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P. Xu

Publications and source records attributed to P. Xu.

At least 19 recordsLinked to original sources
arXiv2025

Symmetric $C_Z$ gate for ultracold neutral atoms based on counterdiabatic driving at Rydberg excitation

We designed a scheme for a neutral atom Rydberg blockade $C_Z$ gate based on the double sequence of adiabatic pulses applied symmetrically to both atoms and using counterdiabatic driving for Rydberg excitation. This provides a substantial reduction in the quantum gate operation time compared to previously proposed double adiabatic schemes, and makes our scheme competitive with modern time-optimal protocols for high-fidelity entangling gates with neutral atoms. Our approach creates a bridge between fully adiabatic and time-optimal gate schemes. The use of adiabatic passage reduces the sensitivity of gate fidelity to variations in laser intensity, while counterdiabatic driving provides short gate times. The intensity and phase profiles of the laser pulse acting on the atoms are described analytically depending only on the gate duration. We demonstrated the applicability of this scheme for single-photon and two-photon schemes of Rydberg excitation in rubidium and cesium atoms, and, for the first time, discussed the implementation of a $C_Z$ gate using three-photon excitation of rubidium atoms. In contrast to many modern $C_Z$ gate protocols, our scheme does not generate intrinsic single-qubit phase shifts, although they still appear in two-photon configuration. We also designed a numerically optimized amplitude-robust gate with an analytically defined phase profile of the laser pulse and compared its performance with the counteradiabatic gate scheme.

quant-ph↗
arXiv2025

An Approach to Use Depletion Charges for Modifying Band Profiles for Field-Effect Transistors

We present the study of using depletion charges for tailoring lateral band profiles and applying it to the promising gate-all-around field-effect transistors (GAAFET). Specifically, we introduce heavily p-type doped Si next to the channel, but outside the channel, of a transistor. They are connected to the heavily n-type doped source and drain for generating the depletion charges. The finite difference method was used for simulations and the results show significant modifications of the conduction band along the channel. The depletion charges act as built-in electrodes capable of significantly modifying the band profiles of field-effect transistors. Quantum confinement within the channel has been attempted with different approaches, such as additional electrodes and point contacts. The results presented show two aspects of this approach, namely, realizing quantum confinement in an all-Si structure and tailoring band profiles within channels to modify their transport properties.

cond-mat.mes-hall↗
arXiv2024

Role of microstructure on flux expulsion of superconducting radio frequency cavities

The trapped residual magnetic flux during the cool-down due to the incomplete Meissner state is a significant source of radio frequency losses in superconducting radio frequency (SRF) cavities. Here, we show a clear correlation between the niobium microstructure in elliptical cavity geometry and flux expulsion behavior. In particular, a traditionally fabricated Nb cavity half cell from an annealed poly-crystalline Nb sheet after an 800 $^\circ$C heat treatment leads to a bi-modal microstructure that ties in with flux trapping and inefficient flux expulsion. This non-uniform microstructure is related to varying strain profiles along the cavity shape. A novel approach to prevent this non uniform microstructure is presented by fabricating a 1.3 GHz single cell Nb cavity with a cold-worked sheet and subsequent heat treatment leading to better flux expulsion after 800 $^\circ$C/3 h. Microstructural evolution by electron backscattered diffraction-orientation imaging microscopy on cavity cutouts, and flux pinning behavior by dc-magnetization on coupon samples confirms a reduction in flux pinning centers with increased heat treatment temperature. The heat treatment temperature dependent mechanical properties and thermal conductivity are reported. The significant impact of cold-work in this study demonstrates clear evidence for the importance of microstructure required for high-performance superconducting cavities with reduced losses caused by magnetic flux trapping.

physics.acc-ph↗
arXiv2024

Quantization of Charge Carriers in Conduction Channels of Si-Based Field-Effect Transistors for Multinary Computation

