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An optomechanical sensor suitable for the study of quantum effects has been developed and characterized. The sensor reads out the vibrations of a microfabricated miniature silicon mechanical oscillator which forms one end mirror of a high finesse Fabry-Pérot cavity. The mechanical quality factor is up to Qϭ300 000 at 300 K and rises up to Qϭ4ϫ10 6 at 4 K. The thermal noise of the oscillator has been measured in the time and frequency domains at room temperature and at 4.5 K. The prospects for observing the standard quantum limit are discussed.

We report the reconstruction of the quantum state of squeezed vacuum generated by a continuouswave optical parametric amplifier. Homodyne detection and tomographic reconstruction methods were used to obtain the density matrix in the Fock (number state) representation. The photon number distribution exhibits odd-even oscillations, a manifestation of the photon pair production in the secondorder nonlinear medium.

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We demonstrate the simultaneous quantum state reconstruction of the spectral modes of the light field emitted by a continuous wave degenerate optical parametric amplifier. The scheme is based on broadband measurement of the quantum fluctuations of the electric field quadratures and subsequent Fourier decomposition into spectral intervals. Applying the standard reconstruction algorithms to each bandwidth-limited quantum trajectory, a "spectrum" of density matrices and Wigner functions is obtained. The recorded states show a smooth transition from the squeezed vacuum to a vacuum state. In the time domain we evaluated the first order correlation function of the squeezed output field, showing good agreement with theory.The experimental techniques of quantum state reconstruction, first applied five years ago, have opened a new field of research, wherein simple quantum mechanical systems can be characterized completely by density matrices and Wigner functions [1]. Our system consists of electromagnetic field modes at optical frequencies. We have previously generated the whole family of squeezed states of light using an optical parametric amplifier (OPA) and reconstructed these states using the method of optical homodyne tomography [2]. The reconstructions presented therein were limited to essentially one particular pair of modes at frequencies ω ± Ω, where Ω is a radio frequency and ω the optical frequency. The spectral bandwidth of this mode pair was ∆Ω/2π = 100 kHz. Since an OPA pumped below threshold emits a frequency spectrum, with a bandwidth determined by the cavity linewidth in the order of several MHz, the output of the OPA is described more precisely by a whole spectrum of quantum states [3]. General schemes for multimode reconstruction become quite complicated already at the two-mode level [4]. So far only one experiment demonstrating photon-