Spontaneous Down-conversion Research Articles

Conservation of orbital angular momentum in stimulated down-conversion

We report on an experiment demonstrating the conservation of orbital angular momentum in stimulated down-conversion. The orbital angular momentum is not transferred to the individual beams of the spontaneous down-conversion, but it is conserved when twin photons are taken individually. We observe the conservation law for an individual beam of the down-conversion through cavity-free stimulated emission.

Quantum Zeno effect in a nonlinear coupler

The distortion of a spontaneous downconversion process caused by an auxiliary mode coupled to the idler wave is analyzed. In general, a strong coupling with the auxiliary mode tends to hinder the downconversion in the nonlinear medium. On the other hand, provided that the evolution is disturbed by the presence of a phase mismatch, the coupling may increase the speed of the downconversion. These effects are interpreted as being manifestations of quantum Zeno or anti-Zeno effects, respectively, and they are understood by using the dressed modes picture of the device. The possibility of using the coupling as a nontrivial phase-matching technique is pointed out.

Quantum images in double-slit experiments with spontaneous down-conversion light.

Summary form only given. This work analyzes patterns seen when a signal beam of spontaneous parametric downconverted light illuminates a double-slit (Young) and a coincidence pattern (object) appears on scanning behind the double-slit arrangement and, conversely, when the conjugated idler beam is scanned spatially keeping the signal detector fixed and a Young coincidence pattern (image) is revealed. Mandel's theory for downconversion is applied, in the (r)-space, to this analysis.

Quantum nonlocality and interference in frequency domain

The experiment is proposed which should demonstrate that if the filter providing the spectral selection is placed in the route of one photon of the entangled pair produced by spontaneous down-conversion and the photon is detected behind it then the interference appears in the (distant) Mach-Zehnder interferometer placed in the route of the other photon of the pair even if the optical path difference through the interferometer greatly exceeds the coherence length of the light and if the spectra of these two photons do not overlap. The theoretical analysis is carried out and physical interpretation is discussed.

Observation of quantum interference effects in the frequency domain.

Interference has been observed in the mixing of signal photons produced by spontaneous down-conversion in two nonlinear crystals, when the optical path difference through the interferometer greatly exceeds the coherence length of the light. The interference shows up as a periodic modulation of the spectrum. We demonstrate that the interference is nonclassical and connected with the entangled state of the signal and idler photons, by transferring the long optical delay from the path of the signal photons to the path of the idler photons. Even though the idlers are not interfering directly, the effect is found to be unchanged, but the interference is lost when the idler photons are blocked. We discuss the extent to which one photon carries the modulated spectrum.

Coherence in two-photon down-conversion induced by a laser.

We discuss the situation in which idler beams from two parametric down-converter crystals are allowed to interfere. We show that, when two mutually coherent signal beams derived from a common laser are injected into the down-converters, the two idler beams can become mutually coherent also. Moreover, the resulting interference pattern can, in principle, have 100% visibility when the number of injected photons per unit down-converter bandwidth is large. This is just the condition for stimulated down-conversion to dominate over spontaneous down-conversion.

Quantum theory of coupled parametric down-conversion and up-conversion with simultaneous phase matching

A two-step nonlinear optical interaction consisting of (i) a spontaneous frequency down-conversion process coupled with (ii) a frequency up-conversion process is treated quantum mechanically in the experimentally observed case of simultaneous collinear phase matching of both processes. The temporal behavior of the output signals is found to depend on which process, (i) or (ii), predominates. The nature of the photon statistics for each of the output modes is found to be "super-Poisson," regardless of which process, (i) or (ii), is dominant, and the zero-point fluctuations characteristic of parametric amplification are exhibited.

Spontaneous Emission in the Frequency Up-Conversion Process in Nonlinear Optics

The spontaneous emission in the frequency up-conversion process in nonlinear optics due to the second-order process consisting of the mismatched spontaneous down-conversion process followed by the phase-matched up-conversion process is studied in detail as a quantum-mechanical scattering problem. The calculated integrated intensity of the spontaneous emission within the spectral and angular widths for the first-order phase-matched up-conversion process is found to be exactly the same as that obtained by Smith and Townes classically. By carefully choosing the parameters involved, it may be possible to eliminate the spontaneous emission from the detected output at the sum frequency of the up-conversion process. It therefore appears that the up-conversion process may indeed be made essentially noise-free and, with further improvements in the conversion efficiency, it is a very promising means of detecting low-level infrared radiation.