What is entanglement?

Answer 1

Entanglement is a state that sub-atomic patricles can enter, where the properties of one particle mimic exactly the properties of another. No matter the physical distance between the two particles, they will continue to mimic each others properties. If one particle is infused with energy, the other particle will act as though it has also been infused. This opens many doors for transportation of energy, as well as information.

AnswerI don't think the two particles necessarily have the same property - they could just as easily have opposite properties. For example, suppose you are given an unstable particle, and you measure its velocity (and let's say for the sake of argument its velocity is 0). Suppose it then decays into two identical particles. Suppose you know it has decayed, but you haven't measured the velocities of either of the new particles. But you know that they are moving apart in opposite directions, and at the same speed (by the law of conservation of momentum). Hence, if you measure the velocity (speed and direction) of one of them, you will know the velocity of the other one. In this case, the two particles are entangled.

On the issue of long-distance communication mentioned above, I don't think it is possible to send a message using entanglement. It is true that a measurement of one particle will affect the measurement of the other, even if they are very far apart. But it is not possible to control what the measured state of your particle is, and therefore one cannot control the state of the other particle.

However, it is (theoretically) possible to use entanglement to make long-distance communication secure. The sender creates many pairs of entangled particles, and sends one from each pair. By the uncertainty principle, it is impossible to completely measure the state of a pair. The two people make partial measurements of the states of their particles. Then they tell each other (publicly) which observables they have measured (eg "I measured the position/momentum [delete one] of particle 37"). If both measured the position, or if both measured the momentum, then they know they have observed the same value (because the particles were entangled). These measurements, which they agree on, become the key for the encryption/decryption using some (symmetric) cipher. Now suppose someone intercepts the particles. The interloper will not be able to re-create particles entangled with the original ones. This means that the sender and receiver will not in fact have the same key. In that case, they can detect the intrusion by taking a small random sample of the particles and telling each other (publicly) what position/momentum they observed. They will then know it is not safe to use the key, and must try again.