The Bridge - February 2018 - 31

message that needs to be sent to Bob in order for
him to make the correction, quantum teleportation
is not accomplished instantaneously, but rather
takes time to complete - the time it takes for the
communication to travel to Bob, which cannot travel
faster than the speed of light. A final fascinating
aspect is that once the state of S is teleported to
system B, S′s state changes completely. In fact, it
contains absolutely no information
about the state prior to teleportation whatsoever.
This is crucial, since it ensures that in the end there
will not be two copies of the same state. This in itself
is another deep and important property of quantum
theory - unlike classical information that can be
copied at will, copying an unknown quantum states
is actually forbidden [5]).
Although very simple on paper, actually
demonstrating teleportation in the laboratory is
very difficult. First of all, it requires Alice and Bob to
share systems in a maximally entangled state, over
large distances. Entanglement is however a very
fragile quantum property that quickly deteriorates in
practice, and hence huge efforts have to be made to
generate and distribute the entanglement necessary
for teleportation. Second, Alice has to be able to
make the joint Bell state measurement on her
systems S and A. This measuremnt is also particularly
demanding, requiring extremely precise interactions
to be engineered and controlled by individual atoms
or photons. Finally, Bob has to wait until he recieves
the message with the result of Alice's measurement,
before he knows how to correct his state accordingly,
the whole while protecting his system from external
sources of noise which 2 again destroy and
decohere its fragile quantum state.
It was a 4 year race before the first experimental
group managed to overcome the above obstacles
and finally demonstrate quantum teleportation.
All of the first demonstrations were realised using
photons [6-8]. Soon after, it was also demonstrated
using atoms, nuclear magnetic resonance (NMR),
and solid-state sysytems [9]. By now it has been

demonstrated using a huge array of different setups.
One important fact about quantum teleportation is
that it can be used as a building block to accomplish
other tasks in quantum
information. As a first example, if the system S is
itself entangled with a system S′ held by another
far away party (let's call them Charlie), then after
teleportation has been performed, the systems S′
and B will end up in an entangled state - even
though they were never in the same place. This is
known as entanglement swapping and is a method
of generating entanglement between distant systems
(Bob and Charlie) by performing a measurement
at an intermediate location (Alice's laboratory). This
process is the basis of a so-called quantum repeater,
which is a leading candidate method for building
future large-scale quantum networks which might
one day span the planet. As a second example in
which quantum teleportation can play a role is in
quantum computing - a computer which replaces
bits with two-state quantum systems, referred to as
qubits. This however is another to-become-reality-scifi story!

REFERENCES
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Wootters, Teleporting an unknown quantum state via dual classical and
Einstein-Podolsky-Rosen channels, Phys. Rev. Lett. 70, 1895 (1993).
[3] Ji-Gang Ren et al., Ground-to-satellite quantum teleportation, Nature
549, 70 (2017).
[4] J. F. Sherson, et al., Quantum teleportation between light and matter,
Nature 443, 557 (2006).
[5] W. K. Wooters and W. H. Zurek, A single quantum cannot be cloned,
Nature 299, 802 (1982).
[6] D. Boschi, S. Branca, F. De Martini, L. Hardy, and S. Popescu,
Experimental Realization of Teleporting an Unknown Pure Quantum State
via Dual Classical and Einstein-Podolsky-Rosen Channels Phys. Rev. Lett.
80, 1121 (1998).
[7] D. Bouwmeester, J.-W. Pan, K. Mattle, M. Eibl, H.Weinfurter, and A.
Zeilinger, Experimental quantum teleportation, Nature 390, 575-579
(1997).
[8] A. Furusawa, et al. Unconditional quantum teleportation, Science 282,
706709 (1998).
[9] S. Pirandola, J. Eisert, C.Weedbrook, A. Furusawa, and S. L. Braunstein,
Advances in quantum teleportation, Nature Photonics 9, 641652 (2015).

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