Transcript Ultrafast Meets Ultrasmall: Quantum Dynamics in Nanostructures

Ultrafast Meets Ultrasmall:
Dancing Electrons in Nanostructures
Dr. Xiaoqin (Elaine) Li
March 31th, 2007
Outline
• What are the scientific questions we are trying to
answer?
• What is our main tool?
• What are quantum dots? Why are they useful?
• The world’s most powerful computer in the future?
• Questions
Probing fast dynamics
• Chemical reactions
Chemical bonds breaking and formation,
energy transfer between molecules happen
on very fast time scales; Take pictures of
molecules during reaction
• Cell Biology
•Many processes such as uptake of oxygen
of blood cells, vision start in a very fast
step
•Views how cell react to drugs
• Physical systems: solids and nanostructures
Electrons
Picture of cells in response to photoactivated cancer treatment drugs
How do we observe fast events?
Use a fast “stop action” camera! (stroboscopy)
• Eadweard Muybridge (1830-1904)
• A brilliant and eccentric photographer
• Photographing animals
• Hired by a rich guy named Leland Stanford
to find out “is there a moment that all four
hooves of a race horse leave the ground?”
• used 12 camera to begin with
• 11 years: 1867-1878
Not fast enough!!
How Fast is Fast?
1 ps=10-12 s
1s
1 fs
=
1 fs=10-15 s
1 as=10-18 s
=
How to make the fastest shutter
Intensity
Mode locking
Frequency
30 modes
all in phase
30 modes
random phases
Intensity
Time
Shortest Laser Pulses:
R. Ell et al. Opt. Lett. 2001
~200 as soft-X-ray pulses
Phys. Today April, 2003 & Oct, 2004
Applications: Laser Machining
A hole drilled in teeth with (a) conventional
lasers and (b) femtosecond lasers
• High precision: machining via ionization atom by atom
• No collateral damages: too fast to deliver heat or shock
• Applied to a wide range of materials: steel, heart tissues
What are Quantum Dots?
Bulk (3D)
Quantum Well Quantum Wires Quantum Dot
(2D)
(1D)
(0D)
Engineering material properties, i.e., emission wavelength
Customized solid-state atoms
• Si nanocrystals formed by solid
phase crystallization
• Colloidal chemically- synthesized
CdSe nanocrystals in solution or
polymer thin film
TEM Image of Si nanocrytals
From G. F. Grom et al. Nature
Vol 407, 358
• Lithographically fabricated
electrostatic gate defined quantum
dots
• Self-assembled quantum dots
formed at interfaces of a strained
system during heteroepitaxial growth
Customized solid-state atoms
CdSe nanocrystals.
From X. Peng, Nature
404, 59
• Si nanocrystals formed by solid
phase crystallization
• Colloidal chemically- synthesized
CdSe nanocrystals in solution or
polymer thin film
• Lithographically fabricated
electrostatic gate defined quantum
dots
• Self-assembled quantum dots
formed at interfaces of a strained
system during heteroepitaxial growth
Customized solid-state atoms
• Si nanocrystals formed by solid
phase crystallization
• Colloidal chemically- synthesized
CdSe nanocrystals in solution or
polymer thin film
AFM image and illustrations of two
quantum dots defined electrostatic
gates. A. W. Holleitner et.al. Science
vol 297, 70, 2002
• Lithographically fabricated
electrostatic gate defined quantum
dots
• Self-assembled quantum dots
formed at interfaces of a strained
system during heteroepitaxial growth
Customized solid-state atoms
• Si nanocrystals formed by solid
phase crystallization
SEM image
taken by A.
Hartmann et. al,
PRL, 84, 5648
• Colloidal chemically- synthesized
CdSe nanocrystals in solution or
polymer thin film
• Lithographically fabricated
electrostatic gate defined quantum
dots
• Self-assembled quantum dots
formed at interfaces of a strained
system during heteroepitaxial
growth
Applications of Quantum Dots
•Biological labeling
Fluorescent Labels in an Easyto-Use Protein Labeling Kit
•Solar cells
•Transistor and light sources
•Quantum logic gates
Single Electron Transistor
made from CdSe Nanocrystal.
From D. L. Klein, Nature,
389,699
Single photon
source. The microdisk contains MBE
grown InAS
quantum dots.
From P. Michler.
Science 290, 2282.
The World’s Most Powerful Computer?
