Bosons can act as fermions with their velocities

Quantum copycat!


Bosons and fermions are two classes in which all particles can be sorted, behave very differently under most circumstances.

In any case, in one dimension, particles that can move on a line—bosons can become as unfriendly as fermions so that no two occupy the same position.

Now, a new study has shown that bosons acting like fermions—can happen with their velocities.

David Weiss, Distinguished Professor of Physics at Penn State and one of the leaders of the research team, said, “All particles in nature come in one of two types, depending on their ‘spin,’ a quantum property with no real analog in classical physics. Bosons, whose spins are whole integers, can share the same quantum state, while fermions, whose spins are half integers, cannot. When the particles are cold or dense enough, bosons behave entirely differently from fermions. Bosons form ‘Bose-Einstein condensates,’ congregating in the same quantum state. Fermions, on the other hand, fill available states one by one to create what is called a ‘Fermi sea.'”

During the experiment, scientists expanded boson in one dimension—the line of atoms is allowed to spread out to become longer and formed a Fermi sea.

Marcos Rigol, professor of physics at Penn State and the other leader of the research team, said, “Identical fermions are antisocial, you can’t have more than one in the same place so when they are very cold, they don’t interact. Bosons can be in the same place, but this becomes energetically too costly when their interactions are powerful. As a result, when constrained to move in one-dimension, their spatial distribution can look like that of non-interacting fermions. Back in 2004, David’s research group experimentally demonstrated this phenomenon, which was theoretically predicted in the 1960s.”

Although spatial properties of strongly interacting bosons and non-interacting fermions are the same in one dimension, bosons can still have the same velocities as each other, while fermions cannot.

Scientists created an array of ultracold one-dimensional gases made up of bosonic atoms. They used an optical lattice, which uses laser light to trap the atoms.

In the light trap, the system is at equilibrium, and the firmly interfacing Bose gases have spatial distributions like fermions, yet at the same time have the velocity distribution of bosons. At the point when the scientists shut off a portion of the trapping light, the atoms extend in one dimension. During this extension, the speed distrust of the bosons quickly changes into a one that is indistinguishable from fermions. The scientists can follow this change as it occurs.

Rigol said, “The dynamics of ultracold gases in optical lattices is the source of many novel fascinating phenomena that only recently have started to be explored. My collaborators and I related this finding to a beautiful underlying mathematical property of the theoretical models describing his experiments, known as ‘integrability.’ Integrability plays a central role in our newly observed dynamical fermionization phenomenon.”

Weiss said, “In the last half-century, many universal properties of equilibrium quantum systems have been elucidated. It has been harder to identify universal behavior in dynamical systems. By fully understanding the dynamics of one-dimensional gases, and then by gradually making the gases less integrable, we hope to identify universal principles in dynamical quantum systems.”

“Dynamical, interacting quantum systems are an important part of fundamental physics. They are also increasing technologically relevant, as many actual and proposed quantum devices are based on them, including quantum simulators and quantum computers.”

Rigol said, “We now have experimental access to things that if you would have asked any theorist was working in the field ten years ago ‘will we see this in our lifetime?’ they would have said ‘no way.'”

In addition to Rigol and Weiss, the research team at Penn State includes Joshua M. Wilson, Neel Malvania, Yuan Le, and Yicheng Zhang. The research was funded by the U.S. National Science Foundation and the U.S. Army Research Office. Computations were performed at the Penn State Institute for Computational and Data Sciences.

Journal Reference:
  1. Observation of dynamical fermionization. DOI: 10.1126/science.aaz0242


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