Xiang-Dong Chen

Plasmon modes of silver nanowire on a silica substrate

Plasmon mode in a silver nanowire is theoretically studied when the nanowire is placed on or near a silica substrate. It is found that the substrate has much influence on the plasmon mode. For the nanowire on the substrate, the plasmon (hybrid) mode possesses not only a long propagation length but also an ultrasmall mode area. From the experimental point of view, this cavity-free structure holds a great potential to study a strong coherent interaction between the plasmon mode and single quantum system (for example, quantum dots) embedded in the substrate.

Broadband integrated polarization beam splitter with surface plasmon

A broadband integrated waveguide polarization beam splitter consisting of a metal nanoribbon and two dielectric waveguides is proposed and numerically investigated. This surface plasmon based device provides a unique approach for polarization sensitive manipulation of light in an integrated circuit and will be essential for future classical and quantum information processes.

Temperature dependent energy level shifts of nitrogen-vacancy centers in diamond

Magnetic resonance and fluorescence spectra of nitrogen-vacancy (NV) color centers ensemble in high purity diamond sample were measured, with temperature ranging from 5.6 K to 295 K. Both microwave and optical transition energies have similar nonlinear temperature dependent changes, which might mainly originate from the local thermal expansion. As the frequency shifts will reduce the fidelity of resonant quantum control, the present results demonstrate the necessity of taking temperature fluctuation into consideration. For temperature below 100 K, the transition energies show tendencies to be constant, which indicate higher stability and performance in applications with NV centers.

Movable Fiber-Integrated Hybrid Plasmonic Waveguide on Metal Film

A waveguide structure consisting of a tapered nanofiber on a metal film is proposed and analyzed to support highly localized hybrid plasmonic modes. The hybrid plasmonic mode can be efficiently excited through the in-line tapered fiber based on adiabatic conversion and collected by the same fiber, which is very convenient in experimentation. Due to the ultrasmall mode area of plasmonic mode, the local electromagnetic field is greatly enhanced in this movable waveguide, which has the potential for enhanced coherence light emitter interactions, such as waveguide quantum electrodynamics, single emitter spectrum, and nonlinear optics.

Quantum statistical imaging of particles without restriction of the diffraction limit.

A quantum measurement method based on the quantum nature of antibunching photon emission has been developed to detect single particles without the restriction of the diffraction limit. By simultaneously counting the single-photon and two-photon signals with fluorescence microscopy, the images of nearby nitrogen-vacancy centers in diamond at a distance of 8.5±2.4  nm have been successfully reconstructed. Also their axes information was optically obtained. This quantum statistical imaging technique, with a simple experimental setup, can also be easily generalized in the measuring and distinguishing of other physical properties with any overlapping, which shows high potential in future image and study of coupled quantum systems for quantum information techniques.

Optical manipulation of the charge state of nitrogen-vacancy center in diamond

The conversion between two nitrogen vacancy (NV) center charge states (NV0 and NV−) was experimentally studied with laser in the range from 378 to 492 nm. The wavelength and power dependent charge state conversion was proved to be single-photon process at short wavelength and two-photon process at long wavelength. Also, the presence of metastable state in NV− significantly affected the conversion process, which can be used to increase NV− population from 72.5% to 80.5% by applying a magnetic field under 532 nm excitation. The method can be used for improving the fidelity of charge state polarization and subsequently optimizing the quantum control with NV center and resolution in the magnetometric and biological sensoring.

Generation of Nitrogen-Vacancy Center Pairs in Bulk Diamond by Molecular Nitrogen Implantation

The coupled negatively charged nitrogen-vacancy (NV−) center system is a promising candidate for scalable quantum information techniques. In this work, ionized nitrogen molecules are implanted into bulk diamond to generate coupled NV− center pairs. The two-photon autocorrelation measurement and optically detected magnetic resonance measurement are carried out to confirm the production of the NV− center pair. Also, both 1.3 μs decoherence time and 4.9 kHz magnetic coupling strength of the NV− center pair are measured by controlling and detecting the spin states. Along with nanoscale manipulation and detection methods, such coupled NV− centers through short distance dipole-dipole interaction would show high potential in scalable quantum information processes.

Vector magnetic field sensing by a single nitrogen vacancy center in diamond

In this letter, we proposed and experimentally demonstrated a method to detect the vector magnetic field with a single nitrogen vacancy (NV) center in diamond. The magnetic field in parallel with the axis of the NV center can be obtained by detecting the electron Zeeman shift, while the Larmor precession of an ancillary nuclear spin close to the NV center can be used to measure the field perpendicular to the axis. Experimentally, both the Zeeman shift and Larmor precession can be measured through the fluorescence from the NV center. By applying additional calibrated magnetic fields, complete information on the vector magnetic field can be achieved with such a method. This vector magnetic-field detection method is insensitive to temperature fluctuation and it can be applied to nanoscale magnetic measurements.

Photonic simulation of system-environment interaction: Non-Markovian processes and dynamical decoupling

The system-environment interaction is simulated by light propagating in coupled photonic waveguides. The profile of the electromagnetic field provides intuitive physical insight to study the Markovian and non-Markovian dynamics of open quantum systems. The transition from non-Markovian to Markovian process is demonstrated by increasing the size of environment, as the energy evolution changes from oscillating to an exponential decay, and the revival period increases. Moreover, the dynamical decoupling with a sequence of phase modulations is introduced to such a photonic open system to form a band structure in time dimension, where the energy dissipation can be significantly accelerated or inhibited. It opens the possibility to tune the dissipation in photonic system, similar to the dynamic decoupling of spins.

Non-Markovianity-assisted high-fidelity Deutsch–Jozsa algorithm in diamond

The memory effects in non-Markovian quantum dynamics can induce the revival of quantum coherence, which is believed to provide important physical resources for quantum information processing (QIP). However, no real quantum algorithms have been demonstrated with the help of such memory effects. Here, we experimentally implemented a non-Markovianity-assisted high-fidelity refined Deutsch–Jozsa algorithm (RDJA) with a solid spin in diamond. The memory effects can induce pronounced non-monotonic variations in the RDJA results, which were confirmed to follow a non-Markovian quantum process by measuring the non-Markovianity of the spin system. By applying the memory effects as physical resources with the assistance of dynamical decoupling, the probability of success of RDJA was elevated above 97% in the open quantum system. This study not only demonstrates that the non-Markovianity is an important physical resource but also presents a feasible way to employ this physical resource. It will stimulate the application of the memory effects in non-Markovian quantum dynamics to improve the performance of practical QIP.Non-Markovianity: an important quantum resourceScientists have successfully applied the non-Markovianity of environment as an important quantum resource to improve the performance of quantum information processing. Quantum system unavoidably suffers from notorious decoherence, which is a primary hurdle for wide applications of quantum information techniques in realistic quantum system. Unlike to passively shield the decoherence, a team led by Fang-Wen Sun at University of Science and Technology of China actively utilized the memory effect of a non-Markovian environment, a typical quantum decoherence environment, to revive quantum coherence and correlation and to improve the fidelity of quantum Deutsch-Jozsa algorithm in a diamond nitrogen-vacancy center. This study not only demonstrates that the non-Markovianity is an important quantum resource but also presents a feasible way to employ this physical resource for quantum information techniques.