8th June 2006 Rob Lambert - UoE PPE Seminar 1 Towards a Quantum Dot-Based Single Photon Source Supervisor: R. J. Warburton Assistant Supervisor: P. Dalgarno Moderator: P. Barker photon antibunching from a single charge-tuneable quantum dot
8th June 2006 Rob Lambert - UoE PPE Seminar 1
Towards a Quantum Dot-Based Single Photon Source
Supervisor: R. J. Warburton
Assistant Supervisor: P. Dalgarno
Moderator: P. Barker
photon antibunching from a single charge-tuneable quantum dot
8th June 2006 Rob Lambert - UoE PPE Seminar 2
SET Award
Best Physics Student:
• National competition
• Round 1: Reference
• Round 2: Current Performance
• Round 3: Project Synopsis
• Round 4: Interview
Sponsors/Judges Include:
• NPL
• IOP
• UCL
• Imperial
8th June 2006 Rob Lambert - UoE PPE Seminar 3
Single Photon Applications
Quantum Computing:
• Spin-interaction and interference for binary 1 or 0 states
• Quantum Teleportation
• Ultrafast low-energy computation
Quantum Cryptography:
• Indeterministic nature of quanta
• Verifiably secure information transfer
8th June 2006 Rob Lambert - UoE PPE Seminar 4
Single Photon Applications
Quantum Cryptography:
• Imagine you’re an Octopus
8th June 2006 Rob Lambert - UoE PPE Seminar 5
Single Photon Applications
…… providing single photons can be reliably produced on demand.
Quantum Computing:
• Spin-interaction and interference for binary 1 or 0 states
• Quantum Teleportation
• Ultrafast low-energy computation
Quantum Cryptography:
• Indeterministic nature of quanta
• Verifiably secure information transfer
8th June 2006 Rob Lambert - UoE PPE Seminar 6
Single Photon Production
Highly Attenuated Laser Pulse Trains (RT):
• Multi-photon pulses (Poissonian probability)
Single Atom/Molecule emission (~RT):
• Difficult selection and microscopy <Å
• Bleaching and Blinking … Not Ideal
Single-QD regimes (4.2K):
• nm-scale island of semiconductor
• Artificial atom
• 3D confining potential
• ~Parabolic quantum well
• None of above problemsAtomic force micrograph image
Axel Lorke
8th June 2006 Rob Lambert - UoE PPE Seminar 7
Looking at Single QDs at 4.2K
Confocal Microscopy:
• high-stability, liquid-helium-cooled, solid-immersion-lens-enhanced, diffraction-limited, confocal microscope
Liquid He @ 4.2K
AR Window
To motor controller Liquid He @ 4.2K
AR Window
To motor controller
BS1
BS2
8° Angled Single-Mode (980nm) Fibre
IP
SampleSampleSample
Piezoelectric
X-Y-Z motors
ITO Gate
Vg
Objective lens NA=0.65
SIL
Annealed Contact
Lock-in Amplifier
I
Vg
vosc VDAC
AC+DC Mixer
Lock-in Amplifier
I
Vg
vosc VDAC
Lock-in Amplifier
I
Vg
vosc VDAC
AC+DC Mixer
826nm ExitationLaser Diode
CCD camera
Photodiode8° Angled Fibre
Collected PL
BandpassFilter
Holographic notch filter
8th June 2006 Rob Lambert - UoE PPE Seminar 8
Looking at Single QDs at 4.2K
Confocal Microscopy:
• Not as complicated as it looks…. Slightly
• Light from other dots thrown away.
