Why quantum logic must be reversible
Quantum computation must use only reversible logic gates because quantum systems evolve according to Schrödinger's equation, which is unitary and deterministic until the moment of measurement. Dissipating heat mid-computation would destroy the quantum state, so every operation before the final measurement must be reversible. The one irreversible act in quantum mechanics is observation itself, the moment you open the box and collapse the superposition. This constraint sharply distinguishes quantum systems from classical ones, where irreversibility is routine, and raises the question of whether useful computation under reversibility is even possible.
- Krishna briefly misstates the rule, saying 'you cannot have reversible logic' before immediately correcting himself, underscoring how counterintuitive the reversibility requirement is.
- The hosts use a poached egg as an analogy: poking the yolk while it cooks ruins it, just as an ill-timed irreversible operation destroys the quantum state being computed on.
Transcript
This chapter, from the episode video's captions · 696 words
28:28quantum magic that happens with the entangling that stuff that I was talking about with Bell's theorem and like you know these two states are entangled. Now um in order to keep the quantum state sort of happy, you better not heat it up. You better not let random other crap go in. But if you're dissipating heat in the middle of your computation, that is going to destroy all of the quantum mechanics of your system, right? >> Because your your your quantum system almost has to maintain this single Lego block state. >> Mhm. >> And it can't be messed around with >> because then that fundamentally changes. >> Yeah. >> The quantum magic of the system. >> Exactly. Yeah. It's like um the only
29:09part and this is the next fundamental thing. The only part of quantum mechanics that is irreversible meaning you can't go backwards is the opening of the box or the measurement the observation >> the observation >> right the observation is the thing where you can't go back before observing quantum mechanics is like kind of deterministic because Schroinger's equation just like tells you how the quantum states are going to evolve. It's only when you open the box and find out if the cat is dead or alive is the cat dead or alive. Otherwise there's like you know states where the cat is dead and the cat is alive and there's a complex amplitude that is attached to both and they both go forward like Schroinger's equations. This this means that quantum mechanics has unitary
29:50evolution. And so if I want to do computation with quantum mechanics I cannot have reversible logic which is kind of key. >> Okay. >> You cannot have reversible logic. >> Yeah. Wait, did I say that right? Um, no, you can only have reversible logic. Yeah, good, good, good catch. Right. Because the point is a system has to basically continuously maintain >> like like basically until the point of observation, which you want to be able to decide when that happens. >> You want to just be able to have this system >> operate in this state, which means it needs to be able to traverse between >> Yeah. >> all of the quantity >> and I need to be able to save like all
30:30of my progress in some sense, you know? >> Okay. Yeah. Yeah, I I I I get the point that you're saying, which is like the benefit of a quantum system is that um >> you don't mess with it until the end, >> right? Everything is reversible. >> Yeah. You don't Yeah. Yeah. All of the quantum magic stays quantum magic and you're like messing with the quantum magic, but now you have to do it in a >> irreversible way, right? You have to obey the laws of Schroinger's equations and like unitary dynamics and like poke it in a very specific way that you're not like destroying the thing that is giving you all the magic. We have a little poached egg. >> We don't want to poke the yolk. >> Yes. >> While it's cooking, >> right? And then ruin our poached egg or
31:12sule or any other kind of cooking analogy. Um I think that's a really interesting point though because now we're sort of helping to define the fundamentals differences between a classical and classical and quantum systems such that we are going to build to this reason of why quantum systems give us these interesting other things we can do. >> Yeah. >> That fundamentally because classical systems are irreversible. >> Uh you cannot get out of a classical system no matter how much power compute. Yeah. >> How well you dissipate the heat, none of that matters. >> None of that matters. Okay. In order to work with the quantum magic, you got to you got to work in the constraints of
31:52quantum mechanics and equations >> or Heisenberg if you're in that camp, right? So, um it's unclear whether this is even possible though, right? >> Like can you compute >> with reversible logic, right? because
From How Quantum Computing Actually Works (Part 1)
Part I of our quantum computing deep dive traces the field from Bell and Feynman to Deutsch and Shor—and explains what quantum computers actually do differently from classical machines.