16,030 research outputs found
Quantum state transfer in imperfect artificial spin networks
High-fidelity quantum computation and quantum state transfer are possible in
short spin chains. We exploit a system based on a dispersive qubit-boson
interaction to mimic XY coupling. In this model, the usually assumed
nearest-neighbors coupling is no more valid: all the qubits are mutually
coupled. We analyze the performances of our model for quantum state transfer
showing how pre-engineered coupling rates allow for nearly optimal state
transfer. We address a setup of superconducting qubits coupled to a microstrip
cavity in which our analysis may be applied.Comment: 4 pages, 3 figures, RevTeX
Landauer's principle in multipartite open quantum system dynamics
We investigate the link between information and thermodynamics embodied by
Landauer's principle in the open dynamics of a multipartite quantum system.
Such irreversible dynamics is described in terms of a collisional model with a
finite temperature reservoir. We demonstrate that Landauer's principle holds,
for such a configuration, in a form that involves the flow of heat dissipated
into the environment and the rate of change of the entropy of the system. Quite
remarkably, such a principle for {\it heat and entropy power} can be explicitly
linked to the rate of creation of correlations among the elements of the
multipartite system and, in turn, the non-Markovian nature of their reduced
evolution. Such features are illustrated in two exemplary cases.Comment: 5 pages, 3 figures, RevTeX4-1; Accepted for publication in Phys. Rev.
Let
Extraction of Singlet States from Noninteracting High-Dimensional Spins
We present a scheme for the extraction of singlet states of two remote
particles of arbitrary quantum spin number. The goal is achieved through
post-selection of the state of interaction mediators sent in succession. A
small number of iterations is sufficient to make the scheme effective. We
propose two suitable experimental setups where the protocol can be implemented.Comment: 4 pages, 2 figure
Normal form decomposition for Gaussian-to-Gaussian superoperators
In this paper we explore the set of linear maps sending the set of quantum
Gaussian states into itself. These maps are in general not positive, a feature
which can be exploited as a test to check whether a given quantum state belongs
to the convex hull of Gaussian states (if one of the considered maps sends it
into a non positive operator, the above state is certified not to belong to the
set). Generalizing a result known to be valid under the assumption of complete
positivity, we provide a characterization of these Gaussian-to-Gaussian (not
necessarily positive) superoperators in terms of their action on the
characteristic function of the inputs. For the special case of one-mode
mappings we also show that any Gaussian-to-Gaussian superoperator can be
expressed as a concatenation of a phase-space dilatation, followed by the
action of a completely positive Gaussian channel, possibly composed with a
transposition. While a similar decomposition is shown to fail in the multi-mode
scenario, we prove that it still holds at least under the further hypothesis of
homogeneous action on the covariance matrix
Structural change in multipartite entanglement sharing: a random matrix approach
We study the typical entanglement properties of a system comprising two
independent qubit environments interacting via a shuttling ancilla. The initial
preparation of the environments is modeled using random-matrix techniques. The
entanglement measure used in our study is then averaged over many histories of
randomly prepared environmental states. Under a Heisenberg interaction model,
the average entanglement between the ancilla and one of the environments
remains constant, regardless of the preparation of the latter and the details
of the interaction. We also show that, upon suitable kinematic and dynamical
changes in the ancilla-environment subsystems, the entanglement-sharing
structure undergoes abrupt modifications associated with a change in the
multipartite entanglement class of the overall system's state. These results
are invariant with respect to the randomized initial state of the environments.Comment: 10 pages, RevTeX4 (Minor typo's corrected. Closer to published
version
Limitation of entanglement due to spatial qubit separation
We consider spatially separated qubits coupled to a thermal bosonic field
that causes pure dephasing. Our focus is on the entanglement of two Bell states
which for vanishing separation are known as robust and fragile entangled
states. The reduced two-qubit dynamics is solved exactly and explicitly. Our
results allow us to gain information about the robustness of two-qubit
decoherence-free subspaces with respect to physical parameters such as
temperature, qubit-bath coupling strength and spatial separation of the qubits.
Moreover, we clarify the relation between single-qubit coherence and two-qubit
entanglement and identify parameter regimes in which the terms robust and
fragile are no longer appropriate.Comment: 7 pages, 3 figures; revised version, accepted for publication in
Europhys. Let
Accumulation of entanglement in a continuous variable memory
We study the accumulation of entanglement in a memory device built out of two
continuous variable (CV) systems. We address the case of a qubit mediating an
indirect joint interaction between the CV systems. We show that, in striking
contrast with respect to registers built out of bidimensional Hilbert spaces,
entanglement superior to a single ebit can be efficiently accumulated in the
memory, even though no entangled resource is used. We study the protocol in an
immediately implementable setup, assessing the effects of the main
imperfections.Comment: 4 pages, 3 figures, RevTeX
The squashed entanglement of the noiseless quantum Gaussian attenuator and amplifier
We determine the maximum squashed entanglement achievable between sender and
receiver of the noiseless quantum Gaussian attenuators and amplifiers and we
prove that it is achieved sending half of an infinitely squeezed two-mode
vacuum state. The key ingredient of the proof is a lower bound to the squashed
entanglement of the quantum Gaussian states obtained applying a two-mode
squeezing operation to a quantum thermal Gaussian state tensored with the
vacuum state. This is the first lower bound to the squashed entanglement of a
quantum Gaussian state and opens the way to determine the squashed entanglement
of all quantum Gaussian channels. Moreover, we determine the classical squashed
entanglement of the quantum Gaussian states above and show that it is strictly
larger than their squashed entanglement. This is the first time that the
classical squashed entanglement of a mixed quantum Gaussian state is
determined
Information-flux approach to multiple-spin dynamics
We introduce and formalize the concept of information flux in a many-body
register as the influence that the dynamics of a specific element receive from
any other element of the register. By quantifying the information flux in a
protocol, we can design the most appropriate initial state of the system and,
noticeably, the distribution of coupling strengths among the parts of the
register itself. The intuitive nature of this tool and its flexibility, which
allow for easily manageable numerical approaches when analytic expressions are
not straightforward, are greatly useful in interacting many-body systems such
as quantum spin chains. We illustrate the use of this concept in quantum
cloning and quantum state transfer and we also sketch its extension to
non-unitary dynamics.Comment: 7 pages, 4 figures, RevTeX
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