# Quantum Computers and Associated Physics



## Tristan427 (Dec 9, 2011)

Striver said:


> 3) is simply fallacious. In any physical system, you cannot get more output than input. Second law of thermodynamics.
> 
> *A quantum system disentangles when information about the system, leaves the system. This happens when a measurement is made on the system like when we do an experiment to measure the system's spin. It also happens when the system is no longer isolated from the surrounding environment.*
> 
> It's like a ball of yarn that is inside a special box that isolates it from everything outside, i.e., the box has a constant temperature and when the box is closed, not a single particle leaves or enters the box. When you open the box to see what's inside, the ball of yarn unravels in one of a finite number of specific ways (yes, balls of yarn do this).


I'm pretty sure I know what you're saying, but I'm not 100% positive I do. Could you explain the bold further? I do apologize, it just irks me very much when I'm not sure I understand something. By the way, I've never taken physics. I understand the first line, I'm pretty sure. It's mainly the second one. 

How does spin affect the measurements, precisely?


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## an absurd man (Jul 22, 2012)

Tristan427 said:


> I'm pretty sure I know what you're saying, but I'm not 100% positive I do. Could you explain the bold further? I do apologize, it just irks me very much when I'm not sure I understand something. By the way, I've never taken physics. I understand the first line, I'm pretty sure. It's mainly the second one.
> 
> How does spin affect the measurements, precisely?


It's the act of measurement that affects the state of the quantum system as @Staysys and @FlightsOfFancy have mentioned. A quantum system's state is easily identified by a set of what are called quantum numbers: *n* (represents energy), *l* (represents angular momentum), *m* (represents shift in energy due to magnetic field), *ms* (spin). 

I don't know *how *particles become entangled except that they interact in some way. But let's say we have an system of two entangled electrons with the same n, l, m quantum numbers. By conservation laws, the electrons must have opposite spins states*, called them up and down so now we can identify a particle by its spin and not the other quantum numbers. 

In an entangled state, the electrons are in a superposition of possible states meaning that the electrons' spin states are not certain at any time. Say that the probability of either electron being in the spin up or spin down states upon measurement is 50%. Now lets say that you want to measure the spin of one of the electrons and the other electron is ten feet away. By measuring, you extract information from the system and you find that the particular electron you measured has spin state up. Necessarily this means the other electron has spin state down.

Now without changing the system in any way, taking another measurement of the electrons will show that they are in the same spin states resultant from the first measurement. That is, the electron in front of you is definitely in the spin up state, and the electron ten feet away is definitely in the spin down state forever and always as long as nothing changes the system. Likewise, if you had measured the electron in front of you to be spin down (50% chance), it would be in the spin down state permanently and the electron ten feet away would be spin up permanently given that you don't change the system in anyway.

The electrons are effectively separated or disentangled now; their states are now independent of each other so altering one the them does not affect the other. By making measurements on the system you have irreversibly altered its state. This is called collapse of the wave function, or collapse of the state vector and is a fundamental result of QM.



*Electrons are spin 1/2 particles. Thus for electrons ms = -1/2 or 1/2. Arbitrarily, you can call ms = 1/2 spin up and ms = -1/2 spin down.


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## HypoTempes (Nov 25, 2013)

Since it's not realy a topic easily explained (well not without a lot of typing from my part :wink Try these guys. 

Quantum Computing (Stanford Encyclopedia of Philosophy)

Very thorough explanation and all the parts involved are easily explained.

Or if you can find this book, its also a very nice source concerning the topic.

The Feynman Processor : Quantum Entanglement and the Computing Revolution (Helix Books Series): Gerard J. Milburn: 9780738201733: Amazon.com: Books


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## RobynC (Jun 10, 2011)

@flights of fancy



> Using the magnet analogy, if you provide a strong enough magnet to overpower the attraction that the other magnet has, it will "disentangle" with the weaker magnet and become "entangled" with the other. Again, this is using a classical analogy, so it is not 100% aligned with quantum (which would deal with energy levels more distinctly).


So a magnet cause two atoms to entangle and disentangle? Or is that some kind of analogy?

I'm confused...


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