Spin qubit quantum computer
The spin qubit quantum computer is a quantum computer based on controlling the spin of charge carriers in semiconductor devices. The first spin qubit quantum computer was first proposed by Daniel Loss and David P. DiVincenzo in 1997,. The proposal was to use the intrinsic spin-1/2 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.
Loss–DiVicenzo proposal
The Loss–DiVicenzo quantum computer proposal tried to fulfill DiVincenzo's criteria for a scalable quantum computer, namely:- identification of well-defined qubits;
- reliable state preparation;
- low decoherence;
- accurate quantum gate operations and
- strong quantum measurements.
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 for implementing the controlled NOT 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:
- the level spacing in the quantum-dot is much greater than
- the pulse time scale is greater than, so there is no time for transitions to higher orbital levels to happen and
- the decoherence time is longer than
From the pulsed Hamiltonian follows the time evolution operator
where is the time-ordering symbol.
We can choose a specific duration of the pulse such that the integral in time over gives and becomes the swap operator
This pulse run for half the time results in a square root of swap gate,
The "XOR" gate may be achieved by combining operations with individual spin rotation operations:
The operator is a conditional phase shift for the state in the basis of. It can be made into a CNOT gate by surrounding the desired target qubit with Hadamard gates.