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Science

Chirality by dissipation

Jelena Stajic

Published Dec 19, 2019• 0 citations
Publisher

Abstract

Quantum Gases Quantum many-body systems can display exotic dynamics in the presence of dissipation. Dogra et al. studied such dynamics in a system consisting of an atomic Bose-Einstein condensate located in an optical cavity and exposed to a standing wave of laser light. Light scattering off the atomic cloud and into the cavity resulted in two distinct, spatially patterned collective modes for the atoms. When the researchers then introduced dissipation to couple the two modes, the system followed a directed circular path through phase space, rotating between the modes. Science , this issue p. [1496][1] [1]: /lookup/doi/10.1126/science.aaw4465

DOI
10.1126/science.366.6472.1467-t

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Quantum Gases
Quantum many-body systems can display exotic dynamics in the presence of dissipation. Dogra et al. studied such dynamics in a system consisting of an atomic Bose-Einstein condensate located in an optical cavity and exposed to a standing wave of laser light. Light scattering off the atomic cloud and into the cavity resulted in two distinct, spatially patterned collective modes for the atoms. When the researchers then introduced dissipation to couple the two modes, the system followed a directed circular path through phase space, rotating between the modes.
Science , this issue p. 1496
Quantum Optics Our textbook understanding of quantum systems tends to come from modeling these systems isolated from the environment. However, an emerging focus is understanding how many-body quantum systems behave when interacting with their surroundings and how they subsequently become dissipative, or non-Hermitian, systems. Ozturk et al. formed a quantum condensate of light by trapping photons in an optical cavity, a system that is naturally dissipative. By altering the trapping conditions, they demonstrated that the system provides a powerful platform with which to explore the complex dynamics and phase transitions occurring in dissipative quantum systems. Science , this issue p. [88][1] [1]: /lookup/doi/10.1126/science.abe9869
Summary form only given. Bose-Einstein condensed atomic gases are a new class of quantum fluids with many novel properties. We have studied collective modes and dissipation by perturbing the condensate with focused laser beams. The scattering of photons imparts momentum to the condensate and creates excitations. Consequently, the coherence and collective behavior of the condensate can strongly affect optical properties. We used light scattering to demonstrate that the coherence length of a condensate is equal to its physical size, and obtained dramatic evidence for the presence of correlated momentum excitations in the many-body condensate wavefunction. At higher laser intensity, a new form of superradiance was found which provided a gain mechanism for the generation of directed beams of atoms and light. This process amplified matter waves in a phase coherent way.
Dissipative and unitary processes define the evolution of a many-body system. Their interplay gives rise to dynamical phase transitions and can lead to instabilities. In this study, we observe a nonstationary state of chiral nature in a synthetic many-body system with independently controllable unitary and dissipative couplings. Our experiment is based on a spinor Bose gas interacting with an optical resonator. Orthogonal quadratures of the resonator field coherently couple the Bose-Einstein condensate to two different atomic spatial modes, whereas the dispersive effect of the resonator losses mediates a dissipative coupling between these modes. In a regime of dominant dissipative coupling, we observe the chiral evolution and relate it to a positional instability.
Atomic Physics Atomic interactions in a Bose-Einstein condensate (BEC) can lead to complex collective behavior. Experimentally, these interactions are often tuned by varying an external magnetic field. Feng et al. modulated the interaction among cesium atoms in a BEC. The collisions between atoms exposed to the modulated field sent the atoms flying out of the condensate in jets of seemingly random directions. A pattern-recognition technique revealed that certain directions were associated with particularly large numbers of scattered atoms. The pattern of the scattering maxima could be attributed to secondary collisions. Science , this issue p. [521][1] [1]: /lookup/doi/10.1126/science.aat5008
Creating quantum gases with long-range atom-atom interactions is a vibrant area of current research, possibly leading to the observation of novel quantum phases and phase transitions. In our approach, we couple the external degree of freedom of a Bose-Einstein condensate to the vacuum mode of a high-finesse optical cavity using a non-resonant standing-wave transverse pump beam. This gives rise to a driven-dissipative system with cavity-mediated atom-atom interactions of global range, which are tunable both in magnitude and sign.
The recent experimental observation of dissipation-induced structural instability provides new opportunities for exploring the competition mechanism between stationary and nonstationary dynamics [N. Dogra et al., Science 366, 1496 (2019)]. In that study, two orthogonal quadratures of cavity field were coupled to two different Zeeman states of a spinor Bose-Einstein condensate (BEC). Here we propose a scheme to couple two density-wave degrees of freedom of a BEC to two quadratures of the cavity field. Different from previous studies, the light-matter quadratures coupling in our model is endowed with a tunable coupling angle. Apart from the uniform and self-organized phases, we unravel a dynamically unstable state induced by the cavity dissipation. Interestingly, the dissipation defines a particular coupling angle, across which the instabilities disappear. Moreover, at this critical coupling angle, one of the two atomic density waves can be independently excited without affecting one another. It is also found that our system can be mapped into a reduced three-level model under the commonly used low-excitation-mode approximation. However, the effectiveness of this approximation is shown to be broken by the dissipative nature of some special system parameters, hinting that the low-excitation-mode approximation is insufficient in capturing some dissipation-sensitive physics. Our work enriches the quantum simulation toolbox in the cavity-quantum-electrodynamics system and broadens the frontiers of light-matter interaction.