Summary form only given. The use of parametric down-conversion as a convenient source of correlated photon pairs dates back to the early 1970's but it Was not until the late 1980's and early 1990's that these Sources were shown to be useful for fundamental studies of the phenomenon of two particle entanglement. In recent experiments three photon interference effects have been studied by entangling initially separate photons which has led on to demonstration of quantum teleportation and three photon entangle via GHz states. Due to the necessity for overlapping weak beams of randomly timed photon pairs the triple coincidence rates in these experiments is low. Furthermore the three-photon interference effects are post-selected from a product state of two-photon entanglements which opens the experiments to criticism from a local realistic viewpoint. An improved source would create entangled states of three photons directly in a way analogous to two-photon downconversion. This work extends the quantum theory of spontaneous two-photon downconversion in the presence of a /spl chi//sup (2)/ non-linearity to the three photon case based on the nonlinear susceptibility /spl chi//sup (3)/. The resulting gain is much lower and the efficiency for the three-photon creation process is small. However we show by simple arguments that the number of modes scale quadratically with bandwidth. This leaves us with the hope of detecting of order one three photon per second when phase matching.
Summary form only given. The use of parametric down-conversion as a convenient source of correlated photon pairs dates back to the early 1970's but it Was not until the late 1980's and early 1990's that these Sources were shown to be useful for fundamental studies of the phenomenon of two particle entanglement. In recent experiments three photon interference effects have been studied by entangling initially separate photons which has led on to demonstration of quantum teleportation and three photon entangle via GHz states. Due to the necessity for overlapping weak beams of randomly timed photon pairs the triple coincidence rates in these experiments is low. Furthermore the three-photon interference effects are post-selected from a product state of two-photon entanglements which opens the experiments to criticism from a local realistic viewpoint. An improved source would create entangled states of three photons directly in a way analogous to two-photon downconversion. This work extends the quantum theory of spontaneous two-photon downconversion in the presence of a /spl chi//sup (2)/ non-linearity to the three photon case based on the nonlinear susceptibility /spl chi//sup (3)/. The resulting gain is much lower and the efficiency for the three-photon creation process is small. However we show by simple arguments that the number of modes scale quadratically with bandwidth. This leaves us with the hope of detecting of order one three photon per second when phase matching.
Summary form only given. Correlated and entangled single photon states have made possible many fundamental studies in quantum optics such as the Bell-type test of hidden variables and non-locality, explorations of quantum interference properties, and investigations of quantum information processing and quantum communications technologies such as quantum cryptography, quantum computing, and quantum teleportation. These correlated single photon states are traditionally generated via a spontaneous parametric down-conversion process using a /spl chi//sup (2)/ material. We propose and analyze the generation of entangled single photons using /spl chi//sup (2)/ parametric processes in optical fibers. Such processes provide a simple and inexpensive correlated photon source readily compatible with commercial communications technologies. It further affords high efficiency for coupling into single mode fibers for applications in quantum cryptography.
Summary form only given. Parametric down-conversion is the most standard source of pairs of polarization-entangled photons, which is relevant to many problems of quantum information. However, the realization of the entanglement between three or more photons is considered so far as a difficult problem. In this report we present a novel scheme for producing three-photon entangled states. The scheme involves cascaded second-order /spl chi//sup (2)/-nonlinearities inside the same twice resonant cavity pumped by a frequency /spl omega/.
