Observing different quantum trajectories in cavity QED

The experimental observation of quantum jumps is an example of single open quantum systems that, when monitored, evolve in terms of stochastic trajectories conditioned on measurements results. Here we present a proposal that allows the experimental observation of a much larger class of quantum trajectories in cavity QED systems. In particular, our scheme allows for the monitoring of engineered thermal baths that are crucial for recent proposals for probing entanglement decay and also for entanglement protection. The scheme relies on the interaction of a three-level atom and a cavity mode that interchangeably play the roles of system and probe. If the atom is detected the evolution of the cavity fields follows quantum trajectories and vice-versa.Comment: 5 pages, 2 figure

Continuous Quantum Error Correction Through Local Operations

We propose local strategies to protect global quantum information. The protocols, which are quantum error correcting codes for dissipative systems, are based on environment measurements, direct feedback control and simple encoding of the logical qubits into physical qutrits whose decaying transitions are indistinguishable and equally probable. The simple addition of one extra level in the description of the subsystems allows for local actions to fully and deterministically protect global resources, such as entanglement. We present codes for both quantum jump and quantum state diffusion measurement strategies and test them against several sources of inefficiency. The use of qutrits in information protocols suggests further characterization of qutrit-qutrit disentanglement dynamics, which we also give together with simple local environment measurement schemes able to prevent distillability sudden death and even enhance entanglement in situations in which our feedback error correction is not possible.Comment: Accepted for publication in Phys. Rev.

Cooperativity of a few quantum emitters in a single-mode cavity

We theoretically investigate the emission properties of a single-mode cavity coupled to a mesoscopic number of incoherently pumped quantum emitters. We propose an operational measure for the medium cooperativity, valid both in the bad and in the good cavity regimes. We show that the opposite regimes of subradiance and superradiance correspond to negative and positive cooperativity, respectively. The lasing regime is shown to be characterized by nonnegative cooperativity. In the bad cavity regime we show that the cooperativity defines the transitions from subradiance to superradiance. In the good cavity regime it helps to define the lasing threshold, also providing distinguishable signatures indicating the lasing regime. Increasing the quality of the cavity mode induces a crossover from the solely superradiant to the lasing regime. Furthermore, we verify that lasing is manifested in a wide range of positive cooperative behavior, showing that stimulated emission and superradiance can coexist. The robustness of the cooperativity is studied with respect to experimental imperfections, such as inhomogeneous broadening and pure dephasing

Universal and deterministic manipulation of the quantum state of harmonic oscillators: a route to unitary gates for Fock State qubits

We present a simple quantum circuit that allows for the universal and deterministic manipulation of the quantum state of confined harmonic oscillators. The scheme is based on the selective interactions of the referred oscillator with an auxiliary three-level system and a classical external driving source, and enables any unitary operations on Fock states, two-by-two. One circuit is equivalent to a single qubit unitary logical gate on Fock states qubits. Sequences of similar protocols allow for complete, deterministic and state-independent manipulation of the harmonic oscillator quantum state.Comment: 4 pages, 4 figure

Distant entanglement protected through artificially increased local temperature

In composed quantum systems, the presence of local dissipative channels causes loss of coherence and entanglement at a rate that grows with the temperature of the reservoirs. However, here we show that if temperature is artificially added to the system, entanglement decay can be significantly slowed down or even suppressed conditioned on suitable local monitoring of the reservoirs. We propose a scheme to implement the joint reservoir monitoring applicable in different experimental setups like trapped ions, circuit and cavity QED or quantum dots coupled to nanowires and we analyze its general robustness against detection inefficiencies and non-zero temperature of the natural reservoir

Geometric phase induced by a cyclically evolving squeezed vacuum reservoir

We propose a new way to generate an observable geometric phase by means of a completely incoherent phenomenon. We show how to imprint a geometric phase to a system by "adiabatically" manipulating the environment with which it interacts. As a specific scheme we analyse a multilevel atom interacting with a broad-band squeezed vacuum bosonic bath. As the squeezing parameters are smoothly changed in time along a closed loop, the ground state of the system acquires a geometric phase. We propose also a scheme to measure such geometric phase by means of a suitable polarization detection.Comment: 4 pages, 1 figur

