-
[show abstract]
[hide abstract]
ABSTRACT: We propose that real-space properties of the two-impurity Kondo model can be
obtained from an effective spin model where two single-impurity Kondo spin
chains are joined via an RKKY interaction between the two impurity spins. We
then use a DMRG approach, valid in all ranges of parameters, to study its
features using two complementary quantum-entanglement measures, the negativity
and the von Neumann entropy. This non-perturbative approach enables us to
uncover the precise dependence of the spatial extent $\xi_K$ of the Kondo
screening cloud with the Kondo and RKKY couplings. Our results reveal an
exponential suppression of the Kondo temperature $T_K \sim 1/\xi_K$ with the
size of the effective impurity spin in the limit of large ferromagnetic RKKY
coupling, a striking display of "Kondo resonance narrowing" in the two-impurity
Kondo model. We also show how the antiferromagnetic RKKY interaction produces
an effective decoupling of the impurities from the bulk already for
intermediate strengths of this interaction, and, furthermore, exhibit how the
non-Fermi liquid quantum critical point is signaled in the quantum entanglement
between various parts of the system.
01/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: We propose a new fast scalable method for achieving a two-qubit entangling gate between arbitrary distant qubits in a network by exploiting dispersionless propagation in uniform chains. This is achieved dynamically by switching on a strong interaction between the qubits and a bus formed by a nonengineered chain of interacting qubits. The quality of the gate scales very efficiently with qubit separations. Surprisingly, a sudden switching of the couplings is not necessary. Moreover, our gate mechanism works for multiple gate operations without resetting the bus. We propose a possible experimental realization in cold atoms trapped in optical lattices and near field Fresnel trapping potentials.
Physical Review Letters 04/2011; 106(14):140501. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We study the quality of state and entanglement transmission through quantum
channels described by spin chains varying both the system parameters and the
initial state of the channel. We consider a vast class of one-dimensional
many-body models which contains some of the most relevant experimental
realizations of quantum data-buses. In particular, we consider spin-1/2 XY and
XXZ model with open boundary conditions. Our results show a significant
difference between free-fermionic (non-interacting) systems (XY) and
interacting ones (XXZ), where in the former case initialization can be
exploited for improving the entanglement distribution, while in the latter case
it also determines the quality of state transmission. In fact, we find that in
non interacting systems the exchange with fermions in the initial state of the
chain always has a destructive effect, and we prove that it can be completely
removed in the isotropic XX model by initializing the chain in a ferromagnetic
state. On the other hand, in interacting systems constructive effects can arise
by scattering between hopping fermions and a proper initialization procedure.
Remarkably our results are the first example in which state and entanglement
transmission show maxima at different points as the interactions and
initializations of spin chain channels are varied.
Physical Review A 04/2011; · 2.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We show that the inherent entanglement of the ground state of strongly
correlated systems can be exploited for both classical and quantum
communications. Our strategy is based on a single qubit rotation which encodes
information in the entangled nature of the ground state. In classical
communication, our mechanism conveys more than one bit of information in each
shot, just as dense coding does, without demanding long range entanglement. In
our scheme for quantum communication, which may more appropriately be
considered as a remote state preparation, the quality is higher than the highly
studied attaching scenarios. Moreover, we propose to implement this new way of
communication in optical lattices where all the requirements of our proposal
have already been achieved.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We propose a new fast scalable method for achieving a two-qubit entangling
gate between arbitrary distant qubits in a network by exploiting dispersionless
propagation in uniform chains. This is achieved dynamically by switching on a
strong interaction between the qubits and a bus formed by a non-engineered
chain of interacting qubits. The quality of the gate scales very efficiently
with qubit separations. Surprisingly, a sudden switching of the coupling is not
necessary and our gate mechanism is not altered by a possibly gradual
switching. The bus is also naturally reset to its initial state making the
complex resetting procedure unnecessary after each application of the gate.
Moreover, we propose a possible experimental realization in cold atoms trapped
in optical lattices and near field Fresnel trapping potentials, which are both
accessible to current technology.
12/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We propose a mechanism where high entanglement between very distant boundary spins is generated by suddenly connecting two long Kondo spin chains. We show that this procedure provides an efficient way to route entanglement between multiple distant sites. We observe that the key features of the entanglement dynamics of the composite spin chain are well described by a simple model of two singlets, each formed by two spins. The proposed routing mechanism is a footprint of the emergence of a Kondo cloud in a Kondo system and can be engineered and observed in varied physical settings.
Physical Review Letters 10/2010; 105(18):187204. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We exploit the nondissipative dynamics of a pair of electrons in a large square quantum dot to perform singlet-triplet spin measurement through a single charge detection and show how this may be used for entanglement swapping and teleportation. The method is also used to generate the Affleck-Kennedy-Lieb-Tasaki ground state, a further resource for quantum computation. We justify, and derive analytic results for, an effective charge-spin Hamiltonian which is valid over a wide range of parameters and agrees well with exact numerical results of a realistic effective-mass model. Our analysis also indicates that the method is robust to the choice of dot-size and initialization errors, as well as decoherence.
