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Increased Fidelity via Quantum Correlated X-Rays: IF via QCX

Gary Walker

Published Mar 6, 2023• 0 citations
OpenAlex

Abstract

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

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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
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.
Quantum interference forces two indistinguishable input photons to depart a beamsplitter in the same (of two possible) spatial output modes; this is known as the Hong-Ou-Mandel (HOM) effect. It provides a quantitative way of measuring the distinguishability between two photons and also has been utilised to determine their relative temporal delays [1, 2]. HOM interferometry offers numerous advantages over its classical counterpart, most importantly it lacks any dependence on the phase of the photons and therefore can potentially provide reliable measurements in environments where the phase is unstable. To date there have been very few investigations on the precision that is achievable with HOM interferometry. Experiments with collinear geometries have yielded sensitivities as low as 0.1fs however these approaches are limited to measurements of birefringence and other polarisation dependent effects [3, 4], as the precision derives from the inherently stable shared path of the photon pairs. Typical non-shared path HOM interferometers exhibit resolution of roughly 1–3 fs (0.3–1 μm).
We show that sub-attosecond delays and sub-Angstrom optical path differences can be measured by using Hong-Ou-Mandel interference measurements with x-rays. We propose to use a system comprising a source based on spontaneous parametric down-conversion for the generation of broadband x-ray photon pairs and a multilayer-based interferometer. The correlation time of the photon pairs and the Hong-Ou-Mandel dip are shorter than 1 attosecond, hence the precision of the measurements is expected to be better than 0.1 attosecond. We anticipate that the scheme we describe in this work will lead to the development of various techniques of quantum measurements with ultra-high precision at x-ray wavelengths.
Abstract Hong–Ou–Mandel (HOM) dip from a biphoton source in a two-photon interferometer provides a myriad of quantum tools for quantum communication and sensing. But the stringent requirements for spatial coherence between the photon pair makes it prohibitively difficult to observe high-fidelity HOM dip in long-distance free-space implementations, e.g., for the photon pairs involved in quantum communication need to match the two path lengths within a few 10 s of micron because of the short coherence width of the two-photon wave-packet. While many techniques for the pathlength balancing of two interferometric arms have been studied and applied extensively, such balancing is further complicated in presence of turbulence in long-distance free-space propagation. On a table-top testbed with each arm of a Mach–Zehnder interferometer (MZI) ∼10\sim 10∼10 ∼ 10 -meter long in our turbulent atmospheric simulator for qubit (TASQ), we have employed below-threshold lasers to achieve classical low-coherence interferometry and show that we could narrow down the interrogation window for the pathlength balancing to obtain interference peak at the center. This method allowed us to reduce the interrogation distance for searching the HOM dip by four orders of magnitude, i.e., from tens of centimeters to a few tens of micron. Such pathlength balancing allowed realizing the HOM dip within $$160\,\upmu \textrm{m}$$ 160 μ m distance from the peak of the classical interference fringe. This technique could be helpful in a wide variety of long-pathlength interference-based quantum sensing.