The work, led by Lihong Wang, the Bren Professor of Medical Engineering and Electrical Engineering and the Andrew and Peggy Cherng Medical Engineering Leadership Chair at Caltech, builds on the lab's 2023 demonstration of quantum microscopy by coincidence (QMC). The approach relies on one of those bizarre quantum-mechanical phenomena called entanglement, in which two particles are linked such that the state of one particle is intimately tied to the state of the other no matter how far apart they might be.

In QMC, entangled pairs of photons, called biphotons, are split so that one photon, called the signal photon, passes through the sample while its entangled partner, called the idler photon, travels a separate parallel path. In some ways, the pair behaves as a single particle that has twice the momentum of an individual photon.

According to the laws of quantum mechanics, a particle's wavelength is inversely related to the particle's momentum. That means that either one of the photons in a biphoton pair effectively images with a wavelength that is half that of the original light. And because microscope resolution improves as wavelength shrinks, this creates a twofold improvement in resolution.

Sending the idler back through

In the new setup, the team still passes a signal photon through the object to be imaged just once. However, the idler photon is routed back through the same pair of lenses three times before it reaches the detector. This is accomplished by applying a magnetic field and using optical tools, including special beam splitters that help the scientists control a property of light called polarization, which is roughly the direction in which the electric field of a light wave travels.