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Experiment results. Curve (a) is the associated shot noise power spectrum, (b) is the beam intensity difference noise power spectrum and (c) the electrical noise level. 

Experiment results. Curve (a) is the associated shot noise power spectrum, (b) is the beam intensity difference noise power spectrum and (c) the electrical noise level. 

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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 p...

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... the detection system, ξ T /(T δ) is the OPO output coupling efficiency, T is the transmission coefficient of the cavity output coupling mirror and δ is extra-cavity loss. The experimental set-up is shown in figure 1. The OPO operates above threshold. The pump laser is a frequency- doubled and stabilized Nd:YAP laser, its frequency is stabilized on an external Fabry-Pérot cavity. The long- term frequency stability is about 2 MHz. The total output is 800 mW with 4% intensity stability. A half-wave plate and a polarizing beam splitter are used to adjust the power in front of the OPO. A Faraday rotator (FR) and a polarizing beam splitter form an optical isolator. L 11 and L 22 are mode-matching lenses of 400 and 150 mm focal length respectively. In order to reduce the transmission losses and the reflection disturbance, all surfaces encountered by the green beam are antireflection coated. The OPO cavity consists of a 10 mm long α -cut semimonolithic KTP crystal and a concave mirror of 20 mm curvature. The front face of the KTP crystal is coated for use as the input coupler with a transmission of 15% at 0 . 54 μ m and high reflectivity at 1 . 08 μ m. The output coupler is highly reflective for 0 . 54 μ m wavelengths and with a certain transmission for 1 . 08 μ m. The length of the OPO cavity is 19 mm. When the output transmissions are 1.5%, 3% and 5%, the measured cavity finesses for 1 . 08 μ m are 170, 140 and 110 respectively. The total extra losses (surface scattering, crystal absorption and residual reflection, etc) are estimated to be 1%, 0.7% and 0.3% and output coupling efficiencies are 0.6, 0.81 and 0.94. The finesses for 0 . 54 μ m are 43, 44 and 47 respectively. The b -axis of the crystal KTP is parallel to the horizontal (less than 0 . 5 ◦ ), so that the output light is polarized in horizontal and vertical directions. In our experiment, we ensure the pump mode is e 2 by rotating the half-wave plate, i.e. the pump polarization is parallel to the b -axis of the crystal. The KTP crystal is cut according to the need for frequency doubling at 1 . 08 μ m with type II 90 ◦ non-critical phase matching, so that the downconverted twin infrared beams are cross polarized with near degeneracy at 1 . 08 μ m. The characteristics of the detection system have been carefully checked. The imperfection of the polarizing beam splitter used in our experiment was less than 1%. The length of the OPO can be tuned by a PZT attached to the output coupler. The temperature of the crystal is actively controlled at the phase-matching temperature. A twin-beam is produced when the OPO is operating above the threshold. Compared with previously used Nd:YAG systems, our Nd:YAP system has a much lower pump threshold for parametric downconversion. The thresholds for output transmissions of 1.5%, 3% and 5% are only 20, 50 and 80 mW. When the pump powers are 30, 60 and 90 mW, the output powers of subharmonic fields are 6, 16 and 20 mW. Slight leaks of pump light from the OPO are blocked by a filter. The outgoing downconverted beams are separated by a polarizing beam splitter and monitored by photodiodes D1 and D2 (ETX500T). The outputs of the photodiodes are amplified and subtracted; the noise of the intensity difference is recorded by a spectrum analyser. To ensure balance of the detection system, the photodiodes were carefully chosen and electronic compensation is included. The total quantum efficiency of the detection system is 88%. A half-wave plate is insert in the twin-beam before the polarizing beam splitter. As shown by Heidmann et al [4], when the polarization of the two beams is rotated by an angle of 45 ◦ , the noise measured in the intensity difference is the shot noise limit, and when the two beams are rotated by an angle of 0 ◦ , the noise measured in the intensity difference is the intensity difference spectrum between the twin-beams. Figure 2 shows the noise spectrum of a twin-beam generated by the OPO with 5% transmission of the output mirror. The frequency range is from 1 to 11 MHz. Curve (a) is the shot noise limit, curve (b) is the noise spectrum of the twin-beam intensity difference and curve (c) is the electronic noise level. The maximum quantum noise reduction is 80% (7 dB) near 1.5 MHz which corresponds to 90% of actual squeezing. Figure 3 shows the comparison between experimental and theoretical results; and full curve represents the theoretical results obtained from equation (1) with parameters ξ = 0 . 94 and η = 0 . 88. The triangles are the experimental results which fit the theoretical curve well. When the transmissions of the output mirror are 1.5%, 3% and 5%, we observe quantum noise reductions of 50% (3 dB), 68% (5 dB) and 80% (7 dB) respectively. Figure 4 shows the relation between the quantum noise reduction and the pump power, where the full lines are the theoretical data and the symbols are the measured results which are slightly lower than those given by the theory. This is probably because the detection system efficiency η used in equation (1) is slightly higher than that in the actual system. The squeezing is not sensitive to pump power and increases with output coupling efficiency. We observed a quantum noise reduction of up to 80% in the intensity difference between twin-beams generated by a non-degenerate type II 90 ◦ non-critical phase-matching optical parametric oscillator pumped by 0 . 54 μ m green light operating above threshold. We verify that the quantum noise reduction is insensitive to the pump power but varies with the output coupling efficiency. The experimental results fit the theory quite well. Compared with an OPO pumped by a Nd:YAG laser, our system has several advantages: higher conversion efficiency, lower pump threshold, and relatively simple configuration. If the Nd:YAP crystal were pumped by laser diode (LD), an all-solid mini ‘squeezer’ may be designed. This non- classical ‘laserlike’ system with a certain intensity may be extensively used in many fields, such as high-sensitivity spectroscopy, the measurement of amplitude modulation and optical ...

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Citations

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