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Field: Physics/Astrophysics
Session Topic:
Slow Light
Speaker:
Mikio Kozuma/Tokyo Institute of Technology
1. Introduction
While lights are the fastest and the most robust carriers of information, it is difficult
to localize and store them. Recently, a novel scheme to store the photonic information
in an atomic ensemble was proposed [1], which is based on the phenomenon of
ultraslow light propagation [2]. Ultraslow light propagation is made possible by
electromagnetically induced transparency (EIT) [3], which is a technique for turning
an opaque medium for a weak probe light into a transparent one with the help of an
additional control light. There is a steep dispersion within the transparency window,
so that the speed of the probe light pulse is significantly reduced in the EIT medium.
Eventually the probe pulse is completely localized in the atomic medium. Cutting off
the control light enables us to map the photonic information on the atomic spin
information.
Control
Probe

Ultraslow light
propagation
Storage of light
Retrieval of the light
2. How to realize quantum memory
Proof-of-principle experiments were simultaneously performed by two groups [4,5],
where a weak laser pulse was stored in an atomic ensemble and after a while the light
pulse whose temporal waveform was similar to the original one was retrieved. These
great demonstrations triggered the research to realize the quantum memory. In order
to demonstrate the quantum memory, what kind of thing should we perform?
Classical electromagnetism says the light is an oscillating electromagnetic field and it
is thus represented as a dot in a plane where the transverse and the longitudinal axes
are sin  t and cos  t , respectively. However, quantum mechanics says these two
components behave as a position and a momentum of a particle, which means the
light can not be represented as a dot due to the uncertainty principle. In other words,
quantum property of the light field exists in its fluctuation. In order to demonstrate
quantum memory, we have to store such a quantum fluctuation of the light field in the
atomic medium.
cos t 
E
Classical
.
time
sin  t 

E
Quantum

time
Pˆ
X  P 
Xˆ

Squeezed
Vacuum
E

Pˆ

time
ˆ
P

Xˆ


P  2
X


3. Our experimental challenge
X

2
P

2


The uncertainty principle allows us to generate very special light field, i.e., the
squeezed vacuum state, where the fluctuation of X is squeezed and that of P is
enhanced. Since the squeezed vacuum is the field whose quantum fluctuation is
manipulated, storing such a state should be the best demonstration of the quantum
memory. EIT was successfully observed for the squeezed vacuum state [6] and very
recently ultraslow propagation of the squeezed vacuum was also realized [7,8]. Now
the final success is very close.

References
[1] “Quantum memory for photons: Dark-state polaritons”, M. Fleischhauer and M. D.
Lukin, Physical Review A 65, 022314 (2002).
[2] “Light speed reduction to 17 meters per second in an ultracold atomic gas”, L. V.
Hau, S. E. Harris, Z. Dutton, and C. H. Behroozi, Nature 397, 594 (1999).
[3] “Electromagnetically induced transparency”, S. E. Harris, Physics Today 50, 36
(1997).
[4] “Observation of coherent optical information storage in an atomic medium using
halted light pulses”, C. Liu, Z. Dutton, C. H. Behroozi, and L .V. Hau, Nature 409, 490
(2001).
[5] “Storage of light in atomic vapor”, D. F. Phillips, A. Fleischhauer, A. Mair, R. L.
Walsworth, and M. D. Lukin, Physical Review Letters 86, 783 (2001).
[6] “Electromagnetically induced transparency with squeezed vacuum”, D. Akamatsu,
K. Akiba, and M. Kozuma, Physical Review Letters 92, 203602 (2004).
[7] “Generation of a squeezed vacuum resonant on a rubidium D1 line with
periodically poled KTiOPO4”, T. Tanimura, D. Akamatsu, Y. Yokoi, A. Furusawa and M.
Kozuma, Optics Letters 31, 2344 (2006).
[8] “Ultraslow propagation of a squeezed vacuum with electromagnetically induced
transparency”, D. Akamatsu, Y. Yokoi, T. Tanimura, A. Furusawa, and M. Kozuma,
arXiv.org e-print archive, quant-ph/0611097.