Quantum computers use GKP states, dreamed up by Alexei Kitaev, John Preskill, and Daniel Gottesman. Their hallmark is built-in stabilizer checks that catch errors. For a long time, it was believed that a successful check guaranteed a nearly perfect state. Alas, it's an illusion.
Stabilizers are like checking a passport by its silhouette: the outline matches, and you trust the person is who they claim to be. But behind those general contours, anyone could lurk. Here it's the same: spectroscopy and photometry measurements (system response) only show an upper bound on quality. The actual similarity to the ideal could be abysmally low.
This mistake has been costly: for years, researchers relied on deceptive reliability. Now it's clear: we need direct accuracy tests or entropy analysis (a measure of chaos). The irony is that these very 'unreliable' states are key to the quantum internet of the future.
Initially, GKP states were developed for optical quantum computing, but it was they that became the foundation for the concept of the quantum internet.