Quantum feedback control in superconducting qubits: Towards creating and stabilizing entanglement in remote qubits

Recent advances in superconducting parametric amplifiers have enabled quantum limited measurements of superconducting qubits, ushering in a new era of measurement based control using quantum feedback. Quantum entanglement is a key aspect of the measurement process. Measurement creates a pointer state which is entangled with the system being measured. Typically, one analyzes the pointer state which in turn determines the state of the original system. I will discuss experiments where we entangle the state of a 3D transmon qubit with a coherent microwave field (the pointer) using the circuit QED architecture. The use of parametric amplifiers to analyze the microwave field enables us to actually observe this entanglement and the resulting strong correlations between the states of the pointer and the qubit. We reconstruct quantum trajectories of the qubit state as it evolves during measurement and show that the final state of the qubit is consistent with the trajectories. Further, we use quantum feedback to actively steer the state of the qubit and demonstrate Rabi oscillations which persist indefinitely [1]. Finally, I will discuss how we can use the pointer states to generate entanglement between remote qubits and stabilize them using feedback. Applications to quantum computing and quantum error correction will also be discussed.\\[4pt] [1] R.Vijay et al., Nature 490, 77-80 (2012)