We show that it is possible to produce time- and path-length-dependent oscillations in the two-photon detection probability of an entangled photon pair when none exists in the detection probability for single photons.
We present experimental observations of a nonclassic and nonlocal interference effect with frequency-entangled photon pairs. We observed two-photon interference in a Hong–Ou–Mandel interferometer by monitoring the coincidence-counting rate as we varied the path-length difference or the relative time delay between two photons. The quantum-mechanical probabilities of joint detection by two detectors that respond to different frequency components exhibit differences that depend not only on the arrangement of the detector pairs but on the spectral filtering. The experimental results provide more understanding of two-photon entanglement and of the interference effects that arise from superposition of indistinguishable probability amplitudes.
A necessary condition in order to obtain interference fringes in QED is the existence of at least two possible paths and unknown which-path information. If the photon beams have different frequencies, stability of the sources and fast enough detectors are also required. In this experiment, first order interference between two truly independent CW Nd:YAG monomode laser sources was produced. Contrary to what is expected, interference is observed although the photon beams are distinguishable and the path is unequivocally known for each photon beam. Segments of the ultra-stabilized CW wavetrains are selected with an acousto optic modulator. Temporal and spatial interference are integrated in a single combined phenomenon via streak camera detection. In this way, interference fringes and frequency beating are both present in the streak camera images. The photon beams are labeled with two slightly different frequencies. In addition, the inclination of the beams establish the projection of their respective wave vectors in the detection plane, which may be seen as perturbations coming from two different slits. The fringes displacement in the time-space interferograms reveal the trajectories of the frequency labeled photons. Since the detection probability of any one photon is independent from the detection of the rest, the trajectory can be drawn for each frequency labeled event. These results indicate that in non-degenerate frequency schemes, the ontology has to be refined and the which path criterion must be precisely stated. If reference is made to the frequency labeled photons, the path of each photon is known, whereas if the query is stated in terms of the detected photons, the path is unknown.
The spin-entangled exciton states produced by singlet fission provide an experimental route to generate entangled states with nondegenerate branches. Nondegenerate entangled pair states possess an internal "clock" that leads to quantum beating in various detected quantities. The implications of this internal clock for Bell's inequality measurements and correlated particle detection are analyzed using two- and three-state spin models. In a Bell's inequality experiment, we find that the choice of detection times can determine whether quantum or classical correlations are observed. The conditions under which the detection events could be time- or spacelike separated are analyzed in order to clarify how the temporal evolution of one particle can influence the time-dependent detection probability of the other. Possible routes to the detection of individual correlated triplet excitons are discussed, emphasizing both physical questions concerning the separation and propagation of triplet excitons over macroscopic distances and experimental challenges concerning decoherence, detection, and interpretation of the signals. We argue that spin-entangled triplet exciton states produced by singlet fission could provide a new way to probe entangled state detection and collapse, complementing schemes based on polarization-entangled photon states.