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 lifetimePhoton

•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 toclassical communication, where an eavesdropper (Dr. Lukishova) is able tomeasure the transmitted signals without arousing Pierce’s or Meng-Chun’sattention, in quantum cryptography eavesdropping can be detectedby 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 thefirst 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, onecan form a histogram of time delay between two photons and thecoincidence count

Histogram

Experimental Setup

APD 2

AP

D 1

Non-polarizing beam splitter

Dichro

ic mirr

or

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 fluorescentdyes and colloidal semiconductor nanocrystals in cavities are theemitters’ bleaching.

Samples we created ourselves using nanodiamonds in liquid crystal

Samples are later placed onto the microscope using magnets

217

100

120

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200200

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47.0 99.0 105

5 by 5 micron scan

217

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47.0 99.0 83

328

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47.0 99.0 83

350.0

0.0

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100.0

150.0

200.0

250.0

300.0

time (ms)800000.00.0 100000.0 200000.0 300000.0 400000.0 500000.0 600000.0 700000.0

APD1

APD2

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: NanodiamondsSample: Nanodiamonds

300.0

25.0

50.0

75.0

100.0

125.0

150.0

175.0

200.0

225.0

250.0

275.0

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

574

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2000 25 50 75 100 125 150 175

3 0.0 99.0 60

25 by 25 micron scanScan of single line

Sample moves as laser scans it line by line.

Photons detected of one line

Sample: NanodiamondsSample: Nanodiamonds

No antibunching

Sample: NanodiamondsSample: Nanodiamonds

Some antibunching – minimum at 0 time interval

Sample: Nanodiamonds—Index Matching Fluid

Sample: Nanodiamonds—Index Matching Fluid

220.0

80.0

100.0

120.0

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160.0

180.0

200.0

time (ms)50000.00.0 5000.0 10000.0 15000.0 20000.0 25000.0 30000.0 35000.0 40000.0 45000.0

APD1

APD2

321

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12

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38

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62

75

88

1000 12 25 38 50 62 75 88

37.0 65.0 60

5 by 5 micron scan

Confocal microscope focuses on emitter

Fluorescence of color centers in nanodiamonds

intensity over time

Sample: Nanodiamonds—Index Matching Fluid

Sample: Nanodiamonds—Index Matching Fluid

321

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38

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1000 12 25 38 50 62 75 88

3

14.0 29.0 62

200.0

60.0

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100.0

120.0

140.0

160.0

180.0

time (ms)300000.00.0 50000.0 100000.0 150000.0 200000.0 250000.0

APD1

APD2

Confocal microscope focuses on different emitter

160.0

60.0

80.0

100.0

120.0

140.0

time (ms)300000.00.0 50000.0 100000.0 150000.0 200000.0 250000.0

APD1

APD2

321

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100

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250

100

0

12

25

38

50

62

75

88

1000 12 25 38 50 62 75 88

3

32.0 55.0 27

Sample: Nanodiamonds—Index Matching Fluid

Sample: Nanodiamonds—Index Matching Fluid

Confocal microscope focuses on different emitter

Sample: Nanodiamonds in Cholesteric Liquid Crystal

Sample: Nanodiamonds in Cholesteric Liquid Crystal

430

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2000 25 50 75 100 125 150 175

165.0 81.0 80

300.0

50.0

75.0

100.0

125.0

150.0

175.0

200.0

225.0

250.0

275.0

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

25 by 25 micron scan

Sample: Quantum DotsSample: Quantum Dots

305

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2000 25 50 75 100 125 150 175

160.0 33.0 58

700.0

0.0

100.0

200.0

300.0

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500.0

600.0

time (ms)800000.00.0 100000.0 200000.0 300000.0 400000.0 500000.0 600000.0 700000.0

APD1

APD2

Laser focused on single quantum dot

11.2 by 11.2 micron scan

Sample: Quantum DotsSample: Quantum Dots

700.0

0.0

100.0

200.0

300.0

400.0

500.0

600.0

time (ms)750000.00.0 250000.0 500000.0

APD1

APD2

2000

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2000 25 50 75 100 125 150 175

121.0 137.0 368

Blinking of quantum dots

Sample: Quantum DotsSample: Quantum Dots

Antibunching – minimum at 0 time interval

Research done….

Thanks to Dr. Lukishova