With its suspended metallic spheres that clack back and forth, Newton’s cradle is more than a popular desktop plaything. It has taught a generation of students about conservation of momentum and energy. It is also the inspiration for an experiment Benjamin Lev, associate professor of physics and of applied physics at Stanford University, has created to study quantum systems.
Lev and his group built their own quantum version of Newton’s cradle in order to answer questions about how the chaotic motion of quantum particles eventually leads to thermal equilibrium in a process called thermalization. Answering how this occurs in quantum systems could help in developing quantum computers, sensors and devices, which Lev characterizes as a “quantum engineering revolution.”
“If we want to be able to create devices that are robust and useful, we need to understand how quantum systems behave out of equilibrium – when they’re kicked, like the Newton’s cradle – at a level as fundamental as we understand that for classical systems,” Lev said.
With the cradle, the researchers observed for the first time how, after inducing small amounts of chaotic motion, a quantum system reaches thermal equilibrium. They published their findings May 2 in Physical Review X.
The results of these experiments, which did not fit previous predictions, have led to a theory of how this process works in quantum systems.
Extremely cold, strongly magnetic
The turbulent swirl of milk as it’s added to coffee is a familiar example of chaos in the non-quantum world. Over time, the coffee mixture becomes homogenous and, therefore, reaches equilibrium. What the Lev lab wanted to know is how this evolution occurs in quantum systems after they induce just a touch of chaos. Through experiments with their cradle, the researchers were the first to observe this process as it happened.
The Lev lab’s quantum Newton’s cradle is different from anything you’ve seen in your co-worker’s cubicle. The researchers shine laser beams through an airtight chamber to cool a gas of atoms down to nearly absolute zero – one of the coldest known gases in the universe – and then they load those atoms into an array of laser tubes that act as the structure for the Newton’s cradle. Each of the 700 parallel cradles contains around 50 atoms in a row. Then, another laser kicks the atoms, starting the movement of the cradle.
Unlike a previous quantum Newton’s cradle developed by David Weiss at Penn State, where weakly magnetic atoms took the place of the cradle’s metal spheres, the Lev lab’s cradle includes strongly magnetic atoms.