Now showing 1 - 10 of 51
  • Publication
    Open Access
    Effective dynamics of cold atoms flowing in two ring-shaped optical potentials with tunable tunneling
    (American Physical Society, 2013)
    Aghamalyan, Davit
    ;
    Amico, Luigi
    ;
    We study the current dynamics of coupled atomic condensates flowing in two ring-shaped optical potentials. We provide a specific setup where the ring-ring coupling can be tuned in an experimentally feasible way. It is demonstrated that the imaginary time effective action of the system in a weak coupling regime provides a two-level-system dynamics for the phase slip across the two rings. Through two-mode Gross- Pitaevskii mean-field equations, the real-time dynamics of the population imbalance and the phase difference between the two condensates is thoroughly analyzed analytically, as a function of the relevant physical parameters of the system. In particular, we find that the macroscopic quantum self-trapping phenomenon is induced in the system if the flowing currents assume a nonvanishing difference.
    WOS© Citations 37Scopus© Citations 40  156  194
  • Publication
    Open Access
    Effective Hamiltonian approach to adiabatic approximation in open systems
    (IOP Publishing, 2013)
    Yi, X. X.
    ;
    Tong, Dianmin
    ;
    ;
    Oh, Choo Hiap
    The adiabatic approximation in open systems is formulated through the effective Hamiltonian approach. By introducing an ancilla, we embed the open system dynamics into a non-Hermitian quantum dynamics of a composite system, the adiabatic evolution of the open system is then defined as the adiabatic dynamics of the composite system. Validity and invalidity conditions for this approximation are established and discussed. A High-order adiabatic approximation for open systems is introduced. As an example, the adiabatic condition for an open spin- 1 2 particle in time-dependent magnetic fields is analyzed.
    WOS© Citations 33Scopus© Citations 34  137  114
  • Publication
    Open Access
    All-versus-nothing proof of Einstein-Podolsky-Rosen steering
    (Springer, 2013)
    Chen, Jing-Ling
    ;
    Ye, Xiang-Jun
    ;
    Wu, Chunfeng
    ;
    Su, Hong-Yi
    ;
    Cabello, Adan
    ;
    ;
    Oh, Choo Hiap
    Einstein-Podolsky-Rosen steering is a form of quantum nonlocality intermediate between entanglement and Bell nonlocality. Although Schro¨dinger already mooted the idea in 1935, steering still defies a complete understanding. In analogy to ‘‘all-versus-nothing’’ proofs of Bell nonlocality, here we present a proof of steering without inequalities rendering the detection of correlations leading to a violation of steering inequalities unnecessary. We show that, given any two-qubit entangled state, the existence of certain projective measurement by Alice so that Bob’s normalized conditional states can be regarded as two different pure states provides a criterion for Alice-to-Bob steerability. Asteering inequality equivalent to the all-versus-nothing proof is also obtained. Our result clearly demonstrates that there exist many quantum states which do not violate any previously known steering inequality but are indeed steerable. Our method offers advantages over the existing methods for experimentally testing steerability, and sheds new light on the asymmetric steering problem.
    WOS© Citations 65Scopus© Citations 66  371  164
  • Publication
    Open Access
    Quantum contextuality for a relativistic spin-1/2 particle
    (2013)
    Chen, Jing-Ling
    ;
    Su, Hong-Yi
    ;
    Wu, Chunfeng
    ;
    Deng, Dong-Ling
    ;
    Cabello, Adan
    ;
    ;
    Oh, Choo Hiap
    The quantum predictions for a single nonrelativistic spin-1/2 particle can be reproduced by noncontextual hidden variables. Here we show that quantum contextuality for a relativistic electron moving in a Coulomb potential naturally emerges if relativistic effects are taken into account. The contextuality can be identified through the violation of noncontextuality inequalities. We also discuss quantum contextuality for the free Dirac electron as well as the relativistic Dirac oscillator.
