Novel Fiber-Optic Device Lays Foundation for Quantum-Enhanced Measurements

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By analyzing a pattern formed by the intersection of two beams of light, researchers can capture elusive details regarding the behavior of mysterious phenomena such as gravitational waves. Creating and precisely measuring these interference patterns would not be possible without instruments called interferometers.

Image Caption: Joseph Lukens, Raphael Pooser, and Nick Peters (from left) of ORNL’s Quantum Information Science Group developed and tested a new interferometer made from highly nonlinear fiber in pursuit of improved sensitivity at the quantum scale. Credit: Carlos Jones/ORNL

For over three decades, scientists have attempted to improve the sensitivity of interferometers to better detect how the number of photons—particles that make up visible light and other forms of electromagnetic energy—leads to changes in light phases. Attempts to achieve this goal are often hampered by optical loss and noise, both of which can decrease the accuracy of interferometer measurements.

But now a team of researchers at the US Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL) has developed and tested a new interferometer to study the factors that contribute to these conditions, and they have devised solutions to overcome them. Their findings were published in the journal Applied Physics Letters, which promoted their paper to Editors’ Pick status. The editors award this distinction to noteworthy publications compiled in an exclusive list.

Most interferometers contain either a beam splitter or a parametric amplifier to divide one beam of light into two, which allows researchers to measure changes in light phases relative to one another. The ORNL team of Joseph Lukens, Raphael Pooser, and Nick Peters, however, employed a specialized version of these devices called a highly nonlinear fiber-based phase-sensitive amplifier, which classifies their creation as a nonlinear interferometer (NLI).

“The concept of interferometry involves splitting light into two modes—one senses a phase change and the other remains unchanged as a reference,” said Peters, a senior R&D staff member who leads the Quantum Communications Team at ORNL. “We want to build the most sensitive instruments allowed by quantum mechanics.”

According to Peters, this NLI demonstrates the potential for eventual quantum-enhanced sensitivity improvements in later iterations of the instrument, which could advance fundamental science, as well as improve various engineering and practical applications. Among the devices that would benefit from more precise interferometers are gyroscopes, which can help stabilize and navigate airplanes and ships, and transducers, which convert energy from one form to another. For example, hydrophones—microphones that detect sound waves underwater—convert sound waves from natural disasters and marine wildlife into electrical signals researchers can successfully interpret.

Despite the beneficial qualities of NLIs, they typically are not compatible with optical fibers used in sensors for these applications. The team hopes to bridge this gap by constructing their NLI out of a compatible material called highly nonlinear fiber.

“The real significance of our work is that this interferometer is the first built with this special kind of fiber, which is important for two reasons,” Peters said. “One is that it provides potential

for a notable sensitivity enhancement, and the other is that this fiber is commercial, which means its use could become widespread, once perfected.”

Determining how to increase the sensitivity of interferometers can be difficult because of the standard quantum limit, which restricts the accuracy of measurements at the quantum scale. Beyond this barrier is the impassable Heisenberg limit, which marks the optimal level of performance for any interferometer.

Although no one has reached this limit to date, the ORNL team consider their NLI to be an important stepping stone in their ongoing quest to get as close to the limit as possible. In the meantime, staff members in ORNL’s Quantum Information Science Group and the Quantum Sensing Team led by Pooser routinely build quantum sensors that surpass the quantum-classical boundary. They consider this work to be a significant achievement and an indication that the new NLI can exceed that boundary as well.

“The standard quantum limit is defined by the number of available photons,” Peters said. “Using a special kind of light field can help lower the amount of noise in measurements taken by an interferometer, which increases sensitivity and helps measurements inch closer to the Heisenberg limit.”

Upon deeper inspection of the interference pattern’s alternate light and dark bands of light, called bright and dark “fringes,” the researchers discovered the presence of noise cancellation at dark fringes by comparing output noise with shot-noise, which refers to the number of photons striking an object.

“We had a favorable noise scaling at this dark fringe that will help us detect data at the shot-noise limit, which is what we want to do if we’re going to be able to show a sensitivity advantage with our NLI,” Peters said.

Because this NLI demonstrates both phase-sensitive optical gain and correlated noise cancellation, the instrument provides useful guidelines for the future production of more advanced amplifiers for highly sensitive interferometers.

Currently, the researchers are planning additional experiments with modified instrumentation to verify their conclusions and better understand the exact parameters of NLIs, hoping to replicate the functionality of the current device while minimizing loss and noise. According to Peters, they will have achieved their ultimate goal when an NLI built from practical nonlinear optical fibers demonstrates a notable sensitivity advantage compared with conventional interferometers.

“These experiments suggest an improvement in the phase sensitivity of these instruments, and what we’ve done is take some steps toward reaching the Heisenberg limit,” Peters said.

The team received funding for this project from the Office of Naval Research.