Quantum Entanglement#
When qubits interact, something even stranger happens…
Schrödinger’s cat lvl 2 🐈⬛🐈#
There are now two quantum cats in one box. We manage to separate them into two separate boxes on opposite ends of the room without looking. Two observers decide to look into the boxes at the same time causing the quantum cats states to collapse. On the first observation, the cats are both in the dead state. On the second observation, they are both in the alive state. After repeating the experiment multiple times, the cats are always found to be in the same state once observed.
The outcomes of the observations are said to be correlated. This is called entanglement since the quantum cats were once in contact.
Why might this result be considered strange? 🧠
Answer
It’s strange because the final outcome for each cat was not decided until the moment of observation. The quantum cats states collapse probabilistically into their final outcomes.
For the measurements to be correlated in this way, one might think that they had to “communicate” at the exact moment of measurement what state they were about to collapse to. However, this explanation indicates that information could travel faster than the speed of light between the quantum objects, which contradicts Albert Einstein’s special theory of relativity. Einstein therefore was of the belief that quantum mechanics was an incomplete theory since some unrepresented variable must come into play in deciding the final measurement outcomes.
As it stands in our current understanding of quantum mechanics, Albert Einstein was both correct and wrong. The current theory, which is continuously being experimentally demonstrated, is what we call quantum non-locality. It does not contradict the special theory of relativity, so Einstein is correct in saying that nothing travels faster than light. However, it contradicts Einstein’s belief of a hidden variable in quantum mechanics.
Quantum non-locality is super strange though. It is the idea that no matter how far we separate the qubits after the interaction, their states can no longer be described independently of the other. It’s like they behave as “one” system, no matter if we place an entire universe between them! Spatial separation means nothing to entangled qubits.
What in the… psychic telepathy?!
This theory might sound a bit crazy, but experimental demonstrations of quantum non-locality was the subject of the 2022 Nobel Prize in Physics.
How might the entangled qubit state be described for our example? 🧠
Answer
In this example, notice that the states 01 and 10 are not possible because of the entanglement.