1,720,966 research outputs found
Photon losses depending on polarization mixedness
No full textWe introduce a quantum channel describing photon losses depending
on the degree of polarization mixedness. This can be regarded as
a model of quantum channel with correlated errors between
discrete and continuous degrees of freedom. We consider classical
information over a continuous alphabet encoded on weak coherent
states as well as classical information over a discrete alphabet
encoded on single photons using dual rail representation. In both
cases we study the one-shot capacity of the channel and its
behaviour in terms of correlation between losses and polarization
mixedness
Removing correlations in signals transmitted over a quantum memory channel
No full textWe consider a model of a bosonic memory channel, which induces correlations among the transmitted signals. The application of suitable unitary transformations at the encoding and decoding stages allows the complete removal of correlations, thereby mapping the memory channel into a memoryless one. However, such transformations, being global over an arbitrarily large number of bosonic modes, are not realistically implementable. We then introduce a family of efficiently realizable transformations, which can be used to partially remove correlations among errors, and we quantify the reduction of the gap with memoryless channels
Exact Steady State of the Open XX-Spin Chain: Entanglement and Transport Properties
Full text linkWe study the reduced dynamics of open quantum spin chains of arbitrary length N with nearest-neighbor XX interactions, immersed within an external constant magnetic field along the z direction, the end spins of which are weakly coupled to heat baths at different temperatures, via energy-preserving couplings. We find the analytic expression of the unique stationary state of the master equation obtained in the so-called global approach based on the spectralization of the full-chain Hamiltonian. Hinging upon the explicit stationary state, we reveal the presence of sink and source terms in the spin-flow continuity equation and compare their behavior with that of the stationary heat flow. Moreover, we also obtain analytic expressions for the steady-state two-spin reduced density matrices and for their concurrence. We then set up an algorithm suited to compute the stationary bipartite entanglement along the chain and to study its dependence on the Hamiltonian parameters and on the bath temperatures
Quantum capacity of a bosonic dephasing channel
Full text linkWe study the quantum capacity of a continuous-variable dephasing channel, which is a notable example of a non-Gaussian quantum channel. We prove that a single-letter formula applies. The optimal input state is found to be diagonal in the Fock basis and with a distribution that is a discrete version of a Gaussian. We discuss how its mean and variance are related to the dephasing rate and input energy. We then show that by increasing the input energy, the capacity saturates to a finite value. We also show that it decays exponentially for large values of dephasing rates
Bath-assisted transport in a three-site spin chain: Global versus local approach
No full textWithin the standard weak-coupling limit, the reduced dynamics of open quantum spin chains with their two end spins coupled to two distinct heat baths at different temperatures are mainly derived using the so-called global and local approaches, in which, respectively, the spin self-interaction is and is not taken into account. In order to compare the differences between the two regimes, we concentrate on an open three-site XX spin chain, provide systematic techniques to address the global and local asymptotic states, and then compare the asymptotic spin-transport features by studying the spin flux through the middle site. Based on the analytical expressions of the stationary states in the two regimes, we find that the local approach misses important global effects emerging as spin sink and source terms that can only be due to nonnegligible interspin interactions. Moreover, we show that the local asympotic transport features cannot be recovered from the global ones by letting the interspin coupling vanish, thus pointing to the existence of different coupling ranges where only one approach is physically tenable and possibly a region where the two descriptions may coexist
Stationary states of open XX-spin chains
Full text linkWe study an open quantum spin chain of arbitrary length with nearest neighbor X X interactions of strength g, immersed in an external constant magnetic field Δ along the z direction, whose end spins are weakly coupled to two heat baths at different temperatures. In the so-called global approach, namely, without neglecting interspin interactions, using standard weak-coupling limit techniques, we first derive the open chain master equation written in terms of fermionic mode operators. Then, we focus on the study of the dependence of the resulting open dynamics from the ratio r ≡ g/Δ. By increasing r, some of the chain Bohr transition frequencies become negative; when this occurs, both the generator of the dissipative time evolution and its stationary states behave discontinuously. As a consequence, the asymptotic spin and heat flows also exhibit discontinuities, but in a different way: while source terms in the spin flow continuity equation show jumps, the heat flow instead is continuous but with discontinuous first derivatives with respect to r. These two behaviors might be experimentally accessible; in particular, they could discriminate between the global and the local approaches to open quantum spin chains. Indeed, the latter one, which neglects interspin interactions in the derivation of the master equation, does not show any kind of discontinuous behavior
Discrimination of dephasing channels
Full text linkThe problem of dephasing channel discrimination is addressed for
finite-dimensional systems. In particular, the optimization with respect to
input states without energy constraint is solved analytically for qubit, qutrit
and ququart. Additionally, it is shown that resorting to side entanglement
assisted strategy is completely useless in this case
PERSISTENT HOMOLOGY ANALYSIS OF MULTIQUBIT ENTANGLEMENT
Full text linkWe introduce a homology-based technique for the classification of multiqubit state vectors with genuine entanglement. In our approach, we associate state vectors to data sets by introducing a metric-like measure in terms of bipartite entanglement, and investigate the persistence of homologies at different scales. This leads to a novel classification of multiqubit entanglement. The relative occurrence frequency of various classes of entangled states is also shown
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