Seeing clearly with entangled biphotons

Entangled photons are a cornerstone of quantum information science, enabling secure communication, enhanced sensing, and novel imaging approaches. Among their many applications, quantum imaging^1–4 is particularly notable for its ability to utilize the spatial correlations of photon pairs to achieve effects that classical optics cannot. Well-known examples include ghost imaging⁵, imaging with undetected photons⁶, and interaction-free imaging^7–9, all of which rely on the peculiar correlations between entangled photons to extract information about objects in unconventional ways. These techniques support applications ranging from fundamental quantum tests¹⁰ to low-light biological imaging^11,12 and pattern recognition^13,14. However, a persistent challenge remains: phase aberrations introduced by atmospheric turbulence or imperfections in optical elements degrade image quality in quantum imaging systems.

In the classical setting, adaptive optics provides a powerful remedy for wavefront correction. A representative example is Shack-Hartmann wavefront sensing¹⁵, in which a microlens array measures phase distortions induced by turbulence or optical imperfections, enabling their subsequent correction using spatial light modulators or deformable mirrors. Sensorless approaches¹⁶ utilize the zero-frequency component of the beam as a feedback metric to optimize the correction phase until this component reaches its maximum.

In quantum imaging, efforts to counteract phase distortions in entangled photons have included biphoton holography using polarization entanglement¹⁷ or a reference beam¹⁸, as well as strategies that use classical light to infer aberrations by leveraging the consistency between the propagation of the pump beam and the entangled biphoton correlations^19,20. Researchers have also explored adapting classical adaptive optics to the realm of quantum imaging^21,22. A particularly creative approach was demonstrated by Cameron et al., who used the sharpness of the biphoton centroid distribution at the Fourier plane as the metric for phase correction, thereby realizing label-free adaptive quantum imaging²³.

Recently, Yi Zheng et al. conducted a new study to introduce an elegant solution: a position-correlated biphoton Shack-Hartmann wavefront sensor (PCB-SHWS)²⁴. As illustrated in Fig. 1, this technique achieves single-shot measurement of the phase gradients experienced by position-correlated entangled photon pairs as they traverse turbulence. A compensating phase is then applied via a spatial light modulator to correct phase gradients induced by imperfect optics or turbulent media, enabling clear quantum imaging even under adverse conditions. This development is significant for two reasons. First, it brings a cornerstone tool of classical adaptive optics, the Shack-Hartmann sensor, into the quantum domain, bridging conceptual and technical gaps. Second, by enabling direct and efficient phase measurement of entangled photons, it paves the way for robust quantum imaging systems capable of operating in realistic, aberration-prone environments.

Fig. 1  Schematics of the proposed PCB-SHWS for quantum adaptive imaging. (Left) Schematic of turbulence phase measurement. Owing to phase aberrations, entangled photon pairs (represented by spheres) passing through a microlens array (MLA) exhibit centroid deviations, which are recorded by an electron-multiplying charge-coupled device (EMCCD) camera. The phase gradient is then retrieved from the recorded photon distribution. (Middle) Measured phase gradient of a plastic film. (Right) Phase aberrations induced by the film are compensated. Measured images reproduced from Ref. 24.

From biomedical microscopy to free-space quantum communication, the capability to maintain clear imaging with entangled photons in the presence of phase disturbances may accelerate the transition of quantum imaging from laboratory demonstrations to practical applications. This work illustrates a broader trend in quantum technologies: adapting classical techniques in ways that respect and leverage unique quantum resources. In the future, as researchers continue to refine tools such as the PCB-SHWS, the prospect of practical, high-resolution, and noise-resilient quantum imaging will increasingly come into focus.