Showing 1–50 of 57 results for author: Kwek, L

Manipulation of Topological Corner States via Subchiral Symmetry

Authors: Hai-Tao Ding, Tianqi Chen, Leong-Chuan Kwek, Jiangbin Gong

Abstract: Higher-order topological phases provide robust corner modes, but their use requires controllable creation, isolation, and transfer of individual modes and their superpositions. Here we demonstrate, using the two-dimensional Benalcazar-Bernevig-Hughes model as an example, that subchiral symmetry provides a general control principle for manipulating topological corner modes. The conventional chiral… ▽ More Higher-order topological phases provide robust corner modes, but their use requires controllable creation, isolation, and transfer of individual modes and their superpositions. Here we demonstrate, using the two-dimensional Benalcazar-Bernevig-Hughes model as an example, that subchiral symmetry provides a general control principle for manipulating topological corner modes. The conventional chiral symmetry decomposes into four subchiral symmetries, each associated with one zero-energy corner mode. By selectively breaking these subsymmetries with controlled intercell hoppings, we reduce the fourfold corner-state manifold step by step to single isolated modes. We further design adiabatic protocols that transfer either a single corner state or a superposition of two corner states between selected corners, while preserving the relative phase in the latter case. Both numerical simulations and IBM quantum-processor implementations show that the proposed protocols can be executed with high fidelity, establishing subchiral symmetry as a route to programmable higher-order topological state manipulation. △ Less

Submitted 16 June, 2026; originally announced June 2026.

Enhanced Density Fluctuations Near a Disordered Chiral Topological Transition

Authors: Hai-Tao Ding, Sen Mu, Leong-Chuan Kwek, Gabriel Lemarié, Jiangbin Gong

Abstract: The universal statistics of density fluctuations of localized quantum states may offer unprecedented opportunities to probe and understand quantum transport in connection with dimensionality, coherence, symmetry and disorder. To date, the possible role of topological phase transitions in the fluctuation statistics is not studied yet. Using a Su-Schrieffer-Heeger chain subject to off-diagonal disor… ▽ More The universal statistics of density fluctuations of localized quantum states may offer unprecedented opportunities to probe and understand quantum transport in connection with dimensionality, coherence, symmetry and disorder. To date, the possible role of topological phase transitions in the fluctuation statistics is not studied yet. Using a Su-Schrieffer-Heeger chain subject to off-diagonal disorder (so that chiral symmetry is preserved), this work investigates how a disorder driven topological phase transition impacts on the spatial fluctuations of the logarithmic wave-packet density $\ln P(r)$ at distance $r$ from the initial excitation. Away from the transition, in both topological and trivial localized phases, the standard deviation follows the conventional one-dimensional scaling $σ[\ln P(r)]\sim r^θ$ with $θ\simeq 1/2$. Near the transition, however, the fluctuation growth is enhanced: the fitted exponent $θ$ increases above $1/2$ in a nonmonotonic manner before returning close to $1/2$ at criticality. We interpret this behavior from the energy-resolved density of states and localization length. Near the transition, several energy sectors carry appreciable spectral weight and exhibit competitive decay rates, preventing a single localization scale from dominating the accessible wave-packet tail and thereby enhancing the fluctuations of $\ln P(r)$. Our results establish wave-packet fluctuation statistics as a dynamical diagnostic of disordered chiral topological transitions and motivate broader studies of fluctuation phenomena in disordered topological quantum systems. △ Less

Submitted 28 May, 2026; originally announced May 2026.

A passive atomtronics filter for Fermi gases

Authors: Jun Hao Hue, Martin-Isbjörn Trappe, Piotr T. Grochowski, Jonathan Lau, Leong-Chuan Kwek

Abstract: We design an atomtronic filter device that spatially separates the components of a two-component Fermi gas with repulsive contact interactions in a two-dimensional geometry. With the aid of density--potential functional theory (DPFT), which can accurately simulate Fermi gases in realistic settings, we propose and characterize a barbell-shaped trapping potential, where a bridge-shaped potential con… ▽ More We design an atomtronic filter device that spatially separates the components of a two-component Fermi gas with repulsive contact interactions in a two-dimensional geometry. With the aid of density--potential functional theory (DPFT), which can accurately simulate Fermi gases in realistic settings, we propose and characterize a barbell-shaped trapping potential, where a bridge-shaped potential connects two ring-shaped potentials. In the strongly repulsive regime, each of the ring traps eventually stores one of the fermion species. Our simulations are a guide to designing component filters for initially mixed, weakly repulsive spin components. We demonstrate that the functioning of this barbell design is robust against variations in experimental settings, for example, across particle numbers, for small deformations of the trap geometry, or if interatomic interactions differ from the bare contact repulsion. Our investigation marks the first step in establishing DPFT as a comprehensive simulation framework for fermionic atomtronics. △ Less

Submitted 9 November, 2025; originally announced November 2025.

Comments: 9 pages, 8 figures

Amplification, Mitigation and Energy Storage via Constrained Thermalization

Authors: Harshank Shrotriya, Midhun Krishna, Leong-Chuan Kwek, Varun Narasimhachar, Sai Vinjanampathy

Abstract: Amplification (mitigation) is the increase (decrease) in the change of thermodynamic quantities when an initial thermal state is thermalized to a different temperature in the presence of constraints, studied thus far only for permutationally invariant baths. In this manuscript, we generalize amplification and mitigation to accommodate generic strong symmetries of open quantum systems and connect t… ▽ More Amplification (mitigation) is the increase (decrease) in the change of thermodynamic quantities when an initial thermal state is thermalized to a different temperature in the presence of constraints, studied thus far only for permutationally invariant baths. In this manuscript, we generalize amplification and mitigation to accommodate generic strong symmetries of open quantum systems and connect the phenomenon to Landauer's erasure. We exemplify our general theory with a new bath-induced battery charging protocol that overcomes the passivity of KMS-preserving transitions. △ Less

Submitted 3 November, 2023; originally announced November 2023.

Comments: Harshank Shrotriya and Midhun Krishna contributed equally, 8 pages, comments welcome

Symmetry-protected topological corner modes in a periodically driven interacting spin lattice

Authors: Kelvin Koor, Raditya Weda Bomantara, Leong Chuan Kwek

Abstract: Periodic driving has the longstanding reputation for generating exotic phases of matter with no static counterparts. This work explores the interplay among periodic driving, interaction effects, and $\mathbb{Z}_2$ symmetry that leads to the emergence of Floquet symmetry protected second-order topological phases in a simple but insightful two-dimensional spin-1/2 lattice. Through a combination of a… ▽ More Periodic driving has the longstanding reputation for generating exotic phases of matter with no static counterparts. This work explores the interplay among periodic driving, interaction effects, and $\mathbb{Z}_2$ symmetry that leads to the emergence of Floquet symmetry protected second-order topological phases in a simple but insightful two-dimensional spin-1/2 lattice. Through a combination of analytical and numerical treatments, we verify the formation of 0 and $π$ modes, i.e., corner localized $\mathbb{Z}_2$ symmetry broken operators that respectively commute and anticommute with the one-period time evolution operator. We further verify the topological nature of these modes by demonstrating their presence over a wide range of parameter values and explicitly deriving their associated topological invariants under special conditions. Finally, we propose a means to detect the signature of such modes in experiments and discuss the effect of imperfections. △ Less

Submitted 16 November, 2022; v1 submitted 14 June, 2022; originally announced June 2022.

Comments: 16 pages, 10 figures. Comments are most welcome

Journal ref: Phys. Rev. B 106, 195122 (2022)

NISQ algorithm for the matrix elements of a generic observable

Authors: Rebecca Erbanni, Kishor Bharti, Leong-Chuan Kwek, Dario Poletti

Abstract: The calculation of off-diagonal matrix elements has various applications in fields such as nuclear physics and quantum chemistry. In this paper, we present a noisy intermediate scale quantum algorithm for estimating the diagonal and off-diagonal matrix elements of a generic observable in the energy eigenbasis of a given Hamiltonian. Several numerical simulations indicate that this approach can fin… ▽ More The calculation of off-diagonal matrix elements has various applications in fields such as nuclear physics and quantum chemistry. In this paper, we present a noisy intermediate scale quantum algorithm for estimating the diagonal and off-diagonal matrix elements of a generic observable in the energy eigenbasis of a given Hamiltonian. Several numerical simulations indicate that this approach can find many of the matrix elements even when the trial functions are randomly initialized across a wide range of parameter values without, at the same time, the need to prepare the energy eigenstates. △ Less

Submitted 2 December, 2022; v1 submitted 20 May, 2022; originally announced May 2022.

