ACTIVE QUANTUM CIRCUITS: INTEGRATED PHOTON PAIR GENERATION AND QUANTUM WALKS

Abstract

We demonstrate experimentally simultaneous photon-pair generation and quantum walks in a lithium niobate waveguide array, where the output photon correlations can be controlled by varying the temperature-dependent phase-matching, switching from classical to quantum statistics. Integrated optical quantum circuits are increasingly gaining attention as a solution for scalable quantum technologies with important application to quantum simulations [1]. A key mechanism for such quantum simulations can be provided by the process of quantum walks, which in quantum optics is enabled in an array of coupled optical waveguides. However, in all integrated quantum walk schemes to date, the correlated photon pairs have been generated externally to the waveguide array (WGA), making it only a passive circuit. Here we experimentally demonstrate an active quantum circuit, where the WGA is used for both photonpair generation and quantum walks of the generated photons. We show that the output photon statistics can be controlled classically by changing the phase matching conditions for photon-pair generation, which allows for controllable engineering of the spatial quantum correlations exhibited through a quantum walk [4]. Importantly, this integrated scheme also allows avoiding input losses from the state generation to the waveguides. This is crucial as input losses in devices with linear coupled waveguides currently limit feasible experimental data acquisition times and hinder possible applications. In our experiments we use an x-cut LiNbO3 WGA [see Fig. 1(a)] fabricated by titanium indiffusion [5], which consists of 40 waveguides with a spacing of 14mm. A single waveguide of the WGA is excited by a continuous wave (CW) solid state laser at a wavelength of 671nm [see Fig. 1(b)]. The WGA is kept at a temperature of ~360C (with an accuracy of 0.1C) to phase-match the Type-I (oo-e) spontaneous parametric down-conversion (SPDC) process. At the pump wavelength the mode is well confined to the central waveguide and does not show evanescent coupling to the neighboring waveguides [see Fig. 1(a)]. However, for the generated low-frequency SPDC photons the waveguide modes extend towards the neighboring waveguides and can couple through the array experiencing quantum walks. Thus the generated output photon pairs demonstrate nontrivial photon statistics. Experimentally, we measure the photon-pair correlations at a wavelength of 1342 ± 6nm (filtered by an interference filter) on two cooled InGaAs single photon detectors (IDQuantique 210) in free-running mode [see Fig. 1(b)]. The non-polarizing beamsplitter is used to separate two photons of a pair with 50% efficiency, while irises allow for individual waveguide output selection. The resulting correlation measurements are shown in Fig. 2(a), where the coincidence counts are normalized with respect to the coupling efficiency. We observe that for a sample temperature of T = 364.5C close to degenerate SPDC phase-matching, photons from down-converted pairs leave the WGA from the same waveguide {-1,-1}, {0,0} or {1,1} (here two numbers correspond to two SPDC photons), defined as bunching, or from the opposite waveguides {-1,1} or {1,-1} defined as antibunching. The highest coincidence rate is achieved in the {0,0} case, since photon pairs are generated in waveguide 0 along the whole length of the WGA. Photon pairs coming from the other waveguides, namely {-1,0}, {1,0}, {0,-1} and {0,1} lead to less coincidences. This means that probabilities of photon bunching and antibunching are simultaneously dominant, which implies a highly non-trivial Paper code: Fig. 1. (a) Schematic representation of LiNbO3 WGA with pump (blue) coupled to a central waveguide and SPDC photon pairs (red) generated in the central waveguide and propagating in the WGA in the regime of quantum walks. (b) Schematic of the experimental setup. entangled state [3]. The theoretical quantum simulations of the process based on Ref. [4] [Fig. 2(b)] are in good agreement with our experiment. To demonstrate the reconfigurability of the device we also measure the correlation of the antibunched photons in waveguide {-1,1} as the temperature is varied, shown in Fig. 2(c). The theory developed for WGAs (blue curve), taking into account structural and temperature-related inhomogeneities in the sample, fits well the experimental data (red circles with error bars). This demonstrates that changing the temperature affects the coincidence counts from various waveguides, allowing control of the output photon statistics and revealing a route to reconfigurable quantum simulations based on walks. To conclude, our work opens a path for implementing combined photon-pair generation and quantum walks for engineering of spatial quantum states in novel active quantum integrated circuits.