20220905, Pontresina Speakers: https://spinqubit5.nccr-spin.ch/speakers/program?

Vandersypen - Spin qubits and simulators - more, better, easier

• 25 years ago was the proposition
• why are spin qubits great?

Vision of a semiconductor qubit processor

• local registers

• quantum links

• integrated circuits

  • overcome the wiring bottleneck

• Linear six qubit array

• visibility of 96%, one qubit control at a time, decreases with simultaneous measurement

• spin resonance frequency shifts with microwave driving and with temperature, non-monotonous temperature dependence

  • should we operate at 200mK?
    • no RF heating effects, and T2 barely reduced.

• Cross talk during simultaneous driving of two qubits by EDSR

  • after you reach a certain Rabi frequency you see a plateau - this is expected, as the maximum distance that an electron can move inside a dot is capped.

• 2x2 Si/SiGe quantum dot array

  • multi-layer design
    • one electron regime reached
    • independent control tunnel coupling achieved

Quantum links

Electron shuttling

• take an electron and move it across the chip in hope that the spin state will be preserved
  • Charge shuttling in a 4-dot array, arXiv 2209.000920
  • estimated spin-flip probability per hop below 0.01%
  • Vande thinks that electron shuttling will be a very attractive approach (Boter PRA 2022)

Superconducting resonators

• coupling distance spins through a superconducting resonator
  • try a dispersive regime.
    • spins are resonant with each other but detuned from the resonator
• quantum simulation of Fermi Hubbard physics
• can get exciton physics in 4x ladder

Silvano De Franceschi - Long life to holes

• 100MHz you can drive holes
  • electrically addressable, but exposed to charge noise defects
  • "sweet spots" where a single hole spin has enhanced coherence in silicon
• Fast single-shot readout of single-hole spin in SiMOS
• Strong coupling between a photon and a hole spin in silicon (see Copenhagen conference)
  • create coupling with electric field in the device and electric field in the cavity

Dominik Zumbuhl - Hot Hole spin qubits in Ge/Si nanowires

• Holes could have a great future
• NCCR Spin - developing fast, compact and scalable electron and hole spin qubits
• Spin-orbit interaction: Hso L, fast all electrical qubits, price charge noise coupling
• no contact hyperfine interaction -> longer coherence times
• g-factor can be modulated strongly with gate voltages

finfets

• Rabi oscillations: 100MHz, Q factor of 100
• operable at 5K, readout actually ends up being the limiting factor via Pauli spin blockade
• Anisotropy of G-factor is strong

nanowires

• spin mixing transitions in spin blockade
• strong spin-orbit coupling ISO ~65nm
• all electrical spin manipulation (EDSR)
• Rabi oscillations can be made extremely fast, 1ns for spin flips
• tunable g-factor is useful for coupling to a resonator

Guido Burkard - Finger prints qubits noise in cavity QED

• Two-qubit gates between spin qubits AC/DC
• Qubit noise limiting gate fidelities and qubit coherence
• describe the system using quantum Langevin equations
• Semiconductor spin qubits arXiv 2112.08863

Andreas Wallraff - repeated quantum error correction in surface code

• google computer fidelity was 10-3, so way less than 1
• Challenges in quantum error correction: bit flips and phase flips
• why surface code?
  • can realise it in a planar geometry
  • has the largest error threshold, 1%
• fast QEC cycle of 1.1 us

Pasquale Scarlino - superconducting and semi-conducting, hybrid quantum circuits

• Barely reached the strong coupling regime until fairly recently (2017/2018)
• High impedance technology: josephson junction array
• NbN disordered thin films
• High impedance SQUID array resonator (100x smaller than competition)