Programmable two-photon quantum interference in103channels in opaque scattering media

Abstract

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...

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