Probing Quantum Magnetism with Near Absolute Zero SU(N) Atoms

^{By Amal Pushp, Affiliate Physicist at the Resonance Science Foundation}

Absolute zero is the temperature at which all physical dynamics come to a halt. The laws of physics however do not allow us to attain absolute zero. This fact unfolds from a fundamental feature of quantum mechanics according to which fluctuations are always occurring at the quantum level and the quantum particles always have enough energy to continue their dynamical motion unlike in a classical system. Such a system contains quantum mechanical energy even at absolute zero and this energy is technically called zero-point energy. However, physicists can achieve temperatures close to absolute zero in an advanced laboratory. Examples where working near absolute zero is common include quantum phenomena like Bose-Einstein condensation, superconductivity, superfluidity, etc.

Now in yet another situation, physicists from Japan and the US have succeeded in cooling atoms of Ytterbium (an element also used in making atomic clocks) close to about one-billionth of a degree of absolute zero, as reported in the press release by Rice university. To get an idea of how cold this is, one might compare it with the temperature of interstellar space which is about 2.7 Kelvin. Note that this temperature of space is determined by measuring the cosmic microwave background radiation (CMB). In the new work researchers have cooled atoms to a temperature that is 3 billion times colder than outer space, let that sink in! The research team primarily used laser to cool the atoms to ultra-cold levels and subsequently built a quantum magnet based on a spin-like property.

^{A chain of single-atom magnets (red spheres with black arrows indicating north-south orientation) that repel one another. Image and Text Source: Harvard University}

In condensed matter theory, physicists generally use an ansatz or an approximation known as the Hubbard model to describe the behavior of solids, high-temperature superconductors, quantum magnets, etc.  In the new work which got published in nature physics [1], physicists used Hubbard models with a special symmetric property arising from a mathematical framework called group theory. The property is known as SU(N), where SU stands for special unitary group and N signifies the possible spin states of particles considered within the model. The complexity of magnetic behavior inside the system under study is also directly proportional to the number N.

The atoms of Ytterbium have six spin states so that the symmetry becomes SU(6). Moreover, the Hubbard model that was simulated in this work is interestingly the first simulation to reveal magnetic relationships in an SU(6) model, calculations of which are impossible to make even on a powerful modern-day supercomputer. As the corresponding author of the study, Kaden Hazzard says, “That's the real reason to do this experiment. Because we're dying to know the physics of this SU(N) Hubbard model”.

The novel work paves the way for further developments and new tools are anticipated which will be able to measure atomic correlations and discover subtler and more exotic phases of matter as well as inflate the boundaries of research within condensed matter in general and quantum magnetism specifically.

References

[1] Shintaro Taie et al., Observation of antiferromagnetic correlations in an ultracold SU(N) Hubbard model, Nature Physics (2022). DOI: 10.1038/s41567-022-01725-6