# quantum teleportation

Quantum teleportation is the only form of teleportation that is currently a scientific reality and the only form of teleportation in which it is known that an absolutely perfect copy of the original is created. Quantum teleportation, an outgrowth of quantum information science, enables the transfer of a quantum state to an arbitrarily distant location. However, the so-called "no-cloning theorem" stipulates that, in the process, the original quantum state is destroyed. Because the physics if quantum teleportation is somewhat esoteric it may be useful to outline it in familiar terms.

## A birthday analogy

A basic quantum teleportation involves three parties – say, three friends: Alice, Bob, and Claire. Claire wants to send Bob, who lives some distance away, a present for his birthday but has left it to the last minute. The only way it will arrive on time is by teleportation. She also doesn't have much money. All she can afford is the polarization state of a single light particle – the direction in which a solitary photon is vibrating.

Claire isn't very good at teleporting things, so she asks more technosavvy
Alice to help. Alice can't simply look at Claire's polarized photon (*X*)
and send the result to Bob because the act of looking – making a direct
measurement – would cause a random change, in accordance with the uncertainty principle) so
that the measurement wouldn't be identical to the photon's original state.
The key to getting a perfect copy to Bob, as Alice knows, is by not looking,
even surreptitiously, but by instead using the weird phenomenon of quantum
entanglement. What's needed are two more photons, *A* and *B*,
that have been created in such a way that they form an entangled pair. One
member of the pair, *B*, is sent directly to Bob, while the other goes
to Alice. Alice now takes her entangled photon, *A*, and combines it
with Claire's unseen photon gift, *X*. To be precise she measures *A* and *X* together in a special way known as a Bell-state measurement.
This measurement does two things: it causes *X* to lose its original
quantum state identity, and it also causes an instantaneous change in the
entangled photon that Bob has received. Bob's photon alters to correlate
with a combination of the result of Alice's measurement and the original
state of *X*. In fact, Bob's photon is now in either *exactly* the same polarization state as the photon that Claire bought for him or
in a state that's closely related to it. He doesn't yet know which.

The final step is for Alice to send Bob a message by conventional means,
such as a phone call, to tell him the result of her Bell-state measurement.
Using this information, Bob can transform his photon so that, if it isn't
already, it becomes an exact replica of the original photon *X*. The
transformation he has to apply depends on the outcome of Alice's measurement.
There are four possibilities, corresponding to four quantum relations between
photons *A* and *X*. Which one of these Alice obtains is completely
random and has nothing to do with *X*'s original state. Bob therefore
doesn't know how to process his photon until he hears from Alice what she
found out. He may, for example, have to rotate the polarization through
180 degrees, or he may have to do nothing at all. When he's made whatever
change is necessary, he's guaranteed to have a perfect copy of the present
that Claire got for him – a photon with exactly the same polarization
state as the original *X*.

## Aspects of quantum teleportation

A few points are worth emphasizing. First, for all practical purposes, photon *B* has *become* the original photon *X*, while *X* itself has been permanently altered (it has effectively lost all memory
of the quantum state it started out with) so that it is no longer *X* in any meaningful sense. This is why the effect is called teleportation:
it's equivalent to *X* having physically jumped to the new location,
even though it hasn't moved materially at all.

Second, *X*'s state has been transferred to Bob without Alice or Bob
ever knowing what the state is. In fact, this lack of knowledge is the very
reason that teleportation is able to work. Because Alice's measurement of *A* and *X* is completely random, it sidesteps Heisenberg's uncertainty
principle; entanglement then primes Bob's half of the entangled photon
pair automatically.

Third, teleportation relies on there being two channels or conduits for
information – a quantum one and an ordinary or classical one. The
quantum channel supports the link between the entangled photon pair and
operates instantaneously, as if there were no separation between Bob and
Alice. The classical channel carries the information that Alice has to provide
to Bob to be able to ensure that his photon is an exact replica of the original *X* or, if it isn't already, that it can be made into an exact replica
by a simple operation. The necessity of this classical channel, across which
signals can travel only at light-speed or below, means that teleportation
takes time even though the entanglement part of it works instantaneously.

Fourth, there are no limits in principle to the distance over which teleportation is effective. An object or property could theoretically be teleported across many light-years. But, again, the process couldn't happen faster than the speed of light and there's be enormous technical difficulties in making it work over such large distances.