Skipping stones down a washboard road?
Drivers of backcountry dirt roads know it well: that teeth-rattling
feeling of rolling over washboard ripples. How does washboard form?
Why doesn't the passage of all those wheels pound the road flat? Now
researchers from France, the UK and Canada have some answers. The
washboard bounces the car on its suspension, but the existence of
washboard does not depend on having a suspension, or even a wheel! By
dragging a flat, inclined "plow" blade over a flat surface of sand,
the researchers showed that ripples formed spontaneously above a
certain speed, even though the plow had no springy suspension at all.
Instead, the bouncing process was more similar to skipping a stone
over the surface of water. Too slow, and the stone sinks or no
ripples form; move fast enough and the forces developed throw the
stone right off the surface. On a sandy road, the ripples are
amplified by successive passing wheels. The research showed that the
washboard road phenomnenon could be understood using similar
mathematical arguments to those used previously for skipping stones.
Unfortunately, the bad news is that a flat sandy road is intrinsically
unstable: ripples will always form above a threshold speed, just as a
stone must always skip if thrown fast enough.
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AD10607
Joining quantum worlds: manipulating ultracold atoms without touching them
We develop a novel direction in quantum physics, quantum optics with quantum
gases, which will close the gap in the understanding of the interaction between
light and matter. On the one hand, optics, which considers the quantum
particles of light (photons), but classical atomic motion, is one of the most
successful fields of modern physics. On the other hand, a new field, quantum
atom optics, treats the motion of ultracold atoms trapped in light-created
potentials quantum mechanically. However, even in very involved problems, the
light potentials are still considered classically. Here we consider the
ultimate quantum limit of light-matter interaction, where the quantum natures
of both ultracold matter, e.g., a Bose-Einstein condensate (BEC), and light are
equally important. We use one of the most intriguing predictions of quantum
mechanics, which claims that the state of one quantum system (in our case,
ultracold gas) can be changed by the distant measurement of another system
(light), even if they do not interact. The key point is the concept of the
“entanglement”, which is possible only in the quantum world. We show, how to
prepare various quantum states of matter (e.g. Schroedinger cat states) by
simply measuring the photons scattered.
