New Method Shows How Bosons Behave Like Fermions

Fermions and bosons are known to behave quite differently under the majority of circumstances. These are two classes in which all particles, ranging from the sub-atomic to atoms themselves, can be organized.

Researchers at Penn State use this apparatus to create an array of ultra-cold one-dimensional gases made up of atoms. These atoms are bosons, one of two classes into which all particles can be sorted. Generally, bosons and fermions, the other class of particle, behave quite differently. However, when the internal interactions among bosons in a one-dimensional gas are very strong, their spatial distribution is the same as non-interacting fermions. The researchers have now shown that when the gases are allowed to expand while still confined in one dimension, their velocity distribution also becomes the same as a gas of non-interacting fermions. Image Credit: Nate Follmer, Penn State.

While identical fermions are inclined to be antisocial, identical bosons prefer to congregate. But in one dimension (1D)—envisage particles that can only travel on a line—bosons can turn out to be as antisocial as fermions, so that the same position is not occupied by both.

A recent study has revealed that the same phenomenon—bosons behaving similar to fermions—can also occur with their velocities. This discovery adds to one’s significant interpretation of quantum systems and can potentially lead to the ultimate development of quantum devices.

All particles in nature come in one of two types, depending on their ‘spin,’ a quantum property with no real analogue 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 completely differently from fermions.

David Weiss, Research Lead and Distinguished Professor, Department of Physics, The Pennsylvania State University

Weiss continued, “Bosons form ‘Bose-Einstein condensates,’ congregating in the same quantum state. Fermions, on the other hand, fill the available states one by one to create the so-called ‘Fermi sea.’

Now, scientists at The Pennsylvania State University (Penn State) have experimentally shown that when bosons expand in 1D, the line of atoms is permitted to spread out to become longer, potentially forming a Fermi sea. A study illustrating the research was published in the Science journal on March 27th, 2020.

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 very strong. As a result, when constrained to move in one-dimension, their spatial distribution can look like that of non-interacting fermions.

Marcos Rigol, Research Lead and Professor, Department of Physics, The Pennsylvania State University

Rigol continued, “Back in 2004, David’s research group experimentally demonstrated this phenomenon, which was theoretically predicted in the 1960s.”

Although the spatial characteristics of powerfully interacting bosons as well as non-interacting fermions remain the same in 1D, bosons can still continue to have the same velocities as one another, whereas fermions cannot. This phenomenon is attributed to the particles’ underlying nature.

In 2005, Marcos, then a graduate student, predicted that when strongly interacting bosons expand in one dimension, their velocity distribution will form a Fermi sea,” added Weiss. “I was very excited to collaborate with him in demonstrating this striking phenomenon.”

The scientists produced an array of ultra-cold, 1D gases composed of bosonic atoms (Bose gases) with the help of an optical lattice that traps the atoms using laser light. The system remains at equilibrium in the light trap, while the powerfully interacting Bose gases exhibit spatial distributions just like fermions but continue to have the bosons’ velocity distributions.

When some of the trapping light was shut off by the researchers, the atoms expand in 1D. At the time of this expansion, the bosons’ velocity distribution easily changed into a one that is analogous to fermions. The scientists can track this transformation as it takes place.

The dynamics of ultracold gases in optical lattices are the source of many novel fascinating phenomena that only recently have started to be explored. For example, Dave’s group showed in 2006 that something as universal as temperature is not well defined after Bose gases undergo dynamics in one dimension.

Marcos Rigol, Research Lead and Professor, Department of Physics, The Pennsylvania State University

Rigol continued, “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.”

Due to the “integrable” nature of the system, the scientists can understand it in excellent detail, and by analyzing the dynamical behavior of these 1D gases, the Penn State researchers are hoping to deal with wider problems prevalent in physics.

In the last half century many universal properties of equilibrium quantum systems have been elucidated,” added Weiss. “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.”

Interacting and dynamical quantum systems represent an integral part of fundamental physics. These systems are also becoming technologically appropriate, since a majority of the proposed and actual quantum devices are based on them, including quantum computers and quantum simulators.

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

Apart from Rigol and Weiss, Penn State researchers include Joshua M. Wilson, Neel Malvania, Yuan Le, and Yicheng Zhang.

The study was financially supported by the U.S. Army Research Office and the U.S. National Science Foundation. Computations were carried out at the Penn State Institute for Computational and Data Sciences.


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