Coherent evolution via reservoir driven holonomy

We show that in the limit of strongly interacting environment a system initially prepared in a Decoherence Free Subspace (DFS) coherently evolves in time, adiabatically following the changes of the DFS. If the reservoir cyclicly evolves in time, the DFS states acquire an holonomy.Comment: 4 page

Equilibrium and Disorder-induced behavior in Quantum Light-Matter Systems

We analyze equilibrium properties of coupled-doped cavities described by the Jaynes-Cummings- Hubbard Hamiltonian. In particular, we characterize the entanglement of the system in relation to the insulating-superfluid phase transition. We point out the existence of a crossover inside the superfluid phase of the system when the excitations change from polaritonic to purely photonic. Using an ensemble statistical approach for small systems and stochastic-mean-field theory for large systems we analyze static disorder of the characteristic parameters of the system and explore the ground state induced statistics. We report on a variety of glassy phases deriving from the hybrid statistics of the system. On-site strong disorder induces insulating behavior through two different mechanisms. For disorder in the light-matter detuning, low energy cavities dominate the statistics allowing the excitations to localize and bunch in such cavities. In the case of disorder in the light- matter coupling, sites with strong coupling between light and matter become very significant, which enhances the Mott-like insulating behavior. Inter-site (hopping) disorder induces fluidity and the dominant sites are strongly coupled to each other.Comment: about 10 pages, 12 figure

Avaliação do transporte apical e da capacidade de centralização de dois sistemas rotatórios de niquel-titânio

Introduction: Rotary nickel-titanium (NiTi) instruments have become very popular in recent years mainly because they allow an effi cient preparation of the root canal system. New rotary endodontic instruments resulted from the development of new features, such as variable taper, non-cutting safety tip and variable length of cutting blades. The purpose of this study was to evaluate the degree of canal transportation observed with two nickel-titanium rotary systems, K3 (SybronEndo, Orange, CA, USA) and Mtwo (VDW, Munich, Germany), as well as their centering ability, by measuring dentin wall thickness before and after instrumentation. Methods: Thirty extracted mandibular molars were embedded in resin blocks and sectioned 3.5 and 5.0 mm short of the anatomical apex. The mesiobuccal canals were prepared with the K3 system, using instruments 0.12/25, 0.08/25, 0.06/25, 0.04/25, and 0.02/30 progressively until reaching the working length; and the mesiolingual canals were prepared with the Mtwo system, using instruments 0.04/10, 0.05/15, 0.06/20, and 0.06/25 to full working length. Pre- and postoperative sections were photographed and all data were recorded and analyzed statistically using the Mann-Whitney test. Results: Canal transportation and centering ability results were similar for both instruments (p > 0.05). Relevance: The K3 and Mtwo systems allowed the preparation of curved root canals with minimal transportation, 3.5 or 5.0 mm short of the anatomical apex.Introdução: Os instrumentos rotatórios em níquel titânio vêm se tornando populares nos últimos anos principalmente devido à sua efi ciência no preparo de canais radiculares. Novos instrumentos rotatórios resultaram do desenvolvimento de características como conicidade, pontas sem corte seguras, e variação do comprimento da lâmina de corte. A proposta deste estudo foi avaliar o grau de transporte do canal observado com dois sistemas rotatórios de níquel-titânio, K3 (SybronEndo, Orange, CA, EUA) e Mtwo (VDW, Munique, Alemanha), bem como sua habilidade de centralização, por meio da mensuração dos fragmentos de dentina da parede do canal radicular, antes e após a instrumentação. Métodos: Trinta molares inferiores foram inseridos em blocos de resina e seccionados a 3,5 e a 5,0 mm do ápice. Os canais mésio-vestibulares foram preparados com o sistema K3 utilizando-se a sequência #25.12, #25.08, #25.06, #25.04 e #30.02 progressivamente até o comprimento de trabalho; e os canais mésio-linguais foram preparados com o sistema Mtwo, utilizando-se os instrumentos #10.04, #15.05, #20.06 e #25.06 por todo o comprimento de trabalho. Secções pré e pós-operatória foram fotografadas, e todos os dados foram anotados e analisados estatisticamente utilizando-se o teste de Mann-Whitney. Resultados: Os resultados referentes ao grau de transporte do canal e à capacidade de centralização foram similares para ambos os sistemas (p > 0.05). Relevância: Os sistemas K3 e Mtwo permitiram o preparo de canais curvos com mínimo transporte a 3,0 ou a 5,0 mm do ápice anatômico