Improving the lifetime of the NV center ensemble coupled with a superconducting flux qubit by applying magnetic fields

One of the promising systems to realize quantum computation is a hybrid system where a superconducting flux qubit plays a role of a quantum processor and the NV center ensemble is used as a quantum memory. We have theoretically and experimentally studied the effect of magnetic fields on this hybrid system, and found that the lifetime of the vacuum Rabi oscillation is improved by applying a few mT magnetic field to the NV center ensemble. Here, we construct a theoretical model to reproduce the vacuum Rabi oscillations with/without magnetic fields applied to the NV centers, and we determine the reason why magnetic fields can affect the coherent properties of the NV center ensemble. From our theoretical analysis, we quantitatively show that the magnetic fields actually suppress the inhomogeneous broadening from the strain in the NV centers.

Ising interaction between capacitively-coupled superconducting flux qubits

Here, we propose a scheme to generate a controllable Ising interaction between superconducting flux qubits. Existing schemes rely on inducting couplings to realize Ising interactions between flux qubits, and the interaction strength is controlled by an applied magnetic field On the other hand, we have found a way to generate an interaction between the flux qubits via capacitive couplings. This has an advantage in individual addressability, because we can control the interaction strength by changing an applied voltage that can be easily localized. This is a crucial step toward the realizing superconducting flux qubit quantum computation.

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 non-unitary properties of projective measurements so that we can effectively turn the interaction on or off via feedforward. Our scheme is useful to generate 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 pro