arXiv subjects

Y. Wan

Publications and source records attributed to Y. Wan.

53 records · Page 3Linked to original sources

arXiv2016

Femtosecond probing of light-speed plasma wakefields by using a relativistic electron bunch

Relativistic wakes produced by intense laser or particle beams propagating through plasmas are being considered as accelerators for next generation of colliders and coherent light sources. Such wakes have been shown to accelerate electrons and positrons to several gigaelectronvolts (GeV), with a few percent energy spread and a high wake-to-beam energy transfer efficiency. However, complete mapping of electric field structure of the wakes has proven elusive. Here we show that a high-energy electron bunch can be used to probe the fields of such light-speed wakes with femtosecond resolution. The highly transient, microscopic wakefield is reconstructed from the density modulated ultra-short probe bunch after it has traversed the wake. This technique enables visualization of linear wakefields in low-density plasmas that can accelerate electrons and positrons beams. It also allows characterization of wakes in plasma density ramps critical for maintaining the beam emittance, improving the energy transfer efficiency and producing high brightness beams from plasma accelerators.

physics.plasm-ph↗

arXiv2016

Capturing relativistic wake eld structures in plasmas using ultrashort high-energy electrons as a probe

A new method capable of capturing coherent electric field structures propagating at nearly the speed of light in plasma with a time resolution as small as a few femtoseconds is proposed. This method uses a few femtoseconds long relativistic electron bunch to probe the wake produced in a plasma by an intense laser pulse or an ultra-short relativistic charged particle beam. As the probe bunch traverses the wake, its momentum is modulated by the electric field of the wake, leading to a density variation of the probe after free-space propagation. This variation of probe density produces a snapshot of the wake that can directly give many useful information of the wake structure and its evolution. Furthermore, this snapshot allows detailed mapping of the longitudinal and transverse components of the wakefield. We develop a theoretical model for field reconstruction and verify it using 3-dimensional particle-in-cell (PIC) simulations. This model can accurately reconstruct the wakefield structure in the linear regime, and it can also qualitatively map the major features of nonlinear wakes. The capturing of the injection in a nonlinear wake is demonstrated through 3D PIC simulations as an example of the applications of this new method.

physics.plasm-ph↗

arXiv2016

Temporal characterization of ultrashort linearly chirped electron bunches generated from a laser wakefield accelerator

A new method for diagnosing the temporal characteristics of ultrashort electron bunches with linear energy chirp generated from a laser wakefield accelerator is described. When the ionization-injected bunch interacts with the back of the drive laser, it is deflected and stretched along the direction of the electric field of the laser. Upon exiting the plasma, if the bunch goes through a narrow slit in front of the dipole magnet that disperses the electrons in the plane of the laser polarization, it can form a series of bunchlets that have different energies but are separated by half a laser wavelength. Since only the electrons that are undeflected by the laser go through the slit, the energy spectrum of the bunch is modulated. By analyzing the modulated energy spectrum, the shots where the bunch has a linear energy chirp can be recognized. Consequently, the energy chirp and beam current profile of those bunches can be reconstructed. This method is demonstrated through particle-in-cell simulations and experiment.

physics.plasm-ph↗

arXiv2016

Non-Generic Dispersion of Excitons in the Bulk of WSe$_2$

We combine electron energy-loss spectroscopy (EELS) and density functional theory (DFT) calculations to study the dispersion and effective mass of excitons in the bulk of WSe$_2$. Our EELS data suggest substantial deviations from the generic quadratic momentum dependence along the $ΓK$-direction. From the DFT-derived Kohn-Sham states we deduce the EELS response without the inclusion of particle-hole attraction to study the possible role of the single-particle band structure on the exciton behavior. Based on this analysis we argue in favor of a strongly momentum dependent particle-hole interaction in WSe$_2$ and other group VI-transition-metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2016

