Category Archives: Experiments

Sesame Street Physics

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Light and Electrons Cooperate

I can just picture Big Bird:

Can you say, “cooperate?”

Can you say, “surface plasmon polariton?”

When light hits a metal, it can create a surface plasmon polariton–often called simply a “surface plasmon”–which is a traveling wave combining electromagnetic fields with electron oscillations. Researchers are using tiny plasmon antennas to funnel more light into solar cells (photovoltaics), increasing their efficiency. Others studying “plasmonics” hope to develop devices that replace some electric currents with plasmon waves, because plasmons can theoretically carry as much information as light pulses but squeeze it into the nanometer-sized wires used in standard computer chips.

Shape and the Single Photon

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Shaping Single Photons

When you detect a photon, you can say where, when, and with what frequency it arrived, but before the measurement, these parameters are undefined. The photon’s existence is embodied in a wave function, which gives the probability of measuring the photon at any time, place, and frequency. The wave function for a single photon is usually a “wave packet”–nearly zero everywhere except in a narrow range of space and time. But as long as you don’t detect the photon directly, you can manipulate its wave function into any complicated shape, in theory.

Focus, People

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This month’s Physical Review Focus: Nanoparticles Stick a Perfect Landing

They found that for speeds less than 1.2 kilometers per second, the nanoparticle bounces off the surface like a basketball. But at higher speeds, some of the nanoparticle undergoes a phase transition to a compressed state called β-tin, where each atom bonds to six neighbors. This transition is surprising, Dumitrică says, because the collision energy is not high enough to induce a phase transition in a macroscopic object. However, the impact force is applied over a few square nanometers, so the pressure inside the nanoparticle is extremely large–around 200,000 atmospheres, which is more than enough to cause the phase transition.

The β-tin state only lasts a few picoseconds, though. As the nanoparticle begins to bounce back, there is a second phase transition to an amorphous, or disordered, state. The combination of the two phase transitions, plus some heat generation, takes up all of the kinetic energy, and the particle remains on the surface. After all of this action, “the recoil is too weak to beat the adhesion forces between the nanoparticle and the substrate,” Dumitrică says.

However

A silicon nanoparticle flying at 8 times the speed of sound can slam into a surface and stick, but it bounces off if colliding at half that speed.

The speed of sound in what, pray tell? I wish journalists would remember (learn?) that the speed of sound is not a constant of nature.

Who's the Fairest Helium Atom of Them All?

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Smoothest surface ever is a mirror for atoms

Metal materials reflect helium atoms much better but are harder to bend precisely into the right shape. Now materials scientists from the Autonomous University of Madrid led by Amadeo Vázquez de Parga have combined silicon and metal to make what they say is the smoothest surface ever made.

They made the near-perfect mirror by coating a thin layer of lead onto a silicon surface. This is not straightforward, because when a very thin metal layer is deposited onto a flat silicon surface it usually forms an uneven coating of differently sized bumps that perform badly as a mirror for helium atoms.

Playing Hard to Get

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Giving your new results away too soon

[W]here do you announce your results first: in the title? In the abstract? In the introduction? Or, in the results paragraph? If you wait to long your paper will become a whodunit and readers will get bored and stop reading your paper. If the clue of your paper is already in the title you might fear that many of your readers will only read your title and will then go on to read the next paper.

It depends on the type of paper, but I think you generally give your main result in the abstract. The paper gives the details of how you did it, context and information about other related research (but not in that order)

He Helps Us Get High

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August 18, 1868. Jules Janssen “invents” helium. (At least, according to principal Skinner. “Curse the man who invented helium! Curse Pierre Jules César Janssen!)

Janssen was observing an eclipse and measured an emission line with a wavelength of 587.49 nm, which didn’t correspond to any known element. Norman Lockyer also observed the line later that year, and as it could not be reproduced in the lab, proposed that it was a new element, which was named after helios, the sun.

It's Not an Inalienable Right!