    WOS© Citations 2Scopus© Citations 2  146  201
  • Publication
    Open Access
    Operator quantum zeno effect: Protecting quantum information with noisy two-qubit interactions
    (American Physical Society, 2013)
    Wang, Shu-Chao
    ;
    Li, Ying
    ;
    Wang, Xiang-Bin
    ;
    The time evolution of some quantum states can be slowed down or even stopped under frequent measurements. This is the usual quantum Zeno effect. Here, we report an operator quantum Zeno effect, in which the evolution of some physical observables is slowed down through measurements even though the quantum state changes randomly with time. Based on the operator quantum Zeno effect, we show how we can protect quantum information from decoherence with two-qubit measurements, realizable with noisy two-qubit interactions.
    WOS© Citations 38Scopus© Citations 36  112  158
  • Publication
    Open Access
    Structure of optimal state discrimination in generalized probabilistic theories
    (2016)
    Bae, Joonwoo
    ;
    Kim, Dai-Gyoung
    ;
    We consider optimal state discrimination in a general convex operational framework, so-called generalized probabilistic theories (GPTs), and present a general method of optimal discrimination by applying the complementarity problem from convex optimization. The method exploits the convex geometry of states but not other detailed conditions or relations of states and effects. We also show that properties in optimal quantum state discrimination are shared in GPTs in general: (i) no measurement sometimes gives optimal discrimination, and (ii) optimal measurement is not unique.
    WOS© Citations 19Scopus© Citations 21  325  113
  • Publication
    Open Access
    Transitions in the quantum computational power
    (2014)
    Wei, Tzu-Chieh
    ;
    Li, Ying
    ;
    We construct two spin models on lattices (both two and three dimensional) to study the capability of quantum computational power as a function of temperature and the system parameter. There exists a finite region in the phase diagram such that the thermal equilibrium states are capable of providing a universal fault-tolerant resource for measurement-based quantum computation. Moreover, in such a region the thermal resource states on the three-dimensional lattices can enable topological protection for quantum computation. The two models behave similarly in terms of quantum computational power. However, they have different properties in terms of the usual phase transitions. The first model has a first-order phase transition only at zero temperature whereas there is no transition at all in the second model. Interestingly, the transition in the quantum computational power does not coincide with the phase transition in the first model.
    WOS© Citations 4Scopus© Citations 7  120  128
  • Publication
    Open Access
    Entanglement witness via symmetric two-body correlations
    (2016)
    Tan, Ernest Ying Zhe
    ;
    Kaszlikowski, Dagomir
    ;
    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. We show that while the witness is unable to detect Greenberger-Horne-Zeilinger (GHZ) states, it is instead able to detect superpositions of Dicke states with GHZ states.
    WOS© Citations 5Scopus© Citations 5  153  169
  • Publication
    Open Access
    Superfluid qubit systems with ring shaped optical lattices
    (2014)
    Amico, Luigi
    ;
    Aghamalyan, Davit
    ;
    Auksztol, Filip
    ;
    Crepaz, Herbert
    ;
    Dumke, Rainer
    ;
    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 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 an 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 ,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.
    WOS© Citations 100Scopus© Citations 99  343  206
  • Publication
    Open Access
    Probing quantum spin glass like system with a double quantum dot
    (2016)
    Koh, Chee Yeong
    ;
    We study the ground state properties of a 4-qubit spin glass like (SGL) chain with probes at the end of the chain and compare our results with the non-spin glass like (NSGL) case. The SGL is modeled as a spin chain with nearest-neighbor couplings, taking on normal variates with mean J and variance Δ2. The entanglement between the probes is used to detect any discontinuity in the ground state energy spectrum. For the NSGL case, it was found that the concurrence of the probes exhibits sharp transitions whenever there are abrupt changes in the energy spectrum. In particular, for the 4-qubit case, there is a sudden change in the ground state energy at an external magnetic fi eld B of around 0.66 (resulting in a drop in concurrence of the probes) and 1.7 (manifest as a spike). The latter spike persists for fi nite temperature case. For the SGL sample with sufficiently large Δ , however, the spike is absent. Thus, an absence in the spike could act as a possible signature of the presence of SGL eff ects. Moreover, the sudden drop in concurrence at B ≈ 0:66 does not disappear but gets smeared with increasing Δ. However, this drop can be accentuated with a smaller probe coupling. The finite temperature case is also briefly discussed.
    Scopus© Citations 1  96  111