Coldest cubic meter in the universe

My collaborators in CUORE, the Cryogenic Underground Observatory for Rare Events, at the underground Gran Sasso National Laboratory in Assergi, Italy, have recently created (literally) the coldest cubic meter in the universe. For 15 days in September 2014, cryogenic experts in the collaboration were able to hold roughly one contiguous cubic meter of material at about 6 mK (that is, 0.006 degrees above absolute zero, the coldest possible temperature).

At first, a claim like “this is the coldest cubic meter in the [insert spacial scale like city/state/country/world/universe]” may sound like an exaggeration or a headline grabbing ruse. What about deep space? What about ice planets? What about nebulae? What about superconductors? Or cold atom traps? However, the claim is absolutely true in the sense that there are no known natural processes that can reliable create temperatures anywhere near 6 mK over a contiguous cubic meter anywhere in the known universe. Cold atom traps, laser cooling, and other remarkable ultracold technologies are able to get systems of atoms down to the bitter pK scale (a billionth of a degree above absolute zero). However, the key term here is “systems of atoms.” These supercooled systems are indeed tiny collections of atoms in very small spaces, nowhere near a cubic meter. Large, macroscopic superconductors can operate at liquid nitrogen or liquid helium temperatures, but those are very warm compared to what we are talking about here. Even deep space is sitting a at a balmy 2.7 K thanks to the cosmic microwave background radiation (CMBR). Some specialized thermodynamic conditions, such as those found the the Boomerang Nebula, may bring things down to a chilly 300-1000 mK because of the extended expansion of gases in a cloud over long times. The CMB cold spot is only 70 micro-kelvin below the CMBR.

However, the only process capable of reliably bringing a cubic meter vessel down to 6 mK are sentient creatures actively trying to do so. While nature could do it on its own in principle, via some exotic process or ultra-rare thermal fluctuation, the easiest natural path to such cold swaths of space, statistically sampled over a short 13.8 billion years, is to first evolve life, then evolve sentient creatures who then actively perform the project. So the only other likely way for there to be another competing cubic meter sitting at this temperature somewhere in the universe is for there to be sentient aliens who also made it happen. The idea behind the news angle “the coldest cubic meter” was the brainchild of my collaborator Jon Ouelett, a graduate student in physics at UC Berkeley and member of the CUORE working group responsible for achieving the cooldown. His take on this is written up nicely in his piece on the arXiv entitled The Coldest Cubic Meter in the Known Universe.

I’ve been member of the CUORE and Cuoricino collaborations since 2004 when I was a postdoc at Lawrence Berkeley Laboratory. I’m now a physics professor at California Polytechnic State University in San Luis Obispo and send undergraduate students to Gran Sasso help with shifts and other R&D activities during the summers through NSF support. Indeed, my students were at Gran Sasso when the cooldown occurred in September, but were working on another part of the project doing experimental shifts for CUORE-0. CUORE-0 is a precursor to CUORE and is currently running at Gran Sasso. It is cooled down to about 10 mK and is perhaps a top-10 contendeder for the coldest contiguous 1/20th of a cubic meter in the known universe.

I will write more about CUORE and its true purpose in coming posts.

On a speculative note, one must naturally wonder if this kind of technology could be utilized in large scale quantum computing or other tests of macroscopic quantum phenomenon. While there are many phonon quanta associated with so many crystals at these temperatures (and so the system is pretty far from the quantum ground state, and has likely decohered on any time scales we could measure) it is still intriguing to ask if some carefully selected macroscopic quantum states of such a large system could be manipulated systematically. Large-mass gravitational wave antennae, or Weber bars, have been cooled to a point where the system can be considered in a coherent quantum state from the right point of view. Such measurements usually take place with sensative SQUID detectors looking for ultra-small strains in the material. Perhaps this new CUORE technology, involving macroscopic mK-cooled crystal arrays, can be utilized in a similar fashion for a variety of purposes.