Journal ref: SciPost Phys. 15, 180 (2023)

Atomtronic multi-terminal Aharonov-Bohm interferometer

Authors: Jonathan Wei Zhong Lau, Koon Siang Gan, Rainer Dumke, Luigi Amico, Leong-Chuan Kwek, Tobias Haug

Abstract: We study a multi-functional device for cold atoms consisting of a three-terminal ring circuit pierced by a synthetic magnetic flux, where the ring can be continuous or discretized. The flux controls the atomic current through the ring via the Aharonov-Bohm effect. Our device shows a flux-induced transition of reflections from an Andreev-like negative density to positive density. Further, the flux… ▽ More We study a multi-functional device for cold atoms consisting of a three-terminal ring circuit pierced by a synthetic magnetic flux, where the ring can be continuous or discretized. The flux controls the atomic current through the ring via the Aharonov-Bohm effect. Our device shows a flux-induced transition of reflections from an Andreev-like negative density to positive density. Further, the flux can direct the atomic current into specific output ports, realizing a flexible non-reciprocal switch to connect multiple atomic systems or sense rotations. By changing the flux linearly in time, we convert constant matter wave currents into an AC modulated current. This effect can be used to realize an atomic frequency generator and study fundamental problems related to the Aharonov-Bohm effect. We experimentally demonstrate Bose-Einstein condensation into the light-shaped optical potential of the three-terminal ring. Our work opens up the possibility of novel atomtronic devices for practical applications in quantum technologies. △ Less

Submitted 15 May, 2023; v1 submitted 3 May, 2022; originally announced May 2022.

Comments: 17 pages, 11 figures

Atomtronic circuits: from many-body physics to quantum technologies

Authors: Luigi Amico, Dana Anderson, Malcolm Boshier, Jean-Philippe Brantut, Leong-Chuan Kwek, Anna Minguzzi, Wolf von Klitzing

Abstract: Atomtronics is an emerging field that aims to manipulate ultracold atom moving in matter wave circuits for both fundamental studies in quantum science and technological applications. In this colloquium, we review recent progress in matter-wave circuitry and atomtronics-based quantum technology. After a short introduction to the basic physical principles and the key experimental techniques needed t… ▽ More Atomtronics is an emerging field that aims to manipulate ultracold atom moving in matter wave circuits for both fundamental studies in quantum science and technological applications. In this colloquium, we review recent progress in matter-wave circuitry and atomtronics-based quantum technology. After a short introduction to the basic physical principles and the key experimental techniques needed to realize atomtronic systems, we describe the physics of matter-waves in simple circuits such as ring traps and two-terminal systems. The main experimental observations and outstanding questions are discussed. We also present possible applications to a broad range of quantum technologies, from quantum sensing with atom interferometry to future quantum simulation and quantum computation architectures. △ Less

Submitted 14 June, 2022; v1 submitted 18 July, 2021; originally announced July 2021.

Comments: 33 revtex pages; 10 figs. To be published in Reviews of Modern Physics

Journal ref: Rev. Mod. Phys. 94, 041001 (2022)

Stability and Dynamics of Many-Body Localized Systems Coupled to Small Bath

Authors: Shao-Hen Chiew, Jiangbin Gong, Leong-Chuan Kwek, Chee-Kong Lee

Abstract: It is known that strong disorder in closed quantum systems leads to many-body localization (MBL), and that this quantum phase can be destroyed by coupling to an infinitely large Markovian environment. However, the stability of the MBL phase is less clear when the system and environment are of finite and comparable size. Here, we study the stability and eventual localization properties of a disorde… ▽ More It is known that strong disorder in closed quantum systems leads to many-body localization (MBL), and that this quantum phase can be destroyed by coupling to an infinitely large Markovian environment. However, the stability of the MBL phase is less clear when the system and environment are of finite and comparable size. Here, we study the stability and eventual localization properties of a disordered Heisenberg spin chain coupled to a finite environment, and extensively explore the effects of environment disorder, geometry, initial state and system-bath coupling strength. By studying the non-equilibrium dynamics and the eventual steady-state properties of different initial states, our numerical results indicate that in most cases, the system retains its localization properties despite the coupling to the finite environment, albeit to a reduced extent. However, in cases where the system and environment is strongly coupled in the ladder configuration, the eventual localization properties are highly dependent on the initial state, and could lead to either thermalization or localization. △ Less

Submitted 22 January, 2022; v1 submitted 16 July, 2021; originally announced July 2021.

GHZ-like states in the Qubit-Qudit Rabi Model

Authors: Yuan Shen, Giampiero Marchegiani, Gianluigi Catelani, Luigi Amico, Ai Qun Liu, Weijun Fan, Leong-Chuan Kwek

Abstract: We study a Rabi type Hamiltonian system in which a qubit and a d-level quantum system (qudit) are coupled through a common resonator. In the weak and strong coupling limits the spectrum is analysed through suitable perturbative schemes. The analysis show that the presence of the multilevels of the qudit effectively enhance the qubit-qudit interaction. The ground state of the strongly coupled syste… ▽ More We study a Rabi type Hamiltonian system in which a qubit and a d-level quantum system (qudit) are coupled through a common resonator. In the weak and strong coupling limits the spectrum is analysed through suitable perturbative schemes. The analysis show that the presence of the multilevels of the qudit effectively enhance the qubit-qudit interaction. The ground state of the strongly coupled system is a found of Greenberger-Horne-Zeilinger (GHZ) type. Therefore, despite the qubit-qudit strong coupling, the nature of the specific tripartite entanglement of the GHZ state suppress the bipartite entanglement. We analyze the system dynamics under quenching and adiabatic switching of the qubit-resonator and qudit-resonator couplings. In the quench case, we found that the non-adiabatic generations of photons in the resonator is enhanced by the number of levels in the qudit. The adiabatic control represents a possible route for preparation of GHZ states. Our analysis provides relevant information for future studies on coherent state transfer in qubit-qudit systems. △ Less

Submitted 5 July, 2021; v1 submitted 26 April, 2021; originally announced April 2021.

Comments: 15 pages, 7 figures, Submission to SciPost

Journal ref: SciPost Phys. 11, 099 (2021)

Noisy intermediate-scale quantum (NISQ) algorithms

Authors: Kishor Bharti, Alba Cervera-Lierta, Thi Ha Kyaw, Tobias Haug, Sumner Alperin-Lea, Abhinav Anand, Matthias Degroote, Hermanni Heimonen, Jakob S. Kottmann, Tim Menke, Wai-Keong Mok, Sukin Sim, Leong-Chuan Kwek, Alán Aspuru-Guzik

Abstract: A universal fault-tolerant quantum computer that can solve efficiently problems such as integer factorization and unstructured database search requires millions of qubits with low error rates and long coherence times. While the experimental advancement towards realizing such devices will potentially take decades of research, noisy intermediate-scale quantum (NISQ) computers already exist. These co… ▽ More A universal fault-tolerant quantum computer that can solve efficiently problems such as integer factorization and unstructured database search requires millions of qubits with low error rates and long coherence times. While the experimental advancement towards realizing such devices will potentially take decades of research, noisy intermediate-scale quantum (NISQ) computers already exist. These computers are composed of hundreds of noisy qubits, i.e. qubits that are not error-corrected, and therefore perform imperfect operations in a limited coherence time. In the search for quantum advantage with these devices, algorithms have been proposed for applications in various disciplines spanning physics, machine learning, quantum chemistry and combinatorial optimization. The goal of such algorithms is to leverage the limited available resources to perform classically challenging tasks. In this review, we provide a thorough summary of NISQ computational paradigms and algorithms. We discuss the key structure of these algorithms, their limitations, and advantages. We additionally provide a comprehensive overview of various benchmarking and software tools useful for programming and testing NISQ devices. △ Less

Submitted 6 October, 2021; v1 submitted 21 January, 2021; originally announced January 2021.

Comments: Added new content, Modified certain parts and the paper structure

Journal ref: Rev. Mod. Phys. 94, 015004 (2022)

Simulating Energy Transfer in Molecular Systems with Digital Quantum Computers

Authors: Chee-Kong Lee, Jonathan Wei Zhong Lau, Liang Shi, Leong Chuan Kwek

Abstract: Quantum computers have the potential to simulate chemical systems beyond the capability of classical computers. Recent developments in hybrid quantum-classical approaches enable the determinations of the ground or low energy states of molecular systems. Here, we extend near-term quantum simulations of chemistry to time-dependent processes by simulating energy transfer in organic semiconducting mol… ▽ More Quantum computers have the potential to simulate chemical systems beyond the capability of classical computers. Recent developments in hybrid quantum-classical approaches enable the determinations of the ground or low energy states of molecular systems. Here, we extend near-term quantum simulations of chemistry to time-dependent processes by simulating energy transfer in organic semiconducting molecules. We developed a multi-scale modeling workflow that combines conventional molecular dynamics and quantum chemistry simulations with hybrid variational quantum algorithm to compute the exciton dynamics in both the single excitation subspace (i.e. Frenkel Hamiltonian) and the full-Hilbert space (i.e. multi-exciton) regimes. Our numerical examples demonstrate the feasibility of our approach, and simulations on IBM Q devices capture the qualitative behaviors of exciton dynamics, but with considerable errors. We present an error mitigation technique that combines experimental results from the variational and Trotter algorithms, and obtain significantly improved quantum dynamics. Our approach opens up new opportunities for modeling quantum dynamics in chemical, biological and material systems with quantum computers. △ Less

Submitted 2 December, 2021; v1 submitted 18 January, 2021; originally announced January 2021.

