter based on a ballistic point contact with zigzag edges. A quantum point contact (QPC) structure is formed in the middle of scattering region and the geometrical size of the QPC is varied by electrostatic gate potentials. The energy band structures and electronic transport properties are investigated using a two-bands k⋅p Hamiltonian by using parameters from the density functional theory (DFT) calculations. The magnetic proximity effect from an Eu-terminated ferromagnetic substrate (EuS) generates a giant valley splitting in the energy of the monolayer WS2 nanoribbon. The conductance is limited by reducing the width and by increasing the length of the QPC because conductance is strongly related to the number of transmission channels and the amounts of back scatterings at the QPC. The polarity of valley current can be manipulated by applying a local gate potential in the QPC region, where the manipulated polarity is limited to well defined K′ polarization only. The lateral hetero materials on both sides of the QPC or lateral hetero ferromagnetic substrate is necessary to have desired valley and spin polarities of current by manipulating the local gate potential. The experimental realization of this quantum system will make it possible to control the valley degree of freedom in addition to controlling charge and spin degree of freedom of carriers in future electronic devices.

Local gate control of Mott metal-insulator transition in a 2D metal-organic framework

Electron-electron interactions in materials lead to exotic many-body quantum phenomena, including Mott metal-insulator transitions (MITs), magnetism, quantum spin liquids, and superconductivity. These phases depend on electronic band occupation and can be controlled via the chemical potential. Flat bands in two-dimensional (2D) and layered materials with a kagome lattice enhance electronic correlations. Although theoretically predicted, correlated-electron Mott insulating phases in monolayer 2D metal-organic frameworks (MOFs) with a kagome structure have not yet been realised experimentally. Here, we synthesise a 2D kagome MOF on a 2D insulator. Scanning tunnelling microscopy (STM) and spectroscopy reveal a MOF electronic energy gap of ∼200 meV, consistent with dynamical mean-field theory predictions of a Mott insulator. Combining template-induced (via work function variations of the substrate) and STM probe-induced gating, we locally tune the electron population of the MOF kagome bands and induce Mott MITs. These findings enable technologies based on electrostatic control of many-body quantum phases in 2D MOFs.

A Self‐Powered Photodetector Based on Graphene Enhanced WSe2/PtSe2 Heterodiode with Fast Speed and Broadband Response

AbstractNarrow bandgap 2D layered material platinum selenide (PtSe2) with good environmental stability, high carrier mobility, and high light absorption, has been widely investigated for uncooled midwave‐infrared (MWIR) photodetection. However, the phototransistor based on the PtSe2 operation at room temperature suffered from the high dark current and background noise. Here, a graphene (G)‐enhanced G‐WSe2/PtSe2 hetero‐diode placed on a metal electrode is reported. To enhance the photogain, a graphene layer placed on the WSe2/PtSe2 heterodiode as a local gating layer is designed. The device exhibits an ultra‐high light on/off ratio of up to 108 and ultra‐fast photoresponse speed with raising time τr = 0.9 µs and decay time of τd = 1.5 µs in the visible spectra range. Notably, ultrabroad band photoresponse from 405 to 3366 nm is demonstrated under the self‐power model. Notably, the device presented a competitive photovoltaic effect with a high energy conversion efficiency (PCE) of 3.45%. The results pave the way toward a new approach to tuning the performance of the atomic thin layered materials photodetector.

Exponential Hardness of Optimization from the Locality in Quantum Neural Networks

Quantum neural networks (QNNs) have become a leading paradigm for establishing near-term quantum applications in recent years. The trainability issue of QNNs has garnered extensive attention, spurring demand for a comprehensive analysis of QNNs in order to identify viable solutions. In this work, we propose a perspective that characterizes the trainability of QNNs based on their locality. We prove that the entire variation range of the loss function via adjusting any local quantum gate vanishes exponentially in the number of qubits with a high probability for a broad class of QNNs. This result reveals extra harsh constraints independent of gradients and unifies the restrictions on gradient-based and gradient-free optimizations naturally. We showcase the validity of our results with numerical simulations of representative models and examples. Our findings, as a fundamental property of random quantum circuits, deepen the understanding of the role of locality in QNNs and serve as a guideline for assessing the effectiveness of diverse training strategies for quantum neural networks.

A Novel Enhancement-Mode Gallium Nitride p-Channel Metal Insulator Semiconductor Field-Effect Transistor with a Buried Back Gate for Gallium Nitride Single-Chip Complementary Logic Circuits

In this work, a novel enhancement-mode GaN p-MISFET with a buried back gate (BBG) is proposed to improve the gate-to-channel modulation capability of a high drain current. By using the p-GaN/AlN/AlGaN/AlN double heterostructure, the buried 2DEG channel is tailored and connected to the top metal gate, which acts as a local back gate. Benefiting from the dual-gate structure (i.e., top metal gate and 2DEG BBG), the drain current of the p-MISFET is significantly improved from −2.1 (in the conv. device) to −9.1 mA/mm (in the BBG device). Moreover, the dual-gate design also bodes well for the gate to p-channel control; the subthreshold slope (SS) is substantially reduced from 148 to ~60 mV/dec, and such a low SS can be sustained for more than 3 decades. The back gate effect and the inherent hole compensation mechanism of the dual-gate structure are thoroughly studied by TCAD simulation, revealing their profound impact on enhancing the subthreshold and on-state characteristics in the BBG p-MISFET. Furthermore, the decent device performance of the proposed BBG p-MISFET is projected to the complementary logic inverters by mixed-mode simulation, showcasing excellent voltage transfer characteristics (VTCs) and dynamic switching behavior. The proposed BBG p-MISFET is promising for developing GaN-on-Si monolithically integrated complementary logic and power devices for high efficiency and compact GaN power IC.