The equations of quantum mechanics are vastly too complicated to solve in full detail. However, if a system is in equilibrium, general principles such as energy minimization are often enough to guide physicists to good approximate solutions. But for quantum systems that are not in equilibrium — perhaps because they have been pushed out of their comfort zone by bursts of light or electric currents — there is no such road map.
Yet amazing new tools, developed over the past decade, are beginning to allow researchers to investigate and control the remarkable properties of such systems, opening up new frontiers in basic science and the prospect for transformative quantum technologies. “In my view, there’s really a new scientific field forming here,” says Andrew Millis, co-director of the Flatiron Institute’s Center for Computational Quantum Physics.
The newly founded Max Planck-New York City Center for Nonequilibrium Quantum Phenomena aims to put this new field of research on firm footing. Founded in November 2019, the center features a close collaboration between theory and experiment, uniting the complementary strengths of three institutions: the Flatiron Institute and Columbia University in New York City, and the Max Planck Society in Germany (specifically, the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg and the Max Planck Institute for Polymer Research in Mainz).
“We would like to be able to create novel phases of matter on demand,” says Dmitri Basov of Columbia, who will co-direct the center with Millis and Andrea Cavalleri of the MPSD. “When you can do that, that’s the foundation of new technologies.”
Each of the three member institutions brings different expertise to the collaboration. Columbia researchers, for instance, are world leaders in creating new materials by layering extremely thin sheets of graphene and other materials — just a single atom thick — on top of each other. When these layers are positioned at varying angles relative to each other, “all of a sudden you can synthesize new phases of matter completely different from the ones you had on the isolated layers,” says Angel Rubio of the MPSD and the Flatiron Institute, one of the new center’s deputy co-directors.
These thinly layered sandwiches offer a fertile testing ground for studying the phenomena that can emerge when a quantum system is pushed out of equilibrium. Columbia researchers are examining, for example, what happens when such layered systems are placed inside optical cavities, mirrored boxes that are used to make lasers.
The Max Planck Society, meanwhile, has long been a leader in devising new experimental systems. Researchers at the MPSD have built laser systems that can deliver short pulses of extremely intense light and then probe the nonequilibrium phenomena the light stimulates, all in a single device. “From a technical point of view, it’s a remarkable achievement,” Millis says. “It opens up all kinds of directions for turning effects on and off.”
“Building these capabilities has required a decade of dedicated effort,” Basov says. “Now we are in a position to exploit these tools. In combination with the unique materials and structures developed by Columbia scientists, we are poised to make gains.”
MPSD researchers have used their laser-based systems to turn on and off a ‘quantum Hall effect,’ a peculiar phenomenon in which part of an electric current flows sideways from the direction of the applied voltage. Researchers at Columbia and the MPSD have also shown that intense light pulses can briefly turn nonmagnetic materials into magnets, or insulators into superconductors.
“These phenomena were quite unexpected,” Millis says. “If we can understand and control them, that would mean that you can turn fundamentally important electronic properties like superconductivity on and off at the flick of a switch.”
The superconductivity the MPSD researchers reported occurs at a much higher temperature than other known instances of superconductivity, which typically require powerful refrigeration. “If we could figure out how to do this in a steady state, it would revolutionize electrical power transmission,” Millis says.
The third partner in the new collaboration, the Flatiron Institute’s Center for Computational Quantum Physics, brings the theoretical, algorithmic and computational expertise that will help the experimentalists make sense of what they are seeing. “We will be able to develop and implement the theory and concepts that are presently missing,” Millis says.
Flatiron researchers plan to develop algorithms and computer code that will allow the collaborators to model complex nonequilibrium phenomena. The aim of this theoretical framework is to help guide Columbia and Max Planck Society researchers in designing their experiments; the experimental findings, in turn, will keep the theoretical models tethered to reality.
The new center has been funded for five years, with the possibility of being renewed for an additional five years, the maximum allowable time span for a Max Planck Society project. The center aims to promote intense scientific exchange between the institutions, with ample travel support and an annual conference, a summer school and two to four workshops each year. It also plans to support six postdoctoral researchers and three graduate students, to be spread among the member institutions.
And, in an unusual experiment, the center also plans to create two joint junior faculty positions, which will start in Germany and then transition: one to Columbia and the other to the Flatiron Institute. This arrangement will allow the two junior scientists to take advantage of the Max Planck Society’s outstanding technical support and experimental infrastructure, and then segue into long-term career trajectories in New York City. (Most appointments of junior group leaders within the Max Planck Society simply terminate after about five years.) The hope is that these joint appointments will keep the bonds between the member institutions alive long after the center has concluded its activities.
“Science today is about the seamless flow of ideas and people across universities and continents,” Basov says. “That’s what this center will enable.”