The development of large-scale optical quantum information processing circuits ground on the stability and reconfigurability enabled by integrated photonics. We demonstrate a reconfigurable 8×8 integrated linear optical network based on silicon nitride waveguides for quantum information processing. Our processor implements a novel optical architecture enabling any arbitrary linear transformation and constitutes the largest programmable circuit reported so far on this platform. We validate a variety of photonic quantum information processing primitives, in the form of Hong-Ou-Mandel interference, bosonic coalescence/anticoalescence and high-dimensional single-photon quantum gates. We achieve fidelities that clearly demonstrate the promising future for large-scale photonic quantum information processing using low-loss silicon nitride.

We investigate two-photon quantum interference in an opaque scattering medium that intrinsically supports 10 6 transmission channels. By adaptive spatial phase-modulation of the incident wavefronts, the photons are directed at targeted speckle spots or output channels. From 10 3 experimentally available coupled channels, we select two channels and enhance their transmission, to realize the equivalent of a fully programmable 2 × 2 beam splitter. By sending pairs of single photons from a parametric down-conversion source through the opaque scattering medium, we observe two-photon quantum interference. The programmed beam splitter need not fulfill energy conservation over the two selected output channels and hence could be non-unitary. Consequently, we have the freedom to tune the quantum interference from bunching (Hong-Ou-Mandel-like) to antibunching. Our results establish opaque scattering media as a platform for high-dimensional quantum interference that is notably relevant for boson sampling and physical-key-based authentication.PACS numbers: 42.50. Dv, 42.25.Dd, 42.50.Ex Light waves propagating through an opaque scattering medium exhibit a random walk inside the medium, which is caused by multiple scattering from spatial inhomogeneities [1]. An alternative description describes this by a transmission matrix [2,3]. The transmission matrix describes how a large amount of input channels is coupled to a similarly large amount of output channels, see Fig. 1. The number of these channels can be controlled, and easily made to exceed millions, by increasing the illuminated area on the medium. Recent advances in control of light propagation through complex wavefront shaping allow for complete control over these channels in multiple-scattering media [3][4][5]. Because of their large number of controllable channels, we explore the use of multiple-scattering media to study quantum interference between multiple photons. Employed as a platform for highdimensional quantum interference, over a large number of channels, multiple-scattering media are of relevance to boson sampling [6][7][8][9][10][11][12][13][14], quantum information processing [15][16][17][18], and physical-key-based authentication [19].It has previously been observed that quantum states are robust against multiple scattering. Correlations in two-photon speckle patterns in single-scattering media have been studied [20,21]. Further, propagation of quantum noise [22][23][24] and propagation of single-photon Fock states through multiplescattering media [25,26] have also been explored. So far it has remained an open question if quantum interference of multiple photons could be demonstrated inside a multiplescattering medium. A hurdle one might expect in an experimental implementation is the low transmission of almost all channels in the multiple-scattering medium. Remarklably, the transmission per channel is not necessarily low since complex wavefront shaping allows funneling of light into selected output modes [3,5].Here we report on an experiment in which we...

We propose and experimentally verify a method to program the effective transmission matrix of general multiport linear optical circuits in random multiple-scattering materials by phase modulation of incident wavefronts. We demonstrate the power of our method by programming linear optical circuits in white paint layers with 2 inputs and 2 outputs, and 2 inputs and 3 outputs. Using interferometric techniques we verify our ability to program any desired phase relation between the outputs. The method works in a deterministic manner and can be directly applied to existing wavefront-shaping setups without the need of measuring a transmission matrix or to rely on sensitive interference measurements. electronic Kerr and free-carrier effects in an ultimate-fast optically switched semiconductor microcavity," J. Opt.

We theoretically investigate quantum interference of two single photons at a lossy asymmetric beam splitter, the most general passive 2×2 optical circuit. The losses in the circuit result in a non-unitary scattering matrix with a non-trivial set of constraints on the elements of the scattering matrix. Our analysis using the noise operator formalism shows that the loss allows tunability of quantum interference to an extent not possible with a lossless beam splitter. Our theoretical studies support the experimental demonstrations of programmable quantum interference in highly multimodal systems such as opaque scattering media and multimode fibers.

We demonstrate a reconfigurable 8×8 photonic integrated circuit suitable for implementing universal gates for quantum information processing protocols. The processor is implemented as a square mesh of tunable beam splitters based on stoichiometric silicon nitride waveguides, containing 128 tunable elements. In order to demonstrate its versatility, we perform a variety of photonic quantum information processing primitives, in the form of Hong-Ou-Mandel interference, bosonic coalescence/anti-coalescence and highdimensional single-photon quantum gates exploiting the whole mode structure of the processor. We achieve fidelities that demonstrate the potential for large-scale photonic quantum information processing using stoichiometric silicon nitride.