arXiv subjects

L. G. Helt

Publications and source records attributed to L. G. Helt.

24 records · Page 2Linked to original sources

arXiv2014

Bi-photon spectral correlation measurements from a silicon nanowire in the quantum and classical regimes

The growing requirement for photon pairs with specific spectral correlations in quantum optics experiments has created a demand for fast, high resolution and accurate source characterization. A promising tool for such characterization uses the classical stimulated process, in which an additional seed laser stimulates photon generation yielding much higher count rates, as recently demonstrated for a $χ^{(2)}$ integrated source in A.~Eckstein \emph{et al.}, Laser Photon. Rev. \textbf{8}, L76 (2014). In this work we extend these results to $χ^{(3)}$ sources, demonstrating spectral correlation measurements via stimulated four-wave mixing for the first time in a integrated optical waveguide, namely a silicon nanowire. We directly confirm the speed-up due to higher count rates and demonstrate that additional resolution can be gained when compared to traditional coincidence measurements. As pump pulse duration can influence the degree of spectral entanglement, all of our measurements are taken for two different pump pulse widths. This allows us to confirm that the classical stimulated process correctly captures the degree of spectral entanglement regardless of pump pulse duration, and cements its place as an essential characterization method for the development of future quantum integrated devices.

physics.optics↗

arXiv2014

Spontaneous parametric downconversion in waveguides: What's loss got to do with it?

We derive frequency correlation and exit probability expressions for photons generated via spontaneous parametric downconversion (SPDC) in nonlinear waveguides that exhibit linear scattering loss. Such loss is included within a general Hamiltonian formalism by connecting waveguide modes to reservoir modes with a phenomenological coupling Hamiltonian, the parameters of which are later related to the usual loss coefficients. In the limit of a low probability of SPDC pair production, the presence of loss requires that we write the usual lossless generated pair state as a reduced density operator, and we find that this density operator is naturally composed of two photon, one photon, and zero photon contributions. The biphoton probability density, or joint spectral intensity (JSI), associated with the two-photon contribution is determined not only by a phase matching term, but also by a loss matching term. The relative size of the loss coefficients within this term lead to three qualitatively different regimes of SPDC JSIs. If either the pump or generated photon loss is much higher than the other, the side lobes of the phase matching squared sinc function are washed out. On the other hand, if pump and generated photon loss are appropriately balanced, the lossy JSI is identical to the lossless JSI. Finally, if the generated photon loss is frequency dependent, the shape of the JSI can be altered more severely, potentially leading to generated photons that are less frequency correlated though also produced less efficiently when compared to photons generated in low-loss waveguides.

quant-ph↗

arXiv2014

Tuneable quantum interference in a 3D integrated circuit

Integrated photonics promises solutions to questions of stability, complexity, and size in quantum optics. Advances in tunable and non-planar integrated platforms, such laser-inscribed photonics, continue to bring the realisation of quantum advantages in computation and metrology ever closer, perhaps most easily seen in multi-path interferometry. Here we demonstrate control of two-photon interference in a chip-scale 3D multi-path interferometer, showing a reduced periodicity and enhanced visibility compared to single photon measurements. Observed non-classical visibilities are widely tunable, and explained well by theoretical predictions based on classical measurements. With these predictions we extract a Fisher information approaching a theoretical maximum, demonstrating the capability of the device for quantum enhanced phase measurements.

quant-ph↗

arXiv2013

Parasitic nonlinearities in photon pair generation via integrated spontaneous four-wave mixing: critical problem or distraction?

We consider integrated photon pair sources based on spontaneous four-wave mixing and derive expressions for the pump powers at which various nonlinear processes become relevant for a variety of source materials and structures. These expressions serve as rules of thumb in identifying reasonable parameter regimes for the design of such sources. We demonstrate that if pump powers are kept low enough to suppress cross-phase modulation, multi-pair events as well as many other nonlinear effects are often also constrained to negligible levels.

quant-ph↗

arXiv2012

Ultra-low power generation of twin photons in a compact silicon ring resonator

We demonstrate efficient generation of correlated photon pairs by spontaneous four wave mixing in a 5 μm radius silicon ring resonator in the telecom band around 1550 nm. By optically pumping our device with a 200 μW continuous wave laser, we obtain a pair generation rate of 0.2 MHz and demonstrate photon time correlations with a coincidence-to-accidental ratio as high as 250. The results are in good agreement with theoretical predictions and show the potential of silicon micro-ring resonators as room temperature sources for integrated quantum optics applications.

physics.optics↗

arXiv2012

From Classical Four-Wave Mixing to Parametric Fluorescence in Silicon micro-ring resonators

Four-wave mixing can be stimulated or occur spontaneously. The first process is intrinsically much stronger, and well understood through classical nonlinear optics. The latter, also known as parametric fluorescence, can be explained only in the framework of a quantum theory of light. We experimentally demonstrate that, in a micro-ring resonator, there exists a simple relation between the efficiencies of these two processes, which is independent of the nonlinearity and size of the ring. In particular we show that the average power generated by parametric fluorescence can be immediately estimated from a classical FWM experiment. These results suggest that classical nonlinear characterization of a photonic integrated structure can provide accurate information on its nonlinear quantum properties.

quant-ph↗