401 research outputs found
Quadratic Dynamical Decoupling with Non-Uniform Error Suppression
We analyze numerically the performance of the near-optimal quadratic
dynamical decoupling (QDD) single-qubit decoherence errors suppression method
[J. West et al., Phys. Rev. Lett. 104, 130501 (2010)]. The QDD sequence is
formed by nesting two optimal Uhrig dynamical decoupling sequences for two
orthogonal axes, comprising N1 and N2 pulses, respectively. Varying these
numbers, we study the decoherence suppression properties of QDD directly by
isolating the errors associated with each system basis operator present in the
system-bath interaction Hamiltonian. Each individual error scales with the
lowest order of the Dyson series, therefore immediately yielding the order of
decoherence suppression. We show that the error suppression properties of QDD
are dependent upon the parities of N1 and N2, and near-optimal performance is
achieved for general single-qubit interactions when N1=N2.Comment: 17 pages, 22 figure
Quantum Darwinism in quantum Brownian motion: the vacuum as a witness
We study quantum Darwinism -- the redundant recording of information about a
decohering system by its environment -- in zero-temperature quantum Brownian
motion. An initially nonlocal quantum state leaves a record whose redundancy
increases rapidly with its spatial extent. Significant delocalization (e.g., a
Schroedinger's Cat state) causes high redundancy: many observers can measure
the system's position without perturbing it. This explains the objective (i.e.
classical) existence of einselected, decoherence-resistant pointer states of
macroscopic objects.Comment: 5 page
Deformed Wigner crystal in a one-dimensional quantum dot
The spatial Fourier spectrum of the electron density distribution in a finite
1D system and the distribution function of electrons over single-particle
states are studied in detail to show that there are two universal features in
their behavior, which characterize the electron ordering and the deformation of
Wigner crystal by boundaries. The distribution function has a -like
singularity at the Fermi momentum . The Fourier spectrum of the density
has a step-like form at the wavevector , with the harmonics being absent
or vanishing above this threshold. These features are found by calculations
using exact diagonalization method. They are shown to be caused by Wigner
ordering of electrons, affected by the boundaries. However the common Luttinger
liquid model with open boundaries fails to capture these features, because it
overestimates the deformation of the Wigner crystal. An improvement of the
Luttinger liquid model is proposed which allows one to describe the above
features correctly. It is based on the corrected form of the density operator
conserving the particle number.Comment: 10 pages, 11 figures. Misprints fixe
Decoherence and dissipation of a quantum harmonic oscillator coupled to two-level systems
We derive and analyze the Born-Markov master equation for a quantum harmonic
oscillator interacting with a bath of independent two-level systems. This
hitherto virtually unexplored model plays a fundamental role as one of the four
"canonical" system-environment models for decoherence and dissipation. To
investigate the influence of further couplings of the environmental spins to a
dissipative bath, we also derive the master equation for a harmonic oscillator
interacting with a single spin coupled to a bosonic bath. Our models are
experimentally motivated by quantum-electromechanical systems and micron-scale
ion traps. Decoherence and dissipation rates are found to exhibit temperature
dependencies significantly different from those in quantum Brownian motion. In
particular, the systematic dissipation rate for the central oscillator
decreases with increasing temperature and goes to zero at zero temperature, but
there also exists a temperature-independent momentum-diffusion (heating) rate.Comment: 8 pages, 3 figure
Storing entanglement of nuclear spins via Uhrig Dynamical Decoupling
Stroboscopic spin flips have already been shown to prolong the coherence
times of quantum systems under noisy environments. Uhrig's dynamical decoupling
scheme provides an optimal sequence for a quantum system interacting with a
dephasing bath. Several experimental demonstrations have already verified the
efficiency of such dynamical decoupling schemes in preserving single qubit
coherences. In this work we describe the experimental study of Uhrig's
dynamical decoupling in preserving two-qubit entangled states using an ensemble
of spin-1/2 nuclear pairs in solution state. We find that the performance of
odd-order Uhrig sequences in preserving entanglement is superior to both
