Comments about the article in Nature: Quantum Quest

Following is a discussion about this article in Nature Vol 501 12 September 2013, by Philip Ball
In the last paragraph I explain my own opinion.


The article starts with the following text:
If the truth be told, few physicists have ever felt comfortable with quantum theory.
I think that the issue is that there are some physicists who do not agree (feel uncomfortable) with certain aspects of the quantum theory.
Next make stunningly accurate calculations about molecular structures, high-energy particle collisions, ....
This is the part we all agree about.
... they run straight into a seemingly impenetrable wall of paradoxes:
  1. Can something really be a particle and a wave at the same time?
  2. Is Schrödinger's cat really both alive and dead?
  3. Is it true that even the gentlest conceivable measurement can somehow have an effect on particles halfway across the Universe
It is very important to consider the following remark which is true for all the three issues: Exactly what do you mean.
The answer on each question is:
  1. No. The problem is that a photon is not a particle and a wave at the same time, but behaves like a particle or a wave depending what is measured. For example you can count photons and different photons can interfer which each other.
  2. No. After you have stopped the experiment and some one looks inside the box the cat is either alive or dead. It should be stated, that humans have nothing to do with the state of the cat. The whole issue is: how long is the cat in the box and what is the half life time of the radio active element used.
  3. No. There is no physical effect. See also the comments in paragraph: "A very reasonable proposal"

"A very reasonable proposal"

Hardy began by noting that a classical system can be specified completely by measuring a certain number of 'pure states', which he denoted N. For a coin N equals two. For a dice N equals six.
Physicists by preference should not use the term "classical".
Probability works differently in the quantum world, however.
There is not something as what is called the "quantum world". Those words should not be used. Instead: "Quantum scale" Probability is not an issue for any sort of system, only the outcome of experiments.
You can throw a coin 1000 times and the outcome can be 495 heads and 505 times tail.
You can throw a different coin 1000 times and the outcome can be 752 heads and 248 times tail. Apperently this is a tailor made coin.
Measuring the spin of an electron for example can distinquish two pure states which can be crudely pictured as a rotation clockwise around say a vertical axis.
IMO this picture is misleading because almost never the axis runs vertical. It is a rotation around an axis which can have any direction in space.
If a process creates two electron's than the spin direction of one can be +x,+y and +z. The other electron than is -x,-y,-z. However again 1000 experiments have to be performed to demonstrate to what extend this relation is correct. x, y and z can have any value between 0 and 1
But unlike in the classical world the electron's spin is a mixture of two quantum states before a measurement is made and the mixture varies along a continuum.
IMO before you make a measurement the electron's spin is described by three parameters x,y and z. The problem is that measuring the spin will effect those three parameters. That means it is not possible to measure the direction of the rotation axis of any electron in space.
If you measure the spin of one electron in the x direction than you should first do that also for the other electron in the x direction 1000 times (to establish correlation). secondly you can measure the x direction of one electron and the y direction of the other electron. The y direction of the first is than the -y direction of the second (with a certain probability, earlier established)
Fuchs was eager to reinterpret the concept of entanglement: in which the states of two or more particles are interdependent, meaning that the measurement of one of them will instantaneously allow the measurer to determine the state of the other.
To establish such a relation you have to perform 1000 experiments in which the state of both particles is measured in the same way. The following shows the results of such an experiment:
state (0,0) 4 times, state (0,1) 496 times, state (1,1) 3 times and state (1,) 497 times.
What the results show is that in 496 cases on one side the state 0 is measured and on the other side the state 1 etc. The lessons to be learned from this experiment is that when you only measure the state on one side there is a high chance that you know the state on the other side.
Next is written:
For example: two photons emitted from an atomic nucleus in opposite directions might be entangled so that one is polarized horizontally and the other is polarized vertically.
The same as above: This correlation has to be performed 1000 times by measuring the direction of both photons in horizontal direction.
Before any measurement is made, the polarization of the photons are correlated but not fixed.
Before any measurement is made, the polarization of the photons is not known, specific if they are correlated and to which extend.
Once a measurement on one photon is made, however, the other also becomes instantaneously determined - even if it is already light years away.
Once a measurement on one photon is made you know the direction (the probability) of the other photon. However only for the distance tested in the 1000 experiments.
For example you can perform your 1000 experiments with two detectors each 100 m from the source.
But you can not use those results for detectors each 2 km from the source. This set up requires again 1000 experiments to establish the corelation.
As Einstein and his co-workers pointed out in 1935, such an instantaneous action over arbitrarily large distances seems to violate the theory of relativity, which holds that nothing can travel faster than light.
Einstein was right in a sense that within the context of photon behaviour there is not instantaneous action involved.
If it is possible to have two photons correlated in the sense that when one photon is horizontal polarized the second is vertical polarized (with a certain probability). In that case measuring the first gives you instantaneous knowledge about the state of the second (with a certain probability) but no communication or action is involved.
See also: Instantaneous Communication Contest

Knowledge cap

Fuchs has rewritten standard quantum theory into a form that closely resembles a branch of classical probability theory known as Bayesian inference.
The outcome of physical experiments and probability theory and are two complete different areas of knowledge.
In the Bayesian view, probabilities aren't intrinsic quantities 'attached' to objects. Rather, they quantify an observer's personal degree of belief of what might happen to the object.
My personal beliefs have nothing to do with the outcome of any experiment.
"That new view, if proves valid, could change our understanding of how to build quantum computers and other quantum information kits" he says, noting that all such applications are critically dependent on the behaviour of quantum probability.
The only way to understand "quantum computers" is by building one. Of course you first have to spicify what, as a function of the inputs, the result (output) of the computations should be. To use a Quantum Computer solely as a device to generate random numbers does not make sense.

Poised for succes?

There is plenty room for scepticism. "Reconstructing quantum theory from a set of basic principles seems like an idea with the odds greatly against it" says Daniel Greenberger.
IMO you can be sceptical about quantum theory, however quantum theory is a broad field and requires a detailed analysis what is accepted and what could require modifications.


The most important aspect of science and physisc is to specify as detailed as possible the observations and experiments on which the laws of nature are based.
In the realm of quantum mechanics this is not always the case. Specific in the area of individual photons.

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Created: 7 Oktober 2013

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