I'd say that all the assumptions I have made are far more likely to hold than the assumptions you need to make for quantum key sharing to be safe. That is not to say that I find the quantum key sharing technique highly likely to be broken, but many new discoveries in the field of quantum mechanics are yet to be made.

We simply can't tell what limits apply to non-observed phenomena, that doesn't mean there ain't limits, but since we can't observe non-observed phenomena it is quite hard to know their exact nature. That said, it doesn't really matter how many of them there may be, for computation a non-observed phenomena is as good as a TTL gate with no output pins, even if it is doing the right thing on the inside, we have no way of obtaining the generated data.

Quantum key sharing by the way builds on non-observed phenomena, if the so-called non-observed phenomena should somehow prove observable, and thus theoretically useful in computers, quantum key sharing would be broken.

I have already shown humility by saying 512 bits, I highly doubt that 256 bits will ever be breakable, and certainly not within a time frame where encrypted information snatched now would be useful, but the 512 bits argument is a lot easier to lead.

By the way, for anyone not totally into the concept of non-observed phenomena, it is not non-observed as in "We didn't bother to look.", but as in "It is only physically possible to retrieve one piece of data from this dataset, when we retrieve a piece of data the rest is lost, nothing has been influenced by that data, and nothing ever will be, it might as well never have existed.".

"Short" as in you can transfer it once and be done with it. Given current computers 128 bit will suffice. If you want it unbreakable for eternity use 512 bits. While it is hard to predict how fast computers will be in distant future, it is easy to tell how fast they will not be, there are simple physical limits dictating how many state changes a particle may go through in a given time, current estimates are around 10^40 per second. By that figure, a 1000 000 000 ton computer would in 1000 000 000 years only be able to undergo ~2^300 state changes, where more than 2^512 is required to break a 512 bit encryption.

As clever as the advances of OTP may seem, they are only theoretical relative to a strong key based encryption.

One time pads are not required for safe encryption, with modern computer cryptography that method is obsolete. A short key string can be used to initiate a scrambling function that generates an endless pseudo-random string. Which effectively cannot be distinguished from random without knowing the key used.

I don't get why anyone even bother with the so-called quantum encryption*, a simple pre shared key scheme is perfectly safe, a lot cheaper, well understood and well tested.

*The quantum part has nothing to do with encryption, it's just an over the top high tech attempt at preventing wire taps.

You do sounds a bit paranoid to me, but that is a problem to be solved as well.

A simple cable tester could be foiled by a hardwire tap. My best bid at checking for such a tap would be to measure impedance of each connector at various frequencies and compare those numbers to those of an identical cable. If you can get an advanced cable tester that output results as various electrical properties, then that should do the trick as well.