Papers by Leong Chuan Kwek
Physical review, Jan 25, 2016
We construct an entanglement witness for many-qubit systems, based on symmetric two-body correlat... more We construct an entanglement witness for many-qubit systems, based on symmetric two-body correlations with two measurement settings. This witness is able to detect the entanglement of some Dicke states for any number of particles, and such detection exhibits some robustness against white noise and thermal noise under the Lipkin-Meshkov-Glick Hamiltonian. In addition, it detects the entanglement of spin-squeezed states, with a detection strength that approaches the maximal value for sufficiently large numbers of particles. As spin-squeezed states can be experimentally generated, the properties of the witness with respect to these states may be amenable to experimental investigation. Finally, we show that while the witness is unable to detect GHZ states, it is instead able to detect superpositions of Dicke states with GHZ states.
arXiv: Quantum Physics, 2016
Entanglement is not only fundamental for understanding multipartite quantum systems but also gene... more Entanglement is not only fundamental for understanding multipartite quantum systems but also generally useful for quantum information applications. Despite much effort devoted so far, little is known about minimal resources for detecting entanglement and also comparisons to tomography which reveals the full characterization. Here, we show that all entangled states can in general be detected in an experimental scheme that estimates the fidelity of two pure quantum states. An experimental proposal is presented with a single Hong-Ou-Mandel interferometry in which only two detectors are applied regardless of the dimensions or the number of modes of quantum systems. This shows measurement settings for entanglement detection are in general inequivalent to tomography: the number of detectors in quantum tomography increases with the dimensions and the modes whereas it is not the case in estimation of fidelity which detects entangled states.
Physical Review B, 2018
We propose two schemes to coherently transfer arbitrary quantum states of the two-electron single... more We propose two schemes to coherently transfer arbitrary quantum states of the two-electron singlet-triplet qubit across a chain of 3 quantum dots. The schemes are based on electrical control over the detuning energy of the quantum dots. The first is a pulse-gated scheme, requiring dc pulses and engineering of inter-and intra-dot Coulomb energies. The second scheme is based on the adiabatic theorem, requiring time-dependent control of the detuning energy through avoided crossings at a rate that the system remains in the ground state. We simulate the transfer fidelity using typical experimental parameters for silicon quantum dots. Our results give state transfer fidelities between 94.3% < F < 99.5% at sub-ns gate times for the pulse-gated scheme and between 75.4% < F < 99.0% at tens of ns for the adiabatic scheme. Taking into account dephasing from charge noise, we obtain state transfer fidelities between 94.0% < F < 99.2% for the pulse-gated scheme and between 64.9% < F < 93.6% for the adiabatic scheme.
WORLD SCIENTIFIC eBooks, Jan 24, 2013
Fundamental Lectures: Cavity Quantum Electrodynamics Circuit Quantum Electrodynamics Strong Coupl... more Fundamental Lectures: Cavity Quantum Electrodynamics Circuit Quantum Electrodynamics Strong Coupling in Semi-Conductors Quantum Open Systems Advanced Lectures: Strong Coupling in Plasmonic Systems Polaritons Bose Einstein Condensates Experimental Circuit QED Experimental Aspects of Quantum Dots Coupled to Cavities Quantum Polaritonics.
The relation between the maximal violation of Svetlichny's inequality and the mixedness of quantu... more The relation between the maximal violation of Svetlichny's inequality and the mixedness of quantum states is explored. We pin down the optimal state (i.e., maximally nonlocal mixed states, or MNMS, for each value of linear entropy) to play the Clauser-Horne-Shimony-Holt and Svetlichny's games. It is clearly shown that the two-qubit and three-qubit MNMS are indeed the most tolerant against white noise, thus serving as a valuable quantum resource for such games. In particular, the quantum prediction of the MNMS decreases as the linear entropy increases, and then ceases to be nonlocal when the linear entropy reaches the critical points 2/3 and 9/14 for the two-and three-qubit cases, respectively. The MNMS has a physical origination associated with the process of experimental preparation.
