Quantum physics is the ultimate challenge to humanity’s classical intuition. During a 1964 lecture at MIT, physicist Richard Feynman famously remarked, “I can safely say that nobody understands quantum mechanics.” It’s precisely this unintuitive weirdness that’s driven physicists over the decades to rely on handy metaphors when explaining one of quantum mechanics’ core principles: superposition.
The most famous of these metaphors is Schrödinger’s Cat, a thought experiment first put forward by Austrian theoretical physicist Erwin Schrödinger in which a cat, a radioactive isotope with a 50/50 chance of decaying, a Geiger counter, a flask of poison, and a triggering mechanism are placed in a sealed box. If the Geiger counter detects radioactive decay, the flask is broken, the poison is released, and the cat dies (disclaimer: no felines were actually harmed in the making of this thought experiment). If the radioactive isotope doesn’t decay, the cat lives.
The significance of this metaphorical setup is that it’s mathematically similar to the idea of quantum superposition and the quantum collapse of the wave function. Until the box is opened, we can’t know whether the radioactive isotope has decayed, so the cat in the box is therefore both alive and dead from our perspective. But once an observer makes a measurement (i.e. opens the box), the “wave function” of this superposition of states collapses. After the box is opened, the cat can only be in one state or the other—it’s either dead or alive.
The seeming paradox of Schrödinger’s cat being both alive and dead is analogous to the problem of superposition in subatomic particles, which can appear to be in two states at once until they’re measured. For decades, however, scientists have theorized that more complex variations of Schrödinger’s famous thought experiment might exist beyond the simple “alive or dead” scenario. Now, a study led by scientists at Oxford University has successfully demonstrated these exotic variations in the lab by using a single ion of strontium-88 confined in an ion trap. By manipulating both the ion’s internal quantum state (i.e., its spin) and its motion—the latter behaving as a quantum oscillator—the researchers created superpositions involving multiple quantum properties, not just two opposing states. The results of the study were published in the journal Physical Review X.
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