I will discuss various methods via which the transmission characteristics of optical quantum channels can be enhanced via the manipulation of continuous variables of the optical system. These methods will range from simple post-selection protocols through to quantum repeaters. In the most general cases the channel is improved with respect to the transmission of arbitrary quantum states - i.e. the quantum information encoding protocol; discrete, continuous or hybrid, is unrestricted. As well as theoretical results I will discuss new experimental demonstrations.
One of the greatest challenges in modern science is the realisation of quantum computers which, as their scale increases, will allow enhanced performance of tasks across many areas of quantum information processing. Quantum logic gates play a vital role in realising these applications by carrying out the elementary operations on the qubits; a key aim is minimising the resources needed to build these gates into useful circuits. While the salient features of a quantum computer have been shown in proof-of-principle experiments, e.g., single- and two-qubit gates, difficulties in scaling quantum systems to encode and manipulate multiple qubits has hindered demonstrations of more complex operations. This is exemplified by the classical Fredkin (or controlled-SWAP) gate  for which, despite many theoretical proposals [2,3] relying on concatenating multiple two-qubit gates, a quantum analogue has yet to be realised.
Here, by directly adding control to a two-qubit SWAP unitary , we use photonic qubit logic to report the first experimental demonstration of a quantum Fredkin gate . Our scheme uses linear optics and improves on the overall probability of success by an order of magnitude over previous proposals [2,3]. This optical approach allows us to add control an arbitrary black-box unitary which is otherwise forbidden in the standard circuit model . Additionally, the action of our gate exhibits quantum coherence allowing the generation of the highest fidelity three-photon GHZ states to date.
The quantum Fredkin gate has many applications in quantum computing, quantum measurements  and cryptography [8,9]. Using our scheme, we apply the Fredkin gate to the task of direct measurements of the purity and state overlap of a quantum system  without recourse to quantum state tomography.
Coastal saltmarsh and their constituent components and processes are of an interest scientifically due to their ecological function and services. However, heterogeneity and seasonal dynamic of the coastal wetland system makes it challenging to map saltmarshes with remotely sensed data. This study selected four important saltmarsh species <i>Pragmitis australis, Sporobolus virginicus, Ficiona nodosa and Schoeloplectus sp.</i> as well as a Mangrove and Pine tree species, <i>Avecinia </i>and <i>Casuarina sp </i>respectively. High Spatial Resolution Worldview-2 data and Coarse Spatial resolution Landsat 8 imagery were selected in this study. Among the selected vegetation types some patches ware fragmented and close to the spatial resolution of Worldview-2 data while and some patch were larger than the 30 meter resolution of Landsat 8 data. This study aims to test the effectiveness of different classifier for the imagery with various spatial and spectral resolutions. Three different classification algorithm, Maximum Likelihood Classifier (MLC), Support Vector Machine (SVM) and Artificial Neural Network (ANN) were tested and compared with their mapping accuracy of the results derived from both satellite imagery. For Worldview-2 data SVM was giving the higher overall accuracy (92.12%, kappa =0.90) followed by ANN (90.82%, Kappa 0.89) and MLC (90.55%, kappa = 0.88). For Landsat 8 data, MLC (82.04%) showed the highest classification accuracy comparing to SVM (77.31%) and ANN (75.23%). The producer accuracy of the classification results were also presented in the paper.
Saltmarsh is one of the important communities of wetlands, however, due to a range of pressures, it has been declared as an EEC (Ecological Endangered Community) in Australia. In order to correctly identify different saltmarsh species, development of spectral libraries of saltmarsh species is essential to monitor this EEC. Hyperspectral remote sensing, can explore the area of wetland monitoring and mapping. The benefits of Hyperion data to wetland monitoring have been studied at Hunter Wetland Park, NSW, Australia. After exclusion of bad bands from the original data, an atmospheric correction model was applied to minimize atmospheric effect and to retrieve apparent surface reflectance for different land cover. Large data dimensionality was reduced by Forward Minimum Noise Fraction (MNF) algorithm. It was found that first 32 MNF band contains more than 80% information of the image. Pixel Purity Index (PPI) algorithm worked properly to extract pure pixel for water, builtup area and three vegetation <i>Casuarina sp., Phragmitis sp.</i> and green grass. The result showed it was challenging to extract extreme pure pixel for <i>Sporobolus</i> and <i>Sarcocornia</i> from the data due to coarse resolution (30 m) and small patch size (<3 m) of those vegetation on the ground . Spectral Angle Mapper, classified the image into five classes: <i>Casuarina</i>, Saltmarsh (<i>Phragmitis</i>), Green grass, Water and Builtup area with 43.55 % accuracy. This classification also failed to classify <i>Sporobolus</i> as a distinct group due to the same reason. A high spatial resolution airborne hyperspectral data and a new study site with a bigger patch of <i>Sporobolus</i> and <i>Sarcocornia</i> is proposed to overcome the issue.
