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Roadmap Towards Quantum Entanglement Positron Emission Tomography (QE-PET)

This roadmap outlines the current research status, underlying physics, and detector technologies for Quantum Entanglement PET (QE-PET), a novel imaging modality that leverages the polarization entanglement of annihilation photons to potentially enhance image reconstruction and introduce new diagnostic biomarkers for tissue pathology, oxygenation, and pH mapping.

Original authors: Paweł Moskal, Shiva Abbaszadeh, Harmanjeet Singh Bilkhu, Peter Caradonna, Pragya Das, Praveen Gurunath Bharathi, Ana Marija Kožuljević, Zdenka Kuncic, Deepak Kumar, Mihael Makek, Marek Nowakowski, Sid

Published 2026-07-15

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Paweł Moskal, Shiva Abbaszadeh, Harmanjeet Singh Bilkhu, Peter Caradonna, Pragya Das, Praveen Gurunath Bharathi, Ana Marija Kožuljević, Zdenka Kuncic, Deepak Kumar, Mihael Makek, Marek Nowakowski, Siddharth Parashari, Kaustav Prasad, Gregory R. Romanchek, Sushil Sharma, Kenji Shimazoe, Ewa Stępień, Sodai Takyu, Miwako Takahashi, Mizuki Uenomachi, Ritesh Verma, Taiga Yamaya

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a pair of magical, invisible twins born from a tiny explosion inside your body. In the world of Positron Emission Tomography (PET) scans, these twins are photons—particles of light—created when a positron and an electron crash into each other. For decades, doctors have used these twins to take pictures of your insides, but they've been looking at them with "blindfolded" eyes. They only tracked where the twins went and when they arrived, ignoring a secret superpower they share: quantum entanglement.

Think of entanglement like a secret handshake. Even if the twins fly in opposite directions to the ends of the room, they still know exactly how the other is spinning. In the language of physics, their "polarization" (the direction they wiggle) is perfectly linked. If one wiggles up, the other wiggles sideways.

The Big Surprise: The Handshake Survives the Crash

For a long time, scientists believed that if one of these twins bumped into something (like a cell in your body) and bounced off—a process called Compton scattering—the secret handshake would instantly break. They thought the twins would become strangers, losing their quantum connection.

But here is the twist: Recent experiments have shown that this belief is wrong. The handshake survives! Even when one photon bounces off an electron inside your body, the two photons remain entangled. They still know how to wiggle in sync.

However, there is a catch. The paper clarifies that you cannot use this surviving handshake to filter out "bad" bounces to make the image clearer for the specific types of bounces that current PET scanners already try to filter.

Why? Current PET scanners use energy windows to ignore photons that have bounced at large angles (greater than 36°). The experiments show that for the "bad" bounces that happen at small angles (less than 36°)—the ones that slip through the scanner's filters—the entanglement stays strong.
The Result: Since the entanglement doesn't disappear for the small-angle bounces that current systems let through, we can't use the "handshake" to tell them apart from the "good" straight-line photons. The paper suggests that trying to filter out these specific scattered photons using this method won't work because the correlation remains too high.

The New Superpower: A Fingerprint of Tissue

If we can't use the handshake to clean up the noise, what can we do? The paper suggests a brand new idea: The handshake itself is a diagnostic tool.

Imagine the twins are born in different neighborhoods. In a "pure" neighborhood (like a specific type of healthy tissue), they are born with a perfect handshake (maximal entanglement). But in a "busy" neighborhood (like fatty tissue or tissue with lots of oxygen), the environment messes with their birth process. The paper found that in these environments, the twins are born with a weaker handshake (non-maximal entanglement).

The Finding: The degree of entanglement isn't always perfect. It changes depending on where the twins were born.
The Analogy: Think of it like a dance. In a quiet studio, the dancers move in perfect, synchronized steps (maximal entanglement). In a crowded, chaotic party, their steps are slightly less synchronized (non-maximal entanglement). By measuring how "in sync" the twins are, we might be able to tell if they were born in a quiet studio (healthy tissue) or a chaotic party (diseased tissue).

