Hawking Radiation in Dielectric Horizon Analogues

黑洞是令人难以置信的迷人物体。它们主要分布在我们生活的宇宙中，吸引着周围的整个星系。它们还吸引了小说作家和科学家的想象力：它们代表了我们的知识和智力可以受到考验的终极前沿。1974年，史蒂芬·霍金基于黑洞有有限温度的假设，预测黑洞周围的事件视界将具有如此强烈的时空扭曲特征的区域分开，以至于光子和粒子实际上被撕裂出真空状态。然后从黑洞外部可以看到这些光子以连续辐射通量的形式发射出来。黑洞发光，就像它们是灯泡一样。不幸的是，这一真正令人惊叹的预测几乎没有希望直接从天体物理黑洞中得到验证。这种“辉光”的温度极低，约为几十纳开尔文量级，无法与宇宙背景温度高得多的温度区分开来。幸运的是，整整30年前，威廉·昂鲁指出，导致黑洞蒸发的相同论点也预测到，流动的流体应该发出声波的热谱，其速度从亚音速到超音速变化。声波将在亚音速和超音速区域之间的过渡处保持阻挡，在所有影响下，都是类似于地平线。现在事实证明，地平线显然比人们想象的要普遍得多。当流动的自来水到达水槽时，它们出现在流动的自来水中，在许多基于水或液体的场景中；它们出现在流动的玻色-爱因斯坦凝聚物中，出现在极化子凝聚物中，最重要的是，出现在移动的介电介质中，这与本项目有关。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。然后，我们会有一个类似于声波在运动流体中的情况：当存在从亚光速到超光速的转变时，光波将不能超过介质速度与光的相速度完全相同的地平点。其中一位PI(U·莱昂哈特)最近提出了一种巧妙的方法，以非常简单的方式实现这种地平线。在玻璃中传播的强激光脉冲将在与脉冲一起传播的折射率中产生局部扰动，即它自然以光速传播。任何接近微扰的光波都将被局部折射率的增加减慢，最终将被阻挡在它永远无法传播的地平线上。使用这个非常简单的方案，另一个项目Pi(D.Faccio)获得了介电视界诱导自发光子发射的第一个证据。正如霍金预测的那样，这种微扰正在通过脱离真空态激发的光子而发光和蒸发。本项目旨在推广这些成果，并将霍金辐射和地平线相关效应的研究提高到一个新的水平。我们现在能够计划真正的实验，第一次给我们提供描述地平线如何与量子真空相互作用的真实数据。此外，霍金发射的核心是一种新的放大机制，由于之前缺乏任何实验可能性，以前从未真正研究过。这种新的放大通道将被研究并用于放大光。人们考虑的目标是创造第一个黑洞激光器，其中光被困在两个不同的视界之间。它来回反弹，在每次反弹时被放大，最后在类似激光的放大过程中指数级爆炸。因此，该项目的影响远远超出了对霍金效应的研究，并投资了从量子场论到非线性光学和光子技术的多个领域。

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.