# Quantum computers?

1. Jan 12, 2005

### kaos

Quantum computers????

Are quantum computers depenent on the many worlds interpretations of QM?
I ask these because i read somewhere about Max Tegmark and he said something about quantum computers wil be the first device to utilize the computing power of other universes.That the computer is using processing elements in other universes to do the calculation.

My point is isn'tthe many worlds interpretation just an opinion?
And so it seems to me that the whole quantum computing project is based on some strange opinion.

Excuse my ignorance but i dont have an education in physics or other sciences.

2. Jan 12, 2005

### caribou

It seems it's the interference effects in quantum theory that they mean by talking about "multiple universes".

Like when an electron seems to go through both slits at the same time in the two-slit experiment, some just interpret it to be "many histories" of the particle interfering but others interpret it as "many universes" interfering.

David Deutsch is famous for quantum computation research and this is his interpretation. Some information about him and the many-universes quantum computer idea here:

http://www.geocities.com/iona_m/Cosmology/DeutschPhysics.html

The same interference effect exists in experiments no matter how people interpret its cause so I'd guess quantum computers are not linked to any particular interpretation.

Last edited: Jan 12, 2005
3. Jan 12, 2005

### kaos

Hmmmm....

Is the power of a quantum computer is a result of the ability to do "and" "or"
decisions simultaneoulsly,that is do many calculations simultaneously,sort of like a massively superscalar computer?Sort of eliminate the wrong permutations and come up with the right answer??

Again my apologies for not framing my questions clearly as i do not have a physics background but i have an IT background.

4. Jan 12, 2005

### caribou

I know very little about quantum computers as I'm still studying quantum theory until I'm happy I understand the theory first.

http://www.qubit.org/
http://www.cs.caltech.edu/~westside/quantum-intro.html [Broken]
http://computer.howstuffworks.com/quantum-computer.htm

Last edited by a moderator: May 1, 2017
5. Jan 12, 2005

### JohnBarchak

The power of the quantum computer is not real!

The massive power increase for the quantum computer can never be realized since it depends on quantum superposition. Quantum superposition has no basis in objective reality. The very definition of quantum superposition is that it is the state of a particle that has NOT been observed. How can you base a computer on a concept that has NEVER been observed?

All the best
John B.

6. Jan 14, 2005

### kaos

Superposition doesnt exist?
John B is that your opinion or is that fact?

7. Jan 14, 2005

### chroot

Staff Emeritus
JohnBarchak has already shown evidence of a few fundamental misconceptions about quantum mechanics in his brief presence here. I wouldn't rely on him for information on QM.

- Warren

8. Jan 14, 2005

### marlon

This is very very untrue...All you state here is WRONG.

First of all, all QM-operations that are the fundation of a quantumcomputer can be excecuted on an ordinary computer, because all they require are operations on vectors, like rotations and translations. Any wavefunction can be represented as a vector so there really is no problem there. You CAN apply QM-algorithms onto some usual computer,but there is ONE catch : don't just try to measure the results. Let me exemplify : In Deutsch's-problem it is proven that with QM you can check whether a function is balanced or not with just 1 single measurement. Basically what you can't do is just measure the function itself because the superposition of the wavefunction (which described the function) will be broken. What you need to figure out (and this is the hard part) is a way to extract this info indirectly by for example measuring the fase of the wavefunction or something like that.

regards
marlon

9. Jan 14, 2005

### kaos

uhhh thx,i think i need to do a lot more reading before i understand......

10. Jan 14, 2005

### marlon

No problem, You'll get there. But indeed, make no mistake about it, you need to know your QM very well in order to understand quantum information theory. This is not some subject that stands on itself independently. It is a "child" of QM...

You can download the Preskill-lectures in this in my "info on the web"entry in my journal

regards
marlon

11. Jan 14, 2005

### marlon

https://www.physicsforums.com/journal.php?s=&action=view&journalid=13790&perpage=10&page=5 [Broken]

marlon

Last edited by a moderator: May 1, 2017
12. Jan 14, 2005

### JohnBarchak

Quantum Superposition

Some people did not seem to understand my comment about superposition never having been observed. I was not questioning how you get information out of a quantum computer. I was questioning the validity of the superposition concept itself. How can you convince anyone of the validity of a concept for something that has never been observed. We know that an electron has a magnetic moment through observations made on electrons. What observations tell us that superposition exists? QM is advertised to be based on "observables." What observable tells us that a particle is in a state of superposition?

All the best
John B.

13. Jan 14, 2005

### chroot

Staff Emeritus
JohnBarchak,

Thousands of experiments indicate superposition is real. Even the lowly Young double-slit experiment.

- Warren

14. Jan 14, 2005

### marlon

You are making a "gedanken"-error here. Every experiment ever performed in the past was backed up by QM or the other way around. So, just like chroot stated, if you are wondering about validity , just look at every successful explanation that QM ever provided. Basically if you are questioning superposition, you are questioning the validity of QM. However in that case you should also be questioning the computer used to write your posts.

The error you make is this one (and it is a classic one). Superposition CANNOT be observed. You are thinking in classical terms when talking about something that is fundamentally different in nature. We only see the manifestations of this superposition-principle in the correct explained experimental data. You give the example of magnetic moment but it is a wrong one. Did you ever observe this magnetic moment. What is it really ??? No, you only see manifestations of this moment via the interactions in which it is involved. The magnetic moment is a tool constructed in theory and used to describe and match experimental data. In a similar manner, the superposition principle was born in order to match the data of the double-slit experiment. More specifically, in order to explain the interference-terms that arise in the probability-distribution when both slits are open...I am sure you know what i am talking about.