Comments: Including full-Hilbert space (i.e. multi-exciton) treatment of energy transfer

Persistent Current of SU(N) Fermions

Authors: Wayne J. Chetcuti, Tobias Haug, Leong-Chuan Kwek, Luigi Amico

Abstract: We study the persistent current in a system of SU($N$) fermions with repulsive interaction confined in a ring-shaped potential and pierced by an effective magnetic flux. By applying a combination of Bethe ansatz and numerical analysis, we demonstrate that, as a combined effect of spin correlations, interactions and applied flux a specific phenomenon can occur in the system: spinon creation in the… ▽ More We study the persistent current in a system of SU($N$) fermions with repulsive interaction confined in a ring-shaped potential and pierced by an effective magnetic flux. By applying a combination of Bethe ansatz and numerical analysis, we demonstrate that, as a combined effect of spin correlations, interactions and applied flux a specific phenomenon can occur in the system: spinon creation in the ground state. As a consequence, peculiar features in the persistent current arise. The elementary flux quantum, which fixes the persistent current periodicity, is observed to evolve from a single particle one to an extreme case of fractional flux quantum, in which one quantum is shared by all the particles. We show that the persistent current depends on the number of spin components $N$, number of particles and interaction in a specific way that in certain physical regimes has universality traits. At integer filling fractions, the persistent current is suppressed above a threshold of the repulsive interaction by the Mott spectral gap. Despite its mesoscopic nature, the current displays a clear finite size scaling behavior. Specific parity effects in the persistent current landscape hold. △ Less

Submitted 14 January, 2022; v1 submitted 2 November, 2020; originally announced November 2020.

Comments: 16 revtex pages, 11 figures. Accepted for Publication in SciPost Physics

Journal ref: SciPost Phys. 12, 033 (2022)

Fully-programmable universal quantum simulator with a one-dimensional quantum processor

Authors: V. M. Bastidas, T. Haug, C. Gravel, L. -C. Kwek, W. J. Munro, Kae Nemoto

Abstract: Current quantum devices execute specific tasks that are hard for classical computers and have the potential to solve problems such as quantum simulation of material science and chemistry, even without error correction. For practical applications it is highly desirable to reconfigure the connectivity of the device, which for superconducting quantum processors is determined at fabrication. In additi… ▽ More Current quantum devices execute specific tasks that are hard for classical computers and have the potential to solve problems such as quantum simulation of material science and chemistry, even without error correction. For practical applications it is highly desirable to reconfigure the connectivity of the device, which for superconducting quantum processors is determined at fabrication. In addition, we require a careful design of control lines and couplings to resonators for measurements. Therefore, it is a cumbersome and slow undertaking to fabricate a new device for each problem we want to solve. Here we periodically drive a one-dimensional chain to engineer effective Hamiltonians that simulate arbitrary connectivities. We demonstrate the capability of our method by engineering driving sequences to simulate star, all-to-all, and ring connectivities. We also simulate a minimal example of the 3-SAT problem including three-body interactions, which are difficult to realize experimentally. Our results open a new paradigm to perform quantum simulation in near term quantum devices by enabling us to stroboscopically simulate arbitrary Hamiltonians with a single device and optimized driving sequences △ Less

Submitted 2 September, 2020; originally announced September 2020.

Comments: 13 pages, 11 Figures. Comments are welcome

Journal ref: Phys. Rev. B 105, 075140 (2022)

Roadmap on Atomtronics: State of the art and perspective

Authors: L. Amico, M. Boshier, G. Birkl, A. Minguzzi, C. Miniatura, L. -C. Kwek, D. Aghamalyan, V. Ahufinger, D. Anderson, N. Andrei, A. S. Arnold, M. Baker, T. A. Bell, T. Bland, J. P. Brantut, D. Cassettari, W. J. Chetcuti, F. Chevy, R. Citro, S. De Palo, R. Dumke, M. Edwards, R. Folman, J. Fortagh, S. A. Gardiner , et al. (34 additional authors not shown)

Abstract: Atomtronics deals with matter-wave circuits of ultra-cold atoms manipulated through magnetic or laser-generated guides with different shapes and intensities. In this way, new types of quantum networks can be constructed, in which coherent fluids are controlled with the know-how developed in the atomic and molecular physics community. In particular, quantum devices with enhanced precision, control… ▽ More Atomtronics deals with matter-wave circuits of ultra-cold atoms manipulated through magnetic or laser-generated guides with different shapes and intensities. In this way, new types of quantum networks can be constructed, in which coherent fluids are controlled with the know-how developed in the atomic and molecular physics community. In particular, quantum devices with enhanced precision, control and flexibility of their operating conditions can be accessed. Concomitantly, new quantum simulators and emulators harnessing on the coherent current flows can also be developed. Here, we survey the landscape of atomtronics-enabled quantum technology and draw a roadmap for the field in the near future. We review some of the latest progresses achieved in matter-wave circuits design and atom-chips. Atomtronic networks are deployed as promising platforms for probing many-body physics with a new angle and a new twist. The latter can be done both at the level of equilibrium and non-equilibrium situations. Numerous relevant problems in mesoscopic physics, like persistent currents and quantum transport in circuits of fermionic or bosonic atoms, are studied through a new lens. We summarize some of the atomtronics quantum devices and sensors. Finally, we discuss alkali-earth and Rydberg atoms as potential platforms for the realization of atomtronic circuits with special features. △ Less

Submitted 11 June, 2021; v1 submitted 10 August, 2020; originally announced August 2020.

Comments: 113 latex pages, 83 figs. To be published in AVS Quantum Science

Journal ref: AVS Quantum Sci. 3, 039201 (2021)

Generalized Measure of Quantum synchronization

Authors: Noufal Jaseem, Michal Hajdušek, Parvinder Solanki, Leong-Chuan Kwek, Rosario Fazio, Sai Vinjanampathy

Abstract: We present a generalized information-theoretic measure of synchronization in quantum systems. This measure is applicable to dynamics of anharmonic oscillators, few-level atoms, and coupled oscillator networks. Furthermore, the new measure allows us to discuss synchronization of disparate physical systems such as coupled hybrid quantum systems and coupled systems undergoing mutual synchronization t… ▽ More We present a generalized information-theoretic measure of synchronization in quantum systems. This measure is applicable to dynamics of anharmonic oscillators, few-level atoms, and coupled oscillator networks. Furthermore, the new measure allows us to discuss synchronization of disparate physical systems such as coupled hybrid quantum systems and coupled systems undergoing mutual synchronization that are also driven locally. In many cases of interest, we find a closed-form expression for the proposed measure. △ Less

Submitted 24 June, 2020; originally announced June 2020.

Comments: 8 pages, 4 figures, comments welcome

Journal ref: Phys. Rev. Research 2, 043287 (2020)

Classifying global state preparation via deep reinforcement learning

Authors: Tobias Haug, Wai-Keong Mok, Jia-Bin You, Wenzu Zhang, Ching Eng Png, Leong-Chuan Kwek

Abstract: Quantum information processing often requires the preparation of arbitrary quantum states, such as all the states on the Bloch sphere for two-level systems. While numerical optimization can prepare individual target states, they lack the ability to find general solutions that work for a large class of states in more complicated quantum systems. Here, we demonstrate global quantum control by prepar… ▽ More Quantum information processing often requires the preparation of arbitrary quantum states, such as all the states on the Bloch sphere for two-level systems. While numerical optimization can prepare individual target states, they lack the ability to find general solutions that work for a large class of states in more complicated quantum systems. Here, we demonstrate global quantum control by preparing a continuous set of states with deep reinforcement learning. The protocols are represented using neural networks, which automatically groups the protocols into similar types, which could be useful for finding classes of protocols and extracting physical insights. As application, we generate arbitrary superposition states for the electron spin in complex multi-level nitrogen-vacancy centers, revealing classes of protocols characterized by specific preparation timescales. Our method could help improve control of near-term quantum computers, quantum sensing devices and quantum simulations. △ Less

Submitted 26 May, 2020; originally announced May 2020.

