Abstract: The Bell-state analyzer includes a semiconductor device having quantum dots formed therein and adapted to support Fermions in a spin-up and/or spin-down states. Different Zeeman splittings in one or more of the quantum dots allows resonant quantum tunneling only for antiparallel spin states. This converts spin parity into charge information via a projective measurement. The measurement of spin parity allows for the determination of part of the states of the Fermions, which provides the states of the qubits, while keeping the undetermined part of the state coherent. The ability to know the parity of qubit states allows for logic operations to be performed on the qubits, i.e., allows for the formation of (two-qubit) quantum gates, which like classical logic gates, are the building blocks of a quantum computer. Quantum computers that perform a parity gate and a CNOT gate using the Bell-state analyzer of the invention are disclosed.

Abstract: The Bell-state analyzer includes a semiconductor device having quantum dots formed therein and adapted to support Fermions in a spin-up and/or spin-down states. Different Zeeman splittings in one or more of the quantum dots allows resonant quantum tunneling only for antiparallel spin states. This converts spin parity into charge information via a projective measurement. The measurement of spin parity allows for the determination of part of the states of the Fermions, which provides the states of the qubits, while keeping the undetermined part of the state coherent. The ability to know the parity of qubit states allows for logic operations to be performed on the qubits, i.e., allows for the formation of (two-qubit) quantum gates, which like classical logic gates, are the building blocks of a quantum computer. Quantum computers that perform a parity gate and a CNOT gate using the Bell-state analyzer of the invention are disclosed.

Abstract: A spin filtering device has: a spin filter (1) having an input region (3) for carrying an electron current, an output region (4) for carrying an electron current, and a three-dimensionally confined quantum region (2a) arranged to operate in the Coulomb blockade regime and separating the input and output regions (3 and 4) whereby electrons can only pass from the input region to the output region by tunnelling through the quantum region; and Zeeman splitting means (5) for causing Zeeman splitting in the spin filter, the quantum region (2a) and input and output regions (3 and 4) being formed such that the Zeeman splitting in the input and output regions (3 and 4) is less than the Fermi energy such that, in operation, the spin filter outputs a tunnelling current predominantly of one spin polarity. The direction of Zeeman splitting may be controlled to control the predominant spin polarity.