Finally, you do not have a problem with the results of a quantum-computer yet you question superposition. Can't you see the craziness in this way of thinking. It would be like saying i trust the computer but i don't trust the micro-processors that govern the computer's "behavior" if you will

regards
marlon

15. Jan 15, 2005

### chrismuktar

I've studied this for my dissertation so i can explain it easily.

A qubit in the analogy of a bit in quantum computer. Unlike a classical computer, which takes values 0 and 1, a qubit is both at the same time. This is called a superposition of states. When you apply your operation to the qubit, out the other side you get the answers for both cases of input, both zero and one. Hence, unlike a normal computer which would have only been able to compute one case at a time, here you have computed both.

The quantum computer relies on entangling input and output qubits in order to produce sequential processing.

In order to get an answer out of the thing, you need to observe the input qubit to be whatever you want it to be (i.e. force it into a state), and by the laws of entanglement, your output qubit always gives you the right answer.

As a matter of fact, quantum computers are quite ingenious, and their computation power increases exponentially with the number qubits used (unlike a normal pc which increases linearly). Some 300-400 qubits can store more numbers than there are atoms in the universe. Now that's something.

16. Jan 15, 2005

### misogynisticfeminist

Basically in classical logic gates, you can only have 1 and 0. But in a quantum logic gate, you can have a superposition of states of both 1 and 0, so essentially you've got 3 states. 1,0 and 1/0.

17. Jan 15, 2005

### chrismuktar

That isn't right; look it up on the net.

18. Jan 15, 2005

### Caroline Thompson

Agreed. People seem to have forgotten that the only evidence we have that superposition is real is the Bell test experiments, and these, as we know, have loopholes that allow for local realist explanations. As Marshall et al said in their seminal 1983 paper,

T W Marshall, E Santos and F Selleri, F, “Local Realism has not been Refuted by Atomic-Cascade Experiments”, Physics Letters A, 98, 5-9 (1983),
Some people here are arguing that there is all sorts of other evidence for superposition. Let them specify it! Back in 1972 John Clauser (of the CHSH Bell test) wrote an interesting paper:
Clauser, J E, “Experimental limitations to the validity of semiclassical radiation theories”, Physical Review A 6, 49 (1972)​
He was able to give "semi-classical" explanations for all the phenomena claimed to be best explained by QM apart from one little area, that of entanglement of separated particles -- precisely the area that is tested in the Bell tests. He, unfortunately, he was under the impression that experiment already showed that in this area QM was correct. The above paper was written before the first actual Bell test (his own, with Freedman, also published in 1972) had been done, and the data he was going on was that from the Kocher-Commins 1967 expt.. I have reason to believe that, like Marshall et al in 1983, he was unknowingly trying to explain data that had been adjusted by subtraction of accidentals. Had he allowed for this, he would have been able to explain this too, using his same semi-classical methods.

Anyway, you are asolutely right, John: the superposition principle has never been proved to be true. Real wave systems can, of course, exist with many frequencies at once -- a bell can ring producing a complicated spectrum of sounds -- but these are just mixtures, not quantum-theoretical "superpositions" of states. Since our general experience of the world suggests that quantum superposition is physically impossible and so does not happen, I think we can safely bet that quantum computers will never achieve any improvement over classical ones. I forsee many simulations of quantum computers, but actual ones few and far between, with unimpressive performance.

Hopefully, all the effort put into them will not have been entirely wasted! Perhaps the algorithms developed will prove useful in their own right.

Caroline
http://freespace.virgin.net/ch.thompson1/

Last edited by a moderator: Apr 21, 2017
19. Jan 15, 2005

### marlon

Hi miso,

You need to look at it like this.
The actual qubit is a state that is defined as the superposition of 0 and 1. The term 01 does not occur in it. Basically the wavefunction of a qubit is the SUM of state 0 and state 1. Now when an operator acts on this state (the operator is often referred to as the black box) entanglement between the two states can occur and the state 01 or 10 is born, depending on what this operator really does...

regards
marlon

20. Jan 16, 2005

### misogynisticfeminist

hmmm understood, thanks marlon......

21. Jan 16, 2005

### kaos

If it is possible, can u guys explain where the computing power comes from in a simple manner? If it isnt possible for a simple explanation then ignore my request.

My understanding is that superposition is a state in btween states, for example somewhere between the 0 or 1 state, is this correct??

Im not educated in physics btw and if you cant answer in a simple way then i dont mind.

22. Jan 16, 2005

### kaos

Btw many worlds interpretation is just a postulate right??It isnt proven or anything right??

23. Jan 17, 2005

### chroot

Staff Emeritus
Superposition is a combination of both pure states, not a state "in between" them.

The many-worlds interpretation is one way to interpret the happenings of quantum mechanics. There are other ways, the most popular of which is known as the Copenhagen interpretation. These interpretations don't affect the hard empirical facts that the theory predicts, only how people choose to interpret the predictions.

- Warren

24. Jan 17, 2005

### masudr

John Barchak seems to have missed the point. Since we start off in a superposition, we end up with a superposition. So with a single operation, we get $$2^n$$ calculations with n qubits.

One way that helps to understand that directly unobservable phenomena have indirect effects on the real world is, for example, virtual photons. As we go into more detail of Feynman diagrams of a single electron, we get better and better corrections to the magnetic moment of the electron. Completely from unobservable phenomena (i.e. the effect of the virtual photons). I'm aware that this isn't exactly the same as what is happening in a quantum computer, but it may help you to understand.