Journal ref: Mach. Learn.: Sci. Technol. 2 (2020)

Machine learning engineering of quantum currents

Authors: Tobias Haug, Rainer Dumke, Leong-Chuan Kwek, Christian Miniatura, Luigi Amico

Abstract: The design, accurate preparation and manipulation of quantum states in quantum circuits are essential operational tasks at the heart of quantum technologies. Nowadays, circuits can be designed with physical parameters that can be controlled with unprecedented accuracy and flexibility. However, the generation of well-controlled current states is still a nagging bottleneck, especially when different… ▽ More The design, accurate preparation and manipulation of quantum states in quantum circuits are essential operational tasks at the heart of quantum technologies. Nowadays, circuits can be designed with physical parameters that can be controlled with unprecedented accuracy and flexibility. However, the generation of well-controlled current states is still a nagging bottleneck, especially when different circuit elements are integrated together. In this work, we show how machine learning can effectively address this challenge and outperform the current existing methods. To this end, we exploit deep reinforcement learning to prepare prescribed quantum current states in circuits composed of lumped elements. To highlight our method, we show how to engineer bosonic persistent currents as they are relevant in different quantum technologies as cold atoms and superconducting circuits. We demonstrate the use of deep reinforcement learning to re-discover established protocols, as well as solve configurations that are difficult to treat with other methods. With our approach, quantum current states characterised by a single winding number or entangled currents of multiple winding numbers can be prepared in a robust manner, superseding the existing protocols. △ Less

Submitted 2 February, 2021; v1 submitted 21 November, 2019; originally announced November 2019.

Comments: 13 pages, 13 figures

Journal ref: Phys. Rev. Research 3, 013034 (2021)

Topological pumping of quantum correlations

Authors: T. Haug, L. Amico, L. -C. Kwek, W. J. Munro, V. M. Bastidas

Abstract: Topological pumping and duality transformations are paradigmatic concepts in condensed matter and statistical mechanics. In this paper, we extend the concept of topological pumping of particles to topological pumping of quantum correlations. We propose a scheme to find pumping protocols for highly-correlated states by mapping them to uncorrelated ones. We show that one way to achieve this is to us… ▽ More Topological pumping and duality transformations are paradigmatic concepts in condensed matter and statistical mechanics. In this paper, we extend the concept of topological pumping of particles to topological pumping of quantum correlations. We propose a scheme to find pumping protocols for highly-correlated states by mapping them to uncorrelated ones. We show that one way to achieve this is to use dualities, because they are non-local transformations that preserve the topological properties of the system. By using them, we demonstrate that topological pumping of kinks and cluster-like excitations can be realized. We find that the entanglement of these highly-correlated excitations is strongly modified during the pumping process and the interactions enhance the robustness against disorder. Our work paves the way to explore topological pumping beyond the notion of particles and opens a new avenue to investigate the relation between correlations and topology. △ Less

Submitted 17 January, 2020; v1 submitted 9 May, 2019; originally announced May 2019.

Comments: 8 Pages, 1 Table, 7 Figures. Accepted for publication in Physical Review Research

Journal ref: Phys. Rev. Research 2, 013135 (2020)

Topological insulator and particle pumping in a one-dimensional shaken optical lattice

Authors: Feng Mei, Jia-Bin You, Dan-Wei Zhang, X. C. Yang, R. Fazio, Shi-Liang Zhu, L. C. Kwek

Abstract: We propose a simple method to simulate and detect topological insulators with cold atoms trapped in a one-dimensional bichromatic optical lattice subjected to a time-periodic modulation. The tight-binding form of this shaken system is equivalent to the periodically driven Aubry-Andre model. We demonstrate that this model can be mapped into a two-dimensional Chern insulator model, whose energy spec… ▽ More We propose a simple method to simulate and detect topological insulators with cold atoms trapped in a one-dimensional bichromatic optical lattice subjected to a time-periodic modulation. The tight-binding form of this shaken system is equivalent to the periodically driven Aubry-Andre model. We demonstrate that this model can be mapped into a two-dimensional Chern insulator model, whose energy spectrum hosts a topological phase within an experimentally accessible parameter regime. By tuning the laser phase adiabatically, such one-dimensional system constitutes a natural platform to realize topological particle pumping. We show that the Chern number characterizing the topological features of this system can be measured by detecting the density shift after one cycle of pumping. △ Less

Submitted 21 February, 2019; originally announced February 2019.

Comments: 8 pages, 2 figures

Journal ref: PRA 90, 063638 (2014)

Environment mediated multipartite and multidimensional entanglement

Authors: Chee Kong Lee, Mojdeh Shikhali Najafabadi, Daniel Schumayer, Leong Chuan Kwek, David A. W. Hutchinson

Abstract: Quantum entanglement is usually considered a fragile quantity and decoherence through coupling to an external environment, such as a thermal reservoir, can quickly destroy the entanglement resource. This doesn't have to be the case and the environment can be engineered to assist in the formation of entanglement. We investigate a system of qubits and higher dimensional spins interacting only throug… ▽ More Quantum entanglement is usually considered a fragile quantity and decoherence through coupling to an external environment, such as a thermal reservoir, can quickly destroy the entanglement resource. This doesn't have to be the case and the environment can be engineered to assist in the formation of entanglement. We investigate a system of qubits and higher dimensional spins interacting only through their mutual coupling to a reservoir. We explore the entanglement of multipartite and multidimensional system as mediated by the bath and show that at low temperatures and intermediate coupling strengths multipartite entanglement may form between qubits and between higher spins, i.e., qudits. We characterise the multipartite entanglement using an entanglement witness based upon the structure factor and demonstrate its validity versus the directly calculated entanglement of formation, suggesting possible experiments for its measure. △ Less

Submitted 21 February, 2019; originally announced February 2019.

Comments: 9 pages, 10 figures

Journal ref: Scientific Reports 9, 9147 (2019)

Quantum Synchronisation in Nanoscale Heat Engines

Authors: Noufal Jaseem, Michal Hajdušek, Vlatko Vedral, Rosario Fazio, Leong-Chuan Kwek, Sai Vinjanampathy

Abstract: Owing to the ubiquity of synchronization in the classical world, it is interesting to study its behavior in quantum systems. Though quantum synchronisation has been investigated in many systems, a clear connection to quantum technology applications is lacking. We bridge this gap and show that nanoscale heat engines are a natural platform to study quantum synchronization and always possess a stable… ▽ More Owing to the ubiquity of synchronization in the classical world, it is interesting to study its behavior in quantum systems. Though quantum synchronisation has been investigated in many systems, a clear connection to quantum technology applications is lacking. We bridge this gap and show that nanoscale heat engines are a natural platform to study quantum synchronization and always possess a stable limit cycle. Furthermore, we demonstrate an intimate relationship between the power of a heat engine and its phase-locking properties by proving that synchronization places an upper bound on the achievable steady-state power of the engine. Finally, we show that the efficiency of the engine sets a point in terms of the bath temperatures where synchronization vanishes. We link the physical phenomenon of synchronization with the emerging field of quantum thermodynamics by establishing quantum synchronization as a mechanism of stable phase coherence. △ Less

Submitted 8 July, 2019; v1 submitted 25 December, 2018; originally announced December 2018.

Comments: 5 pages, 3 pages appendix, 2 figures

Journal ref: Phys. Rev. E 101, 020201 (2020)

Dynamical quantum phase transitions and non-Markovian dynamics

Authors: Thi Ha Kyaw, Victor M. Bastidas, Jirawat Tangpanitanon, Guillermo Romero, Leong-Chuan Kwek

Abstract: In the context of closed quantum systems, when a system prepared in its ground state undergoes a sudden quench, the resulting Loschmidt echo can exhibit zeros, resembling the Fisher zeros in the theory of classical equilibrium phase transitions. These zeros lead to nonanalytical behavior of the corresponding rate function, which is referred to as \textit{dynamical quantum phase transitions} (DQPTs… ▽ More In the context of closed quantum systems, when a system prepared in its ground state undergoes a sudden quench, the resulting Loschmidt echo can exhibit zeros, resembling the Fisher zeros in the theory of classical equilibrium phase transitions. These zeros lead to nonanalytical behavior of the corresponding rate function, which is referred to as \textit{dynamical quantum phase transitions} (DQPTs). In this work, we investigate DQPTs in the context of open quantum systems that are coupled to both Markovian and non-Markovian dephasing baths via a conserved quantity. The general framework is corroborated by studying the non-equilibrium dynamics of a transverse-field Ising ring. We show the robustness of DQPT signatures under the action of both engineered dephasing baths, independently on how strongly they couple to the quantum system. Our theory provides insight on the effect of non-Markovian environments on DQPTs. △ Less

Submitted 22 March, 2019; v1 submitted 12 November, 2018; originally announced November 2018.

