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Laterally confined Semiconductor Quantum dots

Laterally confined Semiconductor Quantum dots. Martin Ebner and Christoph Faigle. Semiconductor Quantum Dots. Material Composition & Fabrication Biasing Energy diagram Coulomb blockade Spin blockade QPC Readout Rabi oscillations Summary. Materials and Fabrication.

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Laterally confined Semiconductor Quantum dots

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  1. LaterallyconfinedSemiconductor Quantum dots Martin Ebner and Christoph Faigle

  2. Semiconductor Quantum Dots • Material Composition & Fabrication • Biasing • Energy diagram • Coulombblockade • Spin blockade • QPC • Readout • Rabioscillations • Summary

  3. Materials and Fabrication AlGaAs layer doped with Si introduces free electrons which accumulate at the AlGaAs/GaAs interface creation of 2D electron gas (height ca. 10 nm) at interface. Throughmolecularbeamepitaxy, electrodesarecreated (~10 nm). Throughchoice of structure, depletedareascanbeisolatedfromtherest of theelectrons -> QDs Byapplyingvoltages to metal electrodes on top of AlGaAslayer, localdepletionareas in the 2DEG arecreated To be able to measure the quantum effects, the device is cooled to 20mK.

  4. Biasingthe Quantum Dot Drain Source VSD IDOT VQPC-L VQPC-R IQPC IQPC QPC-L QPC-R

  5. adjustingtunnelrates T Source Drain ΓD ΓS ΓLR M QPC-L L R QPC-R

  6. Gate voltages Drain Source PL QPC-L PR QPC-R

  7. Energy diagrams

  8. Zeemansplitting by application of a magnetic field -> Zeeman splitting

  9. Zeemansplitting

  10. State transitionenergies

  11. Transitionenergies and potentials StatePreparation: alignsource potential and transitionenergy bytuning VG

  12. Coulombblockade

  13. Hanson, Vandersypenet al. 2004: Coulombdiamonds

  14. Transition potentials and tunneling • Source-drain potential differences lead to single-electron tunneling, depending on the potential of thegate • Tunnelingworksonlywhenground to ground-statetransitionlevelsaretunedintothebiaswindow. Onceinitialcurrentflowsexcitedstatetransitionscancontribute to thetunnelingtwo-pathtunneling • If the next ground state also falls into the potential window, there can be two-electrontunneling • Allowed regions are mapped by the coulomb diagram, in Coulomb blockade regions, there is no tunneling and thus no current flow

  15. Spin blockade • According to the Pauli principle, no two electrons with the same spin are allowed on one orbital

  16. QPC VQPC has to betuned to regime of maximumsensitivity steepestslope

  17. Charge sensing • determine number of electrons on dot • non-invasive (no current flow) • only one reservoir needed • conductance of QPC electrometer depends on electrostatic environment • failure for long tunnel times and high tunnel barriers • measurement time about 10us

  18. Single shot spin readout • Spin-to-charge conversion • Energy selective readout only excited state can tunnel off quantum dot due to potential • Tunnel-rate-selective readout difference in tunnel rate leads to high probability of one state then charge is measured on dot, original state is determined

  19. Energy selectivereadout

  20. Elzermanet al. 2004: fidelity: 82%, T1: 0.5ms @10 T

  21. EOR Problems • requires large Energy splitting: kbT << ΔEZ • sensitive to fluctuations of el.stat. potential • photonassistedtunnelingfrom|> to reservoirdue to high frequencynoise ???

  22. RABI Oscillations • ESR: electronspinresonance • alternatingBacperpendicular to Bextand resonant to statesplitting

  23. continuousRabioscillationdetectionscheme

  24. Koppenset al.: Tuning Bextwith/withoutBac

  25. Koppenset al.: Tuning Bext and Bac

  26. pulsedRabioscillationdetectionscheme

  27. Koppenset al.: Rabioscillations

  28. Rabiperformance • fidelity: 73% (131° instead of 180°) • usestrongerBac • application of compositepulses • measure and compensatenuclearfieldfluctuations (nuclearstatenarrowing) • qbitdiscrimination: • BacorBextgradient • localg-factorengineering • useelectricfield (spin-orbit-interaction)

  29. Summary • A scalablephysicalsystemwithwell-characterizedqubits._✔ • Theability to initializethestate of thequbits to a simple fiducialstate._✔ • Long (relative) decoherencetimes, muchlongerthanthegate-operation time.? (T1 ≈ 1ms, T2* ≈ 10ns, T2 > 1μs [spin echo]) • A universal set of quantumgates.✖NO ENTANGLEMENT YET (2005) • A qubit-specificmeasurementcapability. ✔ (Treadout≈ 10μs)

  30. Thanksforyourattention!

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