Advanced quantum computer made
available to the public for first time
A computer capable of
achieving quantum advantage – a demonstration of supremacy over conventional
machines – is the first that anyone can use over the internet
TECHNOLOGY 1 June 2022
By Alex Wilkins
The Borealis quantum computer consists of many fibre-optic
loops
Xanadu Quantum Technologies Inc
A quantum computer that encodes information
in pulses of light has solved a task in 36 microseconds
that would take the best supercomputer at least 9000 years to complete. The researchers
behind the machine have also connected it to the internet, allowing others to
program it for their own use – the first time such a powerful quantum computer
has been made available to the public.
Quantum computers rely on the
strange properties of quantum mechanics to theoretically perform
certain calculations far more quickly than conventional computers. A
long-standing goal in the field, known as quantum advantage or quantum supremacy, has been to demonstrate that
quantum computers can actually beat regular machines. Google was the first to do so in 2019 with its Sycamore
processor, which can solve a problem involving sampling random numbers that is
essentially impossible for classical machines.
Now, Jonathan Lavoie at Xanadu Quantum
Technologies in Toronto, Canada, and his colleagues have built a quantum
computer called Borealis that uses particles of light, or photons, travelling
through a series of fibre-optic loops to solve a problem known as boson sampling. This involves measuring the
properties of a large group of entangled, or quantum-linked, photons that have
been separated by beam splitters.
Boson sampling is a difficult
task for ordinary computers because the complexity of the calculations
drastically rises as the number of photons increases. Borealis essentially
computes the answer by directly measuring the behaviour
of up to 216 entangled photons.
Solving this problem isn’t
particularly useful outside of establishing that quantum advantage has been achieved,
but it is an important test. “By demonstrating these results using Borealis, we
have validated key technologies that we need for the quantum computers of the
future,” says Lavoie.
Borealis is the second device
to demonstrate quantum advantage in boson sampling. The first is a machine
called Jiuzhang, created by researchers at
the University of Science and Technology of China (USTC). It first showed
quantum advantage in 2020 with 76 photons and then again in an improved version in 2021 using 113 photons. The
USTC team also demonstrated quantum advantage last year in the
random-number-sampling problem, with a machine called Zuchongzhi.
More power
Borealis is an advance on Jiuzhang because it is a more powerful system, capable of
calculating with a larger number of photons, and has a simplified architecture,
says Peter Knight at Imperial College London. “We all thought that
the Chinese experiment was a tour de force, but we couldn’t see that it was
going to go any further because there was a limit to how much stuff you could
cram onto your optical table,” he says.
Compared with Borealis, Jiuzhang uses a larger number of beam splitters to send
entangled photons in lots of different directions. But Borealis takes a
different approach, using loops of optical fibre to
delay the passage of some photons relative to others – separating them in time,
rather than space.
An added benefit of the
stripped-back design is that this computer is more easily controllable, so it
can also be reprogrammed remotely for people to run it with their own settings.
“Borealis is the first machine capable of quantum computational advantage made
publicly available to anyone with an internet connection,” says Lavoie.
People will probably begin by
testing variations of boson sampling, says Knight, but, later on, it may be
possible to apply Borealis to different problems. So far, no one has been able
to demonstrate quantum advantage for a “useful” computational task – the
random-sampling problem first tackled by Googleessentially has no
applications beyond demonstrating quantum advantage.
While Borealis is an
impressive jump forward in scale over Jiuzhang, it
falls short of being a fully programmable quantum computer like Sycamore or Zuchongzhi, says Raj Patel at the University of
Oxford. This is because a component called an interferometer, which measures
interference patterns to extract information from the photons, has been limited
to only record certain photon interactions in an effort to get clearer
readings. “To create a machine that is programmable and can tackle real-world
problems, you would really want the interferometer to be fully connected,” says
Patel.
Lavoie and his colleagues are
now working to turn a blueprint they released last year into a scalable,
fault-tolerant photonic processor built on an integrated chip, which would
improve the quantum machine’s capabilities even further.