Spin qubit quantum computer
The spin qubit quantum computer is a quantum computer based on controlling the spin of charge carriers (electrons and electron holes) in semiconductor devices. The first spin qubit quantum computer was first proposed by Daniel Loss and David P. DiVincenzo in 1997, also known as the Loss–DiVicenzo quantum computer. The proposal was to use the intrinsic spin-½ degree of freedom of individual electrons confined in quantum dots as qubits. This should not be confused with other proposals that use the nuclear spin as qubit, like the Kane quantum computer or the nuclear magnetic resonance quantum  computer. Spin qubits for have been implemented by locally depleting two-dimensional electron gases in semiconductors such a gallium arsenide, silicon and germanium. Spin qubits have also been implemented in graphene. Loss–DiVicenzo proposal. The Loss–DiVicenzo quantum computer proposal tried to fulfill DiVincenzo's criteria for a scalable quantum computer, namely: A candidate for such a quantum computer is a lateral quantum dot system. Earlier work on applications of quantum dots for quantum computing was done by Barenco et al. Implementation of the two-qubit gate. The Loss–DiVincenzo quantum computer operates, basically, using inter-dot gate voltage for  implementing swap operations and local magnetic fields (or any other local spin manipulation) for implementing the controlled NOT gate (CNOT gate). The swap operation is achieved by applying a pulsed inter-dot gate voltage, so the exchange constant in the Heisenberg Hamiltonian becomes time-dependent: This description is only valid if: From the pulsed Hamiltonian follows the time evolution operator We can choose a specific duration of the pulse such that the integral in time over formula_9 gives formula_10 and formula_11 becomes the swap operator formula_12. The CNOT gate may be achieved by combining formula_13 (square root of swap) operations  with individual spin operations: This operator gives a conditional phase for the state in the basis of formula_15.