A fascinating principle underlies our understanding of how large systems behave: despite being made up of countless tiny particles, the overall properties of many systems, like gases, liquids, or solids, can often be described using just a few basic parameters, such as temperature. By adjusting these parameters, one can change how a material behaves and even switch it between different phases, like turning water into ice.
Usually, achieving such changes requires influencing the entire system. But in some special cases, a small, localized disturbance can have a big impact. A common example is supercooled water, which can stay liquid even below freezing point, until it touches a tiny piece of ice, which causes the whole thing to suddenly freeze. This shows how a small local change can trigger a global transformation.
Things get much more complex when we look at systems that are not in equilibrium. If interactions with their surroundings can be neglected, these systems evolve purely according to the laws of quantum mechanics. Even though they cannot fully settle into a traditional equilibrium state, it was found that they often behave as if they are in an exotic or "effective" equilibrium, especially when observed locally.
One well-studied case is the so-called quantum quench, where a system is suddenly changed (like flipping a switch on some global setting) and then allowed to evolve. In homogeneous one-dimensional systems, the long-term behavior after a quench is now fairly well understood.
LoCoMacro took this a step further. It asked: What happens if the disturbance is not global, but highly localized? Can small, local changes still affect the system as a whole when it is out of equilibrium?
Through this lens, LoCoMacro explored how inhomogeneities (small irregularities or local perturbations) can influence the large-scale behavior of quantum systems over time. The project’s key achievements include:
- Showing new ways to create exotic quantum states of matter.
- Designing methods to locally control large-scale behavior in out-of-equilibrium systems.
- Discovering conditions where large-scale entanglement (a core feature of quantum mechanics) can emerge, potentially useful for future technologies like quantum computing.
The project focused on answering several key questions:
(a) What does the system look like at late times after introducing inhomogeneities?
(b) Can we calculate exactly how properties like correlations and entanglement evolve over time?
(c) Which physical quantities remain conserved and shape the system's long-term behavior?
(d) How and when do quantum systems reach thermal-like states, or fail to?
(e) What role do symmetries play in how the system evolves?
(f) How do quantum measurements affect what we observe locally in such systems?
(g) Can subsystems become multipartite entangled in isolated many-body systems?
By addressing these questions, LoCoMacro has helped deepen our understanding of how local actions can influence entire quantum systems, offering not only fundamental insights into quantum physics but also opening doors to potential applications in quantum technologies.
