This document summarizes an experiment on spontaneous parametric down-conversion (SPDC) that aimed to observe quantum entanglement and prove Bell's Theorem. SPDC produces entangled photon pairs through a nonlinear optical process. The experiment showed conservation of momentum, single photon interference, and that measuring the polarization of one photon affects the measurement of its entangled pair, disproving local realism as predicted by quantum mechanics. Key steps included optimizing the down-converted photon streams and quartz plate angle to compensate phase differences and maximize entanglement visibility.

1. Spontaneous Parametric Down-
Conversion
Presented by
Kaleb Niall & Kyle Chesnut
Graduate Internship Program
University of Oregon
September 2, 2016

2. What is the purpose of this
experiment?
Goal – Observation of quantum entanglement
and proof of Bell’s Theorem
Why – This proves some of the fundamental
postulates of Quantum Mechanics
* Existence of single photons
* Dual particle-wave nature of photons
* QM does not follow local realism

3. What are the applications of SPDC?
Single Photon Source
- Fundamental QM research
Quantum Information
- Quantum Communication
- Quantum Computing

4. SPDC – Non-linear optical process where an
incident photon is down-converted to a dual
photon pair each with half the original frequency

5. Type I SPDC – Produced entangled signal and
idler photons, which have parallel polarization
and are phase matched
Bell’s Theory – No physical theory of local
hidden variables can ever reproduce all of the
predictions of quantum mechanics

6. Plan of Attack for Experiments
1. Show SPDC follows Conservation of
Momentum
2. Proof of the existence of single photons
3. Single photon interference
4. Entanglement measurements

7. Down-Converted photon streams are
symmetric about central axis

8. Down-Converted photon streams are
symmetric about central axis
Optimum angle found to be 3.4˚ +/- 0.1˚

9. Second order of coherence does not
follow classical expectations

10. Second order of coherence does not
follow classical expectations
Second order coherence g(2) - Probability of
detecting more than one photon at the same
time [Classical: g(2)= 1 QM Ideal: g(2)=0]
2-Det Classical Test – g(2) = 5.54
3-Det QM Test – g(2) = 0.193 Std Dev = 0.0187

11. Observed single photon constructive
and destructive interference

12. Observed single photon constructive
and destructive interference


13. Initial observations of entanglement
Introduced half-wave plate to put down-converted photons in a
superposition state of vertical and horizontal polarization and a
linear polarizer in front of Detector A

14. Initial observations of entanglement



15. Superposition State
Linear Polarization: 0˚ Linear Polarization: 45˚ Linear Polarization: 90˚
Visibility – 84% Visibility – 75% Visibility – 65%
Detector A Counts – 125,087 Detector A Counts – 115,795 Detector A Counts – 92,614
i
A H H e V V
     

16. Vertically Polarized State
Linear Polarization: 0˚ Linear Polarization: 45˚ Linear Polarization: 90˚
Visibility – 90% Visibility – 85% Visibility – 83%
Detector A Counts – 197,174 Detector A Counts – 130,931 Detector A Counts – 39,594
V V 

17. Optimizing visibility of interference
pattern
Qualities of the non-linear crystal introduce a
phase mismatch between the horizontal and
vertical polarization state
We will introduce a quartz plate at an optimized
angle to compensate for this phase difference
i
A H H e V V
   
 

18. Finding quartz plate angle for phase
compensation

19. Finding quartz plate angle for phase
compensation

20. Measure of entanglement by
polarization

21. Entanglement polarization
measurement

22. Superposition state interference
Linear Polarization: 0˚ Linear Polarization: 45˚ Linear Polarization: 90˚
Visibility – 81% Visibility – 57% Visibility – 68%
Detector A Counts – 69,320 Detector A Counts – 66,697 Detector A Counts – 60,051
1
2
H H V V    

23. Vertical polarization state interference
Linear Polarization: 0˚ Linear Polarization: 45˚ Linear Polarization: 90˚
Visibility – 92% Visibility – 90% Visibility – 92%
Detector A Counts – 140,877 Detector A Counts – 102,287 Detector A Counts – 39,888
V V 

24. Conclusions
• Showed conservation of momentum in the
signal and idler beams
• Indirectly verified the existence of photons
• Observed single photon interference
• Demonstrated the theory of local realism does
not apply to QM by having a polarization
measurement on one photon affect the
polarization measurement of its’ entangled
pair