Nobody has seen a wormhole.
Nobody has produced one.
Nobody knows if it is just beautiful math, but without a physical reality.
Well, we'll take the world's best quantum computer
and see if we can map that into building a wormhole.
And then it became just at the edge of what's possible.
I think the discovery of the Higgs, which came about 10 years ago, was up to that
point the most exciting point of my entire career. This, to me, is equally exciting.
It can begin to explore quantum states that are very hard to realize in nature directly.
Now, the wormhole becomes traversable. It opens. You really can go through.
So, the story begins in 1935 when Einstein wrote
a paper with Rosen, where the first time Einstein-Rosen bridges appear.
And these are bridges that connect two seemingly unconnectable pieces of space-time.
ER–Einstein and Rosen–they pointed out that solutions to general relativity allowed for two
black holes, which are connected, but with a kind of bridge–what we call a wormhole.
But that wormhole has the property of being non-traversable.
Wormhole is a very unstable passage.
It opens and it closes and it gets chaotic and it gets destroyed very fast.
So Einstein, when he proposed the wormhole, he was very frustrated,
because you couldn't actually go through a wormhole. And he was able to realize that
himself. And so, what good is a wormhole if you can't put anything through it?
And it turns out that that's because he wasn't putting in the quantum physics.
1935: same year, another paper.
The EPR paper, which is Einstein, Rosen, Podolsky.
EPR is about quantum entanglement and what Einstein called spooky action at a distance,
something he was not happy with.
Entanglement is this property when subatomic particles–they are partners. Even if you take
them apart, and take them at the other edge of the universe, they hold information that if you
measure the one, you know what the other will be. So entanglement is a strong correlation.
Einstein didn't like quantum entanglement, and this made it very challenging for him
to understand how to reconcile quantum physics and gravitational physics. So it
has been a challenge over decades to understand better how quantum
physics and general relativity fit together into a coherent picture.
Einstein's dream was to figure out how to connect them.
There was no reason to think at the time or for decades afterward that those two papers had
anything to do with one another, apart from there being some authors in common.
And then comes 2013: Maldecena and Lenny Susskind. They say ER equals EPR.
They are the same. This is very daring.
Wormholes and entanglement are the same thing.
That was totally unexpected. One of them has to do with black holes. One of them
has to do with quantum mechanics. Didn't seem like they had anything to do with each other.
A lot of times in physics, you can have two different descriptions of the same
thing. And we call that a duality. It's just sort of a remapping of the same physics.
The basic idea was that when you have quantum entanglement between
two black holes, necessarily there will be a wormhole connecting them.
So there we have a picture of quantum gravity.
We have this dictionary that relates gravitational phenomena to ordinary quantum physics.
You can tackle the same physics from these two different angles, and that's one of the
most concrete paths forward we have in building a full theory of quantum gravity.
The physics of the black holes, we think, is physics of quantum gravity. But we cannot
send an experiment in a black hole and validate that.
So instead we can ask, can there be quantum systems
whose dual description looks like space-time?
I think it might have actually been during a talk
by Juan Maldacena about ER = EPR that I started to wonder to myself:
Could traversable wormholes exist? And people thought it was fascinating as an exotic
configuration of space-time. But I think many people probably suspected that it couldn't work.
So we saw that it could.
One of the magical things about quantum mechanics is you can actually have negative
energy. And this allows you to do things that you would've thought otherwise were impossible.
Daniel Jafferis showed was that if you introduce what’s known as a negative energy shockwave,
you can support your wormhole and get something from one end and have it come out the other.
Now, I'm an experimentalist, so I look at that and I ... All right, what can I do about that?
And so Maria and I were thinking, we have the ability to do things in quantum in
the laboratory. We're physicists. So could we imagine actually doing something with wormholes?
It almost occurred to us immediately that we should be using the best quantum system
that we know how to manipulate. And what is that? It's a quantum computer.
Quantum computers speak a different language than the ordinary language of
bits. A bit–you look at it, okay, it's a zero or one, that's it.
The correlations among qubits are richer and a lot more interesting than correlations
among bits. It’s this language of quantum entanglement. And so there's a world out there,
this extravagant quantum world, which we just haven't been able to explore.
I've worked on a few projects at Google Quantum, and got to get to know the device a bit.
The Google device is the best quantum computer that is around.
All the magic happens in this little chip. You have the actual qubits as these superconducting
pieces of aluminum on a silicon chip. And that's where the entanglement happens.
We had access to the Google quantum computer. And so Maria and Daniel and
Joe and I said, “We have a proposal to try to observe a wormhole on the quantum computer."
And to put the proposal together, it was obvious that these are the people.
In physics, you have to have the courage of your own convictions. And you say, "Well,
maybe it won't work the way that I thought, but we'll learn something by trying." And
that's really the attitude we had going into this. I don't think we really thought
it was going to work, but we thought we would learn a lot by seeing how far you could get.
