Atoms trapped in magic-wavelength optical tweezer will have the same light shift for the desired ground and excited states. In this case the position-dependence differential light shift of the desired transition can be eliminated. For cesium 6S_1/2 (Fg = 4, mF = +4) - 6P_3/2 (Fe = 5, mF = +5) cycling transition (852 nm), the magic wavelength was calculated theoretically for a linearly-polarized optical tweezer, and also it was verified experimentally to be 937.6 nm. We have demonstrated 852-nm triggered single-photon sources based on single cesium atom trapped in linearly-polarized optical tweezers implemented by using of 1064-nm and 937.6-nm lasers, respectively. The photon statistics were characterized by using of the Hanbury Brown - Twiss (HBT) scheme based on Si single-photon detectors (SSPDs). Strong anti-bunching effect [g^2(t=0) = 0.09] was recognized, and it shown perfectly the single-photon characters. The Hong-Ou-Mandel (HOM) two-photon interference measurements based on SSPDs were employed to evaluate the photon indistinguishability. Our preliminary experimental results indicated that the indistinguishability of single photons has been improved ~ 20% for the case of magic-wavelength optical tweezer. References: [1] Phys. Rev. A 94 (2016) 013409; [2] Appl. Phys. Express 9 (2016) 072702; [3] Opt. Express 25 (2017) p.15861
Nowadays the long-range dipole-dipole interaction between highly-excited Rydberg atoms at micrometer distance become promising way to establish the atom-atom quantum entanglement and to implement the two-qubit logic gate. For alkali metal atoms, single-photon excitation from the ground state to the desired Rydberg state demands powerful narrow-linewidth ultra-violet (UV) laser, which is very challenging. Maybe just because of this, the studies of single-photon Rydberg excitation of alkali metal atoms are rare. Alternatively people have employed the two-photon or three-photon Rydberg excitation scheme. However, comparing with the single-photon Rydberg excitation, the two-photon or three-photon scheme has following drawbacks: atomic decoherence due to the photon scattering from the lower and upper transitions, and light shift of the involved ground state and Rydberg state due to the lower and upper transition laser beams. Thanks to the efficient laser frequency conversion technology with PPXX material and the well-developed commercial fiber laser as well as fiber amplifier, we have implemented a tunable 318.6-nm UV laser system based on the cavity-enhanced second-harmonic generation following the single-pass sum-frequency generation of 1560.5-nm and 1076.9-nm fiber-amplified lasers. More than 2-Watt output of the 318.6-nm UV laser has been achieved with a typical linewidth of smaller than 10 kHz. Employing the UV laser system we have demonstrated a single-photon Rydberg excitation spectroscopy of cesium (Cs) atoms. Partial Cs atoms can be directly excited from 6S_1/2 ground state to nP_3/2 (n = 70 - 100) Rydberg states, and Rydberg excitation spectra are obtained with transmission enhancement of a probe beam locked to Cs 6S_1/2 (F = 4) - 6P_3/2 (F’ = 5) cycling transition because partial population on the ground state (F = 4) are transferred to Rydberg state. The quantum defect for Cs nP_3/2 (n = 70 -100) Rydberg states is determined experimentally. Further more, the demodulated single-photon Rydberg excitation spectrum is employed to stabilize the UV laser to specific Cs Rydberg transition to improve the laser frequency stability. References: [1] Opt. Express 25 (2017) p.22510; [2] J. Opt. 19 (2017) 045501; [3] J. Opt. Soc. Am. B 33 (2016) p.2020; [4] Opt. Commun. 370 (2016) p.150. Funding: the National Natural Science Foundation of China (61475091).
We report on experimental preparation of the second-harmonic-wave laser and the single-mode squeezed vacuum state of 795 nm (rubidium atom D1 line) with periodically-poled KTiOPO4 (PPKTP) bulk crystals. By using a four-mirror bow-tie type ring doubling cavity we achieved ~111 mW of continuous-wave single-frequency ultra-violet (UV) laser radiation at 397.5 nm with ~191 mW of 795 nm fundamental-wave laser input. The corresponding doubling efficiency is 58.1%. To our knowledge, this is the highest doubling efficiency at 795 nm so far. Employing the 397.5 nm UV laser as a pump source of an optical parametric oscillator (OPO) with a PPKTP crystal, we achieved 5.6 dB of 795 nm single-mode squeezed vacuum output at analyzing frequency of 2 MHz. To our knowledge, this is the highest squeezing level of 795 nm single-mode squeezed vacuum so far. We analyzed the pump power dependence of the squeezing level, and concluded that UV laser induced losses of PPKTP crystal are main limiting factors for further improving the squeezing level. The generated 795 nm vacuum squeezing has huge potential applications in quantum memory and ultra-precision measurement with rubidium atoms.
