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Quantum memory for light...

Quantum memory for light... - Physics Forum

Quantum memory for light... - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

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Old 12-05-2004, 11:40 AM
Ken Kubos
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Default Quantum memory for light...

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Quantum memory for light
December 05, 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.

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

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. Fiurásek, und E.S. Polzik
Experimental demonstration of quantum memory for light
Nature 432, 482 (2004)

Source: Max Planck Institute

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