Using an ultracold atom quantum simulator, the Chinese team accurately determined the quantum critical point!
The forces of nature have symmetries that physicists embody in so-called "gauge theories".These theories are mathematically elegant, but it is challenging to apply them to systems whose components interact strongly. Performing calculations on a discrete lattice can ease the computational burden, but even this approach can be difficult to implement; another approach would be to model the system in question with another, trickier system with the same quantum-mechanical description.
Now, Han-Yi Wang of the University of Science and Technology of China and collaborators at Lanzhou University and Tsinghua University have done just that. Using an optical lattice of 19 sites, the team has simulated the behavior of massive particles in the fundamental canonical field theory. These experiments open the way for more in-depth calculations.
The results were published Aug. 1 in Physical Review Letters under the title "Interrelated Thermalization and Quantum Criticality in a Lattice Gauge Simulator.
Canonical field theory and thermalization are both topics of critical importance to modern quantum science and technology. The recently realized lattice gauge theory (LGT) atomic quantum simulator provides a unique opportunity to study thermalization in gauge theory, and theoretical studies have shown that quantum thermalization can be used as a signal for quantum phase transitions.
However, experimental studies remain a challenge in accurately determining the critical point and controllably exploring the dynamics of thermalization due to the lack of techniques to locally manipulate and probe the matter and gauge fields.
Atoms trapped in a one-dimensional optical lattice can model how massive particles reach/fail to reach thermal equilibrium in fundamental quantum field theories.
The team's optical lattice consists of alternating "wells" that can be occupied by ultracold rubidium-87 atoms. By adjusting various parameters, the team was able to suppress single-particle hopping and encourage pair hopping to produce results that match the interactions of massive particles, which are described by a one-dimensional lattice version of canonical field theory (i.e., the Schwinger equation model).
After building the system, the researchers populated the lattice with ground-state atoms with alternating top and bottom spins - a quantum critical state analogous to antiferromagnetism. They then followed the thermalization of the system.
Unlike classical quantum systems, closed quantum systems do not necessarily reach a maximum entropy state after an arbitrarily long time. As expected, stagnant thermalization occurred in the USTC experiment. However, it was associated with a quantum critical state, which was unexpected.
The experimental team believes that this finding helps to elucidate how thermalization occurs in canonical field theories.
Experimental system. (a) Schematic of the initial state of an ultracold atom microscope and preparation; (b) Physical modeling of bosons in a one-dimensional optical lattice; (c) Ising quantum phase transition.
Adiabatic warming and phase transition.
Time evolution after quenching.
This experimental research paper has experimentally investigated quantum critical points in lattice canonical field theories from both equilibrium and nonequilibrium thermalization, with the help of "single-site addressing" and atomic number resolution detection capabilities.
In the paper, the experimental team states, "We precisely determine the quantum critical point and observe that the Néel state (an antiferromagnetic ground state) thermalizes only in the critical regime. This result exemplifies the interplay between quantum many-body wounding, quantum criticality and symmetry breaking."
"These experimental results demonstrate the quantitative validity of our ultracold atom quantum simulator and prove that the simulator is a powerful platform for studying nonequilibrium dynamics in gauge theory."
"In the near future, when the size of our system is scaled up by a factor of several, it will surpass the current capabilities and accuracy. The experimental control and detection capabilities developed in this work can be used to study other interesting dynamical phenomena in the system: e.g., string breaking, dynamical transitions between quantum phases, etc."
Reference link:
[1]https://physics.aps.org/articles/v16/s115
[2] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.131.050401