even-order Uhrig sequences and periodic spin-flip sequences. We also find that
there exists an optimal length of the Uhrig sequence at which the decoherence
time gets boosted from a few seconds to about 30 seconds.Comment: 6 pages, 7 figure
Indistinguishability and Interference in the Coherent Control of Atomic and Molecular Processes
The subtle and fundamental issue of indistinguishability and interference
between independent pathways to the same target state is examined in the
context of coherent control of atomic and molecular processes, with emphasis
placed on possible "which-way" information due to quantum entanglement
established in the quantum dynamics. Because quantum interference between
independent pathways to the same target state occurs only when the independent
pathways are indistinguishable, it is first shown that creating useful
coherence (as defined in the paper) between nondegenerate states of a molecule
for subsequent quantum interference manipulation cannot be achieved by
collisions between atoms or molecules that are prepared in momentum and energy
eigenstates. Coherence can, however, be transferred from light fields to atoms
or molecules. Using a particular coherent control scenario, it is shown that
this coherence transfer and the subsequent coherent phase control can be
readily realized by the most classical states of light, i.e., coherent states
of light. It is further demonstrated that quantum states of light may suppress
the extent of phase-sensitive coherent control by leaking out some which-way
information while "incoherent interference control" scenarios proposed in the
literature have automatically ensured the indistinguishability of multiple
excitation pathways. The possibility of quantum coherence in photodissociation
product states is also understood in terms of the disentanglement between
photodissociation fragments. Results offer deeper insights into quantum
coherence generation in atomic and molecular processes.Comment: 26 pages, based on one Chapter from first author's Ph.D thesis in
200
Decoherence suppression via environment preparation
To protect a quantum system from decoherence due to interaction with its
environment, we investigate the existence of initial states of the environment
allowing for decoherence-free evolution of the system. For models in which a
two-state system interacts with a dynamical environment, we prove that such
states exist if and only if the interaction and self-evolution Hamiltonians
share an eigenstate. If decoherence by state preparation is not possible, we
show that initial states minimizing decoherence result from a delicate
compromise between the environment and interaction dynamics.Comment: 4 pages, 2 figure
Decoherence in an accelerated universe
In this paper we study the decoherence processes of the semiclassical
branches of an accelerated universe due to their interaction with a scalar
field with given mass. We use a third quantization formalism to analyze the
decoherence between two branches of a parent universe caused by their
interaction with the vaccum fluctuations of the space-time, and with other
parent unverses in a multiverse scenario.Comment: 11 pages, 2 figure
Collective versus Single--Particle Motion in Quantum Many--Body Systems: Spreading and its Semiclassical Interpretation
We study the interplay between collective and incoherent single-particle
motion in a model of two chains of particles whose interaction comprises a
non-integrable part. In the perturbative regime, but for a general form of the
interaction, we calculate the spectral density for collective excitations. We
obtain the remarkable result that it always has a unique semiclassical
interpretation. We show this by a proper renormalization procedure which allows
us to map our system to a Caldeira-Leggett--type of model in which the bath is
part of the system.Comment: 4 page
Quantum decoherence in noninertial frames
Quantum decoherence, which appears when a system interacts with its
environment in an irreversible way, plays a fundamental role in the description
of quantum-to-classical transitions and has been successfully applied in some
important experiments. Here, we study the decoherence in noninertial frames for
the first time. It is shown that the decoherence and loss of the entanglement
generated by the Unruh effect will influence each other remarkably. It is
interesting to note that in the case of the total system under decoherence, the
sudden death of entanglement may appear for any acceleration. However, in the
case of only Rob's qubit underging decoherence sudden death may only occur when
the acceleration parameter is greater than a "critical point."Comment: 4 pages, 3 figure
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