Physical Review Letters 08/2010; 105(8):080502. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We discuss how to prepare an Ising chain in a GHZ state using a single global control field only. This model does not require the spins to be individually addressable and is applicable to quantum systems such as cold atoms in optical lattices, some liquid- or solid-state NMR experiments, and many nanoscale quantum structures. We show that GHZ states can always be reached asymptotically from certain easy-to-prepare initial states using adiabatic passage, and under certain conditions finite-time reachability can be ensured. To provide a reference useful for future experimental implementations, three different control strategies to achieve the objective—adiabatic passage, Lyapunov control, and optimal control—are compared, and their advantages and disadvantages discussed, in particular in the presence of realistic imperfections such as imperfect initial state preparation, system inhomogeneity, and dephasing.
Phys. Rev. A. 07/2010; 82(1).
-
[show abstract]
[hide abstract]
ABSTRACT: We investigate the entanglement properties of the Kondo spin chain when it is prepared in its ground state as well as its dynamics following a single bond quench. We show that a true measure of entanglement such as negativity enables to characterize the unique features of the gapless Kondo regime. We determine the spatial extent of the Kondo screening cloud and propose an ansatz for the ground state in the Kondo regime accessible to this spin chain; we also demonstrate that the impurity spin is indeed maximally entangled with the Kondo cloud. We exploit these features of the entanglement in the gapless Kondo regime to show that a single local quench at one end of a Kondo spin chain may always induce a fast and long lived oscillatory dynamics, which establishes a high quality entanglement between the individual spins at the opposite ends of the chain. This entanglement is a footprint of the presence of the Kondo cloud and may be engineered so as to attain - even for very large chains- a constant high value independent of the length; in addition, it is thermally robust. To better evidence the remarkable peculiarities of the Kondo regime, we carry a parallel analysis of the entanglement properties of the Kondo spin chain model in the gapped dimerised regime where these remarkable features are absent.
06/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: Quantum dot arrays are a promising media for transferring quantum information between two distant points without resorting to mobile qubits. Here we study two most common disorders namely, hyperfine interaction and exchange coupling fluctuations, in quantum dot arrays and their effects on quantum communication through these chains. Our results show that the hyperfine interaction is more destructive than the exchange coupling fluctuations. The average optimal time for communication is not affected by any disorder in the system and our simulations show that anti-ferromagnetic chains are much more resistive than the ferromagnetic ones against both kind of disorders. Even when time modulation of a coupling and optimal control is employed to improve the transmission, the anti-ferromagnetic chain performs much better. We have assumed the quasi-static approximation for hyperfine interaction and time dependent fluctuations in the exchange couplings. Particularly, for studying exchange coupling fluctuations we have considered the static disorder, white noise and $1/f$ noise. Comment: 10 pages, 12 figures. Comments are welcome!
05/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We discuss how to prepare an Ising chain in a GHZ state using a single global control field only. This model does not require the spins to be individually addressable and is applicable to quantum systems such as cold atoms in optical lattices, some liquid- or solid-state NMR experiments, and many nano-scale quantum structures. We show that GHZ states can always be reached asymptotically from certain easy-to-prepare initial states using adiabatic passage, and under certain conditions finite-time reachability can be ensured. To provide a reference useful for future experimental implementations three different control strategies to achieve the objective, adiabatic passage, Lyapunov control and optimal control are compared, and their advantages and disadvantages discussed, in particular in the presence of realistic imperfections such as imperfect initial state preparation, system inhomogeneity and dephasing. Comment: 13 pages, 11 figures
04/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate how near-perfect entanglement (in fact arbitrarily close to maximal entanglement) can be generated between the end spins of an antiferromagnetic isotropic Heisenberg chain of length N, starting from the ground state in the N/2 excitation subspace, by applying a magnetic field along a given direction, acting on a single spin only. Temporally optimal magnetic fields to generate a singlet pair between the two end spins of the chain are calculated for chains up to length 20 using optimal control theory. The optimal fields are shown to remain effective in various nonideal situations including thermal fluctuations, magnetic field leakage, random system couplings, and decoherence. Furthermore, the quality of the entanglement generated can be substantially improved by taking these imperfections into account in the optimization. In particular, the optimal pulse of a given thermal initial state is also optimal for any other initial thermal state with lower temperature.
Phys. Rev. A. 03/2010; 81(3).
