We measure the correlation spectrum of the Hawking radiation emitted by an analogue black hole and find it to be thermal at the Hawking temperature implied by the analogue surface gravity. The Hawking radiation is in the regime of linear dispersion, in analogy with a real black hole. Furthermore, the radiation inside of the black hole is seen to be composed of negative-energy partners only. This work confirms the prediction of Hawking's theory regarding the value of the Hawking temperature, as well as the thermality of the spectrum. The thermality of Hawking radiation is the root of the information paradox. The correlations between the Hawking and partner particles imply that the analogue black hole has no analogue firewall.It was a profound realization that the entropy of a black hole [1] and Hawking radiation [2,3] should have the same temperature, within a numerical factor on the order of unity. It was further asserted that Hawking radiation should have a thermal spectrum, which creates an information paradox [4,5]. Furthermore, it was proposed that the physics of Hawking radiation could be verified in an analogue system [6]. This proposal was carefully studied and developed theoretically [7][8][9][10][11][12][13][14][15][16][17][18][19]. Classical white and black-hole analogues were also studied experimentally [20][21][22][23], as well as a variety of other analogue gravitational systems [24][25][26][27][28][29][30]. The theoretical works, combined with our long-term study of this subject [14,[31][32][33][34], allowed for the observation of spontaneous Hawking radiation in an analogue black hole [35]. Several theoretical works studied our observation [35], and made predictions about the thermality and Hawking temperature [36][37][38][39][40]. During the years since our observation [35], we have made many improvements to the experimental apparatus. This allows us to study the thermality of the Hawking spectrum, and compare its temperature with the prediction given by the analogue of the surface gravity. In this work, we find that the spectrum of Hawking radiation agrees well with a thermal spectrum, and its temperature is given by Hawking's prediction.The analogue black hole consists of a flowing Bose-Einstein condensate. The flow velocity out in the region < 0 is less than the speed of sound out , as indicated in Fig. 1a. This region corresponds to the outside of the black hole. For > 0, the flow is supersonic ( in > in ),

We observe the time dependence of the Hawking radiation in an analogue black hole. Soon after the formation of the horizon, there is little or no Hawking radiation. The Hawking radiation then ramps up during approximately one period of oscillation, until it reaches the quantity expected for spontaneous emission. This is similar to a black hole created from gravitational collapse. The spectrum remains approximately constant at the spontaneous level for some time, similar to a stationary black hole. An inner horizon then forms, in analogy with a charged black hole. The inner horizon causes stimulated Hawking radiation. Both types of stimulation predicted by Ted Jacobson and coworkers likely contribute, but the monochromatic stimulation probably contributes more than does the black-hole lasing.

Entanglement is a central subject in quantum mechanics. Due to its genuine relativistic behavior and fundamental nature, high-energy colliders are attractive systems for the experimental study of fundamental aspects of quantum mechanics. We propose the detection of entanglement between the spins of top–antitop–quark pairs at the LHC, representing the first proposal of entanglement detection in a pair of quarks, and also the entanglement observation at the highest energy scale so far. We show that entanglement can be observed by direct measurement of the angular separation between the leptons arising from the decay of the top–antitop pair. The detection can be achieved with high statistical significance, using the current data recorded during Run 2 at the LHC. In addition, we develop a simple protocol for the quantum tomography of the top–antitop pair. This experimental technique reconstructs the quantum state of the system, providing a new experimental tool to test theoretical predictions. Our work explicitly implements canonical experimental techniques in quantum information in a two-qubit high-energy system, paving the way to use high-energy colliders to also study quantum information aspects.

We numerically study an analogue black hole with two horizons with similar parameters to a recent experiment. We find that the Hawking radiation exists on a background which contains a density oscillation, a zero-frequency ripple. The Hawking radiation evolves from spontaneous to self-amplifying, while the background ripple grows steadily with no qualitative change. It is seen that the self-amplifying Hawking radiation has a non-zero frequency. The background ripple appears even before the inner horizon is created, in contrast to predictions. This work is in agreement with the recent observation of selfamplifying Hawking radiation, and explains some of the features seen. In contrast to recent works, our study differentiates between the Hawking radiation observed, and the evolution of the background.Spontaneous Hawking radiation from a black hole with one horizon should have a thermal energy distribution [1,2]. This is also true for analogue black holes [3][4][5][6][7][8][9][10][11][12][13][14]. On the other hand, an analogue black hole with two horizons and a superluminal dispersion relation can exhibit selfamplifying Hawking radiation, or "black hole lasing" [15]. This phenomenon has been studied extensively theoretically [15][16][17][18][19][20][21][22] and experimentally [23]. The outer and inner horizons are analogous to the horizons in a charged black hole. The self-amplifying Hawking radiation appears as a growing standing wave between the horizons, oscillating with a single frequency. Due to the stochastic nature of the Hawking radiation, the standing wave is visible in the densitydensity correlation function computed from an ensemble of repetitions of the experiment.It was predicted by Jain, et al. [16] that the background density between the horizons contains a "ripple", a zero-frequency wave which is a feature of the stationary background flow. The selfamplifying Hawking radiation grows upon this background. Naturally, the ripple does not appear in the density-density correlation function since it does not fluctuate.

The violation of a classical Cauchy-Schwarz (CS) inequality is identified as an unequivocal signature of spontaneous Hawking radiation in sonic black holes. This violation can be particularly large near the peaks in the radiation spectrum emitted from a resonant boson structure forming a sonic horizon. As a function of the frequency-dependent Hawking radiation intensity, we analyze the degree of CS violation and the maximum violation temperature for a double barrier structure separating two regions of subsonic and supersonic condensate flow. We also consider the case where the resonant sonic horizon is produced by a space-dependent contact interaction. In some cases, CS violation can be observed by direct atom counting in a time-of-flight experiment. We show that near the conventional zero-frequency peak, the decisive CS violation cannot occur.