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Mass is commonly regarded as an intrinsic property of matter, but modern physics reveals particle masses to have complex origins, such as the Higgs mechanism in high-energy physics. In crystal lattices such as graphene, relativistic Dirac particles can exist as low-energy quasiparticles with masses imparted by lattice symmetry-breaking perturbations. These mass-generating mechanisms all assume Hermiticity, or the conservation of energy in detail. Using a photonic synthetic lattice, we show experimentally that Dirac masses can be generated via non-Hermitian perturbations based on optical gain and loss. We then explore how the space-time engineering of the gain/loss-induced Dirac mass affects the quasiparticles. As we show, the quasiparticles undergo Klein tunnelling at spatial boundaries, but a local breaking of a non-Hermitian symmetry can produce a novel flux nonconservation effect at the domain walls. At a temporal boundary that abruptly flips the sign of the Dirac mass, we observe a variant of the time reflection phenomenon: in the nonrelativistic limit, the Dirac quasiparticle reverses its velocity, while in the relativistic limit the original velocity is retained.

In the classical Hong-Ou-Mandel (HOM) effect pairs of photons with bosonic (fermionic) spatial wavefunction coalesce (anti-coalesce) when mixed on a lossless beamsplitter. Here we report that the presence of dissipation in the beamsplitter allows the observation of the anti-HOM effect, where bosons anti-coalesce and fermions show coalescent-like behavior. We provide an experimental demonstration of the anti-HOM effect for both bosonic and fermionic two-photon entangled states. Beyond its fundamental significance, the anti-HOM effect offers applications in quantum information and metrology where states of entangled photons are dynamically converted., Comment: 10 pages, 4 figures

Multimode fibers (MMFs) have the potential to carry complex images for endoscopy and related applications, but decoding the complex speckle patterns produced by mode-mixing and modal dispersion in MMFs is a serious challenge. Several groups have recently shown that convolutional neural networks (CNNs) can be trained to perform high-fidelity MMF image reconstruction. We find that a considerably simpler neural network architecture, the single hidden layer dense neural network, performs at least as well as previously-used CNNs in terms of image reconstruction fidelity, and is superior in terms of training time and computing resources required. The trained networks can accurately reconstruct MMF images collected over a week after the cessation of the training set, with the dense network performing as well as the CNN over the entire period., Comment: 17 pages, 10 figures

Coherent optical fibre networks are extremely sensitive to thermal, mechanical and acoustic noise, which requires elaborate schemes of phase stabilization with dedicated auxiliary lasers, multiplexers and photodetectors. This is particularly demanding in quantum networks operating at the single-photon level. Here we propose a simple method of phase stabilization based on single-photon counting and apply it to quantum fibre networks implementing single-photon interference on a lossless beamsplitter and coherent perfect absorption on a metamaterial absorber. As a proof of principle, we show dissipative single-photon switching with visibility close to 80%. This method can be employed in quantum networks of greater complexity without classical stabilization rigs, potentially increasing efficiency of the quantum channels., Comment: 4 pages, 4 figures

We study the lensing effect experienced by a weak probe field under conditions of electromagnetically induced transparency (EIT) involving a Rydberg state. A Gaussian coupling beam tightly focused on a laser-cooled atomic cloud produces an inhomogeneity in the coupling Rabi frequency along the transverse direction and makes the EIT area acting like a gradient-index medium. We image the probe beam at the position where it exits the atomic cloud, and observe that a red-detuned probe light is strongly focused with a greatly enhanced intensity whereas a blue-detuned one is de-focused with a reduced intensity. Our experimental results agree very well with the numerical solutions of Maxwell-Bloch equations., Comment: 7 pages, 4 figures

A laser-diode-pumped intracavity frequency-doubled Nd:YAP/KTP laser is presented. Over 110 mw of TEM00 single-frequency output power at 540-nm wavelength was obtained. The output green laser was employed to pump a semimonolithic nondegenerate optical parametric oscillator to produce intensity quantum correlated twin beams at 1080 nm, and the maximum quantum noise squeezing of 74 %(5.9 dB) on the intensity difference fluctuation between the twin beams is observed. The threshold was reduced and the stability was increased significantly when compared with similar lamp-pumped systems., Comment: Published on April 2002