46 research outputs found
Security proof of quantum key distribution with detection efficiency mismatch
In theory, quantum key distribution (QKD) offers unconditional security based
on the laws of physics. However, as demonstrated in recent quantum hacking
theory and experimental papers, detection efficiency loophole can be fatal to
the security of practical QKD systems. Here, we describe the physical origin of
detection efficiency mismatch in various domains including spatial, spectral,
and time domains and in various experimental set-ups. More importantly, we
prove the unconditional security of QKD even with detection efficiency
mismatch. We explicitly show how the key generation rate is characterized by
the maximal detection efficiency ratio between the two detectors. Furthermore,
we prove that by randomly switching the bit assignments of the detectors, the
effect of detection efficiency mismatch can be completely eliminated.Comment: 35 pages, 7 figure
Demonstrating multilevel entanglement and optimal quantum measurements
Optimal generalised quantum measurements are important for quantum information applications
in both photonic and solid state systems. However, until now, the implementations
of such measurements have been optical. Entanglement is also a very important
resource in quantum communication and information processing. However, highdimensional
entangled states and corresponding Bell-inequality violations are challenging
to detect and demonstrate experimentally. This thesis focuses on these two aspects of
signal detection.
A cavity quantum electrodynamics (QED) scheme to realise an optimised quantum
measurement demonstrating the superadditivity of quantum channel capacity is proposed
and analysed. The measurement is shown to be feasible using atoms in a cavity QED setup
even in the presence of rather high levels of experimental errors. This is interesting because
cavity QED realisations could potentially be more easily scaled to increase quantum
coding gain. Experimental unambiguous discrimination between non-orthogonal states is
also carried out for the first time in the solid state using the nuclear spin of a nitrogen
atom associated with a defect in bulk diamond—an important step for implementations
of solid-state quantum computing.
This thesis presents a method for verifying entanglement dimension using only Bell
inequality test measurements. It also shows experimental results demonstrating genuine
eleven-dimensional two-photon orbital angular momentum (OAM) entanglement and violations
of generalised Bell inequalities up to dimension twelve. The demonstrated highdimensional
entanglement is potentially useful for closing the detection loophole in Belltest
experiments and for real-world large-alphabet quantum-cryptography applications
Non-Locality of Experimental Qutrit Pairs
The insight due to John Bell that the joint behavior of individually measured
entangled quantum systems cannot be explained by shared information remains a
mystery to this day. We describe an experiment, and its analysis, displaying
non-locality of entangled qutrit pairs. The non-locality of such systems, as
compared to qubit pairs, is of particular interest since it potentially opens
the door for tests of bipartite non-local behavior independent of probabilistic
Bell inequalities, but of deterministic nature
Hashing for Similarity Search: A Survey
Similarity search (nearest neighbor search) is a problem of pursuing the data
items whose distances to a query item are the smallest from a large database.
Various methods have been developed to address this problem, and recently a lot
of efforts have been devoted to approximate search. In this paper, we present a
survey on one of the main solutions, hashing, which has been widely studied
since the pioneering work locality sensitive hashing. We divide the hashing
algorithms two main categories: locality sensitive hashing, which designs hash
functions without exploring the data distribution and learning to hash, which
learns hash functions according the data distribution, and review them from
various aspects, including hash function design and distance measure and search
scheme in the hash coding space
Multi-photon entanglement and interferometry
Multi-photon interference reveals strictly non-classical phenomena. Its
applications range from fundamental tests of quantum mechanics to photonic
quantum information processing, where a significant fraction of key experiments
achieved so far comes from multi-photon state manipulation. We review the
progress, both theoretical and experimental, of this rapidly advancing
research. The emphasis is given to the creation of photonic entanglement of
various forms, tests of the completeness of quantum mechanics (in particular,
violations of local realism), quantum information protocols for quantum
communication (e.g., quantum teleportation, entanglement purification and
quantum repeater), and quantum computation with linear optics. We shall limit
the scope of our review to "few photon" phenomena involving measurements of
discrete observables.Comment: 71 pages, 38 figures; updated version accepted by Rev. Mod. Phy
Monocular 3d Object Recognition
Object recognition is one of the fundamental tasks of computer vision. Recent advances in the field enable reliable 2D detections from a single cluttered image. However, many challenges still remain. Object detection needs timely response for real world applications. Moreover, we are genuinely interested in estimating the 3D pose and shape of an object or human for the sake of robotic manipulation and human-robot interaction.
In this thesis, a suite of solutions to these challenges is presented. First, Active Deformable Part Models (ADPM) is proposed for fast part-based object detection. ADPM dramatically accelerates the detection by dynamically scheduling the part evaluations and efficiently pruning the image locations. Second, we unleash the power of marrying discriminative 2D parts with an explicit 3D geometric representation. Several methods of such scheme are proposed for recovering rich 3D information of both rigid and non-rigid objects from monocular RGB images. (1) The accurate 3D pose of an object instance is recovered from cluttered images using only the CAD model. (2) A global optimal solution for simultaneous 2D part localization, 3D pose and shape estimation is obtained by optimizing a unified convex objective function. Both appearance and geometric compatibility are jointly maximized. (3) 3D human pose estimation from an image sequence is realized via an Expectation-Maximization algorithm. The 2D joint location uncertainties are marginalized out during inference and 3D pose smoothness is enforced across frames.
By bridging the gap between 2D and 3D, our methods provide an end-to-end solution to 3D object recognition from images. We demonstrate a range of interesting applications using only a single image or a monocular video, including autonomous robotic grasping with a single image, 3D object image pop-up and a monocular human MoCap system. We also show empirical start-of-art results on a number of benchmarks on 2D detection and 3D pose and shape estimation