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arXiv:quant-ph/0304181 · Journal of the Optical Society of America B

Measurement of one-photon and two-photon wave packets in spontaneous parametric downconversion

Yoon-Ho Kim

Published Sep 1, 2003• 35 citations
arXivHTMLPDF

Abstract

One-photon and two-photon wave packets of entangled two-photon states in spontaneous parametric downconversion (SPDC) fields are calculated and measured experimentally. For type II SPDC, measured one-photon and two-photon wave packets agree well with theory. For type I SPDC, the measured one-photon wave packet agrees with the theory. However, the two-photon wave packet is much bigger than the expected value, and the visibility of interference is low. We identify the sources of this discrepancy as the spatial filtering of the two-photon bandwidth and nonpair detection events caused by the detector apertures and the tuning-curve characteristics of the type I SPDC.

DOI
10.1364/josab.20.001959

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One-photon and two-photon wave packets of entangled two-photon states in spontaneous parametric downconversion (SPDC) fields are calculated and measured experimentally. For type II SPDC, measured one-photon and two-photon wave packets agree well with theory. For type I SPDC, the measured one-photon wave packet agrees with the theory. However, the two-photon wave packet is much bigger than the expected value, and the visibility of interference is low. We identify the sources of this discrepancy as the spatial filtering of the two-photon bandwidth and nonpair detection events caused by the detector apertures and the tuning-curve characteristics of the type I SPDC.
Polarization correlation measurement of two-photon state generated in type-II spontaneous parametric downconversion (SPDC) has been performed in a single input port beam splitter configuration. Two orthogonally polarized entangled photons with degenerate k vectors were directed to the single input port 50%-50% beam splitter. Polarization were measured with linear and circular polarization entangled state by the use of two polarization analyzers followed by two photon counting detectors. A strong correlation of polarization has been observed. The type of two-photon state generated in Type-II SPDC was tested. A strong dependence of the polarization correlation upon the spectral bandwidth of the detector spectral filter was found and a different correlation between linear and circular polarization entangled state were observed. It seems that the standard theoretical models for SPDC need to be improved in order to provide a comprehensive treatment for the result of the experiment.
Photons generated via spontaneous parametric down-conversion (SPDC) have broad spectrums and suffer from dispersive broadening of the temporal wave packets when they are transmitted through dispersive media. In this paper we theoretically and experimentally study the detailed amount of the temporal broadening of the two-photon wave packets generated via both type-I and type-II SPDC with β -BaB 2 O 4 of various lengths, by transmitting them through optical fibers. We interpret the results with respect to the spectral properties of the two-photon wave packets. We believe that our results will contribute to implementing protocols involving long-range distributions of photon pairs.
Spontaneous parametric down-conversion (SPDC) has been extensively studied for the case of a continuous wave pump. In this paper SPDC is studied for the case in which the pump is a pulse. The pump pulse acts like a clock with an uncertainty equal to its width. This makes it possible to distinguish pairs of photons born at sufficiently different depths inside the crystal with a consequent decrease in two-photon interference. We study this effect in detail for degenerate collinear type-II SPDC and degenerate type-I SPDC. It may be possible in the type-II case to eliminate the clock effect of the pump by judicious choice of materials and wavelengths.
Spontaneous parametric down-conversion (SPDC) is the most often used source of entangled photon states in quantum optics. Recently, it has been suggested in theory that the reduction of polarization entanglement between the photon pair generated via the type-II SPDC process pumped by a ultrafast laser is due to the asymmetry in the joint spectrum function, which is the indicative of the frequency entanglement between the two photons. In this paper, we report the measurement of the joint spectrum function (i.e., direct observation of the degree of frequency entanglement) of the two-photon state of type-II SPDC in both cw- and ultrafast-laser pumping conditions. We observed that, in agreement with the theory, the degree of frequency entanglement (the shape of the joint spectrum function) is indeed different for cw- and ultrafast-pumped type-II SPDC. Our results are of importance in "engineering" the two-photon state for specific quantum applications, such as quantum cryptography.
Einstein–Podolsky–Rosen (EPR) correlations produced for two-photon entangled wave packets are analyzed by use of optical Wigner functions. In particular, type-II optical parametric downconversion systems are used, and the effects of the length of the crystal and the filter bandwidth on the Wigner functions are studied. The relation between the optical Wigner functions and the interference experiment with a beam splitter is studied. We find that, whereas in first-order correlation experiments (typified, for example, by Young’s interference experiment) the “photon interferes with itself” [ P. A. M. Dirac , The Principles of Quantum Mechanics (Clarendon, Oxford, UK, 1958), p. 9], in the EPR correlations the two-photon amplitude function interferes with itself. Such interference is analyzed by use of the optical Wigner functions.