Just like human twins who evoke amazement and a ense of mystery by reporting empathetic experiences across great distances, photons born in pairs also astonish us by their quantum-correlated behavior.
Although most physics experiments are carried out with independent particles, it is the collective nature of entangled particles that reveals the most fascinating and unexpected aspects of the quantum world. It was Erwin Schrodinger who first said “entanglement is not one but rather the characteristic trait of quantum mechanics”. One curious aspect of the behavior of a pair of particles in an entangled state is that, though each individual particle exhibits an inherent uncertainty, the joint entity of an entangled pair can exhibit no such uncertainty. As an example, while the time of arrival of an individual particle may be totally random, an entangled pair must always arrive simultaneously. This property offers a unique tool for carrying out absolute measurements. Our goal here is to explore the myriad implications and significance of entanglement and to exploit it for the development of a new type of optical measurement—quantum optical metrology. The existence of unique non-classical correlations between twin photons generated in the nonlinear process of spontaneous parametric origins. The non-classical link between such twin quanta is not diminished by arbitrarily large separations between the twins, even when they lie outside the light cone. Twin states have been used with great effectiveness over the past two decades for carrying out definitive quantum experiments that lead to counterintuitive results; among these are those arising from the EinsteinPodolsky-Rosen (EPR) paradox such as various tests of Bell’s inequalities [1-12], as well as non-local dispersion cancellation, entangled-photon–induced transparency, and entangled-photon spectroscopy with monochromatic light. The availability of these twin beams has made it possible to conduct such experiments without having to resort to
Single-photon detectors are of great importance in spectroscopy, laser ranging, astronomy and wherever weak light detection is required, so a precise knowledge of their quantum efficiency is essential, but their calibration is by no means trivial. Traditional methods need a black body radiation source or a highly attenuated laser beam as a reference standard, or an already calibrated reference detector. It is well known that absolute measurement of the quantum efficiency, absolute in the sense that the calibration is independent of any radiometric standard or the properties of another detector, can be performed using entangled twin photons generated in optical spontaneous parametric down-conversion together with a reference detector. We have performed the first proof-of-principle experiment to determine the quantum efficiency by which no second detector or reference standard is required. This absolute self-calibration method is also based on the time correlation of photons emitted in parametric down-conversion, but with the introduction of a suitable time delay between the arrival times of the twin photons. Furthermore, we describe a scheme for absolute self-calibration of the quantum efficiency at different wavelengths by means of a pulsed source of nondegenerate down-converted photons. This method should have even greater potential for applications.
Single-photon detectors are of great importance in spectroscopy, laser ranging, astronomy and wherever weak light detection is required, so a precise knowledge of their quantum efficiency is essential, but their calibration is by no means trivial. Traditional methods need a black body radiation source or a highly attenuated laser beam as a reference standard, or an already calibrated reference detector. It is well known that absolute measurement of the quantum efficiency, absolute in the sense that the calibration is independent of any radiometric standard or the properties of another detector, can be performed using entangled twin photons generated in optical spontaneous parametric down-conversion together with a reference detector. We have performed the first proof-of-principle experiment to determine the quantum efficiency by which no second detector or reference standard is required. This absolute self-calibration method is also based on the time correlation of photons emitted in parametric down-conversion, but with the introduction of a suitable time delay between the arrival times of the twin photons. Furthermore, we describe a scheme for absolute self-calibration of the quantum efficiency at different wavelengths by means of a pulsed source of nondegenerate down-converted photons. This method should have even greater potential for applications.