arXiv subjects

C. H. Lai

Publications and source records attributed to C. H. Lai.

15 recordsLinked to original sources

arXiv2025

Complex magnetic phases and thermodynamics of CuB_2O_4

The copper metaborate CuB_2O_4(CBO) has been studied within the parametric space of magnetic field (B) and temperature (T), showing a series of distinct phases, including commensurate and incommensurate magnetic orders, as well as magnetic soliton phases. However, no single study has simultaneously demonstrated all these phases. moreover, the existence of additional phases below 2 K or above 2 T for B perpenducular to c has been scarcely explored. This work presents the first comprehensive construction of the complex B-T phase diagram for T = 0.1 to 25 K and B = 0 to 9 T (B perpenducular to c), utilizing thermodynamic probes, magnetic measurements, and neutron scattering on a single batch of CBO crystals. Our findings elucidate new magnetic phases and regimes. First-principles calculations help to gain a deeper understanding of this complex phase diagram. The emergence of various phases within such a narrow B and T regime is attributed to two primary mechanisms: the delicate competition between ferromagnetic and antiferromagnetic interactions, and the weak exchange and superexchange interactions within and between the Cu(A) and Cu(B) sublattices. This competition also leads to a magnetically frustrated ground state at absolute zero where the total magnetic entropy release from high to low temperatures is only 0.76 Rln2, lower than Rln2 expected for copper spin 1/2. Using CBO as a model system, the present work elucidates the intricate mechanisms underlying the formation of complex magnetic phases.

cond-mat.str-el↗

arXiv2015

Complete condition for nonzero quantum correlation in continuous variable systems

Quantum correlation provides a promising measure beyond entanglement. Here, we propose a necessary and sufficient condition for nonzero quantum correlation in continuous variable systems, which is simple and easy to perform in terms of a marker $Q_r$. In order to get this condition, we introduce continuous-variable local orthogonal bases of the operator space, which are generalized from the orthogonal basis sets in local operator space for discrete variables. Based on this, we obtain the marker $Q_r$ for all bipartite continuous variable states, and provide several examples including two-mode Gaussian and non-Gaussian states. Our result may provide a candidate for quantum correlation measures, and can be measured by designed quantum circuits.

quant-ph↗

arXiv2013

Test of Genuine Multipartite Nonlocality Without Inequality

In this letter we propose a set of conditions on the joint probabilities as a test of genuine multipartite nonlocality without inequality. Our test is failed by all non-signaling local models in which even nonlocal correlations among some observables (not all) are allowed as long as these correlations respect the non-signaling principle. A pass of our test by a state therefore indicates that this state cannot be simulated by any non-signaling local models, i.e., the state exhibits genuine multipartite nonlocality. It turns out that all entangled symmetric n-qubit ($n \geq 3$) states pass our test and therefore are n-way nonlocal. Also we construct two Bell-type inequalities from our proposed test whose violations indicate genuine multipartite nonlocal correlations.

quant-ph↗

arXiv2012

All entangled pure states violate a single Bell's inequality

We show that a single Bell's inequality with two dichotomic observables for each observer, which is originated from Hardy's nonlocality proof without inequalities, is violated by all entangled pure states of a given number of particles, each of which may have a different number of energy levels. Thus Gisin's theorem is proved in its most general form from which it follows that for pure states Bell's nonlocality and quantum entanglement are equivalent.

quant-ph↗

arXiv2012

Impact of edge-removal on the centrality betweenness of the best spreaders

The control of epidemic spreading is essential to avoid potential fatal consequences and also, to lessen unforeseen socio-economic impact. The need for effective control is exemplified during the severe acute respiratory syndrome (SARS) in 2003, which has inflicted near to a thousand deaths as well as bankruptcies of airlines and related businesses. In this article, we examine the efficacy of control strategies on the propagation of infectious diseases based on removing connections within real world airline network with the associated economic and social costs taken into account through defining appropriate quantitative measures. We uncover the surprising results that removing less busy connections can be far more effective in hindering the spread of the disease than removing the more popular connections. Since disconnecting the less popular routes tend to incur less socio-economic cost, our finding suggests the possibility of trading minimal reduction in connectivity of an important hub with efficiencies in epidemic control. In particular, we demonstrate the performance of various local epidemic control strategies, and show how our approach can predict their cost effectiveness through the spreading control characteristics.

physics.soc-ph↗

arXiv2011

Test Beam Results of 3D Silicon Pixel Sensors for the ATLAS upgrade

Results on beam tests of 3D silicon pixel sensors aimed at the ATLAS Insertable-B-Layer and High Luminosity LHC (HL-LHC)) upgrades are presented. Measurements include charge collection, tracking efficiency and charge sharing between pixel cells, as a function of track incident angle, and were performed with and without a 1.6 T magnetic field oriented as the ATLAS Inner Detector solenoid field. Sensors were bump bonded to the front-end chip currently used in the ATLAS pixel detector. Full 3D sensors, with electrodes penetrating through the entire wafer thickness and active edge, and double-sided 3D sensors with partially overlapping bias and read-out electrodes were tested and showed comparable performance.