The latest field-effect transistors are entering the regime where quantum effects within the conduction channel can play a significant role because of the increasingly reduced dimensions. We investigate the effects of quantized states in conduction channels in transistors with dimensions close to those presently used. We use the standard configuration of Si-based metal-oxide-semiconductor field-effect transistors (MOSFETs), as a simplified model to provide an estimate of the effect of quantization with respect to the dimensions of the conduction channel. The study shows simulated results of drain currents for various combinations of dimensions, in which distinguishable current levels as a function of the applied gate bias can be obtained at room temperature. The same qualitative dependence on dimensions is expected to apply to the state-of-the-art transistor architectures with dimensions near this range, such as fin field-effect transistors (FinFETs) and gate-all-around field-effect transistors (GAAFETs). The results show that utilizing quantized states in the conduction channel for multinary computation has become a possibility with their present dimensions.

cond-mat.mes-hall↗
arXiv2020

Field-Programmable Topological Array: Framework and Case-Studies

Engineering composite materials and devices with desired topological properties is accelerating the development of topological physics and its applications. Approaches of realizing novel topological hybrids, including in-situ epitaxy growth, planar/layered superlattices, and assembled (artificial) atom/dot arrays, etc., endow the topological systems with a substantial degree of control and tunability. Here, we propose a framework for realizing a field-programmable topological array (FPTA) that enables the implementation of dynamically reconfigurable topological platforms. FPTA allows for the independent, simultaneous, and local programmability of the various platform properties, such as the electromagnetic field, the spin-orbit field, and the superconducting order parameter. To demonstrate the effectiveness of the FPTA in rendering the system topologically-nontrivial and implementing non-Abelian manipulations, we simulate their operation in various case-studies. Our framework provides a playground for unearthing novel topological phases using components of high feasibility and sets the guidelines for run-time dynamic reconfiguration which is crucial for high-performance topological electronic circuits and quantum computing.

cond-mat.mes-hall↗
arXiv2017

Entangling two atoms of different isotopes via Rydberg blockade

Quantum entanglement is crucial for simulating and understanding exotic physics of strongly correlated many-body systems, such as high--temperature superconductors, or fractional quantum Hall states. The entanglement of non-identical particles exhibits richer physics of strong many-body correlations and offers more opportunities for quantum computation, especially with neutral atoms where in contrast to ions the interparticle interaction is widely tunable by Feshbach resonances. Moreover, the inter-species entanglement forms a basis for the properties of various compound systems, ranging from Bose-Bose mixtures to photosynthetic light-harvesting complexes. So far, the inter-species entanglement has only been obtained for trapped ions. Here we report on the experimental realization of entanglement of two neutral atoms of different isotopes. A ${}^{87}\mathrm{Rb}$ atom and a ${}^{85}\mathrm{Rb}$ atom are confined in two single--atom optical traps separated by 3.8 $μ$m. Creating a strong Rydberg blockade, we demonstrate a heteronuclear controlled--NOT (C--NOT) quantum gate and generate a heteronuclear entangled state, with raw fidelities $0.73 \pm 0.01$ and $0.59 \pm 0.03$, respectively. Our work, together with the technologies of single--qubit gate and C--NOT gate developed for identical atoms, can be used for simulating any many--body system with multi-species interactions. It also has applications in quantum computing and quantum metrology, since heteronuclear systems exhibit advantages in low crosstalk and in memory protection.

quant-ph↗
arXiv2016

Experimental simulation of next-nearest-neighbor Heisenberg chain with photonic crystal waveguide array

Next-nearest-neighbor Heisenberg chain plays important roles in solid state physics, such as predicting exotic electric properties of two-dimensional materials or magnetic properties of organic compounds. Direct experimental studies of the many-body electron systems or spin systems associating to these materials are challenging tasks, while optical simulation provides an effective and economical way for immediate observation. Comparing with bulk optics, integrated optics are more of fascinating for steady, large scale and long-time evolution simulations. Photonic crystal is an artificial microstructure material with multiple methods to tune the propagation properties, which are essential for various simulation tasks. Here we report for the first time an experimental simulation of next-nearest-neighbor Heisenberg chain with an integrated optical chip of photonic crystal waveguide array. The use of photonic crystal enhances evanescent field thus allows coupling between next-nearest-neighbor waveguides in such a planar waveguide array, without breaking the weak coupling condition of the coupled mode equation. Particularly, similarities between the model and coherent light propagation could reach 0.99 in numerical simulations and 0.89 in experiment. Localization effect induced by second-order coupling and coupling strengthening with increasing wavelengths were also revealed in both numerical simulations and experiments. The platform proposed here is compatible with mature complementary metal oxide semiconductor technology thus possesses the potential for larger-scale problems and photonic crystals further allows simulations of specific target Hamiltonians.

