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arXiv:1511.00897 · Physical Review A

Programmable two-photon quantum interference in 103 channels in opaque scattering media

Tom A. W. Wolterink, Ravitej Uppu, Georgios Ctistis, Willem L. Vos, Klaus J. Boller, Pepijn W. H. Pinkse

Published May 11, 2016• 71 citations
arXivHTMLPDF

Abstract

We investigate two-photon quantum interference in an opaque scattering medium that intrinsically supports a large number of transmission channels. By adaptive spatial phase modulation of the incident wave fronts, 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 $2\ifmmode\times\else\texttimes\fi{}2$ 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 programed beam splitter need not fulfill energy conservation over the two selected output channels and hence could be nonunitary. 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.

DOI
10.1103/physreva.93.053817

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We investigate two-photon quantum interference in an opaque scattering medium that intrinsically supports a large number of transmission channels. By adaptive spatial phase modulation of the incident wave fronts, the photons are directed at targeted speckle spots or output channels. From 103{10}^{3}103 experimentally available coupled channels, we select two channels and enhance their transmission to realize the equivalent of a fully programmable 2\ifmmode×\else\texttimes\fi22\ifmmode\times\else\texttimes\fi{}22\ifmmode×\else\texttimes\fi2 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 programed beam splitter need not fulfill energy conservation over the two selected output channels and hence could be nonunitary. 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.
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Light transport in opaque scattering media mixes the light across the large number of modes in the system. The mixing results in the scrambling of information encoded on the incident light, which is generally detrimental. However, the multimodal transport hints at the possibility of utilizing them as complex optical networks if the transport could be controlled. In recent years, wavefront shaping through adaptive phase control has been used to create multiport optical devices in opaque scattering media. We study the programmability of one such device created in opaque scattering medium, a two-port beam splitter which is a primitive for any complex linear optical network. Here, I will discuss our latest results on the quantum interference between single photons from an ultrabright quantum source at the two-port beam splitter. The novel feature of the realized beam splitter is the programmability of quantum interference between single photons, creating either bunched or anti-bunched light at the outputs. Our demonstration is a first step towards realizing complex optical networks with programmable quantum correlations using opaque scattering media.
The Hong--Ou--Mandel (HOM) effect is a hallmark of two-photon quantum interference, in which two indistinguishable photons impinging on a balanced beam splitter bunch into the same output port. Here, we show that a giant atom (GA), coupled to two waveguides through two coupling points each, can function as a programmable HOM interferometer, enabling continuous control over single-photon beam splitting and two-photon interference. This programmability arises from coupling-phase differences in the GA, which tune its self-interference and directionality, and thereby its scattering response. To characterize the two-photon interference, we analyze the scattering of two Gaussian single-photon wave packets injected through different waveguides and evaluate the bunching and antibunching (coincidence) probabilities of the resulting four output ports. At the operating point where the GA acts as an effective 50:50 beam splitter for single photons, we observe a pronounced HOM dip as the relative input delay between the two wave packets is varied. Away from this point, adjusting the coupling phases continuously tunes the two-photon output statistics between bunching into the same output port and antibunching across distinct output ports. In addition, we explore an application to quantum parameter estimation, showing that small deviations of coupling phases can be estimated from the two-photon output statistics, with the achievable sensitivity quantified by the classical Fisher information associated with a binary coincidence measurement. Our giant-atom beam splitter thus provides a programmable platform for two-photon interference in waveguide quantum electrodynamics, with potential applications in quantum information processing, quantum communication, and quantum sensing.
Wavefront shaping allows for ultimate control of light propagation in multiple-scattering media by adaptive manipulation of incident waves. We have demonstrated a method to program general multiport linear optical circuits in multiple-scattering materials by phase modulation of incident wavefronts. Applying this method to quantum light makes it possible to use these optical circuits for adaptive quantum optical experiments, with high flexibility in the control of quantum interference between multiple optical modes. To this end we have constructed a bright single-photon source, producing heralded single-photon states with a measured production rate of > 106 s−1. We have demonstrated that we are able to control the propagation of these single-photon states in random-scattering media, and we are currently investigating Hong-Ou-Mandel interference in these circuits.
Hong-Ou-Mandel interference is one of the clearest signatures of quantum behaviour: two identical photons meeting at a beam splitter always leave together. Here we demonstrate that this symmetry can be broken. Using a programmable integrated photonic processor, we interfere imbalanced two-photon states on a variable beam splitter and show that the output state of quantum interference can be continuously tuned, reaching the extreme case where destructive interference suppresses bunching at one output port. The asymmetry creates a forbidden bunching channel: photons may exit together at one port or separately at both ports, but cannot emerge together at the other port.
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.