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Journal of Applied Physics

Proposal for entangled x-ray beams

Stephen M. Durbin

Published Jun 13, 2022• 3 citations
Publisher

Abstract

A promising approach for direct detection of quantum entanglement in solid state materials is probing with entangled particles. X-rays are ideal for characterization of many materials but have not been generated as entangled pairs in useful numbers, even at the most powerful synchrotron sources. This proposal describes how x-rays in fully entangled N00N states (with N = 2) can be produced by applying Hong-Ou-Mandel interferometry at an x-ray synchrotron source. The technical requirements for indistinguishable photons, x-ray beamsplitters, and interferometer design are met with existing resources. The entangled x-ray biphotons can be filtered from the unavoidable single photon background by simple diffraction from an analyzer crystal, resulting in intense beams suitable for characterizing quantum materials.

DOI
10.1063/5.0091947

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A promising approach for direct detection of quantum entanglement in solid state materials is probing with entangled particles. X-rays are ideal for characterization of many materials but have not been generated as entangled pairs in useful numbers, even at the most powerful synchrotron sources. This proposal describes how x-rays in fully entangled N00N states (with N = 2) can be produced by applying Hong-Ou-Mandel interferometry at an x-ray synchrotron source. The technical requirements for indistinguishable photons, x-ray beamsplitters, and interferometer design are met with existing resources. The entangled x-ray biphotons can be filtered from the unavoidable single photon background by simple diffraction from an analyzer crystal, resulting in intense beams suitable for characterizing quantum materials.
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The generation and verification of entanglement is one of the big remaining challenges in x-ray quantum optics. We generalize a criterion for two-mode entanglement based on a Bell inequality, which was put forward by Johansen. We show that the experimentally simple criterion detects entanglement even for arbitrary losses behind the source. Furthermore, a beamsplitter of arbitrary reflectivity can be used to generate two-mode entanglement from a single-photon input. We investigate a range of other interesting input states and find that states similar to the single-photon state produce entanglement as well. The criterion thus provides a robust and practical benchmark for both theoretical and experimental work on single-photon entanglement. By applying a post-selection scheme, we demonstrate the importance of two-photon events in experiments, even for very low event rates. The criterion’s possible experimental implementation at x-ray energies, requiring only an interferometer and photodetectors, is discussed. We investigate Mössbauer nuclei in a thin-film cavity as an interferometer and derive a scheme to extract the first-order coherence g(1) from its Fano spectrum. Analyzing experimental data, we show that thin-film cavity systems are capable of sufficiently high coherence for the interferometry part of the criterion.
This project investigated the use of quantum-correlated x-rays (QCX) for enhanced x-ray interrogation, and Hong-Ou-Mandel (HOM) metrology was identified as a promising approach.Measurements at the Advanced Photon Source (APS) have conclusively demonstrated the production of correlated x-ray photon pairs, but the project ended before an HOM interferometer could be built. BackgroundThere is a long-standing need within the response community to be able to determine the internal characteristics of an enclosed target.Traditional x-ray techniques provide standoff capability to image the interior of the target and its contents.However, other key attributes can only be determined using methods that require physical contact with the enclosure or contents, which may not be allowable or possible.This project has explored the benefits that QCX may provide.Technical Approach
Hong-Ou-Mandel interferometry takes advantage of the quantum nature of two-photon interference to increase the resolution of precision measurements of time-delays. Relying on few-photon probe states, this approach is applicable also in cases of extremely sensible samples and it achieves attosecond (nanometer path length) scale resolution, which is relevant to cell biology and two-dimensional materials. Here, we theoretically analyze how the precision of Hong-Ou-Mandel interferometers can be significantly improved by engineering the spectral distribution of two-photon probe states. In particular, we assess the metrological power of different classes of biphoton states with non-Gaussian time-frequency spectral distributions, considering the estimation of both time-and frequency-shifts. We find that grid states, characterized by a periodic structure of peaks in the chronocyclic Wigner function, can outperform standard biphoton states in sensing applications. The considered states can be feasibly produced with atomic photon sources, bulk non-linear crystals and integrated photonic waveguide devices.