OPT OPT 253 Quantum Optics Laboratory, Final PresentationWednesday, 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 material.

Lab 3/4: Confocal MicroscopeA 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.

• A histogram is built to display the frequency of particular time intervals between incoming photons.

0

2

4

6

8

10

0.07

5.17

10.3

15.4

20.5

25.6

30.7

35.8

40.9 46 51

56.1

61.2

66.3

71.4

76.5

81.6

86.7

91.8

96.9

102

107

112

117

122

128

133

138

143

Gap Time (ns)

Ph

oto

n C

ou

nt

Lab 3/4: Sample Scans

1009

3

200

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600

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200

0

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2000 25 50 75 100 125 150 175

151.0 4.0 465

900.0

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

position (nm)25000.00.0 2500.0 5000.0 7500.0 10000.0 12500.0 15000.0 17500.0 20000.0 22500.0

Forw. or APD1

Backw.or APD2

• Scans were produced line by line.

• The most promising areas were then zoomed in on.

• The sample was refocused as needed to obtain sharp peaks.

Lab 3/4: Results

Antibunching was obtained!

0

2

4

6

8

0.04

4.49

8.94

13.4

17.8

22.3

26.7

31.2

35.6

40.1

44.6 49

53.5

57.9

62.4

66.8

71.3

75.7

80.2

84.6

89.1

93.5 98 102

107

111

116

120

Inter-photon Time (ns)

Ph

oto

n C

ou

nt

Lab 3/4: Results

The fluorescence lifetime of DiI dye molecules was calculated to be 3.42 ns (see figure below).

• In order to measure the fluorescence lifetime we used the APD pulse as the start signal and the laser pulse as the stop signal.

Fluorescence Lifetime of DiI Dye Molecules

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)

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 1

Image 4

Image 3Image 2

Lab 2: Results (Mach-Zehnder)

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 1 Image 3Image 2

Image 6

Image 4

Image 5

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

Conincidence Counts Vs. Quartz Plate Horizontal Angle

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0 20 40 60 80 100

Angle of Quartz Plate

Co

inc

ide

nc

e C

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alpha = 0, beta = 0

alpha = 45, beta = 45

alpha = 90, beta = 90

• 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

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

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250

0 50 100 150 200 250 300 350 400

β Polarizer Angle (Degree)

Co

inci

den

ce C

ou

nts

α=135

α=90

α=45

α=0

Lab 1: Violation of Bell’s Inequality

• S is defined in the following way:

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!

, where:

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 PresentationWednesday, December 10th 2008

By Carlin Gettliffe