The TRANSLTR: A powerful code
breaking machine at NSA
• three million processors would all
work in parallel
• it breaks the code of an
encrypted email in minutes
• No more secrets: what is your
plan this weekend?
•However, NSA kept it as a secret
Susan Fletcher, a brilliant and beautiful mathematician and the head
cryptographer discovers that NSA is being held hostage by a code that
would cripple US intelligence.. As she battles to save the agency, she finds
herself fighting not only for her country but also for her life, and in the end,
for the life of the man she loves…
A Fiction book!
The World’s Most Powerful Computer?
Practically since human being began writing, they
have been writing in code, and ciphers have
decided the fates of empires throughout
recorded history. It has always been a neck-toneck race, with code-breakers battling back when
code-makers seems to be in command, and codemakers inventing new and stronger forms of
encryption when previous methods had been
comprised.
Phip Zimmermann: A golden age of cryptography. It is now possible to make ciphers
in modern cryptography that are really out of reach for code-breakers. And it is
going to stay that way…
William Crowell, deputy director of NSA: If all the personal computers in the
world-approximately 260 million computers-were to put to work on a single PGP
encrypted message, it would take on average an estimated 12 millions times the age
of the universe to break a single message…
Is it ever possible to break
an encrypted email?
Yes, if one can ever build a quantum computer…
• Breaking news, made it to the
state of the union address
• How does a modern code work?
• What is different about a
quantum computer?
Information is represented with 0 and 1; a classical bit is wither 0 or 1
or
In the quantum world, one qubit can be in the superposition of 0 and 1
and
only possible with a nano-switch…
Quantum Computing
For one qubit
  0  1
store exponentially more information…
For N qubits
   0 0102 03...0 N   1 0102 03...1N  ...
  N 111213...1N
2 1
However, extracting this information is tricky…
 Factoring numbers (Shor’s )
 searching database (Grover’s)
 enhanced communication protocols
Elements of quantum computing
• Represent quantum information with proper qubits
• Perform a universal family of unitary transformations
• Single-bit operations
• A two-bit conditional quantum gate: CNOT
A
B

A
B A
• Prepare a set of specified initial states
• Read out the computation output.
A two-bit system in a dot
-
-
|00>
s
|10>
+
|01>
-
s
-
+ -
-
s s
-
+ +
|11>
Qubits are defined in the basis of the Bloch vectors of pseudo-spins
delay
Eprobe
DT (a.u.)
Addressing individual quantum dots
Epump
Esignal
Detector
Eprobe
1.619
1.620 1.621 1.622
Energy (eV)
1.623
DT (a.u.)
1.618
0
20
40
60
Delay (ps)
80
1.624
Dancing electrons
Excited
State
Population
1
0
0
p-pulse
p
2p
Pulse Area (Q)
3p
4p
A Two-bit Quantum Gate
+
Biexciton
Two-exciton molecule
DE
1
2
s3
2
1
2
Coulomb Interaction
s
s+
11
s
3
Excitons
10
01
2
s
s+
Ground state
00
conditional operations:
The excitation of one exciton
affects the resonant energy of the
other exciton
1
3
2
2
1
1
2
s
2
or
3
2
3
2
1
s
2
3
2
A two-bit quantum gate
+
DE
s+
Biexciton
s
11
Excitons
10
01
s
s+
Input
C T
0 0
0 1
1 0
1 1
Output
CT
0 0
0 1
1 -1
1 0
Ground state
00
A p pulse tuned to the transition
operation
10  11 as the gate
truth table for the quantum gate
Physical Truth Table
1
1
0.9
0.8
0.8
0.63
0.7
0.67
0.6
Population 0.5
0.4
0.13
0.3
0
0.2
0
0.1
0
0.2
0
0
0.17
|11>
0.06
0.11
0.14
0
|00>
0.09
|01>
|10>
Input State
|10>
|01> Output State
|00>
|11>
Our Dream Computer
Optics & Photonic News,
September 2004
Trapped Ions
Entangled Photons from optical
parametric down conversion
Cold atoms confined in
optical lattice
Single-atom
cavity QED
Questions
• How to capture a fast event?
(use a camera with a faster shutter)
•What is the duration of the shortest laser pulse ever
created? (200 as)
•What drilling tool most people might prefer when visiting
their dentists in the future? (fs lasers)
• Name 2 possible applications for quantum dots.
(solar cells, transistors, protein labels, etc.)
• Does a super computer that is capable of breaking an
encrypted email currently exist?
(No)
• Can a quantum computer ever be built? (We hope so)