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Quantum entanglement is a key resource that can be exploited for a range of applications such as quantum teleportation, quantum computation, and quantum cryptography. However, efforts to exploit entanglement in imaging systems have so far led to solutions such as ghost imaging, that have since found classical implementations. Here, we demonstrate an optical imaging protocol that relies uniquely on entanglement: Two polarizing patterns imprinted and superimposed on a metasurface are separately imaged only when using entangled photons. Unentangled light is not able to distinguish between the two patterns. Entangled single-photon imaging of functional metasurfaces promises advances towards the use of nanostructured subwavelength thin devices in quantum information protocols and a route to efficient quantum state tomography. MOE (Min. of Education, S’pore) Published version

Many gravitational phenomena that lie at the core of our understanding of the Universe have not yet been directly observed. An example in this sense is the boson star that has been proposed as an alternative to some compact objects currently interpreted as being black holes. In the weak field limit, these stars are governed by the Newton–Schrodinger equation. Here we present an optical system that, under appropriate conditions, identically reproduces such equation in two dimensions. A rotating boson star is experimentally and numerically modelled by an optical beam propagating through a medium with a positive thermal nonlinearity and is shown to oscillate in time while also stable up to relatively high densities. For higher densities, instabilities lead to an apparent breakup of the star, yet coherence across the whole structure is maintained. These results show that optical analogues can be used to shed new light on inaccessible gravitational objects., In the weak field limit, boson star evolution is governed by the Newton-Schrödinger equation. Here the authors report an optical setup that provides a formal analogue of such dynamics via the interaction between vortex beams and a medium with positive thermo-optical nonlinearity.

By using correlated photon pairs, we demonstrate the non-local control of single-photon images formed by a polarisation sensitive metasurface. Non-local polarisation selection of a control photon, changes the image recorded after the metasurface.

Materials with a spatially uniform but temporally varying optical response have applications ranging from magnetic field-free optical isolators to fundamental studies of quantum field theories. However, these effects typically become relevant only for time variations oscillating at optical frequencies, thus presenting a significant hurdle that severely limits the realization of such conditions. Here we present a thin-film material with a permittivity that pulsates (uniformly in space) at optical frequencies and realizes a time-reversing medium of the form originally proposed by Pendry [Science 322, 71 (2008)SCIEAS0036-807510.1126/science.1162087]. We use an optically pumped, 500 nm thick film of epsilon-near-zero (ENZ) material based on Al-doped zinc oxide. An incident probe beam is both negatively refracted and time reversed through a reflected phase-conjugated beam. As a result of the high nonlinearity and the refractive index that is close to zero, the ENZ film leads to time reversed beams (simultaneous negative refraction and phase conjugation) with near-unit efficiency and greater-than-unit internal conversion efficiency. The ENZ platform therefore presents the time-reversal features required, e.g., for efficient subwavelength imaging, all-optical isolators and fundamental quantum field theory studies.

Multi-photon absorption processes have a nonlinear dependence on the amplitude of the incident optical field i.e. the number of photons. However, multi-photon absorption is generally weak and multi-photon events occur with extremely low probability. Consequently, it is extremely challenging to engineer quantum nonlinear devices that operate at the single photon level and the majority of quantum technologies have to rely on single photon interactions. Here, we demonstrate experimentally and theoretically that exploiting coherent absorption of N = 2 N00N states makes it possible to enhance the number of two-photon states that are absorbed. An absorbing metasurface placed inside a Sagnac-style interferometer into which we inject an N = 2 N00N state, exhibits two-photon absorption with 40.5% efficiency, close to the theoretical maximum. This high probability of simultaneous absorption of two photons holds the promise for applications in fields that require multi-photon upconversion but are hindered by high peak intensities., 13 page, 5 figures

Recent results in deeply subwavelength thickness films demonstrated coherent control and logical gate operations with both classical and single photon light sources. However, quantum processing and devices typically involve more than one photon and non-trivial input quantum states. Here we experimentally investigate two-photon N00N state coherent absorption in a multilayer graphene film. Depending on the N00N state input phase, it is possible to selectively choose between single or two photon absorption of the input state in the graphene film. These results demonstrate that coherent absorption in the quantum regime exhibits unique features opening up applications in multiphoton spectroscopy and imaging., 5 pages, 5 figures incl. 1 table

Solitons propagating in media with higher order dispersion will shed radiation known as dispersive wave or resonant radiation, with applications in frequency broadening, deep UV sources for spectroscopy or simply fundamental studies of soliton physics. Starting from a recently proposed equation that models the behaviour of ultrashort optical pulses in nonlinear materials using the analytic signal, we find that the resonant radiation associated with the third-harmonic generation term of the equation is parametrically stimulated with an unprecedented gain. Resonant radiation levels, typically only a small fraction of the soliton, are now as intense as the soliton itself. The mechanism is quite universal and works also in normal dispersion and with harmonics higher than the third. We report experimental hints of this super-resonant radiation stimulated by the fifth harmonic in diamond.