The importance of photon pair generation can never be overemphasized. It has formed the basis of most fundamental quantum optical experiments like Bell-experiments, quantum teleportation and entanglement swapping. However for considerable number of years Quantum Mechanics in general has remained largely theoretical and within the four corners of laboratories. Quantum Key Distribution is one of the first quantum applications to break this barrier. It provides us with unconditionally secured communication by providing us with efficient alternative to classical cryptography. This study forms part of the effort in realizing efficient way of generating photon pairs through the Spontaneous Parametric Down-Conversion process using a nonlinear crystal cut for type-I phase matching. After generating the photon pairs, the paper examined how they can be detected efficiently. We measured the rate of single photon generation (up to 200 kHz) as well as the rate at which they are detected in coincidence (150 coincidences per second). We further investigated a quantum interference effect (the Hong-Ou-Mandel effect) which classical wave theory has failed to describe. This effect has numerous applications in scalable quantum networks and in linear quantum computing. Finally, a remarkable conversion efficiency of 2.2
Summary form only given. Correlated photons are a key ingredient of many experiments testing the foundations of quantum mechanics and in quantum information processing. Many of these experiments harness entanglement by means of the nonclassical interference of photon pairs. A critical issue for such experiments is the mode matching of the photonic wave packets. Spontaneous parametric down-conversion is a well-established and practical method for generating pairs of correlated photons. High visibility interference of photon pairs generated by down-conversion in bulk crystals, a standard source for this type of experiments, can be achieved only by significant spatial filtering, which dramatically reduces the useful fraction of photon pairs. It also reduces the correlations between the presence of photons, thus opening for example the efficiency loophole in tests of Bell's inequalities. Nonlinear waveguides, compared to bulk crystals, offer several advantages as a source of correlated photon pairs. Most importantly, the waveguide structure provides a means to control precisely the spatial characteristics of generated photons, because of the well-defined transverse modes of the guide. in addition, nonlinear waveguides offer both higher pair production rates by utilizing the technique of quasiphase matching, and possibilities for spectral mode engineering.
High quality entangled photon sources are a key requirement for many promising quantum optical technologies. However, the production of multi-photon entangled states with good fidelity is challenging. Current sources of multi-photon entanglement require the use of post-selection, which limits their usefulness for some applications. It has been an open challenge to create a source capable of directly producing three-photon entanglement. An important step in this direction was achieved with the demonstration of photon triplets produced by a new process called cascaded downconversion, but these previous measurements were not sufficient to show whether these photons were in an entangled state and only had detection rates of five triplets per hour. In this thesis, we show the first demonstration of a direct source of three-photon entanglement. Our source is based on cascaded downconversion, and we verify that it produces genuine tripartite entanglement in two degrees of freedom: energy-time and polarization. \n \nThe energy-time entanglement is similar to a three-particle generalization of an Einstein-Podolski-Rosen state; the three photons are created simultaneously, yet the sum of their energies is well defined, which is an indication of energy-time entanglement. To prove it, we use time-bandwidth inequalities which check for genuine tripartite entanglement. Our measurements show that the state violates the inequalities with what constitute, to the best of our knowledge, the strongest violation of time-bandwidth inequalities in a tripartite continuous-variable system to date. \n \nWe create polarization entanglement by modifying our experimental setup so that two downconversion processes producing orthogonally polarized triplets interfere to create Greenberger-Horne-Zeilinger states. By using highly efficient superconducting nanowire single photon detectors, we improve the detected triplet rate by 2 orders of magnitude to 660 triplets per hour. We characterize the state using quantum state tomography, and find a fidelity of 86\\% with the ideal state, beating the previous best value for a three-photon entangled state fidelity measured by tomography. We also use the state to perform two tests of local realism. We violate the Mermin and Svetlichny inequalities by 10 and 5 standard deviations respectively, the latter being the strongest violation to date. Finally, we show that, unlike previous sources of tree-photon entanglement, our source can be used as a source of heralded Bell pairs. We demonstrate this by measuring a CHSH inequality with the heralded Bell pairs, and by reconstructing their state using quantum state tomography.
We consider the quantum state of light produced via direct parametric decay of pump photons into photon triples in a medium with cubic nonlinearity. For this state generated in the near-collinear frequency-degenerate regime, the third- and second-order Glauber's correlation functions are calculated and the intensity distribution over frequency and wave vector is found. It is shown that the number of photons generated into a single mode via the three-photon down-conversion is proportional to the width of the frequency-angular intensity distribution for the corresponding two-photon phase matching (spontaneous parametric down-conversion). The intensity of three-photon parametric down-conversion is shown to have an extremely broad frequency spectrum, even for a fixed angle of scattering.