An international collaboration of researchers from China, Spain, Denmark, and Brazil has successfully simulated spontaneous symmetry breaking (SSB) at zero temperature using a superconducting quantum processor.
A first-time achievement, this is a major milestone for condensed physics and showcases additional quantum computing applications, which have seen rapid progress in recent years.
SSB is a concept seen across all areas of physics and is critical in the emergence of complex structures. SSB helps us explain the laws of conservation better, and much of physics is centered around breaking symmetries. Researchers are keen to observe SSB at temperatures close to absolute zero.
However, it is difficult to achieve physically, since the material system reaches a state of immobility. This is where simulations can help. Classical computers have been used to perform such simulations, but with limited success. The temperatures in these simulations are always above absolute zero and take a long time to process.
Leveraging quantum computing properties of entanglement and superposition, researchers were confident they could overcome these challenges of classical computing.
Where quantum computing excels
In theory, a classic or quantum computer is capable of carrying out simulations for SSB at zero temperature. However, the difference between their computations is in the time they will take to conclude.
While a classic computer attempts to solve this problem linearly, a quantum computer takes multiple approaches at once to arrive at the final answer, exponentially increasing the pace of computation. Researchers compare this to opening a lock with many keys in hand.
While a classic computer attempts to unlock using one key at a time, a quantum computer uses several keys simultaneously in its unlocking attempts. This is referred to as superposition.
How did the researchers do it?
The collaboration, consisting of scientists from the Institute of Fundamental Physics in Spain, Federal University of São Carlos (UFSCar) in Brazil, Aarhus University in Denmark, and the Southern University of Science and Technology in Shenzhen, China, used superconducting qubits based on aluminum and niobium alloys that operate around temperatures of one millikelvin.
The experiment consisted of a quantum circuit composed of seven qubits that allowed interaction only with immediate neighbors, and then applied an algorithm to simulate zero temperature evolution, a press release said.
The system began in a classical antiferromagnetic state where particles had spins in opposite directions. It then evolved into a ferromagnetic quantum state in which particles pointed in the same direction and established quantum relations. This phase transition can be attributed to SSB, researcher Alan Santos, who was involved with the work, said in the press release.
The entanglement was also confirmed using measurements of Rényi entropy, which can help quantify subsystem entanglement.
“The crucial point was simulating dynamics at zero temperature. There had already been previous studies on this type of transition, but always at temperatures other than zero. What we showed was that, by setting the temperature to zero, it’s possible to observe symmetry breaking even in local particle interactions, between first neighbors,” said Santos in the press release.
The research findings were published in the journal Nature Communications.