Feng Mang

Tunable thermal entanglement in an effective spin-star system using coupled microcavities

We theoretically explore the possibility of realizing controllable thermal entanglement of effective spins in a four-qubit anisotropic Heisenberg XXZ coupling spin-star system constructed by coupled microcavities. We analyse the dependence of thermal entanglement in this system on temperature, inhomogeneity of the magnetic field, and anisotropy, which can be readily tuned via the external laser fields. The peculiar characteristic and the full controllability of the thermal entanglement are demonstrated to be useful for quantum information processing.

Demonstration of Cold 40Ca+ Ions Confined in a Microscopic Surface-Electrode Ion Trap

40Ca+ ions are successfully confined, under the cooling of a red-detuned laser, in a home-built microscopic surface-electrode (MSE) trap. With all electrodes deposited on a low-rf-loss substrate, our 500-μm-scale MSE trap is designed involving three potential wells and manufactured by the standard technique of the printed circuit board. Both linear and two-dimensional crystals of 40Ca+ are observed in the trap after preliminary micromotion compensation is carried out. The development of the MSE trap aims at large-scale trapped-ion quantum information processing.

Strings of Ion Crystals in a Linear Trap for Quantum Information Processing

Strings of laser cooled 40Ca+ crystals have been successfully confined in our home-built linear ion trap, and ready for quantum information processing. We find the cloud-crystal phase transition of the trapped ions to be strongly sensitive to the frequencies of the Doppler cooling lasers and to the trapping voltage. The quantum jump of a single ion has been observed by controlling the quadrupole transition of the ion by a weak laser with ultra-narrow bandwidth.

Microscopic Surface-Electrode Ion Trap for Scalable Quantum Information Processing

In this paper we try to develop a scalable surface-electrode architecture for ion trap quantum information processing. The confinement of the ions by the rf pseudopotential and the movement of the ions by changing the rf pseudopotential are investigated by numerical simulation. Particular concern is paid to the +-shaped junction, which is the connection of different components of the architecture, and also on the place which yields heat and escaping ions. We show the feasibility of fabricating and operating on the architecture for quantum information processing with currently available technology.

A Study of the Characteristics of the Wave Packets of a Paul-Trapped Ion

Based on the former studies, the time-dependent characteristics of the wave packets of a Paul-trapped ion are investigated explicitly in cylindrical coordinates by the method of function series expansion (FSE). It is shown that the present work relates the quantum motion of a Paul-trapped ion to the orbit of the classical motion and presents a more clear physical picture than the former studies.

Spectra of 42S1/2→32D5/2 Transitions of a Single Trapped 40Ca+ Ion

We investigate the spectra of the electric quadrupole 42S1/2→32D5/2 transitions in a single 40Ca+ ion confined in a home-built linear trap. We probe the transitions with an ultra-narrow bandwidth laser at 729 nm. In a weak magnetic field, the quadrupole transition splits into ten components with the maximal line strength proportional to their squared Clebsch—Gordan factors. In a magnetic field of the order of Gauss, the observed equidistant sideband reflects the Zeeman substructure modulated by the quantized oscillation due to the secular motion in the trap. The temperature of the trapped ion can be determined by the envelope of the sideband spectrum. We also demonstrate the Rabi oscillation in a carrier transition after the ion has been Doppler cooled, which can be fitted by the model with the thermal state of motion.

Manipulation of Ions in Microscopic Surface-Electrode Ion Traps

The spatial manipulation of ionic qubits in a fast and precise fashion is one of the foremost issues in scalable quantum information processing with trapped ions. We report our recent efforts toward precise manipulation of trapped ions, including separation, recombination and reordering of the ions, in our home-made microscopic surface-electrode trap. We also demonstrate the micromotion compensation for the trapped ions by rf-photon cross-correlation, which ensures the cooling of the ions down to the temperature 23.3 mK.

Irreversibility of a quantum walk induced by controllable decoherence employing random unitary operations

Quantum walk is different from random walk in reversibility and interference. Observation of the reduced reversibility in a realistic quantum walk is of scientific interest in understanding the unique quantum behavior. We propose an idea to experimentally investigate the decoherence-induced irreversibility of quantum walks with trapped ions in phase space via the average fidelity decay. By introducing two controllable decoherence sources, i.e., the phase damping channel (i.e., dephasing) and the high temperature amplitude reservoir (i.e., dissipation), in the intervals between the steps of quantum walk, we find that the high temperature amplitude reservoir shows more detrimental effects than the phase damping channel on quantum walks. Our study also shows that the average fidelity decay works better than the position variance for characterizing the transition from quantum walks to random walk. Experimental feasibility to monitor the irreversibility is justified using currently available techniques.

Micromotion Compensation and Photoionization of Ions in a Linear Trap

The stable confinement of ions in an electromagnetic trap is a prerequisite of sideband cooling and quantum information processing. For a string of ions in a linear ion trap, we report our recent efforts of compensating for micromotion of the ions by three methods, which yields narrower fluorescence spectra and lower temperature. We also achieve a photoionization scheme that loads the ions deterministically into the linear trap from an atomic beam.

Control of a Cloud of Laser-Cooled 40Ca+ in a Linear Ion Trap

A cloud of laser-cooled 40Ca+ is successfully trapped and manipulated under well control in our home-built linear ion trap, which is designed and constructed solely for studying quantum information processing. By exploring the variation of the ion cloud with respect to the trap parameters, we have optimized the trapping condition and obtained very good fluorescence spectra. We observe the dynamics of the ion cloud, and estimate the temperature of the ion cloud to be of the order of milli-Kelvin.