A vertical test system for China-ADS project injector II superconducting cavities

To test superconducting cavities, a vertical test system has been designed and set up at the Institute of Modern Physics （IMP）. The system design is based on VCO-PLL hardware and the NI Labview software. The test of the HWR010#2 superconducting cavity shows that the function of this test system is satisfactory for testing the low frequency cavity.     

Low beta spoke cavity multipacting analysis

The simulation and analysis for electron multipacting phenomenon in a low β spoke superconducting cavity in ADS proton accelerator are proposed. Using both CST and Track3P codes, the electron multipacting calculation for β=0.12 spoke superconducting cavity is implemented. The methods of multipacting calculation on both codes are studied and described. With the comparison between the calculation results and the cavity vertical test result, the accuracy and reliability of different codes on calculating multipacting are analyzed. Multipacting calculation can help to understand the results of vertical test and also can help to do the optimization in cavity design.     

Design of a 450 MHz β=0.2 single spoke cavity at PKU

The design of a 450 MHz β=0.2 superconducting single spoke cavity has been finished at Peking University. A theoretical model and a numerical simulation are used to study the relationship between the RF performance and the geometric parameters of the cavity. In this paper, the optimization of the spoke cavity is described in detail. The RF simulation gives the optimum parameters Epk/Zacc of 2.65 and Bpk/Eacc of 5.22 mT/（MV/m）. The mechanical properties of the cavity are also studied. Two stiff ribs are used to offer a credible mechanical stability.     

Preparation of Entanglement and Schrodinger Cat States of Superconducting Quantum Interference Devices Qubits via Coupling to a Microwave Cavity

An alternative scheme is proposed for the generation of n-qubit W states of superconducting quantum interference devices （SQUID） in cavity QED. In this scheme, Raman coupling of two lower flux states of SQUID system is achieved via a microwave pulse and the cavity mode. Conditioned on no photon leakage from the cavity, the n-qubit W state can be generated whether the effective coupling parameters of the SQUID to cavity mode and classical microwave fields are the same or different. Our strictly numerical simulations of the time evolution of the system including decay show that the success probability of our scheme is almost unity and the interaction time is on the order of 10-9 s. The scheme can also be used to generate the Schrodinger cat states of multi-SQUID.     

Study and design of RF coupler for Chinese ADS HWR superconducting cavity

An RF power coupler is a key component of the superconducting accelerating system in Chinese ADS proton linac injector I, which is used to transmit 15 kW RF power from the power source to the superconducting HWR cavity. According to the requirement of working frequency, power level, transmission capability and cooling condition, the physics design of coupler has been finished, which includes RF structure optimization, thermal simulation, thermal stress analysis and so on. Based on this design, the prototype of HWR coupler has been fabricated, and it has successfully passed the high power test.     

A simple scheme to generate x-type four-charge entangled states in circuit QED

We propose a simple scheme to generate x-type four-charge entangled states by using SQUID-based charge qubits capacitively coupled to a transmission line resonator （TLR）. The coupling between the superconducting qubit and the TLR can be effectively controlled by properly adjusting the control parameters of the charge qubit. The experimental feasibility of our scheme is also shown.     

Error analysis and lattice improvement for the C-ADS Injector- I

The injector Scheme- 1 （or Injector- I ） of the C-ADS linac is a 10 mA 10 MeV proton linac working in C／V mode. It is mainly comprised of a 3.2 MeV room-temperature 4-vane RFQ and twelve superconducting single-spoke cavities housed in a long cryostat. Error analysis including alignment and field errors, and static and dynamic ones for the illjector are presented. Based on detailed numerical simulations, an orbit correction scheme has been designed, which shows that with correction the rms residual orbit errors can be controlled within 0.3 mm and a beam loss rate of 1.7× 10-6 is obtained. To reduce the beam loss rate further, an improved lattice design for the superconducting spoke cavity section has been studied.     