Entanglement distillation is an indispensable ingredient in extended quantum communication networks. Distillation
protocols are necessarily non-deterministic and require non-trivial experimental techniques such as noiseless
amplification. We show that noiseless amplification could be achieved by performing a post-selective filtering of
measurement outcomes. We termed this protocol measurement-based noiseless linear amplification (MBNLA).
We apply this protocol to entanglement that suffers transmission loss of up to the equivalent of 100km of optical
fibre and show that it is capable of distilling entanglement to a level stronger than that achievable by transmitting
a maximally entangled state through the same channel. We also provide a proof-of-principle demonstration
of secret key extraction from an otherwise insecure regime via MBNLA. Compared to its physical counterpart,
MBNLA not only is easier in term of implementation, but also allows one to achieve near optimal probability of
We propose an experiment in which an entangled pair of optical pulses follow different paths through a gravitational field. We use a non-standard technique based on localized operators to analyze this situation. The calculation predicts decorrelation of the optical entanglement under experimentally realistic conditions.
We discuss a model for time displaced entanglement, produced by taking one member of an entangled pair on a round trip at relativistic speeds, thus inducing a time-shift between the pair. We show that decoherence of the entangled pair is predicted. For non-maximal entanglement this then implies the ability to induce a non-unitary, non-linear quantum evolution. Although exhibiting unusual characteristics, we show that these evolutions cannot be dismissed on the basis of entropic or causal arguments.
We experimentally demonstrate a complete, end-to-end, quantum key distribution system using a continuous wave laser and standard optical components. Our implementation encodes random bits as weak Gaussian modulations onto the phase and amplitude quadratures of the laser beam. We process data from the quantum channel using a post-selection procedure and subsequently apply information reconciliation and privacy amplification procedures to generate an absolutely secure secret key. The maximum information that an eavesdropper may have obtained about this secret key, from the quantum channel and classical communications, is bounded to below one bit. Under the assumption of individual Gaussian eavesdropping attacks, we achieve a secret key generation rate of 25 Mbits/s for a lossless channel and 1 kbit/s for 90% channel loss, per 17 MHz of detected bandwidth.
Two-qubit entangling gates allow the realization of new types of generalized quantum measurement. We discuss, and use photonic systems to demonstrate, two instances of this: two-qubit entangling measurements realizing superior discrimination of locally prepared two-qubit quantum states relative to what is achievable with local measurements and classical communication; and nondestructive weak measurements with postselection, leading to quantum weak values.
We discuss progress towards implementing two qubit quantum gates in optics. We review the operation of an optical quantum gate which performs all the operations of a control-NOT (CNOT) gate in the coincidence basis with two, unentangled photons as the input and discuss its implementation.
Quantum optics has proved a fertile field for experimental tests of
quantum information science, from experimental verification of the
violation of the Bell inequalities to quantum teleportation. However it was long believed that quantum optics would not provide a practical path to efficient and scaleable quantum computation, and most current efforts to achieve a scaleable quantum computer have focussed on solid state implementations. This orthodoxy was challenged recently when Knill et al. showed that given single photon sources and single photon detectors, linear optics alone would suffice to implement efficient quantum computation. While this result is surprising, the complexity of the optical networks required is daunting. In this talk we propose an efficient scheme which is elegant in its simplicity. We indicate how fundamental single and two qubit gates can be achieved. By encoding the quantum information in multi-photon coherent states, rather than single photon states, simple optical manipulations acquire unexpected power. As an application of this new information processing ability we investigate
a class of high precision measurements. We show how superpositions of
coherent states allow displacement measurements at the Heisenberg limit. Entangling many superpositions of coherent states offers a significant advantage over a single mode superposition states with the same mean photon number.
We present methods of transforming the standard quadrature amplitude squeezing of a continuous-wave light beam to its Stokes parameters and transverse spatial modes statistics. These two states of light are called polarization squeezing and spatial squeezing, respectively. We present experimental results of the quadrature amplitude, polarization and spatial squeezing obtained with a common experimental setup based on optical parametric amplifiers. The transformations from quadrature amplitude to polarization and spatial squeezing are achieved with only simple linear optics.
We review recent theoretical progress in finding ways to do quantum
processing with linear optics, non-classical input states and
conditional measurements. We focus on a dual rail photonic scheme and a
single rail coherent state scheme.
We demonstrate a novel intensity noise suppression configuration which combines laser injection locking and electronic feedback. We use two feedback loops which together suppress the intensity noise of the injection locked laser to 4 dB above the quantum noise limit.
Solid state laser sources, such as diode-laser pumped Nd:YAG lasers, have given us a cw laser light of high power with unprecedented stability and low noise performance. In these lasers most of the technical sources of noise can be eliminated and thereby allow operation close to the theoretical limit set by the quantum properties of the light. We present progress in the experimental realization of such lasers. These investigations include the control of noise by electronic feedback, passive external cavities; and the reliable generation of amplitude squeezed light through second harmonic generation. At the same time we have developed theoretical models describing the quantum noise properties of coupled systems of lasers and cavities. The agreement between our experimental results with noise spectra calculated with our realistic theoretical models demonstrates the ability to predict the performance of various laser systems.