Caltech quantum microscopy technique quadruples resolution using entangled photons

A team at the California Institute of Technology has demonstrated a quantum microscopy method that improves spatial resolution by roughly four times compared with a classical light microscope. The work, led by Lihong Wang, Bren Professor of Medical Engineering and Electrical Engineering, was published in Science Advances on August 6, 2026.

The technique builds on the lab's 2023 demonstration of quantum microscopy by coincidence (QMC), which used entangled photon pairs, or biphotons, to double resolution. In QMC, a signal photon passes through the sample while its entangled idler photon travels a separate path. Because the pair behaves as a single particle with twice the momentum, each photon effectively images with half the wavelength, yielding a twofold resolution improvement.

In the new configuration, the signal photon still traverses the sample once, but the idler photon is routed back through the same pair of lenses three times before detection. The researchers accomplish this by applying a magnetic field and using polarization-controlling beam splitters to direct the idler beam along a folded optical path.

The team tested the system against a standard USAF resolution target, comparing classical imaging, the earlier twofold quantum setup, and the new triple-pass arrangement. The twofold configuration improved resolution by about 1.8 times over classical imaging, while the triple-pass setup achieved an enhancement of roughly four times.

Wang describes the result as entering a new physical regime, noting that conventional understanding suggested a single photon pair could at most double resolution. He says the approach points toward possible tenfold or hundredfold improvements in the future.

The method uses much lower light intensity than conventional high-resolution imaging, reducing the risk of damage to living tissue. The researchers suggest this could enable detailed visualization of structures such as cell nuclei and mitochondria without harming them.

Potential applications also include semiconductor inspection, where quality-control imaging often relies on longer wavelengths that limit classical resolution. A fourfold gain at the same wavelengths could substantially improve defect detection in chips.

Wang acknowledges that a complete theoretical model explaining the multi-pass enhancement is still under development. The experimental work began as a hunch that faced initial skepticism within the lab before a postdoctoral researcher confirmed it worked.