Los Alamos National Lab physicist Wojciech Zurek of the Theoretical (T) Division and
his team of students, recently proved a mathematical theorem supporting quantum Darwinism
— a quantum form of natural selection. Quantum Darwinism sheds new light on
the workings of environment-induced superselection or einselection — a process proposed
quarter century ago to explain the behavior of quantum systems that are
open (that is, that continue to interact, however, weakly, with their surroundings). In
quantum Darwinism “survival of the fittest” is key.
Zurek presented the theory three years ago in order to explain how objective, classical
properties — the essence of our familiar everyday reality — emerge from a
quantum substrate of our universe. Now, the results obtained by Zurek and his coworkers
support this initial hypothesis. Before diving into the significance of the
equations, however, one must comprehend the underpinnings of the theory.
Simply put, instead of the classical world most individuals thought they were
casually viewing, people are actually observing the multiple imprints of the “most
fit” quantum states called pointer states that are made by the system on the state of
the environment. These special stable quantum states emerge from the quantum
mush to become good candidates for classical states. They can persist for a long time
without being affected by the environment. Their stability is the reason for the success
in making multiple imprints on the environment — multiple “copies of
themselves” — that we then detect.
Because of the abundance of the information about these pointer states and the
indirect nature of our observations that involve only a small part of the environment,
these states do not get “messed up.” Rather, they continue to persist and
propagate surviving multiple observations.
Zurek says that quantum Darwinism is a natural extension of decoherence, a
theory that explains how open quantum systems interacting with their environments
differ from closed, completely isolated systems.
To understand the origins of decoherence, a short history lesson is in order. “The
old way of thinking of the forefathers of quantum mechanics assumed that all systems
are isolated, and that measurement involves a direct interaction — that one
must ‘bump into the system’ to observe it. Scientists at the time did not recognize
that the environment was bumping into the system as well,” said Zurek.
Decoherence describes what happens as a result of such “measurements” carried out
by fragments of the environment. In effect, it shows that an open quantum system
ceases to respect the quantum principle of superposition, which is the key to its
“quantumness.”
Theory of decoherence — developed by Zurek and others over the past quarter century
— is now especially relevant in quantum engineering. For instance, to build a quantum computer one must make sure to limit the impact of the environment to eliminate decoherence.
But quantum Darwinism shows that decoherence is not the whole story. Zurek explains, “Recently, we realized that there was an extra twist — we never directly bump into a system to measure its state. We actually use the environment that has already bumped into the system to find out about it.”
For instance, now, when you are looking at this post, you are not interacting with the screen directly. Rather, your eyes are intercepting photons that have already interacted with thescreen. According to Zurek, this is how you observe — how you get information.
“In quantum Darwinism the environment becomes the middle man, the communication
channel through which the information is propagated from the systems to the
observer,” he said.
Another piece of the puzzle that eventually led to the culmination of what is now known as quantum Darwinism is the fact that one never observes the entirety of the environment. Instead, individuals observe merely a fraction of the environment (e.g, the tiny fraction of all photons that have interacted with this screen fall into our eyes), but still can see the same systems in the same states — all the observers get the same big picture. This means that many copies of the same information must have proliferated throughout the environment.
How did this “advertising” happen? Why is the information about some states readily proliferatingwhile data about competing alternatives (their quantum superpositions, which according to thequantum superposition principle are “equally good” when a quantum system is closed) are in effect extinct?
Zurek and his team have demonstrated in a sequence of papers that the already familiar pointer states that are distinguished by their ability to survive decoherence are the same states that advertise best — states that are easiest to find by intercepting small fragments of the environment.
This makes sense: pointer states live on. Survival is the precondition to reproduction — be it proliferation of the species or of information. And only pointer states can continue to be measured by the environment without suffering any ill effects of such inquisition (in contrast to their fragile superpositions). So, as time passes, they tend to leave a redundant, prolific and, thus, noticeable imprint on the environment.
Zurek and his team of students were able to prove the identity of pointer states and states the are easiest to find from a small fraction of the environment through a complex, yet rigorous, sequence of equations. They show, for example, how much of the environment one needs to intercept to find out all one can find out about the system without intercepting all of the environment — e.g. every last photon.
For those who are intrigued by the unique wisdom behind quantum Darwinism but still unclear as to its real-world implications, the whole idea sounds a bit intangible. However, of late, such seemingly abstract results as illustrated in the photon example above are increasingly valuable for applications.
For instance, “smaller is better” is the mantra of nanotechnology, computer hardware and other high-tech areas. And, things begin to be more susceptible to “quantum weirdness” as they get smaller. For instance, if the size of the smallest fragments of computer chips continues its downward spiral in size at a present rate (halving the size every 16 months or so — the so called “Moore’s law”) then in adecade or two researchers will have to deal with individual atoms, and, hence, individual quanta. According to Zurek, that is where quantum Darwinism enters the picture.
“Understanding what happens on the quantum-classical interface helps us prepare the necessary documents for this inevitable border crossing.”