Comments: 19 pages, 11 figures. Added and updated references. Comments are welcome

Journal ref: Phys. Rev. A 101, 012111 (2020)

Topological pumping in Aharonov-Bohm rings

Authors: Tobias Haug, Rainer Dumke, Leong-Chuan Kwek, Luigi Amico

Abstract: Topological Thouless pumping and Aharonov-Bohm effect are both fundamental effects enabled by the topological properties of the system. Here, we study both effects together: topological pumping of interacting particles through Aharonov-Bohm rings. This system can prepare highly entangled many-particle states, transport them via topological pumping and interfere them, revealing a fractional flux qu… ▽ More Topological Thouless pumping and Aharonov-Bohm effect are both fundamental effects enabled by the topological properties of the system. Here, we study both effects together: topological pumping of interacting particles through Aharonov-Bohm rings. This system can prepare highly entangled many-particle states, transport them via topological pumping and interfere them, revealing a fractional flux quantum. The type of the generated state is revealed by non-trivial Aharonov-Bohm interference patterns that could be used for quantum sensing. The reflections induced by the interference result from transitions between topological bands. Specific bands allow transport with a band gap scaling as the square-root of the particle number. Our system paves a new way for a combined system of state preparation and topological protected transport. △ Less

Submitted 23 September, 2019; v1 submitted 19 October, 2018; originally announced October 2018.

Comments: to be published in Communications Physics

Journal ref: Commun Phys 2, 127 (2019)

Nonclassical states in strongly correlated bosonic ring ladders

Authors: Nicolas Victorin, Tobias Haug, Leong-Chuan Kwek, Luigi Amico, Anna Minguzzi

Abstract: 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 inte… ▽ 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. △ Less

Submitted 8 October, 2018; originally announced October 2018.

Journal ref: Phys. Rev. A 99, 033616 (2019)

Monitoring currents in cold-atom circuits

Authors: S. Safaei, L. -C. Kwek, R. Dumke, L. Amico

Abstract: Complex circuits of cold atoms can be exploited to devise new protocols for the diagnostics of cold-atoms systems. Specifically, we study the quench dynamics of a condensate confined in a ring-shaped potential coupled with a rectilinear guide of finite size. We find that the dynamics of the atoms inside the guide is distinctive of the states with different winding numbers in the ring condensate. W… ▽ More Complex circuits of cold atoms can be exploited to devise new protocols for the diagnostics of cold-atoms systems. Specifically, we study the quench dynamics of a condensate confined in a ring-shaped potential coupled with a rectilinear guide of finite size. We find that the dynamics of the atoms inside the guide is distinctive of the states with different winding numbers in the ring condensate. We also observe that the depletion of the density, localized around the tunneling region of the ring condensate, can decay in a pair of excitations experiencing a Sagnac effect. In our approach, the current states of the condensate in the ring can be read out by inspection of the rectilinear guide only, leaving the ring condensate minimally affected by the measurement. We believe that our results set the basis for definition of new quantum rotation sensors. At the same time, our scheme can be employed to explore fundamental questions involving dynamics of bosonic condensates. △ Less

Submitted 23 July, 2019; v1 submitted 13 August, 2018; originally announced August 2018.

Comments: Figures are enlarged. Section IV is added. Journal reference added

Journal ref: Phys. Rev. A 100, 013621 (2019)

Andreev-reflection and Aharonov-Bohm dynamics in atomtronic circuits

Authors: Tobias Haug, Rainer Dumke, Leong-Chuan Kwek, Luigi Amico

Abstract: We study the quantum transport through two specific atomtronic circuits: a Y-junction and a ring-shaped condensate pierced by an effective magnetic flux. We demonstrate that for bosons, the two circuits display Andreev-like reflections. For the Y-junction, the transport depends on the coupling strength of the Y-junction. For the ring-shaped condensate, the transport crucially depends on the partic… ▽ More We study the quantum transport through two specific atomtronic circuits: a Y-junction and a ring-shaped condensate pierced by an effective magnetic flux. We demonstrate that for bosons, the two circuits display Andreev-like reflections. For the Y-junction, the transport depends on the coupling strength of the Y-junction. For the ring-shaped condensate, the transport crucially depends on the particle statistics. For interacting bosons we find that the Aharonov-Bohm interference effect is absent. By breaking the translational invariance of the ring, the flux dependence can be restored. A complementary view of the problem is obtained through a specific non-equilibrium quench protocol. We find that the steady-state is independent of the flux, however the actual time-dynamics depends on the flux. The dynamics of the full closed system can be fitted with an approximated open system approach. For all the protocols we studied, we find striking differences in the dynamics of the Bose-Hubbard model and the Gross-Pitaevskii equation. △ Less

Submitted 23 September, 2019; v1 submitted 10 July, 2018; originally announced July 2018.

Journal ref: Quantum Sci. Technol. 4.4 (2019)

Squeezing Enhances Quantum Synchronization

Authors: Sameer Sonar, Michal Hajdušek, Manas Mukherjee, Rosario Fazio, Vlatko Vedral, Sai Vinjanampathy, Leong-Chuan Kwek

Abstract: It is desirable to observe synchronization of quantum systems in the quantum regime, defined by low number of excitations and a highly non-classical steady state of the self-sustained oscillator. Several existing proposals of observing synchronization in the quantum regime suffer from the fact that the noise statistics overwhelms synchronization in this regime. Here we resolve this issue by drivin… ▽ More It is desirable to observe synchronization of quantum systems in the quantum regime, defined by low number of excitations and a highly non-classical steady state of the self-sustained oscillator. Several existing proposals of observing synchronization in the quantum regime suffer from the fact that the noise statistics overwhelms synchronization in this regime. Here we resolve this issue by driving a self-sustained oscillator with a squeezing Hamiltonian instead of a harmonic drive and analyze this system in the classical and quantum regime. We demonstrate that strong entrainment is possible for small values of squeezing, and in this regime the states are non-classical. Furthermore, we show that the quality of synchronization measured by the FWHM of the power spectrum is enhanced with squeezing. △ Less

Submitted 31 January, 2018; originally announced January 2018.

Comments: 5 pages, 3 figures

Journal ref: Physical Review Letters 120, 163601 (2018)

Cluster state generation in one-dimensional Kitaev honeycomb model via shortcut to adiabaticity

Authors: Thi Ha Kyaw, Leong-Chuan Kwek

Abstract: We propose a mean to obtain computationally useful resource states also known as cluster states, for measurement-based quantum computation, via transitionless quantum driving algorithm. The idea is to cool the system to its unique ground state and tune some control parameters to arrive at computationally useful resource state, which is in one of the degenerate ground states. Even though there is s… ▽ More We propose a mean to obtain computationally useful resource states also known as cluster states, for measurement-based quantum computation, via transitionless quantum driving algorithm. The idea is to cool the system to its unique ground state and tune some control parameters to arrive at computationally useful resource state, which is in one of the degenerate ground states. Even though there is set of conserved quantities already present in the model Hamiltonian, which prevents the instantaneous state to go to any other eigenstate subspaces, one cannot quench the control parameters to get the desired state. In that case, the state will not evolve. With involvement of the shortcut Hamiltonian, we obtain cluster states in fast-forward manner. We elaborate our proposal in the one-dimensional Kitaev honeycomb model, and show that the auxillary Hamiltonian needed for the counterdiabatic driving is of M-body interaction. △ Less

Submitted 28 March, 2018; v1 submitted 18 December, 2017; originally announced December 2017.

Comments: 13 pages, 3 figures. Added detailed derivation to arrive at the shortcut Hamiltonian plus numerical simulations. Invited manuscript to be appeared in Focus on Shortcuts to Adiabaticity, NJP (IOP)

Journal ref: New J. Phys. 20, 045007 (2018)

Readout of the atomtronic quantum interference device

Authors: Tobias Haug, Joel Tan, Mark Theng, Rainer Dumke, Leong-Chuan Kwek, Luigi Amico

Abstract: 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 pro… ▽ 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 self-heterodyne 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. △ Less

Submitted 9 February, 2018; v1 submitted 28 July, 2017; originally announced July 2017.

