Ultrafast Meets Ultrasmall

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Ultrafast Meets Ultrasmall Dancing Electrons in
Nanostructures
Dr. Xiaoqin (Elaine) Li
March 31th, 2007
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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 worlds most powerful computer in the
  future?
• Questions

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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 photo-activated
cancer treatment drugs
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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!!
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How Fast is Fast?
1 ps10-12 s
1 fs10-15 s
1 as10-18 s


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How to make the fastest shutter
Intensity
Mode locking
Frequency
Intensity
Time
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Shortest Laser Pulses
R. Ell et al. Opt. Lett. 2001
200 as soft-X-ray pulses
Phys. Today April, 2003 Oct, 2004
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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

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What are Quantum Dots?
Engineering material properties, i.e., emission
wavelength
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Customized solid-state atoms

• 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

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Customized solid-state atoms

• 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

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Customized solid-state atoms

• 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

AFM image and illustrations of two quantum dots
defined electrostatic gates. A. W. Holleitner
et.al. Science vol 297, 70, 2002
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Customized solid-state atoms

• 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

SEM image taken by A. Hartmann et. al, PRL, 84,
5648
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Applications of Quantum Dots
Fluorescent Labels in an Easy-to-Use Protein
Labeling Kit

• Biological labeling

• Solar cells

• Transistor and light sources

• Quantum logic gates

Single photon source. The micro-disk contains
MBE grown InAS quantum dots. From P. Michler.
Science 290, 2282.
Single Electron Transistor made from CdSe
Nanocrystal. From D. L. Klein, Nature, 389,699
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The Worlds 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!
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The Worlds 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-to-neck race,
with code-breakers battling back when code-makers
seems to be in command, and code-makers 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
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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
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Quantum Computing
For one qubit
store exponentially more information
For N qubits
However, extracting this information is tricky

• Factoring numbers (Shors )
• searching database (Grovers)
• enhanced communication protocols

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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
• Prepare a set of specified initial states
• Read out the computation output.

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A two-bit system in a dot
00gt
01gt
11gt
10gt
Qubits are defined in the basis of the Bloch
vectors of pseudo-spins
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Addressing individual quantum dots
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Dancing electrons
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A Two-bit Quantum Gate
Coulomb Interaction
s
s -
conditional operations The excitation of one
exciton affects the resonant energy of the other
exciton
or
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A two-bit quantum gate
Input C T 0 0 0 1 1 0 1 1
Output C ? T ? 0 0 0 1 1 -1 1 0
A ? pulse tuned to the transition
as the gate operation
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truth table for the quantum gate
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Our Dream Computer
Optics Photonic News, September 2004
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Trapped Ions
Entangled Photons from optical parametric down
conversion
Single-atom cavity QED
Cold atoms confined in optical lattice
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Questions

• How to capture a fast event?
• What is the duration of the shortest laser pulse
  ever created?
• What drilling tool most people might prefer when
  visiting their dentists in the future?
• Name 2 possible applications for quantum dots.
• Does a super computer that is capable of
  breaking an encrypted email currently exist?
• Can a quantum computer ever be built?

(use a camera with a faster shutter)
(200 as)
(fs lasers)
(solar cells, transistors, protein labels, etc.)
(No)
(We hope so)