Does an 8 Year Old Physics Experiment Prove Quantum Computers Can't Exist?

Quantum computers aren't strangers to these pages.  I've written on them several times and some quotes from a piece last January will serve well as a quick introduction.

Most people have heard of the term "bits" - a shortened form of "Binary digITS" - and they know that computers process bits.  Most have also heard that "it's all ones and zeroes"; that is, computers process signals that are considered either "on or off", the logic one and logic zero levels.  At the lowest circuit level in most computers, those bits are handled by logic gates, which are simply groups of transistors used as switches.  In the quantum computers, ones and zeroes are the direction of current flow in super-cooled Niobium wire.

... in his quantum computer, a loop of super cold niobium wire has all of its electrons going in one direction and at the exact same time the same piece of wire has all of its electrons going in the opposite direction.  Their niobium wire is exhibiting quantum behavior in a macroscopic object.

The dirty little secret is that it's very hard to determine if a computer is really operating on quantum principles.  Companies and government agencies are paying millions of dollars for them and they're not even sure they're getting a quantum computer and not a conventional computer.

Surely it can't be that hard right?  Why not take it apart and probe it?  Or why not run a sample computation, a benchmark if you will, on it and another computer to see which is faster.  Unfortunately, it's not that easy.  It violates the laws of quantum physics to be able to look inside (probe it), so the system can't be probed.  When a quantum system is observed, it collapses to one state or the other.  In the example from the original  paragraph, if one measures the current flow in any of the 512 niobium wire loops it will only appear to be moving in one direction, not both.

Got that?  Since quantum theory says it's impossible to observe the system without changing it, nobody knows if these quantum computers being sold are relying on quantum principles.

In June of 2011, an international team of researchers, led by University of Toronto physicist Aephraim Steinberg of the Centre for Quantum Information and Quantum Control, have found a way to observe photons in a dual slit experiment without changing them by applying a new, modern measurement technique to the historic two-slit interferometer experiment in which a beam of light shone through two slits results in an interference pattern on a screen behind.

That famous experiment, and the 1927 Neils Bohr and Albert Einstein debates, seemed to establish that you could not watch a particle go through one of two slits without destroying the interference effect: you had to choose which phenomenon to look for.

These researchers demonstrated that they can watch a photon and see both phenomena; they can observe the system without changing it. 

With this new experiment, the researchers have succeeded for the first time in experimentally reconstructing full trajectories which provide a description of how light particles move through the two slits and form an interference pattern. Their technique builds on a new theory of weak measurement that was developed by Yakir Aharonov's group at Tel Aviv University. Howard Wiseman of Griffith University proposed that it might be possible to measure the direction a photon (particle of light) was moving, conditioned upon where the photon is found. By combining information about the photon's direction at many different points, one could construct its entire flow pattern, i.e. the trajectories it takes to a screen.

"In our experiment, a new single-photon source developed at the National Institute for Standards and Technology in Colorado was used to send photons one by one into an interferometer constructed at Toronto. We then used a quartz calcite, which has an effect on light that depends on the direction the light is propagating, to measure the direction as a function of position. Our measured trajectories are consistent, as Wiseman had predicted, with the realistic but unconventional interpretation of quantum mechanics of such influential thinkers as David Bohm and Louis de Broglie," said Steinberg.

The original double-slit experiment played a central role in the early development of quantum mechanics, leading directly to Bohr's formulation of the principle of complementarity. Complementarity states that observing particle-like or wave-like behaviour in the double-slit experiment depends on the type of measurement made: the system cannot behave as both a particle and wave simultaneously. Steinberg's recent experiment suggests this doesn't have to be the case: the system can behave as both.

The implications here for quantum computing are important.  The friend who found this article says it proves quantum computers are impossible.  I'm not sure it goes that far, but it shows that at least it's not impossible that quantum computers can be probed; the method used in this paper to observe these photons might not be amenable to measuring currents, counting electrons or other measurements required on a computer but another method might work.  On a more abstract level, what does this say about the superposition of currents in the niobium wires in the D-Wave computer?  It was hard for me to grasp how "a loop of super cold niobium wire has all of its electrons going in one direction and at the exact same time the same piece of wire has all of its electrons going in the opposite direction" could be used to make decisions; how it could be useful.  As others commented in my post a year ago, that sounds a lot like zero current flowing.

Sorry, but I love this picture of Shroedinger's cat.  We found that Schrodinger's cat learned to create a quantum superposition of kibbles in each of the parallel universes he visited, so that alive or dead, he had an unlimited supply of kibbles.  Photo from Quartz  magazine article.