Can Schrödinger's Astronaut act as a quantum computer?

Question

I am trying to reconcile Schrödinger's cat vs. quantum computing at the intuitive level. We all know that in the Schrödinger's cat thought experiment, the cat exists in a superposition of states, but once the box is opened, we see no evidence any quantum hanky-panky ever happened. We also know that quantum computing is expected to accomplish tasks that no ordinary computer can match, and this is vitally dependent on preventing decoherence (let alone collapse) of the qubits.

I'd like to form a simple visual image of how Schrödinger's cat does a math problem. Let's not use a literal cat - let's make him an android (feloid?) capable of taking orders and working advanced math, but lacking any "consciousness" that could be taken to have QM connotations. We'll send him up in a space capsule well sealed and insulated to most electromagnetic radiation. I suppose hard X-rays will still pass through the astronaut to the Earth - will that let the cat out of the bag regarding what state he is in? I imagine feloids are immune to cyanide, so we'll use the radioactive random number generator as a seed to generate random numbers to test as factors for the public key of a major cryptocurrency holding, which seems more practical anyway.

Now the feloid works for a predetermined time. If he finds no answer, he sends no message. But if he has gotten the answer, he turns on the radio transmitter and blasts out the secret key at the agreed upon hour. The world gets the average broadcast of all these eureka rich kitties. I've been assured that even if the shielding is perfect, this won't really work, but I am afraid I still don't truly understand the reason. How would it work to be consistent with the principles of an actual quantum computer?

Note: there are several superficially similar questions on this site, but they do not address how the computation is actually done.

Answer 1

Trying to use Schrödinger's cat as an intuitive instrument to understand quantum mechanics is not a very good strategy. Schrödinger's cat is not meant to illustrate how QM works, but meant as a counterexample, illustrating how QM is dumb and couldn't possibly work. Indeed, a cat is never in a quantum state, observed or not, since macroscopic objects decohere on their own / "collapse themselves".

While your image is very charming, it is difficult to untangle what exactly is wrong with it.

But let me try to guide you in a different direction. My preferred "intuitive" approach to quantum computing is to think of a computer as a physics experiment. We know that the physics in the experiment obey the laws of physics. If we have a logical problem, we know that the solution follows the laws of logic. By mapping the laws of logic onto the laws of physics we can run the experiment, and the result is the answer to our problem.

A classical computer does this by running a physics experiment with electricity, onto which we can map logic very well through circuits. The quantum computer is an attempt to map the logic onto some other physics. Since it is the same logic we are using, the two machines can answer the same questions. However, some questions are very inefficient to map to an electric circuit, and much more efficient to map to a quantum system. An obvious question is "what is the ground state energy of this molecule", which in a classical computer requires us to make a bunch of circuitry act like a quantum system. But in a quantum computer, we can map the problem much more directly unto the physics, since the qubit and the molecule obey the same laws.

Answer 2

Now the feloid works for a predetermined time. If he finds no answer, he sends no message. But if he has gotten the answer, he turns on the radio transmitter and blasts out the secret key at the agreed upon hour. The world gets the average broadcast of all these eureka rich kitties.

Then the average broadcast is measured to be a broadcast from one kitty. The broadcast collapses when measured.