Tuesday, May 8, 2012
Plants are quantum mechanical
A plant's antenna complex captures light energy as excitations of chlorophyll molecules. It makes sense that the photon-molecule interaction is quantum mechanical. Apparently long-lived quantum coherence, protected from the environment by the protein scaffold of the antenna complex, also plays a role as the energy is transferred to the reaction center of the complex and stored there in chemical bonds.
Recent experimental work on EPR/Bell's inequalities
I mentioned in class yesterday that researchers are still refining their tests of the EPR correlations and Bell's theorem. Here from 2008 is a report (and the technical article) of an experiment verifying correlations between photons sent from their source, through optical fibers, to detectors in two Swiss towns separated by 18 km.
In googling I came across a short sweet essay in Nature summarizing why these experiments undo our intuitive notions of local realism. The author, Alain Aspect, performed the seminal experiment in the early 1980s.
In googling I came across a short sweet essay in Nature summarizing why these experiments undo our intuitive notions of local realism. The author, Alain Aspect, performed the seminal experiment in the early 1980s.
Friday, April 13, 2012
Superfluids and neutron stars, or, highlights of my afternoon on youtube
1. A beaker of Helium atoms (which are bosons), all in the same quantum state.
2. A neutron star collision.
I mean, I always liked Ziggy Stardust, but I never realized physics has its own genre of rock music.
2. A neutron star collision.
I mean, I always liked Ziggy Stardust, but I never realized physics has its own genre of rock music.
Friday, March 9, 2012
STM links
One of the key people behind the STM images we saw in class is Mike Crommie, now a professor at UC Berkeley. There are some fine new STM images on the Crommie group website. (Wait for the images to cycle.) The links from class were to nano-scale research at UW Madison and to IBM Almaden labs.
Thursday, March 1, 2012
Do I have a secret admirer?
Someone, whose email address I don't recognize, sent me a link to a New York Times article about the many inventions that came out of Bell Labs in the middle part of the 20th century. Transistors, lasers, solar cells, CCDs, fiber optics.... They all depend on quantum physics. Seems like something I might be interested in, no?
Thursday, February 23, 2012
more homework hints
Wikipedia now has good introductions to specialized topics in physics and mathematics. You might enjoy the page on linear transformations. Or the one on eigenvectors.
Wednesday, February 22, 2012
homework hints
-- For problem 2, from Griffiths p. 441: "If you know what a particular linear transformation does to a set of basis vectors, you can easily figure out what it does to any vector." The trick is to work out how you would like a transformation to act on the basis vectors. You'll find (and you might check) that your result works for an arbitrary vector.
-- For problem 3: Based on what Griffiths has told you, you can work on the first two parts of the problem. As Donaldo pointed out to me in the reading questions, Griffiths and I haven't told you enough to understand equation A.75, which is important for the third part (the part about eigenvectors of L_x). Take a stab by guess-and-check, or we'll talk about it in class Friday.
-- For problem 3: Based on what Griffiths has told you, you can work on the first two parts of the problem. As Donaldo pointed out to me in the reading questions, Griffiths and I haven't told you enough to understand equation A.75, which is important for the third part (the part about eigenvectors of L_x). Take a stab by guess-and-check, or we'll talk about it in class Friday.
Monday, February 20, 2012
Quantum computing
The Vancouver Sun reports from the annual meeting of the American Association for the Advancement of Science, held in Vancouver over the weekend, about the state-of-the-art in quantum computing.
Here's the basic idea behind it: Instead of representing the digital 1 and 0 of the computer by the "on" and "off" states of a transistor, you represent them by something like the two spin states of an electron, spin "up" and spin "down." As you know, the general state of an electron is a superposition of these two states. This would correspond to a superposition of a 1 and a 0. The computer could compute on both values at once. A string of 1s and 0s would be a series of electron spins, superpositions of 00010101, 00010111, 00010001..., an exponentially large number of possibilities going at the same time.
It's not often you come across a field at the cutting edge of experiment and theory, where the practical applications are within reach. The folks at D-Wave started selling some sort of primitive ($10,000,000) machine last year, to much controversy. You might enjoy Scott Aaronson's blog wrap on D-wave (dated by a few years). It's a window into the practice of science and the competing priorities of academic and industrial researchers.
Hopefully we can discuss quantum computing in more detail later, either online or in class toward the end of term. If I could choose my path in physics all over again...??
Oh, and the New York Times reports yesterday on a transistor created from a single phosphorus atom.
Here's the basic idea behind it: Instead of representing the digital 1 and 0 of the computer by the "on" and "off" states of a transistor, you represent them by something like the two spin states of an electron, spin "up" and spin "down." As you know, the general state of an electron is a superposition of these two states. This would correspond to a superposition of a 1 and a 0. The computer could compute on both values at once. A string of 1s and 0s would be a series of electron spins, superpositions of 00010101, 00010111, 00010001..., an exponentially large number of possibilities going at the same time.
It's not often you come across a field at the cutting edge of experiment and theory, where the practical applications are within reach. The folks at D-Wave started selling some sort of primitive ($10,000,000) machine last year, to much controversy. You might enjoy Scott Aaronson's blog wrap on D-wave (dated by a few years). It's a window into the practice of science and the competing priorities of academic and industrial researchers.
Hopefully we can discuss quantum computing in more detail later, either online or in class toward the end of term. If I could choose my path in physics all over again...??
Oh, and the New York Times reports yesterday on a transistor created from a single phosphorus atom.
Midterm exam dates
The first midterm will be on Wednesday, March 7. The second midterm will be on Friday, March 30.
Monday, February 13, 2012
Textbooks in the library
This afternoon I added a copy of Griffiths to the copy of Feynman on course reserve. It may take 24 hours for it to hit the shelves.
Quantum in the news: laser-entangled diamonds
We talked today about how an atom entering an open T apparatus will be in a superposition of the three T base states. We wrote: |S+> = c_+|T+> + c_0|T_0> + c_-|T_->. Those states, or the paths they represent, interfere with each other so that the atom leaves the apparatus in the same |S+> state.
It sometimes happens that two particles interact so as to enter a superposition in which the spin state of one depends on the spin state of the other. Neither is in a definite spin state by itself. These particles are said to be entangled, and they can remain entangled over long distances. (This has been tested for separations of many meters, but in principle there's no limit.) Griffiths pp. 421-422 discusses some of the "spooky" consequences of entanglement.
Recently researchers have been demonstrating entanglement in macroscopic (roughly, human-sized) systems. Usually on this scale the effects of entanglement are washed out by uncontrolled interactions with the environment. Here's an article describing a sweet experiment in which physicists, wielding lasers, managed to entangle two chips of diamond. The diamond chips were entangled for all of 350 femtoseconds. (That's 350 * 10^-15 seconds, for those counting.)
It sometimes happens that two particles interact so as to enter a superposition in which the spin state of one depends on the spin state of the other. Neither is in a definite spin state by itself. These particles are said to be entangled, and they can remain entangled over long distances. (This has been tested for separations of many meters, but in principle there's no limit.) Griffiths pp. 421-422 discusses some of the "spooky" consequences of entanglement.
Recently researchers have been demonstrating entanglement in macroscopic (roughly, human-sized) systems. Usually on this scale the effects of entanglement are washed out by uncontrolled interactions with the environment. Here's an article describing a sweet experiment in which physicists, wielding lasers, managed to entangle two chips of diamond. The diamond chips were entangled for all of 350 femtoseconds. (That's 350 * 10^-15 seconds, for those counting.)
Sunday, February 12, 2012
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