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Lu*** M*n*e*'s avatar

in the year 2059 I have a definite prediction, although it is not really a prediction because a YouTuber has sent transmissions back in time from the year 2059. I think 2042 will be when quantum computers take over and nothing is safe, some place gets bombed by some type of missile, and in that place the YouTuber andymation will move to Rhode island. 2047: massive event occurs. 2059: all humans are "non-organic" and they have weird glasses.

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suman suhag's avatar

According to quantum mechanics, things can have properties that take on more than one value simultaneously - but when you measure that property, they instantly "choose" one of those values. This idea can also be applied to pairs of things, explained in more detail below. An object is said to be entangled with another when it has some property that takes on multiple values in a way that depends on the other object. The instantaneous "choosing" of one value (called "collapse") has interesting implications. It means that by measuring one of the entangled objects, you can cause the other object to instantly "choose" a particular value, wherever it is. In some sense, this could be called transmitting information faster than the speed of light. But, there is a (big) catch: we have no way of controlling which outcome the measured object will choose. In this sense, you could say that entanglement allows you to transmit random information faster than light. This is perhaps not useful.

Long answer: I will give an example of quantum entanglement. But first, some basic principles of quantum mechanics. An object is described by its quantum state. For the simple example of a photon, its state tells you where the photon is in space, its momentum, and its polarization. In describing the photon's state, however, you don't just give a number for each of those quantities, you essentially give a probability for getting a particular outcome if you were to measure them. (If you recall the uncertainty principle, this means that the narrower the probability distribution of position, the wider the probability distribution of momentum). For now, we'll ignore position and momentum, and just consider polarization.

Two possible states of a photon's polarization could be horizontal, denoted |H>, or vertical |V>. So the polarization state could generally be written as a certain probability of |H> and a certain probability of |V>, written a|H>+b|V>. (Technically, the probability of measuring |H> is |a|^2 and the probability of measuring |V> is |b|^2, where a and b may be complex and |a|^2+|b|^2 = 1)

The photon state, described by a|H>+b|V> should be understood to mean that the photon is simultaneously polarized both horizontally and vertically. This is an important point: In quantum mechanics, something can have parameters that take on two (or more) values at the same time (be in two places, have two energies, have two polarizations, etc.) Once you measure the system, then one of the options is instantly chosen, and then the parameter that you have measured has a well-defined value. (If this sounds weird, it is, and it is not known why this happens.)

OK, on to entanglement finally. Consider the state of a pair of photons. It turns out that it is possible to generate a pair of photons whose probabilities for |H> and |V> depend on the others probabilities. An example of such a state could be written a|H>|V>+b|V>|H>, where |a|^2 and |b|^2 are the probabilities of |H>|V> and |V>|H> respectively. Here, |H>|V> means the situation where photon 1 is |H> and photon 2 is |V>.

The state a|H>|V>+b|V>|H> means that the pair of photons is simultaneously in the situation with (photon 1 |H> and photon 2 |V>) and the situation with (photon 1 |V> and photon 2 |H>).

So what happens if you measure one of the photons? Say you measure the polarization of photon 1. You get either |H> or |V> as your result. You could get either, only the probabilities are given by |a|^2 and |b|^2. Say you measure |H>. Now the state of the pair of photons immediately collapses into |H>|V>. On the other hand, if you measure |V>, the state immediately becomes |V>|H>.

This is quite odd. As far as we know, this collapse of the state happens instantaneously, no matter how far apart the two photons are. But can it be used to transmit information?

The idea for a communication device would be to generate a pairs of entangled photons in the state (|H>|V>+|V>|H>) in your lab, send one of them through an optical fiber to Bob on the other side of the world, and send the other into an optical fiber of the same length inside your lab. When your photon comes out the other end of the fiber, you measure its polarization. The protocol is that a |H> photon is a 1 and a |V> photon is a 0. Let's say you measure the first 8 photons to come out and get 01101010. You know, that at that moment, on the other side of the world, Bob is measuring 10010101. You could say that you instantaneously sent the message "10010101" - the only problem is that you had no control over what the message was. It was totally random. This is a general problem with transmitting information using entanglement - the whole idea is based on this quantum indeterminacy. As far as we know there is no way around it.

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