Imagine the following gedankenexperiment. Observer Alice is right here on Earth.
ID: 1376992 • Letter: I
Question
Imagine the following gedankenexperiment. Observer Alice is right here on Earth. Observer Bob is at say Alpha Centauri. A pair of maximally entangled qubits is formed with one qubit handed over to Observer Alice and the other sealed in a box on Pluto shielded from decoherence. It takes a few years for any signal to reach Observer Bob. Observer Alice measures her qubit and "collapses" it. To her, the pair of qubits is no longer entangled. However, for a few years, will Observer Bob think there is an entanglement? We have to be careful here. Once the qubit on Earth has been measured by Earthlings, the entanglement is "shared". By the well known monogamy of entanglement theorem, any correlation between both qubits by themselves will have become classical. However, Observer Bob, who has a very good working knowledge of quantum mechanics and relativity, knows that for a few hours, until light signals can reach Pluto, the system consisting of an expanding bubble around Earth and the Pluto qubit are in an entangled state. Alice, who lives inside this bubble, will beg to differ. Is quantum entanglement an objective or subjective property?
Closely related to Wigner's friend and intersubjectivity in quantum mechanics and Given entanglement, why is it permissible to consider the quantum state of subsystems?.
Explanation / Answer
Quantum entanglement is an objective property.
Naughty Troublemaker described a very interesting situation that make quantum entanglement looks like a subjective property. However, even in such a situation, quantum entanglement is still an objective property.
After Observer Alice measures her qubit and "collapses" it, the quantum entanglement is still there, but now the Bob's qubit is entangled with a combined system formed by Alice and her qubit. So before the measurement, Bob's qubit only entangles with Alice's qubit. After Alice measures her qubit, Bob's qubit entangles with a combined system formed by Alice and her qubit.