Just now, research on holographic wormholes appeared on the cover of Nature, and was also called “the first wormhole ever created” by Quanta Magazine.
Previously, in 2019, Google researchers were working on wormhole-related research in the laboratory.
Unexpectedly, scientists not only created wormholes, but also observed the phenomenon of information passing between wormholes——
They constructed a sparse Sachdev-Ye-Kitaev (SYK) model on a 9-qubit circuit and observed the characteristics of wormholes.
However, don’t rush to fantasize about “space jumping”.
Unlike the gravitational wormhole we imagined, this wormhole is a quantum wormhole and cannot travel through time and space.
The progress of the holographic wormhole this time lies in the successful transfer of the quantum state from one quantum system to another through the wormhole.
So, what exactly is this quantum wormhole, and how is it simulated?
2D space-time “simplified version” wormhole
A wormhole is a theory proposed by Einstein and Nathan Rosen, and is postulated to be the connection of a black hole and a white hole.
It is like a tunnel, and its characteristic is that a so-called “mirror universe” can be obtained on the other side.
But with the deepening of research, wormholes have also been divided into many types.
The “gravitational wormhole” that people imagine can travel in time and space is more intuitively called “space-time hole”; as for the quantum wormhole in the quantum state, it is called “miniature wormhole”. There is a big difference between the two .
So why are scientists so obsessed with studying quantum wormholes?
This is because, although general relativity and quantum mechanics have been developed for a long time, there is still a fundamental “conflict” between them——
quantum gravity.
These two theories have not reached a consensus on the theory of quantum gravity. One of the solutions is to prove the holographic principle, that is, use a low-dimensional quantum system to describe a system involving gravity.
A very popular implementation of the holographic principle is the AdS/CFT dual (anti-de Sitter/conformal field theory dual), which links the two theories of quantum field theory and quantum gravity.
If we can find a way to prove the theoretical conjecture of AdS/CFT, it is equivalent to proving the principle of holography, which will further promote the research of quantum gravity a big step forward.
The “wormhole” on the cover of Nature this time is also a quantum wormhole simulated by Google’s quantum computer, and it is still two-dimensional space-time.
Based on the AdS/CFT theory, physicists at Google proposed an experimental hypothesis in 2019 that a quantum state that can be reproduced in a physics laboratory can be interpreted as a wormhole between two black holes. crossing information.
Now, scientists from Google, MIT, Fermilab and Caltech have simulated the corresponding quantum dynamics with 9 qubits and 1 quantum computer.
In the same quantum chip, they created two entangled quantum systems and put a qubit into one of them. As a result, they observed the information from this qubit “traveling through the wormhole” in another quantum system, and the result was consistent with the expected gravitational properties.
However, the wormhole simulated by Google’s quantum computer has caused considerable controversy in the academic community.
One side argues that it doesn’t help much with the theory being studied:
Renate Loll, a quantum gravity theorist at Radboud University in the Netherlands, believes that this wormhole experiment only explores the situation in two-dimensional space-time, that is, the research is carried out in the case of one-dimensional space + one-dimensional time.
△Two-dimensional space-time simulation wormhole
But in the four-dimensional space-time (three-dimensional space + one-dimensional time) we actually live in, quantum gravity is more complicated:
Doing this kind of experiment tends to lead people into the research of 2D toy model (a deliberately simplified model), but ignores the difference of quantum gravity between four-dimensional space-time and two-dimensional space-time.
I don’t see how quantum computers can help with the theory (of what we’re working on)…but I’m happy to be corrected if I’m wrong.
The other party believes that although there are differences between two-dimensional space-time and four-dimensional space-time, this experiment can still gain a lot of “general” experience.
And with the emergence of this holographic wormhole, more wormholes will be simulated and further studied carefully.
So, how is this wormhole simulated?
How is this wormhole simulated?
To understand the creation process of this wormhole, time has to go forward along the research.
The story has been told since at least 2013.
After a meeting that year, Daniel Jafferis from Harvard University—the lead developer of the wormhole transmission protocol and the co-author of this Nature cover—had an idea:
Through speculative duality, specific wormholes can be engineered by tuning the entanglement patterns.
△Daniel Jafferis
Specifically, it is conceivable to put a wire or any other physical connection between two groups of entangled particles, so that the particles encode two mouths of the wormhole.
Under this coupling effect, manipulating the particles on one side will cause changes in the particles on the other side.
In this way, it is possible to open a wormhole between the particles on both sides.
Just do it. Jafferis teamed up with then-Harvard graduate student Ping Gao and visiting scholar Aron Wall to start the research.
Until 2016, the trio finally calculated:
By coupling two groups of entangled particles, when an operation is performed on the left group of particles, in the dual high-latitude space-time image, the wormhole opening to the right side is opened, and a qubit can be pushed through it.
The wormhole they discovered is holographic and traversable.
A few months later, the researchers further demonstrated that traversable wormholes can be achieved in a simple environment.
A quantum system is a “simple environment” that is simple enough to try to manufacture.
Having said that, a new concept needs to be introduced: the SYK (Sachdev-Ye-Kitaev) model.
