Photos of the quantum-classical transition
The transition between the quantum and the classical ¿world¿ has been
visualized experimentally for the first time, providing vivid evidence
of the correctness of the basic ideas of the theory of decoherence. This
theory resolves the longstanding problem of the incompatibility of the
quantum mechanical superposition principle with our everyday experience
of a ¿classical¿ world, a problem most drastically illustrated by the
famous Schrödinger¿s cat paradox. Understanding of decoherence is
essential from a fundamental point of view, and experimental control of
decoherence is crucial for applications, e.g. quantum computers. In the
present experiment, a novel mechanism of decoherence has been studied,
Coulomb interaction of elementary particles without inner degrees of
freedom, namely free electrons in a biprism interferometer, with a truly
macroscopic and dissipative environment, namely the electron gas inside
a semiconducting plate.
The closer the electrons pass to the surface of the plate, the stronger is the disturbance (e.g. heating) of the
electron gas beneath the flight paths of the beam electrons. In turn,
which-path information, entanglement and decoherence increase. This
manifests itself in decreasing contrast of the interference fringes
(which are perpendicular to the surface of the plate) with decreasing
altitude of the electrons above the plate. The decrease in contrast
demonstrates the continuous transition from quantum to classical.
***
How to Rip a Fluid
In a simple experiment on a mixture of water, soap, and salt, we show thata rigid object (like a knife) passes through a gel-like material as if it
were a liquid at slow speeds, but rips it up like a soft solid if it is
pulled rapidly. Most materials in real life do not follow the textbook
cases of solid, liquid, or gas; examples like blood, saliva, toothpaste,
and cell cytoplasm are called viscoelastic (viscous like a fluid, elastic
like a solid). This article focuses on the response of such a material to
increasingly extreme conditions of flow.
As a child will swish its finger
through an unknown liquid to discover what it is, in this experiment we
pull a cylinder through a viscoelastic gel of surfactant and organic salt
in water, to learn its responses. What happens is: flow at slow speeds,
cutting at intermediate speeds, and tearing at the highest speeds. Because
the material is not a solid however, it heals in the wake of the tear, and
recovers completely after several hours. We find that the material
strength of the solid is essentially the surface tension of the liquid -
this fact unifies the response across the time scales from flow to
fracture.
