Hawking Radiation in Dielectric Horizon Analogues

黑洞是非常迷人的物体。它们大量存在于我们生活的宇宙中，吸引着周围的整个星系。它们还吸引着小说作家和科学家的想象力：它们代表着我们的知识和智力可以接受考验的终极前沿。1974年，史蒂芬·霍金在黑洞温度有限的假设的基础上预测，黑洞周围的视界将以强烈时空扭曲为特征的区域分开，以至于光子和粒子被从真空状态中撕裂出来。这些光子从黑洞外被观测到，作为连续的辐射流发射出去。黑洞就像灯泡一样发光。不幸的是，这个真正惊人的预测几乎没有希望直接从天体物理学黑洞中得到验证。“辉光”的温度极低，只有几十纳米开尔文的数量级，无法在高得多的宇宙背景温度中区分出来。幸运的是，就在30年前，威廉·安鲁（William Unruh）注意到，导致黑洞蒸发的相同论点也预测，从速度从亚音速到超音速变化的流动流体中应该发出声波的热谱。声波在亚声速区和超音速区之间的过渡阶段仍将被阻挡，在这一过渡阶段，无论如何，都相当于视界。现在证明，视界显然比人们想象的要普遍得多。它们出现在流入水槽的自来水中，也出现在许多以水或液体为基础的场景中；它们出现在流动的玻色-爱因斯坦凝聚体中，在极化子凝聚体中，最重要的是，在这个项目中，在运动的介电介质中。我们可以想象以接近光速的速度移动一个透明的玻璃样品。那么我们就会有一种类似于声波在流动流体中的情况：在存在从亚光速到超光速的过渡时，光波将无法移动超过视界点，在这个视界点上，介质速度正好等于光的相速度。其中一位pi （U. Leonhardt）最近提出了一种巧妙的方法，以一种非常简单的方式实现这种视野。在玻璃中传播的强激光脉冲会在与脉冲一起传播的折射率中产生局部扰动，即它自然地以光速传播。任何接近扰动的光波都会因局部折射率的增加而变慢，并最终被阻挡在视界上，超过视界它将永远无法传播。利用这个非常简单的建议，另一个项目PI （D. Faccio）获得了介电水平诱导自发光子发射的第一个证据。这种扰动通过释放真空状态激发的光子而发光和蒸发，就像霍金预言的黑洞应该做的那样。本项目旨在推广这些成果，并将霍金辐射和视界相关效应的研究提升到一个新的水平。我们现在能够计划真正的实验，这将首次为我们提供描述视界如何与量子真空相互作用的真实数据。此外，霍金辐射的核心是一种新的放大机制，由于缺乏任何先前的实验可能性，以前从未真正研究过。这种新的放大通道将被研究并用于光的放大。他们的目标是创造第一个黑洞激光器，其中的光被困在两个独立的视界之间。它在每次反弹时被放大，最后在类似激光的放大过程中呈指数级爆炸。因此，这个项目的影响远远超出了霍金效应的研究，并投资了许多领域，从量子场论到非线性光学和光子技术。

Black holes are incredibly fascinating objects. They largely populate the Universe we live in, attracting whole galaxies around them. They also attract the imagination of novel writers and scientists alike: they represent the ultimate frontier at which our knowledge and intellect can be put to the test. In 1974 Stephen Hawking, building upon suggestions that black holes have a finite temperature, predicted that the event horizon surrounding a black hole separates regions characterized by such an intense space-time distortion that photons and particles are literally ripped out of vacuum state. These photons are then seen from outside the black hole to be emitted as a continuous flux of radiation. Black holes glow, just as if they were light bulbs. Unfortunately, this truly amazing prediction has little hope of being verified directly from astrophysical black holes. The "glow" has an extremely low temperature, of the order of tens of nano-Kelvins and cannot be distinguished amongst the much higher cosmic background temperature.Fortunately, exactly 30 years ago, William Unruh noted that the same arguments that lead to black hole evaporation also predict that a thermal spectrum of sound waves should be given out from a flowing fluid whose velocity is made to vary from sub-sonic to super-sonic velocities. Sound waves will remain blocked at the transition between the sub- and super-sonic regions at what, to all effects, is the analogue of an horizon. It now turns out that horizons are apparently far more common than one may imagine. They appear in flowing tap water as it hits the sink and in a number of water or liquid based scenarios; they appear in flowing Bose-Einstein-Condensates, in polariton condensates and, most importantly for what concerns this project, in moving dielectric media. We may imagine moving a transparent glass sample at velocities close to that of light. We would then have a situation analogous to that of sound waves in a moving fluid: in the presence of a transition from sub-luminal to super-luminal speeds, light waves will not be able to move beyond the horizon point at which the medium velocity is exactly equal to the phase velocity of light. One of the PIs (U. Leonhardt) recently proposed an ingenious method to achieve such horizons in a very simple manner. An intense laser pulse propagating in glass will create a local perturbation in the refractive index that travels together with the pulse, i.e. it naturally travels at light speeds. Any light wave approaching the perturbation will be slowed down by the local increase in refractive index and will eventually be blocked at the horizon beyond which it will be never be able to propagate. Using this very simple proposal, the other project PI (D. Faccio) obtained the first evidence of spontaneous photon emission induced by the dielectric horizon. The perturbation is glowing and evaporating by shedding photons excited from the vacuum state, just as Hawking predicted black holes should do. This project aims at taking forth these results and taking studies on Hawking emission and horizon related effects to the next level. We are now able to plan real experiments that can give us for the first time real data describing how horizons interact with the quantum vacuum. Moreover, at the heart of Hawking emission lies a novel amplification mechanism that, due to the lack of any previous experimental possibilities, has never been truly investigated before. This new amplification channel will be studied and used to amplify light. The goal in mind is to create the first black hole laser in which light is trapped in between two separate horizons. Bouncing back and forth it is amplified at each rebound and finally exponentially explodes in laser-like amplification process. The impact of this project therefore goes well beyond investigation of Hawking effects and invests a number of fields, ranging from quantum field theories to nonlinear optics and photonic technologies.