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.