Hawking Radiation in Dielectric Horizon Analogues

黑洞是令人难以置信的迷人物体。它们在很大程度上占据了我们生活的宇宙，吸引了整个星系围绕它们。它们也吸引了小说家和科学家的想象力：它们代表了我们的知识和智力可以接受考验的最终前沿。1974年，斯蒂芬·霍金基于黑洞具有有限温度的假设，预测黑洞周围的事件视界将以强烈时空扭曲为特征的区域分开，光子和粒子实际上被从真空状态中撕裂出来。然后，这些光子从黑洞外部被视为作为连续的辐射流发射。黑洞会发光，就像灯泡一样。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。这种“辉光”具有极低的温度，只有几十纳开尔文的量级，在高得多的宇宙背景温度中无法区分。幸运的是，就在30年前，William Unruh指出，导致黑洞蒸发的相同论点也预测，声波的热谱应该从流动的流体中发出，其速度从亚音速到超音速不等。声速声波将在亚音速和超音速区域之间的过渡处被阻挡，在所有的效果上，这是一个类似于视界的地方。现在看来，地平线显然比人们想象的要普遍得多。它们出现在流动的自来水中，因为它击中了水槽，并在许多水或液体的情况下;它们出现在流动的玻色爱因斯坦凝聚体中，极化子凝聚体中，最重要的是，对于这个项目来说，在移动的电介质中。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。这样，我们就有了一种类似于运动流体中的声波的情况：在存在从亚光速到超光速的转变时，光波将无法移动到视界点之外，在视界点处介质速度正好等于光的相速度。其中一个PI（U。Leonhardt）最近提出了一种巧妙的方法，以非常简单的方式实现这样的视野。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，即它自然地以光速传播。任何接近微扰的光波都将被折射率的局部增加所减慢，并最终被阻挡在视界处，超过视界就永远无法传播。使用这个非常简单的建议，另一个项目PI（D。Faccio）首次获得了介电视界诱导的自发光子发射的证据。扰动通过从真空态激发的光子脱落而发光和蒸发，就像霍金预测的黑洞应该做的那样。本项目旨在利用这些成果，将霍金辐射和视界相关效应的研究推向新的水平。我们现在能够计划真实的实验，这些实验可以第一次给我们描述视界如何与量子真空相互作用的真实的数据。此外，霍金辐射的核心是一种新的放大机制，由于缺乏任何以前的实验可能性，以前从未真正研究过。这种新的放大通道将被研究并用于放大光。我们的目标是创造第一个黑洞激光器，其中光被困在两个独立的视界之间。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呈指数级爆炸。因此，该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.