The team of JILA researchers and NIST fellow James K. Thompson has for the first time managed to combine two of the most “frightening” features of quantum mechanics to create a better quantum sensor: the entanglement between atoms and the delocalization of atoms.
Einstein originally referred to the entanglement of creation as a frightening action at a distance – the strange effect of quantum mechanics in which what happens to one atom somehow influences another atom elsewhere. Entanglement is at the heart of long-awaited quantum computers, quantum simulators and quantum sensors.
A second rather frightening aspect of quantum mechanics is delocalization, the fact that a single atom can be in several places at the same time. As described in their recently published article in Naturethe Thompson group combined the frightening nature of entanglement and delocalization to achieve a matter-wave interferometer capable of detecting accelerations with an accuracy that exceeds the standard quantum limit (a limit on the accuracy of an experimental measurement at a quantum level) for the first time.
By doubling the creepiness, future quantum sensors will be able to provide more accurate navigation, explore necessary natural resources, more accurately determine fundamental constants such as fine structure and gravitational constants, more accurately search for the dark matter, or maybe even one day detect gravitational waves.
To entangle two objects, you usually have to bring them very, very close to each other so that they can interact. The Thompson group has learned to entangle thousands, if not millions, of atoms, even when they are a few millimeters or more apart. They do this by using light bouncing between mirrors, called an optical cavity, to allow information to jump between atoms and bind them together in an entangled state. Using this unique light-based approach, they created and observed some of the most entangled states ever generated in any system, whether atomic, photonic, or solid state.
The group designed two distinct experimental approaches, both of which they have used in their recent work. In the first approach, called quantum measurement without demolition, they pre-measure the quantum noise associated with their atoms and simply subtract the quantum noise from their final measurement.
In a second approach, light injected into the cavity twists the atoms about an axis, a process in which the quantum noise of each atom becomes correlated with the quantum noise of all other atoms so that they can conspire together to become quieter. . “Atoms are kind of like children shutting themselves up to be quiet so they can hear about the party the professor has promised them, but here it’s the entanglement that’s doing the silence,” says Thompson .
Matter wave interferometer
One of the most precise and accurate quantum sensors today is the matter wave interferometer. The idea is that one uses pulses of light to make the atoms move and not move simultaneously by having both absorbed and not absorbed the laser light. This causes the atoms over time to be simultaneously in two different places at once.
As graduate student Chengyi Luo explains, “We shine laser beams on the atoms, so we split the quantum wave packet of each atom into two, in other words, the particle actually exists in two distinct spaces at the same time.” Subsequent pulses of laser light then reverse the process bringing the quantum wave packets closer together so that any changes in the environment, such as accelerations or rotations, can be detected by a measurable amount of interference affecting both parts of the packet of atomic waves, a bit like is done with light fields in normal interferometers, but here with de’Broglie waves, or waves made of matter.
JILA’s team of graduate students figured out how to do all of this inside an optical cavity with highly reflective mirrors. They were able to measure how far atoms traveled along the vertically oriented cavity due to gravity in a quantum version of Galileo’s gravity experiment by dropping objects from the Leaning Tower of Pisa, but with all the precision and accuracy benefits that come from quantum mechanics.
double the dread
By learning how to operate a matter-wave interferometer inside an optical cavity, the team of graduate students led by Chengyi Luo and Graham Greve were then able to take advantage of light-matter interactions to create entanglement between the different atoms to create a quieter and more accurate measurement of acceleration due to gravity. This is the first time anyone has been able to observe a matter wave interferometer with an accuracy that exceeds the standard quantum limit of accuracy set by the quantum noise of non-entangled atoms.
With the increased precision, researchers like Luo and Thompson see many future benefits for using entanglement as a resource in quantum sensors. Thompson says, “I think one day we can 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 every day application such as navigation or geodesy.”
With this major experimental breakthrough, Thompson and his team hope others will use this new entangled interferometer approach to lead to further advances in physics. Optimistically, Thompson says, “As we learn to harness and control all the scares we already know, we may be able to discover new scary things about the universe that we haven’t even thought of yet.
A quantum network of entangled atomic clocks
Graham P. Greve et al, High Finesse Cavity Entanglement Enhanced Matter Wave Interferometry, Nature (2022). DOI: 10.1038/s41586-022-05197-9
Provided by the National Institute of Standards and Technology
Quote: An entangled matter wave interferometer. Now with double the spookiness (2022, October 21) retrieved October 21, 2022 from https://phys.org/news/2022-10-entangled-matter-wave-interferometer-spookiness.html
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