A hypothetical clarketech artifact. Such a structure would almost certainly be composed of Q-matter, but retains two fundamental mysteries:
1. A method for containing Q-Balls efficiently is unknown.
2. Q-Balls typically reflect fermions into anti-fermions, and vice versa. External transmission is not observed in quantum field theory. The means for inverting transmission and reflection coefficients remains unknown and of dubious physical realizability.
Nevertheless, the Q-Mirror meme is particularly strong, and apocryphal stories continue to exist on the Known Net, usually with one of the following variations:
1. A modosophont invents/finds/earns a perfect antimatter rocketship, capable of any relativistic velocity with any fuel. The core contains a one-way Q-Mirror.
2. A modosophont undertakes some vast cunning/diabolical maze/quest with two doors at the end. A logic riddle ensues, and the modosophont solves the correct door to discover fabulous wealth and riches on the other side (thus avoiding certain death from the other door). But "solving" the puzzle, the protagonist passes through a one-way Q-Mirror, thus trapping/defeating/killing them in the end (Never Deal with an Archai).
3. An Archailect instantiated in a binary neutron star system wishes to solve some final puzzle that requires more information density than available within millions of parsecs. The being converts half of itself into antimatter via a vast Q-Mirror, colliding itself with perfect fidelity packing maximum computational 4-volume. Many variations on what the puzzle is, and what the repercussions are of solving it.
4. Variant of #1, involving a Hider Clade and a diabolical Archai.
5. Variant of #3, involving the Fermi Paradox and sterilization of life throughout a particular volume.