A New, Better Way of Thinking About Quantum Decoherence

Trying to understand quantum mechanics isn’t for the faint hearted. It took me decades to start to think I understood the basics. But today I feel like I “get it”. I can hold my own in most discussions of quantum mechanics as long as I don’t have to start doing non-linear equations.

But if you think you have a decent understanding of the basics of quantum mechanics, here is probably the biggest epiphany I’ve had regarding it this year: It is what decoherence really means. If you don’t understand coherence vs. decoherence, I recommend you read a basic primer on quantum mechanics first. You can try mine here: https://www.dhirubhai.net/pulse/quantum-mechanics-computing-primer-roger-grimes/.

I cover this particular topic and more in my latest book called Cryptography Apocalypse (https://www.amazon.com/Cryptography-Apocalypse-Preparing-Quantum-Computing/dp/1119618193), but I gloss over the difference between how most people traditionally think of decoherence and how it is beginning to be thought of now.

A New Way of Thinking About Quantum Decoherence

The traditional school of thought (called the Copenhagen interpretation) was that when quantum particles follow quantum mechanical laws in the quantum world, they are said to be cohered, and when observed suddenly collapse (Schr?dinger's) wave function into a decohered, classical state, and will never follow quantum laws again. To be honest, that we have believed this to be true is a bit silly and explains how little we really know about quantum mechanics.

The newer thinking now is that there isn’t this sudden collapse (which is only speculated to happen to explain why Schr?dinger's quantum math suddenly stops holding true). Instead the new (and I think better) theory is that all quantum particles stay forever quantum. There is no sudden collapse and shift to the classical world.

Instead, decoherence is simply the observer effect and entanglement of the observer and/or any other quantum particles with the original particle making the original particle’s singular answer impossible (using what we know today) for us to follow after the unwanted entanglement(s). For example, suppose we have a single qubit representing a 1 as an answer in a quantum computer. When it decoheres it means it is getting prematurely entangled with other quantum particles (possibly by being observed/measured) so that it makes it very, very hard for observers to figure out what part of what they are seeing is now is from the original, singular particle and its singular answer, and what part of the answer is now due to entanglement impacts from the other entangled particles.

As each additional entanglement happens, Schr?dinger's equation must be updated for each particle now entangled with the original particle. You don’t get more Schr?dinger's equations for each additional entangled particle. You get additional variables that are now part of the one, single, original Schr?dinger's equation, which now encompasses and must be used to solve for all the entangled particles. So, if the original Schr?dinger's equation was A+B=C (it’s not that easy), then the new Schr?dinger's equation encompassing equation would be A+B+A+B…A+B), with an A+B for each entangled particle. With each additional entanglement, it becomes harder for us to figure out what part of our answer was due to the original A+B and which part is due to all the other newer, unwanted, A+B’s.

Decoherence is just our description of our own inability to track all the additional components/particles that now must be part of the original, single Schr?dinger's math equation. We can’t simply say the equation is a 0 or a 1 or a 0 and a 1, we must say that it is a 0 or a 1 or a 0 and a 1 for each additional entangled particle as they impact each other. It is like dropping a drop of purple ink into an ocean. The drop is still there, but it’s very hard to track using methods we know today.

The best description of this new way of thinking about decoherence I’ve read is in Philip Ball’s awesome Beyond Weird book and surprisingly in Sean Carroll’s Something Deeply Hidden (also a great read but for different reasons). The latter is a book defending the Many World’s Interpretation, but it covers the collapse vs. entanglement theory of decoherence using Schr?dinger's equation the best I’ve seen.

For my whole of my quantum wondering life I was waiting for some grand unifying theorem to unite quantum mechanics and classical physics. That’s because the Copenhagen Interpretation couldn’t explain its own inequities and was claiming that there was this being demarcation between quantum and the classical world. That’s silly. It always has been. The quantum world is the classical world and vice-versa. We don’t need a grand unifying theorem. We’ve always had it - Schr?dinger's equation.

Now for the blatant plug again. Nothing is free. <grin> I cover all of this and more, in far more detail, in my latest book (my 11th), called Cryptography Apocalypse (https://www.amazon.com/Cryptography-Apocalypse-Preparing-Quantum-Computing/dp/1119618193).

The chapter list is:

Part I – Quantum Computing Primer

Chap. 1- What is Quantum?

Chap. 2 – Quantum Computers

Chap. 3- How Can Quantum Computing Break Today’s Cryptography?

Chap. 4- When Will the Quantum Break Happen?

Chap. 5- What Will A Post-Quantum World Look Like

Part II - Preparing for the Quantum Break

Chap. 6- Quantum Resistant Cryptography

Chap. 7 - Quantum Cryptography

Chap. 8- Quantum Networking

Chap. 9- Preparing Now

Appendix of Quantum Information Sources

 If you want to know more about any of these subjects, consider purchasing my book.

 If you have any questions or comments, feel free to email me at [email protected] or [email protected].