Frontiers of quantum physics

Abstract:

Quantum systems prepared in pure states evolve into mixtures under environmental action. Continuously realizable ensembles (or physically realizable) are the pure state decompositions of those mixtures that can be generated in time through continuous measurements of the environment. Here, we define continuously realizable entanglement as the average entanglement over realizable ensembles. We search for the measurement strategy to maximize and minimize this quantity through observations on the independent environments that cause two qubits to disentangle in time. We then compare it with the entanglement bounds (entanglement of formation and entanglement of assistance) for the unmonitored system. For some relevant noise sources the maximum realizable entanglement coincides with the upper bound, establishing the scheme as an alternative to protect entanglement. However, for local strategies, the lower bound of the unmonitored system is not reached. © 2011 American Physical Society.

Abstract:

In artificial systems, quantum superposition and entanglement typically decay rapidly unless cryogenic temperatures are used. Could life have evolved to exploit such delicate phenomena? Certain migratory birds have the ability to sense very subtle variations in Earth's magnetic field. Here we apply quantum information theory and the widely accepted "radical pair" model to analyze recent experimental observations of the avian compass. We find that superposition and entanglement are sustained in this living system for at least tens of microseconds, exceeding the durations achieved in the best comparable man-made molecular systems. This conclusion is starkly at variance with the view that life is too "warm and wet" for such quantum phenomena to endure.