Friday, April 27, 2007

Entanglement Sudden Death

There is a very fascinating paper in Science this week.

M.P. Almeida et al. "Environment-Induced Sudden Death of Entanglement", Science v.316, p.579 (2007).

Abstract: We demonstrate the difference between local, single-particle dynamics and global dynamics of entangled quantum systems coupled to independent environments. Using an all-optical experimental setup, we showed that, even when the environment-induced decay of each system is asymptotic, quantum entanglement may suddenly disappear. This "sudden death" constitutes yet another distinct and counterintuitive trait of entanglement.

A perspective on this paper, written by Eberly and Yu, is also in the same issue of Science.

They have devised an elegantly clean way to check and to confirm the existence of so-called "entanglement sudden death" (ESD) (7), a two-body disentanglement that is novel among known relaxation effects because it has no lifetime in any usual sense--that is, entanglement terminates completely after a finite interval, without a smoothly diminishing long-time tail.

But what was more interesting, at least to me (and I'm still reading the paper the 2nd time trying to understand this), is this part of that perspective article:

It is often implied and sometimes said explicitly, in textbooks as well as in physics colloquia, that our evidence for the quantum character of natural phenomena comes from the existence of wave-particle duality in the microworld. But this is misleading at best. Wave mechanics is just optics for particles, and it contains effects no more exotic than are found in physical optics (rays, diffraction, tunneling, etc.). In striking contrast, quantum mechanics exhibits features that have no classical wave counterpart at all. Duality is no help in understanding the entangled nature of Schrödinger's Cat, which "exists" in a strange entangled state, equally likely dead and alive (19-21).

To investigate quantitatively the time development of a property such as the degree of entanglement of two or more quantum systems is to enter what is probably the largest nonclassical sector of the world we live in, and the report by Almeida et al. brings new evidence to bear on these questions. They have used a photonic Cat: a pair of qubits (quantum bits in the form of photon polarizations) whose degree of mutual entanglement they can study in the clear absence of mutual interaction. Each of these photonic qubits interacts only with its own individual environment, and this produces smooth dephasing of each individual photon's polarization angles. But this provides very misleading guidance to a quantitative understanding of the photons' Cat-like properties. One-body information about the photons is useless to explain the sudden death of their entanglement.

This is interesting because I have several entries on Schrodinger Cat-type states and measurements. While this is not quite the same thing, it would be interesting to see where they match.


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