Ashley Lyons

Coherent absorption of N00N states

We experimentally investigate two-photon N00N state coherent absorption in a multilayer graphene film and show that coherent loss can be used as a resource for quantum operations.

Geometries for the coherent control of four-wave mixing in graphene multilayers

Deeply sub-wavelength two-dimensional films may exhibit extraordinarily strong nonlinear effects. Here we show that 2D films exhibit the remarkable property of a phase-controllable nonlinearity, i.e., the amplitude of the nonlinear polarisation wave in the medium can be controlled via the pump beam phase and determines whether a probe beam will “feel” or not the nonlinearity. This is in stark contrast to bulk nonlinearities where propagation in the medium averages out any such phase dependence. We perform a series of experiments in multilayer graphene that highlight some of the consequences of the optical nonlinearity phase-dependence, such as the coherent control of nonlinearly diffracted beams, single-pump-beam induced phase-conjugation and the demonstration of a nonlinear mirror characterised by negative reflection. The observed phase sensitivity is not specific to graphene but rather is solely a result of the dimensionality and is therefore expected in all 2D materials.

Observation of image pair creation and annihilation from superluminal scattering sources

Researchers experimentally investigate the superluminal kinematics of scattering light sources through ultrafast time-resolved imaging. The invariance of the speed of light is one of the foundational pillars of our current understanding of the universe. It implies a series of consequences related to our perception of simultaneity and, ultimately, of time itself. Whereas these consequences are experimentally well studied in the case of subluminal motion, the kinematics of superluminal motion lack direct evidence or even a clear experimental approach. We investigate kinematic effects associated with the superluminal motion of a light source. By using high-temporal-resolution imaging techniques, we directly demonstrate that if the source approaches an observer at superluminal speeds, the temporal ordering of events is inverted and its image appears to propagate backward. Moreover, for a source changing its speed and crossing the interface between subluminal and superluminal propagation regions, we observe image pair annihilation and creation, depending on the crossing direction. These results are very general and show that, regardless of the emitter speed, it is not possible to unambiguously determine the kinematics of an event from imaging and time-resolved measurements alone. This has implications not only for light, but also, for example, for sound and other wave phenomena.

Attosecond-resolution Hong-Ou-Mandel interferometry.

A new Hong-Ou-Mandel interferometer protocol achieves few-attosecond (nanometer) photon path delay resolution. When two indistinguishable photons are each incident on separate input ports of a beamsplitter, they “bunch” deterministically, exiting via the same port as a direct consequence of their bosonic nature. This two-photon interference effect has long-held the potential for application in precision measurement of time delays, such as those induced by transparent specimens with unknown thickness profiles. However, the technique has never achieved resolutions significantly better than the few-femtosecond (micrometer) scale other than in a common-path geometry that severely limits applications. We develop the precision of Hong-Ou-Mandel interferometry toward the ultimate limits dictated by statistical estimation theory, achieving few-attosecond (or nanometer path length) scale resolutions in a dual-arm geometry, thus providing access to length scales pertinent to cell biology and monoatomic layer two-dimensional materials.

High precision metrology from the fisher information of a Hong-Ou-Mandel interferometer

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).

Coherent absorption of two-photon states in metamaterials

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%.