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Modified Bose-Einstein condensation in an optical quantum gas

Carl Ulrich Mattschas, Mario Vretenar, Chris Toebes, Jan Andre Klärs

Published Oct 14, 2021• 0 citations
OpenAlex

Abstract

Open quantum systems can be systematically controlled by making changes to their environment. A prerequisite for a deliberate control of an open quantum system is a sufficient understanding of its underlying physical mechanisms. Here, we investigate the Bose-Einstein condensation of a photonic Bose gas in an environment with controlled dissipation and feedback realized by a potential landscape that effectively acts as a Mach-Zehnder interferometer. Our measurements offer a highly systematic picture of Bose-Einstein condensation under non-equilibrium conditions. We show that the condensation process becomes an interplay between minimizing the energy of the condensate, minimizing the particle losses and maximizing the constructive feedback from the environment. [1] M. Vretenar et al., arXiv:2105.10708 [cond-mat.quant-gas]

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Modified Bose-Einstein condensation in an optical quantum gas

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Open quantum systems can be systematically controlled by making changes to their environment. A well-known example is the spontaneous radiative decay of an electronically excited emitter, such as an atom or a molecule, which is significantly influenced by the feedback from the emitter's environment, for example, by the presence of reflecting surfaces. A prerequisite for a deliberate control of an open quantum system is to reveal the physical mechanisms that determine its state. Here, we investigate the Bose-Einstein condensation of a photonic Bose gas in an environment with controlled dissipation and feedback. Our measurements offer a highly systematic picture of Bose-Einstein condensation under non-equilibrium conditions. We show that by adjusting their frequency Bose-Einstein condensates naturally try to avoid particle loss and destructive interference in their environment. In this way our experiments reveal physical mechanisms involved in the formation of a Bose-Einstein condensate, which typically remain hidden when the system is close to thermal equilibrium.

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Quantum Feedback Control Outside of the Controlled System

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We propose a novel method for controlling quantum systems via feedback. Rather than modifying parameters of the controlled system, the feedback loop changes the way it interacts with environment (the unraveling of the quantum operation describing this interaction). As an example, a problem of controlling the state of two-mode atomic Bose–Einstein condensate, with one mode subject to interferometric phase-contrast imaging in the Mach–Zehnder interferometer is considered. Different unravelings of photodetection operation are implemented by varying the phase shift in the condensate-free arm of the interferometer. A master equation governing the evolution of the condensate is derived. Based on its steady-state solution, the Kullback–Leibler divergence between the atomic distributions with and without feedback is calculated. The results show that this divergence reaches maximum at non-trivial values of the parameters controlling unraveling.

