On November 30, 2022, it was reported in various media that a research group in the USA had created a wormhole with a quantum computer. But that was too simplistic even for headlines. In reality, what happened in a Californian laboratory and was published that November day as the cover story of the scientific journal "Nature" is a lot more complicated.
Worm holes are a purely theoretical construct. According to some calculations, they arise from black holes and link far -distant areas of space -time. That is why they play a role in films and literature where they otherwise bridge unreachable distances in space. However, worm holes are hypothetical objects that - in contrast to black holes - have so far not been discovered in the universe. In particular, they have not yet appeared in any laboratory.
Rather, what was successfully implemented in the experiment is the distillate of a series of abstractions: an experimental protocol evaporated to a few computational units of a quantum computer from an already simplified variant of a quantum mechanical model facilitated by gravity, which in turn corresponds to a space-time enriched by gravity, which, however, takes place in a differently dimensioned and curved type of universe than the one we are familiar with. A confusing cascade that sounds much less like a headline, but remains scientifically exciting. Because this is about concepts that are as influential as they are hotly disputed in the search for quantum gravity.
Is the universe a hologram?
The focus is on the so -called holographic principle. It solves a dilemma for the long -term and previously unsuccessful association of two approaches: on the one hand of the theory of relativity, which describes the influence of gravity on space and time and in the context of which black holes arise, and on the other hand the quantum mechanics that take place on the scale of individual particles And play no role in gravitation. The question is how to access the quantum mechanical information of a particle after it has fallen into a black hole.
The holographic principle provides the answer. According to him, the actually inaccessible interior of the black hole is encoded on its edge. One can find mathematical relations, a so-called duality, between the enclosed volume and its visible surface. On top of that, one only needs a quantum theory without gravity for the description of the edge region. That's why you can now calculate back and forth between a world with and one without gravity as you like. Some speculate, perhaps even the content of our entire universe could be described by a purely quantum mechanical theory. The catch: the holographic principle only applies to a space that functions according to different rules than our familiar universe. Nevertheless, the idea has been intensively discussed and continued in theoretical physics since its development in the 1990s.
A crucial advance came through two pioneers of the field in 2013, the theorists Juan Maldacena from the Institute for Advanced Study in Princeton and Leonard Susskind from Stanford University. They looked at two entangled particles - that is, their quantum mechanical conditions are connected - and argued that this was equivalent to a link between the two by a worm hole. Based on the holographic principle, a worm hole would be found for each quantum restriction. Since worm holes bend space and time, you could perhaps even construct the entire geometry of space -time at a microscopic level through entanglements.
Due to the worm holes that are about these radical thoughts, you could not send anything through like a cinematic adventure. Rather, it is a related black holes under the special conditions of the holographic model universe that works completely differently than ours. Nevertheless, theorist Daniel Jafferis from Harvard University in Cambridge tried to transfer the idea to worm holes that can be crossed. In fact, he succeeded with two colleagues in 2016.
From abstract concepts to practical qubits
But all of this is pure theory. Can the thoughts be applied to something that can be used to experiment, for example on a simple quantum system with particles in our real world? The answer has a description of quantum changes that had already been designed by Subir Sachdev and Jinwu Ye in the 1990s, both of which at the time at Yale University in New Haven. It makes the interactions of many coupled particles predictable. Later Alexei Kitaev from the California Institute of Technology in Pasadena combined with the holographic principle.
This SYK model named according to its three developers provided Jafferis hand-made basics for an experiment with a quantum computer whose information units, the qubits, work with entanglements. In 2019, together with his colleague Ping Gao from the Massachusetts Institute of Technology in Cambridge, he finally presented in a detailed "teleportation protocol" on how to send information in a way with quBITs that correspond to the crossing of a worm hole from a holographic point of view. One that has only one room dimension true to the SYK model, but that is only a further simplification under many assumptions that were necessary so far.
Certain quantum mechanical operations on the particles involved in the quantum computer trigger something that, from the perspective of the one-dimensional wormhole, acts as if an impulse of negative energy had been shot into it. Although there is no negative energy in classical physics, it has a fixed place in quantum mechanics, for example when particles emerge from a vacuum. It can be interpreted as a state of positive energy that moves backwards in time. It is what is needed to keep a wormhole open and to cross it. (The fact that negative energy has meaning only on such tiny scales should also explain why there are no wormholes through which a spaceship can fit.)
Together with the experimental physicist Maria Spiropulu from the California Institute of Technology, Jafferis now wanted to transfer his teleportation protocol to the quantum computer processor called Sycamore developed by Google and test whether the concept works in practice. Sycamore consists of only 53 qubits. On the other hand, the SyK model and thus also Jafferis' protocol for a solution needs the borderline case of infinite many particles. The big challenge now was to evaporate the whole thing on only a few quBITs; in this way that its holographic properties still preserve and remain measurable.
Pseudo-wormhole with nine qubits
The team around Spiropulu and Jafferis succeeded by filtering the most valuable among the qubit interactions with an artificial neural network. The algorithm left those who are at least necessary to create the key features of a successful teleportation. In fact, the neural network identified the simplest circuit required for the protocol. It consists of only nine qubits. Seven of them simulate particles that are intertwined on the two sides of the system. On the one hand you bring an additional eighth qubit. Due to interactions, information about his condition is distributed among the neighbors. This is followed by the manipulation, which corresponds to an impulse of negative energy. This defines the states and fishes out the enclosed qubit. The ninth qubit serves as a reference to check whether the original information has really been returned to the qubit. Careful measurements and evaluations of the research group showed that the experiment has been successful! From a holographic point of view, a teleportation has taken place on the other side of the worm hole.
Now conventional computers have long been able to simulate the behavior of dozens of qubits. Since only nine of these were used, a quantum computer would not necessarily be necessary for the whole thing. In any case, we are already at the end of a chain of models, each of which is only representative of something remotely similar. So another abstraction step would hardly be devastating. In any case, in the search for a reduced variant that could be implemented with real qubits, it was necessary to generate the data for it in a classical way in advance using an artificial neural network. But if you have contact with the Google laboratories anyway, you can at least save yourself this one detour and use the entanglement there directly instead of faking it. It also helps with marketing, after all, the trendy quantum computers attract more attention and research funds than conventional computers.
However, this work does not yet make any better understanding of whether and how our usual space -time restrictions can be seen from quantum restrictions, let alone statements about the existence of worm holes in the universe. Rather, the scientific value lies in a closer look at simplified models in which there are such dualities and in their implementation in a laboratory. Worm holes are still not real, but maybe a investigation based on it really brings a decisive insight in the decades of attempt to travel back and forth between different worlds with and without gravity.
