Researchers find new way bosons act like fermions

Researchers from the state of Pennsylvania use this apparatus to create an array of ultracold one-dimensional gases consisting of atoms. These atoms are bosons, one of two classes into which all particles can be sorted. As a rule, bosons and fermions, another class of particles, behave in completely different ways. However, when the internal interactions between bosons in a one-dimensional gas are very strong, their spatial distribution is the same as that of non-interacting fermions. Now researchers have shown that when gases are allowed to expand while they are still in one dimension, their velocity distribution also becomes the same as that of a gas of non-interacting fermions. Credit: Nate Fallmer, PA

Bosons and fermions, two classes into which all particles can be sorted – from subatomic to atoms themselves – behave very differently in most cases. While identical bosons love to collect, identical fermions tend to be antisocial. However, in one dimension – imagine particles that can only move along a line – bosons can become as autonomous as fermions, so no two can occupy the same position. Now, a new study shows that the same thing – bosons acting like fermions – can occur at their speeds. This discovery complements our fundamental understanding of quantum systems and can provide information on the possible development of quantum devices.

“All particles in nature are of one of two types, depending on their“ rotation ”, of a quantum property without a real analogue in classical physics"Said David Weiss, a distinguished professor of physics at Penn State and one of the leaders of the research team." Bosons whose backs are integers can have the same quantum state, while fermions whose backs are half integers are not can. When the particles are cold or sufficiently dense, the bosons behave quite differently than fermions. Bosons form "Bose-Einstein condensates", united in the same quantum stateFermions, on the other hand, fill the available states one after another, forming the so-called “Fermi Sea”.

Penn State researchers have now experimentally demonstrated that when bosons expand in one dimension – the line of atoms can expand, becoming longer – they can form the Fermi Sea. An article describing the study appears on March 27, 2020 in a journal. The science,

“Identical fermions are antisocial, you cannot have more than one in the same place, so when they are very cold, they don’t interact,” said Marcos Rigol, a professor of physics at Pennsylvania and another research team leader. “The bosons may be in the same place, but it becomes energetically too expensive when their interactions are very strong. As a result, when they are forced to move in one-dimensional space, their spatial distribution may look like non-interacting fermions. Back in 2004, David’s research team experimentally demonstrated this phenomenon, which was theoretically predicted in the 1960s. ”

Despite the fact that the spatial properties of strongly interacting bosons and non-interacting fermions are the same in one dimension, bosons can have the same speed, but fermions can not. This is due to the fundamental nature of the particles.

“In 2005, Marcos, then a graduate student, predicted that when strongly interacting bosons expand in one dimension, their velocity distribution forms the Fermi Sea,” Weiss said. “I was very happy to collaborate with him in demonstrating this amazing phenomenon.”

Quantum copycat: researchers find a new way to behave bosons as fermions

Evolution of the velocity distribution of a trapped gas of strongly interacting bosons expanding in one dimension. Initially, the peak “bosonic” distribution (purple line) gradually turns into a rounded “fermion” distribution (dark red line). The final shape is similar to the Fermi Sea, which will characterize fermions in the same initial trap. Courtesy: Weiss Laboratory, Penn State.

The research team creates an array of ultracold one-dimensional gases consisting of bosonic atoms ("Bose gases") using an optical array that uses laser light to capture atoms. In a light trap, the system is in equilibrium, and strongly interacting Bose gases have spatial distributions, such as fermions, but still have boson velocity distributions. When researchers turn off part of the light trap, atoms expand in one dimension. During this expansion, the distribution of boson velocity smoothly turns into a distribution identical to fermionsResearchers can follow this transformation as it occurs.

“The dynamics of ultracold gases in optical gratings is the source of many interesting new phenomena that have only recently begun to be studied,” said Rigol. “For example, Dave’s group in 2006 showed that something as universal as temperature is not well defined after the Bose gases undergo dynamics in one dimension. My colleagues and I related this conclusion to the beautiful mathematical characteristics of theoretical models describing his experiments, known as “integrability”. Integrability plays a central role in our recently discovered phenomenon of dynamic fermionization. ”

Because the system is “integrable,” researchers can understand it in great detail, and by studying the dynamic behavior of these one-dimensional gases, the Penn State team hopes to solve wide-ranging physics issues.

“Over the past half century, many of the universal properties of equilibrium quantum systems have been clarified,” Weiss said. “It was more difficult to define universal behavior in dynamical systems. Fully understanding the dynamics of one-dimensional gases, and then gradually making gases less integrable, we hope to define universal principles in dynamic quantum systems. ”

Dynamic, interacting quantum systems are an important part of fundamental physics. They are also becoming increasingly technologically significant, as many relevant and proposed quantum devices, including quantum simulators and quantum computers, are based on them.

“Now we have experimental access to things that, if you asked any theorist working in this field ten years ago,“ would we see this in our lifetime? "They would say" for nothing, "said Rigol.

In addition to Rigol and Weiss, the Pennsylvania research team includes Joshua M. Wilson, Neil Malvania, Yuan Le, and Yicheng Zhang. The study was funded by the US National Science Foundation and the US Army Research Department. The calculations were carried out at the Pennsylvania State Institute of Computational Sciences and Data.

Bose-Einstein condensate miscibility properties open up surprises

Additional Information:
Joshua M. Wilson et al. Observation of dynamic fermionization, The science (2020). DOI: 10.1126 / science.aaz0242

Quantum copycat: researchers find a new way to behave bosons as fermions (2020, March 27)
restored March 27, 2020

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