quantum interference between two single photons … interference between two single photons emitted...
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Quantum interference between two single photons
emitted by two single trapped atoms
Gaétan Messin
Groupe d’Optique Quantique,Institut d’Optique, Orsay, France
http://www.iota.u-psud.fr/~grangier/
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TRAPPING
A SINGLE ATOM
IN A DIPOLE TRAP
Schlosser et al., Nature 411, 1024 (2001)
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TRAPPING SINGLE ATOMS IN AN OPTICAL TWEEZERS
Detection ofFluorescence @ 780 nmCloud of cold
Rb atoms ( 100 μK)
810 nm
NA = 0.7 w = 0.8 μm
0- 5 0- 5
Loading the tweezers
Single atom ?
Tight trap high inelastic collision rate
Schlosser et al., PRL 89, 023005 (2002)
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DETECTING A SINGLE ATOM
Fluorescence light induced bythe MOT beams (780 nm)
CCD camera
780 nm filter
Avalanchephotodiode
MOT & dipole trap
Dipole trap beam(810 nm)
1 μm
time (sec)
0 5 10 15 20 25
0
4
8
12C
ou
nts
/ m
s
Schlosser et al., Nature 411, 1024 (2001)
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SINGLE PHOTONS
FROM
A SINGLE ATOM
Darquié et al, Science 309, 454 (2005)
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THE IDEA
Well defined polarization
In principle = Fourier limited by the 6 MHz linewidth of the transition
Spontaneous photons
time
Emission probability
~ 30 ns 200 ns
Exciting laser
4 ns
- pulse780 nm
|1
|0
Excitationlaser
Lens
+
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IS IT REALLY A SINGLE PHOTON SOURCE ?
A single photon source IS NOT just an attenuated source of « classical » light
Probability to detect 2 photons during t = ( t)2 / 2 0
ph / sec
Laser
Density filter
Start – stop configuration: measure the # of coincidences for different delays
Second–order correlation G(2)(t,t+ ) =
A
B
Counter Single photon source:Ncoincidences(0) = 0
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ANTIBUNCHING
500
400
300
200
100
0
Co
inc
ide
nc
es
-1000 -500 0 500 1000
Delays (ns)
Time between 2 pulses
4 – hour acquisition (4 106 photons)Resolution 1 ns, bining 4No background correction
Antibunching
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Area around 0 / Area under a peak = p(2) / p(1)2 / 2
Probability to emit 2 photons during a pulse, p(2) = 0.018(50 better than an attenuated light)
500
400
300
200
100
0
Co
inc
ide
nc
es
-1000 -500 0 500 1000
Delays (ns)
HOW GOOD A SINGLE - PHOTON SOURCE IS IT?
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QUANTUM INTERFERENCE
BETWEEN TWO SINGLE PHOTONS
EMITTED BY INDEPENDENT ATOMS
Beugnon et al., to appear in Nature (2006)
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TWO–PHOTON INTERFERENCES (“COALESCENCE”)
+ = 0
Destructive quantuminterference
| - | 2
50/50
or
Hong, Ou, Mandel Phys. Rev. Lett. 59, 2044 (1987): parametric downconversion
50/50
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Motivation: CONDITIONAL ENTANGLEMENT … towards a 2-qubit gate
Atom-photon entanglementBlinov, Nature 428, 153 (2004)
Weinfürter (2005)
A
+
1 2
B
+
1 2
|1A, + + |2A, |1B, + + |2B,
Two-photon interference+
Double-clic detection
entanglement swapping
A double-clic prepares: |1A,2B + |1B,2A
2 identical atoms emit indistinguishable photons
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ANOTHER KIND OF MOTIVATION…
Loudon, J. Opt. Soc. Am. B 6, 917 (1989).
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TRAPPING TWO ATOMS
- Detect the presence of two atoms- Start sequences of excitation (about 9000 pulses)- Empty the two traps- Wait until two other atoms are trapped ( ~ 300 ms)
threshold
6 μm
Use two independent optical tweezers
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EXPERIMENTAL SETUP
A
B
APD 2
APD 1
pulses
/2
/2
PBS
PBS
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THE 50/50 BEAMSPLITTER
/2
A
B
/2 axis // cubes axis
/2 axis at 22,5° of cubes axis
B
A
“Beam separator”
A
B
“50/50 beamsplitter”
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“BEAM SEPARATOR CONFIGURATION”
B
A
4-hours accumulation Resolution 3.6 ns
(Calibration experiment)
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6600 two–photon events around 0 delay B
A
4-hours accumulation Resolution 3.6 ns
“BEAM SEPARATOR CONFIGURATION”
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“50/50 BEAMSPLITTER CONFIGURATION”A
B 6600 two–photon events around 0 delay
4-hours accumulation Resolution 3.6 ns
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A
B At non-zero delays, N50/50 / Ncal 1 2 atoms trapped
4-hours accumulation Resolution 3.6 ns
“50/50 BEAMSPLITTER CONFIGURATION”
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Each photon comes from a different atom (single photon source)
Not to be confused with single atom antibuching
“50/50 BEAMSPLITTER CONFIGURATION”
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WHAT IF THE TWO PHOTONS DO NOT INTERFERE?
Comes from the single photon nature of the source
Calibration ( /2 axis // cube)
Always a coincidence
/2 axis @ 22,5°and NO interference
no coincidence coincidence
A coincidence half of the time
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Acal
Ainterf
Ratio = Ainterf / Acal < 1/2
QUANTUM INTERFERENCES
Acal/2
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What’s next ?
We need to adapt the entanglement schemes to theRb level structure (time bin…)
+
1 2
|1
|0
+
Improve the spatial overlap by coupling into single-mode fibers (losses…?)
Further cooling of the atoms (Raman cooling)
Raman transition to encode the qubit and read out the coherences
Currently : 1 interfering event every 2 / 3 seconds. conditionnal entanglement ?
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JérômeBeugnon
Philippe Grangier
Gaétan Messin Antoine Browaeys
BenoîtDarquié
Jos DingjanMatt Jones Yvan Sortais
The single atom project
Harold Marion
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More details…
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VARYING THE SPATIAL OVERLAP
A
B
/ 4
x
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Ratio = Ainterf / Acal
VARYING THE SPATIAL OVERLAP
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Measured waist
(62 μm)
Field amplitude
overlap 80 %
VARYING THE SPATIAL OVERLAP
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Data corrected from
background and
adjacent peaks
Calibration curve
Ratio R
AROUND ZERO DELAY
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THE ATOMS MOVE IN THEIR TRAP …
Beat note between the two
photons at
Averaging the beatnotes over
the lightshifts distribution
broadening
T = 60 μK+ taking into account
the heating during
the emission
T = 180 ± 20 μK
R = 0.73 ± 0.02
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WHAT IF THE ATOMS DID NOT MOVE?
Field overlap R = 0.73
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AND WHAT IF THE OVERLAP WERE PERFECT?
T = 200 μK
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MUTUAL COHERENCE OF THE SOURCES?
The mutual temporal coherence is limited by
- linewidth of the transition: 6 MHz (T1)
- motion of the atoms in the trap inhomogeneous
broadening (T2*)
T = 180 mK = 2 MHz
Depends on the number of excitations, and cooling duty cycle
Contrarily to solid state system, no homogeneous broadening
(T2’, dephasing during T1)
Source suitable for QIP as two atoms can be true « identical »
single-photon sources