Journal ref: Phys. Rev. A 97, 013633 (2018)

Floquet stroboscopic divisibility in non-Markovian dynamics

Authors: V. M. Bastidas, Thi Ha Kyaw, Jirawat Tangpanitanon, Guillermo Romero, Leong-Chuan Kwek, Dimitris G. Angelakis

Abstract: We provide a general discussion of the Liouvillian spectrum for a system coupled to a non-Markovian bath using Floquet theory. This approach is suitable when the system is described by a time-convolutionless master equation with time-periodic rates. Surprisingly, the periodic nature of rates allow us to have a stroboscopic divisible dynamical map at discrete times, which we refer to as Floquet str… ▽ More We provide a general discussion of the Liouvillian spectrum for a system coupled to a non-Markovian bath using Floquet theory. This approach is suitable when the system is described by a time-convolutionless master equation with time-periodic rates. Surprisingly, the periodic nature of rates allow us to have a stroboscopic divisible dynamical map at discrete times, which we refer to as Floquet stroboscopic divisibility. We illustrate the general theory for a Schrödinger cat which is roaming inside a non-Markovian bath, and demonstrate the appearance of stroboscopic revival of the cat at later time after its death. Our theory may have profound implications in entropy production in non-equilibrium systems. △ Less

Submitted 28 September, 2018; v1 submitted 14 July, 2017; originally announced July 2017.

Comments: We changed the title and explained in more detail the definition of non-Markovian dynamics used in the manuscript

Journal ref: New J. Phys. 20, 093004 (2018)

The Aharonov-Bohm effect in mesoscopic Bose-Einstein condensates

Authors: Tobias Haug, Hermanni Heimonen, Rainer Dumke, Leong-Chuan Kwek, Luigi Amico

Abstract: Ultra-cold atoms in light-shaped potentials open up new ways to explore mesoscopic physics: Arbitrary trapping potentials can be engineered with only a change of the laser field. Here, we propose using ultracold atoms in light-shaped potentials to feasibly realize a cold atom device to study one of the fundamental problems of mesoscopic physics, the Aharonov-Bohm effect: The interaction of particl… ▽ More Ultra-cold atoms in light-shaped potentials open up new ways to explore mesoscopic physics: Arbitrary trapping potentials can be engineered with only a change of the laser field. Here, we propose using ultracold atoms in light-shaped potentials to feasibly realize a cold atom device to study one of the fundamental problems of mesoscopic physics, the Aharonov-Bohm effect: The interaction of particles with a magnetic field when traveling in a closed loop. Surprisingly, we find that the Aharonov-Bohm effect is washed out for interacting bosons, while it is present for fermions. We show that our atomic device has possible applications as quantum simulator, Mach-Zehnder interferometer and for tests of quantum foundation. △ Less

Submitted 23 September, 2019; v1 submitted 16 June, 2017; originally announced June 2017.

Comments: 5 pages, 5 figures to be published in Physical Review A Rapid Communications

Journal ref: Phys. Rev. A 100, 041601 (2019)

Scaling of geometric phase versus band structure in cluster-Ising models

Authors: Wei Nie, Feng Mei, Luigi Amico, Leong Chuan Kwek

Abstract: We study the phase diagram of a class of models in which a generalized cluster interaction can be quenched by Ising exchange interaction and external magnetic field. We characterize the various phases through winding numbers. They may be ordinary phases with local order parameter or exotic ones, known as symmetry protected topologically ordered phases. Quantum phase transitions with dynamical crit… ▽ More We study the phase diagram of a class of models in which a generalized cluster interaction can be quenched by Ising exchange interaction and external magnetic field. We characterize the various phases through winding numbers. They may be ordinary phases with local order parameter or exotic ones, known as symmetry protected topologically ordered phases. Quantum phase transitions with dynamical critical exponents z = 1 or z = 2 are found. Quantum phase transitions are analyzed through finite-size scaling of the geometric phase accumulated when the spins of the lattice perform an adiabatic precession. In particular, we quantify the scaling behavior of the geometric phase in relation with the topology and low energy properties of the band structure of the system. △ Less

Submitted 1 March, 2017; v1 submitted 8 February, 2017; originally announced February 2017.

Journal ref: Phys. Rev. E 96, 020106 (2017)

Mesoscopic Vortex-Meissner currents in ring ladders

Authors: Tobias Haug, Luigi Amico, Rainer Dumke, Leong-Chuan Kwek

Abstract: Recent experimental progress have revealed Meissner and Vortex phases in low-dimensional ultracold atoms systems. Atomtronic setups can realize ring ladders, while explicitly taking the finite size of the system into account. This enables the engineering of quantized chiral currents and phase slips in-between them. We find that the mesoscopic scale modifies the current. Full control of the lattice… ▽ More Recent experimental progress have revealed Meissner and Vortex phases in low-dimensional ultracold atoms systems. Atomtronic setups can realize ring ladders, while explicitly taking the finite size of the system into account. This enables the engineering of quantized chiral currents and phase slips in-between them. We find that the mesoscopic scale modifies the current. Full control of the lattice configuration reveals a reentrant behavior of Vortex and Meissner phases. Our approach allows a feasible diagnostic of the currents' configuration through time of flight measurements. △ Less

Submitted 18 January, 2018; v1 submitted 29 December, 2016; originally announced December 2016.

Comments: To be published in Quantum Sci. Technol

Journal ref: Quantum Sci. Technol. 3 035006 (2018)

Atomtronics-enabled Quantum Technologies

Authors: Luigi Amico, Gerhard Birkl, Malcolm Boshier, Leong-Chuan Kwek

Abstract: Atomtronics is an emerging field in quantum technology that promises to realize 'atomic circuit' architectures exploiting ultra-cold atoms manipulated in versatile micro-optical circuits generated by laser fields of different shapes and intensities or micro-magnetic circuits known as atom chips. Although devising new applications for computation and information transfer is a defining goal of the f… ▽ More Atomtronics is an emerging field in quantum technology that promises to realize 'atomic circuit' architectures exploiting ultra-cold atoms manipulated in versatile micro-optical circuits generated by laser fields of different shapes and intensities or micro-magnetic circuits known as atom chips. Although devising new applications for computation and information transfer is a defining goal of the field, Atomtronics wants to enlarge the scope of quantum simulators and to access new physical regimes with novel fundamental science. With this focus issue we want to survey the state of the art of Atomtronics-enabled Quantum Technology. We collect articles on both conceptual and applicative aspects of the field for diverse exploitations, both to extend the scope of the existing atom-based quantum devices and to devise platforms for new routes to quantum technology. △ Less

Submitted 22 December, 2016; originally announced December 2016.

Comments: Focus Issue in New Journal of Physics: Editorial remarks, Table of contents & Abstracts

Quantum State Transmission in a Superconducting Charge Qubit-Atom Hybrid

Authors: Deshui Yu, Maria Martinez Valado, Christoph Hufnagel, Leong Chuan Kwek, Luigi Amico, Rainer Dumke

Abstract: 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 superc… ▽ 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. △ Less

Submitted 9 November, 2016; originally announced November 2016.

Comments: 3 figures, 1 table

Journal ref: Sci. Rep. 6, 38356 (2016)

Two-dimensional network of atomtronic qubits

Authors: Shabnam Safaei, Benoît Grémaud, Rainer Dumke, Leong Chuan Kwek, Luigi Amico, Christian Miniatura

Abstract: Through a combination of laser beams, we engineer a two-dimensional optical lattice of Mexican hat potentials able to host atoms in its ring-shaped wells. When tunneling can be ignored (at high laser intensities), we show that a well-defined qubit can be associated with the states of the atoms trapped in each of the rings. Each of these two-level systems can be manipulated by a suitable configurat… ▽ More Through a combination of laser beams, we engineer a two-dimensional optical lattice of Mexican hat potentials able to host atoms in its ring-shaped wells. When tunneling can be ignored (at high laser intensities), we show that a well-defined qubit can be associated with the states of the atoms trapped in each of the rings. Each of these two-level systems can be manipulated by a suitable configuration of Raman laser beams imprinting a synthetic flux onto each Mexican hat cell of the lattice. Overall, we believe that the system has the potential to form a scalable architecture for atomtronic flux qubits. △ Less

Submitted 6 April, 2018; v1 submitted 22 June, 2016; originally announced June 2016.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. A 97, 042306 (2018)

Coherent transfer of singlet-triplet qubit states in an architecture of triple quantum dots

Authors: MengKe Feng, Chang Jian Kwong, Teck Seng Koh, Leong Chuan Kwek

Abstract: 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 theore… ▽ 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 $94.3\% < \mathcal{F} < 99.5\%$ at sub-ns gate times for the pulse-gated scheme and between $75.4\% < \mathcal{F} < 99.0 \%$ at tens of ns for the adiabatic scheme. Taking into account dephasing from charge noise, we obtain state transfer fidelities between $94.0\% < \mathcal{F} < 99.2\%$ for the pulse-gated scheme and between $64.9\% < \mathcal{F} < 93.6\%$ for the adiabatic scheme. △ Less

Submitted 21 June, 2018; v1 submitted 27 February, 2016; originally announced February 2016.