826nm Laser
Up to 10MHz pulsed
Sample @ 4.2 K
~950nm PL
Delayed Emission
Collected by fibre
Fibre Delivered
High NA lensFibre Apertures
Motorized
Piezo
stage
Vg
8th June 2006 Rob Lambert - UoE PPE Seminar 9
MISFET Structure
gatesubstrate
backc onta ct
tun nelb arr ie r
A lAs/GaA s
blo ck ingb arrier
InAs dots
Vg1
Vg2
Fermi energy
R. J. Warburton, C. Schäflien, D. Haft et al., Nature 405, 926
(2000)Vg
2 > Vg1
n-GaAs
Metal - Insulator - Semiconductor Field Effect Transistor:
8th June 2006 Rob Lambert - UoE PPE Seminar 10
Charge-Tuneable PL
X0 X1-
2X0 2X1-
X2- 2X0h
Example State Configurations (spin omitted)
Exiton Spectroscopy:
8th June 2006 Rob Lambert - UoE PPE Seminar 11
Sample Preparation
Gate Contact:• Higher collection efficiency = better statistics
• Improve transmission, move to Indium Tin Oxide
15000 14000 13000 12000 11000 10000 90000.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0700 750 800 850 900 950 1000 10501100
ITO NiCr
Rel
ativ
e Tr
ansm
issi
on
Wavenumber (cm-1)
Wavelength (nm)
8th June 2006 Rob Lambert - UoE PPE Seminar 12
Sample Preparation
Solid Immersion Lens SIL:
• Lower Refraction at Semiconductor Interface
• Increase Refractive Index
• Wider angle = More Light + Better resolution = Easier selection
With SIL
Without SIL
8th June 2006 Rob Lambert - UoE PPE Seminar 13
Experiment
QD PL
0.001
Start
Stop
BEAM SPLITTER
SPAD 1
SPAD 2
FILTER
FILTER
Photon Counting Statistics:
• Hanbury-Brown Twiss Interferometer
8th June 2006 Rob Lambert - UoE PPE Seminar 14
Auto-Correlation
QD PL
0.001
Start
Stop
BEAM SPLITTER
SPAD 1
SPAD 2
FILTER
FILTER
Photon Counting Statistics:
• Hanbury-Brown Twiss Interferometer
• CW and Pulsed antibunching 2nd-order autocorrelation
-30 -20 -10 0 10 20 300.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Nor
mal
ized
cou
nts
τ (ns)
g(2)trough=0.261
τr = 1.00ns
g(2)(0)=0.24123τr = 0.9131ns
g(2)(0)=0.30437
30 -100 -50 0 50 100-10
0
10
20
30
40
50
60
70
80
X0 Vg= -0.9V
Coi
ncid
ence
s
τ (ns)
0.960.95
1.07
1.011.01
1.00
0.0763
1.00
1.02
1.01
0.98
1.08
0.941.02
8th June 2006 Rob Lambert - UoE PPE Seminar 15
Anti-Bunching
Unexpected results:
• Gate-voltage dependant antibunching
• Difference in dependance between X0 and X1- for the same dot
8th June 2006 Rob Lambert - UoE PPE Seminar 16
Results
Photon Counting Statistics:
X1-
8th June 2006 Rob Lambert - UoE PPE Seminar 17
Results
Photon Counting Statistics:
00.0
0.2
0.4
0.6
-0.75V
-0.76V
-0.80V
-0.84V
-0.90V
0.08
0.15
0.12
0.12
0.08
Sca
led
Cou
nts
t (ns)
g(2)(0)Vg
12.5 12.5
X0
8th June 2006 Rob Lambert - UoE PPE Seminar 18
Time Resolved Photo-Luminescence
Photon Counting Statistics:
• Direct comparison to TRPL data
• Neutral Exciton fine structure
0 10 20 30 40 50
10
100
1000
Coi
ncid
ence
s
time (ns)
X0 Dual Exponential
Decay
DarkBright
Spin-Flip
Fine structure discussed in J.M. Smith, P.A.Dalgarno, R.J. Warburton et. al PRL v. 94 n. 19 (2005) pp 197402
X0 X0
X1- X1-
Always Bright
8th June 2006 Rob Lambert - UoE PPE Seminar 19
Comparison with TRPL
Photon Counting Statistics:
• Decay lifetimes/amplitudes vary with gate voltage
• Coulomb Blockade Model
• Agreement with Autocorrelation
-0.85 -0.80 -0.75 -0.70 -0.65 -0.60
0.1
1
10
Life
time
(ns)
Gate Voltage Vg (V)
Primary
Secondary
Autocorrelation
-0.85 -0.80 -0.75 -0.70 -0.65 -0.600.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Primary % Secondary %
Com
pone
nt R
atio
Gate Voltage Vg (V)00.0
0.2
0.4
0.6
-0.75V
-0.76V
-0.80V
-0.84V
-0.90V
0.08
0.15
0.12
0.12
0.08
Sca
led
Cou
nts
t (ns)
g(2)(0)Vg
8th June 2006 Rob Lambert - UoE PPE Seminar 20
-30 -20 -10 0 10 20 300
40
80
120
160
200
0
0
0
1
1
2
2
Coi
ncid
ence
s
τ (ns)
Cross-Correlation
Photon Counting Statistics:
• Quantum Cascade
Peak g(2) = 2.50
Minimum g(2) = 0.41
exponential decay
τr=0.744ns
≡ τd(X0)=0.746ns
2X0 X0
START STOP
Vacuum
8th June 2006 Rob Lambert - UoE PPE Seminar 21
Summary
• ITO+SIL = Better collection efficiency = Higher statistics+Better Resolution
• Verified Antibunching @ 4.2K = Single Photons from QD
• Single Photons = Proof of Second Quantization
• Agreement with TRPL on the same QD for the first time
• Quantum cascade witnessed = Possible trigger for photons on demand
8th June 2006 Rob Lambert - UoE PPE Seminar 22
Outlook
QDs provide Single Photons
• Secure information
• Good news for Bob and Alice
QD Single photons are indistiguishable
• Quantum Teleportation
• Good news for Scotty/Kirk
QD single photon sources for supercomputers?
•Good news for Boffins everywhere
Possibly a lot more talks on this subject in the future
• Bad news for Jellybabies