Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [26]
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 will tell us about some property of P. According to the Heisenberg uncertainty principle, however, the measurement will inevitably involve us losing knowledge of some other property of P.
But all is not lost. Because P* was entangled with A, it retains knowledge about A. And because A was entangled with P, it retains knowledge about P. This means that P*, though it has never been in touch with P, nevertheless knows its secrets. Furthermore, when the measurement was made on A and P together and information about some property of P seemed to be lost, instantaneously it became available to A’s partner, P*. This is the miracle of entanglement.
Since we already know about the other properties of P, obtained from A, we now have all we need to make sure P* has exactly the attributes of P.
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Thus we have exploited entanglement to circumvent the restrictions of the Heisenberg uncertainty principle.
The amazing thing is that, although we have exploited entanglement to make a particle P* with the exact properties of P, at no time did we ever possess any information about the missing property of P! It was transmitted out of our sight through the ghostly connections of entanglement.
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Calling this scheme teleportation is a bit of a cheeky exaggeration since it solves only one of the many problems in making a Star Trek transporter. The researchers of course knew this. But they also knew a thing or two about how to grab newspaper headlines!
As it happens, the Achilles’ heel of the Star Trek transporter turns out to be neither pinning down the position, and so on, of every atom in a person’s body nor assembling a copy of the person from that information. It’s actually transmitting the sheer volume of information needed to describe a person across space. Billions of times more information is needed than for the reconstruction of a two-dimensional TV image. The obvious way to send the information is as a series of binary “bits”—dots and dashes. If the information is to be sent in a reasonable time, the pulses must obviously be short. But ultrashort pulses are possible only with ultrahigh-energy light. As science fiction writer Arthur C. Clarke has pointed out, beaming up Captain Kirk could easily take more energy than there is in a small galaxy of stars!
Teleportation and nonlocality aside, the most mind-blowing consequence of entanglement is what it means for the Universe as a whole. At one time, all particles in the Universe were in the same state because all particles were together in the Big Bang. Consequently,