Scalable quantum computation architecture using always-on Ising interactions via quantum feedforward

Here, we propose a way to control the interaction between qubits with always-on Ising interaction. Unlike the standard method to change the interaction strength with unitary operations, we fully make use of nonunitary properties of projective measurements so that we can effectively turn the interaction on or off via feedforward. Our scheme is useful for generating two- or three-dimensional cluster states that are universal resources for fault-tolerant quantum computation with this scheme, and it provides an alternative way to realize a scalable quantum processor.

Figure 1

Schematic of our scheme to implement two-qubit gates via projective measurements and quantum feedforward under the effect of always-on Ising interaction. First, we let the state ${|\varphi \rangle }_{AC}\otimes {|+\rangle }_B$ evolve for a time $t_1$ according to the Hamiltonian. Second, we perform a $\pi$ pulse on the middle qubit, B. Third, we let the state evolve for a time $t_2$. Finally, we perform a projective measurement and quantum feedforward on qubit B, so that a controlled-phase gate can be implemented between qubits A and C. Due to the engineered Hamiltonian form that we make, the interaction between qubits is turned off as long as qubit B is in a ground state.

Figure 2

Energy diagrams of qubits A and C. The energies depend on the state of qubit B. The energies of qubits A and C are degenerate when qubit B is in a ground state. However, once qubit B is excited, degeneracy is removed, so that the energy difference occurs between the states of qubits A and C.