Need for Quantum Technologies

1. Need for Quantum Technologies “Quantum physics holds the key to the further advance of computing in the postsilicon era.” - J. Birnbaum and R. S. Williams “Coherent spin packets may offer genuine quantum devices through their wave-like properties.” - D. D. Awschalom • Miniaturization – approaching a physical limit • Quantum effects – statistical, fuzzy, strange – unavoidable • Novel quantum technologies being sought  better performance and new functionality and multi-functionality • Spin-based electronics, quantum electronics based on quantum coherence, interference, and entanglement, … etc.

2. Outline • Semiconductor ‘Spintronics’(or magneto-electronics, spin electronics, or magnetronics) – overview (today) • Devices and Materials for Spintronics (3/31) • Spin Injection and Spin Transport (3/31 & 4/5) • Spin Dynamics (4/12) • Spin Manipulation (4/14)

3. Spintronics • - A new and exciting field of research in semiconductors • Explore synergism between semiconductor devices and magnetic devices (hybrid devices) • Both information processing and storage semiconductors magnetic materials Information Processing • Transistors • Diodes • Lasers • Detectors • Modulators Information Storage • Tapes • Hard Disks • Magneto-optical Disks Spintronics

4. Spin Enhanced and Enabled Semiconductor Electronics • Quantum Spin Electronics • Tunneling/transport of quantum confined spin states: natural frequency scale given by spin splitting: GHz-THz • Spin dependent resonant tunneling devices and spin filtering • Spin FETs (“spin gating”) • Spin LEDs, electroluminescent devices, and spin lasers • Coherent Spin Electronics • Optically generated coherent spin states and coherent control of propagating spin information - optical encoders and decoders • Directly generated coherent spin state and coherent control of propagating spin information • Quantum Information Processing • Qubits using coherent spin states a|0> + b|1>, a2 + b2 = 1 • Spin based quantum computing, teleportation, code breaking and cryptography

5. Giant Magnetoresistance (GMR) • System: Fe/Cr superlattices • Fe layers are anti-magnetically coupled initially • Application of H  alignment of Fe magnetization  resistance drops • Saturates Baibich et al., PRL 61, 2472 (1988)

6. Mott’s Two Spin Channel Model … one can consider the spin-up and spin-down conduction electrons as two independent families of charge carriers, each with its own distinct transport properties --- as long as spin-flip scattering is rare on the time scale of all the other scattering processes N. F. Mott, Proc. R. Soc. 153, 699 (1936)

7. Normal vs. Ferromagnetic Metals “Half-metal,” i.e., 100% spin polarization  “metal” for ↓ electrons and “insulator” for ↑ electrons In reality, Fe’s spin polarization is only ~50% G. A. Prinz, Science 282, 1660 (1998)

8. Spin Bottleneck Magnetoresistance “Spin valve”

9. Actual Devices G. A. Prinz, Science 282, 1660 (1998)

10. GMR Read Head G. A. Prinz, Science 282, 1660 (1998)

11. Magnetic Tunneling Junction Monsma et al., PRL 74, 3273 (1995)

12. GMR- & MTJ-Based Memories Magnetic tunneling junction (MTJ) or “spin valve”  Nonvolatile MRAM: “Microchips that never forget ” Compatibility with Si and GaAs  next phase: semiconductor spintronics S. Parkin (1990)

13. Two Recent Discoveries in Semiconductors • A room temperature, optically induced, very long lived quantum coherent spin state in semiconductors and quantum dots that responds at Terahertz with no dissipation and can be transported by small electric fields (UCSB). • Ferromagnetism in semiconducting GaMnAs (Sendai, Japan).

14. Kikkawa-Awschalom Experiments Awschalom & Kikkawa, Physics Today 52, 33 (1999)

15. Kikkawa-Awschalom Experiments Awschalom & Kikkawa, Physics Today 52, 33 (1999)

16. The Spin Hall Effect Kato et al. (D. D. Awschalom’s group at UCSB)

17. InMnAs: First-Grown III-V Magnetic Semiconductor

18. For a review, see, e.g., H. Ohno, Science 281, 951 (1998)

19. III-V Ferromagnetic Semiconductors Low-temperature MBE grown III1-xMnxV: • InMnAs:Tc < 60 K • GaMnAs:Tc < 250 K Mn-Mn exchange: hole mediated Mn ions (Mn2+) = acceptors & local magnetic moments (3d5, S = 5/2) Carrier density tuning External control of ferromagnetism

20. Carrier-Induced Ferromagnetism Curie Temperature (K) Carrier Density (cm-3) Dietl et al., Science 287, 1019 (2000)

21. Light-Induced Ferromagnetism H. Munekata et al., J. Appl. Phys. 81, 4862 (1997).

22. Electrical Tuning of Farromagntism H. Ohno et al., Nature 408, 944 (2000).

23. Realization of Spin-Based Devices technical issues • How strongly can one create carriers of a given spin? • How long can one sustain the spin polarization? • Can propagate ballistically for > 100 m (Kikkawa & Awschalom) • How can one modulate or control the spin? • An electric field perpendicular to the spin channel acts as an effective magnetic field via spin-orbit coupling (Rashba effect) • How sensitively can one detect the spin?

24. Magnetic Semiconductor as a Spin Aligner Y. Ohno et al., Nature 402, 790 (1999).