physics.optics↗
arXiv2015

Generating Searchable Public-Key Ciphertexts with Hidden Structures for Fast Keyword Search

Existing semantically secure public-key searchable encryption schemes take search time linear with the total number of the ciphertexts. This makes retrieval from large-scale databases prohibitive. To alleviate this problem, this paper proposes Searchable Public-Key Ciphertexts with Hidden Structures (SPCHS) for keyword search as fast as possible without sacrificing semantic security of the encrypted keywords. In SPCHS, all keyword-searchable ciphertexts are structured by hidden relations, and with the search trapdoor corresponding to a keyword, the minimum information of therelations is disclosed to a search algorithm as the guidance to find all matching ciphertexts efficiently. We construct a simple SPCHS scheme from scratch in which the ciphertexts have a hidden star-like structure. We prove our scheme to be semantically secure based on the decisional bilinear Diffie-Hellman assumption in the Random Oracle (RO) model. The search complexity of our scheme is dependent on the actual number of the ciphertexts containing the queried keyword, rather than the number of all ciphertexts. Finally, we present a generic SPCHS construction from anonymous identity-based encryption and collision-free full-identity malleable Identity-Based Key Encapsulation Mechanism (IBKEM) with anonymity. We illustrate two collision-free full-identity malleable IBKEM instances, which are semantically secure and anonymous, respectively, in the RO and standard models. The latter instance enables us to construct an SPCHS scheme with semantic security in the standard model.

cs.CR↗
arXiv2015

Reduction of the dc electric field sensitivity of circular Rydberg states using non-resonant dressing fields

Non-resonant dressing fields can make the transition frequency between two circular Rydberg states insensitive to second order variations in the dc electric field. Perturbation theory can be used to establish the required dressing field amplitude and frequency. The same perturbative approach may be used to understand removal of the first order dependence of the transition frequency on electric field about a bias dc electric field [Hyafil et al. Phys. Rev. Lett., v. 93, 103001 (2004)]. The directional alignment of the dressing and dc fields is critical in determining the electric field sensitivity of the dressed transition frequencies. This sensitivity is significantly larger for circular Rydberg states compared to low-angular momentum Rydberg states of Rb.

physics.atom-ph↗
arXiv2015

Controlling Mn Depth Profiles in GaMnAs During High-Temperature Molecular Beam Epitaxial Growth

Mn-doped GaAs thin films were grown at a high substrate temperature of 580 C. During the growth process, the Mn cell temperature was ramped at different rates, resulting in a variety of different Mn concentration depth profile slopes, as measured using dynamic secondary ion mass spectrometry (SIMS). Results show that controlling the Mn deposition rate via temperature during molecular beam epitaxy (MBE) growth can mitigate the effect of Mn atoms diffusing toward the surface. Most importantly, the slope of the Mn concentration as a function of depth inside the sample can be tuned from negative to positive.

cond-mat.mes-hall↗
arXiv2015

Giant Surface Charge Density of Graphene Resolved From Scanning Tunneling Microscopy and First-Principles Theory

In this work, systematic constant-bias, variable-current scanning tunneling microscopy (STM) measurements and STM simulations from density-functional theory are made, yielding critical insights into the spatial structure of electrons in graphene. A foundational comparison is drawn between graphene and graphite, showing the surface charge density of graphene to be 300 percent that of graphite. Furthermore, simulated STM images reveal that high-current STM better resolves graphenes honeycomb bonding structure because of a retraction which occurs in the topmost dangling bond orbitals.

