How to measure the quantumness of a system

I’ve talked before about the fundamental issue of the behavior of a system near the quantum/classical regime boundary. Simple systems on a small scale are dominated by quantum mechanical characteristics. Large numbers of particles considered on a larger length scale act classically. It’s not at all clear how to think about the boundary or transition from quantum behavior to classical behavior.

Back when I was in grad school studying out of Merzbacker’s text, the suggestion was that as N (the number of particles) became large the quantum equations would naturally tend to classical behavior. This was motivated by the experience of the latter part of the 19th century when it was shown that the classical behavior of Heat described by Thermodynamics could be even better thought of in terms of statistical averaging of random behavior of large numbers of basically inert particles. As the number of particles in the ensemble grew, the odd random mesa-states where washed out by the strong classically understood states.

But it hasn’t worked quite as neatly for Quantum/Classical physics. That may be because the behavior imagined in the quantum realm is completely contradicted by the behavior in the macroscopic. At some point particles apparently cease to be non-local and transition to a localized, deterministic behavior. How? Why?

The first step will be working to better describe or even better to measure quantifiably the “quantumness” of a system. Two physicists in South Korea have published a paper that sketches out a theoretical method to do just this, and which suggests some interesting paths for investigation.

“For the past 10 years or so, physicists have been proposing various ways to define or measure macroscopic quantum superpositions. Many of these proposals start by considering the number of particles or the distance between component states involved in the superposition. Although this approach sounds reasonable, the proposals have run into problems – particularly, they have not been general enough to be applied to different types of states.

The biggest advantage of Lee and Jeong’s method of measuring macroscopic quantum superpositions is its generality, which enables it to be applied to many different types of states and allows for direct comparison between them. The method is based on the quantum interference of a given state in phase space, which is the space in which all possible states of a system are represented.

As the scientists explained, a macroscopic quantum superposition has two (or more) well-separated peaks and some oscillating patterns between them in phase space. The scientists showed that the frequency of these interference fringes reflects the size of the superposition, while the magnitude of the interference fringes relates to the degree of genuine superposition. So using this method, the scientists could simultaneously quantify both the size of the system and its degree of quantum coherence. The method also works for superpositions that are fully or partially decoherent, which occurs when macroscopic superpositions lose quantum coherence due to interactions with their environments.”

Read the full details here.

One of the biggest basic questions in Physics these days seems to be centered around this idea of how the quantum regime extends or interacts with the classical. It’s relatively simple (grin) to see how the Relativistic regime collapses to the classical but it’s not at all that way in the quantum. Getting a tool to at least start to quantify the problem is going to be a major step.

The whole situation reminds of where we were with the study of Critical Phenomenon back in the early ’80’s. Once we worked out how to measure fluctuations near the critical point in different sorts of physical systems, and starting classifying them by dimensionality, it became possible for De Gennes to have his theoretical breakthrough that led to his Nobel Prize.

Author: Nicholas Knisely

Episcopal bishop, dad, astronomer, erstwhile dancer...