arXiv subjects

J. I. Cirac

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

At least 235 records · Page 13Linked to original sources

arXiv1998

Creation of entangled states of distant atoms by interference

We propose a scheme to create distant entangled atomic states. It is based on driving two (or more) atoms with a weak laser pulse, so that the probability that two atoms are excited is negligible. If the subsequent spontaneous emission is detected, the entangled state is created. We have developed a model to analyze the fidelity of the resulting state as a function of the dimensions and location of the detector, and the motional properties of the atoms.

quant-ph↗

arXiv1998

Lower Bounds for Attainable Fidelity in Entanglement Purification

We derive lower bounds for the attainable fidelity of standard entanglement purification protocols when local operations and measurements are subjected to errors. We introduce an error parameter which measures the distance between the ideal completely positive map describing a purification step and the one in the presence of errors. We derive non--linear maps for a lower bound of the fidelity at each purification step in terms of this parameter.

quant-ph↗

arXiv1998

Cold bosonic atoms in optical lattices

The dynamics of an ultracold dilute gas of bosonic atoms in an optical lattice can be described by a Bose-Hubbard model where the system parameters are controlled by laser light. We study the continuous (zero temperature) quantum phase transition from the superfluid to the Mott insulator phase induced by varying the depth of the optical potential, where the Mott insulator phase corresponds to a commensurate filling of the lattice (``optical crystal''). Examples for formation of Mott structures in optical lattices with a superimposed harmonic trap, and in optical superlattices are presented.

cond-mat.stat-mech↗

arXiv1998

Quantum repeaters based on entanglement purification

We study the use of entanglement purification for quantum communication over long distances. For distances much longer than the coherence length of a corresponding noisy quantum channel, the fidelity of transmission is usually so low that standard purification methods are not applicable. It is however possible to divide the channel into shorter segments that are purified separately and then connected by the method of entanglement swapping. This method can be much more efficient than schemes based on quantum error correction, as it makes explicit use of two-way classical communication. An important question is how the noise, introduced by imperfect local operations (that constitute the protocols of purification and the entanglement swapping), accumulates in such a compound channel, and how it can be kept below a certain noise level. To treat this problem, we first study the applicability and the efficiency of entanglement purification protocols in the situation of imperfect local operations. We then present a scheme that allows entanglement purification over arbitrary long channels and tolerates errors on the per-cent level. It requires a polynomial overhead in time, and an overhead in local resources that grows only logarithmically with the length of the channel.

quant-ph↗

arXiv1998

Inhibition of spontaneous emission in Fermi gases

Fermi inhibition is a quantum statistical analogue for the inhibition of spontaneous emission by an excited atom in a cavity. This is achieved when the relevant motional states are already occupied by a cloud of cold atoms in the internal ground state. We exhibit non-trivial effects at finite temperature and in anisotropic traps, and briefly consider a possible experimental realization.

cond-mat.stat-mech↗

arXiv1998

Quantum repeaters for communication

In quantum communication via noisy channels, the error probability scales exponentially with the length of the channel. We present a scheme of a quantum repeater that overcomes this limitation. The central idea is to connect a string of (imperfect) entangled pairs of particles by using a novel nested purification protocol, thereby creating a single distant pair of high fidelity. The scheme tolerates general errors on the percent level.

quant-ph↗

arXiv1998

Entanglement engineering of one-photon wavepackets using a single-atom source

We propose a cavity-QED scheme for the controlled generation of sequences of entangled single-photon wavepackets. A photon is created inside a cavity via an active medium, such as an atom, and decays into the continuum of radiation modes outside the cavity(coupled, for example to an optical fiber). Subsequent wavepackets generated in this way behave as independent logical qubits. This and the possibility of producing maximally entangled multi-qubit states suggest many applications in quantum communication.

quant-ph↗

arXiv1997

Quantum chaos in an ion trap: the delta-kicked harmonic oscillator

We propose an experimental configuration, within an ion trap, by which a quantum mechanical delta-kicked harmonic oscillator could be realized, and investigated. We show how to directly measure the sensitivity of the ion motion to small variations in the external parameters.

quant-ph↗

arXiv1997

Quantum communication and the creation of maximally entangled pairs of atoms over a noisy channel

We show how to create maximally entangled EPR pairs between spatially distant atoms, each of them inside a high-Q optical cavity, by sending photons through a general, noisy channel, such as a standard optical fiber. An error correction scheme that uses few auxiliary atoms in each cavity effectively eliminates photoabsorption and other transmission errors. This realizes the `absorption free channel.' A concatenation protocol using the absorption free channel allows for quantum communication with single qubits over distances much larger than the coherence length of the channel.

