Posted in | Quantum Optics

Study Opens up New Avenue for Exploring Non-Equilibrium and Dissipative Many-Body Physics

Yutaka Shikano, Ph.D., a member of Chapman University Institute for Quantum Studies (IQS) has recently published his work in Scientific Reports. One of the most outstanding phenomena in physics is superconductor with excellent technological implications. MRI machines used to image the human body and highly powerful magnets that levitate trains are two examples of the technologies that cannot operate without superconductivity. Now it is understood that the reason for superconductivity arising is a basic quantum mechanical effect.

The fundamental idea of quantum mechanics is that everything at the microscopic scale possesses a wave property including light and matter. Usually, the wave nature is not obvious as the waves are very tiny, and all the waves are not in sync with each other so that their effects are not significant. Therefore to observe quantum mechanical performance, experiments normally have to be conducted at microscopic length scales and at extremely low temperature.

On the other hand, superconductors have a remarkable effect in the disappearance of resistance, altering the whole property of the material. The main quantum effect that happens is that the quantum waves become highly synchronized and happen at a macroscopic level.

It is now understood to be the same fundamental effect as that observed in lasers. The comparison is that in a laser, the photons producing the light are synchronized, and appear as one solo coherent wave.

In a superconductor, the macroscopic wave is for the quantum waves of the electrons, rather than the photons, but the essential quantum feature remains the same. This type of macroscopic quantum waves has also been noticed in Bose-Einstein condensates, where atoms cooled to nanokelvin temperatures collapse into a solo state.

So far, these interconnected but diverse phenomena have only been noticed separately. However, since superconductors, Bose-Einstein condensates, and lasers, all possess a common feature, it has been anticipated that it should possess the capacity to see these features at the same time. A latest experiment conducted by collaborative teams from Japan, Germany, and the US, revealed for the first time that this anticipation is true.

They handled this problem by highly exciting exciton-polaritons, which are particle-like excitations in semiconductor systems and built by powerful coupling between photons and electron-hole pairs. They noticed high-energy side-peak emission unexplainable by two mechanisms known to date - traditional semiconductor lasing driven by the optical gain from unbound electron hole plasma nor Bose-Einstein condensation of exciton-polaritons.

By integrating the experimental data with their most recent theory, the team discovered a possibility that the peak begins from a powerfully bound e-h pairs, which can carry on in the presence of excellent quality optical cavity even for the lasing state. This situation has been thought to be unattainable as an e-h pair experiencing weakened binding force due to other electrons and/or holes splits up in high-density.

The projected situation is closely connected to the BCS physics, which was initially launched by John Bardeen, Leon Cooper, and John Robert Schrieffer to illustrate the beginning of superconductivity. According to the BCS theory, superconductivity is caused by the condensation of weakly bound electron pairs (Cooper pairs). In the most recent theory of e-h pairs plus photons (e-h-p), survival of bound e-h pairs can be explained in BCS theory of e-h-p system as an analogy of Cooper pairs in superconductivity.

Although a full understanding of this observation has not yet been reached. The discovery provides an important step toward the clarification of the relationship between the BCS physics and the semiconductor lasers. The observation not only deepens the understanding of the highly-excited exciton-polariton systems, but also opens up a new avenue for exploring the non-equilibrium and dissipative many-body physics. In such practical application studies, there are still many quantum foundational questions.

Dr. Tomoyuki Horikiri, Yokohama National University

The research paper was published in Scientific Reports by Nature Publishing Group. Along with Tomoyuki Horikiri, the paper was co-authored by Dr. Makoto Yamaguchi and Dr. Kenji Kamide and an international collaboration team including Tim Byrnes at New York University; Yutaka Shikano at Institute for Molecular Science, National Institutes of Natural Sciences and Institute for Quantum Studies, Chapman University; Tetsuya Ogawa at Osaka University; Alfred Forchel at Universität Würzburg, and YoshihisaYamamoto at Stanford University and National Institute of Informatics.

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