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Acta Sinica Quantum Optica

Experimental Investigation of Intensity Difference Squeezing at 1.3 μm-Quantum Communication Window

郑耀辉 Zheng Yaohui

Published Jan 1, 2010• 0 citations
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Abstract

The stable red laser,which is generated by the laser at 1.3 μm wavelength from an all-solid-state single frequency infrared laser pumping a frequency-doubler,is used to pump a continuous wave nondegenerate optical parametric oscillator(cw-NOPO) consisting of LBO crystal with type-Ⅱ non-critical phase matching.The intensity-difference squeezing of 3.9 dB at a telecommunication wavelength has been generated from the cw-NOPO operating above the oscillation threshold through the frequency down-conversion process.

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The stable red laser,which is generated by the laser at 1.3 μm wavelength from an all-solid-state single frequency infrared laser pumping a frequency-doubler,is used to pump a continuous wave nondegenerate optical parametric oscillator(cw-NOPO) consisting of LBO crystal with type-Ⅱ non-critical phase matching.The intensity-difference squeezing of 3.9 dB at a telecommunication wavelength has been generated from the cw-NOPO operating above the oscillation threshold through the frequency down-conversion process.
The intensity difference squeezed state, which means that the fluctuation of the intensity difference between signal and idler beams is less than that of the corresponding shot noise level (SNL), plays an important role in high sensitivity measurement, quantum imaging, and quantum random numbers generation. When an optical parametric oscillator consisting of a type-II phase-matching periodically poled KTiOPO4 crystal operates above the threshold, an intensity difference squeezed state at a telecommunication wavelength can be obtained. The squeezing of 7.7±0.5 dB below the SNL is achieved in an analysis frequency region of 2.4–5.0 MHz.
Continuous variable (CV) audio-band frequency squeezed states at the fiber telecommunication wavelength is an important quantum resource for the practical applications based on optical fiber. As is well known, the optical power attenuation and phase diffusion effect of light at 1.3 μm in standard telecommunication fibres are low and small, respectively. The audio-band frequency squeezed light at 1.34 μm can be utilized to realize quantum precision measurement, such as quantum-enhanced sensing in the low-frequency range, laser interferometer for gravitational wave detection. In this paper, CV audio-band frequency vacuum squeezed states at 1.3 μm are experimentally generated by using a type-I degenerate optical parametric oscillator (DOPO) below the threshold. A home-made continuous-wave single-frequency dual-wavelength (671 nm and 1.34 μm) Nd:YVO4/LBO laser is used as a pump source for DOPO based on a type-I quasi-phase-matched periodically poled KTiOPO4 (PPKTP) crystal. Mode cleaners with a finesse of 400 and linewidth of 0.75 MHz are used to filter the noise of lasers at 671 nm and 1.34 μm, respectively. The intensity noises of the two lasers reach a shot noise level for analysis frequencies higher than 1.0 MHz and their phase noises reach shot noise level for analysis frequencies higher than 1.3 MHz, respectively. The low noise single-frequency 671 nm laser is utilized as a pump of the DOPO. The threshold power of the DOPO is 450 mW. In order to detect the audio-band frequency vacuum squeezed states, the power of local oscillator of a homodyne detector system is optimized to 60 μupW. Furthermore, the effect of common mode rejection ratio (CMRR) of detectors is discussed in detecting the audio-band frequency vacuum squeezed states. Improvement of CMRR of detectors is a good way to detect the audio-band frequency vacuum squeezed states effectively. When the phase matching temperature of PPKTP crystal is controlled at 53℃ by using a home-made temperature controller and the pump power is 95 mW, the vacuum squeezed states are generated at analysis frequency ranging from 8-100 kHz. A maximum measured squeeze of 5.0 dB is obtained at analysis frequency of 36 kHz. A 3.0 dB squeezed light is obtained at an audio-band frequency of 8 kHz.
The configuration of a semimonolithic continuous nondegenerate optical parameter oscillator consisting of an α-cut KTP crystal pumped by an intracavity frequency-doubled and frequency-stabilized Nd∶YAP laser is designed. Both two-mode quadrature-phase squeezing of 3.7dB and intensity-difference squeezing of 7dB have been generated from the cw-NOPO operating respectively below and above the oscillation threshold. The system can operate stably to generate the squeezed light for over half an hour.
A 106μm all-solid-state single frequency Nd∶YVO4 laser was explored to pump a triply resonant optical parametrie oscillator(OPO) based on the periodically poled lithium niobate. When the OPO operated near degeneracy, the amplitude fluctuation correlation of the signal and idler modes was observed as their wavelengths were separated about 200nm. The measured squeezing is 04dB. The amplitude fluctuation correlation existed in a range of 17nm as we tuned the wavelengths of the signal and idler by change the crystal temperature.
A continuous-wave squeezed vacuum at a telecommunication wavelength was reported. Using an Er-doped fiber amplifier seeded by a single-frequency laser diode as the pump laser source, via a frequency doubler and an optical parametric oscillator based on periodically poled LiNbO3, the squeezed vacuum of 2.4±0.1dB below the shot noise limit at 1560nm was observed (the inferred initial squeezing is about 6.4±0.5dB). This system is compatible with standard telecommunication optical fiber, and will be useful for long-distance quantum communication and distributed quantum computing.
A strong quantum correlation between twin-beams at wavelength was observed. The intensity difference noise of twin-beams, which is generated by a semimonolithic OPO cavity pumped by a frequency-doubled and stabilized Nd:YAP laser, is about 80% (7 dB) below the short noise limit at the measurement frequency of 1.5 MHz. The threshold of the optical parametric oscillator is about 20 mW. The parametric dependence of the quantum correlation is experimentally investigated. The experimental results agree with the theory well.