Formation of epitaxial Tl_2Ba_2Ca_2Cu_3O_(10) superconducting films by dc-magnetron sputtering and triple post-annealing method

For obtaining pure phase T12Ba2Ca2Cu3O10 （T1-2223） films with good superconducting properties, the growth technique is improved by dc magnetron sputtering and a triple post-annealing process. The triple post-annealing process comprises annealing twice in argon and once in oxygen at different temperatures. In the first low-temperature annealing phase in argon, T12Ba2CaCu2O8 （T1-2212） is obtained to effectively minimize evaporation in the next step. With the increase of temperature in the second annealing stage in argon, the previously prepared T1-2212 inter-phase is converted into T1-2223 phase. An additional annealing in oxygen is also adopted to improve the properties of T1-2223 films, each containing an optimal oxygen content value. The results of X-ray diffraction （XRD） θ-2θ scans, 09 scans and rotational φ scans show that each of the T1-2223 films has a high phase purity and an epitaxial structure. Smooth films are observed by scanning electron microscopy （SEM）. The critical temperatures Tc of the films are measured to be about 120 K and the critical current densities Jc can reach 4.0 MA/cm2 at 77 K at self field.     

Generating Squeezed States in Solid State Circuits

We propose a scheme for generating squeezed states in solid state circuits which consist a superconducting transmission line resonator （STLR）, a superconducting Cooper-pair box （CPB） and a nanoelectromechanical resonator （NMR）. The nonlinear interaction between the STLR and the CPB can be implemented by setting the external biased flux of the CPB at some certain points. The interaction Hamiltonian between the STLR and the NMR is derived by performing Fr ohlich transformation on tile total Hamiltonian of tile combined system. Just by adiabatically keeping the CPB at the ground state, we get the standard parametric down-conversion Hamiltonian, and the squeezed states of the STLR can be easily generated, which is similar to the three-wave mixing in quantum optics.     

Bunch length measurement using a traveling wave RF deflector

RF deflectors can be used for bunch length measurement with high resolution. This paper describes a completed S-band traveling wave RF deflector and the bunch length measurement of the electron beam produced by the photocathode RF gun of the Shanghai DUV-FEL facility. This is the first time that such a transverse RF deflector has been developed and used to measure the bunch length of picosecond order in China. The deflector＇s VSWR is 1.06, the whole attenuation 0.5 dB, and the bandwidth 4.77 MHz for VSWR less than 1.1. With a laser pulse width of 8.5 ps, beam energy of 4.2 MeV, and bunch charge of 0.64 nC, the bunch lengths for different RF input power into the deflector were measured, and an averaged rms bunch length of 5.25 ps was obtained. A YAG crystal is used as a screen downstream of the deflector, with the calibrated value of 1 pix = 136 μm.     

Theory study on a photonic-assisted radio frequency phase shifter with direct current voltage control

A photonic-assisted radio frequency phase shifter with direct current voltage control is proposed using a polymer- based integrated Mach-Zehnder modulator. A closed-form expression of radio frequency （RF） signal power and phase is given. Theoretical calculation reveals that by carefully setting the bias voltages, RF signal power variation lower than 1-dB and phase accuracy less than 3° can be achieved and are not degraded by perturbation of modulation index once the bias voltage drift is kept within -3% -- 3%.     

Implementation of quantum partial search with superconducting quantum interference device qudits in cavity QED

We present a method to implement the quantum partial search of the database separated into any number of blocks with qudits,D-level quantum systems.Compared with the partial search using qubits,our method needs fewer iteration steps and uses the carriers of the information more economically.To illustrate how to realize the idea with concrete physical systems,we propose a scheme to carry out a twelve-dimensional partial search of the database partitioned into three blocks with superconducting quantum interference devices(SQUIDs)in cavity QED.Through the appropriate modulation of the amplitudes of the microwave pulses,the scheme can overcome the non-identity of the cavity–SQUID coupling strengths due to the parameter variations resulting from the fabrication processes.Numerical simulation under the influence of the cavity and SQUID decays shows that the scheme could be achieved efficiently within current state-of-the-art technology.     

Beam dynamics simulations of the injector for a compact THz source

Terahertz radiation has broad application prospects due to its ability to penetrate deep into many organic materials without the damage caused by ionizing radiations. A free electron laser （FEL）-based THz source is the best choice to produce high-power radiation. In this paper, a 14 MeV injector is introduced for generating high-quality beam for FEL, is composed of an EC-ITC RF gun, compensating coils and a travelling-wave structure. Beam dynamics simulations have been done with ASTRA code to verify the design and to optimize parameters. Simulations of the operating mode at 6 MeV have also been executed.     

