A humorous thinking experiment might be the best way to understand a new proof in quantum computational complexity. Add some floating bar magnets to the bath water and see what happens. Each magnet will attempt to align itself with its neighbors by reversing its orientation. It will interact with other magnets by pushing and pulling on them, and the other magnets will do the same. Now, attempt to answer the following question: What will the final configuration of the system be?
These issues and others of a similar nature, it appears, are insurmountably difficult to resolve. It would take an absurd amount of time for computer simulations to produce an answer with more than a few hundred magnets.
You can now subject the individual atoms in those magnets to the complex laws of quantum mechanics. As you can expect, things get a whole lot more complicated. Henry Yuen, a Columbia University professor, remarked, “The interconnections become more intricate.” ‘Quantum magnets’ have a more intricate constraint on when they’re happy.
It has been revealed that the limits of computation, in both classical and quantum forms, can be discovered through these simple-looking systems. For classical or non-quantum systems, a seminal theorem from computer science opens the door to new possibilities in quantum theory. According to what’s known as the PCP theorem (short for “probabilistically checkable proof”), not only is the end state of magnets (or aspects associated with it) extremely difficult to compute, but many of the phases leading up to it are as well. As a result, the issue becomes even more complicated, with the end state surrounded by a mystery.
Although it hasn’t been proven yet, a different form of PCP expressly addresses quantum mechanics. Quantum PCP conjecture is believed to be true by computer scientists, and if confirmed, it would fundamentally alter our perception of the complexity of quantum problems. Quantum computational complexity theory considers it to be the most important unsolved problem. However, it has stayed out of grasp to this point.
Two researchers came up with an intermediate aim to assist us get there nine years ago. In order for the quantum PCP conjecture to be true, they came up with a simpler hypothesis known as the “no low-energy trivial state” (NLTS) conjecture. The quantum PCP conjecture would not be any easier to prove if this was proven, but it would answer some of the most intriguing problems.
The NLTS conjecture was proved last month by three computer scientists in a report posted to the scientific preprint site arxiv.org. In computer science and quantum physics, the findings have profound ramifications. Dorit Aharonov of the Hebrew University of Jerusalem stated, “It’s extremely exciting.” “The quantum PCP conjecture is a more difficult problem now that this result has been published.”
Imagine a quantum system, such as a collection of atoms, to get a sense of the new finding. If you think about it in terms of the alignment of a magnet, then the spin of an atom is similar. An atom’s spin, unlike a magnet’s, can be in a state of superposition, which is a mixture of multiple directions. There is also the possibility that the spin of one atom may be impossible to define in isolation from the spins of others from far away. Quantum entanglement occurs when two or more atoms are linked in this way. Astonishing, but also fragile and vulnerable to heat interactions, entanglement is a truly remarkable phenomenon. More heat makes it more difficult to entangle a system.
Now, visualize a swarm of atoms being cooled to near-absolute zero. The system’s energy drops as the entanglement patterns become more stable and the system cools down. Using the lowest possible energy, or “ground energy,” is a simple way to describe how the system will end up. At the very least, if it were possible to do so, it would.
For some systems, researchers learned in the late 1990s that this ground energy could never be calculated in a reasonable amount of time. It was anticipated that a level of energy close to the ground energy (but not quite there) would be easier to calculate because the system was warmer and less entangled, making it simpler.
Computer scientists, on the other hand, were split down the middle. Using the classical PCP theorem, it is just as difficult to calculate energies near the end state as it is to calculate the final energy itself. In other words, if the quantum version of the PCP theorem is correct, the precursor energies of the ground energy are equally as difficult to compute. Many scholars believe that the quantum PCP theorem should be valid as well, since the classical PCP theorem is true. Yuen proclaimed, “Surely, a quantum version must be correct.”
A theorem of this magnitude would have far-reaching effects on the physical world. According to scientists, entanglement breaks down at greater temperatures. However, it has been shown that quantum systems can maintain their entanglement at higher temperatures. The problem is that no one can verify that these systems exist.
The difficulty was narrowed down in 2013 by Microsoft Research Station Q Santa Barbara’s Michael Freedman and Matthew Hastings. Instead of looking for systems with the lowest or nearly lowest energy, they focused on how much circuitry it would require to emulate those systems. Even at their lowest energies, these quantum systems, if found, would have to retain extensive patterns of entanglement. The presence of such systems would not necessarily establish the quantum PCP conjecture, but it would be a step forward.
However, computer scientists were familiar with the area of study known as quantum error correction, where researchers build recipes for entanglement that safeguard atoms from disturbance. Each recipe has its own code, and there are numerous codes of varying importance.
A quantum error-correcting code of an almost perfect nature was created by computer experts at the end of the year 2021. As time passed, several additional research teams built on these findings and produced their own variations.
For the past two years, the three authors of the new study have been working together on similar projects to show that one of the new codes has all the qualities needed to build a quantum system of the type that Freedman and Hastings had hypothesized. They were able to disprove the NLTS hypothesis in this way.
They found that entanglement is not as brittle and temperature sensitive as previously thought. In addition, it supports the quantum PCP conjecture, which claims that even when the energy of a quantum system is removed from the ground, it remains nearly impossible to compute.
That which seemed unlikely to be true has been proven to be so, says University of California, Davis researcher Isaac Kim. In spite of the fact that it’s a strange system. The full quantum PCP conjecture will necessitate the use of a variety of new scientific methods. The current result, however, has given them hope that things will improve in the future.
Perhaps the biggest question on their minds is if and how they can be formed in nature, even if the NLTS quantum systems were discovered to be theoretically viable. According to the existing results, this would necessitate long-range entanglement patterns that have never been created in the lab and could only be constructed with enormous quantities of atoms.
Anurag Anshu from Harvard University and Nikolas Breuckmann from University College London co-wrote a new work alongside U.C. Berkeley computer scientist Chinmay Nirkhe, who described the things as “extremely designed”.
‘I believe you may materialize the system if you have the capability to link incredibly faraway qubits,’ said Anshu. In order to get to the low-energy spectrum, though, we must go on yet another voyage. It has been speculated by Breuckmann that the cosmos might contain an area that is NLTS. “I don’t know.”