Posted on 01/22/2014 2:53:50 PM PST by ETL
In a paper published in the current issue of the scientific journal Nature Communications and titled "Direct measurement of a 27-dimensional orbital-angular-momentum state vector," a team of physicists led by the University of Rochester's Mehul Malik describe how they circumvented a basic principle of uncertainty that requires that some states of a quantum system must be understood poorly if other states are to be understood well.
Determining a quantum state, such as the position of an electron or the momentum of a photon, is tricky, to say the least. That's because subatomic particles behave nothing at all like billiard balls, orbiting moons, or any other kind of object with which we humans are remotely familiar.
A photon, for instance, sometimes acts like a wave, diffracting, interfering, and scattering, as all good waves ought to. Yet sometimes it will also behave like a particle, for instance by bashing into an electron or by traveling with ease through a vacuum.
According to our current understanding, things at the quantum scale can exist simultaneously in these two modes, both as localized particles, with distinct measurable states, and as spread-out probabilistic waves, with multiple contradictory states.
One consequence of this "wave-particle duality" is that it imposes a fundamental limit on how much we can know about the universe. An unobserved electron, say scientists, exists as a wave of mutually contradictory states. As the German physicist Werner Heisenberg first pointed out in 1927, taking a measurement of one state, say, the electron's position, and you irreversibly alter its momentum, and vice versa. In the parlance of quantum physicists, the "wavefunction" of a system's probabilities "collapses" into a specific state when you observe it.
If the quantum-mechanical model sounds bizarre, that's because it is.
(Excerpt) Read more at csmonitor.com ...
Direct measurement of a 27-dimensional orbital-angular-momentum state vector
ABSTRACT: The measurement of a quantum state poses a unique challenge for experimentalists. Recently, the technique of \direct measurement" was proposed for characterizing a quantum state in-situ through sequential weak and strong measurements. While this method has been used for measuring polarization states, its real potential lies in the measurement of states with a large dimensionality. Here we show the practical direct measurement of a high-dimensional state vector in the discrete basis of orbital-angular momentum. Through weak measurements of orbital-angular momentum and strong measurements of angular position, we measure the complex probability amplitudes of a pure state with a dimensionality, d=27. Further, we use our method to directly observe the relationship between rotations of a state vector and the relative phase between its orbital-angular-momentum components. Our technique has important applications in high-dimensional classical and quantum information systems, and can be extended to characterize other types of large quantum state.
(it's an 8-page pdf file)
http://arxiv.org/pdf/1306.0619.pdf
Question: What is Quantum Entanglement?
The classic example of quantum entanglement is called the EPR paradox. In a simplified version of this case, consider a particle with quantum spin 0 that decays into two new particles, Particle A and Particle B. Particle A and Particle B head off in opposite directions. However, the original particle had a quantum spin of 0. Each of the new particles has a quantum spin of 1/2, but because they have to add up to 0, one is +1/2 and one is -1/2.
This relationship means that the two particles are entangled. When you measure the spin of Particle A, that measurement has an impact on the possible results you could get when measuring the spin of Particle B. And this isn't just an interesting theoretical prediction, but has been verified experimentally through tests of Bell's Theorem.
One important thing to remember is that in quantum physics, the original uncertainty about the particle's quantum state isn't just a lack of knowledge. A fundamental property of quantum theory is that prior to the act of measurement, the particle really doesn't have a definite state, but is in a superposition of all possible states. This is best modeled by the classic quantum physics thought experiment, Schroedinger's Cat, where a quantum mechanics approach results in an unobserved cat that is both alive and dead simultaneously.
One way of interpreting things is to consider the entire universe as one single wavefunction. In this representation, this "wavefunction of the universe" would contain a term that defines the quantum state of each and every particle. It is this approach that leaves open the door for claims that "everything is connected," which often gets manipulated (either intentionally or through honest confusion) to end up with things like the physics errors in The Secret.
Though this interpretation does mean that the quantum state of every particle in the universe affects the wavefunction of every other particle, it does so in a way that is only mathematical. There is really no sort of experiment which could ever - even in principle - discover the effect in one place showing up in another location.
ping
Quantum Mechanics ping.
If you’d like on or off this list, please let me know.
Final exam question: Does Schrodeinger’s cat box need cleaning?
So much for a serious quantum thread...
i am woefully lacking in knowledge of QM but it intrigues me to no end, so if you don't mind stooopid questions on a regular basis, yes i'd like to be on the list... thx
The kitty pix are better for us anyway.
Only if you don’t mind stupid answers.
:)
As the German physicist Werner Heisenberg first pointed out in 1927, taking a measurement of one state, say, the electron's position, and you irreversibly alter its momentum, and vice versa. In the parlance of quantum physicists, the "wavefunction" of a system's probabilities "collapses" into a specific state when you observe it. stooopid question #1, i understand how actually measuring/metering could alter an objects state, but... how can observing it(under the assumption it means with your eyes) change it's state???
or is he using the words interchangeably?
Not really. Makes it more enjoyable when you mix in a little fun. QM is wacky anyway.
I have a feeling that at some point, it simultaneously does and doesn’t.
I spent a semester (long ago) studying Schrodinger's work. I've spent a lifetime studying cats. Cats are more confusing, but they are also more fun.
If they can’t see the inherent contradiction in saying that something exists simultaneously in two contradictory states, then no power on Earth can help them.
Just curious if you are still around. I used to enjoy your post’s on “science” threads (unlike what has become of them these days)...
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