Free Quantum Theory Cannot Hurt You by Marcus Chown Page A

against the naked randomness of the microworld). But even though there is no way of knowing the spin of one electron until it isobserved, the spin of the other electron must settle down to being opposite instantaneously—no matter how far away the other particle happens to be.
ENTANGLEMENT
    At the heart of nonlocality is the tendency of particles that interact with each other to become entwined, or “entangled”, so that the properties of one are forever dependent on the properties of the other. In the case of the pair of electrons, it is their spins that become dependent on each other. In a very real sense, entangled particles cease to have a separate existence. Like a much-in-love couple, they become a weird joined-at-the-hip entity. No matter how far apart they are pulled, they remain forever connected.
    The weirdest manifestation of entanglement is, without doubt, nonlocality. In fact, it would seem that if we could harness it we could create an instantaneous communications system. With it we could phone the other side of the world with no time delay. In fact, we could phone the other side of the Universe with no time delay! No longer would we need to be inconvenienced by the pesky speed-of-light barrier.
    Frustratingly, however, nonlocality cannot be harnessed to create an instantaneous communications system. Attempts to use the spin of particles to send a message across large distances might use one direction of spin to code for a “0” and the other for a “1.” However, to know that you were sending a “0” or a “1,” you would have to check the spin of the particle. But checking kills the superposition, which is essential for the instantaneous effect. If you sent a message without first looking, you could be only 50 per cent sure of sending a “1,” a level of uncertainty that effectively scrambles any meaningful message.
    So although instantaneous influence is a fundamental feature of our Universe, it turns out that nature does exactly what is required to make it unusable for sending real information. This is how it permitsthe speed-of-light barrier to be broken without actually being broken. What nature gives with one hand it cruelly takes away with the other.
TELEPORTATION
    Arguably, the sexiest potential use of entanglement involves taking an object and sending a complete description of the object to a faraway place so that a suitably clever machine at the other end can construct a perfect copy. This is of course the recipe for the Star Trek transporter, which routinely “beamed” crew members back and forth between planet and ship.
    The technology to reconstruct a solid object merely from the information describing it is of course way beyond our current technological capabilities. But, actually, the idea of creating a perfect copy of an object at a remote location founders on something much more basic than this. According to the Heisenberg uncertainty principle, it is impossible to perfectly describe an object—the positions of all its atoms, the electrons in each of those atoms, and so on. Without such knowledge, however, how can an exact copy ever be assembled?
    Entanglement, remarkably, offers a way out. The reason is that entangled particles behave like a single indivisible entity. At some level, they know each other’s deepest secrets.
    Say we have a particle, P, and we want to make a perfect copy, P*. It stands to reason that in order to do this it is necessary to know P’s properties. However, according to the Heisenberg uncertainty principle, if we measure one particular property of P precisely—say its location—we inevitably lose all knowledge of some other property—in this case, its velocity. Nevertheless, this crippling limitation can be circumvented by an ingenious use of entanglement.
    Take another particle, A, which is similar to both P and P*. The important thing is that A and P* are an entangled pair. Now, entangle A with P and make a measurement of the pair together. This