cond-mat.mes-hall↗
arXiv2015

Atomic Control of Strain in Freestanding Graphene

In this study, we describe a new experimental approach based on constant-current scanning tunneling spectroscopy to controllably and reversibly pull freestanding graphene membranes up to 35 nm from their equilibrium height. In addition, we present scanning tunneling microscopy (STM) images of freestanding graphene membranes with atomic resolution. Atomic-scale corrugation amplitudes 20 times larger than the STM electronic corrugation for graphene on a substrate were observed. The freestanding graphene membrane responds to a local attractive force created at the STM tip as a highly-conductive yet flexible grounding plane with an elastic restoring force. We indicate possible applications of our method in the controlled creation of pseudo-magnetic fields by strain on single-layer graphene.

cond-mat.mes-hall↗
arXiv2015

High-Percentage Success Method for Preparing and Pre-Evaluating Tungsten Tips for Atomic-Resolution Scanning Tunneling Microscopy

A custom double-lamella method is presented for electrochemically etching tungsten wire for use as tips in scanning tunneling microscopy (STM). For comparison, tips were also manufactured in-house using numerous conventional methods and examined using an optical microscope. Both sets of tips were used to obtain STM images of highly-oriented pyrolytic graphite, the quality of which varied. The clarity of the STM images was found to be correlated to the optically-measured cone angle of the STM tip, with larger cone angles consistently producing atomically resolved images. The custom etching procedure described allows one to create larger cone angles and consequently proved superior in reliably producing high-quality tips.

cond-mat.mes-hall↗
arXiv2015

A Pathway between Bernal and Rhombohedral Stacked Graphene Layers with Scanning Tunneling Microscopy

Horizontal shifts in the top layer of highly oriented pyrolytic graphite, induced by a scanning tunneling microscope (STM) tip, are presented. Excellent agreement is found between STM images and those simulated using density functional theory. First-principle calculations identify that the low-energy barrier direction of the top layer displacement is toward a structure where none of the carbon pz orbitals overlap, while the high-energy barrier direction is toward AA stacking. Each directional shift yields a real-space surface charge density similar to graphene; however the low-energy barrier direction requires only one bond length to convert ABA (Bernal) to ABC (rhombohedral).

cond-mat.mes-hall↗
arXiv2015

Schottky barrier and attenuation length for hot hole injection in non-epitaxial Au on p-type GaAs

Ballistic electron emission microscopy (BEEM) was performed to obtain nanoscale current versus bias characteristics of non-epitaxial Au on p-type GaAs in order to accurately measure the local Schottky barrier height. Hole injection BEEM data was averaged from thousands of spectra for various metal film thicknesses and then used to determine the attenuation length of the energetic charge carriers as a function of tip bias. We report the marked increase in attenuation length at biases near the Schottky barrier, providing evidence for the existence of coherent BEEM currents in Schottky diodes. These results provide additional evidence against the randomization of a charge carrier's momentum at the metal-semiconductor interface.

cond-mat.mes-hall↗
arXiv2015

Electromechanical properties of freestanding graphene functionalized with tin oxide (SnO2) nanoparticles

Freestanding graphene membranes were functionalized with SnO2 nanoparticles. A detailed procedure providing uniform coverage and chemical synthesis is presented. Elemental composition was determined using scanning electron microscopy combined with energy dispersive X-ray analysis. A technique called electrostatic-manipulation scanning tunneling microscopy was used to probe the electromechanical properties of functionalized freestanding graphene samples. We found ten times larger movement perpendicular to the plane compared to pristine freestanding graphene, and propose a nanoparticle encapsulation model.

cond-mat.mes-hall↗
arXiv2015

Electronic transition from graphite to graphene via controlled movement of the top layer with scanning tunneling microscopy

A series of measurements using a technique called electrostatic-manipulation scanning tunneling microscopy (EM-STM) were performed on a highly-oriented pyrolytic graphite surface. The electrostatic interaction between the STM tip and the sample can be tuned to produce both reversible and irreversible large-scale movement of the graphite surface. Under this influence, atomic-resolution STM images reveal that a continuous electronic transition from triangular symmetry, where only alternate atoms are imaged, to hexagonal symmetry can be systematically controlled. Density functional theory (DFT) calculations reveal that this transition can be related to vertical displacements of the top layer of graphite relative to the bulk. Evidence for horizontal shifts in the top layer of graphite is also presented. Excellent agreement is found between experimental STM images and those simulated using DFT.

cond-mat.mes-hall↗