70 Science and technology The EconomistFebruary 24th 2018
2 cal computing in the days before silicon.
The slightest puff of interference from the
outside world risks disrupting a qubit and
scuppering a calculation. Every one of
Google’squbits isheld in a chip that has
120 wires coming out of it, each of which is
capable of introducing troublesome noise.
Nor can quantum computers relyon the er-
ror-correction techniques that classical
computers use. Those duplicate data out-
puts and check them against each other.
Duplicating the outputof qubits would
mean having to measure them premature-
ly. That would change the qubits’ quantum
states, wrecking the calculation. Instead,
everythingmust be and remain perfect.
Such reliability has been mostlyachieved in classical computing. Hardware
problems are not unheard of, but they are
relatively rare. (The software is another
matter.) But that dependability isthe result
of 60 years of continuous improvement of
solid-state silicon transistors. Quantum
computing is now in the equivalent of the
days of vacuum tubes running calcula-
tions in room-sized computers. And that
was a world in which the tubes often blew,
and bugsin the system were literal ones,
namely insects that caused short circuits.
Such behemoths were able to turn into to-
day’s sleek machines because, at every
stage of the journey, they were useful. And
that, ultimately, is the standard quantum
computerswill have to match. 7T
SUNAMI are terrible things. And part
of their terror lies in their unpredictabil-
ity. Even when a submarine earthquake
that may cause one is detected, the infor-
mation that is needed to determine wheth-
er a giant wave has actually been created
takes time to gather. That is time unavail-
able for the evacuation of coastlines atrisk.
Contrariwise, issuing a warning when no
subsequent wave arrives provokes cyni-
cism and a tendency to ignore future evac-
uation calls.
Such tsunami-warning systems as do
exist rely on seismometers to detect earth-
quakes, and tide gauges and special buoys
to track a wave’s passage. That is reliable,
but can often be too late to get people away
from threatened coastlines. What thesewarning systems cannot do reliably is
predict immediately whether a given
earthquake will cause a tsunami. And that,
in the view of some seismologists, is a
scandal. For, as the AAASmeeting learned
from Gerald Bawden ofNASA, Paul Huang
of America’s National Tsunami Warning
Centre, Tim Melbourne of Central Wash-
ington University, and Meghan Miller of
UNAVCO, a geoscience research consor-
tium, the tools for accurate tsunami predic-
tion already exist. All that needs to happen
is to connect them up.
The nub of the problem is that it is hard
to distinguish immediately whether a sub-
marine earthquake is powerful enough to
cause a tsunami. Big quakes (those above
about magnitude 7.3) involve slippagealong many kilometres of a fault. That
means their energy is not radiating from a
point. A single seismometer therefore has
difficulty distinguishing between a quake
of magnitude 7.3, 8.3 or even 9.3 (about as
large as they get). The logarithmic nature of
the earthquake-magnitude scale, though,
means the third of these is 1,000 times
more powerful than the first. And the more
powerful the shock, the more likely it is
that a dangerous tsunami will result. Once
the seismic waves from an earthquake
have reached enough seismometers, the
distinction becomes clear. But near any
given quake there are rarely enough
seismometers around.
Except, asthe panellists pointed out,
there are. America’s satellite-based Global
Positioning System and subsequent simi-
lar efforts from other countries (known col-
lectively asGNSS, the Global Navigation
Satellite System) have permitted the cre-
ation in many places of networks of sen-
sors that measure, within millimetres, lo-
cal distortions of Earth’s crust. The main
reason for doing this is to understand the
build-up of earthquake-causing strain in
the crust, so such monitors are most abun-
dant where tremors are commonest. And,
if a tremor does happen, monitors nearby
will be shaken by it.
There are, by the panellists’ estimates,
about 17,000 such monitoring devices
around the world. Of those, around 2,300
make their data available instantly. If these
instant monitors’ signals could all be gath-
ered together and run through suitable
software, the true nature of a big subma-
rine earthquake would be apparent al-
most at once, and appropriate warnings
could be issued.
At the moment, two regional projects
are testing this idea. One, READI, on the Pa-
cific coast of America, is under the aegis of
NASA. The other, GEONET, in Japan, is or-
ganised by that country’s land-mapping
agency. The hope isthat, if these local ven-
tures work, other countries will join in and
a global network can be created over the
next decade.
Really clever use of the GNSS, more-
over, might be able to do even better than
this, by tracking a tsunami as it travels.
Though the most visible consequence of a
tsunami is a wave in the ocean, it also
creates one in the atmosphere. This affects
the arrival time ofGNSSradio waves in a
way that, with enough ground-based de-
tectors, would permit the passage of the
wave to be followed. And these detectors,
too, will soon be commonplace. For many
years, smartphones have contained GNSS
receivers, so a phone’s apps can use loca-
tion information. The latest phones have
equipment so sensitive that it could, in
principle, detect a passingtsunami in the
atmosphere. All this would require is for
someone to write a suitable app, and for
enough phone users to download it. 7Tsunami detectionAhead of the wave
AUSTIN
Finding more time to detect threats and evacuate coastlines