117 research outputs found
Extraction of an Entanglement by Repetition of the Resonant Transmission of an Ancilla Qubit
A scheme for the extraction of entanglement in two noninteracting qubits
(spins) is proposed. The idea is to make use of resonant transmission of
ancilla qubit through the two fixed qubits, controlled by the entanglement in
the scatterers. Repetition of the resonant transmission extracts the singlet
state in the target qubits from their arbitrary given state. Neither the
preparation nor the post-selection of the ancilla spin is required, in contrast
to the previously proposed schemes.Comment: 14 pages, 7 figure
Hot-electron noise suppression in n-Si via the Hall effect
We investigate how hot-electron fluctuations in n-type Si are affected by the
presence of an intense (static) magnetic field in a Hall geometry. By using the
Monte Carlo method, we find that the known Hall-effect-induced redistribution
of electrons among valleys can suppress electron fluctuations with a
simultaneous enhancement of the drift velocity
Effect of Static Disorder in an Electron Fabry-Perot Interferometer with Two Quantum Scattering Centers
In a recent paper -- F. Ciccarello \emph{et al.}, New J. Phys. \textbf{8},
214 (2006) -- we have demonstrated that the electron transmission properties of
a one-dimensional (1D) wire with two identical embedded spin-1/2 impurities can
be significantly affected by entanglement between the spins of the scattering
centers. Such effect is of particular interest in the control of transmission
of quantum information in nanostructures and can be used as a detection scheme
of maximally entangled states of two localized spins. In this letter, we relax
the constraint that the two magnetic impurities are equal and investigate how
the main results presented in the above paper are affected by a static disorder
in the exchange coupling constants of the impurities. Good robustness against
deviation from impurity symmetry is found for both the entanglement dependent
transmission and the maximally entangled states generation scheme.Comment: 4 pages, 5 figure
On demand entanglement in double quantum dots via coherent carrier scattering
We show how two qubits encoded in the orbital states of two quantum dots can
be entangled or disentangled in a controlled way through their interaction with
a weak electron current. The transmission/reflection spectrum of each scattered
electron, acting as an entanglement mediator between the dots, shows a
signature of the dot-dot entangled state. Strikingly, while few scattered
carriers produce decoherence of the whole two-dots system, a larger number of
electrons injected from one lead with proper energy is able to recover its
quantum coherence. Our numerical simulations are based on a real-space solution
of the three-particle Schroedinger equation with open boundaries. The computed
transmission amplitudes are inserted in the analytical expression of the system
density matrix in order to evaluate the entanglement.Comment: 20 pages, 5 figure
Selective writing and read-out of a register of static qubits
We propose a setup comprising an arbitrarily large array of static qubits
(SQs), which interact with a flying qubit (FQ). The SQs work as a quantum
register, which can be written or read-out by means of the FQ through quantum
state transfer (QST). The entire system, including the FQ's motional degrees of
freedom, behaves quantum mechanically. We demonstrate a strategy allowing for
selective QST between the FQ and a single SQ chosen from the register. This is
achieved through a perfect mirror located beyond the SQs and suitable
modulation of the inter-SQ distances.Comment: 14 pages, 4 figure
Photon localization versus population trapping in a coupled-cavity array
We consider a coupled-cavity array (CCA), where one cavity interacts with a two-level atom under the rotating-wave approximation. We investigate the excitation transport dynamics across the array, which arises in the atom's emission process into the CCA vacuum. Due to the known formation of atom-photon bound states, partial field localization and atomic population trapping in general take place. We study the functional dependence on the coupling strength of these two phenomena and show that the threshold values beyond which they become significant are different. As the coupling strength grows from zero, field localization is exhibited first
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
Electron Fabry-Perot interferometer with two entangled magnetic impurities
We consider a one-dimensional (1D) wire along which single conduction
electrons can propagate in the presence of two spin-1/2 magnetic impurities.
The electron may be scattered by each impurity via a contact-exchange
interaction and thus a spin-flip generally occurs at each scattering event.
Adopting a quantum waveguide theory approach, we derive the stationary states
of the system at all orders in the electron-impurity exchange coupling
constant. This allows us to investigate electron transmission for arbitrary
initial states of the two impurity spins. We show that for suitable electron
wave vectors, the triplet and singlet maximally entangled spin states of the
impurities can respectively largely inhibit the electron transport or make the
wire completely transparent for any electron spin state. In the latter case, a
resonance condition can always be found, representing an anomalous behaviour
compared to typical decoherence induced by magnetic impurities. We provide an
explanation for these phenomena in terms of the Hamiltonian symmetries.
Finally, a scheme to generate maximally entangled spin states of the two
impurities via electron scattering is proposed.Comment: 19 page
Quantum non-Markovianity induced by Anderson localization
As discovered by P. W. Anderson, excitations do not propagate freely in a disordered lattice, but, due to destructive interference, they localise. As a consequence, when an atom interacts with a disordered lattice, one indeed observes a non-trivial excitation exchange between atom and lattice. Such non-trivial atomic dynamics will in general be characterised also by a non-trivial quantum information backflow, a clear signature of non-Markovian dynamics. To investigate the above scenario, we consider a quantum emitter, or atom, weakly coupled to a uniform coupled-cavity array (CCA). If initially excited, in the absence of disorder, the emitter undergoes a Markovian spontaneous emission by releasing all its excitation into the CCA (initially in its vacuum state). By introducing static disorder in the CCA the field normal modes become Anderson-localized, giving rise to a non-Markovian atomic dynamics. We show the existence of a functional relationship between a rigorous measure of quantum non-Markovianity and the CCA localization. We furthermore show that the average non-Markovianity of the atomic dynamics is well-described by a phenomenological model in which the atom is coupled, at the same time, to a single mode and to a standard - Markovian - dissipative bath
Toward computability of trace distance discord
It is known that a reliable geometric quantifier of discord-like correlations
can be built by employing the so-called trace distance. This is used to measure
how far the state under investigation is from the closest "classical-quantum"
one. To date, the explicit calculation of this indicator for two qubits was
accomplished only for states such that the reduced density matrix of the
measured party is maximally mixed, a class that includes Bell-diagonal states.
Here, we first reduce the required optimization for a general two-qubit state
to the minimization of an explicit two-variable function. Using this framework,
we show next that the minimum can be analytically worked out in a number of
relevant cases including quantum-classical and X states. This provides an
explicit and compact expression for the trace distance discord of an arbitrary
state belonging to either of these important classes of density matrices.Comment: 24 pages, 2 figures. Added a new section featuring an applicatio
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