Quantum memory for light

R.A. Hettinga rah at shipwright.com
Fri Dec 3 11:37:05 PST 2004


<http://www.physorg.com/news2227.html>

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Quantum memory for light

December 03, 2004


Realization of quantum memory for light allows the extension of quantum
communication far beyond 100 km

In the macroscopic classical world, it is possible to copy information from
one device into another. We do this everyday, when, for example, we copy
files in a computer or we tape a conversation. In the microscopic world,
however, it is not possible to copy the quantum information from one system
into another one. It can only be transferred, without leaving any trace on
the original one. The manipulation and transfer of quantum information is,
in fact, a very active field of research in physics and informatics, since
it is the basis of all the protocols and algorithms in the fields of
quantum communication and computation, which may revolutionize the world of
information. In the work published in Nature, November 25, 2004, scientists
from the Max Planck Institute for Quantum Optics in Garching and the Niels
Bohr Institute in Copenhagen have proposed a scheme to transfer the quantum
state of a pulse of light onto a set of atoms and have demonstrated it
experimentally.
------    
 Image: Experimental set-up: Atomic memory unit consisting of two caesium
cells inside magnetic shields 1 and 2. The path of the recorded and
read-out light pulses is shown with arrows. (Max Planck Institute of
Quantum Optics / Niels Bohr Institute Copenhagen)
-----
In the experiment, a pulse of light is prepared in a certain quantum state
whose properties (polarization) are randomly chosen. Then, the light is
sent through a set of atoms which are contained in a small transparent box
(an atomic cell) at room temperature. In the cell, the light and atoms
interact with each other, giving rise to an "entangled" state in which the
two systems remain correlated. After abandoning the atomic sample, the
pulse of light is detected. Due to the fact that the light and atoms are
entangled, the process of measurement on the light affects the quantum
state of the atoms in such a way that they acquire the original properties
of the light. In this way, the state of polarization of the photons is
transferred into the polarization state of the atoms. This "action at a
distance", in which by performing a measurement on a system it affects the
state of another system which is at a different location is one of the most
intriguing manifestations of Quantum Mechanics, and is the basis of
applications such as quantum cryptography or phenomena like teleportation.

In order to check that the transfer of polarization has indeed taken place,
the researcher measured the polarization of the atoms at the beginning of
the experiment and compared it with the original state of polarization of
the light. In the experiment, these two polarizations coincided up to a 70%
of the time. The main reason for the imperfections where the due to
spontaneous emission, a process in which the atoms absorb the photons but
then emit them in a different direction such that they do not go towards
the photo-detector.

A question that the authors of the paper had to carefully analyze was to
what extent 70% percent of coincidence is enough to claim that the process
was successful. Or, in other words, could they obtain the same result by
measuring the state of polarization of the photons and then preparing the
state of the atoms accordingly? The answer is no. Due to the basic
properties of quantum mechanics, the state of polarization of a laser pulse
cannot be fully detected. Due to the Heisenberg uncertainty principle, it
is impossible to measure the full polarization exactly. In fact, as some of
the authors together with K. Hammerer and M. Wolf (from the Max Planck
Institute of Quantum Optics) have recently shown, the best one can do using
this latter method would be 50%. This implies that the experiment indeed
has successfully demonstrated the transfer beyond what one could do without
creating the entangled state.

The current experiment paves the way for new experiments in which the
information contained in light can be mapped onto atomic clusters and then
back into the light again. In this way, one could not only store the state
of light in an atomic clusters, but also retrieve it. This process will be
necessary if we want to build quantum repeaters, that is, devices which
will allow the extension of quantum communication far beyond the distances
(of the order of 100 km) which are achieved nowadays.
 Original work:

B. Julsgaard, J. Sherson, J.I. Cirac, J. Fiuras
ek, und E.S. Polzik
Experimental demonstration of quantum memory for light
Nature 432, 482 (2004)

Source: Max Planck Institute


-- 
-----------------
R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'





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