Observation of photon antibunching in pulsed squeezed light
T. Hirano, M. Koashi, K. Kono, and M. Matsuoka
Institute for Solid State Physics, University of Tokyo
Roppongi 7-22-1, Minatoku, Tokyo 106, Japan
The phenomenon of photon antibunching is an important manifestation of nonclassical nature of light. We have performed an experiment to show the nonclassical nature of the squeezed light by measuring the second order correlation function (intensity correlation). The squeezed coherent light is generated in a parametric amplifier. When two photons are simultaneously absorbed in parametric process, intensity correlation for =0 decreases (antibunching). On the other hand when two photons are simultaneously emitted it increases (bunching). The direction of the energy flow between the input field at frequency and the pump filed at frequency depends on the relative phase of the two fields. So both antibunching and bunching can be realized by changing the relative phase.
Figure 1 shows our experimental apparatus used to observe antibunching. We used pulsed light in order to obtain a large parametric gain even from a single-pass parametric amplifier. A cw mode-locked Nd:YAG laser generates IR pulses with FWHM100psec at a repetition rate of 82MHz. Second harmonic light () is generated in crystal and is used to pump a single-pass parametric amplifier (PA, crystal). The residual IR pulses are attenuated by harmonic mirrors and used as an input signal () for PA. A collinear configuration is chosen to stabilize the relative phase between the signal () and the pump (). Precise adjustment of intensity of the signal and the pump and their relative phase () is essential to observe antibunching. Two harmonic wave plates (HWP), a polarizer (P) and a birefringent filter (BRF) are used for this adjustment. The squeezed vacuum (parametric fluorescence) component, which is strongly bunched, also exists at the frequency region where we do not have input signal components. In order to remove such fluorescence an etalon and an interference filter are used to restrict the observed frequency bandwidth. Photon counting is performed by two silicon avalanche photodiodes (APD). A time-to-amplitude converter (TAC) is used to measure the intensity correlation.
The observed intensity correlation is shown in Fig.2 as a function of the time-delay. They show peaks at 12nsec intervals corresponding to the repetition rate of the mode-locked laser. The total number of counts in a single peak is shown above each peak. In Fig.2 experimental results obtained under the same condition except for the relative phase are shown. The value of normalized intensity correlation, , changes below and above unity when the relative phase is varied: and , respectively. The Poisson levels used for the normalization were determined by the numbers of start and stop pulses. Fig.2 unmistakably indicates that the squeezed coherent state exhibits the phenomenon of antibunching, which is understandable only in terms of quantized electromagnetic field. The dependence of on the input-photon number and the degree of squeezing will be discussed.