In a world first, researchers combine two of the 'spookiest' features of quantum mechanics

Future quantum sensors will be able to provide more precise navigation, explore for needed natural resources, more precisely determine fundamental constants, look more precisely for dark matter, or maybe someday discover gravitational waves thanks to a team of researchers led by Fellow James K. Thompson from the Joint Institute for Laboratory Astrophysics (JILA) and the National Institute of Standards and Technology (NIST).

Thompson and his team have for the first time successfully combined two of the "spookiest" features of quantum mechanics: entanglement between atoms and delocalization of atoms. By doubling down on these "spooky" features, better quantum sensors can be made.

What are these 'spooky' features?

Entanglement was originally referred to by the one and only Albert Einstein as "spooky action at a distance." This refers to an interesting effect of quantum mechanics where what happens to one atom influences another atom somewhere else. This part of quantum mechanics is crucial for the future of quantum computers, quantum simulators, and quantum sensors.

The second spooky feature refers to delocalization, which sees one single atom can be in more than one place at the same time.

In a world first, the team led by Thompson managed to successfully combine both of these spooky quantum mechanics features to create a matter-wave interferometer that can sense accelerations with a precision that surpasses the standard quantum limit.

What is a matter-wave interferometer?

A matter-wave interferometer simply means the most accurate and precise quantum sensors available to date. It works by using pulses of light to make atoms simultaneously move and not move by having both absorbed and not absorbed laser light. Over time, the atoms are in two places at once.

As graduate student Chengyi Luo explained in Phys.org, "We shine laser beams on the atoms, so we actually split each atom's quantum wave packet in two, in other words, the particle actually exists in two separate spaces at the same time."

The team managed to create this inside an optical cavity with highly-reflective mirrors. This meant the researchers could measure how far the atoms fell along the vertically-oriented cavity due to gravity, but with all the benefits of precision and accuracy that come along from quantum mechanics.

By seeing how to operate a matter-wave interferometer inside of an optical cavity means that the team, for the first time ever, has been able to take advantage of the light-matter interactions to create entanglement between the different atoms to make a quieter and more precise measurement of the acceleration due to gravity.

Useful for the future, Thompson says, "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."

The team hopes its discovery will inspire further advancements in the field of physics. In Thompson's own words, "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."

The study paper was published in the journal Nature.

Abstract:

An ensemble of atoms can operate as a quantum sensor by placing atoms in a superposition of two different states. Upon measurement of the sensor, each atom is individually projected into one of the two states. Creating quantum correlations between the atoms, that is entangling them, could lead to resolutions surpassing the standard quantum limit1,2,3 set by projections of individual atoms. Large amounts of entanglement4,5,6 involving the internal degrees of freedom of laser-cooled atomic ensembles4,5,6,7,8,9,10,11,12,13,14,15,16 have been generated in collective cavity quantum-electrodynamics systems, in which many atoms simultaneously interact with a single optical cavity mode.