Comments: Includes text and supplemental material, 11 Pages, 6 figures

Parity-preserving light-matter system mediates effective two-body interactions

Authors: Thi Ha Kyaw, Sebastian Allende, Leong-Chuan Kwek, Guillermo Romero

Abstract: We study the equilibrium and non-equilibrium physics of two qubits interacting through an ultrastrong coupled qubit-cavity system. By tuning the qubits energy gap while keeping the ultrastrong coupling system to its ground state, we demonstrate a strong two-qubit interaction as well as an enhanced excitation transfer between the two qubits. Our proposal has twofold implications: a means to attain… ▽ More We study the equilibrium and non-equilibrium physics of two qubits interacting through an ultrastrong coupled qubit-cavity system. By tuning the qubits energy gap while keeping the ultrastrong coupling system to its ground state, we demonstrate a strong two-qubit interaction as well as an enhanced excitation transfer between the two qubits. Our proposal has twofold implications: a means to attain multipurpose parity-protected quantum information tasks in superconducting circuits, and a building block for ultrastrong coupled cavity-enhanced exciton transport in disordered media. △ Less

Submitted 26 April, 2017; v1 submitted 22 January, 2016; originally announced January 2016.

Comments: Accepted by Quantum Science and Technology, 19 pages, 10 figures

Journal ref: Quantum Sci. Technol. 2 (2017) 025007

An Atomtronic Flux Qubit: A ring lattice of Bose-Einstein condensates interrupted by three weak links

Authors: D. Aghamalyan, N. T. Nguyen, F. Auksztol, K. S. Gan, M. Martinez Valado, P. C. Condylis, L. -C. Kwek, R. Dumke, L. Amico

Abstract: We study a physical system consisting of a Bose-Einstein condensate confined to a ring shaped lattice potential interrupted by three weak links. The system is assumed to be driven by an effective flux piercing the ring lattice. By employing path integral techniques, we explore the effective quantum dynamics of the system in a pure quantum phase dynamics regime. Moreover, the effects of the density… ▽ More We study a physical system consisting of a Bose-Einstein condensate confined to a ring shaped lattice potential interrupted by three weak links. The system is assumed to be driven by an effective flux piercing the ring lattice. By employing path integral techniques, we explore the effective quantum dynamics of the system in a pure quantum phase dynamics regime. Moreover, the effects of the density's quantum fluctuations are studied through exact diagonalization analysis of the spectroscopy of the Bose-Hubbard model. We demonstrate that a clear two-level system emerges by tuning the magnetic flux at degeneracy. The lattice confinement, platform for the condensate, is realized experimentally employing a spatial light modulator. △ Less

Submitted 14 July, 2016; v1 submitted 28 December, 2015; originally announced December 2015.

Comments: Accepted in New Journal of Physics

Entanglement convertibility by sweeping through the quantum phases of the alternating bonds $XXZ$ chain

Authors: Yu-Chin Tzeng, Li Dai, M. -C. Chung, Luigi Amico, Leong-Chuan Kwek

Abstract: We study the entanglement structure and the topological edge states of the ground state of the spin-1/2 XXZ model with bond alternation. We employ parity-density matrix renormalization group with periodic boundary conditions. The finite-size scaling of Rényi entropies $S_2$ and $S_\infty$ are used to construct the phase diagram of the system. The phase diagram displays three possible phases: Halda… ▽ More We study the entanglement structure and the topological edge states of the ground state of the spin-1/2 XXZ model with bond alternation. We employ parity-density matrix renormalization group with periodic boundary conditions. The finite-size scaling of Rényi entropies $S_2$ and $S_\infty$ are used to construct the phase diagram of the system. The phase diagram displays three possible phases: Haldane type (an example of symmetry protected topological ordered phases), Classical Dimer and Néel phases, the latter bounded by two continuous quantum phase transitions. The entanglement and non-locality in the ground state are studied and quantified by the entanglement convertibility. We found that, at small spatial scales, the ground state is not convertible within the topological Haldane dimer phase. The phenomenology we observe can be described in terms of correlations between edge states. We found that the entanglement spectrum also exhibits a distinctive response in the topological phase: the effective rank of the reduced density matrix displays a specifically large "susceptibility" in the topological phase. These findings support the idea that although the topological order in the ground state cannot be detected by local inspection, the ground state response at local scale can tell the topological phases apart from the non-topological phases. △ Less

Submitted 24 May, 2016; v1 submitted 17 December, 2015; originally announced December 2015.

Comments: Final version

Journal ref: Sci. Rep. 6, 26453 (2016)

Simulation and Detection of Photonic Chern Insulators in One-Dimensional Circuit Quantum Electrodynamics Lattice

Authors: Feng Mei, Jia-Bin You, Wei Nie, R. Fazio, Shi-Liang Zhu, L. C. Kwek

Abstract: We introduce a simple method to realize and detect photonic topological Chern insulators with one-dimensional circiut quantum electrodynamics arrays. By periodically modulating the couplings of the array, we show that this one-dimensional model can be mapped into a two-dimensional Chern insulator model. In addition to allowing the study of photonic Chern insulators, this approach also provides a n… ▽ More We introduce a simple method to realize and detect photonic topological Chern insulators with one-dimensional circiut quantum electrodynamics arrays. By periodically modulating the couplings of the array, we show that this one-dimensional model can be mapped into a two-dimensional Chern insulator model. In addition to allowing the study of photonic Chern insulators, this approach also provides a natural platform to realise experimentally Laughlin's pumping argument. Based on scattering theory of topological insulators and input-output formalism, we show that the photonic edge state can be probed directly and the topological invariant can be detected from the winding number of the reflection coefficient phase. △ Less

Submitted 14 October, 2015; v1 submitted 22 April, 2015; originally announced April 2015.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. A 92, 041805 (2015)

Coherent superposition of current flows in an Atomtronic Quantum Interference Device

Authors: Davit Aghamalyan, Marco Cominotti, Matteo Rizzi, Davide Rossini, Frank Hekking, Anna Minguzzi, Leong-Chuan Kwek, Luigi Amico

Abstract: We consider a correlated Bose gas tightly confined into a ring shaped lattice, in the presence of an artificial gauge potential inducing a persistent current through it. A weak link painted on the ring acts as a source of coherent back-scattering for the propagating gas, interfering with the forward scattered current. This system defines an atomic counterpart of the rf-SQUID: the atomtronics quant… ▽ More We consider a correlated Bose gas tightly confined into a ring shaped lattice, in the presence of an artificial gauge potential inducing a persistent current through it. A weak link painted on the ring acts as a source of coherent back-scattering for the propagating gas, interfering with the forward scattered current. This system defines an atomic counterpart of the rf-SQUID: the atomtronics quantum interference device (AQUID). The goal of the present study is to corroborate the emergence of an effective two-level system in such a setup and to assess its quality, in terms of its inner resolution and its separation from the rest of the many-body spectrum, across the different physical regimes. In order to achieve this aim, we examine the dependence of the qubit energy gap on the bosonic density, the interaction strength, and the barrier depth, and we show how the superposition between current states appears in the momentum distribution (time-of-flight) images. A mesoscopic ring lattice with intermediate-to-strong interactions and weak barrier depth is found to be a favorable candidate for setting up, manipulating and probing a qubit in the next generation of atomic experiments. △ Less

Submitted 29 April, 2015; v1 submitted 18 November, 2014; originally announced November 2014.

Comments: 17 pages, 10 figures

Journal ref: New J. Phys. 17 (2015) 045023

Creation of quantum error correcting codes in the ultrastrong coupling regime

Authors: T. H. Kyaw, D. A. Herrera-Martı, E. Solano, G. Romero, L. -C. Kwek

Abstract: We propose to construct large quantum graph codes by means of superconducting circuits working at the ultrastrong coupling regime. In this physical scenario, we are able to create a cluster state between any pair of qubits within a fraction of a nanosecond. To exemplify our proposal, creation of the five-qubit and Steane codes is numerically simulated. We also provide optimal operating conditions… ▽ More We propose to construct large quantum graph codes by means of superconducting circuits working at the ultrastrong coupling regime. In this physical scenario, we are able to create a cluster state between any pair of qubits within a fraction of a nanosecond. To exemplify our proposal, creation of the five-qubit and Steane codes is numerically simulated. We also provide optimal operating conditions with which the graph codes can be realized with state-of-the-art superconducting technologies. △ Less

Submitted 12 February, 2015; v1 submitted 25 September, 2014; originally announced September 2014.

Comments: Added a new appendix section

Journal ref: Phys. Rev. B 91, 064503 (2015)

Optimal scaling of persistent currents for interacting bosons on a ring

Authors: Marco Cominotti, Matteo Rizzi, Davide Rossini, Davit Aghamalyan, Luigi Amico, Leon C. Kwek, Frank Hekking, Anna Minguzzi

Abstract: We consider the persistent currents induced by an artificial gauge field applied to interacting ultra-cold 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 t… ▽ More We consider the persistent currents induced by an artificial gauge field applied to interacting ultra-cold 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. △ Less

Submitted 23 April, 2015; v1 submitted 5 September, 2014; originally announced September 2014.

