Photon entanglement for life-science imaging: rethinking the limits of the possible

Quantum entanglement is a powerful resource that revolutionizes information science, opens new horizons in communication technologies, and pushes the frontiers of sensing and imaging. Whether or not the methods of quantum entanglement can be extended to life-science imaging is far from clear. Live biological systems are eluding quantum-optical probes, proving, time and time again, too lossy, too noisy, too warm, and too wet to be meaningfully studied by quantum states of light. The central difficulty that puts the main roadblock on the path toward entanglement-enhanced nonlinear bioimaging is that the two-photon absorption (TPA) of entangled photons can exceed the TPA of uncorrelated photons only at the level of incident photon flux densities as low as one photon per entanglement area per entanglement time. This fundamental limitation has long been believed to rule out even a thinnest chance for a success of bioimaging with entangled photons. Here, we show that new approaches in nonlinear and quantum optics, combined with the latest achievements in biotechnologies, open the routes toward efficient photon-entanglement-based strategies in TPA microscopy that can help confront long-standing challenges in life-science imaging. Unleashing the full potential of this approach will require, however, high throughputs of virus-construct delivery, high expression efficiencies of genetically encodable fluorescent markers, high-brightness sources of entangled photons, as well as a thoughtful entanglement engineering in time, space, pulse, and polarization modes. We demonstrate that suitably tailored nonlinear optical fibers can deliver entangled photon pairs confined to entanglement volumes many orders of magnitude smaller than the entanglement volumes attainable through spontaneous parametric down-conversion. These ultracompact modes of entangled photons are shown to enable a radical enhancement of the TPA of entangled photons, opening new avenues for quantum entanglement in life-science imaging.