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OPT 253 Quantum Optics Laboratory,
Final Presentation
Wednesday, December 10th 2008
By Carlin Gettliffe
Introduction
Three laboratory experiments were conducted, each of
which demonstrated a principle of quantum mechanics:
• Single Emitter Fluorescence and
Antibunching
• Single Photon Interference
• Bell’s Inequalities and Quantum
Entanglement
Lab 3/4: Introduction
In this lab we investigated the quantum dot
excitation method of single photon production.
• We learned how to use a confocal microscope and
Hanbury Brown and Twiss setup.
• We prepared samples of quantum dots and excited
them with a pump laser to cause fluorescence.
• We verified antibunching.
Lab 3/4: Background
Reliable antibunched single photon sources are of
great interest because of their potential for use in
unbreakable quantum cryptography systems.
• Antibunching - when single photons
are separated in space and time.
• Quantum dots - molecules that can
act similarly to a single atom.
• Liquid crystals - materials that display
properties of both liquids and
crystals. Planar aligned cholesteric
LCs can act as a photonic bandgap
Lab 3/4: Confocal Microscope
A confocal microscope was used for preliminary imaging
and location of quantum dots, while a Hanbury Brown
and Twiss Setup was used to show antibunching.
• A confocal microscope uses a
pinhole to eliminate off axis and
out of plane light packets.
• A 532 nm laser was used to excite the
quantum dots and cause fluorescence,
which was then imaged with a cooled
CCD camera (not confocal).
Lab 3/4: Hanbury Brown and Twiss
• A 50/50 beam splitter sends
incoming light to two avalanche
photo diodes (APDs).
• Pulses from the photo diodes are
sent to a TimeHarp card, which
measures the time delay between
pulses.
10
6
4
2
Gap Time (ns)
143
138
133
128
122
117
112
107
102
96.9
91.8
86.7
81.6
76.5
71.4
66.3
61.2
51
46
56.1
40.9
35.8
30.7
25.6
20.5
15.4
10.3
0
5.17
Photon Count
8
0.07
• A histogram is built to display
the frequency of particular time
intervals between incoming
photons.
Lab 3/4: Sample Scans
151.0
4.0
465
1009
200
• Scans were produced
line by line.
800
175
600
150
400
125
200
100
• The most promising
areas were then zoomed
in on.
3
75
50
25
0
0
25
50
75
100
125
150
175
200
Forw. or APD1
Backw.or APD2
900.0
• The sample was
refocused as needed to
obtain sharp peaks.
800.0
700.0
600.0
500.0
400.0
300.0
200.0
100.0
0.0
0.0
2500.0
5000.0
7500.0
10000.0 12500.0 15000.0 17500.0 20000.0 22500.0 25000.0
posit ion (nm )
Inter-photon Time (ns)
120
116
111
107
102
98
93.5
89.1
84.6
80.2
75.7
71.3
66.8
62.4
57.9
53.5
49
44.6
40.1
35.6
31.2
26.7
22.3
17.8
13.4
8.94
4.49
0.04
Photon Count
Lab 3/4: Results
Antibunching was obtained!
8
6
4
2
0
Lab 3/4: Results
The fluorescence lifetime of DiI dye molecules was
calculated to be 3.42 ns (see figure below).
Fluorescence Lifetime of DiI Dye Molecules
• In order to
measure the
fluorescence
lifetime we used the
APD pulse as the
start signal and the
laser pulse as the
stop signal.
Lab 3/4: Discussion
Quantum dot excitation as a method
of single photon production:
Pros and Cons
Difficulties included locating single quantum dots,
ensuring that the sample was in focus, and observing
antibunching.
Lab 3/4: Suggestions
• More info about quantum dots and how they work.
• A little more in depth discussion of technique in the
lab (how to get non-clustered quantum dots,
Lab 2: Introduction
• We demonstrated the
wave-particle duality of light
by observing single photon
interference patterns
• Young’s double slit
experiment.
• Mach-Zehnder
interferometer.
Lab 2: Background
Wave particle duality: what does it mean?
• Under certain conditions light behaves as a particle,
while under others it behaves as a wave.
• Any direct measurement of light collapses the wave
function and results in particle behavior.
