EPR paradox
In $1935$, Einstein, Podolsky, and Rosen (EPR) published a paper where they introduced the EPR paradox to question the meaning of quantum mechanics. 1
Let’s suppose we take two qubits, prepare them in the $|Φ^+⟩$ state and then separate the two qubits so that two scientists, Alice and Bob (sufficiently far apart from one another), each have one qubit. Now, say Alice measures her qubit, and then Bob measures his qubit so quickly after Alice’s measurement that any influence from Alice’s qubit would have to travel faster than light in order to affect Bob’s qubit.
According to the laws of quantum mechanics, whenever Alice measures $|0⟩$, Bob should also measure $|0⟩$, since the moment Alice measures $|0⟩$, the state of the qubits is now $|00⟩$. The same goes for $|1⟩$ and $|1⟩$.
EPR proposed that for this to be true, either
- Alice’s measurement was able to influence Bob’s measurement faster than light,
- or the measurement outcomes were predetermined by some hidden variable that quantum mechanics did not account for, meaning quantum mechanics was incomplete.
EPR’s proposal became known as the EPR paradox.
EPR considered the former option absurd as it would disprove the Locality principle and advocated the later option, that the measurement outcomes are determined before the measurement in a way that quantum mechanics does not account for. This belief is called realism, that the qubits have actual, real values before measurement.