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Caltech's entangled photons boost microscopy performance

04 May 2023

"Quantum microscopy by coincidence" offers route to high resolution imaging.

A project at the Caltech lab of Lihong Wang has made use of entangled photons to enhance the resolution of a quantum microscopy platform.

The researchers believe that their approach could allow super-resolution imaging at the Heisenberg limit, the point at which measurement precision cannot theoretically be improved, and without the disadvantages of existing quantum techniques operating at such exacting scales.

Quantum entanglement involves particles whose properties remain connected even though they themselves are relatively far apart. The 2022 Nobel Prize in physics went to researchers investigating how this famously "spooky" connection could be exploited in sensing and encryption.

Entanglement can also have implications for optical data transfer, potentially offering a way to avoid signal loss or interference, as per a project at Heriot-Watt University researching whether high-dimensional photon entanglement can survive noise or decay.

The Caltech project has developed a platform termed quantum microscopy by coincidence (QMC) in which a pair of entangled photons, or a "biphoton", traversing symmetric balanced optical pathlengths behaves like a single photon with half the wavelength. Reported in Nature Communications, the result can be a two-fold improvement in resolution.

In particular, QMC could alleviate some of the problems inherent in using ultraviolet illumination as a source of short wavelengths for analysis of living systems, where the energy of the UV photons damages the delicate cells being observed.

"Cells don't like UV light," said Lihong Wang. "But if we can use 400-nanometer light to image the cell and achieve the effect of 200-nanometer light, which is UV, the cells will be happy and we are getting the resolution of UV."

Nondestructive imaging of cancer cells

Caltech's platform uses a nonlinear crystal and bandpass filter to generate entangled 532-nanometer photon pairs and diverts them along two balanced optical paths, with one of the paired photons passing through the object being imaged. High-NA objectives are integrated into each arm, marking an improvement over previous implementations of similar principles in which the two arms shared the same lenses, according to the project.

The photons continue until they reach a detector, with numerical methods then comparing the wavevectors of the altered photon and its unchanged sibling to build up an image of the target object. In the world of quantum correlation, the spread of incoming signal from the photon after encountering the target is more tightly constrained thanks to entanglement with the unaltered photon than a classical system would produce, so the resolution of the microscope is enhanced.

In trials imaging first manufactured targets and then cancer cells, QMC achieved 1.4-micron resolution and a 100 by 50-square-micron field of view, along with "5 times higher speed, 2.6 times higher contrast-to-noise ratio, and 10 times more robustness to stray light," than classical signals, according to the project's paper.

Although the current set-up can't compete with contrast-to-noise ratios of classical microscopy techniques, not least because of the relatively low efficiency of biphoton creation from the nonlinear crystal, the project believes that QMC's properties will make it attractive for nondestructive bioimaging at a cellular level, with low-intensity illumination revealing details that cannot be resolved by a classical counterpart.

"We developed what we believe is a rigorous theory as well as a faster and more accurate entanglement-measurement method," said Wang. "We reached microscopic resolution and imaged cells."

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