2 November 2019 | New Scientist | 7computers. This means there is
always a new classical algorithm
to test against the quantum ones,
so the bar for quantum supremacy
is continually raised. The question
is whether quantum computers
will be able to outpace their
classical competitors.
The row shouldn’t detract too
much from Google’s achievement,
says Knight. “There are always
going to be clever ways to tweak
classical algorithms, but until
we’ve had a chance to digest the
methodology IBM are proposing,
it is hard to judge,” he says.
Even if you accept IBM’s claims
at face value, Google’s quantum
computer is still a big step forward,
says Gilligan-Lee. “IBM is claiming
that, even when running the
world’s largest computer for
two-and-half days, and running
petabytes of memory, they can
simulate what the quantum chip
does in 200 seconds. When you
put it into context, it is still a pretty
impressive achievement.”
It doesn’t mean quantum
computers are ready to tackle
real-world problems though – that
remains decades away. Instead, it
is a proof of concept. “But it is thefirst baby step on a long road
to getting useful quantum
computers,” says Gilligan-Lee.
He is looking forward to the
next milestone: proof that we have
sufficient control over the qubits
that we can overcome the small
errors they accumulate during
calculations. “We are now in a
phase we call noisy intermediate-
scale quantum computing,” he
says. “To get beyond that, we need
to start doing error correction.
The nice thing is that we can
see from this paper that the
architecture of the Google chip
is already optimised for that.”“We always like to say that
quantum computing is a very
long-term research project, and
it’s going to take years more to get
to the point where we’re doing
more useful work,” said Martinis.
The Sycamore device is just the
first step, he said, and Google is
planning an experiment with
around 1000 qubits in the next
few years. “There are clearly a lot
of hard things to solve, but we are
very excited about that.” ❚See Comment, page 21QUANTUM computers are
machines that use the properties
of quantum physics to store data
and perform computations. This
can be extremely advantageous
for certain tasks where they
could vastly outperform even
our best supercomputers.
Classical computers,
which include smartphones
and laptops, encode
information in binary “bits”
that can either be 0s or 1s.
In a quantum computer,
the basic unit of memory
is a quantum bit or qubit.
Qubits are made using
physical systems, such as
the spin of an electron or
the orientation of a photon.
These systems can be in many
different arrangements all
at once, a property known
as quantum superposition.
Qubits can also be inextricably
linked together through a
phenomenon called quantum
entanglement. The result
is that a series of qubits can
represent different things
simultaneously.
For instance, eight bits
is enough for a classical
computer to represent any
number between 0 and 255.
But eight qubits is enough
for a quantum computer
to represent every number
between 0 and 255 at the
same time. A few hundred
entangled qubits would be
enough to represent morenumbers than there are atoms
in the universe.
This is where quantum
computers get their edge over
classical ones. In situations
where there are a large number
of possible combinations,
quantum computers can
consider them simultaneously.
Examples include trying to
find the prime factors of a
very large number or the best
route between two places.
However, there may also
be plenty of situations in which
classical computers will still
outperform quantum ones.
So the computers of the future
may be a combination of both
these types.
For now, quantum
computers are highly sensitive:
heat, electromagnetic fields
and collisions with air
molecules can cause a qubit
to lose its quantum properties.
This process, known as
quantum decoherence,
leads to the system crashing,
and it happens more quickly
the more particles that
are involved.
Quantum computers
need to protect qubits from
external interference, either
by physically isolating
them, keeping them cool or
zapping them with carefully
controlled pulses of energy.
Additional qubits are needed
to correct for errors that creep
into the system. ❚ExplainerDonna Lu... but what actually is
a quantum computer?
“Quantum supremacy
is all about beating
something, namely,
classical computation.
The latter can, at least
for a while, fight back”
Scott Aaronson,
University of Texas at Austin
Sycamore’s
54-qubit
processorHANNAH BENET/GOOGLE2001
A team at IBM and
Stanford University
use Shor’s algorithm
on a 7-qubit
quantum computer,
made from billions
of molecules in a
test tube, to find
the factors of 152009
The first electronic
quantum processor
is built. The 2-qubit
superconducting
chip is made from
more than a billion
aluminium atoms in
a superconducting
electronic circuit2012
John Preskill coins
the term “quantum
supremacy”
to describe the
potential ability of
quantum computers
to solve problems
that classical
computers can’t2013
Google, NASA and
the US Universities
Space Research
Association
announce the
launch of a
Quantum Artificial
Intelligence Lab2019
January
IBM unveils its
first commercial
quantum computer,
the 20-qubit
Q System One2019
October
Google says its
quantum computer
took 200 seconds to
complete a task that
would take our most
powerful computer
10,000 years,
achieving quantum
supremacyLearn more about the quantum world
Join six experts at our one-day masterclass on 9 November
newscientist.com/qwbristol