Ian Walmsley

Abstract:

In this work, we use wavefront shaping methods to control non-classical states of light propagating through a multiply scattering medium. We experimentally show guiding of a single-photon into a selected single-mode fiber after propagation through the medium, and demonstrate generation of a one-photon entangled state. © 2014 OSA.

Abstract:

We demonstrate a novel interferometric characterization scheme that allows the complete reconstruction of two interfering electric fields. The phase profiles of both beams, and their relative phase, can be retrieved simultaneously as a function of any degree of freedom in which it is possible to shear one of the beams. The method has applications in wavefront sensing or ultrashort-pulse measurement, especially also in the domain of extreme light sources where it is difficult to generate a reference field or to replicate the beam in order to perform a self-referencing measurement. We demonstrate the technique experimentally by measuring simultaneously two ultrashort pulses in a single laser shot.

Abstract:

We present a scheme for linear optical quantum computing using time-bin-encoded qubits in a single spatial mode. We show methods for single-qubit operations and heralded controlled-phase (cphase) gates, providing a sufficient set of operations for universal quantum computing with the Knill-Laflamme-Milburn [Nature (London) 409, 46 (2001)] scheme. Our protocol is suited to currently available photonic devices and ideally allows arbitrary numbers of qubits to be encoded in the same spatial mode, demonstrating the potential for time-frequency modes to dramatically increase the quantum information capacity of fixed spatial resources. As a test of our scheme, we demonstrate the first entirely single spatial mode implementation of a two-qubit quantum gate and show its operation with an average fidelity of 0.84±0.07.

Abstract:

The integrated optical circuit is a promising architecture for the realization of complex quantum optical states and information networks. One element that is required for many of these applications is a high-efficiency photon detector capable of photon-number discrimination. We present an integrated photonic system in the telecom band at 1550 nm based on UV-written silica-on-silicon waveguides and modified transition-edge sensors capable of number resolution and over 40 % efficiency. Exploiting the mode transmission failure of these devices, we multiplex three detectors in series to demonstrate a combined 79 % ± 2 % detection efficiency with a single pass, and 88 % ± 3 % at the operating wavelength of an on-chip terminal reflection grating. Furthermore, our optical measurements clearly demonstrate no significant unexplained loss in this system due to scattering or reflections. This waveguide and detector design therefore allows the placement of number-resolving single-photon detectors of predictable efficiency at arbitrary locations within a photonic circuit - a capability that offers great potential for many quantum optical applications. © 2013 Optical Society of America.

Abstract:

We study the simultaneous estimation of multiple phases as a discretized model for the imaging of a phase object. We identify quantum probe states that provide an enhancement compared to the best quantum scheme for the estimation of each individual phase separately as well as improvements over classical strategies. Our strategy provides an advantage in the variance of the estimation over individual quantum estimation schemes that scales as O(d), where d is the number of phases. Finally, we study the attainability of this limit using realistic probes and photon-number-resolving detectors. This is a problem in which an intrinsic advantage is derived from the estimation of multiple parameters simultaneously.