Researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL) used quantum optics to advance state-of-the-art microscopy and illuminate a path to detecting material properties with greater sensitivity than is possible with traditional tools. Their work is the first practical application of nonlinear interferometry.
Unlike today’s classical microscopes, Pooser and Lawrie’s 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,” Pooser said. “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.” He added, “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 is a miniature diving board that methodically scans a sample and bends when it senses physical changes. Lawrie and Pooser showed that the quantum microscope they invented could measure the displacement of a microcantilever with 50% better sensitivity than is classically possible.
“Squeezed light sources have been used to provide quantum-enhanced sensitivity for the detection of gravitational waves generated by black hole mergers,” Pooser said. “Our work is helping to translate these quantum sensors from the cosmological scale to the nanoscale.”
Their 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, meaning trough amplitudes subtract from peak amplitudes to make the resulting wave smaller. This effect can be seen in waves in a pond or in an electromagnetic wave of light like a laser.
“Interferometers split and then mix two light beams to measure small changes in phase that affect the interference of the two beams when they are recombined,” Lawrie said. “We employed nonlinear interferometers, which use nonlinear optical amplifiers to do the splitting and mixing to achieve classically inaccessible sensitivity.”
ORNL’s approach to quantum microscopy is broadly relevant to any optimized sensor that conventionally uses lasers for signal readout. “For instance, conventional interferometers could be replaced by nonlinear interferometry to achieve quantum-enhanced sensitivity for biochemical sensing, dark matter detection or the characterization of magnetic properties of materials,” Lawrie said.
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Image – ORNL researchers developed a quantum or squeezed, light approach to atomic force microscopy, which enables measurement of signals otherwise buried by noise. Courtesy of Raphael Pooser/ORNL, U.S. Dept. of Energy.
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