arXiv subjects

J. Ignacio Cirac

Publications and source records attributed to J. Ignacio Cirac.

302 records · Page 17Linked to original sources

arXiv2006

Efficient quantum memory and entanglement between light and an atomic ensemble using magnetic fields

We present two protocols, one for the storage of light in an atomic ensemble and the subsequent retrieval, and another one for the generation of entanglement between light and atoms. They rely on two passes of a single pulse through the ensemble, Larmor precessing in an external field. Both protocols work deterministically and the relevant figures of merit - such as the fidelity or the EPR variance - scale exponentially in the coupling strength. We solve the corresponding Maxwell-Bloch equations describing the scattering process and determine the resulting input-output relations which only involve one relevant light mode that, in turn, can be easily accessed experimentally.

quant-ph↗

arXiv2005

Quantum phase transitions in matrix product systems

We investigate quantum phase transitions (QPTs) in spin chain systems characterized by local Hamiltonians with matrix product ground states. We show how to theoretically engineer such QPT points between states with predetermined properties. While some of the characteristics of these transitions are familiar, like the appearance of singularities in the thermodynamic limit, diverging correlation length, and vanishing energy gap, others differ from the standard paradigm: In particular, the ground state energy remains analytic, and the entanglement entropy of a half-chain stays finite. Examples demonstrate that these kinds of transitions can occur at the triple point of `conventional' QPTs.

cond-mat.stat-mech↗

arXiv2005

Extremality of Gaussian quantum states

We investigate Gaussian quantum states in view of their exceptional role within the space of all continuous variables states. A general method for deriving extremality results is provided and applied to entanglement measures, secret key distillation and the classical capacity of Bosonic quantum channels. We prove that for every given covariance matrix the distillable secret key rate and the entanglement, if measured appropriately, are minimized by Gaussian states. This result leads to a clearer picture of the validity of frequently made Gaussian approximations. Moreover, it implies that Gaussian encodings are optimal for the transmission of classical information through Bosonic channels, if the capacity is additive.

quant-ph↗

arXiv2004

Experimental demonstration of quantum memory for light

The information carrier of today's communications, a weak pulse of light, is an intrinsically quantum object. As a consequence, complete information about the pulse cannot, even in principle, be perfectly recorded in a classical memory. In the field of quantum information this has led to a long standing challenge: how to achieve a high-fidelity transfer of an independently prepared quantum state of light onto the atomic quantum state? Here we propose and experimentally demonstrate a protocol for such quantum memory based on atomic ensembles. We demonstrate for the first time a recording of an externally provided quantum state of light onto the atomic quantum memory with a fidelity up to 70%, significantly higher than that for the classical recording. Quantum storage of light is achieved in three steps: an interaction of light with atoms, the subsequent measurement on the transmitted light, and the feedback onto the atoms conditioned on the measurement result. Density of recorded states 33% higher than that for the best classical recording of light on atoms is achieved. A quantum memory lifetime of up to 4 msec is demonstrated.

quant-ph↗

arXiv2004

Quantum entanglement theory in the presence of superselection rules

Superselection rules severly constrain the operations which can be implemented on a distributed quantum system. While the restriction to local operations and classical communication gives rise to entanglement as a nonlocal resource, particle number conservation additionally confines the possible operations and should give rise to a new resource. In [Phys. Rev. Lett. 92, 087904 (2004), quant-ph/0310124] we showed that this resource can be quantified by a single additional number, the superselection induced variance (SiV) without changing the concept of entanglement. In this paper, we give the results on pure states in greater detail; additionally, we provide a discussion of mixed state nonlocality with superselection rules where we consider both formation and distillation. Finally, we demonstrate that SiV is indeed a resource, i.e., that it captures how well a state can be used to overcome the restrictions imposed by the superselection rule.

quant-ph↗

arXiv2003

Theory of Plasmon-assisted Transmission of Entangled Photons

The recent surface plasmon entanglement experiment [E. Altewischer et al., Nature (London) 418, 304 (2002)] is theoretically analyzed. The entanglement preservation upon transmission in the non-focused case is found to provide information about the interaction of the biphoton and the metallic film. The entanglement degradation in the focused case is explained in the framework of a fully multimode model. This phenomenon is a consequence of the polarization-selective filtering behavior of the metallic nanostructured film.

