1,318 research outputs found
Error Avoiding Quantum Codes
The existence is proved of a class of open quantum systems that admits a
linear subspace of the space of states such that the restriction of
the dynamical semigroup to the states built over is unitary. Such
subspace allows for error-avoiding (noiseless) enconding of quantum
information.Comment: 9 pages, LaTe
Decoherence suppression for oscillator-assisted geometric quantum gates via symmetrization
We propose a novel symmetrization procedure to beat decoherence for
oscillator-assisted quantum gate operations. The enacted symmetry is related to
the global geometric features of qubits transformation based on ancillary
oscillator modes, e.g. phonons in an ion-trap system. It is shown that the
devised multi-circuit symmetrized evolution endows the system with a two-fold
resilience against decoherence: insensitivity to thermal fluctuations and
quantum dissipation.Comment: 4 pages, 2 figure
Virtual Quantum Subsystems
The physical resources available to access and manipulate the degrees of
freedom of a quantum system define the set of operationally relevant
observables. The algebraic structure of selects a preferred tensor
product structure i.e., a partition into subsystems. The notion of compoundness
for quantum system is accordingly relativized. Universal control over virtual
subsystems can be achieved by using quantum noncommutative holonomiesComment: Presentation improved, to appear in PRL. 4 Pages, RevTe
Quantum phase transitions and quantum fidelity in free fermion graphs
In this paper we analyze the ground state phase diagram of a class of
fermionic Hamiltonians by looking at the fidelity of ground states
corresponding to slightly different Hamiltonian parameters. The Hamiltonians
under investigation can be considered as the variable range generalization of
the fermionic Hamiltonian obtained by the Jordan-Wigner transformation of the
XY spin-chain in a transverse magnetic field. Under periodic boundary
conditions, the matrices of the problem become circulant and the models are
exactly solvable. Their free-ends counterparts are instead analyzed
numerically. In particular, we focus on the long range model corresponding to a
fully connected directed graph, providing asymptotic results in the
thermodynamic limit, as well as the finite-size scaling analysis of the second
order quantum phase transitions of the system. A strict relation between
fidelity and single particle spectrum is demonstrated, and a peculiar gapful
transition due to the long range nature of the coupling is found. A comparison
between fidelity and another transition marker borrowed from quantum
information i.e., single site entanglement, is also considered.Comment: 14 pages, 5 figure
Ground-State Entanglement in Interacting Bosonic Graphs
We consider a collection of bosonic modes corresponding to the vertices of a
graph Quantum tunneling can occur only along the edges of
and a local self-interaction term is present. Quantum entanglement of one
vertex with respect the rest of the graph is analyzed in the ground-state of
the system as a function of the tunneling amplitude The topology of
plays a major role in determining the tunneling amplitude
which leads to the maximum ground-state entanglement. Whereas in most of the
cases one finds the intuitively expected result we show that it
there exists a family of graphs for which the optimal value of is pushed
down to a finite value. We also show that, for complete graphs, our bi-partite
entanglement provides useful insights in the analysis of the cross-over between
insulating and superfluid ground statesComment: 5 pages (LaTeX) 5 eps figures include
Computation on a Noiseless Quantum Code and Symmetrization
Let be the state-space of a quantum computer coupled with the
environment by a set of error operators spanning a Lie algebra
Suppose admits a noiseless quantum code i.e., a subspace annihilated by We show that a universal set of
gates over is obtained by any generic pair of -invariant
gates. Such gates - if not available from the outset - can be obtained by
resorting to a symmetrization with respect to the group generated by Any computation can then be performed completely within the coding
decoherence-free subspace.Comment: One result added, to appear in Phys. Rev. A (RC) 4 pages LaTeX, no
figure
Surface ages of mid-size Saturnian satellites
The observations of the surfaces of the mid sized Saturnian satellites made
by Cassini Huygens mission have shown a variety of features that allows study
of the processes that took place and are taking place on those worlds. Research
of the Saturnian satellite surfaces has clear implications for Saturn history
and surroundings. In a recent paper, the production of craters on the mid sized
Saturnian satellites by Centaur objects was calculated considering the current
Solar System. We have compared our results with crater counts from Cassini
images and we have noted that the number of observed small craters is less than
our calculated number. In this paper we estimate the age of the surface for
each observed terrain on each mid sized satellite of Saturn. We have noticed
that since there are less observed small craters than calculated (except on
Iapetus), this results in younger ages. This could be the result of efficient
endogenous or exogenous process(es) for erasing small craters and or crater
saturation at those sizes. The size limit from which the observed number of
smaller craters is less than the calculated is different for each satellite,
possibly indicating processes that are unique to each, but other potential
common explanations would be crater saturation and or deposition of E ring
particles. These processes are also suggested by the findings that the smaller
craters are being preferentially removed, and the erasure process is gradual.
On Enceladus, only mid and high latitude plains have remnants of old terrains;
the other regions could be young; the regions near the South Polar Terrain
could be as young as 50 Myr old. On the contrary for Iapetus, all the surface
is old and it notably registers a primordial source of craters. As the crater
size is decreased, it would be perceived to approach saturation until D less
than 2 km craters, where saturation is complete.Comment: Accepted for publication in Icarus, 40 pages, 11 figure
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