It is well known that, on the smallest scales, the laws of physics become very non-intuitive. Particles can tunnel through otherwise impenetrable barriers. They have no position or momentum, in the classical sense. In fact, particles can be described as waves, which exist across a region in space and can interfere with one another. These properties make little sense in classical physics, but the rules governing them—the laws of quantum mechanics—are very well known. In fact, the theory of quantum mechanics is perhaps the most tested and verified theory in all of science.
This is a model of the chip used to create the GGE. Credit: TU Wien.
Far less understood in physics are systems that bridge between the classical and quantum worlds. On the very small scale, quantum physics works well. But on larger spatial scales, classical physics works well. Scientists would like to understand the middle ground as well—the place where quantum physics gives rise to classical mechanics.
In a recent study, scientists from the Vienna University of Technology and Heidelberg University probed this in-between realm of physics. In particular, they looked at how the evolution of a quantum state gives rise to the more familiar properties of statistical mechanics (1). Statistical mechanics is a branch of physics describing systems with too many particles to keep track of individually. Properties like temperature and pressure are rooted in statistical mechanics.
For the first time, these scientists explicitly showed the creation of a system called a generalized Gibbs ensemble (GGE). A GGE is formed when an isolated quantum many-body system is excited and then allowed to relax to a state of maximum entropy (1). In statistical mechanics, when an isolated system is allowed to do so, the temperature of the system becomes uniform. This process is constrained by conserved quantities, like particle number and total energy. In the evolution of a GGE, though, there are quantities other quantities that must be conserved too (1). Describing these quantities requires a full understanding of quantum mechanics, but they create constrain the evolution of the system and create interesting and measurable properties. For example, when prepared in the right way, a GGE can have more than one temperature at the same time—the constraints placed on the GGE by these other conserved quantities prevent the system from reaching a uniform temperature (2).
To create a GGE, the scientists collected thousands of Rb-87 atoms near absolute zero on a chip (2). They then split this system into two identical systems, and used matter-wave interferometry between the two systems to extract information about the two halves (1). Matter-wave interferometry utilizes the wave-like properties of matter at the quantum scale. Scientists can get information about the system by measuring the interference patterns between the matter waves of the two halves.
Besides demonstrating some cool quantum properties, this research verifies theory regarding how large quantum systems behave. The physics of large quantum systems is difficult at best, and any positive experimental result is a step in the right direction.
References
1. Langen, T. et. al. (2015). Experimental observation of a generalized Gibbs ensemble. Science, 348, 6231, 207-211. DOI: 10.1126/science.1257026
2. Vienna University of Technology. (2015, April 9). Quantum Physics: Hot and cold at the same time.ScienceDaily. Retrieved April 10, 2015 fromhttp://www.sciencedaily.com/releases/2015/04/150409143037.htm