Based on feedback control techniques and our realization of nearly complete transferring cold cesium (Cs) atoms from a
magneto-optical trap (MOT) to a far-off-resonance microscopic optical tweezer, we investigated the possibility for nearly
deterministic loading of a single Cs atom in a MOT and in a microscopic optical tweezer. We combined feedback
controls on the gradient of the MOT quadrupole magnetic field (QMF) and on blue-detuned light-assisted collisions
(LAC) of confined cold atoms in the tweezer. Using active feedback on QMF of the MOT, we have achieved ~ 98% of
probability of single atom loading in a MOT. In a microscopic optical tweezer, by combining the feedback controls on
the QMF and the LAC, we finally achieved ~ 95.2% of probability of single atom loading in the tweezer. This two-path
feedback control scheme may be extended to load a small-size 2D tweezer array with exact single atom trapped in each
site simultaneously. This is very important and promising to implement an addressable multiple-qubit system for
demonstrating quantum register and quantum processor.
We implemented and compared three different quasi-phase-matching (QPM) frequency-doubling configurations for 1560nm laser of single pass, double pass and cascade by using of MgO:PPLN bulk crystals. Also a fiber-pigtailed MgO:PPLN waveguide is utilized in single-pass frequency doubling configuration in the case of low-power 1560nm fundamental wave (FW) laser. Employing the second-harmonic wave (SHW) output at 780nm and a rubidium (Rb) vapor cell, we also performed the modulation transfer spectroscopy (MTS). MTS is insensitive to the fluctuation of laser intensity and the temperature drift of atomic vapor cell, so it is a good choice for laser frequency stabilization against atomic hyperfine transition line. The laser frequency stability is significantly improved after being locked via MTS scheme compared with the free-running case.
The narrow electromagnetically-induced transparency (EIT) resonance peaks are observed with two low-power counter-propagating diode lasers in cesium (Cs) 6S1/2 - 6P1/2 - 8S1/2 ladder-type atomic system. To precisely determine the centers of resonance peaks, multiple background-free EIT signals are achieved using a novel scanning scheme in which the coupling laser driving Cs 6P1/2 - 8S1/2 transition is scanned and the probe laser driving Cs 6S1/2 – 6P1/2 is frequency locked. A temperature-stabilized fiber-pigtailed waveguide-type phase electro-optical modulator (EOM) and a stable confocal Fabry-Perot cavity are used as a precise frequency marker to measure the hyperfine splitting of Cs 8S1/2 state. The impact of the external magnetic field on the measurement is also investigated. Furthermore, the hyperfine structure constant (here it is the hyperfine magnetic dipole constant, A) of Cs 8S1/2 state is determined to be A = 219.06 MHz ± 0.12 MHz based on the measured hyperfine splitting (Δhfs = 876.24 MHz ± 0.50 MHz).
We report experimental investigations of two kinds of high-contrast optical filters by utilizing a laser-pumped atomic
vapor and properly-designed Fabry-Perot bulk etalon, which are based on the demand of the detection of quantum
correlated Stokes and anti-Stokes photon pairs in a Λ-type three-level atomic ensemble, respectively. Laser-pumped
cesium (Cs) atomic filter achieves typical peak transmission ~ 74.3% and the distinction ratio between excitation channel
and 9.19GHz- frequency-detuned signal channel is ~ 26.7 dB at 47.15°C. The transmission peak can be tuned within the
range of Doppler line-width of Cs D2 line. The temperature-stabilized narrow-band etalon filter with dozens of GHz
resonant transmission tunability is realized with typical peak transmission of ~ 91.7% and the distinction ratio between
pump and signal channel of ~27.5 dB. These techniques are useful for atom-photon interaction experiments, especially
for Stokes and anti-Stokes photon pairs generation experiments based on collective Raman excitation of Cs atomic ensemble.
