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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Broadband energy-entangled photon for high resolution temporal sensing

Christof Bernhard, Bänz Bessire, Thomas Feurer, André Stefanov

Published Mar 8, 2016• 1 citation
Publisher

Abstract

Energy-time entangled photon pairs exhibit at the same time narrowband and short time features. We show how to make use of those quantum properties to realize measurements beyond the capabilities of classical devices. As proof of principle experiment we show imaging through a scattering medium by selecting the ballistic photons only, using optical coincidence measurements.

DOI
10.1117/12.2212450

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Energy-time entangled photon pairs exhibit at the same time narrowband and short time features. We show how to make use of those quantum properties to realize measurements beyond the capabilities of classical devices. As proof of principle experiment we show imaging through a scattering medium by selecting the ballistic photons only, using optical coincidence measurements.
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This talk introduces our recent works on how to use photonic energy-time entanglement in quantum communications. On one hand, we realized entanglement based dispersive optics quantum key distribution over optical fibers of 20 kilometers, in which time encoding based on energy-time entanglement was used. On the other hand, we proposed and demonstrated a scheme of temporal quantum ghost imaging, which could be realized by correlations in either frequency or time in energy-time entangled photon pairs. Based on it, a scheme of quantum secure ghost imaging was proposed and demonstrated, and it was applied on a new protocol of multi-bit QDS recently.
Improving the image contrast of objects immersed in weakly scattering media can be achieved using various strategies. One common approach is to reject events associated with scattered photons in favor of the detection of ballistic photons. While this is traditionally done via time gating or spatial filtering, we propose a different approach based on probing the object with spatio-temporally entangled photon pairs. We show that coincidence detection, followed by post-selection on spatially correlated events, allows us to isolate ballistic from scattered bi-photons, thereby enhancing image contrast relative to a single-photon detection strategy, and simultaneously removes events due to background light. Our predictions are obtained via numerical simulations and confirmed by experiments conducted in two configurations where either both photons or only one illuminates the scene. In both scenarios, correlation post-selection shows an improvement in image contrast at the expense of higher shot noise due to the lower number of events. The latter can be partially compensated for by appropriately combining events from several post-selection windows. Our findings will enable extending imaging through scattering media into the quantum imaging framework in settings where adaptive optics, time gating, and spatial filtering are impractical.
The resilience of quantum two-photon interference against optical loss and background sets it apart from classical single-photon interference for the purpose of sensing. However, achieving fast measurements at the nanometer scale with two-photon interference is challenging, as attaining high resolution typically requires either ultrabroadband photons or many probe photons. To realize nanometer-scale measurements in a matter of seconds or less, we greatly increase the per-photon information content via highly non-degenerate energy entanglement. Our fast measurements thereby enable us to probe time-varying signals at the nanometer scale. To further develop our capabilities, we have drawn on single-photon imaging techniques to implement a robust protocol for reconstructing periodic signals from our relatively sparse detection data, complete with amplitude and frequency information. With this analysis regimen, we have successfully extracted complex multi-frequency signals as well as kHz-range acoustic vibrations. We review our method and present the results of an example measurement to illustrate our capabilities.
Energy-time entangled photons are critical in many quantum optical phenomena and have emerged as important elements in quantum information protocols. Entanglement in this degree of freedom often manifests itself on ultrafast time scales, making it very difficult to detect, whether one employs direct or interferometric techniques, as photon-counting detectors have insufficient time resolution. Here, we implement ultrafast photon counters based on nonlinear interactions and strong femtosecond laser pulses to probe energy-time entanglement in this important regime. Using this technique and single-photon spectrometers, we characterize all the spectral and temporal correlations of two entangled photons with femtosecond resolution. This enables the witnessing of energy-time entanglement using uncertainty relations and the direct observation of nonlocal dispersion cancellation on ultrafast time scales. These techniques are essential to understand and control the energy-time degree of freedom of light for ultrafast quantum optics.