Sergey Frolov

    • Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA, USA

&Bullet; physics 14, 68

Take a step back and check again. Don’t think that unexpected dates point to something new.

Physicists looking for new states of matter should carefully examine all possible explanations for their data before claiming a discovery.

“No, you probably haven’t discovered a new state of matter,” I say to myself every time a student in my laboratory brings me a special item of data. I try to remain skeptical and initially assume that the data comes from a breakdown, a coincidence or some other triviality. But even then, I can’t help but be hopeful. Maybe we found something new.

It is human nature to want miracles. But miracles don’t happen. And in physics, as in many areas of life, craving for them can keep us from seeing reality and prevent us from asking the right questions. Personally, I just get a kick out of true discoveries – fooling myself with illusions is not a problem as I know they will eventually fall apart. I encourage other physicists to keep this in mind, particularly condensed matter physicists who are looking for new states of matter that I believe are often too quick to make claims about.

Gas, liquid and solid are the three main states of matter. However, physicists differentiate much more. For example, there are electrically conductive states, insulating states and superconducting states in solids.

To discover a completely new state of matter is a great price to pay. Many physicists, including myself, are actively working on it. One place we are looking for is the mesoscopic regime. Mesoscopic systems lie between the microscopic quantum world and the macroscopic classical world. Research on these systems largely laid the foundation for quantum computers. But it is the possibility of finding matter in exotic quantum states that is currently attracting the attention of the field.

And there are many ways to do this. The “meso” regime is like a newly discovered coral reef in the ocean – it can be rich in previously invisible creatures. And indeed, in our explorations of this world at the quantum frontier, we find evidence of new behaviors. One class of these behaviors is called “topological” and an example of a topological state is a topological isolator. When materials are in this state, their surface remains conductive regardless of their shape, while their inside is insulating.

The topological states of matter that have generated great interest are plotted against two key metrics in this qualitative graph: the measured or predicted energy scale of the state and the “quality” of the material. It is almost certain that conditions are observed in the upper left corner of the graph. A state in the lower right corner has no chance of being verified (the energy scale is too small or the perturbation in the material is too great to make the state experimentally accessible). Between these two regions lies “the fog of hope”. Here physicists observe promising – but clouded – signals that are consistent with predictions of state behavior, but these signals can easily be masked by the signals caused by interference. One way to make a definitive discovery is to change the material parameters and move the state out of the fog and out into the open.The topological states of matter that have generated great interest are plotted against two key metrics in this qualitative graph: the measured or predicted energy scale of the state and the “quality” of the material. States in the upper left corner … show more

Another example of a heavily studied topological state is topological superconductivity, a state that I’m interested in. Topological superconductivity is difficult to describe, but loosely speaking, materials that exhibit this behavior have the inside that superconductivity and surfaces that contain exotic quantum states are known as Majorana Null Modes (MZMs). These modes are interesting because they are predicted to be distinguishable from one another. This behavior violates a basic rule for all known particles in the universe. Researchers believe that these states could be used to make quantum computers more robust against noise.

These predictions raise the need for experimental confirmation of MZMs, and the search for them has become widespread, spanning systems from nanowires (my focus) to chains of magnetic atoms and spin fluids. In nanowires, the predicted MZM signature is a peak in the wire’s tunnel conductivity when the wire is connected in a transistor-like circuit. When you find this peak, you will discover MZMs. It sounds simple, except that the MZM signal can be masked by similar peaks from non-topological, non-topological states.

This doubtfulness is why researchers in the field should question their data first and only make tentative claims about a discovery when other explanations are excluded. Extraordinary claims require compelling evidence: there is no room for unsolved miracles within the scientific method. If we think we’ve made an important discovery, the first thing we should do is check our data for internal consistency and tie up any loose ends. If afterwards we can honestly tell ourselves – and our colleagues – that our claim may still be justified, perhaps with reservations, we can announce it cautiously. But even then, we should carefully present all possible explanations. Magazines, referees, peers, and news organizations should consider these alternative options when interpreting or reporting new results.

For MZMs, I hope their discovery will soon occur in either current or future systems. Cleaner materials – for example, higher purity nanowires – are likely to have clearer MZM signals, and there may be better, less-explored systems to investigate. But until we can come up with indisputable evidence, it is important that we all keep reminding ourselves that “no, we probably haven’t discovered a new state of matter”.

About the author

Image by Sergey Frolov

Sergey Frolov is an experimenter in condensed matter physics and conducts electronic transport and microwave experiments on nanoscale devices. He is interested in topological superconductivity, quantum materials and quantum computers. He received his Ph.D. from the University of Illinois at Urbana-Champaign and is currently Associate Professor at the University of Pittsburgh, Pennsylvania.

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