Nature Communications, 2021
Complex-valued neural networks have many advantages over their real-valued counterparts. Conventi... more Complex-valued neural networks have many advantages over their real-valued counterparts. Conventional digital electronic computing platforms are incapable of executing truly complex-valued representations and operations. In contrast, optical computing platforms that encode information in both phase and magnitude can execute complex arithmetic by optical interference, offering significantly enhanced computational speed and energy efficiency. However, to date, most demonstrations of optical neural networks still only utilize conventional real-valued frameworks that are designed for digital computers, forfeiting many of the advantages of optical computing such as efficient complex-valued operations. In this article, we highlight an optical neural chip (ONC) that implements truly complex-valued neural networks. We benchmark the performance of our complex-valued ONC in four settings: simple Boolean tasks, species classification of an Iris dataset, classifying nonlinear datasets (Circle a...
Physical Review A, 2016
We study the relation between the maximal violation of Svetlichny's inequality and the mixedness ... more We study the relation between the maximal violation of Svetlichny's inequality and the mixedness of quantum states and obtain the optimal state (i.e., maximally nonlocal mixed states, or MNMS, for each value of linear entropy) to beat the Clauser-Horne-Shimony-Holt and the Svetlichny games. For the two-qubit and three-qubit MNMS, we showed that these states are also the most tolerant state against white noise, and thus serve as valuable quantum resources for such games. In particular, the quantum prediction of the MNMS decreases as the linear entropy increases, and then ceases to be nonlocal when the linear entropy reaches the critical points 2/3 and 9/14 for the two-and threequbit cases, respectively. The MNMS are related to classical errors in experimental preparation of maximally entangled states.
ACS Photonics, 2021
Recent advances in silicon photonic chips have made huge progress in optical computing owing to t... more Recent advances in silicon photonic chips have made huge progress in optical computing owing to their flexibility in the reconfiguration of various tasks. Its deployment of neural networks serves as an alternative for mitigating the rapidly increased demand for computing resources in electronic platforms. However, it remains a formidable challenge to train the online programmable optical neural networks efficiently, being restricted by the difficulty in obtaining gradient information on a physical device when executing a gradient descent algorithm. Here, we experimentally demonstrate an efficient, physics-agnostic, and closed-loop protocol for training optical neural networks on chip. A gradient-free algorithm, that is, the genetic algorithm, is adopted. The protocol is on-chip implementable, physical agnostic (no need to rely on characterization and offline modeling), and gradientfree. The protocol works for various types of chip structures and is especially helpful to those that cannot be analytically decomposed and characterized. We confirm its viability using several practical tasks, including the crossbar switch and the Iris classification. Finally, by comparing our physics-agonistic and gradient-free method to the off-chip and gradient-based training methods, we demonstrate the robustness of our system to perturbations such as imperfect phase implementation and photodetection noise. Optical processors with gradient-free genetic algorithms have broad application potentials in pattern recognition, reinforcement learning, quantum computing, and realistic applications (such as facial recognition, natural language processing, and autonomous vehicles).
AAPPS Bulletin
In this short review article, we aim to provide physicists not working within the quantum computi... more In this short review article, we aim to provide physicists not working within the quantum computing community a hopefully easy-to-read introduction to the state of the art in the field, with minimal mathematics involved. In particular, we focus on what is termed theNoisy Intermediate Scale Quantumera of quantum computing. We describe how this is increasingly seen to be a distinct phase in the development of quantum computers, heralding an era where we have quantum computers that are capable of doing certain quantum computations in a limited fashion, and subject to certain constraints and noise. We further discuss the prominent algorithms that are believed to hold the most potential for this era, and also describe the competing physical platforms on which to build a quantum computer that have seen the most success so far. We then talk about the applications that are most feasible in the near-term, and finish off with a short discussion on the state of the field. We hope that as non-e...
arXiv (Cornell University), Feb 23, 2016
Quantum teleportation transfers unknown quantum states from one node in a quantum network to anot... more Quantum teleportation transfers unknown quantum states from one node in a quantum network to another. It is one of the crucial architectures in quantum information processing. The teleportation of high-dimensional quantum states remains challenging due to the difficulties in executing high-dimensional Bell state measurement. Here, we propose a Quantum Autoencoder-Facilitated Teleportation (QAFT) protocol for high-dimensional quantum teleportation, and report the first demonstration of QAFT on qutrits using an integrated photonic platform for future scalability. The key strategy is to reduce the dimension of the input states by erasing redundant information and reconstruct its initial state after chip-to-chip teleportation. Machine learning is applied in training the autoencoder to facilitate the teleportation of any state from a particular high-dimensional subspace and achieve the reconstruction of the unknown state (by the decoder) with high fidelities (~ 0.971). Experimentally, we...
