6 research outputs found
99%-fidelity ballistic quantum-state transfer through long uniform channels
Quantum-state transfer with fidelity higher than 0.99 can be achieved in the
ballistic regime of an arbitrarily long one-dimensional chain with uniform
nearest-neighbor interaction, except for the two pairs of mirror symmetric
extremal bonds, say x (first and last) and y (second and last-but-one). These
have to be roughly tuned to suitable values x ~ 2 N^{-1/3} and y ~ 2^{3/4}
N^{-1/6}, N being the chain length. The general framework can describe the
end-to-end response in different models, such as fermion or boson hopping
models and XX spin chains.Comment: 12 pages, 11 figures, 1 tabl
Optimal dynamics for quantum-state and entanglement transfer through homogeneous quantum wires
It is shown that effective quantum-state and entanglement transfer can be
obtained by inducing a coherent dynamics in quantum wires with homogeneous
intrawire interactions. This goal is accomplished by tuning the coupling
between the wire endpoints and the two qubits there attached, to an optimal
value. A general procedure to determine such value is devised, and scaling laws
between the optimal coupling and the length of the wire are found. The
procedure is implemented in the case of a wire consisting of a spin-1/2 XY
chain: results for the time dependence of the quantities which characterize
quantum-state and entanglement transfer are found of extremely good quality and
almost independent of the wire length. The present approach does not require
`ad hoc' engineering of the intrawire interactions nor a specific initial pulse
shaping, and can be applied to a vast class of quantum channels.Comment: 5 pages, 5 figure
Using the J1-J2 Quantum Spin Chain as an Adiabatic Quantum Data Bus
This paper investigates numerically a phenomenon which can be used to
transport a single q-bit down a J1-J2 Heisenberg spin chain using a quantum
adiabatic process. The motivation for investigating such processes comes from
the idea that this method of transport could potentially be used as a means of
sending data to various parts of a quantum computer made of artificial spins,
and that this method could take advantage of the easily prepared ground state
at the so called Majumdar-Ghosh point. We examine several annealing protocols
for this process and find similar result for all of them. The annealing process
works well up to a critical frustration threshold.Comment: 14 pages, 13 figures (2 added), revisions made to add citations and
additional discussion at request of referee
Long quantum channels for high-quality entanglement transfer
High-quality quantum-state and entanglement transfer can be achieved in an
unmodulated spin bus operating in the ballistic regime, which occurs when the
endpoint qubits A and B are coupled to the chain by an exchange interaction
comparable with the intrachain exchange. Indeed, the transition amplitude
characterizing the transfer quality exhibits a maximum for a finite optimal
value , where is the channel length. We show that
scales as for large and that it ensures a
high-quality entanglement transfer even in the limit of arbitrarily long
channels, almost independently of the channel initialization. For instance, the
average quantum-state transmission fidelity exceeds 90% for any chain length.
We emphasize that, taking the reverse point of view, should be
experimentally constrained, high-quality transfer can still be obtained by
adjusting the channel length to its optimal value.Comment: 12 pages, 9 figure
Effective cutting of a quantum spin chain by bond impurities
Spin chains are promising media for short-haul quantum communication. Their
usefulness is manifested in all those situations where stationary information
carriers are involved. In the majority of the communication schemes relying on
quantum spin chains, the latter are assumed to be finite in length, with well
addressable end-chain spins. In this paper we propose that such configuration
could actually be achieved by a mechanism that is able to effectively cut a
spin ring through the insertion of bond defects. We then show how suitable
physical quantities can be identified as figures of merit for the effectiveness
of the cut. We find that, even for modest strengths of the bond defect, a ring
is effectively cut at the defect site. In turn, this has important effects on
the amount of correlations shared by the spins across the resulting chain,
which we study by means of a scattering-based mechanism of a clear physical
interpretation.Comment: 7 pages; revised version, jour. ref. adde