Beam tail effect of a performance-enhanced EC-ITC RF gun

The beam tail effect of multi-bunches will influence the electron beam performance in a high intensity thermionic RF gun. Beam dynamic calculations that illustrate the working states of single beam tail and multi-pulse feed-in of a performance-enhanced EC-ITC （external cathode independent tunable cavity） RF gun for an FEL （free electron laser） injector are performed to estimate the extracted bunch properties. By using both Parmela and homemade MATLAB codes, the effects of a single beam tail as well as interactions of multi-pulses are analyzed, where a ring-based electron algorithm is adopted to calculated RF fields and the space-charge field. Furthermore, the procedure of unexpected deviated-energy particles mixed with an effective bunch head is described by the MATLAB code as well. As a result, the performance-enhanced EC-ITC RF gun is proved to have the capability to extract continual stable bunches suitable for a high requirement THz-FEL.     

Design of a 450 MHz β=0.2 single spoke cavity at PKU

The design of a 450 MHz β=0.2 superconducting single spoke cavity has been finished at Peking University. A theoretical model and a numerical simulation are used to study the relationship between the RF performance and the geometric parameters of the cavity. In this paper, the optimization of the spoke cavity is described in detail. The RF simulation gives the optimum parameters Epk/Eacc of 2.65 and Epk/Eacc of 5.22 mT/(MV/m). The mechanical properties of the cavity are also studied. Two stiff ribs are used to offer a credible mechanical stability.     

Emittance measurement & optimization for the photocathode RF gun with laser profile shaping

The Laser Undulator Compact X-ray source （LUCX） is a test bench for a compact high brightness X-ray generator, based on inverse Compton Scattering at KEK, which requires high intensity multi-bunch trains with low transverse emittance. A photocathode RF gun with emittance compensation solenoid is used as an electron source. Much endeavor has been made to increase the beam intensity in the multi-bunch trains. The cavity of the RF gun is tuned into an unbalanced field in order to reduce space charge effects, so that the field gradient on the cathode surface is relatively higher when the forward RF power into gun cavity is not high enough. A laser profile shaper is employed to convert the driving laser profile from Gaussian into uniform. In this research we seek to find the optimized operational conditions for the decrease of the transverse emittance. With the uniform driving laser and the unbalanced RF gun, the RMS transverse emittance of a 1 nC bunch has been improved effectively from 5.46 πmm·mrad to 3.66 πmm·mrad.     

Design study on very low Beta spoke cavity for China-ADS

Very low Beta superconducting spoke cavity is one of the key challenges for the China-ADS project. In this paper, a new structure of 3βλ/2 spoke cavity is first studied in detail. Its RF and mechanical properties are simulated using CST-MWS and ANSYS, and compared with the traditional βλ/2 spoke structure.     

CSNS-II双谐波加速研究

The Rapid Cycling Synchrotron （RCS） of the China Spallation Neutron Source （CSNS） complex is designed to provide 1.56×10^13 protons per pulse （ppp） during the initial stage, and it is upgradeable to 3.12× 10^13 ppp during the second stage and 6.24× 10^13 ppp during the ultimate stage. The high beam intensity in the RCS requires alleviation of space charge effects to reduce beam losses, which is key in such high beam power accelerators. With higher intensities in the upgrading phases, a dual-harmonic RF system is planned to produce flat-topped bunches that are useful to reduce the space charge effects. We have studied different schemes to apply the dual-harmonic acceleration in CSNS-Ⅱ, and have calculated the main parameters of the RF svstems, which are presented in this Darter.     

Entanglement Purification Through Cavity Input-Output Process

We propose a scheme to purify entanglement of two atoms from not-too-impure entangled states by checking the parity of the two atoms through the cavity input-output process. As the parity check is made by measurement on single-photon polarization, which would not affect the entanglement of the two atoms, our scheme has the successful probability double of that in a previous well-known scheme with linear optical elements [Nature （London） 410 （2001） 1067], and is insensitive to the photon loss and the detection inefficieney. Experimental feasibility of our scheme with current technology is discussed.     

Progress in superconducting qubits from the perspective of coherence and readout

Superconducting qubits are Josephson junction-based circuits that exhibit macroscopic quantum behavior and can be manipulated as artificial atoms.Benefiting from the well-developed technology of microfabrication and microwave engineering,superconducting qubits have great advantages in design flexibility,controllability,and scalability.Over the past decade,there has been rapid progress in the field,which greatly improved our understanding of qubit decoherence and circuit optimization.The single-qubit coherence time has been steadily raised to the order of 10 to 100μs,allowing for the demonstration of high-fidelity gate operations and measurement-based feedback control.Here we review recent progress in the coherence and readout of superconducting qubits.