High-Fidelity Universal Gate Set for $^9$Be$^+$ Ion Qubits

We report high-fidelity laser-beam-induced quantum logic gates on magnetic-field-insensitive qubits comprised of hyperfine states in $^{9}$Be$^+$ ions with a memory coherence time of more than 1 s. We demonstrate single-qubit gates with error per gate of $3.8(1)\times 10^{-5}$. By creating a Bell state with a deterministic two-qubit gate, we deduce a gate error of $8(4)\times10^{-4}$. We characterize the errors in our implementation and discuss methods to further reduce imperfections towards values that are compatible with fault-tolerant processing at realistic overhead.

quant-ph↗

arXiv2016

Physical mechanism of the transverse instability in radiation pressure ion acceleration

The transverse stability of the target is crucial for obtaining high quality ion beams using the laser radiation pressure acceleration (RPA) mechanism. In this letter, a theoretical model and supporting two-dimensional (2D) Particle-in-Cell (PIC) simulations are presented to clarify the physical mechanism of the transverse instability observed in the RPA process. It is shown that the density ripples of the target foil are mainly induced by the coupling between the transverse oscillating electrons and the quasi-static ions, a mechanism similar to the transverse two stream instability in the inertial confinement fusion (ICF) research. The predictions of the mode structure and the growth rates from the theory agree well with the results obtained from the PIC simulations in various regimes, indicating the model contains the essence of the underlying physics of the transverse break-up of the target.

physics.acc-ph↗

arXiv2016

Preparation of entangled states through Hilbert space engineering

Entangled states are a crucial resource for quantum-based technologies such as quantum computers and quantum communication systems (1,2). Exploring new methods for entanglement generation is important for diversifying and eventually improving current approaches. Here, we create entanglement in atomic ions by applying laser fields to constrain the evolution to a restricted number of states, in an approach that has become known as "quantum Zeno dynamics" (3-5). With two trapped $^9\rm{Be}^+$ ions, we obtain Bell state fidelities up to $0.990^{+2}_{-5}$, with three ions, a W-state (6) fidelity of $0.910^{+4}_{-7}$ is obtained. Compared to other methods of producing entanglement in trapped ions, this procedure is relatively insensitive to certain imperfections such as fluctuations in laser intensity, laser frequency, and ion-motion frequencies.

quant-ph↗

arXiv2016

Physical origin of Davydov splitting and resonant Raman spectroscopy of Davydov components in multilayer MoTe2

We systematically study the high-resolution and polarized Raman spectra of multilayer (ML) MoTe2. The layer-breathing (LB) and shear (C) modes are observed in the ultralow-frequency region, which are used to quantitatively evaluate the interlayer coupling in ML MoTe2 based on the linear chain model, in which only the nearest interlayer coupling is considered. The Raman spectra on three different substrates verify the negligible substrate effect on the phonon frequencies of ML MoTe2. Ten excitation energies are used to measure the high-frequency modes of N-layer MoTe2 (NL MoTe2; N is an integer). Under the resonant excitation condition, we observe N-dependent Davydov components in ML MoTe2 , originating from the Raman-active A'1(A21g) modes at ~172 cm-1. More than two Davydov components are observed in NL MoTe2 for N larger than 4 by Raman spectroscopy. The N-dependent Davydov components are further investigated based on the symmetry analysis. A van der Waals model only considering the nearest interlayer coupling has been proposed to well understand the Davydov splitting of high-frequency A'1(A21g) modes. The different resonant profiles for the two Davydov components in 3L MoTe2 indicate that proper excitation energy of ~1.8-2.2 eV must be chosen to observe the Davydov splitting in ML MoTe2 . Our work presents a simple way to identify layer number of ultrathin MoTe2 flakes by the corresponding number and peak position of Davydov components. Our work also provides a direct evidence from Raman spectroscopy of how the nearest van der Waals interactions significantly affect the frequency of the high-frequency intralayer phonon modes in multilayer MoTe2 and expands the understanding on the lattice vibrations and interlayer coupling of transition metal dichalcogenides and other two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2015

