swans on tea

Just another SFN Blogs site
  • Home
  • A very little about me
  • Sample Page

Another Path to Degeneracy

Published by swansont on January 28, 2013 03:00 am under Physics

One of my colleagues mentioned this in group (as in meeting, not therapy) — research that has formed a Bose-Einstein Condensate directly from laser cooling and trapping: Laser cooling to quantum degeneracy

In order to form a BEC, you have to get enough atoms together in a potential well, and with a low enough energy in order to trigger the transition that leaves the bulk of them in the ground state (that’s the condensation). Normally, atoms under such confinement will distribute themselves among the various quantized energy states, but under the proper circumstances the condensation will happen. The tipping point occurs when the phase space density — the number of atoms in a volume defined by the cube of the deBroglie wavelength (which is momentum dependent, so it’s related to the kinetic energy) is above some critical value. The trick, then, is getting enough atoms cold enough and confined such that their deBroglie waves are overlapping.

Up until now, the path to BECs have involved a stage of magnetic trapping, but since magnetic traps are shallow (energetically speaking) you laser-cooled and trapped atoms to load into the magnetic trap. The trap “walls” were then lowered, allowing the most energetic atoms to escape, much like the most energetic molecules leave a hot cup of water, so this is called evaporative cooling. It results in the average energy, and thus temperature, going down. A drawback is that you also lose atoms, but if the phase-space volume decreases faster than atoms are lost it results in the phase space density going up, and a condensate.

So why not simply laser-cool the atoms? The problem with that is that atoms absorbing the photons that are cooling them also re-emit those photons, and when the physical density is large, those emitted photons hit other atoms rather than escaping, and this limits both the density and the temperature. This interaction can’t be present where the condensate is formed.

So the researchers turned it off, in a manner of speaking. They added a second form of trap, a dipole trap, which doesn’t involve the absorption and emission of photons — the light is far from resonance, and you use the electric field gradient of the light to form the trap. In the region where this trap was present, the laser scattering was turned off by shifting the energy of the excited state, which you can do if you add an external field. This was accomplished by the addition of another laser, which gives rise to the AC Stark shift (Johannes, not Tony), or a light shift. The trapping light is no longer near resonance, so the scattering is greatly suppressed. It’s also localized, so it’s only in effect where you shine the laser.

The area where the trapping is still occurring still contains very cold atoms, so the atoms in the dipole trap can still collide and thermalize with them, but once that second trap fills sufficiently, the critical phase-space density is achieved and a BEC forms. Very neat.

No Comment

Comments are closed.

Posting your comment.

  • Recent Posts

    • When Does the Decade End?
    • This is the Hardest Job a Manager Has …
    • 5 Things You Should Know About Light
    • See Spot "Run"
    • The System Works
  • Recent Comments

    • Joey Cook on One Ringy-Dingy, Two Ringy-Dingy
    • Science Fanatic on Talk Like a Physicist Day
    • Chris Gould on Brian Cox is Full of **it
    • LaurieAG on Inmates Running the Asylum
    • Uncle Al on Curious About Curies
  • Archives

    • December 2019
    • February 2015
    • January 2015
    • December 2014
    • November 2014
    • October 2014
    • September 2014
    • August 2014
    • July 2014
    • June 2014
    • May 2014
    • April 2014
    • March 2014
    • February 2014
    • January 2014
    • December 2013
    • November 2013
    • October 2013
    • September 2013
    • August 2013
    • July 2013
    • June 2013
    • May 2013
    • April 2013
    • March 2013
    • February 2013
    • January 2013
    • December 2012
    • November 2012
    • October 2012
    • September 2012
    • August 2012
    • July 2012
    • June 2012
    • May 2012
    • April 2012
    • March 2012
    • February 2012
    • January 2012
    • December 2011
    • November 2011
    • October 2011
    • September 2011
    • August 2011
    • July 2011
    • June 2011
    • May 2011
    • April 2011
    • March 2011
    • February 2011
    • January 2011
    • December 2010
    • November 2010
    • October 2010
    • September 2010
    • August 2010
    • July 2010
    • June 2010
    • May 2010
    • April 2010
    • March 2010
    • February 2010
    • January 2010
    • December 2009
    • November 2009
    • October 2009
    • September 2009
    • August 2009
    • July 2009
    • June 2009
    • May 2009
    • April 2009
    • March 2009
    • February 2009
    • January 2009
    • December 2008
    • November 2008
    • October 2008
    • September 2008
    • August 2008
    • July 2008
    • June 2008
    • May 2008
    • April 2008
    • March 2008
    • February 2008
    • January 2008
  • Categories

    • admin
    • Antiscience
    • Art
    • Blog Compendia
    • Blogging
    • Body
    • Books
    • Bureaucracy
    • Business
    • Cartoon
    • Conference stories
    • Cool stuff
    • DIY science
    • Education
    • Environment
    • Experiments
    • Food
    • Game
    • Geocaching
    • History
    • Humor
    • Illusions
    • Journalism
    • Lab Stories
    • Language
    • Links
    • Math
    • Metaphysics
    • Misc
    • Movies
    • Music
    • Navy
    • Not Really Science at All
    • Other science
    • peeve
    • photography
    • Photos
    • Physics
    • Politics
    • Quotes
    • Rants
    • Religion
    • Satire
    • Sci-Fi
    • Science-general
    • science-y observation
    • Security
    • Shameless self promotion
    • Sick sick sick
    • Silly
    • Sports
    • Tech
    • The Lab
    • Thermal IR
    • Time
    • TMI
    • Toys
    • trivia
    • TV
    • TYAGFITI
    • Typography
    • Uncategorized
    • Video
    • Weird
    • World Events
    • Writing
  • Meta

    • Log in
    • Entries feed
    • Comments feed
    • WordPress.org

Copyright © 2022 swans on tea
WordPress Theme based on Light Theme