2,158 research outputs found
Dissipative dynamics of a two - level system resonantly coupled to a harmonic mode
We propose an approximation scheme to describe the dynamics of the spin-boson
model when the spectral density of the environment shows a peak at a
characteristic frequency which can be very close (or even equal) to
the spin Zeeman frequency . Mapping the problem onto a two-state system
(TSS) coupled to a harmonic oscillator (HO) with frequency we show
that the representation of displaced HO states provides an appropriate basis to
truncate the Hilbert space of the TSS-HO system and therefore a better picture
of the system dynamics. We derive an effective Hamiltonian for the TSS-HO
system, and show it furnishes a very good approximation for the system dynamics
even when its two subsystems are moderately coupled. Finally, assuming the
regime of weak HO-bath coupling and low temperatures, we are able to
analytically evaluate the dissipative TSS dynamics.Comment: 12 pages, 2 figures; V2: Published versio
Conservation law for distributed entanglement of formation and quantum discord
We present a direct relation, based upon a monogamic principle, between
entanglement of formation (EOF) and quantum discord (QD), showing how they are
distributed in an arbitrary tripartite pure system. By extending it to a
paradigmatic situation of a bipartite system coupled to an environment, we
demonstrate that the EOF and the QD obey a conservation relation. By means of
this relation we show that in the deterministic quantum computer with one pure
qubit the protocol has the ability to rearrange the EOF and the QD, which
implies that quantum computation can be understood on a different basis as a
coherent dynamics where quantum correlations are distributed between the qubits
of the computer. Furthermore, for a tripartite mixed state we show that the
balance between distributed EOF and QD results in a stronger version of the
strong subadditivity of entropy.Comment: Published versio
Protecting the operation from general and residual errors by continuous dynamical decoupling
We study the occurrence of errors in a continuously decoupled two-qubit state
during a quantum operation under decoherence. We consider a
realization of this quantum gate based on the Heisenberg exchange interaction,
which alone suffices for achieving universal quantum computation. Furthermore,
we introduce a continuous-dynamical-decoupling scheme that commutes with the
Heisenberg Hamiltonian to protect it from the amplitude damping and dephasing
errors caused by the system-environment interaction. We consider two
error-protection settings. One protects the qubits from both amplitude damping
and dephasing errors. The other features the amplitude damping as a residual
error and protects the qubits from dephasing errors only. In both settings, we
investigate the interaction of qubits with common and independent environments
separately. We study how errors affect the entanglement and fidelity for
different environmental spectral densities.Comment: Extended version of arXiv:1005.1666. To appear in PR
Loss of quantum coherence in a system coupled to a zero-temperature environment
We discuss the influence of a zero-temperature environment on a coherent
quantum system. First, we calculate the reduced density operator of the system
in the framework of the well-known, exactly solvable model of an oscillator
coupled to a bath of harmonic oscillators. Then, we propose the sketch of an
Aharonov--Bohm-like interferometer showing, through interference measurements,
the decrease of the coherence length of the system due to the interaction with
the environment, even in the zero temperature limit
Non-Markovian Dynamics of Quantum Discord
We evaluate the quantum discord dynamics of two qubits in independent and
common non-Markovian environments. We compare the dynamics of entanglement with
that of quantum discord. For independent reservoirs the quantum discord
vanishes only at discrete instants whereas the entanglement can disappear
during a finite time interval. For a common reservoir, quantum discord and
entanglement can behave very differently with sudden birth of the former but
not of the latter. Furthermore, in this case the quantum discord dynamics
presents sudden changes in the derivative of its time evolution which is
evidenced by the presence of kinks in its behavior at discrete instants of
time.Comment: 6 pages, 4 figure
Single-Spin Measurement and Decoherence in Magnetic Resonance Force Microscopy
We consider a simple version of a cyclic adiabatic inversion (CAI) technique
in magnetic resonance force microscopy (MRFM). We study the problem: What
component of the spin is measured in the CAI MRFM? We show that the
non-destructive detection of the cantilever vibrations provides a measurement
of the spin component along the effective magnetic field. This result is based
on numerical simulations of the Hamiltonian dynamics (the Schrodinger equation)
and the numerical solution of the master equation.Comment: 5 pages + 5 figures (PNG format
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