Multi-Element Logic Gates for Trapped-Ion Qubits

Precision control over hybrid physical systems at the quantum level is important for the realization of many quantum-based technologies. In the field of quantum information processing (QIP) and quantum networking, various proposals discuss the possibility of hybrid architectures where specific tasks are delegated to the most suitable subsystem. For example, in quantum networks, it may be advantageous to transfer information from a subsystem that has good memory properties to another subsystem that is more efficient at transporting information between nodes in the network. For trapped-ions, a hybrid system formed of different species introduces extra degrees of freedom that can be exploited to expand and refine the control of the system. Ions of different elements have previously been used in QIP experiments for sympathetic cooling, creation of entanglement through dissipation, and quantum non-demolition (QND) measurement of one species with another. Here, we demonstrate an entangling quantum gate between ions of different elements which can serve as an important building block of QIP, quantum networking, precision spectroscopy, metrology, and quantum simulation. A geometric phase gate between a $^9$Be$^+$ ion and a $^{25}$Mg$^+$ ion is realized through an effective spin-spin interaction generated by state-dependent forces induced with laser beams. Combined with single-qubit gates and same-species entangling gates, this mixed-element entangling gate provides a complete set of gates over such a hybrid system for universal QIP. Using a sequence of such gates, we demonstrate a Controlled-NOT (CNOT) gate and a SWAP gate. We further demonstrate the robustness of these gates against thermal excitation and show improved detection in quantum logic spectroscopy (QLS). We also observe a strong violation of a CHSH-type Bell inequality on entangled states composed of different ion species.

quant-ph↗

arXiv2015

Nano-scale electron bunching in laser-triggered ionization injection in plasma accelerators

Ionization injection is attractive as a controllable injection scheme for generating high quality electron beams using plasma-based wakefield acceleration. Due to the phase dependent tunneling ionization rate and the trapping dynamics within a nonlinear wake, the discrete injection of electrons within the wake is nonlinearly mapped to discrete final phase space structure of the beam at the location where the electrons are trapped. This phenomenon is theoretically analyzed and examined by three-dimensional particle-in-cell simulations which show that three dimensional effects limit the wave number of the modulation to between $> 2k_0$ and about $5k_0$, where $k_0$ is the wavenumber of the injection laser. Such a nano-scale bunched beam can be diagnosed through coherent transition radiation upon its exit from the plasma and may find use in generating high-power ultraviolet radiation upon passage through a resonant undulator.

physics.plasm-ph↗

arXiv2015

Colliding ionization injection in a beam driven plasma accelerator

The proposal of generating high quality electron bunches via ionization injection triggered by an counter propagating laser pulse inside a beam driven plasma wake is examined via two-dimensional particle-in-cell simulations. It is shown that electron bunches obtained using this technique can have extremely small slice energy spread, because each slice is mainly composed of electrons ionized at the same time. Another remarkable advantage is that the injection distance is changeable. A bunch with normalized emittance of 3.3 nm, slice energy spread of 15 keV and brightness of $7.2\times 10^{18}$ A m$^{-2}$ rad$^{-2}$ is obtained with an optimal injection length which is achieved by adjusting the launch time of the drive beam or by changing the laser focal position. This makes the scheme a promising approach to generate high quality electron bunches for the fifth generation light source.

physics.acc-ph↗

arXiv2015

Generation of Low Absolute Energy Spread Electron Beams in Laser Wakefield Acceleration Using Tightly Focused Laser through Near-Ionization-Threshold Injection

An enhanced ionization injection scheme using a tightly focused laser pulse with intensity near the ionization potential to trigger the injection process in a mismatched pre-plasma channel has been proposed and examined via multi-dimensional particle-in-cell simulations. The core idea of the proposed scheme is to lower the energy spread of trapped beams by shortening the injection distance. We have established theory to precisely predict the injection distance, as well as the ionization degree of injection atoms/ions, electron yield and ionized charge. We have found relation between injection distance and laser and plasma parameters, giving a strategy to control injection distance hence optimizing beam's energy spread. In the presented simulation example, we have investigated the whole injection and acceleration in detail and found some unique features of the injection scheme, like multi-bunch injection, unique longitudinal phase-space distribution, etc. Ultimate electron beam has a relative energy spread (rms) down to 1.4% with its peak energy 190 MeV and charge 1.7 pC. The changing trend of beam energy spread indicates that longer acceleration may further lower the energy spread down to less than 1%, which may have potential in applications related to future coherent light source driven by laser-plasma accelerators.