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Jennifer’s enumeration of the PARTICLE BILL OF RIGHTS reminds me of a neat effect. I hope the second amendment

The right of unstable Particles to decay shall not be infringed.

only applies to militias fundamental particles, because people have been messing with that “right” for a while in atoms. These are demonstrated by some fascinating experiments in cavity QED I read about while I was in grad school. Probably the most familiar cavity QED phenomenon is the Casimir force, which arises from modifying the electromagnetic modes that are allowed to exist. In free space, waves of any and all frequencies and polarizations can exist, but when conducting surfaces are present, these alter the boundary conditions. Two flat conductors, closely spaced, will exclude many mode of electromagnetic oscillation, and because each mode carries 1/2 hf of energy even when there are no photons in that mode, this exclusion gives rise to the attractive force.

But there’s more fun to be had with this.

Imagine placing an atom inside a cavity under circumstances similar to where the Casimir force could be observed. You can prepare the atom in a state so that it can only decay by one mode — one transition, with a particular polarization of photon (either linear or circular), and you can also orient the atom so that the photon’s direction of emission (perpendicular pr parallel to the surface) will correspond to a photon mode that isn’t supported by the cavity configuration. When you do this, what is an atom to do? There is no vacuum fluctuation to induce it to decay, nor would a photon from that decay be able to exist. The atom is forced to sit there, grudgingly (or perhaps happily, I don’t think anyone’s asked) not decaying.

You can also choose your system so that there is a higher mode density, and get atoms to decay more quickly than they would in free space. (You can also repeatedly measure an atom’s state and keep it from decaying, in a phenomenon called the quantum Zeno effect, but I’m not going to go there. Or even halfway there)

So I must conclude that since this action is routinely taken on atoms and molecules, without writ or warrant, and we have declared that we do not torture inhibit decay, that this right does not apply to composite systems.

——

Some references
Heinzen, et.al, Phys Rev Lett. 58 1320 (1987)
Jhe, et. al, Phys Rev. Lett. 58, 666 (1987)
Haroche and Kleppner, Physics Today Jan 1989 24-30

Spooky Speeding

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A pretty cool experiment that puts a lower bound on a speed of entanglement has been performed. The experimenters entangled photons, separated them, and then made their measurements.

Physicist Nicolas Gisin and colleagues at the University of Geneva in Switzerland split off pairs of quantum-entangled photons and sent them from the university’s campus through two fiber-optic cables to two Swiss villages located 18 kilometers apart. Thinking of the photons like traffic lights, each passed through specially designed detectors that determined what “color” they were when entering the cable and what color they appeared to be when they reached the terminus. The experiments revealed two things: First, the physical properties of the photons changed identically during their journey, just as predicted by quantum theory–when one turned “red,” so did the other. Second, there was no detectable time difference between when those changes occurred in the photons, as though an imaginary traffic controller had signaled them both.

The results show that any information connection between them would have to occur at at least 10,000 times the speed of light, which is interpreted as a pretty good indication that it’s an inherent behavior of quantum mechanics, and this “communication” isn’t actually taking place. (see also Bohm’s Bummed and the summary at Physics and Physicists)

Or not.

nature news has an article entitled Physicists spooked by faster-than-light information transfer. LiveScience’s article is Spooky Physics: Signals Seem to Travel Faster Than Light. Which is really strange, because at least in the nature summary, they discuss how it isn’t evidence of superluminal communication

A second test ensured that the scientists in the two villages weren’t missing some form of communication thanks to Earth’s motion through space. According to Einstein’s theory of relativity, observers moving at high speeds can have different ‘reference frames’, so that they can potentially get different measurements of the same event. The Geneva results could possibly be explained if the two photons were communicating through a frame of reference that wasn’t readily apparent to the scientists.”

But theoretical calculations have shown that performing tests over a full spin of the globe would test all possible reference frames. The team did just that, and they got the same result in all cases.

The bottom line, says Gisin is that “there is just no time for these two photons to communicate”.

So why use a headline that says or implies that there is FTL information transfer, when the conclusion is that there isn’t?

Threading the Needle

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Another cool find by Zapperz: Threading Light Through the Opaque

Freshly fallen snow is blinding white because the jumble of flakes scatter light in all directions. Such scattering also implies that little light passes through snow, so that if you’re ever buried deep in it, you’ll find yourself in the dark. But according to theoretical physicists, it should always be possible to fiddle with light waves to make them wend their way through such a disordered material, no matter how thick. And now a duo of experimenters has demonstrated that feat.