Xinye Xu

Theory of atom guidance in a hollow laser beam: dressed-atom approach

We present a general theory of atom guiding in a blue-detuned hollow laser beam. Using the dressed-atom approach, we obtain the mean dipole gradient force, the radiation pressure force, and the momentum diffusion coefficients for three-level Λ-type cold atoms. Using Monte Carlo simulation, we calculate the guiding efficiencies and the final velocity distributions of atoms for various conditions. We find that the guiding efficiency depends not only on the intensity and detuning of the guiding hollow beam but also on the atom-guiding direction with respect to the propagation direction of the hollow laser beam. Comparing our analyses with recent experimental results, we find that they are mutually consistent. The results that we present can also be applied to atom guiding by hollow optical fibers.

Effects of noises on joint remote state preparation via a GHZ-class channel

Using a GHZ-class state as quantum channel, we investigate the joint remote preparation of a qubit state in Pauli noise environments. By analytically solving the master equation in Lindblad form, we calculate the time evolution of the GHZ-class channel under different noisy conditions and then obtain the fidelity of the joint remote state preparation (JRSP) process and the corresponding average fidelity. We find that the fidelity depends on the noise type, the GHZ-class state, the initial state to be remotely prepared, and the Pauli decoherence rate. We also find that how two senders share the polar angle information of initial state plays an important role in the fidelity, and information sharing reduces the ability to resist the influence of Pauli noises in our JRSP protocol. Furthermore, how the two senders share the phase information affects the intensity of the bit-phase flip noise and the bit flip noise acting on the average fidelity. Besides, the fidelity of our JRSP protocol achieved via the maximally entangled channel is larger than that achieved via the partially entangled channel.

Deterministic joint remote preparation of an arbitrary two-qubit state in noisy environments

Using four Einstein–Podolsky–Rosen (EPR) states as the shared quantum channel, we investigate the deterministic joint remote preparation of an arbitrary two-qubit state in the presence of noisy environments through the analytical solution of the master equation in the Lindblad form. By means of unitary matrix decomposition method, quantum logic circuit for the deterministic joint remote state preparation (JRSP) protocol is first constructed. Then, we analytically derive the average fidelities of the deterministic JRSP process under the influence of Pauli noises, zero-temperature and high-temperature reservoirs acting on the four EPR pairs. It is found that the average fidelities under the action of different noises display different evolution behaviors. Moreover, for the specific noises examined in this paper, in the long-time limit, the dephasing noise and the zero-temperature environment have the relatively weak effect on their respective average fidelities, whereas the isotropic noise and the high-temperature environment have the relatively strong effect.

Quantum-network generation based on four-wave mixing

We present a scheme to realize versatile quantum networks by cascading several four-wave mixing (FWM) processes in warm rubidium vapors. FWM is an efficient

Multimode entanglement in reconfigurable graph states using optical frequency combs

{\ensuremath{\chi}}^{(3)}

Highly efficient single-pass second harmonic generation in a periodically poled MgO:LiNbO 3 waveguide pumped by a fiber laser at 1111.6 nm

nonlinear process, already used as a resource for multimode quantum state generation and which has been proved to be a promising candidate for applications to quantum information processing. We analyze theoretically the multimode output of cascaded FWM systems, derive its independent squeezed modes, and show how, with phase controlled homodyne detection and digital postprocessing, they can be turned into a versatile source of continuous variable cluster states.

Multi-wavelength fiber lasers operated in stretched-pulse mode-locked regimes with a very compact configuration

Multimode entanglement is an essential resource for quantum information processing and quantum metrology. However, multimode entangled states are generally constructed by targeting a specific graph configuration. This yields to a fixed experimental setup that therefore exhibits reduced versatility and scalability. Here we demonstrate an optical on-demand, reconfigurable multimode entangled state, using an intrinsically multimode quantum resource and a homodyne detection apparatus. Without altering either the initial squeezing source or experimental architecture, we realize the construction of thirteen cluster states of various sizes and connectivities as well as the implementation of a secret sharing protocol. In particular, this system enables the interrogation of quantum correlations and fluctuations for any multimode Gaussian state. This initiates an avenue for implementing on-demand quantum information processing by only adapting the measurement process and not the experimental layout.

A quantum scattering interferometer.

A green light at 556 nm is generated by direct frequency doubling of a fiber laser at 1111.6 nm with a periodically poled MgO:LiNbO3 waveguide. We have investigated optical inhomogeneities by measuring the temperature tuning curve of second harmonic generation, and the obtained parameters are used for identifying the uniformity of the waveguide. The thermal dephasing could be diminished by adjusting the crystal temperature, and the conversion efficiency was maximized. Finally, an output power of 111.8 mW at 556 nm was generated with 213 mW of coupled fundamental light under optimum conditions, which corresponds to 52.5% conversion efficiency.

Study on the clock-transition spectrum of cold 171Yb ytterbium atoms

A novel and compact configuration for the generation of tunable multi-wavelength fiber ring lasers by exploiting the intracavity birefringence filter is demonstrated. The fiber laser is passively mode locked by using a nonlinear polarization rotation technique. A single polarization controller (PC) and a polarization-dependent isolator combined with birefringence act as a tunable comb filter and the mode locker to generate the single-wavelength ultrafast oscillation and multi-wavelength lasing by finely adjusting the PC. Multi-wavelength lasing of up to 50 lines with a wavelength spacing of 0.8 nm within a 3 dB bandwidth has been obtained. The total group velocity dispersion of the ring cavity is designed to the near-zero regime, which indicates a stretched-pulse mode-locked state.

Analysis of inhomogeneous-excitation frequency shifts of ytterbium optical lattice clocks

The collision of two ultracold atoms results in a quantum mechanical superposition of the two possible outcomes: each atom continues without scattering, and each atom scatters as an outgoing spherical wave with an s-wave phase shift. The magnitude of the s-wave phase shift depends very sensitively on the interaction between the atoms. Quantum scattering and the underlying phase shifts are vitally important in many areas of contemporary atomic physics, including Bose–Einstein condensates, degenerate Fermi gases, frequency shifts in atomic clocks and magnetically tuned Feshbach resonances. Precise experimental measurements of quantum scattering phase shifts have not been possible because the number of scattered atoms depends on the s-wave phase shifts as well as the atomic density, which cannot be measured precisely. Here we demonstrate a scattering experiment in which the quantum scattering phase shifts of individual atoms are detected using a novel atom interferometer. By performing an atomic clock measurement using only the scattered part of each atom’s wavefunction, we precisely measure the difference of the s-wave phase shifts for the two clock states in a density-independent manner. Our method will enable direct and precise measurements of ultracold atom–atom interactions, and may be used to place stringent limits on the time variations of fundamental constants.