A clock that will last forever: Researchers propose a way to build the first space-time crystal

[Jim__]

From Phys.org

(Phys.org)—Imagine a clock that will keep perfect time forever, even after the heat-death of the universe. This is the "wow" factor behind a device known as a "space-time crystal," a four-dimensional crystal that has periodic structure in time as well as space. However, there are also practical and important scientific reasons for constructing a space-time crystal. With such a 4D crystal, scientists would have a new and more effective means by which to study how complex physical properties and behaviors emerge from the collective interactions of large numbers of individual particles, the so-called many-body problem of physics. A space-time crystal could also be used to study phenomena in the quantum world, such as entanglement, in which an action on one particle impacts another particle even if the two particles are separated by vast distances.

A space-time crystal, however, has only existed as a concept in the minds of theoretical scientists with no serious idea as to how to actually build one – until now. An international team of scientists led by researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) has proposed the experimental design of a space-time crystal based on an electric-field ion trap and the Coulomb repulsion of particles that carry the same electrical charge.

"The electric field of the ion trap holds charged particles in place and Coulomb repulsion causes them to spontaneously form a spatial ring crystal," says Xiang Zhang, a faculty scientist with Berkeley Lab's Materials Sciences Division who led this research. "Under the application of a weak static magnetic field, this ring-shaped ion crystal will begin a rotation that will never stop. The persistent rotation of trapped ions produces temporal order, leading to the formation of a space-time crystal at the lowest quantum energy state."

Because the space-time crystal is already at its lowest quantum energy state, its temporal order – or timekeeping – will theoretically persist even after the rest of our universe reaches entropy, thermodynamic equilibrium or "heat-death."

more ...

[longship]

1. But Cap'n! The dilithium crystals are about melting as it is!

Couldn't resist.

[littlemissmartypants]

2. What a great site!

Thank you so much for posting this!

[Jim__]

4. It is an interesting site. - n/t

[antiquie]

3. Time crystals could behave almost like perpetual motion machines

Thank you for sending me to the site. This article is linked from the page:

As every young science student knows, moving objects have kinetic energy. But just how much energy does something need to move? In a new study, a pair of physicists has shown that it’s theoretically possible for a system in its lowest energy state, or ground state, to exhibit periodic motion. This periodically moving system can be thought of as the temporal equivalent of a crystal, which is defined by its spatial periodicity. What’s even more intriguing about these "time crystals" is that, by exhibiting motion at their state of lowest energy, they break a fundamental symmetry called time translation symmetry and become "perilously close" to looking like perpetual motion machines.

Full at http://phys.org/news/2012-02-crystals-perpetual-motion-machines.html#nRlv

[Jim__]

5. Perpetual motion machine was the first thing that popped into my head.

The article says it isn't one:

Li is quick to point out that their proposed space-time crystal is not a perpetual motion machine because being at the lowest quantum energy state, there is no energy output. However, there are a great many scientific studies for which a space-time crystal would be invaluable.

I understand that there is no energy output; it's not clear to me why that eliminates it as one.

[antiquie]

6. "perilously close"

I dream; thanks for not totally popping my bubble.

[drm604]

7. Does it require energy output to be considered perpetual motion?

If it's in motion, and it's perpetual, and it requires no energy input, then how is it not perpetual motion?

I agree that it's not an energy source, but that doesn't mean it's not perpetual motion.

[caraher]

10. I think the lack of energy output is what eliminates it as a perpetual motion *machine*

It doesn't do external work, which is usually a key feature of any kind of engine (which I think is what "machine" implicitly means here).

I think the notion that this device can survive the "heat death" of the universe is experimentally impossible. The ions need to be in a particular magnetic field, and in principle one could supply a persistent permanent field with, say, a superconducting magnet. But I don't think any such physical magnet could be stable against the forces that drive the long-term fate of the universe. Bear in mind that the authors are theorists (albeit with substantial insights into experiment; one was on my dissertation committee) and they're more interested in the potential implications of time crystals than the details of limits on the possibilities for their operation as "perpetual" clocks.

[Jim__]

13. The magnetic field does need to remain?

I wasn't sure how to interpret "weak static magnetic field":

"The electric field of the ion trap holds charged particles in place and Coulomb repulsion causes them to spontaneously form a spatial ring crystal," says Xiang Zhang, a faculty scientist with Berkeley Lab's Materials Sciences Division who led this research. "Under the application of a weak static magnetic field, this ring-shaped ion crystal will begin a rotation that will never stop. The persistent rotation of trapped ions produces temporal order, leading to the formation of a space-time crystal at the lowest quantum energy state."

I read that paragraph as meaning that once the magnetic field started the rotation, the rotation would persist. If the magnetic field is removed, the rotation would stop?

[caraher]

14. The applied field definitely needs to be maintained

The rotating motion is the lowest-energy state for the system of ions in a particular potential, which comes from the combination of their mutual electrostatic repulsion and the applied external magnetic field. The key words are "trapped ions." The magnetic field is essential to the trap; without it, you have untrapped ions, which simply repel one another.

[Jim__]

15. Thanks.

[phantom power]

8. in terms of using the thing for a clock...

I wonder how you can observe the thing in a way meaningful for measuring time, without disturbing it in a way that breaks it.

[Jim__]

9. I'm not sure, but that may be an unsolved problem.

From the original paper:

... While it has been proved mathematically that time crystals can exist in principle [5, 6], it was not clear how to realize and observe time crystals experimentally.

In this paper, we propose a method to create a space-time crystal (Fig. 1a), which is also a time crystal, with cold ions in a cylindrically symmetric trapping potential. ...

Based on that, it sounds like they may have solved the realize part of the problem. There is a lot more in the paper, but I can't follow the math.

[caraher]

11. That's a big focus of the paper

I just skimmed it but there's a 2-laser fluorescence scheme that they'd use to read out the state of the system. The claim is that this should do the trick.

[skor584_2il]

16. wow, pretty cool stuff