Experimental Realization of Squeezing

A typical layout of a squeezing experiment based on a Mach-Zehnder interferometer (Sect. 4.2.3) is shown in Fig. 14.64. The output of a well-stabilized laser is split into two beams, a pump beam bi and a reference beam b2. The pump beam with the frequency co] generates by nonlinear interaction with a medium (e.g., four-wave mixing or parametric interaction) new waves at frequencies o l /. After superposition with the reference beam, which acts as a local oscillator, the resulting beat spectrum is detected by the photodetectors D1 and D2 as a function of the phase difference A0, which can be controlled by a wedge in one of the interferometer arms. The difference between the two detector output signals is monitored as a function of the phase difference A0. Contrary to the situation in Fig. 14.62, the spectral noise power density p(/, 0) (= Pnep per frequency interval d/ = 1 s ) shows a periodic variation with 0. This is due to the nonlinear interaction of one of the beams with the nonlinear medium, which preserves phase relations. At certain values of 0 the noise power density Pn(/, 0) drops below the photon noise limit 

The first successful experimental realization of squeezing was reported by Slusher and coworkers [14.163], who used the four-wave mixing in a Na-atomic beam as nonlinear process (Fig. 14.65). The Na atoms are pumped by a dye laser at the frequency col = coo h S, which is slightly detuned from the resonance frequency cuq. In order to increase the pump power, the Na beam is placed inside an optical resonator tuned to the pump frequency col Because of parametric processes (Sect. 5.8.8) of the two pump waves (col, A l) inside the resonator, new waves ai col S are generated at this four-wave mixing process (signal and idler wave). Conservation of energy and momentum demands 

A second resonator with a properly chosen length, such that the mode spacing is Av = 8, enhances the signal as well as the idler wave. 

Another example, where a beam of squeezed light was realized with 3.2 mW and 52% noise reduction, is the second-harmonic generation with a cw NdiYAG laser in a monolithic resonator [14.165]. The experimental setup is shown in Fig. 14.67a. 

The output of the cw Nd YAG laser is frequency doubled in a MgOiLiNbOs nonlinear crystal. The endfaces of the crystal form the resonator mirrors and by a special modulation technique the resonator is kept in resonance both for the fundamental and for the second harmonic wave. A balanced homodyne interferometric detection of the fundamental wave yields the intensity noise of the light beam (/+ detected by Dl) and the shot-noise level (/ detected by D2) as a reference (Fig. 14.67b). 

The essential point is that there are definite phase relations between pump, signal and idler waves, and this fact establishes a correlation between amplitude and phase of the signal wave. This correlation is illustrated in Fig. 9.100 The four-wave mixing generates new sidebands. If the input contains the frequencies and u)l + 5, the output has the additional sideband co — 8. Thus, the amplitude of one sideband is increased at the cost of the other sideband until both sidebands have equal amplitudes. This maximizes the amplitude modulation of the output. Since the phases 

The best squeezing results with a noise suppression of 60 % ( -4 dB) below the quantum noise limit po were obtained by Kimbel and coworkers [1336] with an optical parametric oscillator, where the parametric interaction in a MgOiLiNbOs crystal was used for squeezing. 

The first successful experimental realization of squeezing was reported by Slusher and co-workers [14.133] who used the four-wave mixing in a Na-atomic beam as nonlinear process (Fig. 14.58). The Na atoms are pumped by a dye laser at the frequency = Wq+5 slightly detuned from the resonance 

The essential point is that there are definite phase relations between pump, signal and idler waves, and this fact establishes a correlation between amplitude and phase of the signal wave. As has been shown in detail 

---