Quantum optics advances atomic force microscopy

September 11, 2020 // By Rich Pell
Quantum optics advances atomic force microscopy
Researchers at the Department of Energy's Oak Ridge National Laboratory (ORNL) say they have developed a quantum - or "squeezed" - light approach for atomic force microscopy that enables measurement of signals otherwise buried by noise.

Using quantum optics to advance state-of-the-art microscopy illuminates a path to detecting material properties with greater sensitivity than is possible with traditional tools, say the researchers.

"We showed how to use squeezed light - a workhorse of quantum information science - as a practical resource for microscopy," says Ben Lawrie of ORNL's Materials Science and Technology Division, who led the research with Raphael Pooser of ORNL's Computational Sciences and Engineering Division. "We measured the displacement of an atomic force microscope microcantilever with sensitivity better than the standard quantum limit."

The researchers' quantum microscope requires quantum theory to describe its sensitivity. The nonlinear amplifiers in ORNL's microscope generate a special quantum light source known as squeezed light.

"Imagine a blurry picture," says Pooser. "It's noisy and some fine details are hidden. Classical, noisy light prevents you from seeing those details. A 'squeezed' version is less blurry and reveals fine details that we couldn't see before because of the noise. We can use a squeezed light source instead of a laser to reduce the noise in our sensor readout."

The microcantilever of an atomic force microscope methodically scans a sample and bends when it senses physical changes. The researchers showed that the quantum microscope they invented could measure the displacement of a microcantilever with 50% better sensitivity than is classically possible. For one-second-long measurements, the quantum-enhanced sensitivity was 1.7 femtometers - about twice the diameter of a carbon nucleus.

"Squeezed light sources have been used to provide quantum-enhanced sensitivity for the detection of gravitational waves generated by black hole mergers," says Pooser. "Our work is helping to translate these quantum sensors from the cosmological scale to the nanoscale."

The researchers' approach to quantum microscopy relies on control of waves of light. When waves combine, they can interfere constructively, meaning the amplitudes of peaks add to make the resulting wave bigger, or they can interfere destructively, where trough amplitudes subtract from peak amplitudes

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