The physics at the event horizon resembles the behavior of waves in moving media. Horizons are formed where the local speed of the medium exceeds the wave velocity. We use ultrashort pulses in microstructured optical fibers to demonstrate the formation of an artificial event horizon in optics. We observed a classical optical effect, the blue-shifting of light at a white-hole horizon. We also show by theoretical calculations that such a system is capable of probing the quantum effects of horizons, in particular Hawking radiation.

We have studied dissipation in a Bose-Einstein condensed gas by moving a blue detuned laser beam through the condensate at different velocities. Strong heating was observed only above a critical velocity.PACS 03.75.Fi, 67.40.Vs,67.57.De Macroscopic quantum coherence and collective excitations are key features in our understanding of the phenomenon of superfluidity. The superfluid velocity is proportional to the gradient of the phase of a macroscopic wavefunction. Collective excitations determine a critical velocity below which the flow is dissipationless. This velocity is given by Landau's criterion [1],where ε is the energy of an excitation with momentum p. [4]. Previous work has explored some aspects related to superfluidity such as the macroscopic phase [5] and the phonon nature of low-lying collective excitations [4,6]. In this Letter we report on the measurement of a critical velocity for the excitation of a trapped BoseEinstein condensate. In analogy with the well known argument by Landau and the vibrating wire experiments in superfluid helium [7], we study dissipation when an object is moved through the fluid. Instead of a massive macroscopic object we used a blue detuned laser beam which repels atoms from its focus to create a moving boundary condition.The experiment was conducted in a new apparatus for the production of Bose-Einstein condensates of sodium atoms. The cooling procedure is similar to previous work [8]-the new features have been described elsewhere [9]. Briefly, laser cooled atoms were transferred into a magnetic trap in the Ioffe-Pritchard configuration and further cooled by rf evaporative cooling for 20 seconds, resulting in condensates of between 3 and 12 ×10 6 atoms. After the condensate was formed, we reduced the radial trapping frequency to obtain condensates which were considerably wider than the laser beam used for stirring. This decompression was not perfectly adiabatic, and heated the cloud to a final condensate fraction of about 60%. The final trapping frequencies were ν r = 65 Hz in the radial and ν z = 18 Hz in the axial direction. The resulting condensate was cigar-shaped with Thomas-Fermi diameters of 45 and 150 µm in the radial and axial directions, respectively. The final chemical potential, transition temperature T c and peak density n 0 of the condensate were 110 nK, 510 nK and 1.5 × 10 14 cm −3 , respectively.The laser beam for stirring the condensate had a wavelength of 514 nm and was focused to a Gaussian 1/e 2 beam diameter of 2w = 13µm. The repulsive optical dipole force expelled the atoms from the region of highest laser intensity. A laser power of 400 µW created a 700 nK barrier resulting in a cylindrical hole ∼ 13µm in diameter within the condensate. The laser barrier created a soft boundary, since the Gaussian beam waist was more than 10 times wider than the healing length ξ = (8πan 0 ) −1/2 = 0.3µm, a being the two-body scattering length. The laser was focused on the center of the cloud. Using an acousto-optic deflector, it was scanned back and forth along t...

Optical solitons or solitonlike states shed light to blueshifted frequencies through a resonant emission process. We predict a mechanism by which a second propagating mode is generated. This mode, called negative resonant radiation, originates from the coupling of the soliton mode to the negative-frequency branch of the dispersion relation. Measurements in both bulk media and photonic-crystal fibers confirm our predictions.

We have generated a new type of biphoton state by cavity-enhanced down-conversion in a type-II phase-matched, periodically-poled KTiOPO4 (PPKTP) crystal. By introducing a weak intracavity birefringence, the polarization-entangled output was modulated between the singlet and triplet states according to the arrival-time difference of the signal and idler photons. This cavity-enhanced biphoton source is spectrally bright, yielding a single-mode fiber-coupled coincidence rate of 0.7 pairs/s per mW of pump power per MHz of down-conversion bandwidth. Its novel biphoton behavior may be utilized in sensitive measurements of weak intracavity birefringence. • , in a ring cavity to produce polarization-entangled photons. We will report the first cavity-enhanced operation of a cw type-II down-converter, resulting in a spectrally bright, narrowband source of frequency-degenerate polarizationentangled photon pairs. More importantly, by controlling a weak intracavity birefringence, our source generates a new type of biphoton state whose polarizationentangled output is modulated between the singlet and triplet states according to the arrival-time difference between the signal and idler photons.Consider conventional cw SPDC in a collinear configuration that is type-II phase matched at frequency degeneracy. The post-selected biphoton state emerging from a 50-50 beam splitter placed after the usual timing-compensation crystal is then |ψ = (|H 1 |V 2 + e iφ |V 1 |H 2 )/ √ 2, in the horizontal-vertical (H-V ) basis, where the subscripts label the beam splitter output ports. Ordinarily we have φ = 0, so |ψ is a triplet, but a half-wave plate in one of the output paths can transform |ψ to a singlet by making φ = π. The situation becomes more complicated-and more interestingwhen that down-converter is embedded in a single-ended cavity that resonates the signal and idler. If the signal and idler photons resulting from down-conversion of a pump photon emerge from the output coupler after the same number of roundtrips within the cavity, then they yield a triplet for the post-selected biphoton * Electronic address: chrisk@alum.mit.edu state. However, the times at which these photons leave the cavity may differ by integer multiples of the cavity roundtrip time, τ c . Taking τ c as a natural time-bin unit for the system, we have that the biphoton state associated with a photon pair whose arrival-time difference is mτ c is |ψ = (|H 1 |V 2 + e imφ |V 1 |H 2 )/ √ 2, where φ is the roundtrip cavity birefringence. By tuning the cavity birefringence to achieve φ = 0-something that is easily done with type-II phase matching-we get a new type of biphoton, which exhibits time-bin modulated polarization entanglement. When φ = π and m is even, this biphoton is a triplet; when φ = π and m is odd, it is a singlet. Before describing our experimental work, we shall provide a more precise characterization of cavityenhanced type-II SPDC.Consider the cw down-conversion configuration shown in Fig. 1. A length-L KTP intracavity compensating crystal (ICC) and a ...