Quantum entanglement could make accelerometers and dark matter sensors more accurate

Quantum entanglement significantly improves the precision of sensors that can be used to navigate without GPS, according to a new study in Nature Photonics.

"By exploiting entanglement, we improve both measurement sensitivity and how quickly we can make the measurement," said Zheshen Zhang, associate professor of electrical and computer engineering at the University of Michigan and co-corresponding author of the study. The experiments were done at the University of Arizona, where Zhang was working at the time.

Optomechanical sensors measure forces that disturb a mechanical sensing device that moves in response. That motion is then measured with light waves. In this experiment, the sensors were membranes, which act like drum heads that vibrate after experiencing a push. Optomechanical sensors can function as accelerometers, which can be used for inertial navigation on a planet that doesn't have GPS satellites or within a building as a person navigates different floors.

Quantum entanglement could make optomechanical sensors more accurate than inertial sensors currently in use. It could also enable optomechanical sensors to look for very subtle forces, such as identifying the presence of dark matter. Dark matter is invisible matter believed to account for five times more of the mass in the universe than what we can sense with light. It would tug on the sensor with gravitational force.

Here's how entanglement improves optomechanical sensors:

Optomechanical sensors rely on two synchronized laser beams. One of them is reflected from a sensor, and any movement in the sensor changes the distance that the light travels on its way to the detector. That difference in distance traveled shows up when the second wave overlaps with the first. If the sensor is still, the two waves are perfectly aligned. But if the sensor is moving, they create an interference pattern as the peaks and troughs of their waves cancel each other out in places. That pattern reveals the size and speed of vibrations in the sensor.

Usually in interferometry systems, the further the light travels, the more accurate the system becomes. The most sensitive interferometry system on the planet, the Laser Interferometer Gravitational-Wave Observatory, sends light on 8-kilometer journeys. But that's not going to fit in a smartphone.

To enable high accuracy in miniaturized optomechanical sensors, Zhang's team explored quantum entanglement. Rather than splitting the light once so that it bounced off a sensor and a mirror, they split each beam a second time so that the light bounced off two sensors and two mirrors. Dalziel Wilson, an assistant professor of optical sciences at the University of Arizona, along with his doctoral students Aman Agrawal and Christian Pluchar, built the membrane devices. These membranes, just 100 nanometers -- or 0.0001 millimeters -- thick, move in response to very small forces.

Doubling the sensors improves the accuracy, as the membranes should be vibrating in sync with each other, but the entanglement adds an extra level of coordination. Zhang's group created the entanglement by "squeezing" the laser light. In quantum mechanical objects, such as the photons that make up light, there is a fundamental limit on how well the position and momentum of a particle can be known. Because photons are also waves, this translates to the phase of the wave (where it is in its oscillation) and its amplitude (how much energy it carries).

"Squeezing redistributes the uncertainty, so that the squeezed component is known more precisely, and the anti-squeezed component carries more of the uncertainty. We squeezed the phase because that is what we needed to know for our measurement," said Yi Xia, a recent Ph.D. graduate from Zhang's lab at the University of Arizona and co-corresponding author of the paper.

In squeezed light, the photons are more closely related to one another. Zhang contrasted what happens when the photons go through a beam splitter with cars coming to a fork in the freeway.

"You have three cars going one way and three cars going the other way. But in quantum superposition, each car goes both ways. Now the cars on the left are entangled with the cars on the right," he said.

Because the fluctuations in the two entangled beams are linked, the uncertainties in their phase measurements are correlated. As a result, with some mathematical wizardry, the team was able to get measurements that are 40% more precise than with two unentangled beams, and they can do it 60% faster. What's more, the precision and speed is expected to rise in proportion to the number of sensors.

"It is envisioned that an array of entanglement-enhanced sensors will offer orders-of-magnitude performance gain over existing sensing technology to enable the detection of particles beyond the present physical model, opening the door to a new world that is yet to be observed," said Zhang.

The team's next steps are to miniaturize the system. Already, they can put a squeezed-light source on a chip that is just half a centimeter to a side. They expect to have a prototype chip with the squeezed-light source, beam splitters, waveguides and inertial sensors within a year or two.

The study was funded by the Office of Naval Research, National Science Foundation, Department of Energy and Defense Advanced Research Projects Agency.