E. Brion

Quantum computing with collective ensembles of multilevel systems.

We propose a new physical approach for encoding and processing of quantum information in ensembles of multilevel quantum systems, where the different bits are not carried by individual particles but associated with the collective population of different internal levels. One- and two-bit gates are implemented by collective internal state transitions taking place in the presence of an excitation blockade mechanism, which restricts the population of each internal state to the values zero and unity. Quantum computers with 10-20 bits can be built via this scheme in single trapped clouds of ground state atoms subject to the Rydberg excitation blockade mechanism, and the linear dependence between register size and the number of internal quantum states in atoms offers realistic means to reach larger registers.

Adiabatic elimination in a lambda system

This paper deals with different ways to extract the effective two-dimensional lower level dynamics of a lambda system excited by off-resonant laser beams. We present a commonly used procedure for elimination of the upper level, and we show that it may lead to ambiguous results. To overcome this problem and better understand the applicability conditions of this scheme, we review two rigorous methods which allow us both to derive an unambiguous effective two-level Hamiltonian of the system and to quantify the accuracy of the approximation achieved: the first relies on the exact solution of the Schrodinger equation, while the second resorts to the Greens function formalism and the Feshbach projection operator technique.

Coherence protection by the quantum Zeno effect and nonholonomic control in a Rydberg rubidium isotope

The protection of the coherence of open quantum systems against the influence of their environment is a very topical issue. A scheme is proposed here which protects a general quantum system from the action of a set of arbitrary uncontrolled unitary evolutions. This method draws its inspiration from ideas of standard error correction (ancilla adding, coding and decoding) and the quantum Zeno effect. A demonstration of our method on a simple atomic system--namely, a rubidium isotope--is proposed.

Conditional dynamics induced by new configurations for Rydberg dipole-dipole interactions

We suggest a way to use strong Rydberg dipole-dipole interactions in order to induce nontrivial conditional dynamics in individual-atom systems and mesoscopic ensembles. Contrary to previous works, we suggest exciting atoms into different Rydberg states, which results in a potentially richer dynamical behavior. Specifically, we investigate systems of individual hydrogenlike atoms or mesoscopic ensembles excited into high-lying hydrogenlike

Error Correction in Ensemble Registers for Quantum Repeaters and Quantum Computers

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Photonic controlled-phase gates through Rydberg blockade in optical cavities

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Implementing a neutral atom Rydberg gate without populating the Rydberg state

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Universal quantum computation in a neutral-atom decoherence-free subspace

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Quantum repeater with Rydberg-blocked atomic ensembles in fiber-coupled cavities

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Quantum statistics of light transmitted through an intracavity Rydberg medium

states, and show how to perform three-qubit conditional dynamics on the information they contain through a proper use of dipole-dipole-interaction-induced energy shifts.