32 research outputs found
Surface Entanglement in Quantum Spin Networks
We study the ground-state entanglement in systems of spins forming the
boundary of a quantum spin network in arbitrary geometries and dimensionality.
We show that as long as they are weakly coupled to the bulk of the network, the
surface spins are strongly entangled, even when distant and non directly
interacting, thereby generalizing the phenomenon of long-distance entanglement
occurring in quantum spin chains. Depending on the structure of the couplings
between surface and bulk spins, we discuss in detail how the patterns of
surface entanglement can range from multi-pair bipartite to fully multipartite.
In the context of quantum information and communication, these results find
immediate application to the implementation of quantum routers, that is devices
able to distribute quantum correlations on demand among multiple network nodes.Comment: 8 pages, 8 figure
Generation and detection of large and robust entanglement between two different mechanical resonators in cavity optomechanics
We investigate a general scheme for generating, either dynamically or in the
steady state, continuous variable entanglement between two mechanical
resonators with different frequencies. We employ an optomechanical system in
which a single optical cavity mode driven by a suitably chosen two-tone field
is coupled to the two resonators. Significantly large mechanical entanglement
can be achieved, which is extremely robust with respect to temperature.Comment: To appear in New J. Phys. Small extensions in response to the points
raised by the referee and Refs adde
Ground state cooling in a bad cavity
We study the mechanical effects of light on an atom trapped in a harmonic
potential when an atomic dipole transition is driven by a laser and it is
strongly coupled to a mode of an optical resonator. We investigate the cooling
dynamics in the bad cavity limit, focussing on the case in which the effective
transition linewidth is smaller than the trap frequency, hence when sideband
cooling could be implemented. We show that quantum correlations between the
mechanical actions of laser and cavity field can lead to an enhancement of the
cooling efficiency with respect to sideband cooling. Such interference effects
are found when the resonator losses prevail over spontaneous decay and over the
rates of the coherent processes characterizing the dynamics.Comment: 6 pages, 5 figures; J. Mod. Opt. (2007
Entanglement replication in driven-dissipative many body systems
We study the dissipative dynamics of two independent arrays of many-body
systems, locally driven by a common entangled field. We show that in the steady
state the entanglement of the driving field is reproduced in an arbitrarily
large series of inter-array entangled pairs over all distances. Local
nonclassical driving thus realizes a scale-free entanglement replication and
long-distance entanglement distribution mechanism that has immediate bearing on
the implementation of quantum communication networks.Comment: 7 pages, 4 figures. To appear in Physical Review Letter
Quantum interference from remotely trapped ions
We observe quantum interference of photons emitted by two continuously
laser-excited single ions, independently trapped in distinct vacuum vessels.
High contrast two-photon interference is observed in two experiments with
different ion species, calcium and barium. Our experimental findings are
quantitatively reproduced by Bloch equation calculations. In particular, we
show that the coherence of the individual resonance fluorescence light field is
determined from the observed interference
Collective effects in the dynamics of driven atoms in a high-Q resonator
We study the quantum dynamics of N coherently driven two-level atoms coupled
to an optical resonator. In the strong coupling regime the cavity field
generated by atomic scattering interferes destructively with the pump on the
atoms. This suppresses atomic excitation and even for strong driving fields
prevents atomic saturation, while the stationary intracavity field amplitude is
almost independent of the atom number. The magnitude of the interference effect
depends on the detuning between laser and cavity field and on the relative
atomic positions and is strongest for a wavelength spaced lattice of atoms
placed at the antinodes of the cavity mode. In this case three dimensional
intensity minima are created in the vicinity of each atom. In this regime
spontaneous emission is suppressed and the dominant loss channel is cavity
decay. Even for a cavity linewidth larger than the atomic natural width, one
regains strong interference through the cooperative action of a sufficiently
large number of atoms. These results give a new key to understand recent
experiments on collective cavity cooling and may allow to implement fast
tailored atom-atom interactions as well as nonperturbative particle detection
with very small energy transfer.Comment: 12 pages, 13 figures, significantly extended version, slightly
different from the published on
Raman spectroscopy of a single ion coupled to a high-finesse cavity
We describe an ion-based cavity-QED system in which the internal dynamics of
an atom is coupled to the modes of an optical cavity by vacuum-stimulated Raman
transitions. We observe Raman spectra for different excitation polarizations
and find quantitative agreement with theoretical simulations. Residual motion
of the ion introduces motional sidebands in the Raman spectrum and leads to ion
delocalization. The system offers prospects for cavity-assisted
resolved-sideband ground-state cooling and coherent manipulation of ions and
photons.Comment: 8 pages, 6 figure
Hybrid Mechanical Systems
We discuss hybrid systems in which a mechanical oscillator is coupled to
another (microscopic) quantum system, such as trapped atoms or ions,
solid-state spin qubits, or superconducting devices. We summarize and compare
different coupling schemes and describe first experimental implementations.
Hybrid mechanical systems enable new approaches to quantum control of
mechanical objects, precision sensing, and quantum information processing.Comment: To cite this review, please refer to the published book chapter (see
Journal-ref and DOI). This v2 corresponds to the published versio
Quantifying decoherence in continuous variable systems
We present a detailed report on the decoherence of quantum states of
continuous variable systems under the action of a quantum optical master
equation resulting from the interaction with general Gaussian uncorrelated
environments. The rate of decoherence is quantified by relating it to the decay
rates of various, complementary measures of the quantum nature of a state, such
as the purity, some nonclassicality indicators in phase space and, for two-mode
states, entanglement measures and total correlations between the modes.
Different sets of physically relevant initial configurations are considered,
including one- and two-mode Gaussian states, number states, and coherent
superpositions. Our analysis shows that, generally, the use of initially
squeezed configurations does not help to preserve the coherence of Gaussian
states, whereas it can be effective in protecting coherent superpositions of
both number states and Gaussian wave packets.Comment: Review article; 36 pages, 19 figures; typos corrected, references
adde
Decoherence control in microwave cavities
We present a scheme able to protect the quantum states of a cavity mode
against the decohering effects of photon loss. The scheme preserves quantum
states with a definite parity, and improves previous proposals for decoherence
control in cavities. It is implemented by sending single atoms, one by one,
through the cavity. The atomic state gets first correlated to the photon number
parity. The wrong parity results in an atom in the upper state. The atom in
this state is then used to inject a photon in the mode via adiabatic transfer,
correcting the field parity. By solving numerically the exact master equation
of the system, we show that the protection of simple quantum states could be
experimentally demonstrated using presently available experimental apparatus.Comment: 13 pages, RevTeX, 8 figure