The Current State of the Art

Right now, this is a frontier of discovery, not a finished product.

What's Proven: Scientists have successfully built special scanners (using plastic strips and crystal blocks) that can detect these "handshakes." They have measured the entanglement and confirmed it survives scattering. They have also measured that the entanglement is not maximal (not perfect) in certain materials, suggesting it depends on the environment.
What's Simulated: The idea that we can map tissue oxygenation or pH levels using this "dance sync" is currently based on simulations and theoretical predictions. The paper says this is a "promising perspective" and an "open question," but it hasn't been fully proven in human patients yet.
The Challenge: Building a scanner that can catch these subtle quantum clues is hard. It requires detectors that can see the tiny angles of the bounce and measure the timing with incredible precision (down to picoseconds). Current scanners are like trying to catch a fly with a baseball glove; the new "Quantum Entanglement PET" (QE-PET) needs to be a high-speed camera.

The Bottom Line

The paper tells us that the "magic handshake" of quantum entanglement is real, it survives bumps inside your body, and it might hold a secret code about the type of tissue it was born in. But we are still in the early days. We have the map, but we haven't built the car to drive it yet. The goal is to turn this quantum "dance sync" into a new kind of medical super-vision that can spot diseases by how the light dances, not just where it lands.

Technical Summary: Roadmap Towards Quantum Entanglement Positron Emission Tomography (QE-PET)

Problem Statement

Conventional Positron Emission Tomography (PET) relies on detecting pairs of 511 keV annihilation photons to localize metabolic activity. However, image quality is significantly degraded by two primary factors: random coincidences (accidental pairings of photons from different annihilations) and scattered coincidences (photons that undergo Compton scattering within the patient before detection). Current filtering methods, utilizing energy and time-of-flight (TOF) windows, are insufficient to fully eliminate these noise sources, leading to reduced contrast and signal-to-noise ratios (SNR).

Furthermore, a fundamental physical property of annihilation photons—their quantum entanglement in polarization—has remained inaccessible in clinical practice. Historically, it was assumed that any Compton scattering event would instantly destroy this entanglement (decoherence), rendering the polarization correlation useless for imaging once a photon scattered. Additionally, the degree of entanglement was presumed to be maximal and constant, regardless of the tissue environment.

Methodology and Scope

This roadmap synthesizes current experimental results, theoretical frameworks, and detector development efforts to establish a new paradigm: Quantum Entanglement PET (QE-PET). The approach involves measuring the polarization correlation of annihilation photon pairs via their Compton scattering interactions within the detector.

The document outlines the development of several detector technologies and methodologies:

Plastic Scintillator Systems (J-PET): Utilizing long strips of plastic scintillators where 511 keV photons interact predominantly via Compton scattering, allowing for the reconstruction of scattering planes and the measurement of the azimuthal angle ( $\phi$ ) between them.
Crystal Scintillator Systems: Adapting pixelated crystal detectors (e.g., GAGG, LYSO) to measure polarization correlations, often employing single-layer Compton polarimeters to distinguish between true and random coincidences.
Semiconductor Systems (CZT): Leveraging the superior energy and spatial resolution of Cadmium Zinc Telluride (CZT) detectors to resolve complex Multiple Interaction Photon Events (MIPEs) and extract scattering kinematics.
Hybrid Systems: Integrating PET with Compton cameras (PET-CC) and Whole-Gamma Imaging (WGI) to utilize single-scattered photons and triple-gamma coincidences.

Theoretical work focuses on the Stokes-Mueller formalism and Kraus-type structures to model the evolution of entanglement through multiple Compton scatterings (2-Compton and 3-Compton scattering). Key metrics include the entanglement witness $R$ (the ratio of coincidence rates at specific azimuthal angles) and the scattering-angle-independent entanglement measure $C_{QE}$ .

Key Contributions and Results

1. Refutation of the Decoherence Hypothesis

A pivotal finding reported in the roadmap is the experimental demonstration that quantum entanglement is not destroyed when one photon undergoes Compton scattering, provided the scattering angle is within the energy acceptance windows of current PET systems (typically $\theta_s < 36^\circ$ ).