Optical analogues have been suggested as a means to explore systems that are described by general relativity. We show an optical analogue of the Schrodinger-Newton equation applied to the specific study of rotating boson stars.

Contrary to conventional coherent shocks, we show theoretically and experimentally that nonlocal turbulent flows lead to the emergence of large-scale incoherent shock waves, which constitute a collective phenomenon of the incoherent field as a whole.

Contrary to conventional coherent shocks, we show theoretically and experimentally that nonlocal turbulent flows lead to the emergence of large-scale incoherent shock waves, which constitute a collective phenomenon of the incoherent field as a whole.

International audience; We discuss the main features and our ongoing of work in the field on analogue gravity in photon fluids.

Understanding turbulent flows arising from random dispersive waves that interact strongly through nonlinearities is a challenging issue in physics. Here we report the observation of a characteristic transition: strengthening the nonlocal character of the nonlinear response drives the system from a fully turbulent regime, featuring a sea of coherent small-scale dispersive shock waves (shocklets) towards the unexpected emergence of a giant collective incoherent shock wave. The front of such global incoherent shock carries most of the stochastic fluctuations and is responsible for a peculiar folding of the local spectrum. Nonlinear optics experiments performed in a solution of graphene nano-flakes clearly highlight this remarkable transition. Our observations shed new light on the role of long-range interactions in strongly nonlinear wave systems operating far from thermodynamic equilibrium, which reveals analogies with, for example, gravitational systems, and establishes a new scenario that can be common to many turbulent flows in photonic quantum fluids, hydrodynamics and Bose–Einstein condensates., Understanding turbulent flows arising from random dispersive waves that interact through nonlinearities is a challenging issue in physics. Here, the authors model and observe experimentally in a nonlinear optics set-up the transition between a sea of small-scale shocklets and a giant collective shock wave.

We theoretically analyze parametric amplification as a means to produce dichroism based on the orbital angular momentum (OAM) of an incident signal field. The nonlinear interaction is shown to provide differential gain between signal states of differing OAM, the peak gain occurring at half the OAM of the pump field.

We study the nonlinear interaction between two non-collinear light beams that carry orbital angular momentum (OAM). Theory and experiments highlight the presence of new phase matching conditions in the presence of OAM.

We present a simple yet effective theory that predicts the existence of resonant radiation bands in the deep normal group velocity dispersion region of a medium, even in absence of a zero-group velocity dispersion point. This radiation is evident when the medium is pumped with high-energy ultrashort pulses, and it is driven by the interplay between the Kerr and the shock terms in the NLSE. Accurate experiments performed in bulk silica fully support the theoretical phase-matching condition found by our theory., Comment: 5 pages, 3 figures

We elucidate the consequences of a phase-matching theory that describes second-harmonic generation of two non-collinear incident light beams that carry orbital angular momentum (OAM). More specifically, the two incident beams generate a third that, depending on the incident OAM, may experience a significantly smaller conversion efficiency in comparison to that based on the conventional phase-matching theory. This is the case even for incident angles substantially less than those required for non-conservation of OAM in the nonlinear interaction. Experiments are performed under different conditions and are in excellent agreement with the theory. Our results have implications beyond the specific case studied here of second-harmonic generation, in particular for parametric down-conversion of photons., 6 pages, 4 figures

Quantum gases of atoms and exciton-polaritons are now well-established theoretical and experimental tools for fundamental studies of quantum many-body physics and suggest promising applications to quantum computing. Given their technological complexity, it is of paramount interest to devise other systems where such quantum many-body physics can be investigated at lesser technological expense. Here we examine a relatively well-known system of laser light propagating through thermo-optical defocusing media: based on a hydrodynamic description of light as a quantum fluid of interacting photons, we investigate such systems as a valid room-temperature alternative to atomic or exciton-polariton condensates for studies of many-body physics. First, we show that by using a technique traditionally used in oceanography it is possible to perform a direct measurement of the single-particle part of the dispersion relation of the elementary excitations on top of the photon fluid and to detect its global flow. Then, using a pump-and-probe setup, we investigate the dispersion of excitation modes of the fluid: for very long wavelengths, a sonic, dispersionless propagation is observed that we interpret as a signature of superfluid behavior. (C) 2015 Optical Society of America.

Three-dimensional light bullets in bulk media are identified, through their specific energy density flux, as polychromatic Bessel-like pulses. We also study resonant radiation emission with applications in rogue event studies and THz detection.