-
Proceedings Sixth Workshop on Developments in Computational Models: Causality, Computation, and Physics; 01/2010
-
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate how near-perfect entanglement (in fact arbitrarily close to maximal entanglement) can be generated between the end spins of an anti-ferromagnetic isotropic Heisenberg chain of length $N$, starting from the ground state in the $N/2$ excitation subspace, by applying a magnetic field along a given direction, acting on a single spin only. Temporally optimal magnetic fields to generate a singlet pair between the two end spins of the chain are calculated for chains up to length 20 using optimal control theory. The optimal fields are shown to remain effective in various non-ideal situations including thermal fluctuations, magnetic field leakage, random system couplings and decoherence. Furthermore, the quality of the entanglement generated can be substantially improved by taking these imperfections into account in the optimization. In particular, the optimal pulse of a given thermal initial state is also optimal for any other initial thermal state with lower temperature. Comment: 10 pages, revtex
11/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: We study the entanglement of an impurity at one end of a spin chain with a block of spins using negativity as a true measure of entanglement to characterize the unique features of the gapless Kondo regime in the spin chain Kondo model. For this spin chain in the Kondo regime we determine- with a true entanglement measure- the spatial extent of the Kondo screening cloud, we propose an ansatz for its ground state and demonstrate that the impurity spin is indeed maximally entangled with the cloud. To better evidence the peculiarities of the Kondo regime, we carry a parallel analysis of the entanglement properties of the Kondo spin chain model in the gapped dimerised regime. Our study shows how a genuine entanglement measure stemming from quantum information theory can fully characterize also non perturbative regimes accessible to certain condensed matter systems. Comment: 5 pages and 4 figures
04/2009;
-
[show abstract]
[hide abstract]
ABSTRACT: We study the transmission of both classical or quantum information through all the phases of a finite XXZ spin chain. This characterizes the merit of the different phases in terms of their ability to act as a quantum wire. As far as quantum information is concerned, we need only consider the transmission of entanglement as the direct transmission of a quantum state is equivalent. The isotropic anti-ferromagnetic spin chain is found to be the optimal point of the phase diagram for the transmission of quantum entanglement when one considers both the amount of transmitted entanglement, as well as the velocity with which it is transmitted. But this optimal point in the phase diagram moves to the Neel phase when decoherence or thermal fluctuations are taken to account. This chain may also be able to transfer classical information even when, due to a large magnitude of the noise, quantum information is not transmitted at all. For a certain range of anisotropies of the model, a curious feature is found in the flow of quantum information inside the chain, namely, a hopping mode of entanglement transfer which skips the odd numbered sites. Our predictions will potentially be testable in several physical systems. Comment: 11 pages, 8 figures. Highly revised edition
11/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: We show that, in the gapless Kondo Regime, a single local quench at one end of a Kondo spin chain induces a fast and long lived oscillatory dynamics. This quickly establishes a high quality entanglement between the spins at the opposite ends of the chain. This entanglement is mediated by the Kondo Cloud, attains a constant high value independent of the length for large chains, and shows thermal robustness. In contrast, when the Kondo cloud is absent, e.g. in the gapped dimer regime, only finite size end to end effects can create some entanglement on a much longer time-scale for rather short chains. By decoupling one end of the chain during the dynamics one can distinguish between this end-end effect which vanishes, and the global Kondo cloud mediated entanglement, which persists. This quench approach paves the way to detect the elusive Kondo cloud through the entanglement between two individual spins. Our results show that non-perturbative cooperative phenomena from condensed matter may be exploited for quantum information. Comment: 4 pages, 4 figures. All comments are welcome.
11/2008;
-
[show abstract]
[hide abstract]
ABSTRACT: We study the possibility of using an uniformly coupled finite antiferromagnetic spin-1/2 Heisenberg chain as a channel for transmitting entanglement. One member of a pair of maximally entangled spins is initially appended to one end of a chain in its ground state and the dynamical propagation of this entanglement to the other end is calculated. We show that compared to the analogous scheme with a ferromagnetic chain in its ground state, here the entanglement is transmitted faster, with less decay, with a much higher purity and as a narrow pulse form rising non-analytically from zero. Here non-zero temperatures and depolarizing environments are both found to be less destructive in comparison to the ferromagnetic case. The entanglement is found to propagate through the chain in a peculiar fashion whereby it hops to skip alternate sites. Comment: 5 pages, 5 figures. Modified version with more explanation
06/2007;
-
[show abstract]
[hide abstract]
ABSTRACT: We generalize an already proposed protocol for quantum state transfer to spin chains of arbitrary spin. An arbitrary unknown $d-$ level state is transferred through a chain with rather good fidelity by the natural dynamics of the chain. We compare the performance of this protocol for various values of $d$. A by-product of our study is a much simpler method for picking up the state at the destination as compared with the one proposed previously. We also discuss entanglement distribution through such chains and show that the quality of entanglement transition increases with the number of levels $d$. Comment: More discussion about the ground state has been added. Accepted in Physical Review A
12/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: We study the effect of thermal fluctuations in a recently proposed protocol for transmission of unknown quantum states through quantum spin chains. We develop a low temperature expansion for general spin chains. We then apply this formalism to study exactly thermal effects on short spin chains of four spins. We show that optimal times for extraction of output states are almost independent of the temperature which lowers only the fidelity of the channel. Moreover we show that thermal effects are smaller in the anti-ferromagnetic chains than the ferromagnetic ones. Comment: 14 pages, Latex, five eps figures, references added
12/2004;