Result: Experiments using plastic and crystal detectors (e.g., Ivashkin et al. 2023; Bordes et al. 2024; Parashari et al. 2024) show that the polarization correlation persists after intermediate scattering.
Implication: This invalidates the previous assumption that scattering events could be filtered out solely based on the loss of entanglement. Instead, the correlation remains a viable signal for noise suppression.

2. Discovery of Non-Maximal Entanglement

Contrary to the belief that all electron-positron annihilations produce maximally entangled photons ( $C_{QE} = 1$ ), recent studies indicate that the degree of entanglement depends on the annihilation mechanism.

Result: In porous media and biological tissues, the presence of "pick-off" annihilation processes (where ortho-positronium annihilates with an external electron) introduces separable (non-entangled) photon pairs.
Data: Experimental measurements in porous polymers yielded an entanglement witness $R \approx 2.00$ , intermediate between the theoretical maximum for maximally entangled photons ( $R \approx 2.84$ ) and separable photons ( $R \approx 1.63$ ).
Implication: The degree of entanglement ( $C_{QE}$ ) varies with tissue type and oxygenation levels, suggesting it can serve as a novel diagnostic biomarker for tissue pathology (e.g., hypoxia).

3. Detector Developments and Performance

Plastic Scintillators (J-PET): Demonstrated the first full-scale QE-PET prototype. While sensitivity for double-scatter events is currently low (~3-5% of standard PET), the system successfully measured the $R$ -value and confirmed non-maximal entanglement in porous media.
Crystal Scintillators: Proof-of-concept studies with GAGG and LYSO matrices showed that single-layer detectors can measure polarization correlations. Results indicated a ~25% improvement in the Signal-to-Background Ratio (SBR) when selecting polarization-correlated events, though spatial resolution was initially degraded due to interaction order ambiguity.
Semiconductors (CZT): Systems like the UCSC 2-Panel PET demonstrated the ability to resolve Double Compton Scatter (DCSc) events. Machine learning classifiers utilizing the azimuthal angle difference ( $\Delta\phi$ ) alongside energy and position data achieved high accuracy in distinguishing true coincidences from randoms and prompt gamma leakage.
Whole-Gamma Imaging (WGI): Prototypes combining PET and Compton imaging have successfully visualized single-gamma emitters (e.g., $^{89}$ Zr) and demonstrated the feasibility of $\beta^+$ - $\gamma$ coincidence imaging (e.g., with $^{44}$ Sc).

4. Theoretical Frameworks

The roadmap details the extension of scattering cross-section theories to include multiple Compton scatterings (3-Compton scattering). Theoretical models now accurately predict the evolution of entanglement through intermediate scattering, aligning with experimental data and providing the necessary physics for Monte Carlo simulations (e.g., in Geant4) to model QE-PET systems.

Significance and Claims

The paper positions QE-PET as a transformative step in medical imaging, offering three primary avenues for advancement:

Noise Suppression: By utilizing the angular correlation of scattering planes ( $\phi$ ), QE-PET can selectively filter random coincidences, potentially improving image contrast without increasing radiation dose.
Novel Biomarkers: The degree of quantum entanglement ( $C_{QE}$ ) is proposed as a new diagnostic parameter. Since $C_{QE}$ is sensitive to the local annihilation environment (specifically the ratio of direct annihilation to pick-off processes), it offers a pathway to non-invasively map tissue oxygenation and identify pathological states like hypoxia.
pH and Radical Sensing: The roadmap highlights the feasibility of using positronium lifetime and entanglement characteristics to sense pH levels and free radical concentrations, expanding the functional capabilities of nuclear medicine beyond simple metabolic uptake.

The authors conclude that while translating these quantum measurements into routine clinical practice remains an open challenge requiring further validation of tissue-specific behaviors and improved detector sensitivity, the field has moved from theoretical possibility to experimental reality. The roadmap serves as an invitation to the scientific community to collaborate on refining detector technologies, reconstruction algorithms, and clinical protocols to realize the full potential of Quantum Entanglement PET.