To create a more effective quantum sensor, a team of researchers at JILA has, for the first time, merged two of the “spookiest” aspects of quantum mechanics: entanglement between atoms and delocalization of atoms.
Entanglement is the strange effect of quantum mechanics in which what happens to one atom somehow influences another atom somewhere else. A second rather spooky aspect of quantum mechanics is delocalization, the fact that a single atom can simultaneously be in more than one place.
In this study, researchers combined the spookiness of both entanglement and delocalization to create a matter-wave interferometer that can sense accelerations with a precision that surpasses the standard quantum limit. Future quantum sensors will be able to provide more accurate navigation, search for necessary natural resources, determine fundamental constants like the fine structure and gravitational constants more precisely, search for dark matter more precisely, and perhaps even detect gravitational waves one day by ratcheting up the spookiness.
Researchers used light bouncing between mirrors, called an optical cavity, for entanglement. This allowed information to jump between the atoms and knit them into an entangled state. Using this special light-based technique, they have produced and observed some of the most densely entangled states ever generated in any system, be it atomic, photonic, or solid-state. Using this technique, the group designed two distinct experimental approaches, which they utilized in their recent work.
In the first method, also known as a quantum nondemolition measurement, they premeasure the quantum noise linked to their atoms and then take that measurement out of the equation. The quantum noise of each atom becomes correlated with the quantum noise of all the other atoms by a process known as one-axis twisting in the second method, where light is injected into the cavity. This allows the atoms to work together to become quieter.
JILA and NIST Fellow James K. Thompson said, “The atoms are kind of like kids shushing each other to be quiet so they can hear about the party the teacher has promised them, but here it’s the entanglement that does the shushing.”
The Matter-wave Interferometer is one of the most precise and accurate quantum sensors today.
Graduate student Chengyi Luo explained, “The idea is that one uses pulses of light to cause atoms to move simultaneously and not move by having both absorbed and not absorbed laser light. This causes the atoms over time to simultaneously be in two different places at once.”
“We shine laser beams on the atoms, so we split each atom’s quantum wave packet in two, in other words, the particle exists in two separate spaces simultaneously.”
Later pulses of laser light reverse the process, bringing the quantum wave packets back together, allowing any changes in the environment, such as accelerations or rotations, to be sensed by a measurably large interference between the two components of the atomic wave packet, much like is done with light fields in conventional interferometers, but here with de Broglie waves, or waves made of matter.
The research team determined how to make this work inside an optical cavity with highly-reflective mirrors. They could measure how far the atoms fell along the vertically-oriented cavity due to gravity in a quantum version of Galileo’s gravity experiment dropping items from the Leaning Tower of Pisa, but with all the benefits of precision and accuracy that comes along from quantum mechanics.
The group of graduate students led by Chengyi Luo and Graham Greve were then able to use the entanglement created by the light-matter interactions to create a matter-wave interferometer inside an optical cavity to detect the acceleration due to gravity more quietly and accurately. This is the first instance in which a matter-wave interferometer has been observed at a level of precision that exceeds the typical quantum limit imposed by the quantum noise of unentangled atoms.
Thompson said, “Thanks to the ehanced precision, researchers like Luo and Thompson see many future benefits for utilizing entanglement as a resource in quantum sensors. I think that one day we will be able to introduce entanglement into matter-wave interferometers for detecting gravitational waves in space or for dark matter searches—things that probe fundamental physics, as well as devices that can be used for everyday applications such as navigation or geodesy.”
“With this momentous experimental advance, Thompson and his team hope that others will use this new entangled interferometer approach to lead to other advances in the field of physics. By learning to harness and control all of the spookiness we already know about, maybe we can discover new spooky things about the universe that we haven’t even thought of yet!”