Quantum optics advances atomic force microscopy: Page 2 of 3

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.
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," says Lawrie. "We employed nonlinear interferometers, which use nonlinear optical amplifiers to do the splitting and mixing to achieve classically inaccessible sensitivity."

A well-known aspect of quantum mechanics - the Heisenberg uncertainty principle = makes it impossible to define both the position and momentum of a particle with absolute certainty. A similar uncertainty relationship exists for the amplitude and phase of light, which creates a problem for sensors that rely on classical light sources like lasers.

The highest sensitivity such sensors can achieve minimizes the Heisenberg uncertainty relationship with equal uncertainty in each variable. Squeezed light sources reduce the uncertainty in one variable while increasing the uncertainty in the other variable, say the researchers, thus "squeezing" the uncertainty distribution. The sensitivity in such quantum sensors is typically limited by optical losses.

"Squeezed states are fragile quantum states," says Pooser. "In this experiment, we were able to circumvent the problem by exploiting properties of [quantum] entanglement. Because of entanglement, if we measure the power of one beam of light, it would allow us to predict the power of the other one without measuring it. Because of entanglement, these measurements are less noisy, and that provides us with a higher signal to noise ratio."

This approach to quantum microscopy, say the researchers, is broadly relevant to any optimized sensor that conventionally uses lasers for signal readout.

"For instance," says Lawrie, "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."

The researchers' interdisciplinary study is said to be the first practical application of nonlinear

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