They didn't say, "You're crazy. Let's do something else."
They said, "Okay, you are crazy. Let's do it."
You can use the holographic duality to create the quantum version of the system.
We look instead at this set of entangled qubits and we evolve
it in a way that has exactly the same physics as the gravitational wormhole.
And so, on the quantum computer, we can directly send a qubit from
one side of the wormhole and watch it come out the other.
If you take the physical system that theoretical physicists say, "this is a wormhole," and you
work out pencil-and-paper, "how good of a quantum computer do I need?" You get an
answer that's like, "a quantum computer 10 years from now." It looks hopeless.
But we thought, “Let's try.”
We are here in our GQ2 lab… the controlled signals
actually are produced in these racks sitting next to the cryostat.
When you try to just run a wormhole as the theorists have prepared it
on a quantum computer, you find that you need to implement this very large system.
So that's big. That's a big fat problem.
We have to do what physicists call coarse-graining. You want
to preserve certain features–in this case,
gravitational features, while throwing away all the unnecessary components.
We knew we needed to get to a smaller system size
and so we were banging our heads against it for a year or something.
And then finally, you come up with a weird idea that might work.
The idea of rewriting it as a neural network is something that's out there. It's a weird
idea because you usually don't think of physical wormholes as neural networks.
The wormhole itself has these many parameters that we treat as neural
network weights. We then have a data set that we're going to train on,
which is the dynamics that we want to observe that correspond to wormhole.
It sounds like a generic machine-learning problem now,
and we can just plug it straight into the machine-learning toolbox.
I proposed the idea and tried it out.
The original system has 210 terms.
And we found out that we can do this with seven.
The machine learning managed to keep all of this intact while making a much smaller system.
It’s still baby enough that it fits in the crib.
That was the first moment where, like, "It might be possible. This
whole wormhole thing we've been talking about for two years, maybe we can actually do it."
At probably like two in the morning, I was running these quantum circuits.
We're looking for a peak. That's the qubit making it through the wormhole.
I'm watching the data as it comes out and there's a little bit of a noisy peak. And at
first I was like, this is probably just a noise artifact. But as the data kept
coming in and the peak kept getting clearer as it was like, “This is is a wormhole.”
“I think I just saw one.” And so I was like sending it off to Maria and the others,
like, “I think we have a wormhole, guys.”
He said, "It's there." I said, "No kidding it's there."
It was nuts. It was nuts.
The qubits of the Google quantum computer are
making a little bit of extra space and that's our wormhole.
What happens is, a pulse of negative energy falls into the wormhole.
Now, the wormhole becomes traversable. It opens.
You put a qubit through one side of the wormhole.
The qubit is now in the interior of this wormhole.
That information will actually spread over the entire quantum system over time. And
this spread of quantum information is chaotic.
It becomes shared by many particles in the form of very complicated quantum entanglement.
After spreading, it has to refocus onto a single qubit.
As the wormhole is closing, the qubit is exiting.
I was absolutely astounded. I thought we would get maybe a few steps toward a real wormhole on a real
quantum computer. And I never really believed that we'd get all the way to something that was real.
The experimentalists have a reaction like they want to hug the machine.
I told my brother and he squealed and he said, "Are you destroying the world? What
are you doing?" It was difficult to explain to him that nobody is in danger from this wormhole.
On the processor, if we had a quantum eye and looked in there,
we wouldn't see a ripple of space-time.
We would see the physics that we expect from
the gravity point of view and from the quantum point of view.
So if that is the case, then we are making contact between quantum and gravity.
That's been the holy grail of my whole career–is to understand quantum gravity
and the things that go with it, like wormholes.
We are studying quantum gravity literally from the ground up.
Einstein committed his life to arrive to some understanding of quantum gravity, and
here we are almost a century later.
I wish he was alive to see this because I would like to know what he would say.
What would have Einstein said? He would've said, “ER = EPR. I told you. I knew that in 1935.”
But he didn't.
I think this is concrete experimental evidence for ER = EPR.
The vector is pointing right from the experimental point of view.
This is a confirmation–a direct exploration–of this connection between quantum entanglement
on the one hand and the way space is knit together and holds itself together.
As Google builds more and more capable quantum computers, we will eventually
be able to get this into a regime where we don't even know how to predict what's going to happen.
And I think we're going to have hundreds of my fellow physicists
now writing papers about all the other things you can think about
doing now–now that we've sort of got this door a little bit open.
And as always in physics, when you are able to explore a new frontier,
there will be discoveries that weren't anticipated. Just like when you build a
more powerful accelerator to study particle collisions or a more powerful telescope to
look at the more distant universe, a more powerful quantum computer will be
able to study highly entangled matter that we've never been able to look at before.
Being very close with quantum, we're going to learn a lot.
There’s so many questions that one could explore using these ideas.
And I think most exciting are the questions that we can’t yet pose.