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

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

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 Fluorescence Lifetime

Application of single photon sources is absolutely secure quantum communication 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. 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.

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 two single-photon counting avalanche photodiodes APD1(T) and APD2(R) 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 Histogram

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

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

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

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

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

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

Sample: Nanodiamonds No antibunching

Sample: Nanodiamonds Some antibunching – minimum at 0 time interval

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

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

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

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

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

Sample: Quantum Dots Blinking of quantum dots

Sample: Quantum Dots Antibunching – minimum at 0 time interval

Research done….

Thanks to Dr. Lukishova