Ultrafast Meets Ultrasmall: Dancing Electrons in Nanostructures

1. Ultrafast Meets Ultrasmall: Dancing Electrons in Nanostructures Dr. Xiaoqin (Elaine) Li March 31th, 2007

2. 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

3. 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

4. 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!!

5. 1s 1 fs How Fast is Fast? 1 ps=10-12 s 1 fs=10-15 s 1 as=10-18 s = =

6. 30 modes all in phase 30 modes random phases How to make the fastest shutter Intensity Mode locking Frequency Intensity Time

7. Shortest Laser Pulses: R. Ell et al. Opt. Lett. 2001 ~200 as soft-X-ray pulses Phys. Today April, 2003 & Oct, 2004

8. 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

9. Quantum Well (2D) Quantum Wires (1D) Bulk (3D) Quantum Dot (0D) What are Quantum Dots? Engineering material properties, i.e., emission wavelength

10. 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

11. 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

12. 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

13. 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

14. Applications of Quantum Dots • Biological labeling Fluorescent Labels in an Easy-to-Use Protein Labeling Kit • 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

15. 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!

16. 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…

17. 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…

18. Quantum Computing For one qubit store exponentially more information… For N qubits However, extracting this information is tricky… • Factoring numbers (Shor’s ) • searching database (Grover’s) • enhanced communication protocols

19.  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.

20. s- s- - - - - + + + + - - s+ s+ - - A two-bit system in a dot |00> |01> |11> |10> Qubits are defined in the basis of the Bloch vectors of pseudo-spins

21. Eprobe delay Epump Eprobe Esignal Detector Addressing individual quantum dots

22. p 2p 3p 4p 0 Dancing electrons 1 Excited State Population 0 Pulse Area (Q) p-pulse

23. + Biexciton Two-exciton molecule DE s- s + Excitons s- s + Ground state 1 1 1 3 - - - - 3 1 3 3 1 + + + + + 2 2 2 2 1 + 2 2 2 2 2 2 s+ s- 3 3 - - 2 2 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

24. + DE Biexciton s- s + Excitons s- s + Ground state 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

25. truth table for the quantum gate

26. Our Dream Computer Optics & Photonic News, September 2004

27. Trapped Ions Entangled Photons from optical parametric down conversion Single-atom cavity QED Cold atoms confined in optical lattice

28. 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)