To understand simply, the SYK model is a system of matter particles that interact in groups, and this model was found to be holographic in 2015.
Quantum gravity theorist Juan Maldacena and collaborators proposed that two SYK models linked together could encode the two mouths of Jafferis’ traversable wormhole.
By 2019, Maldacena and his partners had found a concrete way to transfer a qubit of information from one four-way interacting particle system to another.
In the dual space-time diagram, the rotation of the spin directions of all the particles will be transformed into a kind of negative energy shock wave sweeping across the wormhole.
The shock wave can push the qubit forward and also kick the qubit out of the wormhole at a predictable point in time.
Well, back to Jafferis and his research.
In 2018, Jafferis himself, along with many Google Quantum AI researchers, joined a research team of experimental particle physicists.
The core leader of the team participated in the discovery of the Higgs boson (2012).
The main work of the experimental team is “how to use quantum computers to conduct holographic quantum gravity experiments”.
Be aware that although quantum computers are advanced, they are still prone to errors.
To run Jafferis’ wormhole teleportation protocol on it, the experimental team had to come up with a super simplified version of the protocol.
why?
Because a complete SYK model is composed of almost infinite particles.
When the four-way interaction is maintained throughout the model, the particles couple to each other with random strength.
Therefore, it is almost impossible to calculate the complete process.
To greatly simplify the protocol, the experimental team sparsified the SYK model, encoding only its strongest four-way interactions (ignoring the rest), while preserving the holographic nature of the model.
The idea of sparsification comes from ML, which tries to limit the details of the information in the neural network by setting as many weights as possible to zero.
By analogy, the team regards a large number of subsystems as a neural network, and updates the parameters of the system through backpropagation, one is to maintain the gravity characteristics, and the other is to reduce the size of the system.
△Learn the process of making sparse quantum systems to capture gravitational dynamics
After several years, the team finally used the “clever approach” mentioned above to create this holographic wormhole that only requires 7 qubits and hundreds of operations.
Team members mapped the particle interactions of the SYK model to the connections between neurons in the neural network, and trained the system to delete network connections as much as possible while retaining the characteristics of the wormhole.
In this way, the number of four-way interactions is reduced from several hundred to five.
Things suddenly became (relatively) easy, and the experimental team started programming Sycamore’s qubit.
7 qubits encode 14 matter particles (7 each for the left and right SYK models), and each particle on the left is entangled with a particle on the right.
The 8th qubit is in a probabilistic combination of states 0 and 1, then slows down with a particle in the SYK model on the left.
The possible states of this qubit will soon be entangled with the states of the other particles on the left, and its information will be spread evenly among them, like a drop of ink dropped in water and spread evenly.
And then, the spin directions of all the qubits are rotated, and it is opposite to the negative energy shock wave sweeping across the wormhole, and this will cause the qubits entering from the left SYK model to be transferred to the right SYK model.
They refocus on where the one particle on the right (the entangled object after the left particle was swapped) was.
Then all you have to do is measure the state of those qubits and compare the statistics to the readiness of the qubits coming in from the left to prove that the qubits are being sent from left to right.
If it can be summed up in one word, it is:
Translate from the language of quantum information to space-time physics through the holographic principle, let a particle fall into one side of the wormhole, and observe whether it appears on the other side.
The method is clear, how to observe it?
In the above data, the experimental team looked for peaks representing two situations.
If you can see the peak, it means that the qubits of the dual negative energy shockwaves rotate, allowing the qubits to teleport, while the opposite directions of the dual positive energy shockwaves rotate, not allowing the qubits to teleport (and also causing the wormhole to close).
For two years, the experimental team has been gradually improving to reduce the experimental noise.
This is critical for measuring signals because even 1.5 times the noise can completely mask the signal.
In the middle of the night in January this year, on the computer screens of the team members, the peak appeared!
Next to the peak screenshot, the experimenter wrote:
I think we’re seeing a wormhole now.
The peak is “the first sign of quantum gravity that can be seen on a quantum computer.”
The core members of the team were very surprised. The clear and obvious peak made her as excited as when she saw the Higgs boson data.
More importantly, despite the simple structure of this wormhole, the team detected the second feature of wormhole dynamics, namely “size-winding”.
It’s a subtle pattern of how information travels and doesn’t travel between qubits.
At present, the experimental team has not trained the neural network to save this signal, because this signal makes the SYK model sparse.
Of course, this experiment also discovered another fact: no matter what the SYK model is, the feature of dimensional winding will appear.
Like this, like this, it took several years, and this wormhole was finally simulated by Google’s quantum computer~
It has to be said that a quantum computer is a tool for exploring the theory of quantum gravity.
This work represents just one step in using quantum computers to explore physics.
Although controversial, this unprecedented experiment explored the possibility that space-time could somehow emerge from quantum information.
With the continuous improvement of quantum devices, the error rate will be lower, the chip will be stronger, and the research on gravitational phenomena will be more in-depth.
While gravity is just one example of the unique ability of quantum computers to explore complex physical theories, quantum computers can also provide insights and observations about time crystals, quantum chaos and chemistry.
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