"The Argument Against Quantum Computers"
From Quanta Magazine:
The mathematician Gil Kalai believes that quantum computers can't possibly work, even in principle.
Sixteen years ago, on a cold February day at Yale University, a
poster caught Gil Kalai’s eye. It advertised a series of lectures by Michel Devoret, a well-known expert on experimental efforts in quantum computing.
The talks promised to explore the question “Quantum Computer: Miracle
or Mirage?” Kalai expected a vigorous discussion of the pros and cons of
quantum computing. Instead, he recalled, “the skeptical direction was a
little bit neglected.” He set out to explore that skeptical view
himself.
Today, Kalai, a mathematician at Hebrew University in Jerusalem, is one of the most prominent of a loose group of mathematicians, physicists and computer scientists
arguing that quantum computing, for all its theoretical promise, is
something of a mirage.
Some argue that there exist good theoretical
reasons why the innards of a quantum computer — the “qubits” — will
never be able to consistently perform the complex choreography asked of
them. Others say that the machines will never work in practice, or that
if they are built, their advantages won’t be great enough to make up for
the expense.
Kalai has approached the issue from the perspective of a
mathematician and computer scientist. He has analyzed the issue by
looking at computational complexity and, critically, the issue of noise.
All physical systems are noisy, he argues, and qubits kept in highly
sensitive “superpositions” will inevitably be corrupted by any
interaction with the outside world. Getting the noise down isn’t just a
matter of engineering, he says. Doing so would violate certain
fundamental theorems of computation.
Kalai knows that his is a minority view. Companies like IBM, Intel
and Microsoft have invested heavily in quantum computing; venture
capitalists are funding quantum computing startups (such as Quantum Circuits,
a firm set up by Devoret and two of his Yale colleagues). Other nations
— most notably China — are pouring billions of dollars into the sector.
Quanta Magazine recently spoke with Kalai about quantum
computing, noise and the possibility that a decade of work will be
proven wrong within a matter of weeks. A condensed and edited version of
that conversation follows.
When did you first have doubts about quantum computers?
At first, I was quite enthusiastic, like everybody else. But at a
lecture in 2002 by Michel Devoret called “Quantum Computer: Miracle or
Mirage,” I had a feeling that the skeptical direction was a little bit
neglected. Unlike the title, the talk was very much the usual rhetoric
about how wonderful quantum computing is. The side of the mirage was not
well-presented.
And so you began to research the mirage.
Only in 2005 did I decide to work on it myself. I saw a scientific opportunity and some possible connection with my earlier work from 1999 with Itai Benjamini and Oded Schramm on concepts called noise sensitivity and noise stability.
What do you mean by “noise”?
By noise I mean the errors in a process, and sensitivity to noise is a
measure of how likely the noise — the errors — will affect the outcome
of this process. Quantum computing is like any similar process in nature
— noisy, with random fluctuations and errors. When a quantum computer
executes an action, in every computer cycle there is some probability
that a qubit will get corrupted....
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