Biphoton Digital Holography (BDH) is an advanced imaging method merging quantum mechanics with classical optical techniques. This technology uses photons that are fundamentally linked, a property known as entanglement. By employing these quantum light sources alongside holographic detection, BDH captures a complete three-dimensional (3D) representation of a biological sample. This approach offers high fidelity and detail in viewing living systems, overcoming physical limitations that restrict the depth and clarity of conventional imaging methods in biological tissues.
Understanding the Core Technologies
Biphoton Digital Holography relies on two mechanisms: the creation of quantum light and the computational capture of light waves. The quantum light source uses Spontaneous Parametric Down-Conversion (SPDC). A single, high-energy pump photon interacts with a non-linear crystal, splitting into a pair of lower-energy photons: the signal and idler. These photons are inherently correlated or entangled, conserving energy and momentum regardless of the distance separating them.
These biphotons enable the system’s unique imaging capabilities. The signal photon is directed through the object, while its entangled partner, the idler, follows an unobstructed reference path. The image is formed by measuring the joint state of the two photons, which reveals information about the object with higher fidelity.
The second core component is digital holography, which records the full wave field of the light, capturing both amplitude (brightness) and phase (timing and path length). This phase information contains the depth and structural details of the object. A digital sensor records the interference pattern created when the light interacting with the sample is recombined with the reference light.
This recorded interference pattern, the hologram, is processed by computational algorithms to reconstruct a 3D image. This digital reconstruction allows visualization of the internal structure and surface morphology of cells and tissues with nanometric axial sensitivity. Combining quantum-correlated biphotons with this phase-sensitive digital capture allows BDH to encode more information per photon pair.
Achieving Deep Tissue Penetration
The combination of quantum light and holographic detection allows BDH to image significantly deeper into biological tissue than conventional methods. Light scattering is the primary limitation in tissue imaging, causing photons to randomly deflect off cellular structures and rapidly blurring the image as depth increases. In BDH, the quantum correlation between the signal and idler photons helps mitigate this effect.
If the signal photon scatters while passing through dense tissue, its entangled partner, the idler photon, retains a correlated state that can still be measured. This non-local measurement approach extracts spatial information that would otherwise be lost in the noisy background of scattered light. This effectively reduces noise and increases contrast deep within the sample. Capturing the phase information of the light wave is also crucial for maintaining image quality at depth.
Digital holography’s capability to measure the phase shift caused by the sample allows for quantitative phase microscopy, which relates directly to the object’s 3D structure. Unlike traditional methods where resolution degrades quickly with penetration, computational reconstruction from the hologram helps maintain high axial and lateral resolution. Due to quantum correlations, BDH can potentially image beyond the classical diffraction limit, enabling the visualization of finer details in dense environments.
BDH is also associated with reduced damage to living cells, known as phototoxicity. The biphoton generation process utilizes photons with lower energy and longer wavelengths than those used in single-photon techniques. These longer wavelengths minimize light absorption by the tissue, which is the main cause of heating and oxidative stress in living cells. This reduced phototoxicity makes BDH a non-invasive tool for observing dynamic biological processes over extended periods.
Current Applications in Biomedical Research
The depth, resolution, and non-invasive nature of Biphoton Digital Holography make it suitable for several areas of biomedical study. In neuroscience, BDH is being developed to image the structure and activity of neural circuits deep within the brain without extensive invasive procedures. The ability to penetrate hundreds of micrometers into brain tissue with cellular resolution allows for the study of complex neuronal networks and vascular structures that govern brain function.
In oncology, the technology offers a tool for the high-resolution analysis of cellular dynamics relevant to cancer research. BDH provides detailed structural information about individual cells, assisting in the analysis of tumor margins during surgical procedures. This technique can also observe the structural changes and motility of cancer cells during metastasis and proliferation, helping to identify new cellular biomarkers.
Developmental biology benefits from BDH’s utility for long-term, non-destructive observation of living organisms. Researchers use this holographic method for four-dimensional tracking of biological cells, observing embryonic development and structural changes over time. The gentle imaging process ensures that the natural progression of developmental stages is accurately recorded without inducing artifacts or stress. The precise, label-free structural information provided by phase measurements is invaluable for understanding how complex biological structures assemble and function.