An entangled matter-wave interferometer — now with double the spookiness
A rendering of the entangled atoms within the interferometer. Credit: Steven Burrows, Thompson Group/JILA

JILA and NIST Fellow James K. Thompson have for the first time combined two of the "spookiest" features of quantum mechanics to make a better quantum sensor.

Einstein referred to it as creating spooky action at a distance because of the strange effect of quantum mechanics. It's hoped that quantum computers, quantum simulators and quantum sensors will haveanglement.

Delocalization is the fact that a single atom can be in multiple places at the same time. The Thompson group has combined the spookiness of bothentanglement and delocalization to realize 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 precise navigation, explore for needed natural resources, more precisely determine fundamental constants such as the fine structure and gravitational constants, look more precisely for dark matter, or maybe even detect gravitational waves.

Figuring out howentanglement works.

One must bring the objects very close to each other so they can interact. Thousands to millions of atoms can be entangled by the Thompson group. Light bouncing between mirrors allows information to jump between the atoms and create an entangled state. They have created and observed some of the most highly entangled states ever generated in any system using this unique light based approach.

Two different experimental approaches were designed by the group. They make a premeasurement of the quantum noise associated with their atoms and then subtract it from their final measurement.

In a second approach, light injected into the cavity causes the atoms to undergo one- axis twisting, a process in which the quantum noise of each atom becomes correlated with the quantum noise of all the other atoms so that they can be quieter. Thompson says that the atoms are like kids shushing each other to be quiet so they can hear about the party the teacher has promised them.

The interferometer is made of matter-waves.

The matter-wave interferometer is a quantum sensor. The idea is that one uses light to cause atoms to move in opposite directions, but not at the same time. The atoms are in two places at the same time.

In order to split each atom's quantum wave packet in two, we need to shine laser beams on the atoms. The quantum wave packets can be brought back together after a later pulse of laser light, so that any changes in the environment, such as rotation or acceleration, can be detected by a measurable amount of interference happening to the two parts of the atomic wave packet.

JILA graduate students figured out how to make all of this work inside of an optical cavity with highly reflective mirrors. In a quantum version of Galileo's experiment dropping items from the Leaning Tower of Pisa, they could measure how far the atoms fell because of gravity.

Adding to the spookyness.

The graduate students were able to take advantage of the light-matter interactions to create entanglement between the different atoms to make a quieter and more precise measurement. This is the first time that anyone has been able to observe a matter-wave interferometer with a precision that surpasses the standard quantum limit.

Thanks to the enhanced precision, researchers like Thompson see a lot of future benefits for using entanglement as a resource in quantum sensors. Thompson thinks that one day we will be able to use matter-wave interferometers for detecting dark matter searches, as well as devices that can be used for every day applications such as navigation.

Thompson and his team hope that other people will use the new entangled interferometer approach to make other advances in the field of physics. Thompson says, "With optimism, we can discover new spooky things about the universe that we haven't thought of yet."

More information: Graham P. Greve et al, Entanglement-enhanced matter-wave interferometry in a high-finesse cavity, Nature (2022). DOI: 10.1038/s41586-022-05197-9 Journal information: Nature