快猫短视频

JUST as the classic 鈥渢wo-slits鈥� experiment shows that a beam of light is a
quantum wave, a team of researchers has now done the same trick with a flowing
liquid, showing that it, too, demonstrates quantum interference. Their
experiment is so sensitive it can easily sense the turning of the Earth.

In the classic demonstration of quantum interference, a light beam is split
in two and recombined. If the two waves line up peak to peak, the recombined
beam shines bright, but if peak lines up with trough they cancel out. Physicists
have shown this in light, electricity and neutron and atom beams.

The latest material to show quantum interference is ultra-cold helium-3, a
鈥渟uper-fluid鈥� that behaves like a quantum wave and flows without viscosity, say
J. C. S茅amus Davis, Richard Packard and colleagues at the University of
California, Berkeley.

To detect interference, the team made the liquid equivalent of a
superconducting quantum interference device, or SQUID, which uses interfering
electrical currents to detect tiny changes in magnetic fields. It consists of a
superconducting ring with an input on one side and an output on the other. An
incoming wave of electrons splits, flows through the two halves of the ring, and
recombines on the far side.

Each half of the ring contains a constriction called a Josephson
junction鈥攁 tiny sandbar that a quantum wave of electrons can barely slosh
across. The junction accentuates the wave nature of the current.

A magnetic field passing through the ring affects the current in the two
halves of the ring differently, which changes the synchronisation of the
recombining waves. A tiny increase in the field causes the two waves to line up
peak-to-trough and the output drops to zero. A further increase causes the two
waves to line up peak-to-peak, producing maximum output, and so on.

To make an analogous device for a liquid, the researchers fashioned a narrow
tube into a loop roughly 1.4 centimetres in diameter and filled it with helium-3
cooled to less than 0.001 kelvin. For 鈥淛osephson junctions鈥�, they used delicate
membranes pierced with 4225 holes 100 nanometers in diameter.

The researchers knew that rotation would affect their ring in the same way as
a magnetic field affects a SQUID. The flow of helium should repeatedly swing to
zero and back to its maximum value as they change the rate at which the ring
rotates relative to free space. All they had to do to provide the rotation was
to change its orientation relative to the Earth鈥檚 axis. When they did this, the
flow varied precisely as expected.

The result clinches a long-anticipated connection between superfluid helium
and superconductivity, says William Zimmermann of the University of Minnesota in
Minneapolis. 鈥淚 attach a whole lot of importance to anything that makes things
clear and convincing,鈥� he says.