La lumière in vivo
Igor Dotsenko
Chaire de physique quantique,
Collège de France
Journée de l'Institut de Biologie du Collège de France
Paris, 24 novembre 2009
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1
Light for exploring the nature
Everyday life: Most information on our environment
we obtain through light (about 80%).
Science: From studies of biological cells
to distant galaxies the light is the fist tool to start with.
Journée de l'Institut de Biologie, 24/11/09
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Object of investigation
For many centuries, light itself was
an object of interest and investigation for scientists.
I. Newton, light dispersion
T. Young, light interference
H. Fizeau and L. Foucault,
speed of light
Classical properties: electromagnetic wave
with speed c, frequency n, wavelength l, etc.
Journée de l'Institut de Biologie, 24/11/09
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Photon - intrinsically "quantum" state of light
The smallest bit of light with
unit energy and momentum:
Non-classical, quantized photon-number states like:
|exactly n photons
Quantum superposition allows more "exotic" states like
The story of light is not over:
( |exactly n photons + |exactly k photons )
Light is still very
or like:intriguing and
fascinating
object
to explore
( |all photons fly left
+ |all photons
fly right!!!
)
No way to illustrate and understand such
superposition states with classical intuition !
Journée de l'Institut de Biologie, 24/11/09
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Catching a photon
Several ways to tackle the question "How things work?"
1.
observe and
wonder
2.
disturb and follow
Journée de l'Institut de Biologie, 24/11/09
3.
catch and have a
closer look !
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Catching a photon
mirror
Fabry-Perot resonator
Requirements: perfect reflection off the mirrors !!!
(no absorption, no transmission, no scattering)
Journée de l'Institut de Biologie, 24/11/09
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Microwave superconducting cavity:
Storage box for photons
Tlight = 130 ms
- a light travel distance of 39 000 km
(one full turn around the Earth)
2.8 cm
-1.4 billion bounces off the mirrors
5 cm
Journée de l'Institut de Biologie, 24/11/09
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Study light in vivo ?
But, (usually) to see or explore light means to absorb it,
e.g. by an eye retina or a CCD chip!
Can we use a transparent (like glass) probe?
Yes, use giant (Rydberg) atoms
flying one-by-one across the field.
1/4 mm
Journée de l'Institut de Biologie, 24/11/09
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Rydberg atoms
Rydberg states:
uniform electron distribution
(i.e. no phase information)
Superposition of two orbits:
induced dipole rotates
at atomic frequency watom
(n+1) l/2 = 2pr
number of oscillations
(principle quantum number)
n l/2 = 2pr
Information on watom
is encoded in
the dipole phase 
Journée de l'Institut de Biologie, 24/11/09
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Off-resonant interaction
atom
light
¢e/ n
¢g / ¡ n
 Energy conservation  the field is preserved
Atom-field interaction modifies watom proportional to n
 Phase shift of the atomic dipole (relative to free atom)
Phase shift per photon (depends on interaction strength)
Journée de l'Institut de Biologie, 24/11/09
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Phase measurement: Atomic clock
1. Trigger of the atom clock:
resonant pulse
2. Dephasing of the clock:
interaction with the cavity field
3. Measurement of the clock:
second pulse & state detection
Atomic state
(e/g)
is correlated
number
(1/0)
Phase
shift
per photonwith
adjusted
to of
0 =photons
p
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photon
number
atoms
Birth, life and death of a photon
time [s]
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Birth, life and death of a photon
"Warm" cavity excites a thermal photon
(black body radiation):
photon
number
atoms
(i.e. 5% of time there is one photon;
from Planck's law)
time [s]
Journée de l'Institut de Biologie, 24/11/09
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Larger number of photons
Dephasing per photon 0 < p,
for instance, 0 = p/4
Distinguish up to 7 photons
n=7
n=5
n=0
n=4
n=1
n=3
with probability
depending on (n)
n=6
n=2
Measure dipole orientation with many (~50) atoms
Journée de l'Institut de Biologie, 24/11/09
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Seeing quantum jumps of light
Quantum non-demolition measurement: Light in vivo
Initial state is classical electro-magnetic field injected from
a usual microwave source (number of photons is not defined !)
Photon number, n
Random projection onto one of n values
Repeatability of QND measurement
Quantum jumps between discrete
values of n: damping of the field caught
in the act
Journée de l'Institut de Biologie, 24/11/09
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Perspectives: Non-local light
Cavity 1
Cavity 2
Study non-local states, e.g.:
|all photons in Cavity 1, not in 2 + |all photons in Cavity 2, not in 1
What are their properties?
Why not observed in our classical "macroscopic" world?
Where is the transition from quantum to classical?
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The cavity QED team
Julien Bernu (→ Canberra)
Christine Guerlin (→ Zurich)
Samuel Deléglise (→ Munchen)
Clément Sayrin
Xingxing Zhou
Bruno Peaudecerf
Igor Dotsenko
Sébastien Gleyzes
Michel Brune
Jean-Michel Raimond
Serge Haroche
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