32,816 research outputs found
Experimental observation of weak non-Markovianity
Non-Markovianity has recently attracted large interest due to significant
advances in its characterization and its exploitation for quantum information
processing. However, up to now, only non-Markovian regimes featuring
environment to system backflow of information (strong non-Markovianity) have
been experimentally simulated. In this work, using an all-optical setup we
simulate and observe the so-called weak non-Markovian dynamics. Through full
process tomography, we experimentally demonstrate that the dynamics of a qubit
can be non-Markovian despite an always increasing correlation between the
system and its environment which, in our case, denotes no information backflow.
We also show the transition from the weak to the strong regime by changing a
single parameter in the environmental state, leading us to a better
understanding of the fundamental features of non-Markovianity.Comment: v2: final versio
Weak Quantum Theory: Complementarity and Entanglement in Physics and Beyond
The concepts of complementarity and entanglement are considered with respect
to their significance in and beyond physics. A formally generalized, weak
version of quantum theory, more general than ordinary quantum theory of
material systems, is outlined and tentatively applied to some examples.Comment: Revised version. Chapter 5.2 (old counting) omitted for separate
publication, chapter 5.2 (new counting) reformulate
Quantum states and space-time causality
Space-time symmetries and internal quantum symmetries can be placed on equal
footing in a hyperspin geometry. Four-dimensional classical space-time emerges
as a result of a decoherence that disentangles the quantum and the space-time
degrees of freedom. A map from the quantum space-time to classical space-time
that preserves the causality relations of space-time events is necessarily a
density matrix.Comment: 9 pages, to appear in the Proceedings of the 2nd International
Symposium on Information Geometry and its Application
On the distillation and purification of phase-diffused squeezed states
Recently it was discovered that non-Gaussian decoherence processes, such as
phase-diffusion, can be counteracted by purification and distillation protocols
that are solely built on Gaussian operations. Here, we make use of this
experimentally highly accessible regime, and provide a detailed experimental
and theoretical analysis of several strategies for purification/distillation
protocols on phase-diffused squeezed states. Our results provide valuable
information for the optimization of such protocols with respect to the choice
of the trigger quadrature, the trigger threshold value and the probability of
generating a distilled state
Minimal qudit code for a qubit in the phase-damping channel
Using the stabilizer formalism we construct the minimal code into a
D-dimensional Hilbert space (qudit) to protect a qubit against phase damping.
The effectiveness of this code is then studied by means of input-output
fidelity.Comment: 9 pages, 3 figures. REVTe
Quantum reading capacity: General definition and bounds
Quantum reading refers to the task of reading out classical information
stored in a read-only memory device. In any such protocol, the transmitter and
receiver are in the same physical location, and the goal of such a protocol is
to use these devices (modeled by independent quantum channels), coupled with a
quantum strategy, to read out as much information as possible from a memory
device, such as a CD or DVD. As a consequence of the physical setup of quantum
reading, the most natural and general definition for quantum reading capacity
should allow for an adaptive operation after each call to the channel, and this
is how we define quantum reading capacity in this paper. We also establish
several bounds on quantum reading capacity, and we introduce an
environment-parametrized memory cell with associated environment states,
delivering second-order and strong converse bounds for its quantum reading
capacity. We calculate the quantum reading capacities for some exemplary memory
cells, including a thermal memory cell, a qudit erasure memory cell, and a
qudit depolarizing memory cell. We finally provide an explicit example to
illustrate the advantage of using an adaptive strategy in the context of
zero-error quantum reading capacity.Comment: v3: 17 pages, 2 figures, final version published in IEEE Transactions
on Information Theor
Recent advances in exciton based quantum information processing in quantum dot nanostructures
Recent experimental developments in the field of semiconductor quantum dot
spectroscopy will be discussed. First we report about single quantum dot
exciton two-level systems and their coherent properties in terms of single
qubit manipulations. In the second part we report on coherent quantum coupling
in a prototype "two-qubit" system consisting of a vertically stacked pair of
quantum dots. The interaction can be tuned in such quantum dot molecule devices
using an applied voltage as external parameter.Comment: 37 pages, 15 figures, submitted to New Journal of Physics, focus
issue on Solid State Quantum Information, added reference
Quantumness and memory of one qubit in a dissipative cavity under classical control
Hybrid quantum–classical systems constitute a promising architecture for useful control strategies of quantum systems by means of a classical device. Here we provide a comprehensive study of the dynamics of various manifestations of quantumness with memory effects, identified by non-Markovianity, for a qubit controlled by a classical field and embedded in a leaky cavity. We consider both Leggett–Garg inequality and quantum witness as experimentally-friendly indicators of quantumness, also studying the geometric phase of the evolved (noisy) quantum state. We show that, under resonant qubit-classical field interaction, a stronger coupling to the classical control leads to enhancement of quantumness despite a disappearance of non-Markovianity. Differently, increasing the qubit-field detuning (out-of-resonance) reduces the nonclassical behavior of the qubit while recovering non-Markovian features. We then find that the qubit geometric phase can be remarkably preserved irrespective of the cavity spectral width via strong coupling to the classical field. The controllable interaction with the classical field inhibits the effective time-dependent decay rate of the open qubit. These results supply practical insights towards a classical harnessing of quantum properties in a quantum information scenari
Entanglement versus Quantum Discord in Two Coupled Double Quantum Dots
We study the dynamics of quantum correlations of two coupled double quantum
dots containing two excess electrons. The dissipation is included through the
contact with an oscillator bath. We solve the Redfield master equation in order
to determine the dynamics of the quantum discord and the entanglement of
formation. Based on our results, we find that the quantum discord is more
resistant to dissipation than the entanglement of formation for such a system.
We observe that this characteristic is related to whether the oscillator bath
is common to both qubits or not and to the form of the interaction Hamiltonian.
Moreover, our results show that the quantum discord might be finite even for
higher temperatures in the asymptotic limit.Comment: 14 pages, 8 figures (new version is the final version to appear in
NJP
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