Tiancai Zhang

Surface-Plasmon-Induced Modification on the Spontaneous Emission Spectrum via Subwavelength-Confined Anisotropic Purcell Factor

The mechanism of using the anisotropic Purcell factor to control the spontaneous emission linewidths in a four-level atom is theoretically demonstrated; if the polarization angle bisector of the two dipole moments lies along the axis of large/small Purcell factor, destructive/constructive interference narrows/widens the fluorescence center spectral lines. Large anisotropy of the Purcell factor, confined in the subwavelength optical mode volume, leads to rapid spectral line narrowing of atom approaching a metallic nanowire, nanoscale line width pulsing following periodically varying decay rates near a periodic metallic nanostructure, and dramatic modification on the spontaneous emission spectrum near a custom-designed resonant plasmon nanostructure. The combined system opens a good perspective for applications in ultracompact active quantum devices.

A comparison of two nonclassical measures, entanglement potential and the negativity of the Wigner function

Two measures of nonclassicality, the entanglement potential and the negativity of the Wigner distribution function defined by the volume of its negative domains, are compared based on an investigation of the nonclassicality for Fock states and Schrodinger cat states in a decoherence process. Both the entanglement potential and the total negative probability are reduced in the linear loss process and the partial negative distribution of the Wigner function is wiped out for large losses while the entanglement potential is always positive. We give a bound condition and find that, though not yet mathematically proven in general, the upper bound of 50% is the maximum allowed loss for the survival of the negative distribution of the Wigner function.

Generation of blue light at 426 nm by frequency doubling with a monolithic periodically poled KTiOPO 4

Continuous-wave (cw) blue laser generation at 426 nm by frequency doubling with a monolithic periodically poled KTP (PPKTP) cavity is reported in this paper. Without any free mirrors, the standing-wave cavity solely consists of a monolithic PPKTP crystal, and both ends of which are spherically polished and mirror-coated. An output power of 158 mW is obtained when the pump power is 350 mW. The conversion efficiency is 45%. The dependence of the conversion efficiency on the temperature and the incident fundamental power has been discussed. Such a system is integrally stable and compact for long-time operation under temperature control. The system is much more stable than the usual servo lock system for external cavity doubling.

Transferring cold atoms in double magneto-optical trap by a continuous-wave transfer laser beam with large red detuning

A novel scheme of transferring cold atoms in a double magneto-optical trap (MOT) system has been experimentally demonstrated. Cold cesium atoms trapped in a vapor-cell MOT are efficiently transferred to an ultrahigh-vacuum (UHV) MOT by a continuous-wave divergent Gaussian transfer laser beam. When large red detuning and moderate intensity are adopted for the transfer laser beam, enhancement of the recapturing of atoms in the UHV MOT is clearly observed. Using the divergent transfer laser beam (diameter of approximately 1.60 mm in the vapor-cell MOT region) with typical power of approximately 20.2 mW, up to approximately 85% of transfer efficiency is obtained when the frequency detuning is set to around -1.2 GHz, and it is not sensitive to small detuning variation. This transfer is much efficient compared with that in the case of continuous-wave near-resonance weak transfer laser beam (typical power of order of approximately 100 microW and typical frequency detuning of approximately-10 MHz) which is normally used in double-MOT experiment. The enhancement is ascribed to the guiding effect on cold atomic flux by transverse dipole potential of the large red-detuned transfer laser beam.

Evanescent-Vacuum-Enhanced Photon-Exciton Coupling and Fluorescence Collection

An evanescent optical mode existing in various nanophotonic structures always acts as a cavity mode rather than an electromagnetic vacuum in the study of cavity quantum electrodynamics (CQED). Here we show that taking the evanescent mode as an electromagnetic vacuum in which the nanocavity is located is possible through the optical mode design. The proposed evanescent vacuum enables us to enhance both the reversible photon-exciton interaction and fluorescence collection. By embedding the custom-designed plasmon nanocavity into the evanescent vacuum provided by a metallic or dielectric nanowire, the photon-exciton coupling coefficient can achieve 4.2 times that in vacuum due to the exponential decay of the evanescent wave, and spontaneously emitted photons with Rabi splitting can be guided by an evanescent wave with a collection efficiency of 47% at most. Electromagnetic vacuum engineering at subwavelength scale holds promise for controlling the light-matter interaction in quantum optics, CQED, and on-chip quantum information.

Intrinsic Quantum Beats of Atomic Populations and Their Nanoscale Realization Through Resonant Plasmonic Antenna

In the coherently trapped populations of a four-level atom, we demonstrated the quantum beats with different mechanism, which originate from the interference between transition channels with different dipole moments. The beat frequency is determined by the intrinsic atomic parameters, i.e., the spacing of upper levels and ratio of dipole moments. The resonant plasmonic nanoantenna, as a candidate for the creation of anisotropic vacuum, was proposed to achieve the nanoscale realization of the quantum beats, spontaneous emission cancellation, and Rabi oscillation in two-photon correlations through the enhanced near-field and modified decay rates.

Observation of sub-Doppler absorption in the Λ-type three-level Doppler-broadened cesium system

Thanks to the atomic coherence in the coupling-laser-driven atomic system, sub-Doppler absorption has been observed in Doppler-broadened cesium vapor cell via the Λ-type three-level scheme. The linewidth of the sub-Doppler absorption peak becomes narrower while the frequency detuning of coupling laser increases. The results are in agreement with the theoretical prediction by G. Vemuri et al.

Precision measurement of single atoms strongly coupled to the higher-order transverse modes of a high-finesse optical cavity

We have experimentally demonstrated the strong coupling between single atoms and the higher-order Hermite-Gaussian transverse modes in a high-finesse optical microcavity. Compared to the usual low-order symmetric transverse modes, multiple lobes and the asymmetric spatial pattern of the titled modes provide more information about the motion of single atoms in the cavity. The motional information can be extracted from the measured transmission spectra, which includes the velocities and the positions of the atoms in vertical and off-axis directions. The scheme has great potential in time-resolved atom-cavity microscopy and in tracking the three-dimensional single atom trajectory in real time.

Improvement of the signal-to-noise ratio of laser-induced-fluorescence photon-counting signals of single-atoms magneto-optical trap

Employing grating extended-cavity diode lasers as the cooling/trapping and repumping lasers for preparing and manipulating single atoms, we have implemented a large-magnetic-gradient caesium magneto-optical trap (MOT). To detect and evaluate single caesium atoms trapped in MOT, laser-induced-fluorescence (LIF) photons of trapped atoms driven by MOT lasers are collected and counted by an avalanched photodiode worked in photon-counting mode. The dependences of LIF photon-counting signals of single atoms on a cooling lasers intensity, frequency detuning and frequency fluctuation are analysed and investigated. Remarkable improvement of the signal-to-noise ratio of LIF photon-counting signals is achieved by optimizing the cooling lasers intensity and frequency detuning and using the modulation-free polarization spectroscopic technique with feedback to both the slow channel (piezoelectric transducer channel with typical bandwidth of ~2?kHz in the grating extended cavity) and the fast channel (current modulation channel with typical bandwidth of ~200?kHz in the current driver).

Precision measurement of ultralow losses of an asymmetric optical microcavity

The losses of the transmission, absorption, and scattering of optical mirrors govern the extraction efficiency of a nonclassical state that is generated inside a cavity. By measuring the reflectivities and transmittances and the matching factors from both sides of a super-mirror-made microcavity at various mode-matching efficiencies, the transmission losses and the unwanted losses, including the absorption and scatter losses, of the left and right cavity mirrors were both determined at the parts-per-million level.