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Is quantum entanglement necessary for observing induced coherence between photons generated in separate biphoton sources

Arturo Rojas-Santana, Gerard J. Machado, Dorilian Lopez-Mago, Juan P. Torres

Published May 7, 2020• 1 citation
OpenAlex

Abstract

There is renewed interest on the concept of induced coherence between beams generated by separate biphoton sources, that has led to the demonstration of new schemes for imaging, spectroscopy, microscopy and optical coherence tomography (OCT). These schemes make use of continuous wave pumping in the low parametric gain regime, which reflects the frequency entanglement between the down-converted photons. However, is entanglement a requisite to observe induced coherence? Contrary to some claims, we will show that it is not. This might be an advantage for OCT applications. High axial resolution requires a large bandwidth and for continuous wave pumping this requires the use of short nonlinear crystals. This is detrimental since short crystals generate small photon fluxes. We show that the use of ultrashort pump pulses increases axial resolution even with long crystals.

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There is renewed interest on the concept of induced coherence between beams generated by separate biphoton sources, that has led to the demonstration of new schemes for imaging, spectroscopy, microscopy and optical coherence tomography (OCT). These schemes make use of continuous wave pumping in the low parametric gain regime, which reflects the frequency entanglement between the down-converted photons. However, is entanglement a requisite to observe induced coherence? Contrary to some claims, we will show that it is not. This might be an advantage for OCT applications. High axial resolution requires a large bandwidth and for continuous wave pumping this requires the use of short nonlinear crystals. This is detrimental since short crystals generate small photon fluxes. We show that the use of ultrashort pump pulses increases axial resolution even with long crystals.
There is renewed interest in using the coherence between beams generated in separate down-converter sources for new applications in imaging, spectroscopy, microscopy, and optical coherence tomography (OCT). These schemes make use of continuous-wave (cw) pumping in the low parametric gain regime, which produces frequency correlations and frequency entanglement between signal-idler pairs generated in each single source. But can induced coherence still be observed if there is no frequency correlation, so the biphoton wave function is factorable? We will show that this is the case and might be an advantage for OCT applications. High axial resolution requires a large bandwidth. For cw pumping, this requires the use of short nonlinear crystals. This is detrimental since short crystals generate small photon fluxes. We show that the use of ultrashort pump pulses allows one to improve the axial resolution even for a long crystal that produces higher photon fluxes.
Optical-coherence tomography (OCT) is a technique that employs light in order to measure the internal structure of semitransparent, e.g. biological, samples. It is based on the interference pattern of low-coherence light. Quantum-OCT (QOCT), instead, employs the correlation properties of entangled photon pairs, for example, generated by the process of spontaneous parametric downconversion (SPDC). The usual QOCT scheme uses photon pairs characterised by a joint-spectral amplitude with strict spectral anti-correlations. It has been shown that, in contrast with its classical counterpart, QOCT provides resolution enhancement and dispersion cancellation. In this paper, we revisit the theory of QOCT and extend the theoretical model so as to include photon pairs with arbitrary spectral correlations. We present experimental results that complement the theory and explain the physical underpinnings appearing in the interference pattern. In our experiment, we utilize a pump for the SPDC process ranging from continuous wave to pulsed in the femtosecond regime, and show that cross-correlation interference effects appearing for each pair of layers may be directly suppressed for a sufficiently large pump bandwidth. Our results provide insights and strategies that could guide practical implementations of QOCT.
We demonstrate a different scheme to perform optical sectioning of a sample based on the concept of induced coherence [Zou et al., Phys. Rev. Lett. 67, 318 (1991)]. This can be viewed as a different type of optical coherence tomography scheme where the varying reflectivity of the sample along the direction of propagation of an optical beam translates into changes of the degree of first-order coherence between two beams. As a practical advantage the scheme allows probing the sample with one wavelength and measuring photons with another wavelength. In a bio-imaging scenario, this would result in a deeper penetration into the sample because of probing with longer wavelengths, while still using the optimum wavelength for detection. The scheme proposed here could achieve submicron axial resolution by making use of nonlinear parametric sources with broad spectral bandwidth emission.
Interference of two beams produced at separate biphoton sources was first observed more than two decades ago. The phenomenon, often called coherence without induced emission, has recently gained attention after its applications to imaging, spectroscopy, and measuring biphoton correlations have been discovered. The sources used in the corresponding experiments are nonlinear crystals pumped by laser light. The use of a laser pump makes the occurrence of induced (stimulated) emission unavoidable and the effect of stimulated emission can be observed in the joint detection rate of the two beams. This fact raises the question whether the stimulated emission also lays a role in inducing the coherence. Here we investigate a case in which the crystals are pumped with a single-photon Fock state. We find that coherence is induced even though the possibility of stimulated emission is now fully ruled out. Furthermore, the joint detection rate of the two beams becomes ideally zero and does no longer change with the pump power. We illustrate our results by numerical simulations and by comparisons with experimental findings. Our results rule out any classical or semi-classical explanation of the phenomenon and also imply that similar experiments can be performed with fermions, for which stimulated emission is strictly forbidden.
Interference of two beams produced at separate biphoton sources was first observed more than two decades ago. The phenomenon, often called ``induced coherence without induced emission,'' has recently gained attention after its applications to imaging, spectroscopy, and measuring biphoton correlations have been discovered. The sources used in the corresponding experiments are nonlinear crystals pumped by laser light. The use of a laser pump makes the occurrence of induced (stimulated) emission unavoidable, and the effect of stimulated emission can be observed in the joint detection rate of the two beams. This fact raises the question whether the stimulated emission also plays a role in inducing the coherence. Here we investigate a case in which the crystals are pumped with a single-photon Fock state. We find that coherence is induced even though the possibility of stimulated emission is now fully ruled out. Furthermore, the joint detection rate of the two beams becomes ideally zero. Our results rule out any classical or semiclassical explanation of the phenomenon and also suggest that it is, in principle, possible to perform similar experiments with fermions, for which stimulated emission is strictly forbidden.
Optical coherence tomography (OCT) based on Michelson interferometer has widely been utilized in biology and medicine as a type of optical biopsy and quantum optical coherence tomography (QOCT) based on Hong-Ou-Mandel interferometer has recently been demonstrated. By use of quantum entangled photon pairs generated via spontaneous parametric down conversion (SPDC) process, axial resolution of QOCT can be better than that of OCT in principle for a source of same bandwidth and group velocity dispersion (GVD) effect for QOCT can be automatically cancelled thanks to the frequency correlation of entangled photon pairs. To realize high-resolution QOCT, we need a broadband quantum entangled photon pair source. Then we proposed a novel conventional method to generate broadband spontaneous parametric fluorescence via SPDC by using multiple nonlinear crystals pumped by a CW laser. Our method has controllability to tune the center frequency of generated photons and scalability to extend the number of crystals. This conventional method can enable us to achieve sub-micron axial resolution of QOCT.