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Sebastian Mehl, PhD 2014, FZ Juelich

Optimal Interesting Quantum Gates with Quantum Dot Qubits David DiVincenzo 19.8.2014 Spinqubits summer school, Konstanz. Sebastian Mehl, PhD 2014, FZ Juelich. Not for today…. Poster A38: Michael Hell Virtual fluctuations with real implications: Renormalization-induced torques in

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Sebastian Mehl, PhD 2014, FZ Juelich

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  1. Optimal Interesting Quantum Gates withQuantum Dot QubitsDavid DiVincenzo 19.8.2014Spinqubits summer school, Konstanz

  2. Sebastian Mehl, PhD 2014, FZ Juelich

  3. Not for today… Poster A38: Michael Hell Virtual fluctuations with real implications: Renormalization-induced torques in spin-qubit control and readout virtual fluctuations of quantum dot electrons induce torques affecting the qubit Bloch vector induce spin torque → coherent backaction → anomalous spin resonance ferro- magnet source sensor quantum dot quantum dot charge / spin qubit ferro- magnet drain

  4. QD Arrays – how can we use them to make effective gates? Petta, Science (2005) Shulman, Science (2012) Medford, Nature (2013) Higginbotham, PRL (2014) Title

  5. Outline • double quantum dots • high bias phase gates • how more electrons can be useful • double quantum dots of different sizes • new sweet spot • manipulation with spin-orbit interactions • two-qubit gates for double quantum dots • triple quantum dots

  6. DQD: High bias phase gate singlet-triplet qubit Levy, PRL (2002) PRB 88, 161408 (2013)

  7. DQD: High bias phase gate singlet-triplet qubit Levy, PRL (2002) exchange interaction, low bias magnetic field gradient PRB 88, 161408 (2013)

  8. DQD: High bias phase gate Pauli spin blockade sweet spot PRB 88, 161408 (2013)

  9. DQD: High bias phase gate go deep into (2,0) – triplet using excited state orbital comes into play sweet spot PRB 88, 161408 (2013)

  10. DQD: High bias phase gate charge noise in leading order: orbital picture for doubly occupied states: Can we find ? PRB 88, 161408 (2013)

  11. DQD: High bias phase gate Fock-Darwin states: “atomic” orbitals for quantum dots For Fock-Darwin conditions (circular, harmonic dot) L orbitals are complex conjugates of one another. Story is more complicated for any other shell. PRB 88, 161408 (2013)

  12. DQD: High bias phase gate Fock-Darwin states: “atomic” orbitals for quantum dots 4 electron configuration has unique charge density, independent of spin use (3,3) <-> (4,2) instead of (1,1) <-> (2,1) PRB 88, 161408 (2013)

  13. 2. Playing with confinement and spin-orbit interaction:finding a different kind of sweet spot

  14. DQDs of different sizes Different kind of sweet spot? Can we get rid of ? arXiv 1408.1010

  15. DQDs of different sizes (l0=20nm vs. 100nm) Different kind of sweet spot? singlet-triplet inversion: strongly confined QD favors singlet weakly confined QD favors triplet degeneracy of singlet and triplet in (1,1) sweet spot? arXiv 1408.1010

  16. DQDs of different sizes is sufficient to operate a STQ at the sweet spot Also spin-orbit interactions do it! Estimate: ΔSO=25 MHz (GaAs), 250MHz (InAs) arXiv 1408.1010

  17. 3. Achieving longer-ranged coupling for two-qubit gates“Conventional” ST qubit, but with new type of coupler

  18. Entangling operation of STQs single quantum state couples QD2 and QD3 empty/doubly occupied: (super) exchange singly occupied: exchange PRB 90, 045404 (2014)

  19. Entangling operation of STQs empty/doubly occupied: (super) exchange note: works also for direct exchange between QD2 and QD3 PRB 90, 045404 (2014)

  20. Entangling operation of STQs singly occupied: exchange PRB 90, 045404 (2014)

  21. Entangling operation of STQs • excellent control: • exchange interaction can be controlled • with independent gate at QS • magnetic fields at QD1 and QD4 do not matter • noise discussion, cf. paper PRB 90, 045404 (2014)

  22. Entangling operation of STQs more complicated gates for e.g. for (1,1,0,1,1) and (1,1,2,1,1): PRB 90, 045404 (2014)

  23. Outline • double quantum dots • high bias phase gates • how more electrons can be useful • double quantum dots of different sizes • new sweet spot • manipulation with spin-orbit interactions • two-qubit gates for double quantum dots • triple quantum dots

  24. Triple Quantum Dot Qubit exchange-only qubit theory: DiVincenzo, Nature (2000) experiment: Medford, Nature (2013) Noise discussion: PRB 87, 195309 (2013) Title

  25. Triple Quantum Dot Qubit exchange-only qubit PRB 87, 195309 (2013) Title

  26. Triple Quantum Dot Qubit exchange-only qubit PRB 87, 195309 (2013) Title

  27. Triple Quantum Dot Qubit exchange-only qubit PRB 87, 195309 (2013) Title

  28. Triple Quantum Dot Qubit • Fidelity: • Qubit encoding in • all other states have different quantum numbers • decoherence free subspace But: dominant noise channels are local e.g. hyperfine interactions PRB 87, 195309 (2013) Title

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