Kali Wilson

Optical analogues of the Newton–Schrödinger equation and boson star evolution

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.

Synthetic magnetism for photon fluids

European Research Council under the European Union [(FP/2007-17172013)/ERC GA 306559]; EPSRC (U.K.) [EP/J00443X/1, EP/M024636/1]; EPSRC CM-CDT [EP/L015110/1]

Experimental Methods for Generating Two-Dimensional Quantum Turbulence in Bose-Einstein Condensates

Bose–Einstein condensates of dilute gases are well-suited for investigations of vortex dynamics and turbulence in quantum fluids, yet there has been little experimental research into the approaches that may be most promising for generating states of two-dimensional turbulence in these systems. Here we give an overview of techniques for generating the large and disordered vortex distributions associated with two-dimensional quantum turbulence. We focus on describing methods explored in our Bose–Einstein condensation laboratory, and discuss the suitability of these methods for studying various aspects of two-dimensional quantum turbulence. We also summarize some of the open questions regarding our own understanding of these mechanisms of two-dimensional quantum turbulence generation in condensates. We find that while these disordered distributions of vortices can be generated by a variety of techniques, further investigation is needed to identify methods for obtaining quasisteady-state quantum turbulence in condensates.

Enhancing the recovery of a temporal sequence of images using joint deconvolution

In this work, we address the reconstruction of spatial patterns that are encoded in light fields associated with a series of light pulses emitted by a laser source and imaged using photon-counting cameras, with an intrinsic response significantly longer than the pulse delay. Adopting a Bayesian approach, we propose and demonstrate experimentally a novel joint temporal deconvolution algorithm taking advantage of the fact that single pulses are observed simultaneously by different pixels. Using an intensified CCD camera with a 1000-ps gate, stepped with 10-ps increments, we show the ability to resolve images that are separated by a 10-ps delay, four time better compared to standard deconvolution techniques.

Rotation-dependent nonlinear absorption of orbital angular momentum beams in ruby

We investigate the effect of a rotating medium on orbital angular momentum (OAM)-carrying beams by combining a weak probe beam shifted in frequency relative to a strong pump beam. We show how the rotational Doppler effect modifies the light-matter interaction through the external rotation of the medium. This interaction leads to an absorption that increases with the mechanical rotation velocity of the medium and with a rate that depends on the OAM of the light beam.

Rotating black hole geometries in a two-dimensional photon superfluid

Analogue gravity studies the physics of curved spacetime in laboratory experiments, where the propagation of elementary excitations in inhomogeneous flows is mapped to those of scalar fields in a curved spacetime metric. While most analogue gravity experiments are performed in 1+1 dimensions (one spatial plus time) and thus can only mimic only 1+1D spacetime, we present a 2+1D photon (room temperature) superfluid where the geometry of a rotating acoustic black hole can be realized in 2+1D dimensions. By measuring the local flow velocity and speed of waves in the superfluid, we identify a 2D region surrounded by an ergo sphere and a spatially separated event horizon. This provides the first direct experimental evidence of an ergosphere and horizon in any system, and the possibility in the future to study the analogue of Penrose superradiance from rotating black holes with quantised angular momentum and modified dispersion relations.

Slow light in flight imaging

Slow light has been explored for building quantum networks, with particular interest in slowing the group velocity of single photons [1], and more recently exploited to enhance the measurement of small phase shifts. Generally, slow-light effects have been characterized as the net effect of a pulse propagating through the slow-light medium, i.e., as a pulse delay time Δt measured with a fast photodiode at the output of the medium [2]. In this work, we use a single-photon imaging camera to observe slow light in situ, and thus provide a direct measurement of spatial pulse compression and temporal dispersion as the pulse travels through the slow light medium, in this case a hyperfine absorption doublet in hot Rb vapor. Our method combines light-in-flight imaging techniques with a camera comprised of an array of single-photon avalanche diodes (SPAD camera) [3] to image the photons scattered by the Rb vapor in the direction of the camera as shown in Fig. 1(a). In addition, the single photon nature of the SPAD detector allows us to obtain a measurement of the single photon group velocity. As shown in Fig. 1(c) and (d), we observe a significant delay, on the order of nanoseconds, in the detection of the photons scattered when the pulse first enters the slow-light medium. This lag in scattered-photon arrival time is a direct visualization of the slowing down of the single-photon group velocity. The pulses used here had a temporal full width at half maximum (FWHM) of τ ∼1 ns, with measured group velocities as low as vg ∼ 0.006c. At these low group velocities we observe a full fractional pulse delay of up to F D = Δt/τ ∼ 40 over 7 cm of propagation, and F D ∼ 5 for the scattered single photons, which propagate through ∼ 1 cm of Rb vapor prior to exiting the cell en-route to the camera.

Quantum droplets of light in the presence of synthetic magnetic fields

Recently, quantum droplets have been demonstrated in dipolar Bose-Einstein condensates, where the long range (nonlocal) attractive interaction is counterbalanced by a local repulsive interaction [1]. In this work, we investigate the formation of quantum droplets in a two-dimensional nonlocal fluid of light. Fluids of light allow us to control the geometry of the system, and thus introduce vorticity which in turn creates an artificial magnetic field for the quantum droplet. In a quantum fluid of light, the photons comprising the fluid are treated as a gas of interacting Bose-particles, where the nonlocal interaction comes from the nonlinearity inherent in the material, in our case an attractive third-order thermo-optical nonlinearity. In contrast to matter-wave droplets, photon fluid droplets are not stabilized by local particle-particle scattering, but from the quantum pressure itself, i.e., a balance between diffraction and the nonlocal nonlinearity.

Optical analogues of the Schrödinger-Newton equation and rotating boson stars

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

The role of geometry in nonlocal superfluids

By modifying the geometry of the optical beam, we control the effective nonlocal interaction length of a nonlocal photon fluid. This allows us to access the superfluid flow regime, which would otherwise be unobservable.