Physical Review A, 2019
We study the ground state of a bosonic ring ladder under a gauge flux in the vortex phase, corres... more We study the ground state of a bosonic ring ladder under a gauge flux in the vortex phase, corresponding to the case where the single-particle dispersion relation has two degenerate minima. By combining exact diagonalization and an approximate fermionization approach we show that the ground state of the system evolves from a fragmented state of two single-particle states at weak interparticle interactions to a fragmented state of two Fermi seas at large interactions. Fragmentation is inferred from the study of the eigenvalues of the reduced single-particle density matrix as well as from the calculation of the fidelity of the states. We characterize these nonclassical states by the momentum distribution, the chiral currents and the current-current correlations.
Quantum Science and Technology, 2021
Simulating quantum dynamics is expected to be performed more easily on a quantum computer than on... more Simulating quantum dynamics is expected to be performed more easily on a quantum computer than on a classical computer. However, the currently available quantum devices lack the capability to implement fault-tolerant quantum algorithms for quantum simulation. Hybrid classical quantum algorithms such as the variational quantum algorithms have been proposed to effectively use current term quantum devices. One promising approach to quantum simulation in the noisy intermediate-scale quantum (NISQ) era is the diagonalisation based approach, with some of the promising examples being the subspace Variational Quantum Simulator (SVQS), Variational Fast Forwarding (VFF), fixed-state Variational Fast Forwarding (fs-VFF), and the Variational Hamiltonian Diagonalisation (VHD) algorithms. However, these algorithms require a feedback loop between the classical and quantum computers, which can be a crucial bottleneck in practical application. Here, we present the Classical Quantum Fast Forwarding (CQFF) as an alternative diagonalisation based algorithm for quantum simulation. CQFF shares some similarities with SVQS, VFF, fs-VFF and VHD but removes the need for a classical-quantum feedback loop and controlled multi-qubit unitaries. The CQFF algorithm does not suffer from the barren plateau problem and the accuracy can be systematically increased. Furthermore, if the Hamiltonian to be simulated is expressed as a linear combination of tensored-Pauli matrices, the CQFF algorithm reduces to the task of sampling some many-body quantum state in a set of Pauli-rotated bases, which is easy to do in the NISQ era. We run the CQFF algorithm on existing quantum processors and demonstrate the promise of the CQFF algorithm for current-term quantum hardware. We compare CQFF with Trotterization for a XY spin chain model Hamiltonian and find that the CQFF algorithm can simulate the dynamics more than 10 5 times longer than Trotterization on current-term quantum hardware. This provides a 10 4 times improvement over the previous record.
Physical Review A, 2018
A Bose-Einstein condensate confined in ring shaped lattices interrupted by a weak link and pierce... more A Bose-Einstein condensate confined in ring shaped lattices interrupted by a weak link and pierced by an effective magnetic flux defines the atomic counterpart of the superconducting quantum interference device: the atomtronic quantum interference device (AQUID). In this paper, we report on the detection of current states in the system through a self-heterodyne protocol. Following the original proposal of the NIST and Paris groups, the ring-condensate many-body wave function interferes with a reference condensate expanding from the center of the ring. We focus on the rf-AQUID which realizes effective qubit dynamics. Both the Bose-Hubbard and Gross-Pitaevskii dynamics are studied. For the Bose-Hubbard dynamics, we demonstrate that the selfheterodyne protocol can be applied, but higher-order correlations in the evolution of the interfering condensates are measured to readout of the current states of the system. We study how states with macroscopic quantum coherence can be told apart analyzing the noise in the time of flight of the ring condensate.
Scientific reports, Jan 6, 2016
Hybrids consisting of macroscopic superconducting circuits and microscopic components, such as at... more Hybrids consisting of macroscopic superconducting circuits and microscopic components, such as atoms and spins, have the potential of transmitting an arbitrary state between different quantum species, leading to the prospective of high-speed operation and long-time storage of quantum information. Here we propose a novel hybrid structure, where a neutral-atom qubit directly interfaces with a superconducting charge qubit, to implement the qubit-state transmission. The highly-excited Rydberg atom located inside the gate capacitor strongly affects the behavior of Cooper pairs in the box while the atom in the ground state hardly interferes with the superconducting device. In addition, the DC Stark shift of the atomic states significantly depends on the charge-qubit states. By means of the standard spectroscopic techniques and sweeping the gate voltage bias, we show how to transfer an arbitrary quantum state from the superconducting device to the atom and vice versa.
