Real-time observation of collective quantum modes

December 15, 2017, ETH Zurich
In the ETH-experiment, rubidium atoms were coupled to the light waves in two resonators. In the "energy sombrero" resulting from that coupling, Goldstone and Higgs modes (red dots and arrows) were directly observed. Credit: Tilman Esslinger group / ETH Zurich

A cylindrical rod is rotationally symmetric - after any arbitrary rotation around its axis it always looks the same. If an increasingly large force is applied to it in the longitudinal direction, however, it will eventually buckle and lose its rotational symmetry. Such processes, known as "spontaneous symmetry breaking", also occur in subtle ways in the microscopic quantum world, where they are responsible for a number of fundamental phenomena such as magnetism and superconductivity. A team of researchers led by ETH professor Tilman Esslinger and Senior Scientist Tobias Donner at the Institute for Quantum Electronics has now studied the consequences of spontaneous symmetry breaking in detail using a quantum simulator. The results of their research have recently been published in the scientific journal Science.

Phase transitions caused by symmetry breaking

In their new work, Esslinger and his collaborators took a particular interest in - physical processes, that is, in which the properties of a material change drastically, such as the transition of a material from solid to liquid or the spontaneous magnetization of a solid. In a particular type of phase transition that is caused by , so-called Higgs and Goldstone modes appear. Those modes describe how the particles in a material react collectively to a perturbation from the outside. "Such collective excitations have only been detected indirectly so far," explains Julian Léonard, who obtained his doctorate in Esslinger's laboratory now works as a post-doc at Harvard University, "but now we have succeeded in directly observing the character of those modes, which is dictated by symmetry."

Sombrero in the quantum simulator

For that purpose, the physicists built a quantum simulator - a laboratory system, that is, in which quantum phenomena can be studied in their purest form and under controlled conditions. The quantum simulator used by the ETH researchers consists of extremely cold rubidium atoms that are exposed to several light waves. Using two optical resonators, a coupling between the atoms and the light waves is created that causes the shape of the potential of the rubidium atoms to look like a rotationally symmetric salad bowl. The coordinates of the energy surface correspond to the intensity of the light in the two resonators. A laser beam that creates a so-called optical lattice can then be used to change this salad bowl-like surface in such a way that, above a critical strength of the laser beam, it starts resembling a Mexican sombrero with a bulge in the centre.

Under those circumstances, much like in the case of the cylindrical rod, spontaneous breaking occurs: just as the rod suddenly buckled in a random spatial direction, the atoms in Esslinger's experiment, which started out in the middle of the salad bowl, now all together look for a new energy minimum. That minimum can lie anywhere along the groove of the sombrero, as every point along the groove has the same energy. That also means, however, that (energetically speaking) the atoms can be moved collectively along the groove without any energy input - this corresponds to the so-called Goldstone mode. By contrast, if one wants to nudge them radially, away from the middle of the sombrero or towards it, one has to provide the energy necessary for this Higgs mode. Again, this can be compared to a buckled rod, which is easy to rotate but hard to bend further.

Measuring modes in real time

"Normally, Goldstone and Higgs modes are detected indirectly via that energy", says Andrea Morales, a PhD student and member of the research team, "but we have now been able to study in real time how those modes behave when the system is perturbed". To do so, the researchers sent a short laser pulse into one of the and then measured the light intensity in both resonators as a function of time. This allowed them to calculate the position of the atoms inside the energy sombrero. As expected, after exciting a Goldstone mode, only the angular coordinate along the groove changed, whereas in the Higgs mode it was the radial position that varied.

For Tilman Esslinger, this direct observation of an important and widespread many-body phenomenon - which thus far could only be observed indirectly - represents one of the essential strengths of the quantum simulator: "In those synthetic systems we have a pretty ideal realization of what occurs in nature - in solids and also in single molecules. The direct observation of the dynamics of the Goldstone and Higgs modes in the deepens our understanding of what happens in such natural systems."

Explore further: Neutrons detect elusive Higgs amplitude mode in quantum material

More information: Julian Léonard et al, Monitoring and manipulating Higgs and Goldstone modes in a supersolid quantum gas, Science (2017). DOI: 10.1126/science.aan2608

Related Stories

Three-way battles in the quantum world

April 11, 2016

In phase transitions, for instance between water and water vapor, the motional energy competes with the attractive energy between neighboring molecules. Physicists at ETH Zurich have now studied quantum phase transitions ...

Shaking the topological cocktail of success

November 12, 2014

Graphene is the miracle material of the future. Consisting of a single layer of carbon atoms arranged in a honeycomb lattice, the material is extremely stable, flexible, highly conductive and of particular interest for electronic ...

Supercool breakthrough brings new quantum benchmark

July 4, 2017

By gently prodding a swirling cloud of supercooled lithium atoms with a pair of lasers, and observing the atoms' response, researchers at Swinburne have developed a new way to probe the properties of quantum materials.

Recommended for you

Using organoids to understand how the brain wrinkles

February 20, 2018

A team of researchers working at the Weizmann Institute of Science has found that organoids can be used to better understand how the human brain wrinkles as it develops. In their paper published in the journal Nature Physics, ...

Pattern formation—the paradoxical role of turbulence

February 19, 2018

The formation of self-organizing molecular patterns in cells is a critical component of many biological processes. Researchers from Ludwig-Maximilians-Universitaet (LMU) in Munich have proposed a new theory to explain how ...

Bringing a hidden superconducting state to light

February 16, 2018

A team of scientists has detected a hidden state of electronic order in a layered material containing lanthanum, barium, copper, and oxygen (LBCO). When cooled to a certain temperature and with certain concentrations of barium, ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.