Different types of matter can sometimes behave in surprisingly similar ways when they reach a critical point between two states.
Scientists call this phenomenon “universality.”
At these special points, many of the microscopic details that normally distinguish one material from another become less important, and the systems begin following the same mathematical rules.
Now, researchers have directly measured one of these universal patterns in a quantum experiment for the first time.
The international team, led by researchers at Caltech, used specially designed quantum simulators to test predictions from a mathematical framework known as conformal field theory.
Physicists have relied on these theories for decades to describe how systems behave near critical points, but some of their most important predictions had never been directly observed experimentally.
The researchers focused on two theories called the Ising and tricritical Ising conformal field theories.
Both predict what happens when a quantum system reaches a tipping point between different states.
Unlike familiar changes such as water boiling into steam, the transitions studied here are not driven by heat. Instead, they are caused entirely by quantum effects and occur at temperatures close to absolute zero.
At such a critical point, a quantum system can absorb specific amounts of energy. These allowed energies can be imagined as the rungs of a ladder. Conformal field theories predict exactly how the distances between these rungs should relate to one another.
To test those predictions, the researchers created chains of strontium atoms held in place by tightly focused laser beams called optical tweezers. Other lasers pushed the atoms into highly excited “Rydberg” states, causing neighboring atoms to interact strongly. The researchers then carefully adjusted the system until it reached the desired quantum critical point.
Next, they used a technique called many-body modulation spectroscopy to measure its energy levels. The researchers gently varied the lasers at different frequencies and watched how the atoms responded.
The idea is similar to running a wet finger around the rim of a wine glass. When the movement matches the glass’s natural frequency, it resonates. In the quantum experiment, strong responses revealed the system’s allowed energy levels.
Experiments involving chains of up to 35 atoms produced energy patterns matching predictions from the Ising conformal field theory. When the researchers adjusted the system to reach the more complex tricritical point, they again found the distinctive energy ratios predicted by theory.
They were also able to uncover additional energy levels and change the patterns by modifying atoms at the ends of the chain, providing further tests of theoretical predictions.
The findings provide rare experimental confirmation of mathematical ideas developed over decades. More importantly, the technique could now be applied to quantum systems whose behavior scientists cannot calculate in advance.
The researchers next plan to move beyond one-dimensional chains and arrange atoms in two-dimensional grids. Because conformal field theories in two dimensions are much less understood, quantum simulators could help uncover new patterns of matter that even powerful classical computers struggle to predict.
