Two-photon Interference Effect Research Articles

Quantum interference by two temporally distinguishable pulses

We report a two-photon interference effect, in which the entangled photon pairs are generated from two laser pulses well-separated in time. In a single pump pulse case, interference effects did not occur in our experimental scheme. However, by introducing a second pump pulse delayed in time, quantum interference was then observed. The visibility of the interference fringes shows dependence on the delay time between two laser pulses. The results are explained in terms of indistinguishability of biphoton amplitudes which originated from two temporally separated laser pulses.

Quantum optics of lossy beam splitters

The familiar input-output relations for an optical beam splitter are generalized to allow for linear absorption by the medium forming the mirror. Beam-splitter losses generally affect the noise levels detectable in experiments involving nonclassical Light. When employed to investigate two-photon interference effects, a lossy beam splitter can lead to apparent nonlinear absorption, which, in the most extreme case, leads to either both or neither of the photons being absorbed. The degree of second-order coherence of antibunched light can be maintained on transmission through the beam splitter but any amplitude squeezing in the incident light is degraded.

Interferometric Bell-state analysis.

We present the experimental demonstration of a Bell-state analyzer employing two-photon interference effects. Photon pairs produced by parametric down-conversion allowed us to generate momentum-entangled Bell states and to demonstrate the properties of this device. The performance obtained indicates its readiness for use with quantum communication schemes and in experiments on the foundations of quantum mechanics.

Experimental tests of Bell's inequalities based on space-time and spin variables.

Coincidence detections are measured between the output ports of a beam splitter whose input is an orthogonally polarized two-photon state produced in type-II spontaneous parametric down-conversion. When the delays between the two optical paths leading to each detector are varied by two spatially separated Pockel cells, a two-photon interference effect is seen that highlights a simultaneous entanglement in both spin and space-time variables. In contrast to all previous experimental tests of Bell's in- equalities, which have utilized states entangled with respect to only one type of observable, the double entanglement realized here allows the demonstration of two different types of Bell inequality violations in one experimental setup.

Violation of Bell's inequality over 4 km of optical fiber.

Using energy correlated photon pairs created in parametric down-conversion we demonstrate two-photon interference effects in fiber interferometers separated by 4.3 km of optical fiber. The measured 86% interference visibilities confirm the purely quantum mechanical origin of these correlations.

A two-photon interference experiment using type II optical parametric down conversion

A pair of orthogonally polarized light quanta, produced from type II parametric down conversion, is injected into a single input port of a beamsplitter and detected coincidently by two detectors placed in the two output ports of the beamsplitter. With the help of a set of crystal quartz plates, a Pockel's cell and two linear polarizers, the coincidence counting rate exhibits interference modulation of the pump frequency with 88% visibility when manipulating the voltage across the Pockel's cell, regardless of the optical delay (which is much greater than the coherence length) and the mutual incoherence. This two-photon interference effect is responsible by a nonclassical two-photon state which is entangled both in spin and in space-time.

Using the Pauli exclusion principle to exhibit local-realism violations in overlapping interferometers

Recently Hardy has exhibited a striking new violation of local realism that occurs in a pair of interferometers that have been interconnected so that knowledge about the path chosen by a particle in one interferometer conveys some information about the path or paths chosen by the particle in the other interferometer. It has been demonstrated that particle annihilation and a two-photon interference effect can be exploited to give rise to this local realism violation. Here we show that in the case of fermion interferometers the Pauli exclusion principle can be exploited to give rise to this effect.

Quantum versus stochastic or hidden-variable fluctuations in two-photon interference effects.

In a series of experiments performed by Mandel and co-workers, nonclassical effects have been demonstrated in the interference of two photons generated in a process of parametric down-conversion. The nonclassical effects in the two-photon interference effects can be discussed in the framework of two different descriptions. In the first description, a stochastic theory of electromagnetic field fluctuations can be used in order to calculate the interference pattern. In the second description, a theory of hidden-variable fluctuations can be applied in order to calculate correlations of the interference pattern. A stochastic theory leads to statistical inequalities for the light intensities, while a local hidden-variable theory leads to Bell's inequalities. Using the Schwinger-boson representation of the angular momentum, we show that the two-photon interference effects can be described in terms of spin-correlated states. In particular, we show that the action of a beam splitter on the photons in a parametric down-conversion is equivalent to the production of an entangled state that is very similar to the well-known Einstein, Podolsky, and Rosen spin-singlet state. We show that the stochastic theory of two-photon fluctuations is not equivalent to a hidden-variable theory of photon correlations. We establish a range for which the stochastic theory fails but the hidden-variable theory is still possible. We compare our theoretical predictions with the experimental results and conclude that a violation of the stochastic theory has been clearly observed, while the violation of the hidden-variable theory is less pronounced.

Selective ionization of Ba and Sr isotopes based on a two-photon interference effect

We show that the zero- and nonzero-nuclear-spin isotopes of Ba and Sr can be excited and ionized at different rates by sequential absorption of radiation from two pulsed laser beams. The selective excitation takes place in a J=0\ensuremath{\leftarrow}J=1\ensuremath{\leftarrow}J=0 energy-level scheme (where J is the electronic angular momentum), which, depending on the relative polarization of the laser beams, gives totally destructive interference in the absorption rate to the upper J=0 state of a zero-nuclear-spin atom, or a finite but time-dependent rate for population of the upper state for an atom with nonzero nuclear spin. A theoretical explanation of the effect shows the alignment of the nuclear spin as well as the population of the upper state to be time dependent. Isotope ratios were measured in Ba and Sr as a function of laser polarization and delay time, and compared with theory.