Observation of thermal Hawking radiation and its temperature in an analogue black hole.

The entropy of a black hole1 and Hawking radiation2 should have the same temperature given by the surface gravity, within a numerical factor of the order of unity. In addition, Hawking radiation should have a thermal spectrum, which creates an information paradox3,4. However, the thermality should be limited by greybody factors5, at the very least6. It has been proposed that the physics of Hawking radiation could be verified in an analogue system7, an idea that has been carefully studied and developed theoretically8-18. Classical white-hole analogues have been investigated experimentally19-21, and other analogue systems have been presented22,23. The theoretical works and our long-term study of this subject15,24-27 enabled us to observe spontaneous Hawking radiation in an analogue black hole28. The observed correlation spectrum showed thermality at the lowest and highest energies, but the overall spectrum was not of the thermal form, and no temperature could be ascribed to it. Theoretical studies of our observation made predictions about the thermality and Hawking temperature29-33. Here we construct an analogue black hole with improvements compared with our previous setup, such as reduced magnetic field noise, enhanced mechanical and thermal stability and redesigned optics. We find that the correlation spectrum of Hawking radiation agrees well with a thermal spectrum, and its temperature is given by the surface gravity, confirming the predictions of Hawking's theory. The Hawking radiation observed is in the regime of linear dispersion, in analogy with a real black hole, and the radiation inside the black hole is composed of negative-energy partner modes only, as predicted.

Measurement of the Static Structure Factor in a Paraxial Fluid of Light Using Bragg-like Spectroscopy.

We implement Bragg-like spectroscopy in a paraxial fluid of light by imprinting analogues of short Bragg pulses on the photon fluid using wavefront shaping with a spatial light modulator. We report a measurement of the static structure factor, S(k), and we find a quantitative agreement with the prediction of the Feynman relation revealing indirectly the presence of pair-correlated particles in the fluid. Finally, we improve the resolution over previous methods and obtain the dispersion relation including a linear phononic regime for weakly interacting photons and low sound velocity.

Nonlinear elimination of spin-exchange relaxation of high magnetic moments.

Relaxation of the Larmor magnetic moment by spin-exchange collisions has been shown to diminish for high alkali densities, resulting from the linear part of the collisional interaction. In contrast, we demonstrate both experimentally and theoretically the elimination of spin-exchange relaxation of high magnetic moments (birefringence) in alkali vapor. This elimination originates from the nonlinear part of the spin-exchange interaction, as a scattering process of the Larmor magnetic moment. We find counterintuitively that the threshold magnetic field is the same as in the Larmor case, despite the fact that the precession frequency is twice as large.

Analogue cosmological particle creation in an ultracold quantum fluid of light.

The rapid expansion of the early universe resulted in the spontaneous production of cosmological particles from vacuum fluctuations, some of which are observable today in the cosmic microwave background anisotropy. The analogue of cosmological particle creation in a quantum fluid was proposed, but the quantum, spontaneous effect due to vacuum fluctuations has not yet been observed. Here we report the spontaneous creation of analogue cosmological particles in the laboratory, using a quenched 3-dimensional quantum fluid of light. We observe acoustic peaks in the density power spectrum, in close quantitative agreement with the quantum-field theoretical prediction. We find that the long-wavelength particles provide a window to early times. This work introduces the quantum fluid of light, as cold as an atomic Bose-Einstein condensate.

Repumping ground-state population in a coherently driven atomic resonance.

We experimentally demonstrate an optical pumping technique to pump a dilute rubidium vapor into the m(F) = 0 ground states. The technique utilizes selection rules that forbid the excitation of the m(F) = 0 states by linearly-polarized light. A substantial increase in the transparency contrast of the coherent-population-trapping resonance used for frequency standards is demonstrated.

Direct observation of a sub-Poissonian number distribution of atoms in an optical lattice.

We report single-site resolution in a lattice with tunneling between sites, allowing for an in situ study of stochastic losses. The ratio of the loss rate to the tunneling rate is seen to determine the number fluctuations, and the overall profile of the lattice. Sub-Poissonian number fluctuations are observed. Deriving the lattice beams from a microlens array results in perfect relative stability between beams.

Realization of a sonic black hole analog in a Bose-Einstein condensate.

We have created an analog of a black hole in a Bose-Einstein condensate. In this sonic black hole, sound waves, rather than light waves, cannot escape the event horizon. A steplike potential accelerates the flow of the condensate to velocities which cross and exceed the speed of sound by an order of magnitude. The Landau critical velocity is therefore surpassed. The point where the flow velocity equals the speed of sound is the sonic event horizon. The effective gravity is determined from the profiles of the velocity and speed of sound. A simulation finds negative energy excitations, by means of Bragg spectroscopy.

Direct observation of the phonon energy in a Bose-Einstein condensate by tomographic imaging.

The momentum and energy of phonons in a Bose-Einstein condensate are measured directly from a time-of-flight image by computerized tomography. We find that the same atoms that carry the momentum of the excitation also carry the excitation energy. The measured energy is in agreement with the Bogoliubov spectrum. Hydrodynamic simulations are performed which confirm our observation.