Lately, I’ve been trying to gain a better understanding of quantum decoherence. This is the process of a quantum system in superposition interacting with the environment and, as a result, appearing to lose its quantum nature, notably by having interference between the elements of its superposition become undetectable.
Decoherence is often used synonymously with the wave function collapse (including by me in older posts), but they’re generally acknowledged to not be the same thing. Decoherence explains the disappearance of the interference between the superposition states, but it doesn’t address how or why those multiple states appear to collapse or reduce to one, if in fact they do. The ultimate fate of those states is why we need interpretations such as Copenhagen, many-worlds, etc.
One of the struggles I’ve had is that most popular descriptions of decoherence are a bit shallow, and more meaty treatments tend to quickly fall into dense mathematical formalism, usually ending my ability to follow the discussion. Often these accounts discuss how decoherence is a quantum system becoming entangled with its environment.
While that’s true, the jumble of terms led me down a wrong path, assuming an equivalence between coherence and entanglement which doesn’t exist. As a result, I’ve thrown around the word “coherent” incorrectly in both posts and conversations, most recently on my post on quantum nonlocality.
While rooting around online for a good explanation, I stumbled on a post by Chad Orzel noting the effort he had put into the explanation of decoherence in his book, How to Teach Quantum Physics to Your Dog. I bought this book several years ago, and had read his description before, but not with my current interest. As it turns out, Orzel’s description of decoherence is the best popular account I’ve come across. Highly recommended! (Though you would have to endure dog and rabbit analogies.)
Orzel uses an interferometer with beam splitters and mirrors for his explanation. But I think most people are more familiar with the double slit experiment. (If you’re not familiar with it, Jim Al-Khalili does an excellent nine minute primer.)
The interference that takes place in this experiment happens because the components of the wave, including the parts that make it through both the top and bottom slit, are coherent with each other, that is, they have a phase relation. That’s a fancy way of saying that the timing is right. The wave propagates everywhere at the same time, so it interferes with itself in the right way to produce the diffraction pattern on the back screen.
Decoherence, the loss of quantum coherence, is simply the timing getting screwed up. This can happen by putting a detector at one of the slits, which uses a magnetic field to detect the particle, slightly delaying propagation of that part of the wave and throwing the timing off.
We might accomplish a similar effect by increasing the distances between the particle gun, the first screen, and the second screen, so that the wave interacts with more air molecules in its journey, causing random fluctuations in the propagation of its components. It can also be induced to smaller or greater extents by having the wave pass through perpendicular electromagnetic waves of varying intensity before reaching the screen.
Surprisingly (at least to me), decoherence does not end the interference. The coherency of the wave causes the interference to form noticeable and detectable patterns. When the timing gets messed up, those detectable patterns are disrupted. But the interference itself remains. It’s just fragmented and isolated now and, depending on just how decohered the wave has become, much harder to detect. (It is possible in principle, although it would require knowledge of all the microstates in whatever part of the environment was involved up to that point, so not something happening anytime soon.)
This description of decoherence makes sense, and seems to drain much of the mystery. Decoherence is generally irreversible, but it’s an irreversibility in practice due to the enormous number of moving parts. In other words, while effectively impossible to reverse, it is possible in principle. In fact, if not allowed to progress too far, it actually can be reversed, that is, recohered, as a 2015 experiment managed to do.
So, in the quantum nonlocality post, when discussing a “world” under the many-worlds interpretation, I should have stuck to entanglement language. In that sense, a “world” is a state or branch in the composite superposition of an entanglement network, an entity which is nonseparable and therefore has nonlocal states, even if it doesn’t have nonlocal dynamics.
Coherence itself only refers to the state of a wave function that allows the elements of its superposition to interfere with each other. Once the wave is decohered, its role is over. The term “decoherence” often seems used to refer to the sustained wave of entanglement into the environment, but in that broader sense, coherence itself is no longer a factor. Sorry for propagating my confusion on this.
The learning continues. Any pointers would be much appreciated.