Monday, December 20, 2010

Science's Breakthrough of the Year for 2010

Science Magazine has named the work of a team at UCSB directed by Andrew Cleland and John Martinis as their scientific breakthrough of the year for 2010.  Their achievement:  the demonstration of a "quantum machine".  I'm writing about this for two reasons.  First, it is extremely cool stuff that has a nano+condensed matter focus.  Second, this article and this one in the media have so many things wrong with them that I don't even know where to begin, and upon reading them I felt compelled to try to give a better explanation of this impressive work.

One of the main points of quantum mechanics is that systems tend to take in or emit energy in "quanta" (chunks of a certain size) rather than in any old amount.  This quantization is the reason for the observation of spectral lines, and mathematically is rather analogous to the fact that a guitar string can ring at a discrete set of harmonics and not any arbitrary frequency.   The idea that a quantum system at low energies can have a very small number of states each corresponding to a certain specific energy is familiar (in slightly different language) to every high school chemistry student who has seen s, p, and d orbitals and talked about the Bohr model of the atom.  The quantization of energy shows up not just in the case of electronic transitions (that we've discussed so far), but also in mechanical motion.  Vibrations in quantum mechanics are quantized - in quantum mechanics, a perfect ball-on-a-spring mechanical oscillator with some mechanical frequency can only emit or absorb energy in amounts of size hf, where h is Planck's constant.  Furthermore, there is some lowest energy allowed state of the oscillator called the "ground state".  Again, this is all old news, and such vibrational quantization is clear as a bell in many spectroscopy techniques (infrared absorption; Raman spectroscopy). 

The first remarkable thing done by the UCSB team is to manufacture a mechanical resonator containing millions of atoms, and to put that whole object into its quantum ground state (by cooling it so that the thermal energy scale is much smaller than hf for that resonator).  In fact, that's the comparatively easy part.  The second (and really) remarkable thing that the UCSB team did was to confirm experimentally that the resonator really was in its ground state, and to deliberately add and take away single quanta of energy from the resonator.  This is very challenging to do, because quantum states can be quite delicate - it's very easy to have your measurement setup mess with the quantum system you're trying to study!

What is the point?  Well, on the basic science side, it's of fundamental interest to understand just how complicated many particle systems behave when they are placed in highly quantum situations.  That's where much of the "spookiness" of quantum physics lurks.  On the practical side, the tools developed to do these kinds of experiments are one way that people like Martinis hope to build quantum computers.  I strongly encourage you to watch the video on the Science webpage (should be free access w/ registration); it's a thorough discussion of this impressive achievement.

6 comments:

Don Monroe said...

Nice discussion, Doug.

I have one question of terminology to toss out to the crowds: since when does "machine" imply mechanical? That's the way Science used it. It's also the way New Scientist described it. Aren't computers machines, too? (IBM thinks so!)

To my mind, although I think the research is great, it doesn't seem overwhelmingly profound or surprising that quantum mechanics applies to mechanical systems.

Filipe said...

I think the second paragraf has some "copy and paste" problem. :) Then ending "orbitals and talked about the Bohr ..." should come just after "...who has seen s, p, d and f..."

Doug Natelson said...

Filipe, thanks. I fixed it. Very bizarre - I swear it was correct when I posted it and got munched after the fact.

Don, I know what you're saying about machines and "mechanical". I think it reflects the idea that a (traditional) machine has moving parts. I think the part that captures the imagination of the science writers (and possibly that of the Science editors as well) is that this demonstrates that one can take a solid system of many atoms and put it into a superposition state. (Horribly mangled by the popular press as meaning that macroscopic objects can be "two places at once".)

All of this reminds me of an old fake ad in the classic book Science Made Stupid. Picture the kind of advertisement that used to be on the inside of a matchbook cover. A guy is holding a wrench (that's actually an impossible object) beneath the words "Make Big Money: Be A Quantum Mechanic!"

Austen said...

The terminology that we seem to be stuck with uses electronics to describe the motion of electrons and mechanics to describe motion of the nuclei, or crystal lattice. Of course it's an artificial distinction.

Massimo said...

Aren't you excited that "time travel is possible" ?

Doug Natelson said...

Massimo - the sad thing is, I think I can see where they get that. If you could be in two places at the same time (spacelike separation between events), then a relativistic observer could say that you had gone back in time.

Time to write a hugely sensationalistic take on the many-worlds interpretation of QM. PHYSICIST SAYS WE'RE IN COMMUNICATION WITH ALTERNATE REALITIES!