Squeezed Light to Improve Microscopy

Researchers at the Department of Energy’s Oak Ridge National Laboratory 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. “We showed how to use squeezed light – a workhorse of quantum information science – as a practical resource for micro­scopy,” said 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.”

Researchers developed a quantum, or squeezed, light approach for atomic force microscopy that enables measurement of signals otherwise buried by noise. (Source: R. Pooser, ORNL / DOE)

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. With student interns Nick Savino, Emma Batson, Jeff Garcia and Jacob Beckey, Lawrie and Pooser showed that the quantum micro­scope they invented could measure the displace­ment of a micro­cantilever with fifty percent 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,” 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.” The inter­disciplinary study is the first practical appli­cation of nonlinear inter­ferometry. 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.

That fact creates a problem for sensors that rely on classical light sources like lasers: The highest sensi­tivity they 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, thus squeezing the uncer­tainty distri­bution. For that reason, the scientific community has used squeezing to study phenomena both great and small.

The sensitivity in such quantum sensors is typically limited by optical losses. “Squeezed states are fragile quantum states,” Pooser said. “In this experiment, we were able to circumvent the problem by exploiting proper­ties of entanglement.” Entanglement means inde­pendent objects behaving as one. Einstein called it “spooky action at a distance.” In this case, the intensities of the light beams are correlated with each other at the quantum level.

“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,” he continued. “Because of ent­anglement, these measurements are less noisy, and that provides us with a higher signal to noise ratio.” 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. (Source: ORNL)

Reference: R. C. Pooser et al.: Truncated Nonlinear Interferometry for Quantum-Enhanced Atomic Force Microscopy, Phys. Rev. Lett. 124, 230504 (2020); DOI: 10.1103/PhysRevLett.124.230504

Link: Quantum Information Science, Oak Ridge National Laboratory, Oak Ridge, USA

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