How is CNOT gate physically implemented in IBM Q?

Question

This is because if control qubit is in arbitrary state then how can it be made to control the CNOT gate? What is the interface between Control Qubit and CNOT Gate?

Answer 1

First of all let me welcome you to QCSE! I'm new myself.

IBM operates superconducting qubits based on josephson junctions. The basic architecture of the chip can be visualized as a graph with qubits at the vertices and superconducting, microwave resonators as the edges. Their native two-qubit operation is something called a cross-resonance gate. Basically, the control qubit is driven at the transition frequency of the target qubit. This induces a coupling between the two that depends on the amplitude of the drive signal. By tuning this amplitude, you can obtain an effective hamiltonian that corresponds to the generator of the clifford group $[ZX]^{1/2}$. This, combined with single-qubit rotations, can give you a CNOT gate. For more details I'd recommend this paper.

Hope this helps.