A quantum
nanodevice can simultaneously act as a heat engine and a refrigerator
A
multitasking nanomachine that can act as a heat engine and a refrigerator at
the same time has been created by RIKEN engineers1. The device is one of the
first to test how quantum effects, which govern the behavior of particles on
the smallest scale, might one day be exploited to enhance the performance of
nanotechnologies.
Conventional
heat engines and refrigerators work by connecting two pools of fluid.
Compressing one pool causes its fluid to heat up, while rapidly expanding the
other pool cools its fluid. If these operations are done in a periodic cycle,
the pools will exchange energy and the system can be used as either a heat
engine or a fridge.
It would
be impossible to set up a macroscale machine that does both tasks
simultaneously—nor would engineers want to, says Keiji Ono of the RIKEN
Advanced Device Laboratory. “Combining a traditional heat engine with a
refrigerator would make it a completely useless machine,” he says. “It wouldn’t
know what to do.”
But things
are different when you shrink things down. Physicists have been developing ever
smaller devices, sometimes based on single atoms. At these tiny scales, they
have to account for quantum theory—the strange set of laws that says, for
instance, an electron can exist in two places at the same time or have two
different energies. Physicists are developing new theoretical frameworks and
experiments to try to work out how such systems will behave.
The
quantum version of the heat engine uses an electron in a transistor. The
electron has two possible energy states. The team could increase or decrease
the gap between these energy states by applying an electric field and
microwaves. “This can be analogous to the periodic expanding–compressing
operation of a fluid in a chamber,” says Ono, who led the experiment. The
device also emitted microwaves when the electron went from the high-energy
level to the lower one.
By
monitoring whether the upper energy level was occupied, the team first
demonstrated that the nanodevice could act as either a heat engine or as a
refrigerator. But then they showed something far stranger—the nanomachine could
act as both at the same time, which is a purely quantum effect. The researchers
confirmed this by looking at the occupancy of the upper energy level, which
combined to create a characteristic interference pattern. “There was an almost
perfect match between the experimental interference pattern and that predicted
by theory,” says Ono.
“This may
allow rapid switching between the two modes of operation,” Ono explains. “This
ability could help create novel applications with such systems in the future.”