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Photonics Research

Photon-pair generation in a quadratically nonlinear parity-time symmetric coupler

Diana A. Antonosyan, Alexander S. Solntsev, Andrey A. Sukhorukov

Published Feb 12, 2018• 15 citations
Publisher

Abstract

Integrated nonlinear waveguide structures enable generation of quantum entangled photons. We describe theoretically the effects of spatially inhomogeneous loss on the creation of photon pairs through spontaneous parametric down-conversion in quadratically nonlinear directional couplers, where photons experience effective parity-time (PT) symmetric potential due to the presence of optical loss in one of the waveguides. We show that for losses below the PT-breaking threshold, the quantum photon states can be flexibly tuned similarly to conservative couplers, whereas for stronger losses, the correlations between two waveguide modes are suppressed. We also formulate a quantum-classical correspondence with sum-frequency generation for fast evaluation of device performance. These results can be applied for the design of quantum plasmonic circuits.

DOI
10.1364/prj.6.0000a6

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Integrated nonlinear waveguide structures enable generation of quantum entangled photons. We describe theoretically the effects of spatially inhomogeneous loss on the creation of photon pairs through spontaneous parametric down-conversion in quadratically nonlinear directional couplers, where photons experience effective parity-time (PT) symmetric potential due to the presence of optical loss in one of the waveguides. We show that for losses below the PT-breaking threshold, the quantum photon states can be flexibly tuned similarly to conservative couplers, whereas for stronger losses, the correlations between two waveguide modes are suppressed. We also formulate a quantum-classical correspondence with sum-frequency generation for fast evaluation of device performance. These results can be applied for the design of quantum plasmonic circuits.
We describe theoretically the process of spontaneous parametric down-conversion in quadratic nonlinear waveguide arrays in the presence of linear loss. We derive a set of discrete Schr\"odinger-type equations for the biphoton wave function and the wave function of one photon when the other photon in a pair is lost. We demonstrate effects arising from loss-affected interference between the generated photon pairs and show that nonlinear waveguide arrays can serve as a robust loss-tolerant integrated platform for the generation of entangled photon states with nonclassical spatial correlations.
Integrated optical quantum circuits based on photonic waveguiding structures are increasingly gaining attention as a possible solution for scalable quantum technologies with important applications to quantum simulations. Quantum communication provides secure information transmission, but the distance over which quantum states of light can be distributed without significant disturbance is limited due to inescapable losses and noise in optical elements. Loss is the greatest challenge facing the implementation of integrated photonic technologies, and it is inescapable in experimental reality. In recent years there is a rise of interest in structures with spatially inhomogeneous losses. Light propagation in waveguiding structures with spatially distributed sections of loss can be used for implementation of quantum plasmonic circuits, which are able to strongly confine light to sub-wavelength dimensions, as well as for parity-time (PT) symmetric structures, with phase transition associated with PT-symmetry breaking, which opens new possibilities for light manipulation. The PhD thesis contains research on the controllable classical and quantum dynamics of optical frequency conversion processes in quadratically nonlinear photonic integrated circuites in the presence of losses. Namely, I discuss spontaneous parametric down-conversion (SPDC), sum-frequency generation (SFG) and optical parametric amplification (OPA) in nonlinear structures governed by non-Hermition Hamiltonians. I explore the fundamental features of multi-photon generation in integrated nonlinear waveguides. I have been shown that arrays of coupled nonlinear waveguides can serve as a robust integrated platform for the generation of entangled photon states with nonclassical spatial correlations through spontaneous parametric down-conversion (SPDC), and that the operation of such quantum circuit is tolerant even to relatively high losses. Furthermore, I have studied the bi-photon multimode quantum emission tomography in waveguide structures with spatially inhomegeneous losses. The PhD thesis also covers the research on the effect of these losses in waveguide couplers possessing parity-time (PT) symmetry. I have identified an anti-PT spectral symmetry of a parametric amplifier based on those couplers. Finally, I describe the single-photon conversion to a photon pair, and identify opportunities for the efficient enhancement of this process.
We establish theoretically and demonstrate experimentally that photon-pair generation through spontaneous parametric down-conversion in a nonlinear waveguide with scattering or material losses can be effectively emulated by classical laser light propagation through a specially designed linear waveguide circuit. This platform can represent arbitrary photon and pump losses, with a potential for the emulation of non-Markovian decay. We characterize the photon-pair correlation spectrum and observe its characteristic transformation from the well-known sinc-shape in lossless waveguides towards a Lorentzian shape in the presence of photon loss.
The generation of multi-photon quantum states in integrated waveguiding structures provides a route to the creation of quantum optical chips. In particular, the simultaneous generation of photons through spontaneous parametric down-conversion (SPDC) and their quantum walks in nonlinear waveguide arrays (WGAs) can be used to produce strongly spatially entangled biphoton states [1] with applications for quantum search algorithms [2]. Whereas up to now only arrays of single-mode nonlinear waveguides have been considered for SPDC, in general the waveguides may support several guided modes, which were demonstrated to strongly affect nonlinear wave mixing in the classical regime [3]. Here we show that using higher order modes for SPDC has a significant influence on the generated two-photon states and gives additional means to control them. In particular, we employ the symmetry of the modes to engineer effectively nonlocal photon pair generation by SPDC.
We predict that all-optically-reconfigurable generation of photon pairs with tailored spatial entanglement can be realized via spontaneous parametric down conversion in integrated nonlinear coupled waveguides. The required elements of the output quantum wave function are directly mapped from the amplitudes and phases of the classical laser pump inputs in each waveguide. This is achieved through special nonuniform domain poling, which locally inverts the sign of quadratic nonlinear susceptibility and accordingly shapes the interference of biphoton quantum states generated along the waveguides. We demonstrate a device configuration for the generation of any linear combination of two-photon Bell states.
Integrated optical circuits enable stable and scalable realization of quantum logic devices, which can form a basis for the mass production of photonic chips for quantum communications and computations. An important challenge is the integration of single-photon sources, which should provide on-chip generation and preparation of quantum photon states. Spontaneous parametric down-conversion (SPDC) provides an attractive solution for experimental generation of correlated and entangled photon pairs [1,2]. However, internal losses in the waveguides may still be present, affecting nontrivially the emerging photon state [3].