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 Pic: news.softpedia.com
Since the
beginning of the nineties research into quantum teleportation has been
booming with theoretical and experimental physicists. Transmission of
quantum information involves a fundamental problem: According to
Heisenberg’s uncertainty principle two complementary properties of a
quantum particle, e.g. location and momentum cannot be precisely
measured simultaneously. The entire information of the system thus has
to be transmitted without being completely known. But the nature of the
particles also carries with it the solution to this problem: the
possibility of "entangling" two particles in such a way that their
properties become perfectly correlated. If a certain property is
measured in one of the "twin" particles, this determines the
corresponding property of the other automatically and with immediate
effect.
With
the help of entangled particles, successful teleportation can be
achieved roughly as follows: An auxiliary pair of entangled particles
is created, the one being transmitted to "Alice" and the other to
"Bob". (The names "Alice" and "Bob" have been adopted to describe the
transmission of quantum information from A to B.) Alice now entangles
the object of teleportation with her auxiliary particle and then
measures the joint state (Bell measurement). She sends the result to
Bob in the classical manner. He applies it to his auxiliary particle
and "conjures up" the teleportation object from it.
Are "such
"instructions for use" merely mental games? The great challenge to
theoretical physicists is to devise concepts which can also be put into
practice. The experiment described here has been conducted by a
research team headed by Prof. Eugene Polzik at Niels Bohr Institute in
Copenhagen. It follows a proposal made by Prof. Ignacio Cirac, Managing
Director at MPQ, and his collaborator Dr. Klemens Hammerer (also at MPQ
at that time, now at University of Innsbruck, Austria).
First the twin pair is produced by sending a strong light pulse to a glass tube filled with caesium gas (about 1012
atoms). The magnetic moments of the gas atoms are aligned in a
homogenous magnetic field. The light also has a preferential direction:
It is polarised, i.e. the electric field oscillates in just one
direction. Under theses conditions the light and the atoms are made to
interact with one another so that the light pulse emerging from the gas
that is sent to Alice is "entangled" with the ensemble of 1012 caesium atoms located at Bob’s site.
Alice
mixes the arriving pulse by means of a beam splitter with the object
that she wants to teleport: a weak light pulse containing very few
photons. The light pulses issuing at the two outputs of the beam
splitter are measured with photo-detectors and the results are sent to
Bob.
The measured results tell Bob what has to be done to
complete teleportation and transfer the selected quantum states of the
light pulse, amplitude and phase, onto the atomic ensemble. For this
purpose he applies a low-frequency magnetic field that makes the
collective spin (angular momentum) of the system oscillate. This
process can be compared with the precession of a spinning top about its
major axis: the deflection of the spinning top corresponds to the
amplitude of the light, while the zero passage corresponds to the
phase.
To prove that quantum teleportation has been
successfully performed, a second intense pulse of polarised light is
sent to the atomic ensemble after 0.1 milliseconds and, so to speak,
"reads out" its state. From these measured values theoretical
physicists can calculate the so-called fidelity, a quality-factor
specifying how well the state of the teleported object agrees with the
original. (A fidelity of 1 is equivalent to a perfect agreement, while
the value zero indicates that there has been no transfer at all.) In
the present experiment the fidelity is 0.6, this being well above the
value of 0.5 that would at best be achieved by classical means, e.g. by
communicating measured values by telephone, without the help of
entangled particle-pairs.
Unlike the customary conception of
"beaming", it is not a matter here of a particle disappearing from one
place and re-appearing in another. "Quantum teleportation constitutes
methods of communication for application in quantum cryptography, the
decoding of data, and not new kinds of transportation", as Dr. Klemens
Hammerer emphasizes. "The importance of the experiment is that it is
now possible for the first time to achieve teleportation between
stationary atoms, which can store quantum states, and light, which is
needed to transmit information over great distances. This marks an
important step towards accomplishing quantum cryptography, i.e.
absolutely safe communication over long distances, such as between
Munich and Copenhagen."
Source: Max Planck Society
Related Links:
J. F. Sherson et al.,
"Quantum teleportation between light and matter"
Nature 443, 557-560(5 October 2006).
M. Lukin and Matthew Eisaman
"Atomic physics: Quantum leap from light to atoms"
Nature 443, 512-513 (5 October 2006).
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