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Exploring Unsolved Physical Phenomena
—Quantum many-body physics with ultracold atomic gases
Shun Uchino,  Assistant Professor

The fascination of theoretical physics research

I am a researcher of theoretical physics, who works with a paper and pen (these days with a tablet and stylus, or sometimes a computer). Being able to make predictions and interpret experiments of unknown phenomena is fascinating for a theoretical physicist. We may not be making any world-changing discoveries, but even so, there is no substitute for the joy of finding a natural law that no one else knows.

My field of research is many-body physics of microscopic particles that obey quantum mechanics. I am particularly interested in quantum-transport phenomena. Here, transport is a flow that occurs when some kind of force is added to a system in equilibrium. For example, an electric current is generated in the presence of an electric field, which gives the force for charged particles. It is known that quantum-mechanical properties of electrons become pronounced at low temperatures and the behavior of the system as a whole changes in a drastic manner. Such pronounced transport phenomena are referred to as the “quantum-transport phenomena.”

Experimental techniques improve day by day, and from time to time we encounter a phenomenon that cannot be explained by existing theories. This is when many theoreticians including me are exciting, and we think about a way to explain such a phenomenon.

Why research ultracold atomic gases?

In the past, most research about quantum transport was performed in an electronic system. Since an electronic system is closely related to semiconductor and superconductor technologies, such a system is also of importance in terms of practical applications. However, if our interest is persuit of truth behind quantum transport, an electronic system is not the unique approach. Recently, a promising alternative approach has emerged, which is nothing but an ultracold atomic gas.

An ultracold atomic gases has many useful characteristics. For example, since an ultracold atomic gas is confined in a vacuum chamber, noise has very little effect on it. This is a big advantage when measuring a system in detail. In addition, while the Coulomb force, which is a tough cookie due to the long-range character, works between electrons, a force between atoms is short range and is relatively easy to handle from the theoretical point of view. Furthermore, we can control the strength of the force between atoms in predictable ways by means of a Feshbach resonance. This technique allows us to address questions of a strongly interacting ultracold atomic gas, which has yet to be investigated in other fields of research.

Elucidating anomalous quantum-transport effects

I will discuss one of the phenomena that we have clarified using ultracold atomic gases. Our research is based on an experiment performed by Tilman Esslinger’s group at ETH Zurich.

Figure 1 shows a general view of the experimental setup at ETH we are studying. “Atomic reserviors,” which consist of about 100,000 lithium atoms (6Li) and expand to the left and right, are cooled to a few tens of nanokelvin. The constriction in the center is the part responsible for transport, and it is a tube that only several atoms at a time can pass through. When there is some kind of imbalance between two reservoirs, such as when one atomic reservoir has more atoms, this arrangement results in atoms being transported through the tube.

Figure 1: View of a device for experiments on quantum-transport effects using ultracold atomic gases. The blue parts expanding to the left and right are clusters of atoms. Atoms are transported across the central construction (source: Shun Uchino).

The experiment conducted by Esslinger’s group (Proc. Natl. Acad. Sci. U.S.A. 113, 8144 (2016)) addressed transport of normal (non-superconducting) fluids. By strengthening the interaction between atoms, they discovered something that contradicted the Landauer formula. Here, the Landauer formula is widely used for understanding the behavior of electrons in a semiconductor, and it predicts the quantization of conductance when passing through a fine wire. Conductance, the reciprocal of resistance, is a measure of how readily something flows. This “anomalous phenomenon” can be discovered by an ultracold atomic gas where the interactios between particles can systematically be manipulated. Before the ETH experiment, researchers anticipated that stronger interactions would impede the flow through the tube, and thus tend to reduce conductance, but the opposite happened: conductance increased.

We analyzed this anomalous transport phenomenon in depth. We then formulated a theory that explained this effect in terms of the “superfluid forerunner” phenomenon. Our idea was that, although this was not a superfluid per se, it was in a region extremely close to superfluids. When we calculated the conductance of particles with this in mind, we found that we were able to reproduce the “anomalous” conductance (see Figure 2).

The transport phenomenon observed in an ultracold atomic gas had not yet been discovered in electronic systems, but in principle it may occur in electron transport as well. Our finding may provide important hints for understanding strongly-correlated quantum many-body systems such as high-temperature superconductors, which are still not well understood.

Figure 2: Anomalous conductance in an ultracold atomic gas. The dots and bars extending vertically represent experimental results, and the solid lines represent calculated results. The horizontal axis is an index representing the difficulty with which atoms pass through the tube (more difficult to the right), and the vertical axis is the physical quantity of conductance. Results are colored according to interaction strength. Where 1/kFa is highest (red), the interaction is strongest. (Reproduced from Phys. Rev. Lett. 118, 105303 (2017).)

Using ultracold atomic gases, we plan to further investigate phenomena that could not be investigated using electronic systems.

Once a phenomena is theorized, we may find new phenomena based on the theory and apply to related unsolved phenomena. By elucidating novel physical phenomena, we open the door to technologies that could bring unimaginable changes to the world in 100 or 200 years.

Interview and Composition:kaori Oishi
In cooperation with: Waseda University Graduate School of Political Science J-School

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