We demonstrate a technique of photon-echo interferometry which provides extraordinary sensitivity for the measurement of the relative phase in the coherences for pairs of optical transitions. The method is applied to study collision-induced optical-coherence transfer between adjacent transitions in atomic samarium vapor.
The interference of waves, be they photons, atoms, or electrons, and the measured phase shift induced as the waves take a different route around the arms of an interferometer are widely used in applications ranging from global positioning systems and gravity wave detectors down to the more mundane characterization of optical materials. The sensitivity of the interferometer in measuring the phase shift is affected by noise contributions to the signal. To combat this noise, piezoelectric actuators are typically used to adjust the system to an optimum measuring point for maximum sensitivity. This approach has drawbacks, however, for certain applications where the determination of arbitrary phase shifts is important. Pezzé et al. introduce a method and protocol of photon counting at the output paths to determine the phase. They show that the classical, or thermal, noise can be eliminated without bias (i.e., no tinkering with the phase shift is necessary). At this quantum limit, only quantum uncertainty affects the precision of the phase measurement. — ISO
Collision-induced coherence transfer between infrared molecular transitions is studied using a technique that permits the complete distribution of accompanying velocity changes to be determined in a simple way. The experiments are carried out by measuring the intensity of two pulse photon echoes as a function of applied Stark field at fixed large echo time delay. The velocity change distribution for coherence-transfer collisions is obtained directly as the shape of the echo intensity versus Stark field curve. Previously unpublished data for dipole-quadrupole and van der Waals interactions are presented, and a theory of coherence-transfer velocity-changing kernels is developed that is in good agreement with the data.
Optical quantum metrology uses specially-prepared states of light to probe sensitive samples. Choosing a suitable probe state allows one to extract more information per probe photon about an unknown parameter than is possible using any classical technique. In effect, these techniques improve the sensitivity of the measurement whenever the number of probe photons is constrained in some way. This thesis presents experimental and theoretical results on the use of pairs of time-frequency entangled photons in precision measurements. It begins with an overview of necessary background knowledge about quantum optics and quantum information. This is followed by a theoretical discussion on the estimation of a single delay using two-photon interference. We show that constraints on the probe photon's bandwidth complicate the measurement problem, requiring suitable optimization of the two-photon quantum state to avoid losing precision. We then consider the use of two-photon interference for linear spectroscopic measurements. Linear spectroscopy relates the absorption and phase spectrum of a sample to unknown electronic parameters. We describe a proof-of-principle experiment demonstrating that two-photon interferometric spectroscopy can be used for simultaneously measurement of both absorption and phase. This is followed by an information-theoretic discussion of the precision of this technique. For low losses, we show that the method provides phase sensitivity similar to classical interferometry while approaching the optimal absorption precision of any possible measurement. If the absorption and phase values are both related to a single unknown parameter, then our technique provides improved sensitivity to this parameter compared to both classical measurements and other quantum techniques in the presence of realistic levels of loss. We conclude with a discussion of quantum-optical coherence tomography, which is a two-photon interferometric method for determining the material structure of a sample. We present an ongoing experiment which aims to apply the method to tissue-like samples, where the quantum measurement is limited by a poor signal-to-noise ratio. Preliminary results demonstrate that this noise can be largely reduced by suitable frequency-domain filtering. This allows us to precisely determine the position of a scattering center using a few hundred detected photon pairs despite the presence of large amounts of noise. We follow this with a theoretical discussion of signal artifacts which are known to complicate the measured interferogram. We show these artifacts are due to a distinct interferometric effect, which can be distinguished from the signal of interest using the relative time delay between detected photons. We conclude with a brief discussion on possible techniques for artifact removal.