We have performed cavity ring-down spectroscopy by locking a high-finesse resonator to the probe laser. We have obtained combination overtone spectra of water vapor in the ambient environment with a baseline noise of 5ϫ10 Ϫ9 cm Ϫ1 for decay constants ͑Rϭ99.93% reflectors͒ of 1 s. This cavity-locked approach ensures single transverse mode excitation, reduces shot-to-shot fluctuations in the decay constant to 4ϫ10 Ϫ3 , and eliminates oscillations in spectral backgrounds. This approach also allows ring-down decay acquisition rates limited only by the ring-down and buildup constants of the resonator, and holds the promise of offering truly shot-noise-limited cavity ring-down spectroscopy measurements.

A system is described that employs a diode-pumped Nd:YAG continuous-wave laser source servolocked to a three-mirror optical cavity and an analog detection circuit that extracts the ring-down rate from the exponentially decaying ring-down waveform. This scheme improves on traditional cavity ring-down spectroscopy setups by increasing signal acquisition rates to tens of kilohertz and reducing measurement noise sources. For example, an absorption spectrum of a weak CO 2 transition at 1064 nm is obtained in less than 10 s at a spectral resolution of 75 kHz employing a cavity with an empty-cavity ring-down decay lifetime of 2.8 s and a total roundtrip path length of 42 cm. The analog detection system enables laser frequency scan rates greater than 500 MHz/s. The long-term sensitivity of this system is 8.8ϫ10 Ϫ12 cm Ϫ1 Hz Ϫ1/2 and the short-term sensitivity is 1.0ϫ10 Ϫ12 cm Ϫ1 Hz Ϫ1/2 .

Position and momentum were the first pair of conjugate observables explicitly used to illustrate the intricacy of quantum mechanics. We have extended position and momentum entanglement to bright optical beams. Applications in optical metrology and interferometry require the continuous measurement of laser beams, with the accuracy fundamentally limited by the uncertainty principle. Techniques based on spatial entanglement of the beams could overcome this limit, and high-quality entanglement is required. We report a value of 0.51 for inseparability and 0.62 for the Einstein-Podolsky-Rosen criterion, both normalized to a classical limit of 1. These results are a conclusive optical demonstration of macroscopic position and momentum quantum entanglement and also confirm that the resources for spatial multimode protocols are available.