One interesting challenge in bio-sensing, photovoltaics, imaging and other related fields is the identification of approaches for tuning the broadband absorption in thin slab materials. In recent years, interest has been focused on the study of the effects that two counter-propagating beams produce by interfering with each other. In this configuration both complete absorption or lossless transmission of the incidence radiation can be achieved in an absorbing material. This phenomenon, named Coherent Perfect Absorption (CPA), was first observed in a Fabry-Perot cavity, where the light is trapped due to the multiple reflections. Recently it has been theoretically shown that CPA, also known as time-reversed lasing, can be observed in a light-absorbing film with a deeply sub-wavelength thickness, e.g. typically λ/100 is required [1, 2]. In this case, the coherent interference between the two counter-propagating beams produces a standing wave. If the position of the thin film corresponds to the position of the node with zero-field (resp. antinode with maximum field) of the stationary wave, the material can act like a transparent (resp. perfect absorber) slab.

Light scattering from a Kerr induced inhomogeneity is revisited, unveiling novel emission channels resulting essentially from the formation of shock front instabilities. These exhibit different features in the presence of negative or positive GVD.

Quantum interference forces two indistinguishable input photons to depart a beamsplitter in the same (of two possible) spatial output modes; this is known as the Hong-Ou-Mandel (HOM) effect. It provides a quantitative way of measuring the distinguishability between two photons and also has been utilised to determine their relative temporal delays [1, 2]. HOM interferometry offers numerous advantages over its classical counterpart, most importantly it lacks any dependence on the phase of the photons and therefore can potentially provide reliable measurements in environments where the phase is unstable. To date there have been very few investigations on the precision that is achievable with HOM interferometry. Experiments with collinear geometries have yielded sensitivities as low as 0.1fs however these approaches are limited to measurements of birefringence and other polarisation dependent effects [3, 4], as the precision derives from the inherently stable shared path of the photon pairs. Typical non-shared path HOM interferometers exhibit resolution of roughly 1–3 fs (0.3–1 μm).

Multiple photon absorption processes typically have a nonlinear dependence on the amplitude of the incident optical field. On the other hand, quantum technologies rely on single photon events. It has therefore been of great technical difficulty to achieve nonlinear devices using single photons. This is due to the small cross-section of absorption in room temperature devices, with multi-photon absorption events occurring with extremely low probability. The lack of access to nonlinear processes severely inhibits the use of optics for a large number of applications surrounding quantum technologies. We demonstrate experimentally that by exploiting a coherent absorption mechanism for N=2 N00N states, outlined theoretically by Jeffers in 2000 [1] and experimentally explored by Roger et. al. in 2016 [2], that it is possible to determine and enhance the number of two photon states that are absorbed. Here a 50% absorbing metasurface is placed inside a Sagnac interferometer into which we inject a N00N state. We show that by tuning the phase φ of the input state, |2,0〉 + exp(−ί2φ) |0,2〉, we can selectively tune the output state. For an input phase of φ = π/2 or 3π/2 we find that a single photon is absorbed with 100% probability. However, when we tune the input phase to φ = 0 or π we see that either 0 or 2 photons are absorbed with equal probability. We have developed a theoretical model that, with no free parameters, fits the experimentally measured two-photon contribution and finds the maximum contribution of |0,0) (0,0| to the output state to be 40.5%.

Resonant excitation of dispersive waves from spatiotemporal light bullets exhibits unique features such as rogue statistics. Resonant radiation may also be stimulated on a co-propagating THz pulse, bridging a 6 octave spectral gap.

We investigate superfluid behavior in a room-temperature photon fluid based on a thermal nonlinearity that mediates the photon-photon interactions. Placing an extended obstacle inside the flowing photon fluid leads to the nucleation of quantised vortices.

The technologies of heating, photovoltaics, water photocatalysis and artificial photosynthesis depend on the absorption of light and novel approaches such as coherent absorption from a standing wave promise total dissipation of energy. Extending the control of absorption down to very low light levels and eventually to the single-photon regime is of great interest and yet remains largely unexplored. Here we demonstrate the coherent absorption of single photons in a deeply subwavelength 50% absorber. We show that while the absorption of photons from a travelling wave is probabilistic, standing wave absorption can be observed deterministically, with nearly unitary probability of coupling a photon into a mode of the material, for example, a localized plasmon when this is a metamaterial excited at the plasmon resonance. These results bring a better understanding of the coherent absorption process, which is of central importance for light harvesting, detection, sensing and photonic data processing applications., Understanding the nature of coherent absorption is essential for exploiting it in new technologies. Here, the authors show that a metamaterial can deterministically absorb photons from a travelling wave with nearly unitary probability, down to the single-photon level.