12 | New Scientist | 31 August 2019QUANTUM teleportation has
made a leap in sophistication.
Physicists have teleported more
information at once than has
ever previously been possible,
paving the way for a global
quantum internet that would be
extremely secure from hacking.
This isn’t teleportation as you
might imagine it from science
fiction. Rather than transporting
matter through space, it involves
moving information related to
the quantum state of a particle.
Previously, we have only been
able to teleport quantum bits,
or qubits, the simplest unit of
quantum information in which
a particle can be in two states
at once. For instance, a photon
that is simultaneously vertically
and horizontally polarised would
be a qubit.
Now Jian-Wei Pan at the
University of Science and
Technology of China and Anton
Zeilinger at the University of
Vienna in Austria and their
colleagues have teleported a more
complicated unit of quantum
information called a qutrit for
the first time. If a qubit can be
considered two-dimensional,
a qutrit is three-dimensional:
the photon is polarised in three
perpendicular directions.
“The higher the dimensions
of your quantum system, the
more secure you can ensure your
communication is and the more
information you can encode,” says
Ciarán Lee at University College
London. “But going from a qubit
to a qutrit is especially difficult:
the tricks you use for qubits have
to do with a nice symmetry that
qutrits don’t have.”
To teleport a qubit, you begin
with three particles. One is the
qubit whose information you
want to teleport. The other two
are a pair of particles that have
been entangled in such a waythat making a measurement
on one will affect the result of a
measurement made on the other.
Now imagine two people,
traditionally called Alice and Bob.
Alice has the qubit and one of the
pair of entangled particles. Bob
has the other particle in the pair.
If Alice wants to send the qubit’sinformation to Bob, she performs
a special kind of measurement
on both her particle and the
qubit. Going through this process
means that Alice’s particle is now
entangled with the qubit, as well
as Bob’s particle.
Because of all this entanglement,
Alice’s measurement forces Bob’s
particle into one of four possible
states. He can find out which by
making a measurement. The
results of Alice’s measurement –which she can send to Bob using
non-quantum methods, such as
an email – lets him determine how
the measured state of his particle
is related to that of the original
qubit. Once he knows that, he can
reconstruct the information from
the original qubit. Its information
has been teleported.
Qutrits are a level up in
difficulty because it is much
harder for Alice to perform her
measurement and entangle her
particle with the qutrit.
The researchers got around this
by adding another particle to the
system so Alice is measuring three
particles instead of two (Physical
Review Letters, doi.org/c9ns).
As a result, her measurements
contain more information, which
she sends to Bob, allowing him to
reconstruct the qutrit.
The researchers could teleport
qutrits with 75 per cent fidelity,
meaning Bob’s qutrit was 75 per
cent similar to Alice’s original.
That may not seem high,but the highest fidelity possible if
the quantum entanglement had
failed is 50 per cent.
“Seventy-five per cent is
probably not good enough to
start communicating in this
way with much accuracy, but
this is early days,” says Lee.
The researchers claim that
their method could be used to
teleport even larger packets of
information with higher fidelity.
If that works out, it would be
a further step towards quantum
communication systems. Passing
messages using quantum
entanglement would be far
more secure than current
encryption methods.
Quantum teleportation
could enable information to
be passed over long distances
by secure quantum networks,
says Lee. “The ability to teleport
a high-dimensional system is
going to be one of the bedrocks
on which a future quantum
internet is built.” ❚News
CryptographyEL
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STUDIO/SC
IENCE^PH
OTO^ LIBRARYQuantum teleportation win
Physicists have used the rules of quantum entanglement to teleport
a richer package of information than ever before, reports Leah CraneTeleportation depends
on spooky quantum
connectionsLove quantum theory?
More entanglement and weirdness online
newscientist.com/article-topic/quantum-science“ Passing messages through
quantum entanglement
would be a very secure
means of encryption”