Disentangling photons and atoms to keep quantum systems clean

Entanglement is not always desirable. Careful experiment design could help avoid unplanned-for quantum entanglement between a system and its environment, something known as quantum decoherence.

Chrisopher Hynes

In quantum physics, the divisions between object and observer—the systems and environment—become blurred. Because any measuring device is governed by the laws of quantum mechanics, the act of measurement involves an interaction between two quantum systems. The exact mechanisms by which this works are still unclear in many instances, but much of the quasi-mystical language once used to describe quantum mechanics has given way to precise scientific descriptions.

One remaining frontier is comprehension of how systems gradually lose coherence via interactions with their environment, which prevents their usefulness in quantum computing. A new set of experiments by Yinnon Glickman, Shlomi Kotler, Nitzan Akerman, and Roee Ozeri revealed part of the mechanism by which environment disrupts quantum systems: photons. They found that photons that interacted with a quantum system can end up correlated with the system's state, the hallmark of entanglement. By careful preparation of the atom's state, it may be possible to reduce the loss of quantum information to the environment, and thus extend the life of these systems.

Measurement in quantum physics transforms an indeterminate system—one that behaves as if it's in a number of different states simultaneously—into one with a definite set of physical attributes. For example, the spin of an electron cannot typically be known in the absence of a measurement. However, sending the atom through a (nonuniform) magnetic field will deflect it either up or down relative to the field, showing that the electron is either aligned with or aligned against the magnet.