12 research outputs found
Quantum defragmentation algorithm
In this addendum of our paper [D. Burgarth and V. Giovannetti, Phys. Rev.
Lett. 99, 100501 (2007)] we prove that during the transformation that allows
one to enforce control by relaxation on a quantum system, the ancillary memory
can be kept at a finite size, independently from the fidelity one wants to
achieve. The result is obtained by introducing the quantum analog of
defragmentation algorithms which are employed for efficiently reorganizing
classical information in conventional hard-disks. Our result also implies that
the reduced dynamics in any noisy system can be simulated with finitely many
resources.Comment: 2 pages, 1 figur
Indirect Hamiltonian Identification through a small gateway
Identifying the nature of interactions in a quantum system is essential in
understanding any physical phenomena. Acquiring information on the Hamiltonian
can be a tough challenge in many-body systems because it generally requires
access to all parts of the system. We show that if the coupling topology is
known, the Hamiltonian identification is indeed possible indirectly even though
only a small gateway to the system is used. Surprisingly, even a degenerate
Hamiltonian can be estimated by applying an extra field to the gateway.Comment: 5 pages, 1 figure; see Video Abstract at
http://www.quantiki.org/video_abstracts/0903061
Communication through a quantum link
A chain of interacting spin behaves like a quantum mediator (quantum link)
which allows two distant parties that control the ends of the chain to exchange
quantum messages. We show that over repeated uses without resetting the study
of a quantum link can be connected to correlated quantum channels with finite
dimensional environment (finite memory quantum channel). Then, using coding
arguments for such kind of channels and results on mixing channels we present a
protocol that allows us to achieve perfect information transmission through a
quantum link.Comment: 10 pages, 1 figur
Memory Effects in spin-chain channels for information transmission
We investigate the multiple use of a ferromagnetic spin chain for quantum and
classical communications without resetting. We find that the memory of the
state transmitted during the first use makes the spin chain a qualitatively
different quantum channel during the second transmission, for which we find the
relevant Kraus operators. We propose a parameter to quantify the amount of
memory in the channel and find that it influences the quality of the channel,
as reflected through fidelity and entanglement transmissible during the second
use. For certain evolution times, the memory allows the channel to exceed the
memoryless classical capacity (achieved by separable inputs) and in some cases
it can also enhance the quantum capacity.Comment: 5 pages, 4 figure
Dynamical Recurrence and the Quantum Control of Coupled Oscillators
Controllability-the possibility of performing any target dynamics by applying a set of available operations-is a fundamental requirement for the practical use of any physical system. For finite-dimensional systems, such as spin systems, precise criteria to establish controllability, such as the so-called rank criterion, are well known. However, most physical systems require a description in terms of an infinite-dimensional Hilbert space whose controllability properties are poorly understood. Here, we investigate infinite-dimensional bosonic quantum systems-encompassing quantum light, ensembles of bosonic atoms, motional degrees of freedom of ions, and nanomechanical oscillators-governed by quadratic Hamiltonians (such that their evolution is analogous to coupled harmonic oscillators). After having highlighted the intimate connection between controllability and recurrence in the Hilbert space, we prove that, for coupled oscillators, a simple extra condition has to be fulfilled to extend the rank criterion to infinite-dimensional quadratic systems. Further, we present a useful application of our finding, by proving indirect controllability of a chain of harmonic oscillators
Local controllability of quantum networks
We give a sufficient criterion that guarantees that a many-body quantum
system can be controlled by properly manipulating the (local) Hamiltonian of
one of its subsystems. The method can be applied to a wide range of systems: it
does not depend on the details of the couplings but only on their associated
topology. As a special case, we prove that Heisenberg and
Affleck-Kennedy-Lieb-Tasaki chains can be controlled by operating on one of the
spins at their ends. In principle, arbitrary quantum algorithms can be
performed on such chains by acting on a single qubit.Comment: 4 pages, 3 figure
Efficient generation of a maximally entangled state by repeated on- and off-resonant scattering of ancilla qubits
A scheme for preparing two fixed non-interacting qubits in a maximally
entangled state is presented. By repeating on- and off-resonant scattering of
ancilla qubits, the state of the target qubits is driven from an arbitrary
initial state into the singlet state with probability 1 (perfect efficiency).
Neither the preparation nor the post-selection of the ancilla spin state is
required. The convergence from an arbitrary input state to the unique fixed
point (mixing property) is proved rigorously, and its robustness is
investigated, by scrutinizing the effects of imperfections in the incident wave
of the ancilla, such as mistuning to a resonant momentum, imperfect
monochromatization, and fluctuation of the incident momentum, as well as
detector efficiency.Comment: 18 pages, 12 figure
Memory Effects in spin-chain channels for information transmission
We investigate the multiple use of a ferromagnetic spin chain for quantum and
classical communications without resetting. We find that the memory of the
state transmitted during the first use makes the spin chain a qualitatively
different quantum channel during the second transmission, for which we find the
relevant Kraus operators. We propose a parameter to quantify the amount of
memory in the channel and find that it influences the quality of the channel,
as reflected through fidelity and entanglement transmissible during the second
use. For certain evolution times, the memory allows the channel to exceed the
memoryless classical capacity (achieved by separable inputs) and in some cases
it can also enhance the quantum capacity.Comment: 5 pages, 4 figure