| Frequency conversion technology,especially based on the second order nonlinear optical effect,has important significance for expanding the laser frequency range.Based on this,a wide range of laser frequency can be obtained,covering ultraviolet to infrared band.With the further development of frequency conversion technology,quantum frequency conversion was proposed in 1990(the signal photon can be converted into the target photon of a different frequency while preserving the quantum state).Connecting quantum nodes with fiber telecom bands to build quantum networks.In this paper,the freuency conversions between 780 nm of Rubidium atom D2 line and 852 nm of cesium atom D2 line and 1560 nm of fiber telecom C-band are realized by using periodically-poled quasi-matching block crystal and waveguide.Especially,the freuency conversions are also realized with weak light and even single-photon level.The experiments are mainly based on the maturity of fiber amplifiers and periodically-poled quasi-matching block crystals and waveguides.At present,the optical fiber communication market develops rapidly,and the performances of devices in fiber telecom C-band(1530-1565 nm)are excellent.First,we study the experimental method to yield 780 nm laser(5S1/2–5P3/2 line(D2 line)of rubidium atom),and apply it to yield 852 nm laser(6S1/2–6P3/2 line(D2 line)of cesium atom).The watt-level 780 nm laser can be obtained by second harmonic generation(SHG)with 1560 nm laser.The 852 nm laser can be achieved by sum frequency generation(SFG)with1560 nm and 1878 nm lasers.Combining the two schemes,we can produce single-frequency 780 nm and 852 nm lasers with good properties of narrow bandwidth.So we can conveniently perform laser cooling and trapping,as well as manipulating both Rb and Cs atoms simultaneously,it is useful in the Rb-Cs biatomic interferometer and cold Rb Cs dimer experiments.Quantum state transmission is the key to realize quantum network.The photonic frequency of quantum nodes(such as alkali metal systems)are usually much higher than that of flying photonic qubits transmitted in the optical fiber networks.Therefore,it needs quantum frequency conversion(QFC)of the photonic frequency between the quantum nodes and the flying photonic qubits.Similarly,flying photonic qubits are connected with quantum nodes via quantum frequency up conversion.Single-photon-level frequency conversion is an indispensable step to realize quantum frequency conversion.In our experiments,we realize two-way single-photon-level frequency conversion between 1560 nm and 852 nm respectively.The effects of different pump light on the noise photons were analyzed,and the signal-to-noise ratio(SNR)is effectively improved by selecting narrow band filters and changing the polarization of the noise photons.Based on the study of single-photon-level frequency conversion between cesium atom D2 line and telecom C-band,we can realize the connection between cesium atom nodes through low-loss fiber.In addition,the schemes can be extended to other types of quantum nodes,providing the foundation for the realization of hybrid quantum network in the future.Finally,the quantum frequency conversion from 852 nm to 1560 nm is realized based on the cesium atoms magneto-optical trap by using PPLN waveguide.In addition,we analysize the problems of noise and signal-to-noise ratio(SNR).The characteristic works in this paper are described below:1).The narrow bandwidth tunable single-frequency high beam quality780 nm laser is achieved by single-pass configuration with 1560 nm laser system and period-polarized magnesium-doped lithium niobate crystal(PPMg O:LN).When the power of 1560 nm laser is 14.2 W,the output 780 nm laser is 2.4 W,the conversion efficiency is 17.2%,and the continuous tunable range is greater than 10 GHz.In order to improve the conversion efficiency,cascading two crystals and changing the laser linewidth of the seed were used.When the fundamental laser power is 13.2W,3.5-W 780 nm laser can be obtained by using the configuration of two cascade crystals,and the maximum efficiency is 26.8%.When three lasers with linewidth of~1.2 MHz(DFB),~200 k Hz(ECDL)and ~600 Hz(DFB-EDFL)are used as seed sources,the conversion efficiency are 11.0%,14.3% and 16.6%,respectively;2).Based on the double frequency,the nonlinear frequency chain is used to yield 520 nm green laser combining single-pass sum-frequency configuration.And comparing the output power and efficiency by choosing different nonlinear crystals(periodically polarized potassium titanium oxide phosphate(PPKTP)crystal,periodically polarized lithium magnesium oxide tanate(PPMg O:s LT)crystal).When the power of 1560 nm laser is 11.6 W,545-m W520 nm laser can be generated by using PPKTP crystal;350-m W520 nm can be generated by using PPMg O:s LT crystal.And the continuous tuning range of 520 nm green light is at least 9 GHz.Based on the PPKTP crystal,the two-color optical fields(signal laser at 1560 nm,idler laser at 780 nm)are yielded by a doubly resonant optical parameter oscillator.The system can be applied to the remote quantum information transmission with rubidium atom quantum memory;3).The narrow bandwidth tunable single-frequency high beam quality852 nm laser is achieved by single-pass sum-frequency configuration with the 1560 nm laser system and the 1878 nm laser system and the PPMg O: LN crystal.The efficiency is improved by achromatic method.The maximum power of 852 nm is 276 m W,and the continuous tuning range is at least 9.2 GHz.Then the 1560 nm laser is chopped into single-photon level to realize single-photon-level frequency conversion and analyzed the problems of noise and SNR.When the power of 1878 nm pumped laser is 150 m W,the SNR is 38.8 and the conversion efficiency is1.9%;4).The inverse down conversion process 852 nm laser to 1560 nm laser is realized.The high beam quality 1560 nm laser is achieved by single-pass difference-frequency configuration.In addition,the conversion efficiency is improved by achromatic method.When injected with 450-m W 1878 nm pump light and 10-m W 852 nm signal light,136-?W 1560 nm laser is obtained.Then,the 852 nm laser is chopped into single-photon level to realize single-photon-level frequency conversion.The effects of filters on the SNR are compared.In the case of using a bandpass filter with a bandwidth of 12 nm and a fiber Bragg grating with a bandwidth of0.3 nm,a maximum SNR of 31.3 and a conversion efficiency of1.7% are achieved.Finally,the SNR is further improved by changing polarization of the noise photons.Under the same conditions,the SNR is increased from 31.3 to 58.3;5).Weak 852 nm laser converted to 1560 nm laser based on periodic polarized Lithium niobate(PPLN)waveguide is carried out.By using a bandpass filter with a bandwidth of 12 nm and a fiber Bragg grating with a bandwidth of 0.3 nm,the SNR is 91.3,and the maximum internal conversion efficiency is 6.2%.The experimental equipment of continuous light excitation single-photon source is optimized,the probability of single atom is increased to ~ 80% by magnetic field triggering device in magneto-optical trap.In this state,Hanbury Brown-Twiss(HBT)experiment is carried out,and the normalization of second order coherence at zero delay is 0.11.The magneto-optical trap system connected with the frequency conversion device can realize quantum frequency conversion. |
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