Spontaneous parametric down-conversion is investigated theoretically for a nonlinear crystal placed inside a microcavity of length 100 microns. We find suppression and enhancement in the count rate for detectors placed outside the cavity.
A theory of spontaneous parametric downconversion in a nonlinear crystal that is placed inside a microcavity in the degenerate, collinear, type I arrangement is given. Both atom and field variables are fully quantized. The photon-correlation function for the entangled photon pairs produced has a much longer correlation time (~0.4 ns) than that without the cavity (of the order of subpicoseconds) and can be measured directly with fast photodetectors. We also calculate the resonant suppression and enhancement in the photon-pair count rate as a function of cavity length. The results are in qualitative agreement with recent downconversion experiments [Phys. Rev. Lett. 85, 2556 (1999)] in a 4.5-mm cavity.
Spontaneous parametric down-conversion (SPDC) in a nonlinear crystal has been a workhorse for the generation of entangled and correlated single-photon pairs used for quantum communications applications for nearly three decades. However, as a naturally broadband process, the ability of SPDC to interface with the very narrow energy transitions in atomic ensembles for implementing quantum memories, which are needed for quantum repeaters to extend the reach of quantum communications, was initially limited. To overcome this limitation, the process was enhanced by placing the nonlinear crystal inside a resonating cavity. This modified process has some important advantages, including narrowing the spectral linewidth of generated photons into brighter resonant modes of the cavity, and the ability to lock the desired mode of the cavity to the targeted transition frequency of the atomic ensemble. This paper presents an overview of the principle of cavity-enhanced SPDC, a review of works to date using this technique, and an example of one of these implementations.
The most common way of generating correlated photons is by parametric down-conversion in a nonlinear crystal in free space. However, a drawback of this technique is the extremely low photon flux available. We present a simple analysis which shows that cavity-enhanced parametric downconversion, using an arrangement similar to an optical parametric oscillator and operated well below threshold, can yield a substantially higher photon flux without significantly degrading the correlation between photons.