Reconsidering Rapid Qubit Purification by Feedback

Weak measurement and rapid state reduction in bipartite quantum systems

In this paper we consider feedback control algorithms for the rapid purification of a bipartite state consisting of two qubits, when the observer has access to only one of the qubits. We show 1) that the algorithm that maximizes the average purification rate is not the same as that that for a single qubit, and 2) that it is always possible to construct an algorithm that generates a deterministi...

Optimality of Feedback Control Strategies for Qubit Purification

Rapid-state purification protocols for a Cooper pair box

Quantum feedback control of a superconducting qubit: Persistent Rabi oscillations

The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single, albeit probabilistic, outcome. The time-scale of this “instantaneous” process can be stretched using weak measurements so that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously tra...

Global versus local optimality in feedback-controlled qubit purification: new insights from minimizing Rényi entropies

It was first shown by Jacobs, in 2003, that the process of qubit state purification by continuous measurement of one observable can be enhanced, on average, by unitary feedback control. Here, we quantify this by the reduction in any one of the family of Rényi entropies α S , with α < < ∞ 0 , at some terminal time, revealing the rich structure of stochastic quantum control even for this simple p...