MATT O'DOWD: This episode is supported by the Great Courses
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What exactly are time crystals?
Are they the bling inside your time turner,
the flux in your flux capacitor?
Are they the heart of the Tardis?
In today's edition of "Space Time Journal Club,"
we find out.
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In "Space Time Journal Club," we review new scientific papers
that are making waves.
We pick them apart to turn the technobabble
into simple English, at least as much as is possible.
Then we fight about it in the comments.
This week, we're going to take a look at a recent publication
by Norman Yao et al.
in physical review letters, entitled
"Discreet Time Crystals: Rigidity, Criticality,
and Realizations."
This paper proposed an approach to making
these bizarre objects, a recipe that two other research
teams have now followed and have actually
synthesized these things.
But first up, what on earth are time crystals?
The idea was first proposed in 2012 by Nobel laureate Frank
Wilczek of MIT.
He suggested a type of matter that
exhibits a sort of fundamental oscillation over time.
So some property of the material goes through a repeating cycle,
but how does that make it a time crystal?
The analogy is that regular crystals have a periodic cycle
through space.
Molecular patterns repeat again and again along their lattices.
Time crystals repeat some internal state
with constant separations in time.
The name time crystal is somewhat out there,
but Wilczek wasn't the first to use it
in reference to a regularly repeating system.
That may have been Arthur Winfree
in his "The Geometry of Biological Time,"
where it's used to describe periodic biological systems.
But Wilczek was clever to apply it here,
because the name made the internet go completely bonkers.
And so here we are.
Wilczek came up with a simple model
in which charged particles in a superconducting ring
break what we call continuous time translational symmetry.
That's a fancy way of saying that the system looks
different on a global level from one instant to the next.
Normal matter that is in what we call thermal equilibrium
only has random internal motion.
In solid matter, that would be the vibrational buzz
of its constituent atoms.
But from one instant to the next, that buzz stays random.
In regular matter in equilibrium,
statistical properties stay the same over time.
Wilczek's imaginary system broke this time translational
symmetry, because there are global statistical differences
in the state of the matter, non-random patterns that
change over time.
Big deal.
Lots of things change over time.
Cups of coffee cool down, planets orbit the sun,
the universe expands.
But cups of coffee in the universe
are not in equilibrium, and the planets
are macroscopic moving objects.
Wilczek proposed an actual substance
that was in perpetual motion while in equilibrium.
More, he imagined a substance for which oscillations
were the most fundamental lowest energy or ground states.
This would break time translational symmetry, which
makes most physicists nervous.
Well, physicists can chill.
In 2015, Haruki Watanabe of UC Berkeley and Masaki Oshikawa
of the University of Tokyo showed
from theoretical arguments that time translational symmetry
can't be broken by a quantum system in equilibrium.
That sounds bad for time crystals,
but that's where this new paper by Yao et al.
comes in.
Their answer is to throw away this equilibrium thing.
Thermal equilibrium means a closed system.
No energy in, no energy out.
Norman Yao, also at UC Berkeley, and his team
proposed a way to make time crystals
by using some sort of external input of energy
to force the oscillating states.
The idea goes like this.
Set up a chain of ions, so electrically charged atoms.
These atoms have spin values, quantum mechanical angular
momenta from their electrons.
Spins in nearby atoms like to line up
with each other due to interacting magnetic fields.
Either direct alignment or opposite alignment
are both a lower energy state than random alignment.
This is the same effect that results in magnetic materials.
So you prepare a string of ions where the line spins.
Now cause those spins to flip back and forth using a laser.
A laser is just a very well-ordered
electromagnetic wave with a known period or frequency.
The spin-flip oscillation will be determined
by the period of the laser.
That laser is what takes the system out of equilibrium,
because you are basically pumping in energy.
Causing spins to flip in a laser isn't particularly exciting.
I mean, you're basically grabbing the electrons
and forcing them to oscillate.
But the paper proposes that if you let go of the electrons,
their spin oscillations should continue.
They should be sustained internally.
That means they should resist a change
in the frequency of the input laser,
or continue oscillating at least for a while
if the input EM field is randomized.
In addition, other researchers theorized that spins should not
oscillate at the same period as the laser,
but at an integer multiple of the driving period.
So two, three, four, et cetera spin oscillations for every EM
field oscillation in the laser.
Yao et al.'s work was theoretical,
but it involved numerical calculations that allowed
them to draw a phase diagram.
