2012 Cs Experiment

Research highlight

Observation of quantum criticality with cold atoms in optical lattices

Our work on quantum criticality is published online at Science! This achievement realizes a major goal of our research.

Quantum criticality emerges when a many-body system is in the proximity of a continuous phase transition that is driven by quantum fluctuations. In the quantum critical regime, exotic, yet universal properties are anticipated and discussed across a broad spectrum of physics disciplines. Ultracold atoms provide a clean system to test these predictions. 

In this work, we report the observation of quantum criticality with two-dimensional Bose gases in optical lattices. Based on in situ density measurements, we observe universal scaling of the equation of state at low temperatures, locate the quantum critical point, and constrain the critical exponents. Furthermore, we observe a finite critical entropy per particle which carries a weak dependence on the atomic interaction strength. Our experiment provides a prototypical method to study quantum criticality with ultracold atoms.

Below is a phase diagram that illustrates the quantum phase transition we study in this work:

The vacuum-to-superfluid quantum phase transition in 2D optical lattices.

At zero temperature, a quantum phase transition from vacuum (horizontal thick blue line) to superfluid occurs when the chemical potential µ reaches the critical value µ₀. Sufficiently close to the transition point µ₀, quantum criticality prevails (red shaded area), and the normal-to-superfluid transition temperature T_c (measurements shown as empty circles) is expected to vanish as (µ-µ₀) to the power zν; the blue line is a guide to the eye. From the prediction zν = 1, the linearly extrapolated critical chemical potential is µ₀ = -3.6(6)t, consistent with the theoretical value -4t. Here both the thermal energy scale kT and chemical potential µ are normalized by the tunneling t.

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