In quantum mechanics, superposition refers to the ability of a quantum system to exist in multiple states simultaneously until it is measured or observed. By opening the box, you are "measuring" the state of the rotating cat, therefore causing it to either be upright or upside-down!
While objects in our everyday world exist in definite states (a light is either on or off, a ball is either here or there), quantum particles like electrons, photons, or atoms can exist in a combination of different states at once.
Illustrating Superposition with Schrödinger's Cat
In 1935, physicist Erwin Schrödinger proposed a thought experiment to illustrate the paradoxical nature of quantum superposition when applied to everyday objects:
Imagine a cat placed in a sealed box with a radioactive atom, a Geiger counter, and a vial of poison. If the radioactive atom decays (a quantum event governed by probability), the Geiger counter detects it and triggers a mechanism that breaks the vial, releasing the poison and killing the cat.
According to quantum mechanics, until we open the box and observe the system, the radioactive atom exists in a superposition of "decayed" and "not decayed" states. Since the cat's fate is entangled with this quantum state, the cat must also exist in a superposition of "alive" and "dead" states.
In our case, we used an example of a cat rotating to illustrate the basics of superposition! While a spinning cat may not mean it is in multiple states in the real world, the purpose of our illustration was to represent the idea of superposition as it relates to electron spin.
Spinning electrons can be in superposition of both the "up" and "down" states until it is measured. This property is what makes quantum computing possible, as it is fundamental in the processing of multiple possibilities simultaneously. To understand the magnitude of this difference, however, let's first observe how classical computers do computations!