quant-ph↗

arXiv1997

Quantum gates with "hot" trapped ions

We propose a scheme to perform a fundamental two-qubit gate between two trapped ions using ideas from atom interferometry. As opposed to the scheme considered by J. I. Cirac and P. Zoller, Phys. Rev. Lett. 74, 4091 (1995), it does not require laser cooling to the motional ground state.

quant-ph↗

arXiv1997

Creation of Dark Solitons and Vortices in Bose-Einstein Condensates

We propose and analyze a scheme to create dark solitons and vortices in Bose-Einstein condensates. This is achieved starting from a condensate in the internal state $|a>$ and transferring the atoms to the internal state $|b>$ via a Raman transition induced by laser light. By scanning adiabatically the Raman detuning, dark solitons and vortices are created.

cond-mat↗

arXiv1997

Laser cooling of single trapped atoms to the ground state: a dark state in position space

We propose a scheme that allows to laser cool trapped atoms to the ground state of a one-dimensional confining potential. The scheme is based on the creation of a dark state by designing the laser profile, so that the hottest atoms are coherently pumped to another internal level, and then repumped back. The scheme works beyond the Lamb-Dicke limit. We present results of a full quantum treatment for a one-dimensional model.

quant-ph↗

arXiv1997

Purifying two-bit quantum gates and joint measurements in cavity QED

Using a cavity QED setup we show how to implement a particular joint measurement on two atoms in a fault tolerant way. Based on this scheme, we illustrate how to realize quantum communication over a noisy channel when local operations are subject to errors. We also present a scheme to perform and purify a universal two-bit gate.

quant-ph↗

arXiv1997

On the Improvement of Frequency Stardards with Quantum Entanglement

The optimal precision of frequency measurements in the presence of decoherence is discussed. We analyze different preparations of n two level systems as well as different measurement procedures. We show that standard Ramsey spectroscopy on uncorrelated atoms and optimal measurements on maximally entangled states provide the same resolution. The best resolution is achieved using partially entangled preparations with a high degree of symmetry.

quant-ph↗

arXiv1997

Quantum Superposition States of Bose-Einstein Condensates

We propose a scheme to create a macroscopic ``Schödinger cat'' state formed by two interacting Bose condensates. In analogy with quantum optics, where the control and engineering of quantum states can be maintained to a large extend, we consider the present scheme to be an example of quantum atom optics at work.

quant-ph↗

arXiv1997

Mimicking a Squeezed Bath Interaction: Quantum Reservoir Engineering with Atoms

The interaction of an atomic two-level system and a squeezed vacuum leads to interesting novel effects in atomic dynamics, including line narrowing in resonance fluorescence and absorption spectra, and a suppressed (enhanced) decay of the in-phase and out-of phase component of the atomic polarization. On the experimental side these predictions have so far eluded observation, essentially due to the difficulty of embedding atoms in a 4 pi squeezed vacuum. In this paper we show how to ``engineer'' a squeezed-bath-type interaction for an effective two-level system. In the simplest example, our two-level atom is represented by the two ground levels of an atom with angular momentum J=1/2 -> J=1/2 transition (a four level system) which is driven by (weak) laser fields and coupled to the vacuum reservoir of radiation modes. Interference between the spontaneous emission channels in optical pumping leads to a squeezed bath type coupling, and thus to symmetry breaking of decay on the Bloch sphere. With this system it should be possible to observe the effects predicted in the context of squeezed bath - atom interactions. The laser parameters allow one to choose properties of the squeezed bath interaction, such as the (effective) photon number expectation number N and the squeezing phase phi. We present results of a detailed analytical and numerical study.

quant-ph↗

arXiv1997

Ground State Laser Cooling Beyond the Lamb-Dicke Limit

We propose a laser cooling scheme that allows to cool a single atom confined in a harmonic potential to the trap ground state $|0>$. The scheme assumes strong confinement, where the oscillation frequency in the trap is larger than the effective spontaneous decay width, but is not restricted to the Lamb-Dicke limit, i.e. the size of the trap ground state can be larger than the optical wavelength. This cooling scheme may be useful in the context of quantum computations with ions and Bose-Einstein condensation.

quant-ph↗

arXiv1997

Stability and collective excitations of a two-component Bose-condensed gas: a moment approach

The dynamics of a two-component dilute Bose gas of atoms at zero temperature is described in the mean field approximation by a two-component Gross-Pitaevskii Equation. We solve this equation assuming a Gaussian shape for the wavefunction, where the free parameters of the trial wavefunction are determined using a moment method. We derive equilibrium states and the phase diagrams for the stability for positive and negative s-wave scattering lengths, and obtain the low energy excitation frequencies corresponding to the collective motion of the two Bose condensates.

cond-mat↗