Comments: 4 pages, 2 figures

Journal ref: The European Physical Journal Special Topics, Volume 224, Issue 3, pp 519-524 (2015)

Scalable quantum memory in the ultrastrong coupling regime

Authors: T. H. Kyaw, S. Felicetti, G. Romero, E. Solano, L. -C. Kwek

Abstract: Circuit quantum electrodynamics, consisting of superconducting artificial atoms coupled to on-chip resonators, represents a prime candidate to implement the scalable quantum computing architecture because of the presence of good tunability and controllability. Furthermore, recent advances have pushed the technology towards the ultrastrong coupling regime of light-matter interaction, where the qubi… ▽ More Circuit quantum electrodynamics, consisting of superconducting artificial atoms coupled to on-chip resonators, represents a prime candidate to implement the scalable quantum computing architecture because of the presence of good tunability and controllability. Furthermore, recent advances have pushed the technology towards the ultrastrong coupling regime of light-matter interaction, where the qubit-resonator coupling strength reaches a considerable fraction of the resonator frequency. Here, we propose a qubit-resonator system operating in that regime, as a quantum memory device and study the storage and retrieval of quantum information in and from the Z2 parity-protected quantum memory, within experimentally feasible schemes. We are also convinced that our proposal might pave a way to realize a scalable quantum random-access memory due to its fast storage and readout performances. △ Less

Submitted 2 March, 2015; v1 submitted 23 April, 2014; originally announced April 2014.

Comments: We have updated the title, abstract and included a new section on the open-system dynamics

Journal ref: Sci. Rep. 5, 8621 (2015)

Local convertibility and the quantum simulation of edge states in many-body systems

Authors: Fabio Franchini, Jian Cui, Luigi Amico, Heng Fan, Mile Gu, Vladimir E. Korepin, Leong Chuan Kwek, Vlatko Vedral

Abstract: In some many-body systems, certain ground state entanglement (Renyi) entropies increase even as the correlation length decreases. This entanglement non-monotonicity is a potential indicator of non-classicality. In this work we demonstrate that such a phenomenon, known as non-local convertibility, is due to the edge state (de)construction occurring in the system. To this end, we employ the example… ▽ More In some many-body systems, certain ground state entanglement (Renyi) entropies increase even as the correlation length decreases. This entanglement non-monotonicity is a potential indicator of non-classicality. In this work we demonstrate that such a phenomenon, known as non-local convertibility, is due to the edge state (de)construction occurring in the system. To this end, we employ the example of the Ising chain, displaying an order-disorder quantum phase transitions. Employing both analytical and numerical methods, we compute entanglement entropies for various system bipartitions (A|B) and consider ground states with and without Majorana edge states. We find that the thermal ground states, enjoying the Hamiltonian symmetries, show non-local convertibility if either A or B are smaller than, or of the order of, the correlation length. In contrast, the ordered (symmetry breaking) ground state is always locally convertible. The edge states behavior explains all these results and could disclose a paradigm to understand local convertibility in other quantum phases of matter. The connection we establish between convertibility and non-local, quantum correlations provides a clear criterion of which features a universal quantum simulator should possess to outperform a classical machine. △ Less

Submitted 23 September, 2014; v1 submitted 27 June, 2013; originally announced June 2013.

Comments: Accepted by Physical Review X. 5 pages (+ 2 pages of Methods & SupplementaryMmaterial). 11 figures. Several changes since first submission

Report number: MIT-CTP 4463

Journal ref: Phys. Rev. X 4, 041028 (2014)

Superfluid qubit systems with ring shaped optical lattices

Authors: Luigi Amico, Davit Aghamalyan, H. Crepaz, F. Auksztol, R. Dumke, L. -C. Kwek

Abstract: We study an experimentally feasible qubit system employing neutral atomic currents. Our system is based on bosonic cold atoms trapped in ring-shaped optical lattice potentials. The lattice makes the system strictly one dimensional and it provides the infrastructure to realize a tunable ring-ring interaction. Our implementation combines the low decoherence rates of of neutral cold atoms systems, ov… ▽ More We study an experimentally feasible qubit system employing neutral atomic currents. Our system is based on bosonic cold atoms trapped in ring-shaped optical lattice potentials. The lattice makes the system strictly one dimensional and it provides the infrastructure to realize a tunable ring-ring interaction. Our implementation combines the low decoherence rates of of neutral cold atoms systems, overcoming single site addressing, with the robustness of topologically protected solid state Josephson flux qubits. Characteristic fluctuations in the magnetic fields affecting Josephson junction based flux qubits are expected to be minimized employing neutral atoms as flux carriers. By breaking the Galilean invariance we demonstrate how atomic currents through the lattice provide a implementation of a qubit. This is realized either by artificially creating a phase slip in a single ring, or by tunnel coupling of two homogeneous ring lattices. The single qubit infrastructure is experimentally investigated with tailored optical potentials. Indeed, we have experimentally realized scaled ring-lattice potentials that could host, in principle, $n\sim 10$ of such ring-qubits, arranged in a stack configuration, along the laser beam propagation axis. An experimentally viable scheme of the two-ring-qubit is discussed, as well. Based on our analysis, we provide protocols to initialize, address, and read-out the qubit. △ Less

Submitted 18 February, 2014; v1 submitted 16 April, 2013; originally announced April 2013.

Comments: 14 revtex4-1 pages, 7 figs; to be published in Scientific Reports

Journal ref: Scientific Reports vol. 4, 4298 (2014)

A solenoidal synthetic field and the non-Abelian Aharonov-Bohm effects in neutral atoms

Authors: Ming-Xia Huo, Nie Wei, David A. W. Hutchinson, Leong Chuan Kwek

Abstract: Cold neutral atoms provide a versatile and controllable platform for emulating various quantum systems. Despite efforts to develop artificial gauge fields in these systems, realizing a unique ideal-solenoid-shaped magnetic field within the quantum domain in any real-world physical system remains elusive. Here we propose a scheme to generate a "hairline" solenoid with an extremely small size around… ▽ More Cold neutral atoms provide a versatile and controllable platform for emulating various quantum systems. Despite efforts to develop artificial gauge fields in these systems, realizing a unique ideal-solenoid-shaped magnetic field within the quantum domain in any real-world physical system remains elusive. Here we propose a scheme to generate a "hairline" solenoid with an extremely small size around 1 micrometer which is smaller than the typical coherence length in cold atoms. Correspondingly, interference effects will play a role in transport. Despite the small size, the magnetic flux imposed on the atoms is very large thanks to the very strong field generated inside the solenoid. By arranging different sets of Laguerre-Gauss (LG) lasers, the generation of Abelian and non-Abelian SU(2) lattice gauge fields is proposed for neutral atoms in ring- and square-shaped optical lattices. As an application, interference patterns of the magnetic type-I Aharonov-Bohm (AB) effect are obtained by evolving atoms along a circle over several tens of lattice cells. During the evolution, the quantum coherence is maintained and the atoms are exposed to a large magnetic flux. The scheme requires only standard optical access, and is robust to weak particle interactions. △ Less

Submitted 11 August, 2014; v1 submitted 30 October, 2012; originally announced October 2012.

Journal ref: Scientific Reports 4: 5992 (2014)

Mimicking interacting relativistic theories with stationary pulses of light

Authors: Dimitris G. Angelakis, MingXia Huo, Darrick Chang, Leong Chuan Kwek, Vladimir Korepin

Abstract: One of the most well known relativistic field theory models is the Thirring model (TM). Its realization can demonstrate the famous prediction for the renormalization of mass due to interactions. However, experimental verification of the latter requires complex accelerator experiments whereas analytical solutions of the model can be extremely cumbersome to obtain. In this work, following Feynman's… ▽ More One of the most well known relativistic field theory models is the Thirring model (TM). Its realization can demonstrate the famous prediction for the renormalization of mass due to interactions. However, experimental verification of the latter requires complex accelerator experiments whereas analytical solutions of the model can be extremely cumbersome to obtain. In this work, following Feynman's original proposal, we propose a alternative quantum system as a simulator of the TM dynamics. Here the relativistic particles are mimicked, counter-intuitively, by polarized photons in a quantum nonlinear medium. We show that the entire set of regimes of the Thirring model -- bosonic or fermionic, and massless or massive -- can be faithfully reproduced using coherent light trapping techniques. The sought after correlations' scalings can be extracted by simple probing of the coherence functions of the light using standard optical techniques. △ Less

Submitted 31 July, 2012; originally announced July 2012.

Comments: 6 pages with 3 figures

Journal ref: Phys. Rev. Lett. 110, 100502 (2013)