• Single photons can interfere with themselves because
as long as no measurement has been performed to
determine precisely which path the photon has taken, it
will behave as a wave.
Lab 2: Mach-Zehnder Interferometer
• A 633 nm He-Ne laser attenuated to approximately 1
photon/300 meters was used as a light source.
• So what’s the deal with polarizer D?
Lab 2: Young’s Double Slit
• A classic experiment that clearly demonstrates the
wave nature of light.
• A coherent monochromatic light source is passed
through two slits. An interference pattern then appears
at the detector (in this case a cooled CCD camera)
Lab 2: Results (Young’s Double Slit)
Image 2
Image 1
Image 3
Attenuation
Acquisition Time (s)
Gain
Image 1
1.2 x 10-6
3
255
Image 2
9.4 x 10-6
1
255
Image 3
0.16
0.3
None
Image 4
None
0.3
None
Image 4
Lab 2: Results (Mach-Zehnder)
Image 1
Image 2
Attenuation
Acquisition Time
(s)
Image 1
3.0 x 10-6
1
Image 2
3.0 x 10-6
2
Image 3
3.0 x 10-6
5
Image 4
3.0 x 10-6
10
Image 5
3.0 x 10-6
25
Image 6
3.7 x 10-5
~5
Image 3
Image 4
Image 5
Image 6
Lab 2: Results (Mach-Zehnder)
Which path information preserved
(without polarizer)
Which path information destroyed
(with polarizer)
Lab 1: Introduction
• Entangled photons were produced using a BBO
crystal.
• We aligned the quartz plate in order to create the
appropriate phase shift between the H and V
polarization components of the laser beam.
• We observed the cosine squared dependence of
coincidence count on polarizer angle.
•We confirmed a violation of Bell’s inequality.
Lab 1: Background
• Entangled photons cannot be described in terms of
single particle states
• A measurement performed on one of a pair of
entangled photons will affect the outcome of a
measurement performed on the other one.
• Bell’s inequality is a classical relationship. A
violation of Bell’s inequality implies entanglement and
nonlocality.
Lab 1: Setup
• 406 nm diode laser.
• Spontaneous
parametric down
converted photons
(produced with the
BBO crystals) are
detected by the APDs
• Using the polarizers it
is possible to select for
different polarization
states
Lab 1: Quartz Plate Alignment
• We tried to find the intersection of the curves obtained
from different polarizer positions (with varying quartz
plate angles).
• The quartz plate was used to compensate for the phase
shift induced by the BBO crystals
Conincidence Counts Vs. Quartz Plate Horizontal Angle
250
Coincidence Counts
200
alpha = 0, beta = 0
alpha = 45, beta = 45
alpha = 90, beta = 90
150
100
50
0
0
20
40
60
Angle of Quartz Plate
80
100
Lab 1: Cosine Squared Dependence
• We tried to find the intersection of the curves
obtained from different relative polarizer angles.
Coincidence Counts vs. Angle of Polarizer
250
Coincidence Counts
200
150
α=135
α=90
α=45
α=0
100
50
0
0
50
100
150
200
β Polarizer Angle (Degree)
250
300
350
400
Lab 1: Violation of Bell’s Inequality
• S is defined in the following way:
, where:
When S is greater than or equal to 2, we have a
violation of Bell’s inequality.
In this case, we calculated S to be 2.196!
Lab 1: Discussion
• We encountered many difficulties related to the
alignment of the optical system, and especially the
quartz plate.
• Our value of S was unexpectedly high.
• We successfully demonstrated violation of Bell’s
inequality.
Lab 1: Suggestions
• Have lab isolated so that risk of disalignment is
lower.
• A better theoretical explanation of Bell’s inequality,
perhaps using Joe Eberly’s method.
• Few lab days, longer time period (so that
disalignment is less of a risk)
Overall Suggestions
• A more in depth explanation of some of the
theoretical concepts (prior to questions being asked).
This could be in the form of short “lab lectures”.
• A bit more involvement in the setup process.
• Labs once a week for longer.
• More theory, fewer straight directions.
OPT
OPT 253 Quantum Optics Laboratory,
Final Presentation
Wednesday, December 10th 2008
By Carlin Gettliffe