Photonic methods of radio-frequency waveform generation and processing provide performance and flexibility over electronic methods due to the ultrawide bandwidth offered by the optical carriers. However, they suffer from lack of integration and slow
reconfiguration speed. Here we propose an architecture of integrated photonic RF waveform generation and processing, and implement it on a silicon chip fabricated in a semiconductor manufacturing foundry. Our device can generate programmable RF bursts or continuous waveforms with only the light source, electrical drives/controls and detectors being off chip. It turns on and off an individual pulse in the RF burst within 4 nanoseconds, achieving a reconfiguration speed three orders of magnitude faster than thermal tuning. The on-chip optical delay elements offers an integrated approach to accurately manipulate individual RF waveform features without constrains set by the speed and timing jitter of electronics, and should find broad applications ranging from high-speed wireless to defense electronics.
We investigate the fidelity of the measurement-based quantum computation (MBQC) when it is coupled with boson environment, by measuring cluster state fidelity and gate fidelity. Two different schemes of cluster state preparation are studied. In the C
ontrolled-Z (CZ) creation scheme, cluster states are prepared by entangling all qubits in $|+rangle$ state with CZ gates on all neighboring sites. The fidelity shows an oscillation pattern over time evolution. The influence of environment temperature is evaluated, and suggestions are given to enhance the performance of MBQC realized in this way. In the Hamiltonian creation scheme, cluster states are made by cooling a system with cluster Hamiltonians, of which ground states are cluster states. The fidelity sudden drop phenomenon is discovered. When the coupling is below a threshold, MBQC systems are highly robust against the noise. Our main environment model is the one with a single collective bosonic mode.
Topological insulators are insulating in the bulk but possess spin-momentum locked metallic surface states protected by time-reversal symmetry. The existence of these surface states has been confirmed by angle-resolved photoemission spectroscopy (ARP
ES) and scanning tunneling microscopy (STM). Detecting these surface states by transport measurement, which might at first appear to be the most direct avenue, was shown to be much more challenging than expected. Here, we report a detailed electronic transport study in high quality Bi2Se3 topological insulator thin films. Measurements under in-plane magnetic field, along and perpendicular to the bias current show opposite magnetoresistance. We argue that this contrasting behavior is related to the locking of the spin and current direction providing evidence for helical spin structure of the topological surface states.
We propose one kind of transformation functions for nonmagnetic invisibility cloak with minimized scattering on the basis of generalized transformation. By matching the impedance at the outer surface of the cloak, the transformations with two paramet
ers are determined. To confirm the performance of the cloak, full wave simulation based on the finite element method is carried out. Furthermore, total scattering cross section is computed to better illustrate the scattering characteristics of cloak with different parameters. In addition, based on the effective media theory, alternating layered system composed of two isotropic materials is employed to realize the cloak practically.
It is known that the parity of reflection amplitude can either be even or odd under the mirror operation. Up to now, all the parities of reflection amplitude in the one-mode energy region are even under the mirror operation. In this paper, we give an
example of odd parity for Andreev reflection (AR) in a three-terminal graphene-supercondutor hybrid systems. We found that the parity is even for the Andreev retroreflection (ARR) and odd for specular Andreev reflection (SAR). We attribute this remarkable phenomenon to the distinct topology of the band structure of graphene and the specular Andreev reflection involving two energy bands with different parity symmetry. As a result of odd parity of SAR, the SAR probability of a four-terminal system with two superconducting leads (two reflection interfaces) can be zero even when the system is asymmetric due to the quantum interference of two ARs.
We have demonstrated a metropolitan all-pass quantum communication network in field fiber for four nodes. Any two nodes of them can be connected in the network to perform quantum key distribution (QKD). An optical switching module is presented that e
nables arbitrary 2-connectivity among output ports. Integrated QKD terminals are worked out, which can operate either as a transmitter, a receiver, or even both at the same time. Furthermore, an additional link in another city of 60 km fiber (up to 130 km) is seamless integrated into this network based on a trusted relay architecture. On all the links, we have implemented protocol of decoy state scheme. All of necessary electrical hardware, synchronization, feedback control, network software, execution of QKD protocols are made by tailored designing, which allow a completely automatical and stable running. Our system has been put into operation in Hefei in August 2009, and publicly demonstrated during an evaluation conference on quantum network organized by the Chinese Academy of Sciences on August 29, 2009. Real-time voice telephone with one-time pad encoding between any two of the five nodes (four all-pass nodes plus one additional node through relay) is successfully established in the network within 60km.
DJpsiFDC is an event generator package for the process $ggto J/psi J/psi$. It generates events for primary leading-order $2to 2$ processes. The package could generate a LHE document and this document could easily be embedded into detector simulation
software frameworks. The package is produced in Fortran codes.
We study proximity-induced superconductivity in gold nanowires as a function of the length of the nanowire, magnetic field, and excitation current. Short nanowires exhibit a sharp superconducting transition, whereas long nanowires show nonzero resist
ance. At intermediate lengths, however, we observe two sharp transitions; the normal and superconducting regions are separated by what we call the mini-gap phase. Additionally, we detect periodic oscillations in the differential magnetoresistance. We provide a theoretical model for the mini-gap phase as well as the periodic oscillations in terms of the coexistence of proximity-induced superconductivity with a normal region near the center of the wire, created either by temperature or application of a magnetic field.
Electron charge transport through a quantum point contact (QPC) driven by an asymmetric spin bias is studied. A large charge current is induced when the transmission coefficient of the QPC jumps from one integer plateau to the next. Furthermore, for
an open external circuit, the induced charge bias instead of the charge current is found to be quite large. It provides an efficient and practical way to detect spin bias by using a very simple device, a QPC or a STM tip. In addition, with the aid of magnetic field, polarization direction of the spin bias can also be determined.
We investigate the electron transport through a graphene p-n junction under a perpendicular magnetic field. By using Landauar-Buttiker formalism combining with the non-equilibrium Green function method, the conductance is studied for the clean and di
sordered samples. For the clean p-n junction, the conductance is quite small. In the presence of disorders, it is strongly enhanced and exhibits plateau structure at suitable range of disorders. Our numerical results show that the lowest plateau can survive for a very broad range of disorder strength, but the existence of high plateaus depends on system parameters and sometimes can not be formed at all. When the disorder is slightly outside of this disorder range, some conductance plateaus can still emerge with its value lower than the ideal value. These results are in excellent agreement with the recent experiment.
