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Physical Review Letters

X-Ray Parametric Down-Conversion in the Langevin Regime

S. Shwartz, Ryan Coffee, Jan M. Feldkamp, Yiping Feng, J. B. Hastings, G. Y. Yin, Stephen E. Harris

Published Jul 6, 2012• 96 citations
Publisher

Abstract

We experimentally and theoretically study the coincidence count rate for down-converted x-ray photons. Because of photoionization, parametric down-conversion at x-ray wavelengths generally involves loss and the theoretical description requires a Langevin approach. By working in a transmission geometry (Laue) rather than in the Bragg geometry of previous experiments, we obtain an improvement in the signal-to-noise ratio of 12.5, and find agreement between experiment and theory.

DOI
10.1103/physrevlett.109.013602

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We experimentally and theoretically study the coincidence count rate for down-converted x-ray photons. Because of photoionization, parametric down-conversion at x-ray wavelengths generally involves loss and the theoretical description requires a Langevin approach. By working in a transmission geometry (Laue) rather than in the Bragg geometry of previous experiments, we obtain an improvement in the signal-to-noise ratio of 12.5, and find agreement between experiment and theory.
The coincidence rate in two detectors that register signal and idler photons in parametric down-conversion is calculated using a quantum description of the radiation field that extends the semiclassical theory to the resonance regime. The signal is given by a sum over paths in the joint field plus matter space, each involving a pair of sites (molecules) and six radiation field modes. It may not be factorized into amplitudes representing one-site, three-mode paths. All effects of multiple vacuum modes are accounted for by theses paths without adding a Langevin noise source. The molecular information is given by a time-ordered superoperator Green's function rather than the susceptibility ${\ensuremath{\chi}}^{(2)}$ used in the semiclassical theory.
We present the first experimental demonstration of quantum enhanced detection at x-ray wavelengths.We show that x-ray pairs that are generated by spontaneous down-conversion can be used for the generation of heralded x-ray photons and directly measure the sub-Poissonian statistics of the single photons by using photon number resolving detectors.We utilize the properties of the strong time-energy correlations of the down-converted photons to demonstrate the ability to improve the visibility and the signal-to-noise ratio of an image with a small number of photons in an environment with a noise level that is higher than the signal by many orders of magnitude.A long-term goal of this work is to demonstrate x-ray photon entanglement.However, given the technical challenges, which are much greater than in the visible regime, we take a first step in demonstrating nonclassical (sub-Poisson) photon correlations.In our work, we demonstrate a new protocol for the measurement of quantum effects with x rays using advantages such as background-free measurements that the x-ray regime offers for experiments aiming at testing fundamental concepts in quantum optics.
Quantum imaging leverages correlations between pairs of photons and has the potential to obtain image information beyond what classical sources provide. Extending this approach to the X-ray regime has been limited by low photon-pair generation rates and the lack of suitable detectors. Here, we demonstrate X-ray coincidence imaging using spontaneous parametric down-conversion (SPDC) and a pixelated area detector with time- and energy-resolved capabilities. This configuration enables simultaneous detection of correlated X-ray photon pairs and coincidence-based imaging of test objects, including a biological specimen. The increased coincidence rate and spatially resolved detection establish a basis for future quantum-enhanced and low-dose X-ray imaging.