Journal of Experimental and Theoretical Physics Letters•Oct 25, 2022•3 citations
Open quantum systems can be systematically controlled by making changes to their environment. A prerequisite for a deliberate control of an open quantum system is a sufficient understanding of its underlying physical mechanisms. Here, we investigate the Bose-Einstein condensation of a photonic Bose gas in an environment with controlled dissipation and feedback realized by a potential landscape that effectively acts as a Mach-Zehnder interferometer. Our measurements offer a highly systematic picture of Bose-Einstein condensation under non-equilibrium conditions. We show that the condensation process becomes an interplay between minimizing the energy of the condensate, minimizing the particle losses and maximizing the constructive feedback from the environment. [1] M. Vretenar et al., arXiv:2105.10708 [cond-mat.quant-gas]
Open quantum systems can be systematically controlled by making changes to their environment. A well-known example is the spontaneous radiative decay of an electronically excited emitter, such as an atom or a molecule, which is significantly influenced by the feedback from the emitter's environment, for example, by the presence of reflecting surfaces. A prerequisite for a deliberate control of an open quantum system is to reveal the physical mechanisms that determine its state. Here, we investigate the Bose-Einstein condensation of a photonic Bose gas in an environment with controlled dissipation and feedback. Our measurements offer a highly systematic picture of Bose-Einstein condensation under non-equilibrium conditions. We show that by adjusting their frequency Bose-Einstein condensates naturally try to avoid particle loss and destructive interference in their environment. In this way our experiments reveal physical mechanisms involved in the formation of a Bose-Einstein condensate, which typically remain hidden when the system is close to thermal equilibrium.
Abstract Many bosons can occupy a single quantum state without a limit. It is described by the quantum-mechanical Bose–Einstein statistic, which allows Bose–Einstein condensation at low temperatures and high particle densities. Photons, historically the first considered bosonic gas, were late to show this phenomenon, observed in rhodamine-filled microcavities and doped fibre cavities. These findings have raised the question of whether condensation is also common in other laser systems with potential technological applications. Here we show the Bose–Einstein condensation of photons in a broad-area vertical-cavity surface-emitting laser with a slight cavity-gain spectral detuning. We observed a Bose–Einstein condensate in the fundamental transversal optical mode at a critical phase-space density. The experimental results follow the equation of state for a two-dimensional gas of bosons in thermal equilibrium, although the extracted spectral temperatures were lower than the device’s. This is interpreted as originating from the driven-dissipative nature of the photon gas. In contrast, non-equilibrium lasing action is observed in the higher-order modes in more negatively detuned device. Our work opens the way for the potential exploration of superfluid physics of interacting photons mediated by semiconductor optical nonlinearities. It also shows great promise for enabling single-mode high-power emission from a large-aperture device.
Abstract We consider the problem of decoherence in a Bose – Einstein condensate (BEC) of noninteracting atoms during interferometric probing with a classical monochromatic external field. The condensate is located in one of the arms of the Mach – Zehnder interferometer, while part of the radiation from the interferometer output is fed back to the input, thereby closing the coherent feedback loop. A more general setting of the problem is also considered, in which the condensate is located in a system of interferometers ‘nested’ into each other, while part of the output radiation of each of them is also fed back to its input, closing the so-called multiloop feedback. The possibility of effective control of decoherence rates of various matrix elements is shown. The application of feedback of the proposed type to BEC in a double-well potential is investigated. It is found that this feedback allows efficient control of the distribution of atoms between the wells.
A Bose-Einstein condensate can be made of photons. The photons are held at thermodynamic equilibrium in a dye-filled microcavity and pumped with a laser. Thermalisation can be demonstrated and above the threshold a Bose-Einstein condensate will form. A Mach-Zehnder interferometer is built and used to measure the spatial and temporal first-order coherence under various conditions. We build a momentum-resolved spectrometer and use it to obtain views into the phase-space distribution of the photon condensate. We put an upper bound on the value of the interaction strength parameter and find that the microcavity system is ergodic even when not at thermal equilibrium. We build a setup to stabilise the pump laser power with the aim to observe the λ-point of the condensate.
We report on the experimental observation of stimulated cooling in the non-equilibrium Bose-Einstein condensate (BEC) of weakly interacting exciton-polaritons from approximately room temperature down to 20K. By resolving the condensate in energy-momentum space and performing interferometric measurements, we distinguish the condensate from thermalized particles yet occupying excited states macroscopically. In contrast to the analytical quantum theories of non-equilibrium BEC [Shishkov et al., Phys. Rev. Lett. 128, 065301 (2022)], we observe segmentation of the particle density along the excited states into two fractions both following Bose-Einstein distribution, albeit with different effective temperatures and chemical potentials. Our results indicate that the temperature of the weakly interacting Bose gas is universally set by the density-dependent chemical potential, revealing a defining property of non-equilibrium BECs. Finally, we demonstrate that the stimulated nature of the cooling process directly governs the emergence of quantum coherence of the condensate and shapes the dissipative properties of the excited states.
We propose a novel method for controlling quantum systems via feedback. Rather than modifying parameters of the controlled system, the feedback loop changes the way it interacts with environment (the unraveling of the quantum operation describing this interaction). As an example, a problem of controlling the state of two-mode atomic Bose–Einstein condensate, with one mode subject to interferometric phase-contrast imaging in the Mach–Zehnder interferometer is considered. Different unravelings of photodetection operation are implemented by varying the phase shift in the condensate-free arm of the interferometer. A master equation governing the evolution of the condensate is derived. Based on its steady-state solution, the Kullback–Leibler divergence between the atomic distributions with and without feedback is calculated. The results show that this divergence reaches maximum at non-trivial values of the parameters controlling unraveling.