arXiv subjects

J. I. Cirac

Publications and source records attributed to J. I. Cirac.

At least 127 records · Page 7Linked to original sources

arXiv2005

Sequential generation of entangled multi-qubit states

We consider the deterministic generation of entangled multi-qubit states by the sequential coupling of an ancillary system to initially uncorrelated qubits. We characterize all achievable states in terms of classes of matrix product states and give a recipe for the generation on demand of any multi-qubit state. The proposed methods are suitable for any sequential generation-scheme, though we focus on streams of single photon time-bin qubits emitted by an atom coupled to an optical cavity. We show, in particular, how to generate familiar quantum information states such as W, GHZ, and cluster states, within such a framework.

quant-ph↗

arXiv2005

Quantum benchmark for storage and transmission of coherent states

We consider the storage and transmission of a Gaussian distributed set of coherent states of continuous variable systems. We prove a limit on the average fidelity achievable when the states are transmitted or stored by a classical channel, i.e., a measure and repreparation scheme which sends or stores classical information only. The obtained bound is tight and serves as a benchmark which has to be surpassed by quantum channels in order to outperform any classical strategy. The success in experimental demonstrations of quantum memories as well as quantum teleportation has to be judged on this footing.

quant-ph↗

arXiv2004

Renormalization group transformations on quantum states

We construct a general renormalization group transformation on quantum states, independent of any Hamiltonian dynamics of the system. We illustrate this procedure for translational invariant matrix product states in one dimension and show that product, GHZ, W and domain wall states are special cases of an emerging classification of the fixed points of this coarse--graining transformation.

quant-ph↗

arXiv2004

Entanglement flow in multipartite systems

We investigate entanglement dynamics in multipartite systems, establishing a quantitative concept of entanglement flow: both flow through individual particles, and flow along general networks of interacting particles. In the former case, the rate at which a particle can transmit entanglement is shown to depend on that particle's entanglement with the rest of the system. In the latter, we derive a set of entanglement rate equations, relating the rate of entanglement generation between two subsets of particles to the entanglement already present further back along the network. We use the rate equations to derive a lower bound on entanglement generation in qubit chains, and compare this to existing entanglement creation protocols.

quant-ph↗

arXiv2004

Bose-Einstein Condensation and strong-correlation behavior of phonons in ion traps

We show that the dynamics of phonons in a set of trapped ions interacting with lasers is described by a Bose-Hubbard model whose parameters can be externally adjusted. We investigate the possibility of observing several quantum many-body phenomena, including (quasi) Bose-Einstein condensation as well as a superfluid-Mott insulator quantum phase transition.

quant-ph↗

arXiv2004

Quantum information processing using localized ensembles of nuclear spins

We describe a technique for quantum information processing based on localized en sembles of nuclear spins. A qubit is identified as the presence or absence of a collective excitation of a mesoscopic ensemble of nuclear spins surrounding a single quantum dot. All single and two-qubit operations can be effected using hyperfine interactions and single-electron spin rotations, hence the proposed scheme avoids gate errors arising from entanglement between spin and orbital degrees of freedom. Ultra-long coherence times of nuclear spins suggest that this scheme could be particularly well suited for applications where long lived memory is essential.

cond-mat.mes-hall↗

arXiv2004

Light-Matter Quantum Interface

We propose a quantum interface which applies multiple passes of a pulse of light through an atomic sample with phase/polarization rotations in between the passes. Our proposal does not require nonclassical light input or measurements on the system, and it predicts rapidly growing unconditional entanglement of light and atoms from just coherent inputs. The proposed interface makes it possible to achieve a number of tasks within quantum information processing including teleportation between light and atoms, quantum memory for light and squeezing of atomic and light variables.

quant-ph↗

arXiv2004

Renormalization algorithms for Quantum-Many Body Systems in two and higher dimensions

We describe quantum many--body systems in terms of projected entangled--pair states, which naturally extend matrix product states to two and more dimensions. We present an algorithm to determine correlation functions in an efficient way. We use this result to build powerful numerical simulation techniques to describe the ground state, finite temperature, and evolution of spin systems in two and higher dimensions.

cond-mat.str-el↗

arXiv2004

Spin dynamics for bosons in an optical lattice

We study the internal dynamics of bosonic atoms in an optical lattice. Within the regime in which the atomic crystal is a Mott insulator with one atom per well, the atoms behave as localized spins which interact according to some spin Hamiltonian. The type of Hamiltonian (Heisenberg, Ising), and the sign of interactions may be tuned by changing the properties of the optical lattice, or applying external magnetic fields. When, on the other hand, the number of atoms per lattice site is unknown, we can still use the bosons to perform general quantum computation.

