Twisting Atoms to Push Quantum Limits

The chaos within a black hole scrambles information. Gravity tugs on time in tiny, discrete steps. A phantom-like presence pervades our universe, yet evades detection. These intangible phenomena may seem like mere conjectures of science fiction, but in reality, experimental comprehension is not far, in neither time nor space.

Astronomical advances in quantum simulators and quantum sensors will likely be made within the decade, and the leading experiments for black holes, gravitons, and dark matter will be not in space, but in basements – sitting on tables, in a black room lit only by lasers.

These experiments, generally called quantum precision measurements, are leading the forefront of our fundamental understanding of the universe. These experiments use QIST (Quantum Information Science and Technology) phenomena to study and harness the quantum behavior of atoms, ions, and molecules, yet can fit within a food truck. Their power lies in their precision, or more colloquially, their sensitivity. Current quantum precision experiments, such as JILA’s atomic clock, have sensitivities of nearly 10^-19[1], and with only a little more^[2], this clock will be capable of detecting gravitational waves caused by colliding black holes, or measuring gravity’s influence on time. There’s only one problem: we already hit the limit of quantum precision.

The limit of quantum precision is defined by the Standard Quantum Limit, or SQL for short. Inherent to quantum measurements, SQL defines the inevitable quantum noise that arises from wave function collapse. This noise is not unlike the noise in a coin toss experiment, where more throws better estimate equal outcomes of heads and tails.

But according to JILA Fellows Ana Maria Rey and James Thompson, unordinary quantum experiments could access precisions beyond the standard limit. Quantum noise can be evaded, Thompson explained, by harnessing quantum entanglement. “When two atoms are entangled, the quantum noise of one atom cancels the quantum noise of another atom.”

Atoms can become entangled by undergoing a coherent interaction called spin exchange. “The physics [behind spin exchange] is very easy,” explained Rey, as she described two atoms exchanging a photon to swap energy quantified by spin. When the atoms swap spins, they become entangled, meaning measuring the spin of one atom gives immediate information about the spin of the second atom. If two coins could be entangled, for example, then one coin landing heads would force the second coin to land tails.

The physics of spin exchange may be easy to understand, but the physical feat is anything but easy for the atoms. Two atoms may be many centimeters apart when they exchange spins. For these tiny atoms, that is the spatial equivalent of two humans discussing a coin toss while one is in Europe, and the other on the far side of the sun.

To spur long-distance relationships between two atoms, researchers in the Thompson lab placed tens of thousands of Strontium atoms between two mirrors. The scene then plays out like an apathetic game of catch: Bored of standing around, a random atom tosses a photon into the void. The mirrors then bounce that photon back and forth, back and forth, beating on, borne back ceaselessly into the – boom! A second atom steps up and catches the photon. And just like that, the two atoms, tosser and catcher, swap spins.

The group confirmed spin exchange interactions were indeed occurring by witnessing one-axis twisting. According to Rey, if a circle represents standard quantum noise, then one-axis twisting is “the circle sheering into an ellipse.” By redistributing the quantum noise along one dimension, it is reduced along the other. This one-axis twisting is a first step towards spin squeezing, added Thompson, which is a type of entanglement.

And the group was further pleased when they saw an “energy gap” emerge. An energy gap is like a penalty: atoms that misalign must pay for their rebellion with an energy fee. Thompson said the energy gap is generated by an effective magnetic field that can be sourced to the strong atom alignment. In other words, the aligning atoms encourage all others to align. “They create a kind of peer pressure,” said Thompson, “it’s hard to go against the crowd.”

Because the energy gap encourages atoms to maintain their spins, both Rey and Thompson believe the gap could protect entangled states from becoming disentangled. But the group has yet to observe this protection, as they need to first generate entangled states.

This will happen soon, promised Rey. “We have measured this gap. We have shown that this gap exists. And therefore, for the future, when we can generate actual entanglement, it is going to be useful because it is going to be able to protect our entangled states for longer times.”

This work was published in Science on 19 July 2018. JILA Fellows Dr. Ana Maria Rey and Dr. James Thompson completed this research with help from JILA postdoctoral researchers Dr. Matthew Norcia and Dr. Robert Lewis-Swan, former JILA graduate student Dr. Bihui Zhu, and current JILA graduate student Julia Cline.

Written by Catherine Klauss

^[1] With a current precision of 2.5 x 10-19, the atomic clock loses less than a second within the age of the universe.

^[2] About two orders of magnitude more precision.