Abstract
Barreto Lemos et al. [Nature 512, 409–412 (2014)] reported an experiment in which a non-degenerate parametric downconverter and a non-degenerate optical parametric amplifier—used as a wavelength-converting phase conjugator—were employed to image object transparencies in a manner akin to ghost imaging. Their experiment, however, relied on single-photon detection, rather than the photon-coincidence measurements employed in ghost imaging with a parametric downconverter source. More importantly, their system formed images despite the photons that passed through the object never being detected. Barreto Lemos et al. interpreted their experiment as a quantum imager, as assuredly it is, owing to its downconverter’s emitting entangled signal and idler beams. We show, however, that virtually all the features of their setup can be realized in a quantum-mimetic fashion using classical-state light, specifically a pair of bright pseudothermal beams possessing a phase-sensitive cross correlation. Owing to its much higher signal-to-noise ratio, our bright-source classical imager could greatly reduce image-acquisition time compared to that of Barreto Lemos et al.‘s quantum system, while retaining the latter’s ability to image with undetected photons.
Highlights
Barreto Lemos et al [Nature 512, 409–412 (2014)] reported an experiment in which a non-degenerate parametric downconverter and a non-degenerate optical parametric amplifier—used as a wavelengthconverting phase conjugator—were employed to image object transparencies in a manner akin to ghost imaging
Three-wave mixing in a second-order nonlinear material is the workhorse of nonclassical light-beam generation, with spontaneous parametric downconverters producing entangled signal and idler beams5, optical parametric amplifiers producing squeezed-vacuum states6, and optical parametric oscillators producing photon-twin beams7
The resulting conjugate has a phase-insensitive cross correlation with that companion, which can be sensed via second-order interference. This possibility was exploited, theoretically in Ref. 15 and experimentally in Ref. 16, to realize phase-conjugate optical coherence tomography, in which classical-state signal and idler beams—of the type mentioned in the preceding paragraph—yielded the axial resolution and dispersion immunity afforded by quantum optical coherence tomography (Q-OCT) without the need for nonclassical light
Summary
Three-wave mixing in a second-order nonlinear material is the workhorse of nonclassical light-beam generation, with spontaneous parametric downconverters producing entangled signal and idler beams, optical parametric amplifiers producing squeezed-vacuum states, and optical parametric oscillators producing photon-twin beams7 It follows that imagers using any such sources will be quantum imagers, according to the criterion described in the preceding paragraph. The resulting conjugate has a phase-insensitive cross correlation with that companion, which can be sensed via second-order interference This possibility was exploited, theoretically in Ref. 15 and experimentally in Ref. 16, to realize phase-conjugate optical coherence tomography, in which classical-state signal and idler beams—of the type mentioned in the preceding paragraph—yielded the axial resolution and dispersion immunity afforded by Q-OCT without the need for nonclassical light. For both imagers considered below, the photons that interacted with the object being imaged are not the photons that are detected
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