Q&A
JANUARY/FEBRUARY 2020. DISCOVER 41The subtext: We are venturing into an
age of quantum supremacy — the point
at which quantum computers outper-
form the best classical supercomputers
in solving a well-defined problem.
Engineers test the accuracy of quantum
computing chips by using them to solve
a problem, and then verifying the work
with a classical machine. But in early
2019, that process became problematic,
reported Neven, who runs Google’s
Quantum Artificial Intelligence Lab.
Google’s quantum chip was improving so
quickly that his group had to commandeer
increasingly large computers — and then
clusters of computers — to check its work.
It’s become clear that eventually, they’ll
run out of machines.
Case in point: Google announced
in October that its 53-qubit quantum
processor had needed only 200 seconds
to complete a problem that would have
required 10,000 years on a supercomputer.
Neven’s group observed a “double expo-
nential” growth rate in the chip’s comput-
ing power over a few months. Plain old
exponential growth is already really fast:
It means that from one step to the next, the
value of something multiplies. Bacterial
growth can be exponential if the number
of organisms doubles during an observed
time interval. So can computing power of
classical computers under Moore’s Law,
the idea that it doubles roughly every year
or two. But under double exponential
growth, the exponents have exponents.
That makes a world of difference: Instead
of a progression from 2 to 4 to 8 to 16
to 32 bacteria, for example, a double-
exponentially growing colony in the same
time would grow from 2 to 4 to 16 to 256
to 65,536.
Neven credits the growth rate to two
factors: the predicted way that quantum
computers improve on the compu-
tational power of classical ones, and
quick improvement of quantum chipsthemselves. Some began referring to this
growth rate as “Neven’s Law.” Some theo-
rists say such growth was unavoidable.
We talked to Dowling (who suggests
a more fitting moniker: the “Dowling-
Neven Law”) about double exponential
growth, his prediction and his under-
appreciated Beer Theory of Quantum
Mechanics.Q: You saw double exponential growth
on the horizon long before it showed
up in a lab. How?
A: Anytime there’s a new technology, if
it is worthwhile, eventually it kicks into
exponential growth in something. We see
this with the internet, we saw this with
classical computers. You eventually hit a
point where all of the engineers figure out
how to make this work, miniaturize it and
then you suddenly run into exponential
growth in terms of the hardware. If it
doesn’t happen, that hardware falls off the
face of the Earth as a nonviable technology.Q: So you weren’t surprised to see
Google’s chip improving so quickly?
A: I’m only surprised that it happened
earlier than I expected. In my book, I said
within the next 50 to 80 years. I guessed a
little too conservatively.Q: You’re a theoretical physicist. Are
you typically conservative in your
predictions?
People say I’m fracking nuts when I
publish this stuff. I like to think that I’m
the crazy guy that always makes the least
conservative prediction. I thought this was
far-out wacky stuff, and I was making the
most outrageous prediction. That’s why
it’s taking everybody by surprise. Nobody
expected double exponential growth in
processing power to happen this soon.Q: Given that quantum chips are getting
so fast, can I buy my own quantum
computer now?
A: Most of the people think the quantum
computer is a solved problem. That we
can just wait, and Google will sell you one
that can do whatever you want. But no.
We’re in the [prototype] era. The number
of qubits is doubling every six months,
but the qubits are not perfect. They fail a
lot and have imperfections and so forth.
But Intel and Google and IBM aren’t going
to wait for perfect qubits. The people
who made the [first computers] didn’t
say, “We’re going to stop making bigger
computers until we figure out how to make
perfect vacuum tubes.”Q: What’s the big deal about doing
problems with quantum mechanics
instead of classical physics?
A: If you have 32 qubits, it’s like you have(^232) parallel universes that are working
on parts of your computation. Or like
you have a parallel processor with 2^32
processors. But you only pay the electric
bill in our universe.
Q: Quantum mechanics gets really
difficult, really fast. How do you deal
with that?
A: Everybody has their own interpretation
of quantum mechanics. Mine is the
Many Beers Interpretation of Quantum
Mechanics. With no beer, quantum
mechanics doesn’t make any sense. After
one, two or three beers, it makes perfect
sense. But once you get to six or 10, it
doesn’t make any sense again. I’m on my
first bottle, so I’m in the zone.
NEVEN’S GROUP
OBSERVED A “DOUBLE
EXPONENTIAL” GROWTH
RATE IN THE CHIP’S
COMPUTING POWER OVER
A FEW MONTHS.
i
In his 2013 book, Schrödinger’s Killer App, Louisiana State University theo-
retical physicist Jonathan Dowling predicted what he called “super exponen-
tial growth.” He was right. Back in May, during Google’s Quantum Spring
Symposium, computer engineer Hartmut Neven reported the company’s
quantum computing chip had been gaining power at breakneck speed.