This is sort of like the phase diagram of regular matter
in which you plot pressure versus temperature.
Different materials become solid, liquid, gas, or plasma
at different locations on that phase diagram.
The analogous phase diagram for time crystals
plots interaction strength between atoms
versus imperfection in the spin-flip driving signal.
This triangle at the bottom is where time crystals live.
If the variations in the forcing signal become too messy
and the interaction strength is too weak,
then the time crystal effectively
melts into regular time symmetric matter,
in which the ion chain follows the rhythm of the driving
signal perfectly with no independent rhythm of its own.
This right side of the graph is also interesting.
If the connections between the spins of the ions
become too strong, then a wormhole
forms and sends your graduate students back
to the Paleocene era.
I'm kidding.
At that point, thermal effects take over and the rhythm dies.
Since Yao and team laid out a practical approach
to building time crystals, in August 2016
two teams have synthesized them in the lab
in completely different ways.
Chris Monroe's team at the University of Maryland
followed Yao's suggestions for setting up a chain of ions,
linking 10 ytterbium ions and driving them with a laser.
Mikhail Lukin's Harvard team tried
something completely different.
They used microwaves to generate oscillations
in the spins of nitrogen impurities inside a diamond.
A time crystal within a space crystal.
Both spin systems developed periods
that were integer multiples of the drivers.
The ytterbium ions' oscillations were twice the laser period.
The diamond flaws, three times the microwave period.
Both resisted changes in the driving period,
keeping up their own rhythms.
Finally, both fit the predicted phase diagram,
their time asymmetry melting when
subjected to too much perturbation or too little
interaction strength.
So two teams verified this result
in completely different ways.
That means time crystals, at least by Yao et al.'s
definition of them, can exist.
By the way, these two lab results
have been submitted to journals, but as
of the filming of this "Journal Club,"
the peer review isn't complete.
I should also add that while their systems do break
continuous time translational symmetry,
they have a different type of symmetry--
discrete time symmetry.
That means if you shift forwards or backwards
in time in steps of exactly their period,
they will return to the same state.
Scientists are using the term discrete time crystals
to describe such systems.
Time crystals could have their first application
in quantum computing.
Perhaps the most popular approach
to building a quantum computing memory element
is to use electron spins, which can represent the ones
and zeros of a classical computer
in the up-down direction of the spin.
One of the most serious challenges
is that these quantum states are really hard to maintain.
It doesn't take much random motion from heat
to scramble a carefully prepared array of entangled spin
alignments, completely messing up your calculation.
Time crystals with their resilient spin-flip cycle
could be the next step in building stable quantum memory.
Time crystals could also help bridge the gap
between quantum mechanics and general relativity.
Before this year, time stood out as a major symmetry
that hadn't been broken.
And unlike in relativity, quantum mechanics
treats space and time very differently to each other.
Now that we've seen matter settle
into discrete lattices in time just like in regular crystals,
perhaps it's a first step in a quantum union of space time.
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A couple of weeks ago, we talked about
the recent spectacular discovery of seven terrestrial
and potentially habitable worlds around a nearby red dwarf
star, the TRAPPIST-1 system.
Solano Felicio asks about the possibility of stable orbits
when planets are so close together.
Well, that's where orbital resonance comes in.
The planet's orbital periods have
evolved so that each pair of planets lines
up at regular intervals.
That actually stabilizes the orbits
rather than makes them more chaotic.
Some period of instability would have preceded the system,
finding this nice resonant configuration,
and that probably drove the planets
from much larger orbits to their current locations very close
to the star.
Matthew Pick asks about the transit method,
and wonders how rare it is for a planetary system
to be lined up so we actually see the transit.
The answer is that, yeah, it's rare,
but there are a lot of stars in the galaxy.
The Kepler Space Telescope has so far
discovered 2,330 confirmed exoplanets
in 578 planetary systems.
That's 578 stars out of the 100,000 stars
that Kepler monitors.
That gives you an idea of the average probability
of alignment, but not all of those will have planets.
Yet it's estimated that a system with an Earth-like planet
orbiting a Sun-like star has around a 1% chance
of transiting from our perspective.
I think my favorite comment was from WarriorofCathar,
who pointed out that we basically just learned
all about a solar system light years away by studying
how faint shadows make other faint shadows wobble.
I agree, Warrior.
Damn science, you scary.
Scary awesome.
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