Malcolm N. O'Sullivan

High-order thermal ghost imaging

We show that high-order ghost imaging has higher visibility and contrast-to-noise ratio as compared to conventional thermal ghost imaging. We also obtain the optimal polynomial order that gives the best contrast-to-noise ratio.

Optimization of thermal ghost imaging: high-order correlations vs. background subtraction

We compare the performance of high-order thermal ghost imaging with that of conventional (that is, lowest-order) thermal ghost imaging for different data processing methods. Particular attention is given to high-order thermal ghost imaging with background normalization and conventional ghost imaging with background subtraction. The contrast-to-noise ratio (CNR) of the ghost image is used as the figure of merit for the comparison.We find analytically that the CNR of the normalized high-order ghost image is inversely proportional to the square root of the number of transmitting pixels of the object. This scaling law is independent of the exponents used in calculating the high-order correlation and is the same as that of conventional ghost imaging with background subtraction. We find that no data processing procedure performs better than lowest-order ghost imaging with background subtraction. Our results are found to be able to explain the observations of a recent experiment [Chen et al., arXiv:0902.3713v3 [quant-ph]].

Two-Color Ghost Imaging

We study a quantum ghost imaging system that uses different wavelengths to illuminate the object and the reference detector. We found that the resolution is limited by the wavelength of light illuminating the object.

Temporal coherence and indistinguishability in two-photon interference effects

We show that temporal two-photon interference effects involving the signal and idler photons created by parametric down-conversion can be fully characterized in terms of the variations of two length parameters---called the biphoton path-length difference and the biphoton path-asymmetry-length difference---which we construct using the six different length parameters that a general two-photon interference experiment involves. We perform an experiment in which the effects of the variations of these two parameters can be independently controlled and studied. In our experimental setup, which does not involve mixing of signal and idler photons at a beam splitter, we further report observations of Hong-Ou-Mandel- (HOM-)like effects both in coincidence and in one-photon count rates. As an important consequence, we argue that the HOM and the HOM-like effects are best described as observations of how two-photon coherence changes as a function of the biphoton path-asymmetry-length difference.

High-dimensional Quantum Key Distribution with Photonic Orbital Angular Momentum

We experimentally demonstrate a quantum cryptography system based on photonic orbital angular momentum. The system achieves a channel capacity of 2.1 bits per sifted photon through the use of a 7-dimensional alphabet for encoding information.

Enhancing entangled-state phase estimation by combining classical and quantum protocols

We present a protocol that combines quantum and classical resources to increase the sensitivity of a phase measurement. The superresolution is achieved through the use of N00N states and multiple passes through a prism pair.

Enhancing entangled-state phase estimation by combining classical and quantum protocols.

We present a protocol that combines quantum and classical resources to increase the sensitivity of a phase measurement. The superresolution is achieved through the use of N00N states and multiple passes through a prism pair.

Nonlinear Optics: The Enabling Technology for Quantum Information Science

Nonlinear optical processes such as parametric down conversion and squeezed light generation are key elements of most quantum protocols, leading to crucial applications such as quantum imaging, sub-shot-noise metrology, and secure communication.

Near-perfect sorting of orbital angular momentum: A step towards high-dimensional quantum communications

A novel technique is introduced for separation of orbital-angular-momentum (OAM) states of light. Using this method, we demonstrate the realization of a free-space communication link with a channel capacity of more than 4 bits per photon.

Influence of Atmospheric Turbulence on the Performance of a High Dimensional Quantum Key Distribution System using Spatial Mode Encoding

The effects of atmospheric turbulence on a the channel capacity of a free-space quantum key distribution system with information encoded on the transverse modes of the photon are studied theoretically and experimentally.