Is it already a known fact that if the practical engineering challenges of quantum computing are solved that the physics of quantum computing will work?

Nothing is known until you actually do it, but I think it is fair to say that the ball is in the court of the skeptics at this point. All of the fundamental principles have been proven: qubits work, we know that certain gates form a complete set that allow for universal quantum computing, error correcting has been shown, etc. There could be some looming wall in the physics that we don't know about. For instance, if some objective-collapse theories are correct, then there would be a built-in limit to how large a system can be before it automatically decoheres. This would mean that there is a maximum limit to the number of entangled qubits you can have in a quantum computer. But so far, we haven't seen any evidence of this and we keep blowing right past all the limits that people have derived for what the maximum should be in these theories. Bottom line, there doesn't seem to be anything in the way, but we don't know what we don't know. It remains unknown how long coherence can be maintained for a large quantum state in a "non-spherical cow" real environment. Even in Faraday caged cryogenic cooled area, there will always be thermal noise, there will always be radiation, there will always be electrical noise from the gates themselves. You can minimize some of it to be sure, and error correcting can handle some of it at the cost of making the entanglement even larger and more susceptible to environmental noise. There has been limited progress on maintaining coherency and it is not known even at a theoretical level whether this can be improved sufficiently for practical large problems to be solved. All macro level quantum systems decay rapidly to a conventional state, which is what allows normal physics to work at macro scale. I don’t know what you are talking about. Error correction gives a net reduction in error rate with more qubits. That has been known in theory and shown in practice. It doesn’t make it “more susceptible to noise.” Coherence times are helpful if you can increase them but not fundamentally a barrier because you are constantly cycling in fresh qubits. As long as you can do that fast enough, which again has already been demonstrated is possible, then you can scale it arbitrarily. "You asked about practical challenges but let me assure that all fundamental principles [formulated by Mr. Feynman 70 years ago] hold strong" I do not disagree with you but this is a very distracting answer. A more honest reply is that many major practical challenges are there, they remain unsolved and nobody really knows whether they will ever be solved. Ignorant question, but wouldn't the observation of states of matter such as superfluids already demonstrate that a very large number of particles can be entangled without decohering? Superfluids are not in a coherent superposition. I'm not an expert on it, but the dynamics come from quantum effects within and between atoms locally, not at large scales. The theory backing quantum information processing is indeed perfectly sound. It would be major surprise, and a genuine novel physics discovery, if there was a physical or fundamental reason why it could not work. The problem is that the "remaining engineering challenge" is going to be more and more challenging as easy problems are solved and the systems are scaled up. How can we cool or trap 1 million qubits? How can we calibrate 1 million qubits? How can we improve qubit coherence / physical gate errors after exhausting all clever design strategies? How can we shorten error correction cycle while maintaining logical error rates (sort of clock cycle)? How can we identify rare catastrophic events and mitigate them? How can we verify correctness? The questions themselves may look like engineering problems. However, it is possible that the solutions require disruptive fundamental changes. For example, finding a new family of error correcting codes, finding better material, finding new mathematical methods, invent completely new types of qubits with much lower physical errors, etc. There’s a neat recursive aspect here: the same quantum devices that depend on advances in material science for improved coherence and lower error rates could, once scaled up, be used to simulate and design those very materials. Large-scale quantum simulation could close the loop — quantum computers helping engineer the next generation of quantum hardware. Quantum computing is definitely real, definitely works, and will work better in the future. Also, the stocks of all publicly traded quantum computing companies are all zeros Create your account and connect with a world of communities.