Ile waży światło?

W styczniu tego roku w czasopiśmie Physical Review Letters ukazał się ciekawy artykuł, który mówi, że udało się zatrzymać światło. Oczywiście nie jest to znany efekt fotowoltaiczny, ale przy okazji jest to doświadczenie, które zaczyna dotykać zjawisk, nad którym zastanawiałem się już jakiś czas temu w poście o polu elektromagnetycznym. Jeśli więc potrafimy zatrzymać światło, to powinna powstać masa, a więc powinniśmy ją zważyć. Niestety autorzy tego artykułu nie zastanawiają się nad tym, chociaż jest to bardzo interesujący problem. Poniżej cytuję streszczenie tego artykułu. Jakby rozwinięciem tego problemu jest drugi artykuł, również cytowany poniżej, który mówi o powstaniu ujemnej masy. Ale nie wiem, jak zważyć ujemną masę? Może ktoś wie?

Phys. Rev. Lett. 120, 013901 – Published 3 January 2018

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.013901#!

Light Stops at Exceptional Points

Tamar Goldzak¹, Alexei A. Mailybaev^2,*, and Nimrod Moiseyev^1,†

• ¹Schulich Faculty of Chemistry and Faculty of Physics, Technion—Israel Institute of Technology, Haifa 32000, Israel
• ²Instituto Nacional de Matemática Pura e Aplicada—IMPA, 22460-320 Rio de Janeiro, Brazil

• ^*alexei@impa.br
• ^†nimrod@tx.technion.ac.il

Almost twenty years ago, light was slowed down to less than 10−7 of its vacuum speed in a cloud of ultracold atoms of sodium. Upon a sudden turn-off of the coupling laser, a slow light pulse can be imprinted on cold atoms such that it can be read out and converted into a photon again. In this process, the light is stopped by absorbing it and storing its shape within the atomic ensemble. Alternatively, the light can be stopped at the band edge in photonic-crystal waveguides, where the group speed vanishes. Here, we extend the phenomenon of stopped light to the new field of parity-time (PT) symmetric systems. We show that zero group speed in PT symmetric optical waveguides can be achieved if the system is prepared at an exceptional point, where two optical modes coalesce. This effect can be tuned for optical pulses in a wide range of frequencies and bandwidths, as we demonstrate in a system of coupled waveguides with gain and loss.

Phys. Rev. Lett. 118, 155301 – Published 10 April 2017

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.155301#!

Negative-Mass Hydrodynamics in a Spin-Orbit–Coupled Bose-Einstein Condensate

M. A. Khamehchi¹, Khalid Hossain¹, M. E. Mossman¹, Yongping Zhang^2,3,*, Th. Busch^2,†, Michael McNeil Forbes^1,4,‡, and P. Engels^1,§

• ¹Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, USA
• ²Quantum Systems Unit, OIST Graduate University, Onna, Okinawa 904-0495, Japan
• ³Department of Physics, Shanghai University, Shanghai 200444, China
• ⁴Department of Physics, University of Washington, Seattle, Washington 98105, USA

• ^*yongping11@t.shu.edu.cn
• ^†thomas.busch@oist.jp
• ^‡michael.forbes@wsu.edu
• ^§engels@wsu.edu

A negative effective mass can be realized in quantum systems by engineering the dispersion relation. A powerful method is provided by spin-orbit coupling, which is currently at the center of intense research efforts. Here we measure an expanding spin-orbit coupled Bose-Einstein condensate whose dispersion features a region of negative effective mass. We observe a range of dynamical phenomena, including the breaking of parity and of Galilean covariance, dynamical instabilities, and self-trapping. The experimental findings are reproduced by a single-band Gross-Pitaevskii simulation, demonstrating that the emerging features—shock waves, soliton trains, self-trapping, etc.—originate from a modified dispersion. Our work also sheds new light on related phenomena in optical lattices, where the underlying periodic structure often complicates their interpretation.