physics.ins-det↗

arXiv2010

Strengthened quantum Hamming bound

We report two analytical bounds for quantum error-correcting codes that do not have preexisting classical counterparts. Firstly the quantum Hamming and Singleton bounds are combined into a single tighter bound, and then the combined bound is further strengthened via the well-known Lloyd's theorem in classical coding theory, which claims that perfect codes, codes attaining the Hamming bound, do not exist if the Lloyd's polynomial has some non-integer zeros. Our bound characterizes quantitatively the improvement over the Hamming bound via the non-integerness of the zeros of the Lloyd's polynomial. In the case of 1-error correcting codes our bound holds true for impure codes as well, which we conjecture to be always true, and for stabilizer codes there is a 1-logical-qudit improvement for an infinite family of lengths.

quant-ph↗

arXiv2007

Nonadditive quantum error-correcting code

We report the first nonadditive quantum error-correcting code, namely, a $((9,12,3))$ code which is a 12-dimensional subspace within a 9-qubit Hilbert space, that outperforms the optimal stabilizer code of the same length by encoding more levels while correcting arbitrary single-qubit errors.

quant-ph↗

arXiv2007

Controllable Quantum Switchboard

All quantum information processes inevitably requires the explicit state preparation of an entangled state. Here we present the construction of a quantum switchboard which can act both as an optimal quantum cloning machine and a quantum demultiplexer based on the preparation of a four-qubit state.

quant-ph↗

arXiv2007

Manipulating Quantum States of Molecules Created via Photoassociation of Bose-Einstein Condensates

We show the quantum state transfer technique in two-color photoassociation (PA) of a Bose-Einstein condensate, where a quantized field is used to couple the free-bound transition from atom state to excited molecular state. Under the weak excitation condition, we find that quantum states of the quantized field can be transferred to the created molecular condensate. The feasibility of this technique is confirmed by considering the atomic and molecular decays discovered in the current PA experiments. The present results allow us to manipulate quantum states of molecules in the photoassociation of a Bose-Einstein condensate.

quant-ph↗

arXiv2004

General formalism of Hamiltonians for realizing a prescribed evolution of a qubit

We investigate the inverse problem concerning the evolution of a qubit system, specifically we consider how one can establish the Hamiltonians that account for the evolution of a qubit along a prescribed path in the projected Hilbert space. For a given path, there are infinite Hamiltonians which can realize the same evolution. A general form of the Hamiltonians is constructed in which one may select the desired one for implementing a prescribed evolution. This scheme can be generalized to higher dimensional systems.

quant-ph↗

arXiv2004

Experimental Quantum Cloning with Prior Partial Information

When prior partial information about a state to be cloned is available, it can be cloned with a fidelity higher than that of universal quantum cloning. We experimentally verify this intriguing relationship between the cloning fidelity and the prior information by reporting the first experimental optimal quantum state-dependent cloning, using nuclear magnetic resonance techniques. Our experiments may further have important implications into many quantum information processing protocols.

quant-ph↗

arXiv2003

Experimental demonstration of an efficient quantum phase-covariant cloning and its possible applications to simulating eavesdropping in quantum cryptography

We describe a nuclear magnetic resonance (NMR) experiment which implements an efficient one-to-two qubit phase-covariant cloning machine(QPCCM). In the experiment we have achieved remarkably high fidelities of cloning, 0.848 and 0.844 respectively for the original and the blank qubit. This experimental value is close to the optimal theoretical value of 0.854. We have also demonstrated how to use our phase-covariant cloning machine for quantum simulations of bit by bit eavesdropping in the four-state quantum key distribution protocol.

quant-ph↗

arXiv1992

c = 1 conformal field theory and the fractional quantum Hall effect

We examine the application of $c=1$ conformal field theory to the description of the fractional quantum Hall effect (FQHE). It is found that the Gaussian model together with an appropriate boundary condition for the order parameter furnishes an effective theory for the Laughlin type FQHE. The plateau formation condition corresponds to taking the {\em chiral} portion of the theory.

hep-th↗

arXiv1992

Spinning Braid Group Representation and the Fractional Quantum Hall Effect

The path integral approach to representing braid group is generalized for particles with spin. Introducing the notion of {\em charged} winding number in the super-plane, we represent the braid group generators as homotopically constrained Feynman kernels. In this framework, super Knizhnik-Zamolodchikov operators appear naturally in the Hamiltonian, suggesting the possibility of {\em spinning nonabelian} anyons. We then apply our formulation to the study of fractional quantum Hall effect (FQHE). A systematic discussion of the ground states and their quasi-hole excitations is given. We obtain Laughlin, Halperin and Moore-Read states as {\em exact} ground state solutions to the respective Hamiltonians associated to the braid group representations. The energy gap of the quasi-excitation is also obtainable from this approach.

hep-th↗