47,370 research outputs found
Noise robustness in the detection of non separable random unitary maps
We briefly review a recently proposed method to detect properties of quantum
noise processes and quantum channels. We illustrate in detail the method for
detecting non separable random unitary channels and consider in particular the
explicit examples of the CNOT and CZ gates. We analyse their robustness in the
presence of noise for several quantum noise models.Comment: 10 pages, 1 figur
Detection methods to rule out completely co-positive and bi-entangling operations
In this work we extend the quantum channel detection method developed in
[Phys. Rev. A 88, 042335 (2013)] and [Phys. Script. T153, 014044 (2013)] in
order to detect other interesting convex sets of quantum channels. First we
work out a procedure to detect non completely co-positive maps. Then we focus
on the set of so-called bi-entangling operations and show how a map outside
this set can be revealed. In both cases we provide explicit examples showing
the theoretical technique and the corresponding experimental procedure.Comment: 6 pages, 2 figure
Note on lattice regularization and equal-time correlators for parton distribution functions
We show that a recent interesting idea to circumvent the difficulties with
the continuation of parton distribution functions to the Euclidean region, that
consists in looking at equal time correlators between proton states of infinite
momentum, encounters some problems related to the power divergent mixing
pattern of DIS operators, when implemented within the lattice regularization.Comment: 15 pages, no figures, Physical Review D (2017
Quantum Cloning by Cellular Automata
We introduce a quantum cellular automaton that achieves approximate
phase-covariant cloning of qubits. The automaton is optimized for 1-to-2N
economical cloning. The use of the automaton for cloning allows us to exploit
different foliations for improving the performance with given resources.Comment: 4 pages, 6 figures, 1 table, published versio
Quantum Hypergraph States
We introduce a class of multiqubit quantum states which generalizes graph
states. These states correspond to an underlying mathematical hypergraph, i.e.
a graph where edges connecting more than two vertices are considered. We derive
a generalised stabilizer formalism to describe this class of states. We
introduce the notion of k-uniformity and show that this gives rise to classes
of states which are inequivalent under the action of the local Pauli group.
Finally we disclose a one-to-one correspondence with states employed in quantum
algorithms, such as Deutsch-Jozsa's and Grover's.Comment: 9+5 pages, 5 figures, 1 table, published versio
Quantum channel detection
We present a method to detect properties of quantum channels, assuming that
some a priori information about the form of the channel is available. The
method is based on a correspondence with entanglement detection methods for
multipartite density matrices based on witness operators. We first illustrate
the method in the case of entanglement breaking channels and non separable
random unitary channels, and show how it can be implemented experimentally by
means of local measurements. We then study the detection of non separable maps
and show that for pairs of systems of dimension higher than two the detection
operators are not the same as in the random unitary case, highlighting a richer
separability structure of quantum channels with respect to quantum states.
Finally we consider the set of PPT maps, developing a technique to reveal NPT
maps.Comment: 7 pages, 4 figures, published versio
On lattice chiral gauge theories
The Smit-Swift-Aoki formulation of a lattice chiral gauge theory is presented. In this formulation the Wilson and other non invariant terms in the action are made gauge invariant by the coupling with a nonlinear auxilary scalar field, omega. It is shown that omega decouples from the physical states only if appropriate parameters are tuned so as to satisfy a set of BRST identities. In addition, explicit ghost fields are necessary to ensure decoupling. These theories can give rise to the correct continuum limit. Similar considerations apply to schemes with mirror fermions. Simpler cases with a global chiral symmetry are discussed and it is shown that the theory becomes free at decoupling. Recent numerical simulations agree with those considerations
Localization Transition in Incommensurate non-Hermitian Systems
A class of one-dimensional lattice models with incommensurate complex
potential is found
to exhibit localization transition at . This
transition from extended to localized states manifests in the behavior of the
complex eigenspectum. In the extended phase, states with real eigenenergies
have finite measure and this measure goes to zero in the localized phase.
Furthermore, all extended states exhibit real spectrum provided . Another novel feature of the system is the fact that the
imaginary part of the spectrum is sensitive to the boundary conditions {\it
only at the onset to localization}
- …