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APL Photonics

Turbulence-free interference induced by the turbulence itself

Thomas A. Smith, Yanhua H. Shih

Published Dec 1, 2020• 2 citations
Publisher

Abstract

When a laser beam is incident on a double-slit interferometer without turbulence, the classic Young’s double-slit interference is present in the first-order measurement of the mean photon number (or intensity), while the second-order measurement of photon number fluctuation correlation (or intensity fluctuation correlation) yields a trivial constant. When optical turbulence is introduced, it destroys the classic interference present in the measurement of the photon number; however, two-photon interference appears in the measurement of photon number fluctuation correlation. This interesting observation means that the observed two-photon interference is not only observable through turbulence, i.e., turbulence-free, but also induced by the turbulence itself. Turbulence-free two-photon interference induced by the turbulence itself allows for interferometric sensing through strong turbulence when coherent radiation, such as a laser, is applied.

DOI
10.1063/5.0031474

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When a laser beam is incident on a double-slit interferometer without turbulence, the classic Young’s double-slit interference is present in the first-order measurement of the mean photon number (or intensity), while the second-order measurement of photon number fluctuation correlation (or intensity fluctuation correlation) yields a trivial constant. When optical turbulence is introduced, it destroys the classic interference present in the measurement of the photon number; however, two-photon interference appears in the measurement of photon number fluctuation correlation. This interesting observation means that the observed two-photon interference is not only observable through turbulence, i.e., turbulence-free, but also induced by the turbulence itself. Turbulence-free two-photon interference induced by the turbulence itself allows for interferometric sensing through strong turbulence when coherent radiation, such as a laser, is applied.
This article reports a study on a turbulence-free Young's double-slit interferometer. When the environmental turbulence blurs out the classic Young's double-slit interference completely, a two-photon interference pattern is still observable from the measurement of intensity or photon number fluctuation correlation. This two-photon interferometer always produces a turbulence-free interference pattern, when the double-slit interferometer is utilizing both first-order spatially incoherent light and spatially coherent light. This type of two-photon interferometer establishes new capabilities in optical observations and sensing measurements that require high sensitivity and stability.
Optical turbulence can be detrimental for optical observations. For instance, atmospheric turbulence may reduce the visibility or completely blur out the interference produced by an interferometer in open air. However, a simple two-photon interference theory based on Einstein's granularity picture of light makes a turbulence-free interferometer possible; i.e., any refraction index, length, or phase variations along the optical paths of the interferometer do not have any effect on its interference. Applying this mechanism, the reported experiment demonstrates a two-photon double-slit interference that is insensitive to atmospheric turbulence. The turbulence-free mechanism and especially the turbulence-free interferometer would be helpful in optical observations that require high sensitivity and stability such as for gravitational-wave detection.
The study of entangled states greatly advanced our understanding about two-photon interferometry. Two-photon interference is not the interference between two photons, it is about a pair of photons interfering with the pair itself. In the language of quantum theory, two-photon interference is the result of superposition between different yet indistinguishable two-photon probability amplitudes. Under certain conditions, the observed second-order interference can be factorized into a product of two individual classic first-order interferences. In this case, the interference is not only observable from the intensity fluctuation correlation measurement, or from the photon number fluctuation correlation measurement, of two photodetectors, but also observable in the counting rate of each individual photodetector. It may not be uneasy to accept the quantum concept of two-photon interference for “classical” thermal radiation. Indeed, the radiation Planck dealt with was “classical” thermal light. Perhaps, no one would even expect observing nontrivial second-order correlation from a laser beam, since laser field is non-Gaussin.
Over the past several decades, single photon-based path interference in double-slit experiment has been well demonstrated for the particle nature of photons satisfying complementarity theory, where the quantum mechanical interpretation of the single photon interference fringe is given by Born rule for complex amplitudes in measurements. Unlike most conventional methods using entangled photon pairs, here, a classical coherent light source is directly applied for the proof of the same self-interference phenomenon. Instead of double slits, we use a Mach-Zehnder interferometer as usual, where the resulting self-interference fringe of coherent single photons is exactly the same as that of the coherent photon ensemble of the laser, demonstrating that the classical coherence based on the wave nature is rooted in the single photon self-interference. This unexpected result seems to contradict our common understanding of decoherence phenomena caused by multi-wave interference among bandwidth distributed photons in coherence optics.