Based on a Λ-type three-level system consists of cesium (Cs) 6S1/2 (Fg = 3) and 6S1/2 (Fg = 4) long-lived ground states
and 6P3/2 (Fe = 4) excited state, we have experimentally measured and theoretically analyzed the characters of coherent
population trapping (CPT) resonances in Cs vapor cells with different partial pressure of neon (Ne) as buffer gas around
room temperature. The impact of some experimental parameters, such as the relative intensity ratio between two
phase-locked laser beams, the laser intensity, with or without buffer gas and the partial pressure of Ne, the temperature
and the longitudinal magnetic field, on the CPT resonance was studied in details. With the optimized parameters, we got
typical CPT signal with the full-width half-maximum (FWHM) linewidth as narrow as ~ 181 Hz in a Cs vapor cell filled
with 30 Torr of Ne as buffer gas.
This work deals with the cooling and trapping of single cesium (Cs) atoms in a large-magnetic-gradient magneto-optical
trap (MOT) and the confinement of single Cs atoms in a far-off-resonance optical dipole trap (FORT). The experiment
setup is based on two large-numerical-aperture lens assemblies which allow us to strongly focus a 1064-nm TEM00-mode
Gaussian laser beam to a 1/e2 radius of ~ 2.3 μm to form a microscopic FORT for isolating single atom with environment
and to efficiently collect the laser-induced-fluorescence photons emitted by single atoms for detecting and recognizing
single atom’s internal state. We have tried both of “bottom-up” and “top-down” loading schemes to confine single atoms
in the microscopic FORT. In the “bottom-up” scheme, we have successfully prepared single Cs atoms in the MOT and
transferred it into FORT with a probability of almost 100%. In the “top-down” scheme, we have achieved ~ 74% of
single atom loading probability in the FORT using light-assisted collisions induced by blue detuning laser and with
prepared many Cs atoms in the MOT. The relaxation time in hyperfine level of ground state of trapped single Cs atom is
measured to be ~5.4 s. To coherently manipulate atomic quantum bits (qubit) encoded in the clock states (mF = 0 states in
Fg = 3 and 4 hyperfine levels) of single Cs atom via the two-photon simulated Raman adiabatic passage (STIRAP), we
have prepared two phase-locked laser beams with a frequency difference of ~ 9.192 GHz by optically injecting an
852-nm master laser to lock the +1-order sideband of a 9-GHz current-modulated slave diode laser. The two
phase-locked laser beams are used to drive STIRAP process in the Λ-type three-level system consists of Cs |6S1/2 Fg = 4,
mF = 0> and |6S1/2 Fg = 3, mF = 0< long-lived clock states and Cs |6S1/2 Fe = 4, mF = +1 > excited state with the
single-photon detuning of ~ -20 GHz. Rabi flopping experiments are in progress.
Optical dipole traps (ODT) with far-off-resonance laser are important tools in more and more present cold-atom
experiments, which allow confinement of laser-cooled atoms with a long storage time. Particularly, the magic
wavelength ODT can cancel the position-dependent spatially inhomogeneous light shift of desired atomic transition,
which is introduced by the ODT laser beam. Now it plays an important role in the state-insensitive quantum engineering
and the atomic optical clock. To verify the magic wavelength or the magic wavelength combination for D2 line transition
of cesium (Cs) and rubidium (Rb) atoms, we calculated and analyzed the light shift of the 133Cs 6S1/2 - 6P3/2 transition for
a monochromatic ODT, and also the 87Rb 5S1/2 - 5P3/2 transition for a dichromatic ODT with a laser frequency ratio of
2:1. Also a dichromatic magic-wavelength ODT laser system for 87Rb atoms is proposed and experimentally realized by
employing the quasi-phase-matched frequency doubling technique with telecom laser and fiber amplifier.
Single cesium atom prepared in a large-magnetic-gradient magneto-optical trap (MOT) has been efficiently loaded into a
microscopic far-off-resonance optical trap (FORT, or optical tweezer), and the atom can be transferred back and forth
between two traps with high efficiency. The intensity noise spectra of tweezer laser are measured and the heating
mechanisms in optical tweezer are analyzed. To prolong the lifetime of single atom trapped in optical tweezer, laser
cooling technique is utilized to decrease atom's kinetic energy, and the effective temperature of single atom in tweezer is
estimated by the release-and-recapture (R&R) method. Thanks to laser cooling, typical lifetime of ~ 130.6 ± 1.8 s for
single atom in tweezer is obtained. These works provides a good starting point for coherent manipulation of single atom.
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