Dark matter is everywhere. It’s not just
out in space. We’re flowing through an entire
wind of dark matter.
As you sleep, they’re under your bed, they’re
in your closet. They’re passing through
you right now. The decades long quest to understand
what dark matter is, a mysterious substance
that makes up most of the mass in the universe,
and force it to reveal itself is taking a
new experimental turn. Scientists have built
this advanced instrument with parts from a
quantum computer that’s sensitive enough
to listen for the signal of a dark matter
particle. It’s a scanning experiment. Like
an AM radio we have a knob that we’re very,
very slowly tuning. And if they hit just the
right frequency where dark matter might be
hiding. It’s going to be a fairly narrow
tone, so just a hmmmmm. When you get to particle
physics, it turns out everything is waves.
So even our particles are waves.  Sound
is a wave. You can imagine each particle as
a particular note. They have very specific
energies whereas they sit around there. And energies in physics correspond
to frequency. This is like setting a musical
scale. You’re listening for what would sound
like a tone, amidst a sea of white noise.
There's numerous astrophysical measurements
that look at things in the universe. There's
things out there that are interacting gravitationally
that aren’t stars, they don't seem to be
dust, they're not planets as far as we
can tell. You find that this extra stuff out
there isn't even made of atoms. This is very
peculiar because you're made of atoms, I'm
made of atoms, almost everything we can study
is made out of atoms. And this means there's
something new and different out there, some
new different particle, and we call it, dark
matter. The theorist sees the astrophysical
observations and says a-ha, there's something
new out there. And they've got a set of things
that could exist, but don't necessarily do exist.
The experimentalists job is to basically go
through these...one at a time. On the list
are some dark matter particles with names
as weird and curious as the physics behind
them. People have heard of WIMPS. There’s
also MACHOS, which is kind of the exact opposite
of a WIMP. Those are massive astronomical
compact halo objects. Think black holes. There’s
WIMPzillas. There’s WISPS. Hidden sector
photons. There’s stealth dark matter. And
the star of this episode that’s getting
this big experimental push is called - the
axion. This theoretical particle was named
after laundry detergent in the 1970s because
it could clean up two big problems in physics:
dark matter and the strong CP problem. This
is another perplexing mystery that involves
a surprising balance between two of the fundamental
forces of nature: the strong force and the
weak force. One way to think about it simply
is. If you see a pencil that's kind of just
sitting there on his head and not falling
over. That's strange. It should fall over
unless something else is holding there. The
best idea for that is something called Pecci
and Quinn Symmetry. Which basically cleans
this problem up and explains oh yes there's
this natural cancellation, and the only side
effect is this extra particle called the axion.
It's produced in large amounts in the early
universe and doesn't interact very much so
it's still there, and so just as a consequence
of fixing this nuclear physics problem, you
have stuff out there, gravitating, not interacting,
and it fits the bill for dark matter just
perfectly. It's too good of a coincidence
to not pursue, to go out and try and find the axion.
Okay, so how do physicists set out to find
this hypothetical particle that may or may
not exist? First, follow the theory. It's
almost certainly very light and when I say
very light I mean much lighter than an electron.
Being light actually makes it much more wavelike
than particle like. It would act a lot like
a radio-wave that carries a little bit of
mass. With the right conditions, you can convert
energy between axions and real radio waves.
Basically you just need a strong magnetic
field that can do this conversion process.
Then, build an instrument that’s specially
designed to do this called a haloscope. It's
basically a telescope, but looking for the
dark matter halo. The whole experiment sits
in a large magnet, around 8 Tesla, and that
promotes the conversion of axion dark matter
into detectable radio waves. And we do this
inside a microwave cavity, which is like a
big soda can made out of copper. The cavity
itself is actually tuned by two tuning rods,
those are positioned here and here. They’re
connected all the way to the top by a couple
of gear boxes. And the idea is that within
this cavity, you move the tuning rods slowly,
like you kind of tune an AM radio, and you
tune the resonant frequency of the cavity.
This little doo-hickey right here is the actual
antenna. So, that’s what we put into the
cavity to pull all the power out. From there,
all the power gets sucked into something that’s
stored in here, which is called our quantum
amplifier package. The whole thing is kept
cool by this right here which is our dilution
refrigerator.
Because axion interactions are so weak. You
need almost no background, and there's plenty
of background just from things just having
a temperature, they just radiate. So you need
technology to make yourself very very cold.
And that’s where we've tied in a bit with
quantum computing, because quantum computing
involves making measurements at the bounds
of quantum mechanics. There’s been a lot
of development of radio scale amplifiers and
ultra sensitive electronics that work at these
ultracold temperatures. So while they’re
trying to read out their qubits, the same
sort of devices can be used to detect extremely
small sources of power that might be coming
from dark matter. The difficulty is you want
the cavity to be at a particular frequency
that corresponds to where you want to look
for the axion, that frequency has a lot of
wiggle room according to theorists. We start
around 500 megahertz and working our way up
to 10 gigahertz. We'll look in one region,
one frequency range. We'll not see any power
out. And then we move to the next range and
we have to be able to scan that very quickly.
Most of the experiment is in keeping the experiment
running. It has many moving parts, many complicated
systems, they all have to be maintained, when
they break you have to fix them.
Which is exactly what happened when we came
to visit. So this is a persnickety issue with
doing stuff at cryogenics. We were just putting
signals through the system. As we cool down,
those power levels dropped, which doesn't
make any sense. We’re trying to diagnose
what we think is a fault in the line. It’s
a very critical cable that’s coming out
of our experiment which would measure power
from the axion if the axion were to interact
in our magnetic field. There are experiments
that have many thousands of cables, and you
don’t want to go through and examine them
all by eye, right. We would do a measurement.
We would take the next cable off, do a measurement.
And at that point, we actually got to where
the error was. “I think he just disconnected
it at the top.” “Oh at the top.” “He
just did.” “That’s interesting, because
I think that’s where the break is, just
looking at this.” “Oh hold it, woo!”
Due to strain on the cable, part of the pin
actually just pulled back. Things strain and
contract when things get cold. And so that
caused this little gap to appear. We would
not have been able to take good data with
that. We've recently crossed the threshold
where we are now sensitive enough to the types
of interactions that theorists predict for
axions. Kind of the exciting thing is any
day, you could just hit it and it’s there
and it’ll be obvious and clear. My dream
is that I get a call or I'm looking at the
data. Like that little peek there. Let's zoom
in on that, let’s take a little bit more
data on that. That peak is staying there.
Let's move that magnet down. But so far...
By and large, it has been white noise. The
axion parameter space um, is quite wide and
unexplored. So with this experiment, we’re
going to move eventually into a multi-cavity
system, and that’s to get to higher frequencies.
If we were to explore the entire possible
range, if there is no axion out there, then
we need some new ideas. A no result actually
goes and pulls the floor out of other areas
of physics that are very interesting. Dark
matter is a difficult problem. You have to
be motivated by this mystery. To push the
envelope, to actually discover things, you're
going to have to do that cutting edge work.
You have to be able to fail. Even if it's
a complete failure of the experiment, you
don't find anything, that's not actually a
failure. You’re exploring the boundaries.
