The weird laws of quantum mechanics set a Standard Quantum Limit on our ability to measure things. In the case of atoms, physicists often visualize this quantum fuzziness as an uncertainty blob (orange above) indicating the range of possible directions in which an arrow may point in space. Quantum mechanics dictates that you can only measure one thing at a time, say where the arrow is pointing along North-South or East-West, but not both. A quantum measurement can be visualized as squeezing the uncertainty out of the North-South direction, and into the East-West direction, much like what happens when you squeeze a water balloon.
The squeezed state acts as a sharper arrow for more precise measurements, just like a sharper arrow on a watch would allow you to more precisely time how fast an athlete runs a race. So? The Global Positioning System, mineral and resource exploration, and our understanding of earth geology and the rules of the universe all might be improved by making sharper quantum arrows.
To generate a squeezed state, we make a measurement that also does not destroy the quantum state of the atoms (i.e. collapsing them into North or South in physics speak). We then use the knowledge gained to cancel the original quantum fuzziness--a process known as conditioning. The squeezed state is an entangled state in which the atoms are no longer independent objects, but are now correlated such that their individual quantum fuzzinesses conspire to cancel in one direction, but add in another direction.
We have recently demonstrated the creation of squeezed states with uncertainties below the Standard Quantum Limit. We use lasers and a vacuum chamber to cool and trap a million Rubidium atoms between two mirrors. We then bounce laser light back and forth between the mirrors nearly a thousand times. As the light passes through the atoms, it is modified by the degree to which the atom arrows are pointing North or South.
By measuring the modification to the light, we essentially determine where the arrows are pointing along the North-South direction. Because the light passes through all the atoms many times, we cannot tell which atom caused the light to be modified--this is crucial to not destroying the quantum state, and so we call this a coherence-preserving quantum nondemolition measurement. This is very different from (and much more stringent than) the typical quantum nondemolition measurements that are routinely used for quantum computing. Using the information flowing out of our detector, we can squeeze the quantum uncertainty out of the North-South direction and into the less important East-West direction.
Most recently, we achieved a factor of 60 improvement over the Standard Quantum Limit using laser-cooled rubidium atoms, and also demonstrated the basic concepts for creating squeezing in optical lattice clocks based on strontium atoms.