Cancellation of Ultra-Violet Infinities in One Loop Gravity (V E Korepin) Quantum Discord in a Sp... more Cancellation of Ultra-Violet Infinities in One Loop Gravity (V E Korepin) Quantum Discord in a Spin System with Symmetry Breaking (B Tomasello, D Rossini, A Hamma and L Amico) Entanglement from the Dynamics of an Ideal Bose Gas in a Lattice (S Bose) Aspects of the Riemannian Geometry of Quantum Computation (H E Brandt) Quantum Mechanics and the Role of Time: Are Quantum Systems Markovian? (T Durt) Explicit Formula of the Separability Criterion for Continuous Variables Systems (K Fujikawa) Yang - Baxter Equations in Quantum Information (M-L Ge and K Xue) Nondistillable Entanglement Guarantees Distillable Entanglement (L Chen and M Hayashi) Reduced Density Matrix and Entanglement Entropy of Permutationally Invariant Quantum Many-Body Systems (V Popkov and M Salerno) Solitons Experience for Black Hole Production in Ultrarelativistic Particle Collisions (I Ya Aref'Eva) Sine - Gordon Theory in the Repulsive Regime, Thermodynamic Bethe Ansatz and Minimal Models (H Itoyama) On Some Algebraic and Combinatorial Properties of Dunkl Elements (A N Kirillov) Finite Projective Spaces, Geometric Spreads of Lines and Multi-Qubits (M Saniga) Monogamy of Entanglement, N-Representability Problems and Ground States (T-C Wei).
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
Optics Express, 2017
Quantum state tomography is a key technology for fully determining a quantum state. Unfortunately... more Quantum state tomography is a key technology for fully determining a quantum state. Unfortunately, standard quantum state tomography is intractable for general many-body quantum states, because the number of measurements and the post-processing time increase exponentially with the size of the system. However, for the matrix product states (MPSs), there exists an efficient method using linearly scaled local measurements and polynomially scaled post-processing times. In this study, we demonstrate the validity of the method in practice by reconstructing a four-photon MPS from its local two-or three-photon reduced-density matrices with the presence of statistical errors and systematical errors in experiment.
The European Physical Journal Special Topics, 2015
We consider the persistent currents induced by an artificial gauge field applied to interacting u... more We consider the persistent currents induced by an artificial gauge field applied to interacting ultracold bosonic atoms in a tight ring trap. Using both analytical and numerical methods, we study the scaling of the persistent current amplitude with the size of the ring. In the strongly interacting regime we find a power-law scaling, in good agreement with the predictions of the Luttinger-liquid theory. By exploring all interaction regimes we find that the scaling is optimal, i.e. the current amplitude decreases slower with the system size, at intermediate interactions.
Department of Physics, National University of Singapore, Republic of Singapore(Dated: October28,2... more Department of Physics, National University of Singapore, Republic of Singapore(Dated: October28,2011)Theobservationthatconceptsfromquantuminformationhasgeneratedmanyalternativeindica-tors of quantum phase transitions hints that quantum phase transitions could possess operationalsignificance with respect to the processing of quantum information. Yet, there has remained fewstudies on whether the different quantum phases that result from such transitions differ in theircapacitytoprocessinformation. Weshowthereexistsquantumphasetransitionsthatcauseadis-tinctqualitativechangeinourabilitytosimulatecertainquantumsystemsunderperturbationofanexternalfieldbylocaloperationsandclassicalcommunication. Inparticular,bystudyingthegen-eralXY modelweshowthatincertainquantumphases,theeffectofadiabaticperturbationsoftheexternalmagneticfieldcanbesimulatedbylocalspinoperations,whereastheresultingeffectwithinotherphasesresultsinfundamentallynon-localinteractions. Wediscussthepotentialimplicationsof such phase transitions to adiabatic quantum computation, where a computational advantageexistsonlywhenadiabaticperturbationofgroundstatesresultsinmulti-bodyinteractions.
Uploads
Papers by Leong Chuan Kwek