cond-mat.other↗

arXiv2004

Quantum computation with cold bosonic atoms in an optical lattice

We analyse an implementation of a quantum computer using bosonic atoms in an optical lattice. We show that, even though the number of atoms per site and the tunneling rate between neighbouring sites is unknown, one may perform a universal set of gates by means of adiabatic passage.

quant-ph↗

arXiv2004

Matrix Product Density Operators: Simulation of finite-T and dissipative systems

We show how to simulate numerically both the evolution of 1D quantum systems under dissipation as well as in thermal equilibrium. The method applies to both finite and inhomogeneous systems and it is based on two ideas: (a) a representation for density operators which extends that of matrix product states to mixed states; (b) an algorithm to approximate the evolution (in real or imaginary time) of such states which is variational (and thus optimal) in nature.

cond-mat.other↗

arXiv2004

Key distillation from Gaussian states by Gaussian operations

We study the secrecy properties of Gaussian states under Gaussian operations. Although such operations are useless for quantum distillation, we prove that it is possible to distill a secret key secure against any attack from sufficiently entangled Gaussian states with non-positive partial transposition. Moreover, all such states allow for key distillation, when Eve is assumed to perform finite-size coherent attacks before the reconciliation process.

quant-ph↗

arXiv2004

Adiabatic Path to Fractional Quantum Hall States of a Few Bosonic Atoms

We propose a realistic scheme to create motional entangled states of a few bosonic atoms. It can experimentally be realized with a gas of ultra cold bosonic atoms trapped in a deep optical lattice potential. By simultaneously deforming and rotating the trapping potential on each lattice site it is feasible to adiabatically create a variety of entangled states on each lattice well. We fully address the case of N=2 and N=4 atoms per well and identify a sequence of fractional quantum Hall states: the Pfaffian state, the 1/2-Laughlin quasiparticle and the 1/2-Laughlin state. Exact knowledge of the spectrum has allowed us to design adiabatic paths to these states, with all times and parameters well within the reach of current experimental setups. We further discuss the detection of these states by measuring different properties as their density profile, angular momentum or correlation functions.

cond-mat.mes-hall↗

arXiv2004

Quantum optical implementation of quantum information processing

By popular request we post these old (from 2001) lecture notes of the Varenna Summer School Proceedings. The original was published as J. I. Cirac, L. M. Duan, and P. Zoller, in "Experimental Quantum Computation and Information" Proceedings of the International School of Physics "Enrico Fermi", Course CXLVIII, p. 263, edited by F. Di Martini and C. Monroe (IOS Press, Amsterdam, 2002).

quant-ph↗

arXiv2004

Implementing quantum information processing with atoms, ions and photons

Lecture Notes of the Les Houches Summer School 2003 on Quantum entanglement and information processing to be published as P. Zoller, J. I. Cirac, Luming Duan and J. J. Garcia-Ripoll, in "Quantum entanglement and information processing", Proceedings of the Les Houches Summer School, Session 79, edited by D. Est{è}ve, J.M. Raimond and J. Dalibard (Elsevier, Amsterdam, 2004). This is an updated although shortened version of J. I. Cirac, L. M. Duan, and P. Zoller, in "Quantum optical implementation of quantum information processing", Proceedings of the International School of Physics "Enrico Fermi", Course CXLVIII, p. 263, edited by F. Di Martini and C. Monroe (IOS Press, Amsterdam, 2002).

quant-ph↗

arXiv2004

DMRG and periodic boundary conditions: a quantum information perspective

We introduce a picture to analyze the density matrix renormalization group (DMRG) numerical method from a quantum information perspective. This leads us to introduce some modifications for problems with periodic boundary conditions in which the results are dramatically improved. The picture also explains some features of the method in terms of entanglement and teleportation.

cond-mat.str-el↗

arXiv2004

Implementation of Spin Hamiltonians in Optical Lattices

We propose an optical lattice setup to investigate spin chains and ladders. Electric and magnetic fields allow us to vary at will the coupling constants, producing a variety of quantum phases including the Haldane phase, critical phases, quantum dimers etc. Numerical simulations are presented showing how ground states can be prepared adiabatically. We also propose ways to measure a number of observables, like energy gap, staggered magnetization, end-chain spins effects, spin correlations and the string order parameter.

cond-mat.str-el↗

arXiv2004

Non-additivity of quantum capacity for multiparty communication channels

We investigate multiparty communication scenarios where information is sent from several sender to several receivers. We establish a relation between the quantum capacity of multiparty communication channels and their distillability properties which enables us to show that the quantum capacity of such channels is not additive.

quant-ph↗