“Schrödinger’s cat” — quantum superposition

What just happened?

Measurement off: the particle (a photon or an ion) is in two states at once — the scene shows two overlapping, flickering “probability clouds”. Measurement on: the clouds vanish instantly and the system “chooses” one definite, sharp state. This is not a gradual fading — it is an abrupt collapse at the moment of observation.

Quantum superposition:

— as long as no one measures, the system state $|\psi\rangle$ is a linear combination (a superposition) of both possible outcomes $|0\rangle$ and $|1\rangle$, each with a certain “weight” (amplitude) $c_0, c_1$. A measurement forces a choice of one of them — with a probability that depends on those amplitudes.

Where did “Schrödinger’s cat” come from?

In 1935 Erwin Schrödinger proposed a thought experiment: a cat shut in a box together with a mechanism that (based on a random quantum event) may kill it. Before we open the box, the cat would formally be both “alive” and “dead” at once — in a superposition of both states. Schrödinger did not want to believe it — the experiment was meant to show how absurd it sounds to carry the rules of quantum mechanics straight over to macroscopic objects such as a cat.

What did Haroche and Wineland do?

Serge Haroche (single photons trapped in a microwave cavity) and David Wineland (single ions in an electromagnetic trap) were the first to learn how to measure and control single quantum systems — without destroying them in the act of measurement. This is enormously hard: an ordinary measurement usually “kills” the delicate quantum state. For these breakthrough experimental methods they received the Nobel Prize in Physics in 2012.

Why does it matter today?

The ability to control single photons and ions without destroying their quantum state is the foundation of two fields: quantum computers (qubits have to survive in superposition for as long as possible) and the most accurate atomic clocks in the world (based on single, precisely controlled ions).

Why this matters for your exam:

This goes beyond the core curriculum, but it illustrates a key idea of quantum mechanics: a measurement is not passive “peeking” — it actively changes the system, forcing the superposition to collapse to a single state.

Remember: observation (measurement) destroys superposition — it forces a collapse to a single, definite state. As long as no one measures, the system stays in a superposition of many states at once.