Nature Using quantum entanglement, matter-wave interferometer exceeds the standard quantum limit for the first time

JILA and NIST researcher James K. Thompson has led a team of researchers who have successfully combined two of the "weirdest" features of quantum mechanics - the entanglement and delocalization of atoms - for the first time. delocalization) to create better quantum sensors. They have developed a matter-wave interferometer: this interferometer enables the sensing of acceleration with an accuracy that exceeds the standard quantum limit (the limit on experimental measurement accuracy at the quantum scale).

The research results were published in the journal Nature on October 19 under the title "Entanglement-enhanced matter-wave interferometry in high-finesse cavities" [1].

01Two experimental approaches: generating cavity quantum electrodynamic entanglement

To entangle two objects, they must usually be brought very, very close to each other so that they can interact. the Thompson group has learned how to entangle thousands to millions of atoms together, even when they are millimeters or more apart. They do this by using light that bounces between mirrors (called optical cavities), allowing information to jump between atoms and weave them into an entangled state. Using this unique light-based approach, they created and observed some of the most highly entangled states produced in atomic, photonic and solid-state systems.

The team devised two different experimental approaches, both of which report the generation of cavity quantum electrodynamic entanglement between the external momentum states of different atoms; and, both of which rely on strong collective coupling between the atoms and the optical cavity.

In the first approach, the team achieves cavity-enhanced quantum non-destructive (QND) measurements, where they premeasure the quantum noise associated with the atoms and essentially measure and subtract the quantum noise; in the second approach, the experimental team uses the cavity to mediate a single interaction between the atoms to achieve so-called uniaxial torsion (OAT) or omni-directional Ising interaction. The light injected into the cavity causes the atoms to undergo uniaxial twisting, in which the quantum noise of each atom is correlated with the quantum noise of all the other atoms so that they can conspire together to become "quieter," Thompson says. So they can hear the teacher promise them a party, but here, it's the entanglement that's doing the shushing. "

Both methods have achieved and produced up to 18.5 dB of entanglement between atomic internal states and have achieved only the enhancements directly observed in entangled microwave clocks and magnetometers.

Overview of the experiment. a) Supercooled atoms undergo guided free fall in a vertical high-fine cavity. b) Bloch spheres describe the generation of entanglement and injection into the Mach-Zehnder matter-wave interferometer.

02Matter-wave interferometer exceeds standard quantum limit accuracy for the first time

One of the most accurate and precise quantum sensors available today is the matter-wave interferometer. The principle is that one uses pulses of light to make atoms move/not move at the same time by absorbing/not absorbing laser light. This results in atoms that can be in two different places at the same time for a period of time.

As graduate student Chengyi Luo explained [2], "We irradiate the atoms with a laser beam, so we actually split the quantum wave packet of each atom in two; in other words, the particles actually exist in two different spaces at the same time. Subsequent laser pulses reverse this process and bring the quantum wave packets back together. In this way, any changes in the environment, such as acceleration or rotation, can be sensed by the measurable amount of interference occurring in the two parts of the atomic wave packet, much like the light field in an ordinary interferometer, but here it is a wave made of matter. "

JILA's team of graduate students achieves all this in an optical cavity with a highly reflective mirror. They can measure the distance atoms fall along the vertical cavity under gravity, a quantum version of Galileo's gravity experiment of dropping objects from the Leaning Tower of Pisa, but with all the precision and accuracy benefits that come with quantum mechanics.

Manipulation of matter waves in a highly detailed cavity.

exhibited sensitivity beyond the standard quantum limit (SQL).

By learning how to operate the matter-wave interferometer within an optical cavity, the research team was then able to use light-matter interactions, which create entanglement between different atoms, to make quieter and more accurate measurements of gravitational acceleration. This is the first study to find that the matter-wave interferometer exceeds the standard quantum accuracy limit set by the quantum noise of unentangled atoms.

03Entanglement as a quantum sensor: the future is limitless

Because of the improved accuracy, researchers see many future benefits of using entanglement as a resource for quantum sensors.

I think we will one day be able to introduce entanglement into matter-wave interferometers for detecting gravitational waves in space, or for dark matter searches," Thompson says. These are things that probe fundamental physics and are devices that can be used for everyday applications such as navigation or geodesy. "

With this major experimental advance, Thompson and his team also hope that others will be able to use this new entanglement interferometer approach to make other advances in physics, Thompson says optimistically: "By learning to harness and control all the spooky things we already know, maybe we can discover new spooky things about the universe -- some of which we haven't even thought of yet. "

Reference link：

[1]https://www.nature.com/articles/s41586-022-05197-9

[2]https://phys.org/news/2022-10-entangled-matter-wave-interferometer-spookiness.html