Researchers use webs of lasers to remove entropy from a system causing quantum gases to cool

Dec 22, 2011 by Bob Yirka report
Orbital excitation blockade mechanism in an optical lattice. Image (c) NPG, doi:10.1038/nature10668

( -- Many physicists around the world are hard at work trying to figure out new and exciting ways to create ultra-cold objects, the reason being is that if a system could be created that operates at or at least very near absolute zero, superconductors could be devised that might help create quantum computers, which would of course run at speeds that would make the current generation look quaint. Plus, theory suggests new states of matter might be discovered.

Now, new work by a group of physicists from Harvard appears to be coming closer than ever. They’ve figured out a way to remove entropy from a specialized system leaving much colder behind. In their paper, published in Nature, they discuss how they’ve come up with something called an orbital excitation blockade, a form of interaction blockade, to reach temperatures tens to hundreds of times colder than current methods.

The team did their research in a three step process. In the first they shot atoms that make up rubidium with a laser, forcing them to glow in a way that made them give off more energy then they absorbed, making them cooler of course. By doing so they also created a system whereby they were able to control the atoms due to the pressure created by the laser. Thus they could hold them still, move them around, or even cause them to run into each other.

Next, the team caused the atoms to grow even colder by allowing evaporative cooling to due its work.

After that, the real work began. Here the team used meshes of lasers, called optical lattices to remove entropy from the system. The already cooled atoms were made to knock into one another using lasers ala the method used to start the whole process; this time in the optical lattices. In so doing, the excited activity of atom one dampened the excited activity of the other, a process the team calls an orbital excitation blockade. The team then removed the excited atoms from the system, leaving the unexcited, cold atoms behind, in effect, removing from the system.

In actual experiments done thus, far, the team has demonstrated an ability to actually remove heat from a system using their excitation blockade, but only to a certain point. They believe more research will allow them to reach temperatures tens or even hundreds of a billionth of a degree above absolute zero, which would take them into truly unknown territory.


More information: Orbital excitation blockade and algorithmic cooling in quantum gases, Nature, 480, 500–503 (22 December 2011) doi:10.1038/nature10668

Interaction blockade occurs when strong interactions in a confined, few-body system prevent a particle from occupying an otherwise accessible quantum state. Blockade phenomena reveal the underlying granular nature of quantum systems and allow for the detection and manipulation of the constituent particles, be they electrons, spins, atoms or photons. Applications include single-electron transistors based on electronic Coulomb blockade7 and quantum logic gates in Rydberg atoms. Here we report a form of interaction blockade that occurs when transferring ultracold atoms between orbitals in an optical lattice. We call this orbital excitation blockade (OEB). In this system, atoms at the same lattice site undergo coherent collisions described by a contact interaction whose strength depends strongly on the orbital wavefunctions of the atoms. We induce coherent orbital excitations by modulating the lattice depth, and observe staircase-like excitation behaviour as we cross the interaction-split resonances by tuning the modulation frequency. As an application of OEB, we demonstrate algorithmic cooling of quantum gases: a sequence of reversible OEB-based quantum operations isolates the entropy in one part of the system and then an irreversible step removes the entropy from the gas. This technique may make it possible to cool quantum gases to have the ultralow entropies required for quantum simulation of strongly correlated electron systems. In addition, the close analogy between OEB and dipole blockade in Rydberg atoms provides a plan for the implementation of two-quantum-bit gates in a quantum computing architecture with natural scalability.

A Harvard University press release can be found below:





The catch is that observing the quantum mechanical effects that Greiner, Bakr and colleagues seek requires extreme temperatures.


The hotter an object is, the more its constituent particles move around, obscuring the quantum world much as a shaken camera blurs a photograph.


Akin to preparing precisely one egg per dimple in a carton, this "orbital excitation blockade" process removes excess atoms from a crystal until there is precisely one atom per site.


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User comments : 6

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1.7 / 5 (3) Dec 22, 2011
Researchers create zero entropy system, but somehow can't be sure exactly where it is anymore. ;-)
1 / 5 (2) Dec 22, 2011
It's almost as if time itself is nearly stopped, and that would lead to some interesting discoveries.
1 / 5 (2) Dec 23, 2011

As if hundreds of a billionth would be even lower then tens.
not rated yet Dec 23, 2011
What's that in K?
not rated yet Dec 23, 2011
BTW, the writing style in this article leaves something (maybe a lot) to be desired. It's an article in search of an editor?
not rated yet Jan 05, 2012
It may be that a zero entropy system "collapses" into nothingness. This nothingness may be "dark Mass". This "dark mass" may make up a significant portion of the universe's mass that is not yet accounted for.


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