Hardware of a Quantum Computer #5 Spin Qubit

• Quantum dot : the small space where electrons are pulled in below one gate
  • Quantum bit : spin of that electron
  • Similar to transistors in that they are both involved in controlling the behaviour of electrons by applying local voltages to charge conducting leads
    • Non-linear current-voltage responses
    • Can be fabricated at sizes 10-100 nm
    • BUT quantum dots require more gates to produce and electric field which can trap single electrons
  • Fermi energy : energy of the highest occupied state in the reservoir
  • Coulomb blockade : electrons cannot be removed from / added to the island
  • Individuals added one by one as they pass through the quantum dot
  • Two coupled quantum dots
    • Crosstalk
      • Voltage applied to the first gate electrode affects the potential of the second quantum dot
      • Lines aligned at an angle
    • Change in the number of electrons in one quantum dot changes the alignment of the levels in the second quantum dot
  • Gallium arsenide (GaAs) vs Silicon (Si)
    • Isotopic purification → qubits can have long coherence time in Silicon
    • Similar to conventional silicon electronics → easier industrial development
  • To large scale circuits
    • All Quantum dots to be identical and uniform
    • Cross-bar technology
      • Limited number of wires can be used to adjust a much larger number of components
    • Future
      • Envision true quantum integrated circuits, where different local arrays are interconnected with other local arrays on the same chip using quantum links
        • Quantum links : links that can carry quantum information, that can transfer entanglement
      • Layer of classical electronics co-integrated with the qubits in the same way, to distribute signals on the chip efficiently
  • Strengths
    • Long coherence time
    • High density

Operations on spin qubits

• Magnetic field
  • Absence - spin-up electrons and spin-down electrons have the same energy
  • When introduced - spin-up electrons have higher energy than spin-down electrons
    • Original energy level to split into 2 energy levels
  • Can perform quantum gates by applying an AC magnetic field that has the same frequency as qubit’s resonance frequency
    • Having the resonance is difficult
      • Nuclear spins in the background of the material → unpredictable magnetic field
        → change in qubit’s frequency in an unpredictable way
• Readout qubits
  • Elzerman readout
    • The spin is converted to a charge that can be read out via a nearby electrometer
  • Move the Fermi level of the reservoir in between the two spin states
    • Only a spin-up electron can tunnel out
    • spin-down electron remain trapped inside the quantum dot
    • BUT a spin-down electrons may acquire the energy necessary to tunnel out of the dot from thermal fluctuations
      • Energy gap between the spin-up and spin-down state quite small
        • Energy gap < charging energy of a quantum dot
        • → even small fluctuation can excite a spin-down electron into the reservoir