Single Photon Source for Quantum Communication

1. Single Photon Source for Quantum Communication Sarah Walters, Meng-Chun Hsu, Hubert Zal, Pierce Morgan

2. Single photon source- all photons are separated from each other (antibunching) single photon source attenuated laser pulses (never have antibunching) How to create single photons? Focus the laser beam on a single emitter Single emitter emits single photon at a time because of fluorescence lifetime Photon

3. Fluorescence Lifetime • While the electron is in a higher energy level, no more electrons can be excited • The photon must be emitted before the electron can be excited again • Time electron is in a higher energy level is fluorescence lifetime

4. Application of single photon sources is absolutely secure quantum communication In contrast to classical communication, where an eavesdropper (Dr. Lukishova) is able to measure the transmitted signals without arousing Pierce’s or Meng-Chun’s attention, in quantum cryptography eavesdropping can be detected by Meng-Chun or Pierce. Encode information using different polarization states of photons The problems with creating such technology is due to the difficulties in developing robust sources of antibunched photons on demand.

5. How do we prove that we have single photons? We need to measure the time interval between two consecutive photons and prove that no photons have zero time intervals between them (this is called antibunching) Measure flourescent antibunching using Hanbury Brown and Twiss inteferometer Beam splitter directs about half of the incident photons toward the first APD and half toward the second APD One is used to provide a ‘start’ signal, and the other, which is on a delay, is used to provide a ‘stop’ signal. By measuring the time between ‘start’ and ‘stop’ signals, one can form a histogram of time delay between two photons and the coincidence count two single-photon counting avalanche photodiodes APD1(T) and APD2(R) Histogram

6. Experimental Setup Dichroic mirror Single emitter APD 1 APD 2 Filter Microscope cover slips Non-polarizing beam splitter Microscope objective 532nm laser

7. Confocal Fluorescent Microscope sample is placed here laser beam enters here filters diminish intensity of laser beam Preparing to put the sample on the confocal microscope

8. Two types of emitters were used – single color centers in nanodiamonds and single colloidal semiconductor Cadmium Selinium Tellurium quantum dots The primary problems with using fluorescent dyes and colloidal semiconductor nanocrystals in cavities are the emitters’ bleaching. Both are only Several nanometers Liquid diamond monocrystaline- same diamond as found in jewelry Quantum dots – very small molecules made to act as a single atom

9. Samples we created ourselves using nanodiamonds in liquid crystal Samples are later placed onto the microscope using magnets

10. Specific position Go to a specific position X min and X max Y min and Y max Intensity of photons per time focus on top right emitter 10/28/2009 Area of scan Intensity over time 5 by 5 micron scan

11. Sample: Nanodiamonds Sample moves as laser scans it line by line. Scan of single line 25 by 25 micron scan Photons detected of one line

12. Sample: Nanodiamonds No antibunching

13. Sample: Nanodiamonds Some antibunching – minimum at 0 time interval

14. Sample: Nanodiamonds—Index Matching Fluid 5 by 5 micron scan intensity over time Fluorescence of color centers in nanodiamonds Confocal microscope focuses on emitter

15. Sample: Nanodiamonds—Index Matching Fluid Confocal microscope focuses on different emitter

16. Sample: Nanodiamonds—Index Matching Fluid Confocal microscope focuses on different emitter

17. Sample: Nanodiamonds in Cholesteric Liquid Crystal 25 by 25 micron scan

18. Sample: Quantum Dots 11.2 by 11.2 micron scan Laser focused on single quantum dot

19. Sample: Quantum Dots Blinking of quantum dots

20. Sample: Quantum Dots Antibunching – minimum at 0 time interval

21. Research done….

22. Thanks to Dr. Lukishova