Schrödinger’s cat famously exists in a superposition of dead and alive, but only while the box is closed. Why are we not allowed to view this superposition when we open the box?
Oversimplified: quantum systems are fragile. Opening the box (“measuring”) disturbs the cat, forcing it into one state. What you don’t usually hear, is that the box has a label on it. This label has a mathematical expression written on it, called a “wave function,” which gives us the probabilities that we’ll find the cat in one state or another. No matter how many states a wave function says a quantum system might have, it is forced into one state by an observation – an act of measurement.
This seems to contradict Schrödinger’s equation, which predicts that probabilities will evolve and that superpositions will be preserved, but decoherence is a natural consequence of the equation. A measuring apparatus interacts with a quantum system, ending its isolation. Like opening a refrigerator door, the cold – the quantum information – leaks out into the environment. The wave function “collapses,” the superposition is destroyed, and we can no longer access the information that was leaked. You can’t get the cold air back into the refrigerator. Mathematically speaking, from the quantum system’s point-of-view, each component of the wave function “dephases” until becoming incoherent. Some of the cold air remains in the refrigerator, and some states survive this act of measurement.
Understanding decoherence is crucial in quantum computing, not only because of this measurement problem but also because of the natural information loss that occurs when qubits are left idle. Unwanted interactions with the environment – an improperly sealed refrigerator – can cause leakage over time, corrupting our computation. We need to use Quantum Error Correction (QEC), or we need to design qubits that are inherently resistant to decoherence.