Quantum Optics: Single Photon Interference

1. Quantum Optics: Single Photon Interference

2. Recap on quantum optics • The principle of Wave-particle duality says that light behaves like a particle and a wave at the same time • In a double slit experiment, the wave can go through both slits and interfere with itself on the screen, making fringes. • Does the particle go through one slit, the other, or both? Does the particle interfere with itself?

3. Young’s Double Slit Experiment

4. Video

5. Specifications • 632.8 nm Red laser • Double slit is 77.2 cm away from laser • Power after the double slit was .114 microwatts • Initial beam emitted 3.63 x 10^11 photons / second

6. Defining key terms • Exposure time-how long the camera was collecting photons for • Gain-a function on the EM-CCD camera that can make the image cleaner and clearer. Don’t confuse with gain medium! • Polarizer-allows light that is oriented the same way as the polarizer through, while absorbing all other light • Attenuation-a type of filter that reduces the number of photons by absorption. • Fringe Visibility-A mathematical way of measuring how distinct the fringes appear = (gray value max-min)/(gray value max + min)

7. 1.2 photons per meter Fringe Visibility: .480 Exposure: 0.2s No EM gain 3 orders of attenuation

8. Exposure: 0.1s No gain 3 orders of attenuation 1.2 photons per meter Fringe visibility: .907

9. Exposure: .1s Gain 255 3 orders 1.2 photons per meter Fringe Visibility .842

10. Exposure: 0.1s Gain 255 4 orders of attenuation 121 photons per kilometer Fringe visibility .549

11. 6 orders (1.21 photons per kilometer) Exposure: 10 seconds Fringe Visibility: 0.571

12. Exposure: 5 seconds Fringe Visibility: 0.711

13. Exposure 1 second Fringe Visibility: 0.550

14. Exposure.05 seconds Fringe Visibility : 0.365

15. Exposure .01 seconds Fringe Visibility : ≈ 0

16. Exposure .008 seconds Fringe Visibility : ≈ 0

17. Exposure .005 seconds Fringe Visibility : ≈ 0

18. The quantum conundrum • Heisenberg uncertainty principle-we cant know, precisely, where a particle is and how fast its moving • When we measure the particle’s position in space, we make a ‘realization’ of where it is. • The act of measuring a particle causes its wave function to collapse and it behaves like a particle. No more wave-particle duality. • If you measure the photon and do an interference experiment, will fringes form? Lets find out!

20. Stats • Stats • Power: 1.56 µw • Wavelength: 632.8 nm • Length of System: 65 cm

21. Data for High intensity Polarizer at 45 degrees Fringe visibility: .810 No polarizer Exposure - .1 s No gain 2 order attenuation Polarizer – 45 degrees Number of photons per meter – 164.6

22. Data for Low intensity No polarizer Polarizer at 45 degrees Fringe Visibility: .787 Exposure - 2 s Gain - 255 6 order attenuation Polarizer – 45 degrees Number of photons per kilometer-16.46

23. Data for Low intensity No polarizer Polarizer at 45 degrees Fringe Visibility: .767 Exposure - 5 s Gain - 255 7.5 order attenuation Number of phtonos per every 2 kilometers – 1.04

24. Polarization • By adjusting Polarizer B we can investigate the single photon interference. • When we move the polarizer in a certain direction the probability that the light is coming from a certain polarization becomes higher which will reduce the amount of fringes.

25. Data of Polarizations Exposure - 5 s, Gain – 255, 7 order attenuation , Polarizer – 45 degrees, Number of photons per kilometer – 1.646 Fringe Visibility : .833

26. Data of Polarizations Exposure - 5 s, Gain – 255, 7 order attenuation , Polarizer – 65 degrees Fringe Visibility : .800

27. Data of Polarizations Exposure - 5 s, Gain – 255, 7 order attenuation , Polarizer – 90 degrees Fringe Visibility: .408

28. Data of Polarizations Exposure - 5 s, Gain – 255, 7 order attenuation , Polarizer – 135 degrees Fringe Visibility: .871

29. Knox’s Question 5. Imagine doing Young’s two slit interference experiment with an average of one photon per second incident on the slits. If you cover one slit with your thumb every other second what happens? • Couldn’t cover slits, rotated polarizer instead

30. Fringe Visibility: .7357 Photons per Km: 76.4 Exposure : 10 sec. Gain : 255 5 Orders of attenuation With polarizer at 35, no rotation

31. Exp.: 10s Gain: 255 5 orders of attenuation 5s, polarizer at 35 -- 5s at 80 Fringe Visibility: .602  less than before Photons per Km: 76.4

32. Conclusion • Double Slit • Observed Interference at High Intensity • Reduced to one photon at a time • Interference still observed  photons interfere with themselves • Interferometer • Observed how knowing “Which path” information affects interference • No interference when polarizer was removed, interference when it was left in place

33. Conclusion (cont.) • During the last few decades the research of single photon interference has increased due to its possible applications in the field of quantum information. Examples of these are quantum computing, quantum teleportation, and quantum key distribtution.