Antilaser for an Ultracold Atoms Condensate

The world’s first antilaser for nonlinear Bose-Einstein condensate: It is possible to absorb the selected signal completely, even though the nonlinear system makes it difficult to predict the waves behaviour. (Source: ITMO)

An inte­rnational team of scientists developed the world’s first anti­laser for a nonlinear Bose-Einstein conden­sate of ultracold atoms. For the first time, scientists demon­strated that it is possible to absorb the selected signal com­pletely, even though the nonlinear system makes it diffi­cult to predict the waves behaviour. The results can be used to mani­pulate super­fluid flows, create atomic lasers, and also study nonlinear optical systems.

Suc­cessful infor­mation transfer requires an ability to extin­guish selected electro­magnetic signal completely, without any reflection. This might happen only when the parameters of the electro­magnetic waves and the system around them are coherent with each other. Devices that provide coherent perfect absorp­tion of a wave with given parameters are anti­lasers. They have been used for several years in optics, for example, to create high-precision filters or sensors. The work of standard anti­lasers is based on the destruc­tive inter­ference of waves incident on the absorber. If the parameters of the incident waves are matched in a certain way, then their inter­action leads to the perfect absorption with zero reflec­tion.

However, until now it was not clear whether such an absorp­tion is possible in nonlinear systems, such as optical fiber trans­mitting high-intensity signal in strong external electro­magnetic field. The problem is that it is much more difficult to describe the inter­action of the incident waves propa­gating in the nonlinear medium. At the same time, nonlinear systems are interes­ting due to their capa­bility of controlling wave frequency and shape without energy loss. This can be useful for signal distinc­tion in optical computers. However, the problem is that nonlinear systems often turn out to be unstable and predic­ting their behavior can be diffi­cult.

In studying this problem, scientists from Germany, Russia and Portugal became the first to construct an antilaser for waves propa­gating in a nonlinear medium. In their experiments, scientists used a Bose-Einstein conden­sate of ultracold atoms. In a BEC all atoms form a coherent cloud which supports propa­gation of matter waves. Strong repulsive inter­actions between the condensed atoms induce nonlinear properties in the system. For example, the inter­action of waves ceases to obey the laws of linear inter­ference. To catch the conden­sate, scientists use a periodic optical trap formed by the inter­section of two laser beams. A focused electron beam applied to the central cell of the lattice makes the atoms leak out from this cell. Atoms from neighboring cells go to the central cell, striving to make up for the leak. As a result, two superfluid matter flows directed toward the center are formed in the conden­sate. Once the flows meet in the central cell, they are absorbed perfectly, without reflection.

“The laws that describe the propa­gation of waves in various media are universal. There­fore, our idea can be adapted to implement an antilaser in other nonlinear systems. For example, in nonlinear optical wave­guides or in conden­sates of quasi­particles, such as polari­tons and excitons. This concept can also be used when working with nonlinear acous­tic waves. For example, you can build a device that will absorb sounds of a certain frequency. Although such devices may not be made soon, we have shown that they are possible,” notes researcher Dmitry Zezyulin from ITMO Univer­sity.

Scientists currently plan to shift to nonlinear optical systems, where atoms will be replaced with photons. “Photons, unlike atoms, are difficult to keep in the system for long. However, in this project, my colleagues managed to make a nonlinear atomic system behave as if it consisted of photons. At the same time, they managed to implement an ideal absorption in such conditions. This means that these processes are also possible in nonlinear photonic systems,” adds Ivan Iorsh, the head of the Inter­national Labora­tory of Photo­processes in the Meso­scopic Systems. (Source: ITMO)

Reference: A. Müllers et al.: Coherent perfect absorption of nonlinear matter waves, Sci. Adv. 4, eaat6539 (2018); DOI: 10.1126/sciadv.aat6539

Links: OPTIMAS State Research Center, Technische Universität Kaiserslautern, Kaiserslautern, Germany • Laboratory of Light-Matter-Coupling in Nanostructures, ITMO University, Saint Petersburg, Russia

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