Friday, January 23, 2015

Ivan Deutsch says no true qubit yet

Quanta magazine has an interview:
For more than 20 years, Ivan H. Deutsch has struggled to design the guts of a working quantum computer. He has not been alone. The quest to harness the computational might of quantum weirdness continues to occupy hundreds of researchers around the world. Why hasn’t there been more to show for their work? As physicists have known since quantum computing’s beginnings, the same characteristics that make quantum computing exponentially powerful also make it devilishly difficult to control. The quantum computing “nightmare” has always been that a quantum computer’s advantages in speed would be wiped out by the machine’s complexity.
I do not know whether he is related to David Deutsch, another quantum computing pioneer. Ivan's big hope:
Before I die, I would love to see just one universal logical qubit that can be indefinitely error corrected. It would instantly be classified by the government, of course. But I dream on, regardless.
Remember that, next time you read that someone has achieve 10 qubits, or 20 qubits, or whatever. No one has truly made a single qubit.
QUANTA MAGAZINE: Why would a universal quantum machine be so uniquely powerful?

IVAN DEUTSCH: In a classical computer, information is stored in retrievable bits binary coded as 0 or 1. But in a quantum computer, elementary particles inhabit a probabilistic limbo called superposition where a “qubit” can be coded as 0 and 1.

Here is the magic: Each qubit can be entangled with the other qubits in the machine. The intertwining of quantum “states” exponentially increases the number of 0s and 1s that can be simultaneously processed by an array of qubits. Machines that can harness the power of quantum logic can deal with exponentially greater levels of complexity than the most powerful classical computer. Problems that would take a state-of-the-art classical computer the age of our universe to solve, can, in theory, be solved by a universal quantum computer in hours.
Yes, he is dreaming. Just because we can create states where we do not know whether a measurement will give a 0 or 1, that does not mean that the alternate possibilities of 0 and 1 can be used for exponential computational advantage.

I wonder how many years or decades the Deutsches and others will be able to get away with this. Their argument is essentially:
When you shine a laser thru a double slit, you get a diffraction pattern, just like what you get with water waves or any other waves. But with light it is different, because light is made of particles called photons, not waves, and the pattern is the result of photons being in two places at once. Quantum mechanics explains how something can be two things at once, such as putting Schroedinger's cat in a box and if you do not know whether it is alive or dead then you just say that it is in a superposition of live and dead states. The problem of quantum mechanics is the engineering problem of making devices that act like Schroedinger cat boxes, and so that the interaction between the live cat and the dead cat in the probabilistic limbo can be harnessed to do a computation.
This is a good story, if it works, but it is a fantasy. Yes, I would love to see one universal logical qubit also. There would be a Nobel Prize for that. But when is anyone besides me and a handful of others going to get skeptical? In 10 years? In 20 years? I do not see that the quantum computing failures* have caused any skepticism so far. I have listed my reasons for skepticism.

Marek Zukowski has a new paper on some quantum paradoxes, but it admits that the argument that quantum mechanics disproves "local causality" only holds if you define that term to mean a hidden variable theory that is directly contrary to quantum mechanics. Also, the paradoxes disappear if you just assume that "unperformed experiments have no results". Quantum computing is all about trying to get computational results from unperformed experiments.

* A comment below disputes the term "quantum computing failures".


  1. Just one comment. I don't see any justification for saying "quantum computing failures". While experimental qubits are far behind the most optimistic predictions that were made in 1995, they are also far ahead of the most pessimistic predictions that were made in 1996.

    1. I wouldn't be too Shor of yourself. Fusion has many similar claims and has gotten nowhere near break-even. Using a bunch of fancy lasers just cost the taxpayer billions and now they are replaying failed Tokamak designs. Furthermore, the issue of motivation becomes apparent to experts in numerical methods. Speed and space will not replace the need for algorithmic efficiency.