Counterpropagating twin photons by parametric fluorescence

Efficient Frequency Mixing of Guided Surface Waves by Atomically Thin Nonlinear Crystals

Monolayer transition metal dichalcogenides possess considerable second-order nonlinear coefficients but a limited efficiency of frequency conversion due to the short interaction length with light under the typical direct illumination. Here, we demonstrate an efficient frequency mixing of the guided surface waves on a monolayer tungsten disulfide (WS₂) by simultaneously lifting the temporal and spatial overlap of the guided wave and the nonlinear crystal. Three orders-of-magnitude enhancement of the conversion efficiency was achieved in the counter-propagating excitation configuration. Also, the frequency-mixing signals are highly collimated, with the emission direction and polarization controlled, respectively, by the pump frequencies and the rotation angle of WS₂ relative to the propagation direction of the guided waves. These results indicate that the rules of nonlinear frequency conversion are applicable even when the crystal is scaled down to the ultimate single-layer limit. This study provides a versatile platform to enhance the nonlinear optical response of 2D materials and favor the scalable generation of a coherent light source and entangled photon pairs.

Photonic crystal waveguides as sources of counterpropagating factorizable biphoton states

We demonstrate numerically that photonic crystal slab waveguides can generate spectrally unentangled biphoton states, highly desired for heralding of single photons. We achieve this by modally phase matching a counterpropagating spontaneous parametric down-conversion process, in a fully integrated scheme and without the need for periodic poling. Such a configuration is an ideal integrated source of heralded single photons, as it spatially separates the photons of a pair at the source without any extra components, while allowing for generation of spectrally narrow photons on a very short length scale.

Generation of Counterpropagating Path-Entangled Photon Pairs in a Single Periodic Waveguide

We propose the use of nonlinear periodic waveguides for direct and fully integrated generation of counterpropagating photon pairs by spontaneous parametric down-conversion. Using the unique properties of Bloch modes in such periodic structures, we furthermore show that two counterpropagating phase-matching conditions can be fulfilled simultaneously, allowing for the generation of path-entangled Bell states in a single periodic waveguide. To demonstrate the feasibility of our proposal, we design a photonic crystal slab waveguide made of lithium niobate and numerically demonstrate Bell-state generation.

Direct Bell states generation on a III-V semiconductor chip at room temperature

We demonstrate the direct generation of polarization-entangled photon pairs at room temperature and telecom wavelength in an AlGaAs semiconductor waveguide. The source is based on spontaneous parametric down-conversion with a counterpropagating phase-matching scheme. The quality of the two-photon state is assessed by the reconstruction of the density matrix giving a raw fidelity to a Bell state of 0.83; a theoretical model, taking into account the experimental parameters, provides ways to understand and control the amount of entanglement. Its compatibility with electrical injection, together with the high versatility of the generated two-photon state, make this source an attractive candidate for completely integrated quantum photonics devices.

Time-reversed optical parametric oscillation

In recent works, the idea of time-reversed laser oscillation has been proposed and demonstrated to realize a two-channel coherent perfect absorber [Y. D. Chong et al., Phys. Rev. Lett. 105, 053901 (2010); W. Wan et al., Science 331, 889 (2011)]. Here the time reversal of optical parametric oscillation in a nonlinear χ(2) medium is considered and shown to realize a coherent perfect absorber for colored incident signal and idler fields. A detailed analysis is presented for the time-reversed process of mirrorless optical parametric oscillation in the full nonlinear regime.

Two-photon interference with a semiconductor integrated source at room temperature

We experimentally demonstrate an integrated semiconductor ridge microcavity source of counterpropagating twin photons at room temperature in the telecom range. Based on type II parametric down conversion with a counterpropagating phase-matching, pump photons generate photon pairs with an efficiency of about 10(-11) and a spectral linewidth of 0.3 nm for a 1 mm long sample. The indistiguishability of the photons of the pair is measured via a Hong-Ou-Mandel two-photon interference experiment showing a visibility of 85 %. This work opens a route towards new guided-wave semiconductor quantum devices.

Shaping the waveform of entangled photons

We demonstrate experimentally tunable control of the joint spectrum, i.e., waveform and degree of frequency correlations, of paired photons generated in spontaneous parametric down-conversion. This control is mediated by the spatial shape of the pump beam in type-I noncollinear configurations.

Semiconductor waveguide source of counterpropagating twin photons

We experimentally demonstrate an integrated semiconductor source of counterpropagating twin photons in the telecom range. A pump beam impinging on top of an AlGaAs waveguide generates parametrically two counterpropagating, orthogonally polarized signal/idler guided modes. A 2 mm long waveguide emits at room temperature one average photon pair per pump pulse, with a spectral linewidth of 0.15 nm. The twin character of the emitted photons is ascertained through a time-correlation measurement. This work opens a route towards new guided-wave semiconductor quantum devices.

Enhancing the axial resolution of quantum optical coherence tomography by chirped quasi-phase matching

We show theoretically that a chirped quasi-phase-matching nonlinear crystal structure can significantly enhance the axial resolution in quantum optical coherence tomography by increasing the spectral width of the generated entangled photon pairs. For pulsed pumps we show how the pump-pulse duration affects the maximum resolution attainable.

Orbital angular momentum of entangled counterpropagating photons

We elucidate the paraxial orbital angular momentum of entangled photon pairs generated by spontaneous parametric downconversion in different noncollinear geometries in which the entangled photons counterpropagate. We find, in particular, the orbital angular momentum of entangled pairs generated in transverse-emitting configurations, in which none of the known rules for selecting orbital angular momentum holds.