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 reactionsChemical bonds breaking and formation, energy transfer between molecules happen on very fast time scales; Take pictures of molecules during reaction

Picture of cells in response to photo-activated cancer treatment drugs

• Physical systems: solids and nanostructures

Electrons

• 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

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?

= =1s

1 fs

1 ps=10-12 s 1 fs=10-15 s 1 as=10-18 s

Frequency

Intensity

Time

Intensity

30 modesrandom phases

30 modesall in phase

How to make the fastest shutter

Mode locking

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 (2D)

Quantum Wires (1D)

Quantum Dot (0D)

Engineering material properties, i.e., emission wavelength

TEM Image of Si nanocrytals

From G. F. Grom et al. Nature

Vol 407, 358

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

CdSe nanocrystals.

From X. Peng, Nature 404,

59

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

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

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

Applications of Quantum Dots

Fluorescent Labels in an Easy-to-Use Protein

Labeling Kit

Single Electron Transistor made from CdSe Nanocrystal. From D. L. Klein, Nature, 389,699

Single photon source. The micro-disk contains MBE grown InAS quantum dots.From P. Michler. Science 290, 2282.

•Biological labeling

•Solar cells

•Transistor and light sources

•Quantum logic gates

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-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…

Is it ever possible to break an encrypted email?

Yes, if one can ever build a quantum computer…

Information is represented with 0 and 1; a classical bit is wither 0 or 1

In the quantum world, one qubit can be in the superposition of 0 and 1

• Breaking news, made it to the state of the union address

• How does a modern code work?

• What is different about a quantum computer?

or

and only possible with a nano-switch…

Quantum Computing

However, extracting this information is tricky…

0 1

0 010203...0N 1 010203...1N ...

2N 1

111213...1N

For one qubit

For N qubits

store exponentially more information…

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

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

A

A

B B A

A two-bit system in a dot

|10> |11>

|00> |01>

- - -

-

+

-

-

+

--

++

Qubits are defined in the basis of the Bloch vectors of pseudo-spins

Addressing individual quantum dots

Eprobe

Epump

EprobeEsignal

Detector

delay

1.618 1.619 1.620 1.621 1.622 1.623 1.624Energy (eV)

DT

(a

.u.)

0 20 40 60 80

DT

(a

.u.)

Delay (ps)

Dancing electrons

0

1Excited State Population

0 2 3 4Pulse Area (-pulse

A Two-bit Quantum Gate

212

1

23

23

-

21

21

23

23

2

121

23 2

3

conditional operations:The excitation of one exciton affects the resonant energy of the other exciton

or

Coulomb InteractionCoulomb Interaction

BiexcitonTwo-exciton

moleculeE

+

Ground state

Excitons

+

00

+

10

01

11

A two-bit quantum gate

BiexcitonE

+

Ground state

Excitons

+

00

+

10

01

11

InputC T0 00 11 01 1

OutputC T 0 00 11 -11 0

A pulse tuned to the transition as the gate operation

1110

truth table for the quantum gate

|00>|01>

|10>|11>

|00>

|01>

|10>

|11>

0 0

0.63

0.13

0 00.17

0.67

0

0.8

0.06

0.11

1

0.2

0.14

0.090

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Population

Input State

Output State

Physical Truth Table

Our Dream Computer

Optics & Photonic News, September 2004

Trapped Ions

Entangled Photons from optical parametric down conversion

Single-atom cavity QEDCold atoms confined in

optical lattice

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)