quant-ph↗

arXiv2003

Discrete entanglement distribution with squeezed light

We show how one can entangle distant atoms by using squeezed light. Entanglement is obtained in steady state, and can be increased by manipulating the atoms locally. We study the effects of imperfections, and show how to scale up the scheme to build a quantum network.

quant-ph↗

arXiv2002

Entanglement generation and Hamiltonian simulation in Continuous-Variable Systems

Several recent experiments have demonstrated the promise of atomic ensembles for quantum teleportation and quantum memory. In these cases the collective internal state of the atoms is well described by continuous variables $X_1, P_1$ and the interaction with the optical field ($X_2, P_2$) by a quadratic Hamiltonian $X_1X_2$. We show how this interaction can be used optimally to create entanglement and squeezing. We derive conditions for the efficient simulation of quadratic Hamiltonians and the engineering of all Gaussian operations and states.

quant-ph↗

arXiv2002

The characterization of Gaussian operations and Distillation of Gaussian States

We characterize the class of all physical operations that transform Gaussian states to Gaussian states. We show that this class coincides with that of all operations which can be performed on Gaussian states using linear optical elements and homodyne measurements. For bipartite systems we characterize the processes which can be implemented by local operations and classical communication, as well as those that can be implemented using positive partial transpose preserving maps. As an application, we show that Gaussian states cannot be distilled by local Gaussian operations and classical communication. We also define and characterize positive (but not completely positive) Gaussian maps.

quant-ph↗

arXiv2002

On the effective size of certain "Schrödinger cat'' like states

Several experiments and experimental proposals for the production of macroscopic superpositions naturally lead to states of the general form $|ϕ_1>^{\otimes N}+|ϕ_2>^{\otimes N}$, where the number of subsystems $N$ is very large, but the states of the individual subsystems have large overlap, $|łϕ_1|ϕ_2 \r|^2=1-ε^2$. We propose two different methods for assigning an effective particle number to such states, using ideal Greenberger--Horne--Zeilinger (GHZ)-- states of the form $|0\r^{\otimes n}+|1\r^{\otimes n}$ as a standard of comparison. The two methods are based on decoherence and on a distillation protocol respectively. Both lead to an effective size $n$ of the order of $N ε^2$.

quant-ph↗

arXiv2001

Bound entanglement for continuous variables is a rare phenomenon

We discuss the notion of bound entanglement (BE) for continuous variables (CV). We show that the set of non--distillable states (NDS) for CV is nowhere dense in the set of all states, i.e., the states of infinite--dimensional bipartite systems are generically distillable. This automatically implies that the sets of separable states, entangled states with positive partial transpose, and bound entangled states are also nowhere dense in the set of all states. All these properties significantly distinguish quantum CV systems from the spin like ones. The aspects of the definition of BE for CV is also analysed, especially in context of Schmidt numbers theory. In particular the main result is generalised by means of arbitrary Schmidt number and single copy regime.

quant-ph↗

arXiv2001

Separability Properties of Three-mode Gaussian States

We derive a necessary and sufficient condition for the separability of tripartite three mode Gaussian states, that is easy to check for any such state. We give a classification of the separability properties of those systems and show how to determine for any state to which class it belongs. We show that there exist genuinely tripartite bound entangled states and point out how to construct and prepare such states.

quant-ph↗

arXiv2001

Quantum Correlations in Two-Fermion Systems

We characterize and classify quantum correlations in two-fermion systems having 2K single-particle states. For pure states we introduce the Slater decomposition and rank (in analogy to Schmidt decomposition and rank), i.e. we decompose the state into a combination of elementary Slater determinants formed by mutually orthogonal single-particle states. Mixed states can be characterized by their Slater number which is the minimal Slater rank required to generate them. For K=2 we give a necessary and sufficient condition for a state to have a Slater number of 1. We introduce a correlation measure for mixed states which can be evaluated analytically for K=2. For higher K, we provide a method of constructing and optimizing Slater number witnesses, i.e. operators that detect Slater number for some states.

quant-ph↗

arXiv2000

All inseparable two-mode Gaussian continuous variable states are distillable

We show that all entangled Gaussian states of two infinite dimensional systems can be distilled to maximally entangled states in finite dimensions. The distillation protocol involves local squeezing operations, local homodyne measurements with ancilla systems prepared in coherent states as well as local joint measurements of the total number operator of two copies of the state.

quant-ph↗