physics.acc-ph↗

arXiv2015

Exact phase space matching for staging plasma and traditional accelerator components using longitudinally tailored plasma profiles

Phase space matching between two plasma-accelerator (PA) stages and between a PA and a traditional accelerator component is a critical issue for emittance preservation of beams accelerated by PAs. The drastic differences of the transverse focusing strengths as the beam propagates between different stages and components may lead to a catastrophic emittance growth in the presence of both finite energy spread and lack of proper matching. We propose using the linear focusing forces from nonlinear wakes in longitudinally tailored plasma density profiles to provide exact phase space matching to properly transport the electron beam through two such stages with negligible emittance growth. Theoretical analysis and particle-in-cell simulations show how these structures may work in four different scenarios. Good agreement between theory and simulation is obtained.

physics.acc-ph↗

arXiv2014

Damage-free single-mode transmission of deep-UV light in hollow-core PCF

Transmission of UV light with high beam quality and pointing stability is desirable for many experiments in atomic, molecular and optical physics. In particular, laser cooling and coherent manipulation of trapped ions with transitions in the UV require stable, single-mode light delivery. Transmitting even ~2 mW CW light at 280 nm through silica solid-core fibers has previously been found to cause transmission degradation after just a few hours due to optical damage. We show that photonic crystal fiber of the kagomé type can be used for effectively single-mode transmission with acceptable loss and bending sensitivity. No transmission degradation was observed even after >100 hours of operation with 15 mW CW input power. In addition it is shown that implementation of the fiber in a trapped ion experiment significantly increases the coherence times of the internal state transfer due to an increase in beam pointing stability.

physics.optics↗

arXiv2014

Generating 10~40MeV high quality monoenergetic electron beams using a 5TW 60fs laser at Tsinghua University

A unique facility for laser plasma physics and advanced accelerator research has been built recently at Tsinghua Universtiy. This system is based on Tsinghua Thomson scattering X-ray source (TTX), which combining an ultrafast TW laser with a synchronized 45MeV high brightness linac. In our recent laser wakefield acceleration experiments, we have obtained 10~40MeV high quality monoenergetic electron beams by running the laser at 5TW peak power. Under certain conditions, very low relative energy spread of a few percent can be achieved. Absolute charge calibration for three different scintillating screens has also been performed using the linac system.

physics.acc-ph↗

arXiv2014

Low emittance electron beam generation from a laser wakefield accelerator using two laser pulses with different wavelengths

Ionization injection triggered by short wavelength laser pulses inside a nonlinear wakefield driven by a longer wavelength laser is examined via multi-dimensional particle-in-cell simulations. We find that very bright electron beams can be generated through this two-color scheme in either collinear propagating or transverse colliding geometry. For a fixed laser intensity $I$, lasers with longer/shorter wavelength $λ$ have larger/smaller ponderomotive potential ($\propto I λ^2$). The two color scheme utilizes this property to separate the injection process from the wakefield excitation process. Very strong wakes can be generated at relatively low laser intensities by using a longer wavelength laser driver (e.g. a $10 \micro\meter$ CO$_2$ laser) due to its very large ponderomotive potential. On the other hand, short wavelength laser can produce electrons with very small residual momenta ($p_\perp\sim a_0\sim \sqrt{I}λ$) inside the wake, leading to electron beams with very small normalized emittances (tens of $\nano\meter$). Using particle-in-cell simulations we show that a $\sim10 \femto\second$ electron beam with $\sim4 \pico\coulomb$ of charge and a normalized emittance of $\sim 50 \nano\meter$ can be generated by combining a 10 $\micro\meter $ driving laser with a 400 $\nano\meter$ injection laser, which is an improvement of more than one order of magnitude compared to the typical results obtained when a single wavelength laser used for both the wake formation and ionization injection.

physics.acc-ph↗

arXiv1997

Incipient p-wave Superconductivity in a Si-MOSFET

We suggest here that the recently-reported ``insulator-metal transition'' in Si MOSFET's is consistent instead with a transition from an insulator to a T=0 p-wave superconducting state.

cond-mat.str-el↗